Chapter XXII: Introduction (21)

=455. General Conclusions.=—The nitrogenous food of plants is provided in several ways; _viz._, (1) By the nitrogen brought to soil in rain and snow. This nitrogen is chiefly in the form of ammonia and nitric acid. The nitrogen gas in solution in rain water has no significance as a plant food. (2) By the action of certain anaerobic organisms herding in the rootlets of leguminous plants, free nitrogen may be oxidized and put into form for assimilation. (3) By the action of certain organisms on nitrogenous compounds pre-existing in the soil, ammonia, nitrous acid, and finally, nitric acid, are produced. It is believed that the plant organism, unaided by the activity of a micro-organism, is unable to assimilate nitrogen unless it be fully oxidized to nitric acid. (4) There exist micro-organisms capable of acting directly on free nitrogen independent of other plant growth, but the significance of this possible source of plant food is, at the present time, unknown. (5) The micro-organisms of importance to agriculture may be isolated and developed to the exclusion of other organisms of a similar character. This isolation is best accomplished in culture-media consisting essentially of a mineral gelatin to which is added only pure carbohydrates and the necessary mineral nourishment. (6) The nitrifying ferments consist probably of several species, of different geographic distribution. Different types of soils probably have nitrifying organisms of different properties. This is illustrated by the fact that nitrification is accomplished in dry alkaline soils under conditions in which the ordinary nitrifying organisms would fail to develop. (7) The study of typical soils in respect of the kind, activity, and vigor of their nitrifying organisms has become as important a factor in soil analysis as the usual determination of physical and chemical composition.

DETERMINATION OF NITRIC AND NITROUS ACIDS IN SOILS.

=456. Classification of Methods.=—The minute quantities in which highly oxidized nitrogen exists in soils render the operations of its quantitative estimation extremely delicate. On the other hand, the easy solubility of these forms of combination and the absence of absorptive powers therefor, in the soil, render the separation of them from the soil a matter of great ease. It is possible, therefore, to secure all the nitrates and nitrites present in a large quantity of earth in a solution which can be concentrated under proper precautions to a volume convenient for manipulation. The method of this extraction is the same for all the processes of determination. The methods of analysis suited to soil extracts, as a rule, may also be used in the determination of the same compounds in rain, drainage, and sewage waters, and for the qualitative and quantitative control of the progress of nitrification. The various processes employed may be classified as follows:

1. The conversion of the nitrogen into the gaseous state and the determination of its volume directly. This is accomplished by combustion with copper oxid and metallic copper.

2. The conversion of the nitrogen into nitric oxid and the volumetric determination thereof. The decomposition of a nitrate with ferrous chlorid in the presence of free hydrochloric acid is an instance of this type of methods.

3. The oxidation of colored organic solutions and the consequent disappearance of the characteristic color, or its change into a different tint. The indigo and indigotin processes are examples of this method.

4. The production of color, in a colorless or practically colorless solution, by the treatment thereof with the nitrate in presence of an acid which decomposes it with the liberation of oxidizing compounds. The depth of color produced is compared with that formed by a known quantity of a pure nitrate solution until the two colorations are alike. The methods depending on the use of carbazol or acid phenol sulfate illustrate this class of reactions.

5. The conversion of the nitrogen into ammonia by moist combustion with sulfuric acid in the presence of certain organic compounds, _e. g._, salicylic acid, and the collection of the ammonia in standard acid, the excess of which, is determined by titration.

6. The reduction of nitrates to ammonia by nascent hydrogen and the recovery of the ammonia produced by distillation and collection in standard acid.

7. The reduction of nitrates by electrolytic action and the collection of the ammonia as above.

8. The decomposition of nitrates with the quantitative evolution of a different element, and the direct or indirect measurement of the evolved substance. The quantitative evolution of chlorin on treating a nitrate with hydrochloric acid, the collection of the chlorin in potassium iodid, and the determination of the iodin set free, form a process belonging here.

=457. Relative Merit of Methods.=—The processes mentioned in the classifications embraced under numbers (1) and (5) of the preceding schedule are sufficiently described in the paragraphs devoted thereto, under soil and fertilizers. In practice at the present time it is rare to determine the nitrogen in nitrates by the copper oxid method. The more rapid and equally exact processes of colorimetric comparison or reduction by nascent hydrogen are in all respects to be preferred.

The indigo methods among the colorimetric processes are not so much in use now as those which depend on the development of a color. Lawes and Gilbert considered them far inferior to the Schloesing method. The developed color methods are especially delicate and are to be preferred in all cases where the detection of the merest traces of nitrates is desired. Where nitrates are present in considerable quantities the reduction method with nascent hydrogen is to be preferred over all others. In all these cases the judgment of the analyst must be exercised. The particular method to be employed in any given case can not be determined save by the intelligent discrimination of the operator.

