Chapter IV: Preface: To Fourth Edition (3)

Use 1 liter of a sample containing 0.1 to 1.0 part per million of copper, and proportionate amounts for other concentrations. Evaporate to about 75 cc., and wash into a 100 cc. platinum dish. Add 2 cc. of dilute sulfuric acid for clear and soft waters; add more acid to very alkaline waters to offset the alkalinity; add 5 cc. of acid to waters carrying much organic matter or clay to insure the formation of a soluble copper salt. Then place the dish as the anode in a direct current circuit, suspend a spiral wire cathode in the solution so that it is parallel to and about half an inch from the bottom of the dish, and close the circuit.

Electrolyze for about four hours with occasional stirring, or over night, if convenient. The current may be supplied by two gravity cells in series, yielding a current through the solution of about 0.02 ampere. Lift out the cathode without previously opening the circuit, and immerse the spiral in a small amount of dilute nitric acid previously heated to boiling. Wash off the wire and evaporate the nitric acid solution to dryness on the water bath. If the presence of silver is suspected add a few drops of hydrochloric acid before evaporation. Dissolve the residue in water and wash it into a 50 cc. Nessler tube. Dilute to 50 cc. and add 10 cc. of the potassium sulfide solution. The color of the copper sulfide develops at once and is fairly permanent, lasting at least several hours. Add 10 cc. of the potassium sulfide solution to a similar tube containing 50 cc. of distilled water, and then add to it standard copper solution in 0.2 cc. portions until the colors of the two tubes match. If 1 liter of the sample is used copper in parts per million is equal to the number of cubic centimeters of standard copper solution required to match the color of the sample multiplied by 0.2.

TIN.

Small quantities of tin are occasionally found in waters that have passed through tin or tin-lined pipes. This metal, if present, is precipitated with the iron by ammonia in the lead, zinc, and copper separations. In the method for copper alone, it is removed in the same way and may be further avoided by dissolving the sulfides in concentrated nitric acid. Any tin present will then separate as an insoluble compound, which may be ignited and weighed as the oxide (SnO_{2}).

The following schematic tables illustrate the procedures given.

Table 10.—SCHEME FOR THE SEPARATION OF LEAD, ZINC, AND COPPER.

─────────────────────────────────────────────────────────────────────── 1. Concentrate sample. Add 10 cc. NH_{4}Cl, a few drops NH_{4}OH and saturate with H_{2}S. Allow to stand, add more NH_{4}OH and H_{2}S. Boil, filter, and wash. ────────────────────────────────────────────────────────┬────────────── 2. Dissolve the precipitate in dilute HNO_{3}. Filter │3. Reject the and wash. Evaporate to 10 or 15 cc. Cool. Add 5 cc. │filtrate which concentrated H_{2}SO_{4}, and heat until white fumes are│contains the given off. Dilute slightly and treat with 150 cc. of 50 │coloring per cent alcohol. Allow to stand; filter, and wash with │matter. 50 per cent alcohol. │ ──────────────────────────┬─────────────────────────────┤ 4. The precipitate │5. The filtrate contains the │ contains the Pb. Dissolve │Zn and Cu. Concentrate to │ in NH_{4}C_{2}H_{3}O_{2} │expel alcohol. Add excess of │ solution. Filter into a 50│NH_{4}OH, filter and wash │ cc. Nessler tube and wash │precipitate. │ with water containing ├──────────────┬──────────────┴────────────── NH_{4}C_{2}H_{3}O_{2}. │6. Reject the │7. The filtrate contains the Divide filtrate in halves.│precipitate │Zn and Cu. Neutralize with Saturate one-half with │which contains│H_{2}SO_{4}. Add 10 cc. H_{2}S. Determine the Pb │the Fe. │concentrated H_{2}SO_{4} and in the other half by │ │1 g. urea. Electrolyze for adding HC_{2}H_{3}O_{2} │ │two hours with a current of and H_{2}S and comparing │ │0.5 ampere. Break circuit, with standards containing │ │empty dish and wash. known amounts of Pb. │ │ ──────────────────────────┼──────────────┴───────────────────────────── 8. The deposit is Cu. │9. The solution contains the Zn. Nearly Immerse the cathode in a │neutralize with NH_{4}OH. Concentrate to small amount of hot, │less than the capacity of the dish. Add 2 g. dilute HNO_{3}; wash off │K_{2}C_{2}O_{4} and 1.5 g. K_{2}SO_{4}. and evaporate to dryness. │Electrolyze for 3 hours with a current of Take up in water and wash │0.3 ampere. Siphon off solution, break into a Nessler tube. Make │circuit, wash with water, then alcohol, dry up to mark, and add 10 cc.│at 70° C., cool and weigh. of potassium sulfide ├──────────────────────────────────────────── solution. Compare with │10. The weighed residue is metallic Zn. standard. If large amount │ is present, dry and weigh │ as Cu. │ ──────────────────────────┴────────────────────────────────────────────

Table 11.—SCHEME FOR DETERMINATION OF COPPER ONLY.

─────────────────────────────────────────────────────────────────────── 11. Concentrate sample to 75 cc. Add 2 cc. conc. H_{2}SO_{4} for clear, soft waters and 5 cc. for alkaline or turbid waters. Electrolyze following procedure in 7 and 8. ───────────────────────────────────────────────────────────────────────

Table 12.—SCHEME FOR DETERMINATION OF ZINC ONLY.

─────────────────────────────────────────────────────────────────────── 13. Follow scheme for all three metals as given in Table 10 through section 5. Nearly neutralize the filtrate with H_{2}SO_{4}, concentrate to less than the capacity of the dish and electrolyze as directed in section 9. ───────────────────────────────────────────────────────────────────────

MINERAL ANALYSIS.

RESIDUE ON EVAPORATION.

See description of method (p. 29). The residue should be dried one hour at 180° C. Turbid waters should be filtered, and the composition of the suspended matter should be determined separately or the amount of it reported as suspended matter.

ALKALINITY AND ACIDITY.

See description of method (pp. 35–41).

CHLORIDE.

See description of method (pp. 41–43).

NITRATE NITROGEN.

See description of method (pp. 23–25).

SEPARATION OF SILICA, IRON, ALUMINIUM, CALCIUM, AND MAGNESIUM.[10][48]

SILICA.

