Chapter II: Preface: To Fourth Edition (1)

The Committee on Standard Methods of Bacteriological Water Analysis was reorganized in 1918 with the following membership: F. P. Gorham, chairman, L. A. Rogers, W. G. Bissell, H. E. Hasseltine, H. W. Redfield, with M. Levine as adjunct member. This committee made a report in 1918 which was not acted on by the Laboratory Section, and in 1919 made a revised report, recommending certain changes in Standard Methods, which were adopted by the section and which are now incorporated in this present fourth edition.

Following are the more important changes:

New brands of peptone authorized.

Phenol Red Method of Hydrogen-ion Concentration.

Five-tenths per cent of sugar specified for broths instead of 1 per cent.

Sterilization of sugar is media specified in greater detail.

Preparation of Endo Medium.

Synthetic Medium for the Methyl Red Test.

There are no changes in the chemical methods in this edition.

AMERICAN PUBLIC HEALTH ASSOCIATION.
_LABORATORY SECTION._
STANDARD METHODS FOR THE EXAMINATION OF WATER AND SEWAGE.

Compiled and revised by committees of the American Public Health Association and the American Chemical Society and referees of the Association of Official Agricultural Chemists.

COLLECTION OF SAMPLES.

QUANTITY REQUIRED FOR ANALYSIS.

The minimum quantity necessary for making the ordinary physical, chemical, and microscopical analyses of water or sewage is 2 liters; for the bacteriological examination, 100 cc. In special analyses larger quantities may be required.

BOTTLES.

The bottles for the collection of samples shall have glass stoppers, except when physical, mineral, or microscopical examinations only are to be made. Jugs or metal containers shall not be used.

Sample bottles shall be carefully cleansed each time before using. This may be done by treating with sulfuric acid and potassium bichromate, or with alkaline permanganate, followed by a mixture of oxalic and sulfuric acids, and by thoroughly rinsing with water and draining. The stoppers and necks of the bottles shall be protected from dirt by tying cloth, thick paper or tin foil over them.

For shipment bottles shall be packed in cases with a separate compartment for each bottle. Wooden boxes may be lined with corrugated fibre paper, felt, or similar substance, or provided with spring corner strips, to prevent breakage. Lined wicker baskets also may be used.

Bottles for bacteriological samples shall be sterilized as directed on page 98.

INTERVAL BEFORE ANALYSIS.

In general, the shorter the time elapsing between the collection and the analysis of a sample the more reliable will be the analytical results. Under many conditions analyses made in the field are to be commended, as data so obtained are frequently preferable to data obtained in a distant laboratory after the composition of the water has changed.

The time that may be allowed to elapse between the collection of a sample and the beginning of its analysis cannot be stated definitely. It depends on the character of the sample, the examinations to be made, and other conditions. The following are suggested as fairly reasonable maximum limits.

_Physical and chemical analysis._

Ground waters 72 hours
Fairly pure surface waters 48 "
Polluted surface waters 12 "
Sewage effluents 6 "
Raw sewages 6 "

_Microscopical examination._

Ground waters 72 hours
Fairly pure surface waters 24 "
Waters containing fragile organisms Immediate examination

_Bacteriological examination._

Samples kept at less than 10°C 24 hours

If a longer period elapses between collection and examination the time should be noted. If sterilized by the addition of chloroform, formaldehyde, mercuric chloride, or some other germicide samples for sanitary chemical examination may be allowed to stand for longer periods than those indicated, but as this is a matter which will vary according to circumstances, no definite procedure is recommended. If unsterilized samples of sewage, sewage effluents, and highly polluted surface waters are analyzed after greater intervals than those suggested caution must be used in interpreting analyses of the organic content, which frequently changes materially upon standing.

Determinations of dissolved gases, especially oxygen, hydrogen sulfide, and carbon dioxide, should be made at the time of collection in order to be reasonably accurate, in accordance with the directions given hereafter in connection with each determination.

REPRESENTATIVE SAMPLES.

Care should be taken to obtain a sample that is truly representative of the liquid to be analyzed. With sewages this is especially important because marked variations in composition occur from hour to hour. Satisfactory samples of some liquids can be obtained only by mixing together several portions collected at different times or at different places—the details as to collection and mixing depending upon local conditions.

PHYSICAL EXAMINATION.

TEMPERATURE.

The temperature of the sample, if taken, shall be taken at the time of collection, and shall be expressed preferably in degrees Centigrade, to the nearest degree, or closer if more precise data are required. The thermophone[109] is recommended for obtaining the temperature of water at various depths below the surface.

TURBIDITY.

The turbidity of water is due to suspended matter, such as clay, silt, finely divided organic matter, microscopic organisms, and similar material.

TURBIDITY STANDARD.[110]

The standard of turbidity shall be that adopted by the United States Geological Survey, namely, a water which contains 100 parts per million of silica in such a state of fineness that a bright platinum wire 1 millimeter in diameter can just be seen when the center of the wire is 100 millimeters below the surface of the water and the eye of the observer is 1.2 meters above the wire, the observation being made in the middle of the day, in the open air, but not in sunlight, and in a vessel so large that the sides do not shut out the light so as to influence the results. The turbidity of such water is arbitrarily fixed at 100 parts per million.

For preparation of the silica standard dry Pear’s “precipitated fuller’s earth” and sift it through a 200–mesh sieve. One gram of this preparation in 1 liter of distilled water makes a stock suspension which contains 1,000 parts per million of silica and which should have a turbidity of 1,000. Test this suspension, after diluting a portion of it with nine times its volume of distilled water, by the platinum wire method to ascertain if the silica has the necessary degree of fineness and if the suspension has the necessary degree of turbidity. If not, correct by adding more silica or more water as the case demands.[A]

Footnote A:

This method of correction very slightly alters the coefficient of
fineness of the standard, but does not noticeably affect its use.

Standards for comparison shall be prepared from this stock suspension by dilution with distilled water. For turbidity readings below 20, standards of 0, 5, 10, 15, and 20 shall be kept in clear glass bottles of the same size as that containing the sample; for readings above 20, standards of 20, 30, 40, 50, 60, 70, 80, 90, and 100 shall be kept in 100 cc. Nessler tubes approximately 20 millimeters in diameter.

Comparison with the standards shall be made by viewing both standard and sample sidewise toward the light by looking at some object and noting the distinctness with which the margins of the object can be seen.

The standards shall be kept stoppered, and both sample and standards shall be thoroughly shaken before making the comparison.

In order to prevent any bacterial or algal growths from developing in the standards a small amount of mercury bichloride may be added to them.