=458. The Extraction of Nitric Acid from the Soil.=—The easy solubility of nitric acid and of nitrates in water is taken advantage of in the separation of these bodies from the soil. A convenient quantity, usually about 1,000 grams of the fine soil, is taken for the extraction. Instead of freeing the soil entirely from water, it is better to determine the amount of water in the air-dried or prepared sample, and base the calculation on 1,000 grams of the water-free soil.

All samples of soil, when taken for the purpose of examining for nitrates, should be rapidly dried to prevent the process of nitrification from continuing after the sample is taken. For this purpose the soil should be placed in a thin layer in a warm place, 50°–60°, until air-dried. It still contains in this case a little moisture but not enough to permit nitrification to go on.

One thousand grams of soil prepared as above are treated with 2,000 cubic centimeters of distilled water, free of nitric acid, and allowed to stand for forty-eight hours with frequent shaking. One thousand cubic centimeters of the extract are then filtered, corresponding to 500 grams of the dry soil. A small quantity of pure sodium carbonate should be added to the filtrate which is then evaporated on a water-bath to a volume of about 100 cubic centimeters. Should a precipitate be formed during evaporation it should be separated by filtration, the filter washed thoroughly, and the filtrate again evaporated to a volume of 100 cubic centimeters.

In taking a soil for the determination of nitrates, it is well to extend the sampling to a considerable depth. If the sample be taken only to the depth of nine inches, it should be in dry weather when the nitrates are near the surface.

The temperature at which a soil is dried has also an influence on the quality of nitric nitrogen remaining after desiccation.

If a wet soil be dried at 100°, the nitrates present will suffer partial decomposition. This is probably due to deoxidation by organic matter present. On the other hand, ordinary air-drying affords opportunity for continued nitrification, thus increasing the residuum of oxidized nitrogen. The above method is essentially that followed by Warington at Rothamstead.

The method of drying practiced at Rothamstead, in order to secure results as nearly accurate as possible is the following:[293]

The soil is broken up directly after it is taken from the field, and spread on trays in layers one inch deep. The trays are then placed in a room at 55°. The drying is completed in twenty-four hours. After drying, stones and roots are removed, and the soil is finely powdered and placed in bottles.

For extracting the nitrates, a funnel is prepared by cutting off the bottom from a bottle four and a half inches in diameter. A nicely fitting disk of copper gauze is placed in the bottom of this funnel, and this is covered with two filter papers, the upper one having a slightly greater diameter than the lower. The paper is first moistened, and then from 200 to 500 grams of the dry powdered soil introduced. The funnel is connected with the receiving flask of a filter pump, and pure water poured on the soil until it is thoroughly saturated. The water is then added in small quantities. When the filtrate amounts to 100 cubic centimeters the process may be discontinued, since all the nitrates in the soil will be found in this part of the filtrate.

The extract obtained above is evaporated to convenient bulk for the determination of nitric nitrogen.

THE NITRIC OXID PROCESS.

=459. Method of Schloesing.=—The processes for estimating nitrogen by combustion with copper oxid and by moist combustion with sulfuric acid have both been used for the determination of the quantity of nitrogen existing in a highly oxidized state. These processes will be fully discussed under the head of fertilizers. In the case of soil extracts, drainage waters, etc., it will be sufficient to discuss, for the present, only those processes adapted especially to a quick and accurate estimation of oxidized nitrogen.

The principle of the method of Schloesing depends on the decomposition of nitrates in the presence of a ferrous salt and a strong mineral acid.[294] The nitrogen in the process appears as nitric oxid, the volume of which may be directly measured, or it may be converted into nitric acid and titrated by an alkali.

The typical reactions which take place are represented in the following equation:

6FeCl₂ + 2KNO₃ + 8HCl = 3Fe₂Cl₆ + 2KCl + 4H₂O + 2NO.

=460. Schloesing’s Modified Method.=—The Schloesing method as now practiced by the French chemists is conducted in the apparatus shown in Fig. 75.[295] The carbon dioxid is generated by the action of the hydrochloric acid in F on the fragments of marble in A. After passing the wash-bottle the gas enters the small tubulated retort, C, which contains the nitrate in solution. For ordinary soils 100 grams are placed in an extraction flask, plugged with cotton, and a layer of the same material is placed over the soil for the purpose of securing an even distribution of the extracting liquid. This liquid is distilled water containing in each liter one gram of calcium chlorid. The purpose of using the calcium chlorid is to prevent the soil from becoming compacted which would render the extraction of the nitrate difficult. The extracting liquid is allowed to fall drop by drop from a mariotte bottle until the filtrate amounts to 500 cubic centimeters. This volume is concentrated on a sand-bath until it is reduced to ten or fifteen cubic centimeters when it is transferred to a flat-bottomed dish and the evaporation finished over steam, care being taken not to allow the temperature to exceed 100°.