Evaporate in platinum 100 to 1,000 cc. of the sample or sufficient if possible to form a residue weighing 0.4 to 0.6 gram, and preferably containing 0.1 to 0.2 gram of calcium. When the residue is nearly dry add 1 cc. of hydrochloric acid (1 to 1) and, after moistening the sides of the dish, evaporate to dryness. Dry at 180° C. and if much organic matter is present char it in a radiator. Moisten the residue with dilute hydrochloric acid and expel the excess of acid by heating on the water bath. Add a few drops of hydrochloric acid, dissolve in hot water, and filter. Wash the residue with hot water. Evaporate the filtrate to dryness, repeat the filtration, and combine the two residues. If great accuracy is not required the second evaporation with hydrochloric acid may be omitted. Ignite and weigh the insoluble residue. Add 2 drops of concentrated sulfuric acid and a little hydrofluoric acid, volatilize the acids, ignite, and weigh again. Report the loss in weight as silica (SiO_{2}). A weight of non-volatile matter exceeding 0.5 mg. should be analyzed.

IRON AND ALUMINIUM.

Heat to boiling the filtrate from the insoluble residue, oxidize with concentrated nitric acid or bromine, and concentrate to about 25 cc. Add ammonium hydroxide in slight excess, boil one minute, and filter. Dissolve the precipitate on the filter in a small amount of hot dilute hydrochloric acid. Reprecipitate with ammonium hydroxide, filter, and wash. Unless very accurate results are necessary this solution and reprecipitation may be omitted. Unite the two filtrates for determination of calcium. Ignite and weigh the precipitate. It will comprise oxides of iron and aluminium and phosphate. If much phosphate is present it should be determined in a separate sample and a correction for the amount applied; otherwise it may be neglected. Determine the iron in the ignited precipitate by fusion with sodium or potassium pyrosulfate, reduction with zinc, and titration with potassium permanganate. Aluminium (Al) is calculated as follows:

Al = 0.53[(Fe_{2}O_{3} + Al_{2}O_{3}) − 1.43 Fe]

CALCIUM.

Concentrate the filtrate from the separation of iron and aluminium to about 100 cc., and add an excess of concentrated solution of ammonium oxalate, little by little, to the hot ammoniacal solution. Keep the solution warm and stir at intervals till the precipitate settles readily and leaves a clear supernatant liquid. Filter, dissolve the precipitate in a little hot dilute hydrochloric acid, and reprecipitate with ammonium hydroxide and ammonium oxalate. If great accuracy is not required this solution and reprecipitation may be omitted, and the first precipitate may be washed clean with hot water[64a]. Save the filtrate for determination of magnesium. Ignite the precipitate and weigh it as calcium oxide, 71.5 per cent of which is the equivalent of calcium (Ca); or dissolve the precipitate in hot 2 per cent sulfuric acid and titrate with a standard solution of potassium permanganate.

MAGNESIUM.

Acidify with hydrochloric acid the filtrate from the separation of calcium and concentrate it to about 100 cc. Add 20 cc. of a saturated solution of microcosmic salt, cool, and make slightly but distinctly alkaline by adding ammonium hydroxide, drop by drop. Allow the solution to stand four hours, then filter and wash with 3 per cent ammonium hydroxide. Dissolve the precipitate, especially in the presence of large amounts of sodium or potassium, in a slight excess of dilute hydrochloric acid and reprecipitate the magnesium with ammonium hydroxide and a few drops of microcosmic salt solution. If great accuracy is not required this solution and reprecipitation may be omitted. Ignite the precipitate and weigh it as magnesium pyrophosphate (Mg_{2}P_{2}O_{7}), 21.9 per cent of which is the equivalent of magnesium (Mg.). If manganese is present[64a] it is precipitated with the magnesium and a correction for it should be applied after having determined manganese in a separate sample. The weight of manganese pyrophosphate (Mn_{2}P_{2}O_{7}) is 2.58 times the weight of manganese.

SEPARATION OF SULFATE, SODIUM, AND POTASSIUM.

SULFATE.

Evaporate to dryness 100 to 1,000 cc. of the sample, or sufficient to obtain a residue weighing 0.4 to 0.6 gram and containing preferably 0.05 to 0.2 gram of sodium. Acidify the residue with hydrochloric acid and remove the silica, iron, and aluminium (pp. 56–57). Make acid and add a hot solution of barium chloride in slight excess to the hot filtrate, and warm it, stirring at intervals for one-half hour, until the precipitate settles readily and leaves a clear supernatant liquid. Dry, ignite, and weigh the precipitate of barium sulfate, 41.1 per cent of which is equal to the content of sulfate (SO_{4}).

SODIUM, POTASSIUM, AND LITHIUM.

Evaporate to dryness the filtrate from the precipitation of barium sulfate. Add a few cubic centimeters of hot water and then a saturated solution of barium hydroxide until a slight film collects on the top of the solution. Filter and wash the precipitate with hot water. Add to the filtrate an excess of ammonium hydroxide and ammonium carbonate solution. Filter, evaporate the filtrate to dryness, dry, and ignite at low red heat to expel ammonium salts. Repeat the operations including the addition of barium hydroxide until no precipitate is obtained by barium hydroxide or by ammonium hydroxide and ammonium carbonate. Evaporate the final filtrate to dryness in a weighed platinum dish, dry, cool, and weigh the residue. Dissolve the residue in a few cubic centimeters of water, filter, wash the filter paper twice with hot water, then ignite the filter paper in the platinum dish. Cool and weigh the residue. Subtract this weight from the first weight including the residue. The difference is the weight of the chlorides of sodium and potassium and lithium. If it is not desired to separate sodium and potassium the weight of sodium and potassium as sodium may be calculated from this difference by multiplying it by 0.394.

POTASSIUM.

_First procedure._—Add to the solution of the chlorides of sodium and potassium a few drops of dilute hydrochloric acid (1 to 3) and 1 cc. of 10 per cent platinic chloride (PtCl_{4}) for each 30 mg. of the combined chlorides. Evaporate to a thick syrup on the water bath, then remove dish and allow it to come to dryness at laboratory temperature. Treat the residue cold with 80 per cent alcohol and filter. Wash the precipitate with 80 per cent alcohol until the filtrate is no longer colored. Dry the precipitate and dissolve it in hot water. Evaporate the solution to dryness in a platinum dish and weigh it as potassium platinic chloride (K_{2}PtCl_{6}). The weight of potassium (K) is 16.1 per cent of this weight and the equivalent of potassium chloride (KCl) is 30.7 per cent of this weight. Subtract the equivalent weight of potassium chloride from the weight of the combined chlorides. The weight of the sodium is 39.4 per cent of the difference.