PLATINUM WIRE METHOD.[42]

This method requires a rod with a platinum wire 1 mm. in diameter inserted in it about 1 inch from one end of the rod and projecting from it at a right angle at least 25 mm. Near the other end of the rod, at a distance of 1.2 meters from the platinum wire, a small ring shall be placed directly above the wire through which, with his eye directly above the ring, the observer shall look when making the examination.

The rod shall be graduated as follows: The graduation mark of 100 shall be placed on the rod at a distance of 100 mm. from the center of the wire. Other graduations shall be made according to Table 1, which is based on the best obtainable data. The distances recorded in Table 1 are intended to be such that when the water is diluted the turbidity readings will decrease in the same proportion as the percentage of the original water in the mixture. These graduations are those on what is known as the U. S. Geological Survey Turbidity Rod of 1902.[105]

Table 1.—GRADUATION OF TURBIDITY ROD.

───────────────────────────────────┬───────────────────────────────────
Turbidity │ Vanishing depth of wire (mm.).
(parts per million). │
│
───────────────────────────────────┼───────────────────────────────────
7│ 1095
8│ 971
9│ 873
10│ 794
11│ 729
12│ 674
13│ 627
14│ 587
15│ 551
16│ 520
17│ 493
18│ 468
19│ 446
20│ 426
22│ 391
24│ 361
26│ 336
28│ 314
30│ 296
35│ 257
40│ 228
45│ 205
50│ 187
55│ 171
60│ 158
65│ 147
70│ 138
75│ 130
80│ 122
85│ 116
90│ 110
95│ 105
100│ 100
110│ 93
120│ 86
130│ 81
140│ 76
150│ 72
160│ 68.7
180│ 62.4
200│ 57.4
250│ 49.1
300│ 43.2
350│ 38.8
400│ 35.4
500│ 30.9
600│ 27.7
800│ 23.4
1000│ 20.9
1500│ 17.1
2000│ 14.8
3000│ 12.1
───────────────────────────────────┴───────────────────────────────────

_Procedure._—Lower the rod vertically into the water as far as the wire can be seen and read the level of the surface of the water on the graduated scale. This will indicate the turbidity.

The following precautions shall be taken to insure correct results:

Observations shall be made in the open air, preferably in the middle of the day and not in direct sunlight. The wire shall be kept bright and clean. If for any reason observations cannot be made directly under natural conditions a pail or tank may be filled with water and the observation taken in that, but if this is done care shall be taken that the water is thoroughly stirred before the observation is made, and no vessel shall be used for this purpose unless its diameter is at least twice as great as the depth to which the wire is immersed. Waters which have a turbidity greater than 500 shall be diluted with clear water before the observations are made, but if this is done the degree of dilution shall be reported.

TURBIDIMETRIC METHOD.

Several forms of turbidimeter or diaphanometer[73] have been suggested for use. The simplest and most satisfactory form is the candle turbidimeter.[116] This consists of a graduated glass tube with a flat polished bottom, enclosed in a metal case. This is supported over an English standard candle and so arranged that one may look vertically down through the tube at the flame of the candle. The observation is made by pouring the sample of water into the tube until the image of the flame of the candle just disappears from view. Care shall be taken not to allow soot or moisture to accumulate on the lower side of the glass bottom of the tube so as to interfere with the accuracy of the observations. The graduations on the tube correspond to turbidities produced in distilled water by certain numbers of parts per million of silica standard. In order to insure uniform results it is necessary to have the distance between the top rim of the candle and the bottom of the tube constant, and this distance shall be 7.6 cm. or 3 inches. The observations shall be made in a darkened room or with a black cloth over the head.

It is allowable to substitute for the candle an electric light. Calibrate the apparatus to correspond with the United States Geological Survey scale. The figures in Table 2 on page 8 are believed to be approximately correct for the candle turbidimeter but should be checked by the experimenter. It is allowable to calibrate the tube of the instrument with waters of known turbidity prepared by making a series of dilutions of the silica standard with distilled water. From the figures obtained in calibrating plot a curve from which the turbidity of a sample may be read when the depth of water in the tube has been obtained.

Table 2.—GRADUATION OF CANDLE TURBIDIMETER.

───────────────────────────────────┬───────────────────────────────────
Depth of liquid │ Turbidity
(cm.). │ (parts per million of silica).
───────────────────────────────────┼───────────────────────────────────
2.3│ 1000
2.6│ 900
2.9│ 800
3.2│ 700
3.5│ 650
3.8│ 600
4.1│ 550
4.5│ 500
4.9│ 450
5.5│ 400
5.6│ 390
5.8│ 380
5.9│ 370
6.1│ 360
6.3│ 350
6.4│ 340
6.6│ 330
6.8│ 320
7.0│ 310
7.3│ 300
7.5│ 290
7.8│ 280
8.1│ 270
8.4│ 260
8.7│ 250
9.1│ 240
9.5│ 230
9.9│ 220
10.3│ 210
10.9│ 200
11.4│ 190
12.0│ 180
12.7│ 170
13.5│ 160
14.4│ 150
15.4│ 140
16.6│ 130
18.0│ 120
19.6│ 110
21.5│ 100
───────────────────────────────────┴───────────────────────────────────

The results of turbidity observations shall be expressed in whole numbers which correspond to parts per million of silica and recorded as follows:

Turbidity between │ 1│and│ 50│recorded to nearest│unit
" " │ 51│ " │ 100│ " " " │ 5
" " │ 101│ " │ 500│ " " " │ 10
" " │ 501│ " │ 1000│ " " " │ 50
" " │1001│ " │greater│ " " " │ 100

COEFFICIENT OF FINENESS[80]

The quotient obtained by dividing the weight of suspended matter in the sample by the turbidity, both expressed in the same unit, shall be called the coefficient of fineness. If the quotient is greater than unity the matter in suspension is coarser and if it is less than unity it is finer than the standard.

COLOR.

The “color,” or the “true color,” of water shall be considered the color that is due only to substances in solution; that is, it is the color of the water after the suspended matter has been removed. In stating results the word “color” shall mean the “true color” unless otherwise designated.

The “apparent color” shall be considered as including not only the true color but also any color produced by substances in suspension. It is the color of the original unfiltered sample.

The platinum-cobalt method of measuring color shall be considered as the standard, and the unit of color shall be that produced by 1 part per million of platinum.