FIGURE 75. SCHLOESING’S APPARATUS FOR NITRIC ACID.
]

Another and more rapid method for dissolving the nitrate, may also be practiced. In a flask holding about one liter, place 220 grams of the soil and 660 cubic centimeters of distilled water and shake vigorously, or enough water to make 660 cubic centimeters together with the moisture remaining in the air-dried sample taken. All the nitrates pass into solution. Throw the contents of the flask into a filter and take 600 cubic centimeters of the filtrate which will contain all the nitrates in 200 grams of the sample taken. This filtrate is evaporated as described above.

In the flat dish containing the dried nitrates, pour three or four cubic centimeters of ferrous chlorid solution and stir with a small glass rod until complete solution of the nitrate takes place. By means of a small funnel the solution is poured into C, and the capsule and funnel are well rinsed with two cubic centimeters of hydrochloric acid. The washing is repeated three times as above described, and once with one cubic centimeter of water, which is added cautiously so as to form a layer over the surface of the heavier liquid. The tubulated flask is then connected with the carbon dioxid apparatus, previously freed from air, and the gas allowed to flow evenly until the whole of the apparatus is completely air-free. The other details of the method are essentially the same as those adopted by the Commission of French Agricultural Chemists which will be given below.

=461. The French Agricultural Method.=—The Schloesing method as practiced by the French agricultural chemists is very slightly different from the procedure just described.[296] The process with soils is carried on as follows:

Five hundred grams of the soil are taken and introduced into a flask of about two liters capacity and shaken thoroughly with a liter of distilled water. The whole of the nitrates of the soil is thus passed into solution. The solution is filtered and 400 cubic centimeters of the filtrate are taken, which correspond to 200 grams of the soil. This liquid is evaporated in a flask, adding a fragment of paraffin to prevent foaming, until its volume is reduced to fifteen or twenty cubic centimeters. It is afterwards transferred through a filter into a capsule with a flat bottom in which the evaporation is finished on a steam-bath, taking care that the temperature does not exceed 100°. An important precaution is, not to allow the contact of the water with the soil to be too prolonged, to avoid the reduction of the nitrates which could take place under the influence of the denitrifying organisms which are developed with so great a rapidity in moist earth. The apparatus in which the transformation of the nitrates into nitric oxid takes place is essentially that already described (Fig. 75). The carbon dioxid generator is connected by means of a rubber tube and a small wash-bottle to the small retort in which the reaction takes place, and from which the exit tube leads to a mercury trough. The gas which is disengaged is received under a jar drawn out to a fine point in its upper part, which carries about fifteen cubic centimeters of potash solution containing two parts of water to one of potash.

The operation is conducted as follows:

Into the small capsule which contains the dried matter, three or four cubic centimeters of ferrous chlorid are poured. By means of a stirring rod the residue sticking to the sides of the capsule is detached with care and all the matter is thus collected in the bottom. By means of a small funnel the contents of the capsule are introduced into the retort. About two cubic centimeters of hydrochloric acid are used for washing out the materials and this acid is also introduced into the retort. The washing with hydrochloric acid is repeated three or four times, and finally the apparatus is washed with one cubic centimeter of water, which is also poured in by the small funnel with great care, so that this water may form a layer over the surface of the liquid. The apparatus is now connected and filled completely with carbon dioxid. Since it is necessary that this gas should be completely free of air, the flask, which generates it, is first filled with the acidulated water from the acid flask, and the air is thus almost totally displaced by the liquid. The evolution of carbon dioxid gas which follows, completely frees the apparatus from air. When this is accomplished the retort is connected with the rest of the apparatus and the gas allowed to pass for about two minutes until the air is completely driven out of all the connections. The current is arrested for a moment by pinching the rubber tube which conducts the carbon dioxid into the retort, and the vessel which is to receive the gas is then placed over the delivery-tube, this vessel being filled with mercury and a strong solution of potash. The communication between the retort and the carbon dioxid flask is broken and the flask is heated slightly by means of a small lamp. The first bubbles of gas evolved should be entirely absorbed by the potash. This will be an indication of the complete absence of the air. When the liquid is in a state of ebullition the nitrogen dioxid is set free. The boiling is regulated in such a way that the evolution is regular and the liquid of the retort may not, by a too violent boiling, pass into the receiver. The boiling is continued until the larger part of the liquid is distilled and only three or four cubic centimeters remain in the retort. At this time a few bubbles of carbon dioxid are allowed to flow through in order to cause to pass into the receiver the last traces of nitric oxid. The gas received is left for some minutes in contact with the potash.