_Second procedure._[86][103a]—Add to the hot solution of the combined chlorides 20 per cent perchloric acid (HClO_{4}) slightly in excess of the amount required to combine with the bases. One cubic centimeter of 20 per cent perchloric acid is equivalent to 90 mg. of potassium. Evaporate the solution to dryness, dissolve the residue in 10 cc. of hot water and a small amount of perchloric acid, and again evaporate to dryness. Repeat the addition of water, perchloric acid, and evaporation until white fumes appear on evaporating to dryness. Add to the residue 25 cc. of 96 per cent alcohol containing 0.2 per cent of perchloric acid (1 cc. of 20 per cent perchloric acid in 100 cc. of 98 per cent alcohol). Break up the residue with a stirring rod. Decant the supernatant liquid through a weighed Gooch crucible that has been washed with 0.2 per cent perchloric acid in alcohol. If the precipitate is unusually large dissolve it in hot water and repeat the evaporation with perchloric acid. Wash the precipitate once by decantation with the 0.2 per cent perchloric acid in alcohol, transfer the precipitate to the crucible, and wash it several times with a 0.2 per cent perchloric acid in alcohol. Dry the crucible at 120–130° C. for one hour, cool, and weigh it. The increase in weight is potassium perchlorate (KClO_{4}). The equivalent weight of potassium is 28.2 per cent and the equivalent weight of potassium chloride is 53.8 per cent of the potassium perchlorate. Calculate the content of sodium by difference.

LITHIUM.[34]

Use a large quantity of the sample. Obtain the combined chlorides of sodium, potassium, and lithium (see pp. 58–59). Transfer the combined chlorides to a small Erlenmeyer flask (50 or 100 cc. capacity) and evaporate the solution nearly, but not quite, to dryness. Add about 30 cc. of redistilled amyl alcohol. Connect the flask, the stopper of which carries a thermometer, with a condenser[E] and boil until the temperature rises approximately to the boiling point of amyl alcohol (130° C.), showing that all the water has been driven off. Cool slightly and add a drop of hydrochloric acid to convert small amounts of lithium hydroxide to lithium chloride. Connect with the condenser and continue the boiling to drive off again all water and until the temperature reaches the boiling point of amyl alcohol. The content of the flask at this time is usually 15 to 20 cc. Filter through a small paper or a Gooch crucible into a graduated cylinder and note exact quantity of filtrate, which determines the subsequent correction. Wash the precipitate with small quantities of dehydrated amyl alcohol. Evaporate the filtrate and washings in a platinum dish to dryness on the steam bath, dissolve the residue in water, and add a few drops of sulfuric acid. Evaporate on a steam bath and expel the excess of sulfuric acid by gentle heat over a flame. Repeat until carbonaceous matter is completely burned off. Cool and weigh the dish and contents. Dissolve in a small quantity of hot water, filter through a small filter, wash, and return filter to dish, ignite, and weigh. The difference between the original weight of dish and contents and the weight of the dish and small amount of residue equals the weight of impure lithium sulfate. The purity of the lithium sulfate should be tested by adding small amounts of ammonium phosphate and ammonium hydroxide, which will precipitate any magnesium present with the lithium sulfate. Any precipitate appearing after standing over night should be collected on a small filter and weighed as magnesium pyrophosphate, calculated to sulfate, and subtracted from the weight of impure lithium sulfate. From this weight subtract 0.00113 gram for every 10 cc. of amyl alcohol filtrate exclusive of the amyl alcohol used in washing residue because of the slight solubility of solid mixed chlorides in amyl alcohol. Calculate lithium from the corrected weight of lithium sulfate. Dissolve the mixed chlorides from flask and filter with hot water, evaporate to dryness, ignite gently to remove amyl alcohol, filter and thoroughly wash; concentrate the filtrates and washings to 25 to 50 cc.

Footnote E:

The amyl alcohol may be boiled off without the use of a condenser, but
the vapors are very disagreeable.

To the weight of potassium chloride add 0.00051 gram for every 10 cc. of amyl alcohol used in the extraction of the lithium chloride, which corrects for the solubility of the potassium chloride in amyl alcohol. Calculate to potassium.

The weight of sodium chloride is found by subtracting the combined weights of lithium chloride and potassium chloride (corrected) from the total weight of the three chlorides. Calculate sodium chloride to sodium.

BROMINE, IODINE, ARSENIC, AND BORIC ACID.

Evaporate to dryness a large quantity of the sample to which a small amount of sodium carbonate has been added. Boil the residue with distilled water, transfer it to a filter, and thoroughly wash it with hot water. Dilute the alkaline filtrate to a definite volume, and determine bromine and iodine, arsenic, and boric acid in aliquot portions of it.

BROMINE AND IODINE.[10]

_Reagents._—1. Sulfuric acid. 1 to 5.

2. Potassium nitrite or sodium nitrite. Two per cent solution.

3. Carbon bisulfide. Freshly purified by distillation.

4. Iodine standards. Acidify with dilute sulfuric acid measured quantities of a standard solution of potassium iodide in small tubes. Add 3 or 4 drops of the potassium nitrite solution and extract with carbon bisulfide as in the actual determination. Transfer to small flasks the standards from which the iodine has been removed.

5. Chlorine water. Saturated solution.

6. Bromine standards. Add measured quantities of a standard solution of a bromide to the liquid in each of the small flasks from which the iodine has been removed. Add to each 5 cc. of purified carbon bisulfide, and proceed exactly as with the sample.

_Procedure._—Evaporate to dryness an aliquot portion of the alkaline filtrate. Dissolve the residue in 2 or 3 cc. of water, and add enough absolute alcohol to make the percentage of alcohol about 90. Boil and filter and repeat the extraction of the residue with alcohol once or twice. Add 2 or 3 drops of sodium hydroxide to the combined alcoholic filtrates and evaporate to dryness. Dissolve the residue in 2 or 3 cc. of water and repeat the extraction with alcohol and the filtration. Add a drop of sodium hydroxide to the filtrate and evaporate it to dryness. Dissolve the residue in a little water. Acidify this solution with dilute sulfuric acid, adding 3 or 4 drops excess, and transfer it to a small flask. Add 4 drops of the solution of potassium nitrite or sodium nitrite and about 5 cc. of carbon bisulfide. Shake the mixture until all the iodine is extracted. Separate the acid solution from the carbon bisulfide by filtration. Wash the flask, filter, and contents with cold distilled water, and transfer the carbon bisulfide containing the iodine in solution to Nessler tubes by means of about 5 cc. of pure carbon bisulfide. In washing the filter, dilute the contents of the tube to a definite volume, usually 12 or 15 cc., and compare the color with that of known amounts of iodine dissolved in carbon bisulfide in other tubes.