COMPARISON WITH PLATINUM-COBALT STANDARDS.[43]

_Reagents._—Dissolve 1.246 grams of potassium platinic chloride (PtCl_{4}2KCl), containing 0.5 gram platinum, and 1.00 gram crystallized cobalt chloride (CoCl_{2}.6H_{2}O), containing 0.25 gram of cobalt, in water with 100 cc. concentrated hydrochloric acid, and dilute to 1 liter with distilled water. This solution has a color of 500. Dilute this solution with distilled water in 50 cc. Nessler tubes to prepare standards having colors of 0, 5, 10, 15, 20, 25, 30, 35, 40, 50, 60, and 70. Keep these standards in Nessler tubes of such diameter that the graduation mark is between 20 and 25 cm. above the bottom and of such uniformity that they match within such limit that the distance from the bottom to the graduation mark of the longest tube shall not exceed that of the shortest tube by more than 6 mm. Protect the tubes from dust and light when not in use.

_Procedure._—The color of a sample shall be observed by filling a standard Nessler tube to the height equal to that in the standard tubes with the sample and by comparing it with the standards. The observation shall be made by looking vertically downward through the tubes upon a white or mirrored surface placed at such angle that light is reflected upward through the column of liquid.

Water that has a color greater than 70 shall be diluted before making the comparison, in order that no difficulties may be encountered in matching the hues.

Water containing matter in suspension shall be filtered, before the color observation is made, until no visible turbidity remains. If the suspended matter is coarse, filter paper may be used for this purpose; if the suspended matter is fine, the use of a Berkefeld filter is recommended. The Pasteur filter shall not be used as it exerts a marked decolorizing action.

The apparent color, if determined, shall be determined on the original sample without filtration. The true and the apparent color of clear waters or waters with low turbidities are substantially the same.

The results of color determinations shall be expressed in whole numbers and recorded as follows:

Color between 1 and 50 recorded to nearest unit
" " 51 " 100 " " " 5
" " 101 " 250 " " " 10
" " 251 " 500 " " " 20.

COMPARISON WITH GLASS DISKS.[105]

As the platinum-cobalt standard method is not well adapted for field work, the color of the water to be tested may be compared with that of glass disks held at the end of metallic tubes through which they are viewed by looking toward a white surface. The glass disks are individually calibrated to correspond with colors on the platinum scale. Experience has shown that the glass disks used by the U. S. Geological Survey give results in substantial agreement with those obtained by the platinum determinations, and their use is recognized as a standard procedure.

COMPARISON WITH NESSLER STANDARDS.

Inasmuch as the Nessler scale[62] and the natural water scale[22][49] which agrees with it except for colors less than 20, have been largely used in the past, the old results may be converted[117] into terms of the platinum standard by means of the ratios in Table 3, but they must not be considered as universally applicable as the variable sensitiveness of the Nessler solution introduces an uncertain factor.

Table 3.—VALUES FOR CONVERTING COLORS BY THE NATURAL WATER SCALE INTO
COLORS BY THE PLATINUM STANDARD IN PARTS PER MILLION.[B]

───────────┬─────┬─────┬─────┬─────┬─────┬─────┬─────┬─────┬─────┬─────
Modified │ │ │ │ │ │ │ │ │ │
Nessler or │ │ │ │ │ │ │ │ │ │
natural │0.00.│0.01.│0.02.│0.03.│0.04.│0.05.│0.06.│0.07.│0.08.│0.09.
water │ │ │ │ │ │ │ │ │ │
standard. │ │ │ │ │ │ │ │ │ │
───────────┴─────┴─────┴─────┴─────┴─────┴─────┴─────┴─────┴─────┴─────
Platinum-cobalt standard color.
0.00│ 0│ 2│ 4│ 6│ 8│ 9│ 11│ 13│ 15│ 17
.10│ 18│ 19│ 20│ 20│ 21│ 22│ 23│ 24│ 24│ 26
.20│ 26│ 27│ 27│ 28│ 29│ 29│ 30│ 31│ 32│ 32
.30│ 33│ 34│ 34│ 35│ 35│ 36│ 37│ 37│ 38│ 38
.40│ 39│ 40│ 40│ 41│ 42│ 42│ 43│ 44│ 45│ 45
.50│ 46│ 47│ 47│ 48│ 48│ 49│ 50│ 50│ 51│ 51
.60│ 52│ 53│ 53│ 54│ 54│ 55│ 56│ 56│ 57│ 57
.70│ 58│ 58│ 59│ 59│ 60│ 60│ 61│ 61│ 62│ 62
.80│ 63│ 64│ 64│ 65│ 66│ 66│ 67│ 68│ 69│ 69
.90│ 70│ 71│ 72│ 73│ 74│ 75│ 77│ 78│ 79│ 80
1.00│ 81│ 82│ 82│ 83│ 84│ 84│ 85│ 86│ 87│ 87
1.10│ 88│ 89│ 89│ 90│ 91│ 91│ 92│ 93│ 94│ 94
1.20│ 95│ 96│ 96│ 97│ 98│ 98│ 99│ 100│ 101│ 101
1.30│ 102│ 103│ 103│ 104│ 105│ 105│ 106│ 107│ 108│ 108
1.40│ 109│ 110│ 110│ 111│ 112│ 112│ 113│ 114│ 115│ 115
1.50│ 116│ 117│ 117│ 118│ 118│ 119│ 120│ 120│ 121│ 121
1.60│ 122│ 123│ 123│ 124│ 125│ 125│ 126│ 127│ 128│ 128
1.70│ 129│ 130│ 130│ 131│ 132│ 132│ 133│ 134│ 135│ 136
1.80│ 136│ 137│ 137│ 138│ 139│ 139│ 140│ 141│ 142│ 142
1.90│ 143│ 144│ 144│ 145│ 146│ 146│ 147│ 148│ 149│ 149
2.00│ 150│ │ │ │ │ │ │ │ │
───────────┴─────┴─────┴─────┴─────┴─────┴─────┴─────┴─────┴─────┴─────

Footnote B:

Zero on the true Nessler scale is about 15 on the platinum scale.

LOVIBOND TINTOMETER.

The value of the readings of tint and shade by the Lovibond tintometer[66][82][83] has not been commensurate with the labor involved, but it is necessary to make a record of the reflected tint and shade[50] of some waters. The standard color disks used in teaching optics may be used for the purpose.

_Procedure._—The white disk supports three movable standard color sectors, red, yellow, and blue, and one movable black sector. All are mounted on a device which can be revolved rapidly, blending the colors into a uniform tint or shade. A scale around the circumference of the disk is used to indicate the percentage of each color or white or black in the blend.

Place the sample in a battery jar on a white ground; adjust the sectors so that when blended the tint or shade will match the reflected tint or shade of the sample. Report the percentages of red, yellow blue, white, and black in the blended tint or shade.