Afterward in a small flask, G, the neck of which is drawn out to a fine point, and carrying a bulb-tube, H, and a piece of rubber tubing, there are boiled twenty-five or thirty cubic centimeters of water for five or six minutes in order to drive all the air out of the flask, and while the boiling is continued the rubber tubing is fastened to the drawn-out part of the jar containing the nitric oxid. Within the rubber tubing the drawn-out point is broken and the vapor of water is forced into the jar and drives before it the solution of potash which has filled the capillary part of the drawn-out tube. As soon as the point is broken, the boiling of the flask is stopped and by its cooling the nitric oxid passes into it. It is necessary to press the rubber tubing with the fingers in order that the passage of the gas into the flask be not too rapid. As the solution of potash rises in the bell-jar which contains the nitric oxid near to the point where the rubber tubing covers its drawn-out portion, the fingers are removed and a clamp put in their place. There still remains a little nitric oxid in the flask and to drive this out it is necessary to introduce five or six cubic centimeters of pure hydrogen, which are allowed to pass over into the receiving flask, by releasing the clamp in the same way as the nitric oxid. The hydrogen being introduced in successive portions, finally carries all the nitric oxid into the flask without allowing any of the potash to enter.

The flask containing the nitric oxid is now connected with a reservoir of oxygen. The oxygen is allowed to enter, bubble by bubble, by means of cooling the flask by immersion in water. The transformation of nitric oxid into nitric acid is not entirely complete for twenty-four hours. It is necessary, therefore, to wait that long after the introduction of the oxygen before determining the amount of nitric acid produced.

The contents of the flask are placed in a titration-jar, the flask being washed two or three times and a few drops of tincture of litmus being added. The nitric acid is then determined by a standard solution of calcium hydroxid or some other standard alkali. From the titration the content of nitric acid is calculated.

The French Committee further suggests that this method may be modified in the way of making it more rapid by collecting the nitric acid in a graduated tube filled with mercury and containing some potash. The volume of the gas is determined and the pressure of the barometer and the temperature observed, and the usual calculations made to reduce the volume to zero and to a pressure of 760 millimeters of mercury. Each cubic centimeter of nitric oxid thus measured corresponds to 2.417 milligrams of nitric acid. The presence of organic matter does not interfere with the determination of nitric acid by either of the methods given above.

=462. Modification of Warington.=—The method of procedure and description of apparatus used, as employed by Warington, are as follows:

The vessel in which the reaction takes place is a small tubulated receiver, A (Fig. 76), about four centimeters in diameter, mounted and connected as shown in the illustration. The delivery-tube dips into a jar of mercury in a trough containing the same liquid. The long supply funnel-tube _a_ is of small bore, holding in all only one-half cubic centimeter. The connecting tube F, carrying a clamp, is also of small diameter and serves to connect the apparatus with a supply of carbon dioxid.

FIGURE 76. WARINGTON’S APPARATUS FOR NITRIC ACID.
]

In practice, the supply-tube _a_ is first filled with strong hydrochloric acid and carbon dioxid passed through the apparatus until the air is all expelled. This is indicated when a portion of the gas collected over the mercury, is entirely absorbed by caustic alkali.

At this point the current of carbon dioxid is stopped by the clamp C, and a bath of calcium chlorid, B, heated to 140° is brought under the bulb A, until the latter is half immersed therein. The temperature of the bath is maintained by a lamp. By allowing a few drops of hydrochloric acid to enter the receiver, the carbon dioxid is almost wholly expelled. The end of the delivery-tube is then connected with the tube, T, filled with mercury, and the apparatus is ready for use.