Transfer to a small flask the sample from which the iodine has been removed. Add saturated chlorine water, 1 cc. at a time, shaking after each addition until all the bromine is freed. Care must be taken not to add much more chlorine than that necessary to free the bromine, since an excess of reagent may form a bromochloride that spoils the color reaction. Separate the water solution from the carbon bisulfide by filtration through a moistened filter, wash the contents of the filter two or three times with water, and then transfer them to a Nessler tube by means of about 1 cc. of carbon bisulfide. Repeat this extraction of the filtrate twice, using 3 cc. of carbon bisulfide each time. The combined carbon bisulfide extracts usually amount to 11.5 to 12 cc. Add enough carbon bisulfide to the tubes to bring them to a definite volume, usually 12 to 15 cc., and compare the sample with the standards. If much bromine is present it is not always completely extracted by the amounts of carbon bisulfide recommended. If the extraction is incomplete, therefore, make one or two extra extractions with carbon bisulfide, transfer the extracts to another tube, and compare the color with that of the standards.

ARSENIC.[31]

Evaporate to dryness an aliquot portion of the alkaline filtrate (p. 61). Acidify the residue with arsenic-free sulfuric acid, and subject it to the action of arsenic-free zinc and sulfuric acid in a Marsh-Berzelius apparatus. Compare the mirror obtained with a mirror obtained from an arsenious oxide solution of known strength.

BORIC ACID.

Evaporate to dryness an aliquot portion of the alkaline filtrate (p. 61), treat the residue with 1 or 2 cc. of water, and slightly acidify the solution with hydrochloric acid. Add about 25 cc. of absolute alcohol, boil, filter, and repeat the extraction of the residue. Make the filtrate slightly alkaline with sodium hydroxide, and evaporate it to dryness. Add a little water, slightly acidify with hydrochloric acid, and place a strip of turmeric paper in the liquid. Evaporate to dryness on the steam bath, and continue the heating until the turmeric paper is dry. If boric acid is present the turmeric paper becomes cherry red. It is not usually necessary to determine quantitatively boric acid; the quantitative method devised by Gooch[33] is recommended.

HYDROGEN SULFIDE.[103]

Hydrogen sulfide should be determined preferably in the field; the procedure as far as the final titration with sodium thiosulfate must be carried out in the field.

_Reagents._—1. N/100 sodium thiosulfate.

2. Standard iodine. A N/100 solution containing potassium iodide standardized against the N/100 sodium thiosulfate. To standardize, add 10 cc. of the iodine solution to 500 cc. of boiled distilled water. Add about 1 gram of potassium iodide, and titrate with N/100 sodium thiosulfate in the presence of starch indicator. One cc. of N/100 iodine is equivalent to 0.17 mg. H_{2}S.

3. Potassium iodide. Crystals.

4. Starch. A freshly prepared solution for use as indicator.

_Procedure._—Add 500 cc. of the sample to 10 cc. of the standard iodine solution and 1 gram of potassium iodide in a large glass-stoppered bottle or flask. If the sample is to be collected from a tap lead the water into the bottle through a rubber tube extending to the bottom of the bottle so as to eliminate errors due to aeration. Shake the bottle, allow it to stand for a few minutes, and then titrate the excess of iodine with sodium thiosulfate in the presence of starch indicator. Hydrogen sulfide (H_{2}S) in parts per million is equal to 0.34 times the difference in cubic centimeters between the amount of iodine solution added and the amount of N/100 thiosulfate used in the titration.

CHLORINE.

In waters that have been treated with calcium hypochlorite or liquid chlorine it is frequently advisable to ascertain the presence or absence of chlorine. As the reagents which have been proposed for its detection are not specific for chlorine but give similar or identical reactions with oxidizing agents or reducible substances care must be exercised in interpreting the results of such tests: nitrites and ferric salts are of common occurrence, and chlorates also may lead to misinterpretation in waters treated with calcium hypochlorite.

_Reagents._—1. Tolidin solution. One gram of o-tolidin, purified by being recrystallized from alcohol, is dissolved in 1 liter of 10 per cent hydrochloric acid.

2. Copper sulfate solution. Dissolve 1.5 grams of copper sulfate and 1 cc. of concentrated sulfuric acid in distilled water and dilute the solution to 100 cc.

3. Potassium bichromate solution. Dissolve 0.025 gram of potassium bichromate and 0.1 cc. of concentrated sulfuric acid in distilled water and dilute the solution to 100 cc.

_Procedure._—Mix 1 cc. of the tolidin reagent with 100 cc. of the sample in a Nessler tube and allow the solution to stand at least 5 minutes. Small amounts of free chlorine give a yellow and larger amounts an orange color.

For quantitative determination compare the color with that of standards in similar tubes prepared from the solutions of copper sulfate and potassium bichromate. The amounts of solution for various standards are indicated in Table 13.

Table 13.—PREPARATION OF PERMANENT STANDARDS FOR CONTENT OF CHLORINE.

───────────────────────┬───────────────────────┬───────────────────────
Chlorine. │ Solution of copper │ Solution of potassium
│ sulfate. │ bichromate.
───────────────────────┼───────────────────────┼───────────────────────
_Parts per million._ │ _cc._ │ _cc._
│ │
0.01│ 0.0│ 0.8
.02│ .0│ 2.1
.03│ .0│ 3.2
.04│ .0│ 4.3
.05│ .4│ 5.5
.06│ .8│ 6.6
.07│ 1.2│ 7.5
.08│ 1.5│ 8.7
.09│ 1.7│ 9.0
.10│ 1.8│ 10.0
.20│ 1.9│ 20.0
.30│ 1.9│ 30.0
.40│ 2.0│ 38.0
.50│ 2.0│ 45.0
───────────────────────┴───────────────────────┴───────────────────────

DISSOLVED OXYGEN.[16][65][68][71b][99][100c][120]

_Reagents._—1. Sulfuric acid, concentrated. (Sp. gr. 1.83–1.84.)

2. Potassium permanganate. Dissolve 6.32 grams of the salt in water and dilute the solution to 1 liter.

3. Potassium oxalate. A 2 per cent solution.

4. Manganous sulfate. Dissolve 480 grams of the salt in water and dilute the solution to 1 liter.

5. Alkaline potassium iodide. Dissolve 700 grams of potassium hydroxide and 150 grams of potassium iodide in water and dilute the solution to 1 liter.

6. Hydrochloric acid. Concentrated (Sp. gr. 1.18–1.19).

7. Sodium thiosulfate. A N/40 solution. Dissolve 6.2 grams of chemically pure recrystallized sodium thiosulfate in water and dilute the solution to 1 liter with freshly boiled distilled water. Each cc. is equivalent to 0.2 mg. of oxygen or to 0.1395 cc. of oxygen at 0°C. and 760 mm. pressure. Inasmuch as this solution is not permanent it should be standardized occasionally against a N/40 solution of potassium bichromate. The keeping qualities of the thiosulfate solution are improved by adding to each liter 5 cc. of chloroform and 1.5 grams of ammonium carbonate before diluting to the prescribed volume.