ODOR.[4][14][53][72][92][114][115][121c]

The observation of the odor, cold and hot, of samples of surface water is important as the odors are usually indicative of organic growths or sewage contamination or both. The odor of some ground waters is caused by the earthy constituents of the water-bearing strata. The odor of a contaminated well water is often contributory evidence of its pollution. A study of the organisms as directed under Microscopical Examination (p. 90) is a valuable adjunct to physical and chemical examination of water. Certain odors distinguish or identify certain organisms, as, for example, the “fishy” odor of _Uroglena_, the “aromatic” or “rose geranium” odor of _Asterionella_ and the “pig pen” odor of _Anabaena_. Observe and record the odor, both at room temperature and at just below the boiling point, as follows:

COLD ODOR.

Shake the sample violently in one of the collecting bottles, when it is half to two-thirds full and when the sample is at room temperature (about 20° C.). Remove the stopper and smell the odor at the mouth of the bottle.

HOT ODOR.

Pour about 150 cc. of the sample into a 500 cc. Erlenmeyer flask. Cover the flask with a well-fitting watch glass. Heat the water almost to boiling on a hot plate. Remove the flask from the plate and allow it to cool not more than five minutes. Then agitate it with a rotary movement, slip the watch glass to one side, and smell the odor.

EXPRESSION OF RESULTS.

Express the quality of the odor by a descriptive epithet like the following, which may be abbreviated in the record:

a—aromatic
C—free chlorine
d—disagreeable
e—earthy
f—fishy
g—grassy
m—moldy
M—musty
P—peaty
s—sweetish
S—hydrogen sulfide
v—vegetable.

Express the intensity of the odor by a numeral prefixed to the term expressing quality, which may be defined as follows:

Numerical Term. Definition.
value.

0 None. No odor perceptible.

1 Very An odor that would not be detected ordinarily by
faint. the average consumer, but that could be detected
in the laboratory by an experienced observer.

2 Faint. An odor that the consumer might detect if his
attention were called to it, but that would not
attract attention otherwise.

3 Distinct. An odor that would be detected readily and that
might cause the water to be regarded with
disfavor.

4 Decided. An odor that would force itself upon the attention
and that might make the water unpalatable.

5 Very An odor of such intensity that the water would be
strong. absolutely unfit to drink. (A term to be used
only in extreme cases.)

CHEMICAL EXAMINATION.

EXPRESSION OF RESULTS.

The results of chemical analyses shall be expressed in parts per million, which in most analyses is practically equivalent to milligrams per liter. In some laboratories other forms of expression have been used. Results expressed in parts per 100,000 or in grains per gallon may be transformed to parts per million, or conversely, by the use of the following table:

Table 4.—FACTORS FOR TRANSFORMING RESULTS OF ANALYSES.

───────────────────────────────┬─────────────────────────────────────── Unit. │ Equivalent. ───────────────────────────────┼─────────┬─────────┬─────────┬───────── │ Grains │ Grains │ │ │per U.S. │ per │Parts per│Parts per │ gallon. │Imperial │100,000. │million. │ │ gallon. │ │ ───────────────────────────────┼─────────┼─────────┼─────────┼───────── 1 grain per U. S. gallon │ 1.000│ 1.20│ 1.71│ 17.1 1 grain per Imperial gallon │ .835│ 1.00│ 1.43│ 14.3 1 part per 100,000 │ .585│ .70│ 1.00│ 10.0 1 part per million │ .058│ .07│ .10│ 1.0 ───────────────────────────────┴─────────┴─────────┴─────────┴─────────

The following general rules shall govern the use of significant figures in the expression of results:

1. If the results show quantities greater than 10 parts per million use no decimals; record only whole numbers. If the quantities reach hundreds and thousands of parts record only two significant figures.

2. If the results are between 1 and 10 parts do not retain more than one decimal place.

3. If the results are between 0.1 and 1 part do not retain more than two decimal places.

4. Estimates of ammonia, albuminoid, and nitrite nitrogen alone justify the use of three decimals.

5. If the results of analyses are tabulated ciphers should not be added at the right of the decimal point to make the column uniform.

FORMS OF NITROGEN.

Nitrogenous organic matter passes through several intermediate compounds during its natural decomposition, and that which does not gasify ultimately forms nitrate. Nitrogen in organic matter is determined by the Kjeldahl process.[13][14][58] An indication of the amount present is obtained by the albuminoid nitrogen determination.[14][15][67][106][107] It has not been found possible to differentiate the nitrogen in the organic matter that readily decomposes from that in stable or non-putrescible compounds. Decomposition of organic matter produces nitrogen combined in ammonia, which is the first step between nitrogenous organic matter and the completely mineralized nitrate. Ammonia nitrogen may be determined by distillation and Nesslerization or by direct Nesslerization of the clarified sample. The next step is oxidation to nitrite, and the final step, oxidation to nitrate. It is recommended that all forms of nitrogen be reported as the element nitrogen (N).

AMMONIA NITROGEN.

There are two methods for estimating ammonia nitrogen—distillation and direct Nesslerization. Distillation is recommended for most waters and direct Nesslerization is recommended for sewages, sewage effluents, and highly polluted surface waters.

DETERMINATION BY DISTILLATION.[38][68b][111][121]

_Procedure._—Use a metal or a glass flask connected with a condenser so that the distillate may drop from the condenser tube directly into a Nessler tube or a flask. Free the apparatus from ammonia by boiling distilled water in it until the distillate shows no trace of ammonia. After this has been done empty the distilling flask and measure into it 500 cc. of the sample, or a smaller portion diluted to 500 cc. with ammonia-free water. If the sample is acid or if the presence of urea is suspected add about 0.5 gram of sodium carbonate before distillation. Omit this if possible as it tends to increase “bumping.” Apply heat so that the distillation may proceed at the rate of not more than 10 cc. nor less than 6 cc. per minute. Collect the distillate in four Nessler tubes, 50 cc. to each tube, or if the nitrogen is high in a 200 cc. graduated flask. These receptacles contain the ammonia nitrogen to be measured as hereafter described.

Use Nessler tubes of such diameter that the graduation mark is between 20 and 25 cm. above the bottom and of such uniformity of diameter that the distance from the bottom to the graduation mark of the longest tube shall not exceed that of the shortest tube by more than 6 mm. The tubes must be of clear white glass with polished bottoms.

MEASUREMENT OF AMMONIA NITROGEN.