The nitrate, in which the nitric acid is to be determined, in a dry state, is dissolved in two cubic centimeters of the ferrous chlorid solution (one gram of iron in ten cubic centimeters), one cubic centimeter of strong hydrochloric acid is added, and the whole is then introduced into the receiver through the supply-tube, being followed by successive rinsings with hydrochloric acid, each rinsing not exceeding one-half cubic centimeter. The contents of the receiver are, in a few moments, boiled to dryness; a little carbon dioxid is admitted before dryness is reached, and again afterwards to drive over all remains of nitric oxid. In the recovered gas the carbon dioxid is first absorbed by caustic potash, and afterwards the nitric oxid by ferrous chlorid. All measurements of the gas are made in Frankland’s modification of Regnault’s apparatus. The carbon dioxid should be as free as possible from oxygen. The carbon dioxid generator is formed of two vessels, the lower one consisting of a bottle with a tubule in the side near the bottom; this bottle is supported in an inverted position and contains the marble from which the gas is generated. The upper vessel consists of a similar bottle standing upright and containing the hydrochloric acid required to act on the marble. The two vessels are connected by a glass tube passing from the side tubule of the upper vessel to the inverted mouth of the lower vessel. The acid from the upper vessel thus enters below the marble. Carbon dioxid is generated and removed at pleasure by opening a stop-cock attached to the side tubule of the lower vessel thus allowing hydrochloric acid to descend and come in contact with the marble. A good Kipp’s generator of any approved form may also be used instead of the simple apparatus, above described.

The fragments of marble used are previously boiled in water in a strong flask. After boiling has proceeded for some time, a rubber stopper is fixed in the neck of the flask and the flame removed. Boiling will then continue for some time in a partial vacuum.

The hydrochloric acid is also well boiled and has dissolved in it a moderate quantity of cuprous chlorid. As soon as the acid has been placed in the upper reservoir, it is covered by a layer of oil. The apparatus being thus charged is at once set in active work by opening the stop-cock of the marble reservoir; the acid descends, enters the marble reservoir, and the carbon dioxid produced drives out the air. As the acid reservoir is kept on a higher level than the marble reservoir, the latter is always under internal pressure, and leakage of air from without, into the apparatus, cannot occur.

The presence of the cuprous chlorid in the hydrochloric acid not only insures the removal of dissolved oxygen, but affords an indication to the eye of the maintenance of this condition. While the acid remains of an olive tint, oxygen is absent; but should the color change to a blue-green, more cuprous chlorid must be added. All the reagents employed should be previously boiled.

In order to secure absolute freedom from air, the following modifications on the above process have been adopted by Warington: The apparatus having been mounted as described, the funnel-tube attached to the bulb retort is filled with water, and the apparatus connected with the carbon dioxid generator. Carbon dioxid is then passed through the apparatus until a moderate stream of bubbles rises in the mercury trough. The stop-cock is left in this position, and the admission of gas is controlled by the pinch-cock. The bath of calcium chlorid is so adjusted as to cause the bulb retort to be almost entirely submerged, and the temperature of the bath is kept at 130° to 140°. Small quantities of water are next admitted into the bulb and expelled as steam in the current of carbon dioxid, the supply of this gas being shut off before the evaporation is entirely completed, so as to leave as little carbon dioxid as possible in the apparatus. Previous to very delicate experiments it is advisable to introduce through the funnel-tube a small quantity of potassium nitrate, ferrous chlorid, and hydrochloric acid, rinsing the tube with the latter reagent. Any trace of oxygen remaining in the apparatus is then consumed by the nitric oxid formed; and after boiling to dryness and driving out the nitric acid with carbon dioxid, the apparatus is in a perfect condition for a quantitative experiment.

=463. Preparation of the Materials to be Analyzed.=—According to Warington, soil extracts may be used without other preparation than concentration.

Vegetable juices which coagulate when heated, require to be boiled and filtered or else evaporated to a thin sirup, treated with alcohol, and filtered. A clear solution being thus obtained, it is concentrated over a water-bath to a minimum volume in a beaker of small size. As soon as cool, it is mixed with one cubic centimeter of a cold saturated solution of ferrous chlorid and one cubic centimeter of hydrochloric acid, both reagents having been boiled and cooled immediately before use.

In mixing with the reagents, care must be taken that bubbles of air are not entangled, which is apt to occur with viscid extracts.

The quantity of ferrous chlorid mentioned is amply sufficient for most soil extracts, but it is well to use two cubic centimeters in the first experiment, the presence of a considerable excess of ferrous chlorid in the retort being thus insured. With bulky vegetable extracts more ferrous chlorid should be employed. To the sirup from twenty grams of mangel-wurzel sap, five cubic centimeters of ferrous chlorid and two cubic centimeters of hydrochloric acid are usually added.

=464. Measurement of the Gas.=—The measurement of the gas was for some time conducted by the use of concentrated potash for absorbing the carbon dioxid, and ferrous chlorid for absorbing the nitric oxid. The use of the ferrous chlorid, however, was found to introduce a source of error. The treatment of the gas with oxygen and pyrogallol over potash has therefore been substituted by Warington for its absorption by ferrous chlorid.