8. Starch solution. Mix a small amount of clean starch with cold water until it becomes a thin paste and stir this mass into 150 to 200 times its weight of boiling water. Boil for a few minutes, then sterilize. It may be preserved by adding a few drops of chloroform.

_Collection of sample._—Collect the sample in a narrow-necked glass-stoppered bottle of 250 to 270 cc. capacity. The following procedure should be followed in order to avoid entrainment or absorption of atmospheric oxygen. In collecting from a tap fill the bottle through a glass or rubber tube extending well into the tap and to the bottom of the bottle. To avoid air bubbles allow the bottle to overflow for several minutes, and then carefully replace the glass stopper so that no air bubble is entrained. In collecting from the surface of a pond or tank connect the sample bottle to a bottle of 1 liter capacity. Provide each bottle with a two-hole rubber stopper having one glass tube extending to the bottom and another glass tube entering but not projecting into the bottle. Connect the short tube of the sample bottle with the long tube of the liter bottle. Immerse the sample bottle in the water and apply suction to the outlet of the liter bottle. To collect a sample at any depth arrange the two bottles so that the outlet tube of the liter bottle is at a higher elevation then the inlet tube of the sample bottle. Lower the two bottles, in any convenient form of cage properly weighted, to the desired depth. Water entering during the descent will be flushed through into the liter bottle. When air bubbles cease rising to the surface raise the bottles. Finally replace the perforated stopper of the sample bottle with a glass stopper in such manner as to avoid entraining bubbles of air.

_Procedure._—Remove the stopper from the bottle and add, first, 0.7 cc. of the concentrated sulfuric acid, and then 1 cc. of the potassium permanganate solution. These and all other reagents should be introduced by pipette under the surface of the liquid. Insert the stopper and mix by inverting the bottle several times. After 20 minutes have elapsed destroy the excess of permanganate by adding 1 cc. of the potassium oxalate solution, the bottle being at once restoppered and its contents mixed. If a noticeable excess of potassium permanganate is not present at the end of 20 minutes, again add 1 cc. of the potassium permanganate solution. If this is still insufficient use a stronger potassium permanganate solution. After the liquid has been decolorized by the addition of potassium oxalate add 1 cc. of the manganous sulfate solution and 3 cc. of the alkaline potassium iodide solution. Allow the precipitate to settle. Add 2 cc. of the hydrochloric acid and mix by shaking.

The procedure to this point must be carried out in the field, but after the acid has been added and the stopper replaced there is no further change, and the rest of the test may be performed within a few hours, as convenient. Transfer 200 cc. of the contents of the bottle to a flask and titrate with N/40 sodium thiosulfate, using a few cubic centimeters of the starch solution as indicator toward the end of the titration. Do not add the starch solution until the color has become faint yellow, and titrate until the blue color disappears.

The use of potassium permanganate is made necessary by high nitrite or organic matter. The procedure outlined must be followed in all work on sewage and partly purified effluents or seriously polluted streams or samples whose nitrite nitrogen exceeds 0.1 part per million. In testing other samples the procedure may be shortened by beginning with the addition of the manganous sulfate solution and proceeding from that point as outlined, except that only 1 cc. of alkaline potassium iodide need be added.

_Calculation of Results._—Oxygen shall be reported in parts per million by weight. It is sometimes convenient to know the number of cubic centimeters per liter of the gas at 0°C. temperature and 760 mm. pressure and also to know the percentage which the amount of gas present is of the maximum amount capable of being dissolved by distilled water at the same temperature and pressure. If 200 cc. of the sample is taken the number of cubic centimeters of N/40 thiosulfate used is equal to parts per million of oxygen. Corrections for volume of reagents added amount to less than 3 per cent and are not justified except in work of unusual precision. To obtain the result in cubic centimeters per liter multiply the number of cubic centimeters of thiosulfate used by 0.698. To obtain the result in percentage of saturation divide the number of cubic centimeters of thiosulfate by the figure in Table 14 opposite the temperature of the water and under the proper chlorine figure. The last column of Table 14 permits interpolation for intermediate chlorine values. At elevations differing considerably from mean sea level and for accurate work attention must be given to barometric pressure, the normal pressure in the region being preferable to the specific pressure at the time of sampling. The term “saturation” refers to a condition of equilibrium between the solution and an oxygen pressure in the atmosphere corresponding to 158.8 millimeters, or approximately one-fifth atmosphere. The true saturation or equilibrium between the solution and pure oxygen is nearly five times this value, and consequently values in excess of 100 per cent saturation frequently occur in the presence of oxygen-forming plants.

Table 14.—SOLUBILITY OF OXYGEN IN FRESH WATER AND IN SEA WATER OF
STATED DEGREES OF SALINITY AT VARIOUS TEMPERATURES WHEN EXPOSED TO AN
ATMOSPHERE CONTAINING 20.9 PER CENT OF OXYGEN UNDER A PRESSURE OF 760
MM.[F]

(Calculated by G. C. Whipple and M. C. Whipple from measurements of C.
J. Fox.)[27][119]