The amount of ammonia in the distillates may be measured either by (1) comparison of the Nesslerized distillates with Nesslerized solutions containing known quantities of nitrogen as ammonium chloride, or by (2) comparison of the Nesslerized distillates with permanent standard solutions in which the colors of Nesslerized standard ammonia solutions are duplicated by solutions of platinum and cobalt chlorides.

COMPARISON WITH AMMONIA STANDARDS.

_Reagents._—1. Ammonia-free water.

2. Standard ammonium chloride solution. Dissolve 3.82 grams of ammonium chloride in ammonia-free water and dilute to 1 liter; dilute 10 cc. of this to 1 liter with ammonia-free water. One cc. equals 0.00001 gram of nitrogen.

3. Nessler reagent.[8] Dissolve 50 grams of potassium iodide in a minimum quantity of cold water. Add a saturated solution of mercuric chloride until a slight precipitate persists permanently. Add 400 cc. of 50 per cent solution of potassium hydroxide, made by dissolving the potassium hydroxide and allowing it to clarify by sedimentation before using. Dilute to 1 liter, allow to settle, and decant. This solution should give the required color with ammonia within five minutes after addition and should not produce a precipitate with small amounts of ammonia within two hours.

_Procedure._—Prepare a series of 16 Nessler tubes containing the following amounts of the standard ammonium chloride solution, diluted to 50 cc. with ammonia-free water, namely: 0.0, 0.1, 0.3, 0.5, 0.7, 1.0, 1.4, 1.7, 2.0, 2.5, 3.0, 3.5, 4.0, 4.5, 5.0, and 6.0 cc. These solutions will contain 0.00001 gram of nitrogen for each cubic centimeter of the standard solution.

Nesslerize the standards and the distillates by adding approximately 1 cc. of Nessler reagent to each tube. Do not stir the contents of the tubes. The temperature of the tubes should be practically the same as that of the standards; otherwise the colors will not be directly comparable.[45] Allow the tubes to stand at least 10 minutes after Nesslerizing. Compare the color produced in the tubes with that in the standards by looking vertically downward through them at a white or mirrored surface placed at an angle in front of a window so as to reflect the light upward.

If the color obtained by Nesslerizing the distillates is greater than that of the darkest tube of the standards, mix the contents of the tube thoroughly, pour out half of the liquid, and dilute the remainder to the original volume with ammonia-free water; then make the color comparison and multiply the result by two. If the color is still too dark after pouring out half the liquid, repeat this process of division until a reading can be made. The process of dilution may be shortened by mixing together the distillates from one sample before making the comparison and comparing an aliquot portion with the standards.

After the readings have been recorded add the results obtained by Nesslerizing each portion of the entire distillate. If 500 cc. of the sample is distilled this sum, expressed in cubic centimeters and multiplied by 0.02, will give the number of parts per million of ammonia nitrogen in the sample. If x cc. of sample is used multiply the sum of the readings by 10/x.

If the ammonia is known to be high the distillate may be collected in 200 cc. flasks and an aliquot part Nesslerized.

COMPARISON WITH PERMANENT STANDARDS.[62][65]

_Reagents._—Platinum solution. Dissolve 2.00 grams of potassium platinic chloride (PtCl_{4}.2KCl) in a small amount of distilled water, add 100 cc. of strong hydrochloric acid, and dilute to 1 liter.

Cobalt solution. Dissolve 12 grams of cobaltous chloride (CoCl_{2}.6H_{2}O) in distilled water, add 100 cc. of strong hydrochloric acid, and dilute to 1 liter.

Prepare standards by putting various amounts of these two solutions into Nessler tubes and diluting to the 50 cc. mark with distilled water as indicated in Table 5. These standards may be kept for several months if protected from dust.

Table 5.—PREPARATION OF PERMANENT STANDARDS FOR THE DETERMINATION OF
AMMONIA.

───────────────────────┬───────────────────────┬───────────────────────
Value in standard │ Solution of platinum. │ Solution of cobalt.
ammonium chloride. │ │
───────────────────────┼───────────────────────┼───────────────────────
_cc._│ _cc._│ _cc._
0.0│ 1.2│ 0.0
.1│ 1.8│ .0
.2│ 2.8│ .0
.4│ 4.7│ .1
.7│ 5.9│ .2
│ │
1.0│ 7.7│ .5
1.4│ 9.9│ 1.1
1.7│ 11.4│ 1.7
2.0│ 12.7│ 2.2
2.5│ 15.0│ 3.3
│ │
3.0│ 17.3│ 4.5
3.5│ 19.0│ 5.7
4.0│ 19.7│ 7.1
4.5│ 19.9│ 8.7
5.0│ 20.0│ 10.4
│ │
6.0│ 20.0│ 15.0
7.0│ 20.0│ 22.0
───────────────────────┴───────────────────────┴───────────────────────

The amounts in Table 5 are approximate, and the actual amount necessary will differ with the character of the Nessler solution, the color sensitiveness of the analyst’s eye, and other conditions. The final test of the standard is best obtained by comparing it with Nesslerized standards and modifying the tint accordingly. Such comparison should be made for each new batch of Nessler solution and should be checked by each analyst.

_Procedure._—In comparison with permanent standards, Nesslerize the distillates in the manner above described and compare the resulting colors at the end of about 10 minutes with the permanent standards. The method of calculating results is precisely the same as with the ammonia standards.

MODIFICATION FOR SEWAGE.

Ammonia nitrogen and albuminoid nitrogen in sewages, soils, and other materials of high nitrogen content may be satisfactorily determined by diluting the sample with ammonia-free distilled water and proceeding as described in the preceding sections, but it is permissible to distill with steam.[40]

_Procedure._—Use a 200 cc. long-necked Kjeldahl flask connected with a condenser so that the distillate may drop from the condenser tube directly into a Nessler tube or a flask. Connect the Kjeldahl flask with a steam generator by a tube reaching almost to the bottom of the flask.

After the apparatus is freed from ammonia put the sample to be tested into the flask. Use 10 to 100 cc. of the sample according to its ammonia content. Pass ammonia-free steam through the liquid in the Kjeldahl flask and collect the distillate in the usual way. It is usually convenient to collect the distillate in a 200 cc. flask and to take an aliquot part of it for Nesslerization. Compare with standards and calculate the nitrogen content in the usual manner.

This method has the advantage, when the sample is treated with an alkaline solution of potassium permanganate, of avoiding bumping, permitting the assay of solid matter, and yielding the ammonia more rapidly than by the ordinary process of distillation.

DETERMINATION BY DIRECT NESSLERIZATION.[21][75]

_Reagents._— 1. Ten per cent solution of copper sulfate
(CuSO_{4}.5H_{2}O).