The chief source of error attending the oxygen process lies in the small quantity of carbon monoxid produced during the absorption with pyrogallol; this error becomes negligible if the oxygen be only used in small excess. The amount of oxygen employed can be regulated by the use of Bischof’s gas delivery-tube. This may be made of a test-tube having a small perforation half an inch from the mouth. The tube is partly filled with oxygen over mercury, and its mouth is then closed by a finely perforated stopper made from a piece of wide tube and fitted tightly into the test-tube by means of a covering of rubber. When this tube is inclined, the side perforation being downwards, the oxygen is discharged in small bubbles from the perforated stopper, while mercury enters through the opening. Using this tube, the supply of oxygen is perfectly under control and can be stopped as soon as a fresh bubble ceases to produce a red tinge on entering. Warington concludes his description by stating that in the reaction proposed by Schloesing the analyst has a means of determining a very small quantity of nitric acid with considerable accuracy, even in the presence of organic matter; but to accomplish this, the various simplifications consisting in the omission of the stream of carbon dioxid, and the collection of the gas over caustic soda must be abandoned, and special precautions must be taken to exclude all traces of oxygen from the apparatus.

=465. Spiegel’s Modification.=—Spiegel noticed inaccuracies in the results of the ferrous chlorid method of estimating nitric acid when carbon dioxid is used, which sometimes amounted to three per cent of the nitric acid present in the sample. The following suggestions are made by him for the improvement of the process:[297]

As regards the use of carbon dioxid in the operation, the first difficulty consists in obtaining it entirely free from air. By the use of small pieces of marble, which, before being placed in the Kipp apparatus, are kept for a long while in boiling water, a product is obtained which, after thirty minutes of moderate evolution, leaves only a trace of unabsorbed gas in contact with potash-lye. The apparatus used is illustrated in Fig. 77.

FIGURE 77. SPIEGEL’S APPARATUS FOR NITRIC ACID.
]

A is a round flask of about 150 cubic centimeters capacity, furnished with a well-fitting rubber stopper provided with two holes, one for the entrance of the funnel-tube B and the other for the delivery-tube C. The tube B ends about two centimeters above the bottom of A and carries a bulb-shaped funnel at its top capable of holding about fifty cubic centimeters. The gas-tube D is ground into the bulb of B as shown in the figure.

After the flask had been filled with the solution to be examined, carbon dioxid is conducted through D and the flask is heated to boiling until the gas which escapes through C no longer contains any air. The measuring tube is brought over the end of the delivery-tube C, in the usual manner, but not shown in the figure. In the funnel of B are placed twenty cubic centimeters of previously prepared and boiled ferrous chlorid solution and this liquid is allowed to flow partly into A by lifting slightly the gas-tube, D. About forty cubic centimeters of concentrated, boiled hydrochloric acid are afterwards added to it in the same way. As soon as the liquid in the flask A is again boiling, the stream of carbon dioxid is shut off and allowed to flow again only towards the end of the operation, when the contents of the flask are reduced almost to dryness. As will be seen from the above directions no unboiled liquids of any kind are to be used as reagents in the apparatus described. If the flask A were made much smaller the efficiency of this apparatus would be increased. It appears to have few, if any, advantages over Warington’s process.

=466. Schulze-Tiemann Method.=—The modification of Schulze-Tiemann in the ferrous salt method consists chiefly in the omission of the use of carbon dioxid, and in the simplified form of apparatus, which permits rapid work and gives, also, according to some authorities, very exact and reliable results.[298] The extract, representing 500 grams of the fine soil, is reduced by evaporation to 100 cubic centimeters and placed in a glass flask, _A_ (Fig. 78), of 500 cubic centimeters capacity. The flask is closed with a rubber stopper, carrying two bent glass tubes which pass through it. The tube _a b c_ is drawn out into a point at _a_ and reaches about two centimeters below the surface of the rubber stopper. The tube _e f g_ passes just to the lower surface of the rubber stopper. The two tubes mentioned are connected, by means of rubber tubes and pinch-cocks, with the tubes _d_ and _h_. The pinch-cocks at _c_ and _g_ must be capable of closing the tubes air-tight. The end of the tube _g h_ passes into a crystallizing dish, _B_, and is bent upward to a point passing two to three centimeters into the measuring tube _C_. The point within the tube is covered with a piece of rubber tubing. The measuring tube _C_ is divided into tenths of a cubic centimeter, and together with the crystallizing dish _B_, is filled with a ten per cent solution of boiled soda-lye, which is obtained by dissolving 12.9 parts of sodium hydroxid in 100 parts of water.