─────────────┬────────────────────────────────────────────┬───────────
│ │Difference
Temperature. │ Chloride in sea water (milligrams per │ per 100
│ liter). │ parts of
│ │ chloride.
─────────────┼────────┬────────┬────────┬────────┬────────┼───────────
│ 0. │ 5000. │ 10000. │ 15000. │ 20000. │
─────────────┼────────┴────────┴────────┴────────┴────────┼───────────
_°C._ │_Dissolved oxygen in milligrams per liter._ │_Parts per
│ │ million._
0│ 14.62│ 13.79│ 12.97│ 12.14│ 11.32│ 0.0165
1│ 14.23│ 13.41│ 12.61│ 11.82│ 11.03│ .0160
2│ 13.84│ 13.05│ 12.28│ 11.52│ 10.76│ .0154
3│ 13.48│ 12.72│ 11.98│ 11.24│ 10.50│ .0149
4│ 13.13│ 12.41│ 11.69│ 10.97│ 10.25│ .0144
5│ 12.80│ 12.09│ 11.39│ 10.70│ 10.01│ .0140
│ │ │ │ │ │
6│ 12.48│ 11.79│ 11.12│ 10.45│ 9.78│ .0135
7│ 12.17│ 11.51│ 10.85│ 10.21│ 9.57│ .0130
8│ 11.87│ 11.24│ 10.61│ 9.98│ 9.36│ .0125
9│ 11.59│ 10.97│ 10.36│ 9.76│ 9.17│ .0121
10│ 11.33│ 10.73│ 10.13│ 9.55│ 8.98│ .0118
│ │ │ │ │ │
11│ 11.08│ 10.49│ 9.92│ 9.35│ 8.80│ .0114
12│ 10.83│ 10.28│ 9.72│ 9.17│ 8.62│ .0110
13│ 10.60│ 10.05│ 9.52│ 8.98│ 8.46│ .0107
14│ 10.37│ 9.85│ 9.32│ 8.80│ 8.30│ .0104
15│ 10.15│ 9.65│ 9.14│ 8.63│ 8.14│ .0100
│ │ │ │ │ │
16│ 9.95│ 9.46│ 8.96│ 8.47│ 7.99│ .0098
17│ 9.74│ 9.26│ 8.78│ 8.30│ 7.84│ .0095
18│ 9.54│ 9.07│ 8.62│ 8.15│ 7.70│ .0092
19│ 9.35│ 8.89│ 8.45│ 8.00│ 7.56│ .0089
20│ 9.17│ 8.73│ 8.30│ 7.86│ 7.42│ .0088
│ │ │ │ │ │
21│ 8.99│ 8.57│ 8.14│ 7.71│ 7.28│ .0086
22│ 8.83│ 8.42│ 7.99│ 7.57│ 7.14│ .0085
23│ 8.68│ 8.27│ 7.85│ 7.43│ 7.00│ .0083
24│ 8.53│ 8.12│ 7.71│ 7.30│ 6.87│ .0083
25│ 8.38│ 7.96│ 7.56│ 7.15│ 6.74│ .0082
│ │ │ │ │ │
26│ 8.22│ 7.81│ 7.42│ 7.02│ 6.61│ .0080
27│ 8.07│ 7.67│ 7.28│ 6.88│ 6.49│ .0079
28│ 7.92│ 7.53│ 7.14│ 6.75│ 6.37│ .0078
29│ 7.77│ 7.39│ 7.00│ 6.62│ 6.25│ .0076
30│ 7.63│ 7.25│ 6.86│ 6.49│ 6.13│ .0075
─────────────┴────────┴────────┴────────┴────────┴────────┴───────────

Footnote F:

Under any other barometric pressure, B, the solubility can be obtained
from the corresponding value in the table by the formula:

S´ = S(B/760) = S(B´/29.92) in which S´ = Solubility at B or B´,
S = Solubility at 760 mm. or 29.92
inches,
B = Barometric pressure in mm.,
and B´ = Barometric pressure in
inches.

ETHER-SOLUBLE MATTER.[44]

Evaporate 500 cc. of the sample in a porcelain evaporating dish to a volume of about 50 cc. By means of a rubber-tipped glass rod remove to the bottom of the dish the solid matter attached to the sides, and add normal sulfuric acid to neutralize the alkalinity. Do not use an excess of acid. Then evaporate the contents of the dish to dryness. Treat the dry residue with boiling ether, rubbing the bottom and sides of the dish to insure complete solution of fat. Three extractions with ether are required. Filter the ether solution through a 5 cm. filter into a weighed flask having a wide mouth. Evaporate the ether slowly, and dry the flask at 100° C. for 30 minutes. The increase in weight of the flask gives the amount of fats, or, in more precise language, the ether-soluble matter.

An excess of acid gives too high results because of the formation of fatty-acid residues.

RELATIVE STABILITY OF EFFLUENTS.[78]

_Reagent._—Methylene blue solution. A 0.05 per cent aqueous solution of methylene blue, preferably the double zinc salt or commercial variety.[60b]

_Collection of sample._—Collect the sample in a glass-stoppered bottle holding approximately 150 cc. If the dissolved oxygen is low observe precautions similar to those used in collecting samples for dissolved oxygen (p. 66).

_Procedure._—Add 0.4 cc. of the methylene blue solution to the sample in the 150 cc. bottle. As methylene blue has a slightly antiseptic property be careful to add exactly 0.4 cc. Add the methylene blue solution preferably below the surface of the liquid after filling the bottle with the sample. If the methylene blue is added first do not allow the liquid to overflow as coloring matter will thus be lost. Incubate the sample at 20° C. for ten days. Four days’ incubation may be considered sufficient for all practical purposes in routine plant-control work. If quick results are desired incubate the sample at 37° C. for five days using suitable stoppers[1a][2a] to prevent the loss and reabsorption of dissolved oxygen. The bacterial flora at 37° C. is different from that at 20° C. The lower temperature is more nearly the average temperature of surface waters and therefore the higher temperature should be used only when quick approximate results are essential. Observe the sample at least twice a day during incubation. Give a sample in which the methylene blue becomes decolorized a relative stability corresponding to the time required for reduction (see Table 15). For routine filter control ordinary room or cellar temperature gives fairly satisfactory results. For accurate studies, room temperature incubation is very undesirable, as the fluctuations in temperature which are ordinarily not noticed are responsible for appreciable deviations from the true values of relative stability. If the samples are incubated less than 10 days at 20° C. and are not decolorized place a plus sign after the stability value in order to indicate that the stability might have been higher if more time had been allowed. In applying this test to river waters it often happens that the blue coloring matter is removed either partly or completely through absorption by the clay which many rivers carry in suspension. True relative stabilities cannot be obtained for such waters except by determining the initial available oxygen at the start and the biochemical oxygen demand on incubation at 20° C. for 10 days (pp. 71–73). Germicides, such as hypochlorite of lime, if present in sufficient quantity, vitiate the results. If a sample contains free chlorine, therefore, store it about 2 hours, or until the chlorine is gone, and then add methylene blue.

Table 15[78] gives the relation between the time in days to decolorize methylene blue at 20° C. (_t__{20}) and the relative stability number or ratio of available oxygen to oxygen required for equilibrium, expressed in percentage (S).

Table 15.—RELATIVE STABILITY NUMBERS.

───────────────────────────────────┬───────────────────────────────────
Time required for decolorization at│ Relative stability.
20° C. │
───────────────────────────────────┼───────────────────────────────────
_Days._ │ _Percentage._
0.5│ 11
1.0│ 21
1.5│ 30
2.0│ 37
2.5│ 44
3.0│ 50
4.0│ 60
5.0│ 68
6.0│ 75
7.0│ 80
8.0│ 84
9.0│ 87
10.0│ 90
11.0│ 92
12.0│ 94
13.0│ 95
14.0│ 96
16.0│ 97
18.0│ 98
20.0│ 99
───────────────────────────────────┴───────────────────────────────────

The theoretical relation is, S = 100 (1 − 0.794_t__{20})

The relation between the time of reduction at 20° C. and that at 37° C. is approximately two to one, but if an observer incubates at 37° C. he should work out his own comparative 37° C. table or factor.

A relative stability of 75 signifies that the sample examined contains a supply of available oxygen equal to 75 per cent of the amount of oxygen which it requires in order to become perfectly stable. The available oxygen is approximately equivalent to the dissolved oxygen plus the available oxygen of nitrate and nitrite. Nitrite in sewage is usually so low as to be negligible.