2. Ten per cent solution of lead acetate
(Pb(C_{2}H_{3}O_{2})_{2}.3H_{2}O).

3. Fifty per cent solution of sodium hydroxide (NaOH) or
potassium hydroxide (KOH).

_Procedure._—To 50 cc. of the sample to be tested, diluted if necessary with an equal volume of ammonia-free water, in a short tube, add a few drops of the copper sulfate solution. After thoroughly mixing, add 1 cc. of the alkali hydroxide solution and again thoroughly mix. Allow the tube to stand for a few minutes, when a heavy precipitate should fall to the bottom, leaving a colorless supernatant liquid. Nesslerize an aliquot part. Compare with standards and compute the ammonia nitrogen in the same manner as in the distillation procedure.

Samples containing hydrogen sulfide may require the use of lead acetate in addition to the copper sulfate. Some samples may require a few trials before the right combination of the three solutions to bring about the best results can be found.

Instead of adding copper sulfate to sewages of high magnesium content satisfactory clarification of the sample can be obtained by mixing it with the alkali hydroxide alone.[54]

ALBUMINOID NITROGEN.

The addition of an alkaline permanganate solution to liquids containing nitrogenous organic matter causes the formation of ammonia, which can be distilled and determined by Nesslerization of the distillate. The nitrogen of the ammonia, thus obtained, is called albuminoid nitrogen. As the ratio of nitrogenous organic matter to the ammonia obtained by distillation is decidedly variable[6][30][75] in sewages and other substances containing much nitrogenous organic matter albuminoid nitrogen results on such substances are less accurate[29] than organic (Kjeldahl) nitrogen. Therefore in sewage work, including analysis of influents and effluents of purification plants and the water of highly polluted streams, it is recommended that determinations of organic nitrogen be substituted for determinations of albuminoid nitrogen. For ground waters and surface waters containing but little pollution, the albuminoid nitrogen is approximately one-half the organic nitrogen; accordingly the continuance of albuminoid nitrogen determinations for this class of work is approved.

_Reagents._—Alkaline potassium permanganate. Pour 1,200 cc. of distilled water into a porcelain dish holding 2,500 cc., boil 10 minutes, and turn off the gas. Add 16 grams of C. P. potassium permanganate and stir until solution is complete. Then add 800 cc. of 50 per cent clarified solution of potassium hydroxide or an equivalent amount of sodium hydroxide and enough distilled water to fill the dish. Boil down to 2,000 cc. Test this solution for ammonia by making a blank determination. Correct determinations by the amount of this blank.

_Procedure._—After the collection of the distillate for ammonia nitrogen described on page 15 add 50 cc. (or more if necessary to insure the complete oxidation of the organic matter) of alkaline potassium permanganate and continue the distillation until at least four portions, and preferably five portions, of 50 cc. each, of distillate have been collected in separate tubes. Determine the albuminoid nitrogen in the distillate by Nesslerization. If the albuminoid nitrogen is known to be high it is convenient to collect the distillate in a 200 cc. flask and to Nesslerize an aliquot part of it.

Dissolved albuminoid nitrogen may be determined in a sample from which suspended matter has been removed by filtration either through filter paper or through a Berkefeld filter. Suspended albuminoid nitrogen is the difference between the total and the dissolved albuminoid nitrogen.

ORGANIC NITROGEN.[24b][69][71][76][84]

_Procedure for water._—Boil 500 cc. of the sample in a round-bottomed flask to remove ammonia nitrogen. This usually causes the loss of 200 cc. of the sample, which may be collected for the determination of ammonia nitrogen. Add 5 cc. of nitrogen-free concentrated sulfuric acid and a small piece of ignited pumice. Mix by shaking and place over a flame under a hood. Digest until copious fumes of sulfuric acid are given off and the liquid finally becomes colorless or pale straw color. Remove from the flame, and add potassium permanganate crystals in small portions until a heavy green precipitate persists in the liquid. Cool. Dilute to about 300 cc. with ammonia-free water. Make alkaline with 10 per cent ammonia-free sodium hydroxide. Distill the ammonia, collect the distillate in Nessler tubes, Nesslerize, and compare with standards as described (pp. 16–18).

_First procedure for sewage_[76].—Distill the ammonia nitrogen directly from 100 cc. or less of the sample, diluted to 500 cc. with nitrogen-free water. Collect the distillate and determine the ammonia nitrogen in it. Add 5 cc. of nitrogen-free sulfuric acid and 1 cc. of 10 per cent nitrogen-free copper sulfate, and digest the liquid for half an hour after it has become colorless or pale straw color. Add 0.5 gram of potassium permanganate crystals to the hot acid solution, and dilute to 500 cc. with ammonia-free water. Dilute 10 cc. or more of this liquid, in a Kjeldahl distilling flask, to about 300 cc. with ammonia-free water. Make alkaline with 10 per cent sodium hydroxide, distill, and Nesslerize. With some samples direct Nesslerization may be used. (See p. 19.)

In this determination care must be taken to digest thoroughly, to add potassium permanganate to the point of precipitation, to sample carefully after dilution, and to add enough sodium hydroxide to insure the separation of the ammonia from the precipitated manganese hydroxide. Potassium permanganate should not be added during digestion because it causes loss of nitrogen.

_Second procedure for sewage._—Omit the separation of ammonia nitrogen and determine the ammonia nitrogen and organic nitrogen together. Determine the ammonia nitrogen in a separate sample by direct Nesslerization as described on page 19. The organic nitrogen is equal to the difference.

NITRITE NITROGEN.[51][63a][64][94c][108]

_Reagents._—1. Sulfanilic acid solution. Dissolve 8.00 grams of the purest sulfanilic acid in 1,000 cc. of 5 N acetic acid (sp. gr. 1.041) or in 1,000 cc. of water containing 50 cc. of concentrated hydrochloric acid. This is practically a saturated solution.

2. α-naphthylamine acetate or chloride solution. Dissolve 5.00 grams solid α-naphthylamine in 1,000 cc. of 5 N acetic acid or in 1,000 cc. of water containing 8 cc. of concentrated hydrochloric acid. Filter the solution through washed absorbent cotton or an alundum filter.

3. Sodium nitrite stock solution. Dissolve 1.1 gram silver nitrite in nitrite-free water; precipitate the silver with sodium chloride solution and dilute the whole to 1 liter.

4. Standard sodium nitrite solution. Dilute 100 cc. of solution 3 to 1 liter, then dilute 50 cc. of this solution to 1 liter with sterilized nitrite-free water, add 1 cc. of chloroform, and preserve in a sterilized bottle. One cc. = 0.0005 mg. nitrogen.