FIGURE 78. SCHULZE-TIEMANN’S NITRIC ACID APPARATUS.
]

The liquid which is to be examined for nitric acid, the pinch-cocks being opened and the tube _g h_ not dipping into the crystallizing dish, is boiled for one hour in order to drive the air out of the flask _A_. The end of the tube _e f g h_ is then brought into the crystallizing dish containing the sodium hydroxid solution so that the steam escaping from the flask _A_, escapes partly through the tube _b c d_, and partly through the tube _f g h_, not allowing, however, the bubbles to enter the measuring tube _C_. To determine whether the air is all expelled, the pinch-cock at _g_ is closed and the soda-lye will thereupon rise to _g_ in case no air interferes. It is best to close the tube at _g_ first with the thumb and finger and then the rise of the soda-lye to that point can be determined by the impulse felt. The tube is then firmly closed by means of the pinch-cock _g_. The rest of the steam is allowed to escape through the tube _a b c d_, and the evaporation is continued until the contents of the flask are evaporated to about ten cubic centimeters. The flask into which the tube _c d_ dips, is filled with freshly boiled water. The lamp is removed from the flask _A_, the pinch-cock is closed, whereupon the tube _c d_ becomes filled with the freshly boiled water. The measuring tube _C_, filled with freshly boiled soda-lye is closed with the thumb and brought into the dish _B_, care being taken that no bubble of air enters. It is placed over the end of the tube _g h_.

The pressure of the external air will now flatten the rubber tubes at _c_ and _g_. The flask at the end of _c d_ holding freshly boiled water is then replaced with one filled with a nearly saturated solution of ferrous chlorid containing some hydrochloric acid. The flask containing the ferrous chlorid solution should be graduated so that the amount which is sucked into the flask _A_ can be determined. The pinch-cock _c_ is opened and from fifteen to twenty cubic centimeters of the ferrous chlorid solution allowed to flow into _A_. The end of the tube _c d_ is then placed in another flask containing strong hydrochloric acid, and the latter allowed to flow into the tube in small quantities at a time until all the ferrous chlorid is washed out of the tube _b c d_ into _A_. At the point _b_ there is sometimes formed a little bubble of hydrochloric acid in the state of gas, which by heating the flask _A_ completely disappears.

The flask _A_ is next warmed gently until the rubber tubes at the pinch-cocks begin to assume their normal condition. The pinch-cock at _g_ is now replaced by the thumb and finger, and as soon as the pressure within the flask _A_ is somewhat stronger, caused by the nitric oxid gas evolved from the mixture, it is allowed to pass through the tube _e f g h_ and escape into the measuring cylinder _C_. By a manipulation of the finger and thumb at _g_, it is possible to prevent regurgitation of the sodium hydroxid into _A_, and at the same time to relieve the pressure of the nitric oxid in _A_, which would be difficult to do by means of the pinch-cock alone.

The boiling of the liquid is continued until there is no longer any increase of the volume of gas in the measuring cylinder _C_. After the end of the operation the tube _g h_ is removed from the dish _B_ and the measuring tube _C_ is closed by means of the thumb while its end is still beneath the surface of the soda-lye, and it is shaken until all traces of any hydrochloric acid, which may have escaped absorption, are removed. It is then placed in a large glass cylinder filled with water at the temperature at which the volume of gas is to be read. After being kept at this constant temperature for about half an hour the volume of the nitric oxid can be read. For this purpose the measuring cylinder _C_ is sunk into the water of the large cylinder until the level of the liquids within and without the tube is the same. The usual correction for pressure of the atmosphere, as determined by the barometer, and for the tension of the aqueous vapor at the temperature at which the reading is made, is applied. The correction is made by means of the following formula:

V′ = (V × 273 × (B − f)) ÷ ((273 + t) × 760)

In this formula V′ denotes the volume of the gas at the temperature of zero, and at 760 millimeters barometric pressure; V the volume of the gas as read at the barometric pressure observed, B, and the temperature observed, _t_, while _f_ denotes the tension of the aqueous vapor in millimeters of mercury pressure at the observed temperature _t_. The tension of the aqueous vapor at temperatures from zero to 26°, expressed in millimeters of mercury, is given in the following table:

─────┬─────────────
Temp.│ Tension in
° │mm. mercury.
─────┼─────────────
0│ 4.6
1│ 4.9
2│ 5.3
3│ 5.7
4│ 6.1
5│ 6.5
6│ 6.9
7│ 7.4
8│ 8.0
9│ 8.5
10│ 9.1
11│ 9.7
12│ 10.4
13│ 11.1
14│ 11.9
15│ 12.7
16│ 13.5
17│ 14.4
18│ 15.3
19│ 16.3
20│ 17.4
21│ 18.5
22│ 19.6
23│ 20.9
24│ 22.2
25│ 23.5
26│ 25.0
─────┴─────────────

From the gas volume reduced by the above formula the nitric acid is calculated as follows:

One cubic centimeter of nitric oxid weighs at 0° and 760 millimeters barometric pressure 1.343 milligrams.