BIOCHEMICAL OXYGEN DEMAND OF SEWAGE AND EFFLUENTS.[60a][60c][60d]

RELATIVE STABILITY METHOD.

The relative stability method may be employed to obtain a measure of the putrescible material in sewages and effluents in terms of oxygen demand.

_Procedure for effluents._—Divide the total available oxygen, including the oxygen of nitrite and nitrate, by the relative stability expressed as a decimal.

_Procedure for sewages._—Make one or two dilutions with fully aerated distilled water of known dissolved oxygen content. Tap water may be employed if it is free from nitrates. Vary the relative proportions of sewage and water to be employed to give a relative stability of 50 to 75. Unless seals[1b][2b][52a] are used bring the water as well as the sewage to the temperature at which the mixtures are to be incubated before preparing the dilutions. During the manipulation avoid aeration. Having made the proper dilutions, determine the relative stability of each.

Calculate the oxygen demand in parts per million by the formula:

Oxygen demand = O(1 − p)/Rp

In this formula, O is the initial dissolved oxygen of the diluting water, p is the proportion of sewage; and R is the relative stability of the mixture. Ordinarily the available oxygen in crude sewages, septic tank effluents, settling tank effluents, and trade wastes can be neglected.

SODIUM NITRATE METHOD.

For the determination of the biochemical oxygen demand the sodium nitrate method may be used[60a][60c][60d][52a]. The method is based on the biochemical consumption of oxygen from sodium nitrate by a sewage or polluted water during an incubation period of ten days at 20° C. A reasonable excess of sodium nitrate does not give a higher oxygen demand, as do higher dilutions with aerated water. The oxygen absorbed from the air in applying the method to sewages is negligible.

_Reagent._—Sodium nitrate solution. Dissolve 26.56 grams of pure sodium nitrate in 1 liter of distilled water. One cc. of this solution in 250 cc. of sewage represents 50 parts per million of available oxygen. The strength of the sodium nitrate solution may be varied to suit conditions.

_Procedure for sewages._—Ordinarily disregard the initial available oxygen as it is very small compared with the total biochemical oxygen demand. Add measured amounts of the sodium nitrate solution to the sewage in bottles holding approximately 250 cc. which have been completely filled and stoppered. Incubate for 10 days at 20° C. A seal is not required during incubation. The appearance of a black sediment and the development of a putrid odor during incubation indicates that too little sodium nitrate has been added. Methylene blue solution in proper proportion may be added at the start to serve as an indicator during the incubation. Domestic sewage usually varies in its oxygen demand from 100 to 300 parts per million, approximately 30 per cent of which is used up at 20° C. in the first 24 hours. At the end of the incubation period determine the residual nitrite and nitrate. Determine the nitrate by the aluminium reduction method and direct Nesslerization. To convert the nitrogen into oxygen equivalents, multiply the nitrite nitrogen by 1.7 and the nitrate nitrogen by 2.9. The difference between the available oxygen added as sodium nitrate and that found as nitrite and nitrate at the end of the incubation period is the biochemical oxygen demand.

_Procedure for industrial wastes._—Employ the same procedure using larger quantities of the sodium nitrate solution. Make the reaction alkaline to methyl orange and acid to phenolphthalein. Adjust an acid reaction with sodium bicarbonate and a caustic alkaline reaction with weak hydrochloric acid. If the liquid is devoid of sewage bacteria seed it with sewage after adjusting the reaction.

_Procedure for polluted river waters._—Determine the initial available oxygen. Unless the river water is badly polluted add 10 parts per million of sodium nitrate oxygen. Collect carefully, avoiding aeration, three samples in 250 cc. bottles. To one sample add a definite quantity of sodium nitrate solution and incubate. Incubate the other two samples for the determination of the residual free oxygen, nitrite, and nitrate. If there is free oxygen left, the bottle containing the sodium nitrate solution may be discarded. If there is no free oxygen determine residual nitrite and nitrate as directed under the procedure for sewage (p. 72) and calculate the oxygen demand.

ANALYSIS OF SEWAGE SLUDGE AND MUD DEPOSITS.

COLLECTION OF SAMPLE.

Collect a representative sample of the material. In general more than one sample should be taken from a spot and a large number of samples should be collected rather than a few large samples. If the surface layer is darker and a lower layer consists of pure clay sample only the surface layer. Samples may be analyzed either separately or as composites of careful mixtures. After the sample has settled a few minutes roughly drain or siphon the excess water. Allow sewage sludge to stand for one hour before draining it free from excess water unless it is essential to determine the moisture content of the sample originally collected. If sludge cannot be analyzed within twenty-four hours it is better not to use air-tight bottles and to add small quantities of chloroform and keep in the ice box to retard decomposition. At the time of collection carefully examine mud from the bottom of surface water for evidence of sewage pollution and macroscopic and microscopic animal and plant organisms. Record the predominant species. Note the physical appearance of the material, particularly its color, odor, and consistency. Express all analytical results in percentage on a dry basis.

REACTION.

Determine the reaction by diluting a definite quantity of the wet sludge and titrating by the methods given under alkalinity and acidity (pp. 35–39 and 39–41).

SPECIFIC GRAVITY.

Weigh to the nearest tenth of a gram a wide-mouthed flask of 100 to 300 cc. capacity, according to the quantity of material available. Then completely fill the flask with distilled water to the brim and weigh it again. Empty the flask and fill it completely with fresh sewage sludge or mud. If the material is of such consistency that it flows readily fill the flask to the brim and weigh. The specific gravity is equal to the weight of the sludge or mud divided by the weight of an equal volume of distilled water.

If the material does not flow readily fill the weighed flask as completely as possible without exerting pressure during the procedure. Weigh and then fill the flask to the brim with distilled water. Let it stand for a few minutes, until trapped air has escaped, then add more water if necessary and weigh. Subtract the weight of the added water from the weight of the water that completely fills the flask; the specific gravity is equal to the weight of the material divided by this difference. Record the specific gravity only to the second decimal place.

MOISTURE.

Heat approximately 25 grams of sludge or mud in a weighed nickel dish on the water bath until it is fairly dry. Dry the residue in an oven at 100° C., cool, and weigh. Repeat to approximate constant weight. The loss in weight is moisture.

VOLATILE AND FIXED MATTER.

Ignite, at dull red heat in a hood, the residue from the determination of moisture until all the carbon has disappeared. Cool the residue in a desiccator and weigh it. The residue is the fixed matter. The volatile matter is the difference in weight between the original dried sludge and the ignited sludge.

TOTAL ORGANIC NITROGEN.