5. Fuchsine solution. 0.1 gram per liter.

_Procedure._—Place in a standard Nessler tube 50 cc. of the sample, decolorized if necessary with nitrite-free aluminium hydroxide (see p. 42) or a smaller amount diluted to 50 cc. At the same time prepare in Nessler tubes a set of standards, by diluting to 50 cc. with nitrite-free water, various amounts of the standard nitrite solution. The following amounts of standard solution are suggested: 0.0, 0.1, 0.2, 0.4, 0.7, 1.0, 1.4, 1.7, 2.0, and 2.5 cc. Add 1 cc. of the sulfanilic acid solution and 1 cc. of the α-naphthylamine acetate or hydrochloride solution to the sample and to each standard. Mix thoroughly and allow to stand 10 minutes; then compare the sample with the standards. Do not allow the sample to stand more than one-half hour before making the comparison. If the color of the sample is deeper than that of the highest standard repeat the test on a diluted sample. If 50 cc. of the sample is used 0.01 times the number of cc. of the standard matched equals parts per million of nitrite nitrogen. Satisfactory results can be obtained by using either hydrochloric or acetic acid in preparing the test solutions, but the speed of the reaction is more rapid if acetic acid is used.[112]

Permanent standards may be prepared by matching the nitrite standards with dilutions of the fuchsine solution. Fuchsine standards have been found to be sufficiently accurate for waters high in nitrite and for sewage. The standards should be checked once a month and kept out of bright sunlight.

NITRATE NITROGEN.[16][36][90][100]

Two methods are recommended for the determination of nitrate nitrogen in water, sewage, and sewage effluents.

PHENOLDISULFONIC ACID METHOD.[1][5][32]

_Reagents._—1. Phenoldisulfonic acid. Dissolve 25 grams of pure white phenol in 150 cc. of pure concentrated sulfuric acid. Add 75 cc. of fuming sulfuric acid (15 per cent SO_{3}), stir well, and heat for 2 hours at about 100°C.

2. Potassium hydroxide solution. Prepare an approximately 12 N solution, 10 cc. of which will neutralize about 4 cc. of the phenoldisulfonic acid.

3. Standard nitrate solution. Dissolve 0.72 gram of pure recrystallized potassium nitrate in 1 liter of distilled water. Evaporate cautiously to dryness 10 cc. of the solution on the water bath. Moisten residue quickly and thoroughly with 2 cc. of phenoldisulfonic acid and dilute to 1 liter. This is the standard solution, 1 cc. of which equals 0.001 mg. of nitrate nitrogen.

4. Standard silver sulfate solution. Dissolve 4.4 grams of silver sulfate free from nitrate in 1 liter of water. One cc. of this solution is equal to 1 mg. of chloride.

_Procedure._—The alkalinity, chloride, and nitrite content, and color of the sample must first be determined. If the sample is highly colored decolorize it with freshly precipitated aluminium hydroxide. Measure into an evaporating dish 100 cc. of the sample, or if nitrate is very high such volume as will contain about 0.01 mg. of nitrate nitrogen. Add sufficient N/50 sulfuric acid nearly to neutralize the alkalinity. Then add sufficient standard silver sulfate to precipitate all but about 0.1 mg. of chloride. The removal of chloride may be omitted if the sample contains less than 30 parts per million of chloride. Heat the mixture to boiling, add a little aluminium hydroxide, stir, filter, and wash with small amounts of hot water. Evaporate the filtrate to dryness, and add 2 cc. of the phenoldisulfonic acid, rubbing with a glass rod to insure intimate contact. If the residue becomes packed or appears vitreous because of the presence of much iron, heat the dish on the water bath for a few minutes. Dilute the mixture with distilled water, and add slowly a strong solution of potassium hydroxide or ammonium hydroxide until the maximum color is developed. Transfer the solution to a Nessler tube, filtering if necessary. If nitrate is present a yellow color will be formed. Compare the color with that of standards[52][55] made by adding 2 cc. of strong potassium hydroxide or ammonium hydroxide to various amounts of standard nitrate solution and diluting them to 50 cc. in Nessler tubes. The following amounts of standard nitrate solution are suggested: 0, 0.5, 1.0, 1.5, 2.0, 4.0, 6.0, 8.0, 10.0, 15.0, 20.0, and 40.0 cc. These standards may be kept several weeks without deterioration. If 100 cc. of water is used the number of cubic centimeters of the standard multiplied by 0.01 is equal to parts per million of nitrate nitrogen.

Standards that will remain permanent for several years if stored in the dark may be prepared from tripotassium nitrophenoldisulfonate.[5]

If nitrite nitrogen is present in excess of 1 part per million it should be oxidized by heating the samples a few minutes with a few drops of hydrogen peroxide free from nitrate repeatedly added[95] or by adding dilute potassium permanganate in the cold until a faint pink coloration appears; the nitrogen equivalent of the nitrite thus oxidized to nitrate is then subtracted from the final nitrate nitrogen reading.

REDUCTION METHOD.[2][46]

_Reagents._—1. Sodium or potassium hydroxide solution. Dissolve 250 grams of the hydroxide in 1.25 liters of distilled water. Add several strips of aluminium foil and allow the evolution of hydrogen to continue over night. Concentrate the solution to 1 liter by boiling.

2. Aluminium foil. Use strips of pure aluminium about 10 cm. long, 6 mm. wide, and 0.33 mm. thick and weighing about 0.5 gram.

_Procedure._—To 100 cc. of the sample in a 300 cc. casserole add 2 cc. of the hydroxide solution and concentrate by boiling to about 20 cc. Pour the contents of the casserole into a test tube about 16 cm. long and 3 cm. in diameter, or of approximately 100 cc. capacity. Rinse the casserole several times with nitrogen-free water and add the rinse water to the liquid already in the tube, thus making the contents of the tube approximately 75 cc. Add a strip of aluminium foil. Close the tube by means of a rubber stopper through which passes a bent glass tube about 5 mm. in diameter. Put the shorter arm of the tube flush with the lower side of the rubber stopper and let the longer arm extend below the surface of distilled water in another test tube. This apparatus serves as a trap through which the evolved hydrogen escapes freely. The small amount of ammonia escaping into the trap may be neglected. Allow the action to proceed for a minimum period of four hours or over night. Pour the contents of the tube into a distilling flask, dilute with 250 cc. of ammonia-free water, distill, collect the distillate in Nessler tubes, and Nesslerize. If the nitrate content is high collect the distillate in a 200 cc. flask and Nesslerize an aliquot part. If the supernatant liquid in the reduction tube is clear and colorless the solution may be diluted to a definite volume and an aliquot part Nesslerized without distillation.