_Preparation of sample._—For the determination of organic nitrogen and fats dry approximately 50 to 75 grams of the sludge or mud in a porcelain dish first on the water bath and finally in the hot-water oven until all the moisture has disappeared. Grind the dry material to a fine powder and keep it in a glass-stoppered bottle.

_Reagents._—1. Sulfuric acid. Concentrated, nitrogen-free.
2. Copper sulfate solution. Ten per cent.
3. Potassium permanganate. Crystals.

_First procedure._—Weigh accurately 0.5 gram of dried sludge or 5.0 grams of dried mud and put it into a 500 cc. Kjeldahl flask. Digest it with 20 cc. of sulfuric acid, or more if necessary, and 1 cc. of copper sulfate solution to assist the oxidation. Boil for several hours until the liquid becomes colorless or slightly yellow. Oxidize the residue with 0.5 gram of potassium permanganate, and follow the “Procedure for Sewage” (pp. 21–22).

_Second procedure._—The following method is convenient for routine work at sewage disposal plants. After digestion as described in the first procedure, cool, transfer to a glass-stoppered 100 cc. flask, dilute with distilled water to 100 cc., and mix well. Transfer 50 cc. with a pipette into another 100 cc. volumetric flask, and make this portion alkaline with 50 per cent sodium hydroxide, testing a drop of the liquid on a porcelain plate with phenolphthalein to insure neutralization. The formation of a floc usually indicates that neutralization is complete. Dilute the solution to 100 cc., pour it into a small glass-stoppered bottle and permit it to stand until the next day. Nesslerize an aliquot portion of the clear supernatant liquid, and calculate the percentage of nitrogen in the material.

ETHER-SOLUBLE MATTER.

Fats are usually determined only on sewage sludge, but some mud deposits contain small quantities due to the presence of trade wastes.

_Procedure._—Weigh 0.5 to 25 grams of dry material according to the quality of the sludge or mud. Add water to the weighed portion in a porcelain dish and acidify the mixture with N/50 sulfuric acid in the presence of litmus tincture or azolitmin solution as indicator. Avoid adding too much acid as an excess gives too high results on account of fatty acid residues. Evaporate the acidified mixture to dryness on the water bath, and heat it in the hot air oven at 100° C. two to three hours. Extract the dry residue with boiling ether, rubbing the sides and bottom of the dish to insure complete solution of the fat. Three extractions with ether are usually sufficient. Filter the ether solution through a 5 cm. filter paper into a small flask. Evaporate the ether slowly, dry the fatty extract for half an hour at 100° C., cool in a desiccator, and weigh. If it is desirable, particularly with certain industrial wastes, to determine the quantity of saponified fat determine the fats with and without the addition of acid. The difference between the quantities found by the two determinations is the content of saponified fat.

FERROUS SULFIDE.

The liberation of hydrogen sulfide on adding dilute hydrochloric acid to a sludge indicates the presence of ferrous sulfide. As ferrous sulfide quickly oxidizes on exposure to air a quantitative determination of this constituent must be made immediately after collection of the sample.

_Procedure._—Heat a definite portion of the sludge with hydrochloric acid in a flask. Pass the liberated gas through bromine water or hydrogen peroxide. Determine gravimetrically the sulfate in the oxidizing solution, and calculate the equivalent of ferrous sulfide by multiplying the weight of barium sulfate by 0.376.

BIOCHEMICAL OXYGEN DEMAND.

The quantity of river mud most suitable for the determination of the biochemical oxygen demand ranges within certain limits, largely according to the amount of oxidizable matter present. For examinations of river mud prepare a 1 per cent stock suspension in distilled water or tap water saturated with oxygen and free from nitrate; use in the test a dilution of this stock suspension equivalent to a concentration of 1 to 10 grams per liter of mud. For examinations of fresh sewage sludge prepare a 1 per cent stock suspension in a similar manner, but use in the test a dilution equivalent to only 0.1 to 1.0 gram per liter of wet material. For examinations of dried sludges, which have undergone more or less oxidation higher concentrations may be required.

_Procedure._—Place a measured portion of the sample, or the proper amount of the 1 per cent stock suspension of the sample, in a 300 cc. narrow-mouth glass-stoppered bottle, and dilute it to the desired concentration with water saturated with oxygen. Determine the oxygen content at 20° C. of the waters that are used for dilution. This determination must be made before the mud or sludge is added because iron sulfide in the mud or sludge rapidly consumes part of the dissolved oxygen. Incubate at 20° C. for five days.

Shortly before the determination of the oxygen remaining in solution at the end of five days rotate the bottle once or twice to mix its contents and allow sedimentation for about 30 minutes. Siphon the greater part of the liquid through a narrow-bore siphon into a 150 cc. bottle, which has been filled with carbon dioxide. Reject the first 25 cc. of the siphoned liquid and allow a little to overflow at the end of siphoning. Determine the oxygen content of the solution in the bottle in the usual way (pp. 65–68). Report the oxygen demand in percentage of the dried mud or sludge.

ANALYSIS OF CHEMICALS.

The following sections describe the accepted methods for the analysis of the chemicals commonly used in the treatment of water.

REAGENTS.

1. Distilled water. In practically all the tests of chemicals it is necessary to use exclusively distilled water that has been freshly boiled to free it from carbon dioxide and oxygen.

2. Concentrated hydrochloric acid. Sp. gr. 1.20.

3. Hydrochloric acid, N/2.

4. Hydrochloric acid, N/10.

5. Ammonium hydroxide. Redistilled; sp. gr. 0.90.

6. Dilute sulfuric acid. Dilute 1 part of concentrated sulfuric acid with 3 parts of freshly boiled distilled water.

7. Methyl orange indicator. See page 36.

8. Phenolphthalein indicator. See page 36.

9. Bromine water.

10. Stannous chloride, N/20. This should be frequently standardized by titration against a standard iron solution. One cc. of N/20 stannous chloride is equal to 0.0028 gram of iron (Fe) estimated in the ferrous state.

11. Sodium hydroxide, N/1. Free from carbonate. This should be frequently standardized by titration against a standard acid solution in presence of phenolphthalein indicator. One cc. of N/1 sodium hydroxide is equal to 0.049 gram of sulfuric acid (H_{2}SO_{4}), or to 0.03645 gram of hydrochloric acid (HCl).

12. Sodium hydroxide, N/20. Free from carbonate.

13. Standard potassium permanganate. A N/10 solution. One cc. of N/10 potassium permanganate is equal to 0.0056 gram of iron (Fe) estimated in the ferrous state.

14. Alcohol. Ethyl alcohol, 95 per cent.

15. Sugar. Solid granulated cane sugar.

SULFATE OF ALUMINIUM.