TOTAL NITROGEN.[93]

In sewage work it is frequently of assistance to know the total nitrogen content. This is ordinarily computed by adding together the organic, ammonia, nitrite, and nitrate nitrogen, each of which is determined as already described.

OXYGEN CONSUMED.[24][67][84a][85][94f][101][102]

Oxygen consumed means the oxygen that the oxidizable compounds of sewage and water consume when treated in an acid solution with potassium permanganate. The expression is synonymous with oxygen required, oxygen absorbed, and oxygen-consuming capacity. It should not be confused with biochemical oxygen demand.

As the carbon, not the nitrogen, in organic matter is oxidized by potassium permanganate, oxygen consumed is considered by some an indication of the amount of carbonaceous organic matter present. The determination indicates, however, only part of the carbon, the proportion varying in different samples because the carbon in nitrogenous matter is not so readily oxidized as that in carbonaceous organic matter. Furthermore, it does not directly differentiate the carbon present in unstable organic matter from that in fairly stable organic matter, such as is sometimes referred to as residual humus matter. As nitrite nitrogen, ferrous iron, sulfide, and other oxidizable mineral substances reduce potassium permanganate, corrections for them should be made in the determination.

RECOMMENDED METHOD.

_Reagents._—1. Dilute sulfuric acid. Dilute 1 part of concentrated sulfuric acid with 3 parts of distilled water and free the solution from oxidizable matter by adding potassium permanganate until a faint pink color persists after the solution has stood several hours.

2. Standard ammonium oxalate. Dissolve 0.888 gram of the pure salt in 1 liter of distilled water. One cc. is equivalent to 0.1 mg. of oxygen. An equivalent quantity of oxalic acid or sodium oxalate may be used.

3. Standard potassium permanganate. Dissolve 0.4 gram of the crystallized salt in 1 liter of distilled water. Add 10 cc. of the dilute sulfuric acid and 10 cc. of this solution of potassium permanganate to 100 cc. of distilled water, and digest 30 minutes. Add 10 cc. of the ammonium oxalate solution, and then add potassium permanganate till a pink coloration appears. This destroys the oxygen-consuming capacity of the water used. Now add another 10 cc. of ammonium oxalate solution and titrate with potassium permanganate. Adjust the potassium permanganate solution so that 1 cc. is equivalent to 1 cc. of ammonium oxalate solution or 0.1 mg. of available oxygen.

_Acid digestion._—Place in a flask 100 cc. of the water, or, if the water is of high organic content, a smaller portion diluted to 100 cc. Add 10 cc. of sulfuric acid solution and 10 cc. of standard potassium permanganate and digest the liquid exactly 30 minutes in a bath of boiling water the level of which is kept above the level of the contents of the flask.[70][71a] If the quantity of permanganate is insufficient for complete oxidation repeat the digestion with a larger quantity; at least 5 cc. excess of the standard permanganate should be present when the ammonium oxalate solution is added. Remove the flask, add 10 cc. of the ammonium oxalate solution, and titrate with the standard permanganate until a faint but distinct color is obtained. If 100 cc. of water is used the number of cubic centimeters of potassium permanganate solution in excess of the number of cubic centimeters of ammonium oxalate solution is equal to parts per million of oxygen consumed.

If oxidizable mineral substances, such as ferrous iron, sulfide, or nitrite, are present in the sample corrections should be applied as accurately as possible by suitable procedures. Direct titration of the acidified sample in the cold, using a three-minute period of digestion, serves this purpose quite well for polluted surface waters and fairly well for purified sewage effluents. Few raw sewages containing no trade wastes need such a correction, but raw sewages containing “pickling” liquors do need it. If the sample contains both oxidizable mineral compounds and gaseous organic substances the latter should be driven off by heat and the sample allowed to cool before applying this test for the correction factor. If such corrections are made the fact should be stated with the amount of correction.

_Period and temperature of digestion._—As the practice in regard to the period and temperature of digestion has varied widely it is difficult to compare the results obtained at one laboratory with those obtained at another. None of the methods gives absolute results. They are all relative[26][29][57] at best. Digesting 30 minutes at the boiling temperature is herein designated the recommended method. If samples are analyzed by any other method the method should be noted, and, representative results by the standard method should be placed on record for purposes of comparison.

OTHER METHODS.

_Additional reagents._—1. Potassium iodide solution. Ten per cent solution, free from iodate.

2. Standard sodium thiosulfate. Dissolve 1.0 gram of the pure crystallized salt in 1 liter of distilled water. Standardize this solution against the standard potassium permanganate. As the thiosulfate solution does not keep well determine its actual strength at frequent intervals.

3. Starch indicator. Prepare as directed in the section on dissolved oxygen (pp. 65–66).

4. Sodium hydroxide solution. Dissolve 1 part of pure sodium hydroxide in 2 parts of distilled water.

Certain widely practiced deviations from the standard procedure just described are noted in the following paragraphs.

1. Heat the acidified sample to boiling, add the permanganate solution, and digest for two minutes[16] at boiling temperature. This procedure is facilitated by agitating the liquid constantly with a small current of air to guard against bumping.

2. Same method as No. 1 except that the period of digestion is five minutes.[121a]

3. Same method as No. 2 except that the permanganate solution is added to the acidified sample when cold, and digestion is continued five minutes after the sample reaches the boiling point. The advantage of this method is that there is included the oxygen-consuming power of the volatile matter present in some sewages and sewage effluents, which is driven off by heat and thus escapes when the test is made in accordance with procedures 1 and 2.

4. Same method as No. 3 except that the period of digestion is 10 minutes.[63][68c]

5. Digestion of the sample after the acid and permanganate solutions are added is carried out abroad, especially in England, at approximately the room temperature,[24a][69a][94f][100a] apparently to guard against decomposition[17] of permanganate in the presence of high chloride, for periods of three minutes, fifteen minutes, and four hours; many observers record the oxygen consumed after all three periods, while some record the result only for the four-hour period. At the end of the period of digestion, add 0.5 cc. of potassium iodide solution to discharge the pink color; mix; titrate the liberated iodine with thiosulfate until the yellow color is nearly destroyed, then add a few drops of starch solution and continue titration until the blue color is just discharged. The number of cubic centimeters of potassium permanganate solution in excess of the number of cubic centimeters of sodium thiosulfate solution is equal to parts per million of oxygen consumed.