Since two molecules of NO (molecular weight sixty) correspond to one molecule of N₂O₅ (108) we have the following equation: 60 : 108 = 1.343 : x. Whence x = 2.417 milligrams, the weight of nitric acid corresponding to one cubic centimeter of nitric oxid.

FIGURE 79. DE KONICK’S APPARATUS.
]

FIGURE 80. END OF DELIVERY-TUBE.
]

=467. DeKonick’s Modification of Schloesing’s Method.=—This modification consists in an arrangement of the gas delivery-tube, whereby the regurgitation of the water in the measuring burette into the evolution flask is prevented by a device for sealing the delivery-tube with mercury.[299] The apparatus is arranged as shown in Fig. 79. The flask in which the decomposition takes place is provided with a long neck, into which a side tube is sealed and bent upwards, carrying a small funnel attached to it by rubber tubing. The piece of rubber tubing carries a pinch-cock, by means of which the solution containing the nitrate and hydrochloric acid can be introduced into the flask. The small gas delivery-tube is arranged as shown in the figure, and carries at the end next the burette a device shown in Fig. 80. The cork represented in this device has radial notches cut in it, so as to permit of a free communication between the water in the burette and in the pneumatic trough. The open end of the burette, when the apparatus is mounted ready for use, rests on the notched surface of the cork, and the end of the delivery-tube is placed in the crystallizing dish resting on the bottom of the pneumatic trough.

The end of the delivery-tube, as indicated, has fused onto it a vertical tube open at both ends and six to seven centimeters in length, and carrying the notched cork already described. The crystallizing dish in the bottom of the pneumatic trough is filled with mercury until the point of union of the delivery-tube with the vertical end is sealed to the depth of a few millimeters. As the gas is evolved it bubbles up through the mercury into the measuring tube and the displaced water passes out through the notches in the cork. Should any back pressure supervene the mercury at once rises in the delivery-tube which is of such a length as to prevent its entrance into the flask. The operation can then be carried on with absolute safety.

To make an estimation there are placed in the flask about forty cubic centimeters of ferrous chlorid solution containing about 200 grams of iron to the liter, and also an equal volume of hydrochloric acid of one and one-tenth specific gravity. The side tube is also filled up to the funnel with the acid. The contents of the flask are boiled until all air is expelled, which can be determined by holding a test-tube filled with water over the end of the delivery-tube. The solution containing the nitrate is next placed in the funnel, the pinch-cock opened and the liquid allowed to run into the flask by means of the partial vacuum produced by stopping the boiling and allowing the mercury to rise in the delivery-tube. All the solution is washed into the flask by successive rinsings of the funnel with hydrochloric acid, being careful to allow no bubble of air to enter. The contents of the flask are again raised to the boiling-point and the nitric oxid evolved collected in the nitrometer. The solution examined should contain enough nitrate to afford from sixty to eighty cubic centimeters of gas. Without refilling the flask, from eight to nine determinations can be made by regenerating the ferrous chlorid by treatment with zinc chlorid. Care must be exercised not to add the zinc chlorid in excess, otherwise ammonia and not nitric oxid will be produced. The side tube and funnel must also be carefully freed from zinc chlorid by washing with hydrochloric acid.

=468. Schmidt’s Process.=—In the case of a water, or the aqueous extract of a soil, according to the content of nitric acid, from fifty to one hundred cubic centimeters are evaporated to thirty cubic centimeters, and the residue sucked into the generating flask of the apparatus, Fig. 81, and, with the rinsings with distilled water, evaporated again to from twenty to thirty cubic centimeters, and the flask then connected, as shown in the figure, to a Schliff measuring apparatus, B.[300] This apparatus is previously filled to _i_ with mercury, and the bulb _g_ connected with _k_ by a rubber tube.

FIGURE 81. SCHMIDT’S APPARATUS.
]

The apparatus is then filled with a twenty per cent, previously boiled and still warm, caustic soda solution until the bulb _g_ is partially filled when raised a little above the cock _h_. Then _h_ is closed and _g_ held, by an appropriate support, on about the same level with _h_. The cock at _b_ is then closed and _e_ opened. Meanwhile the ebullition in the flask is continued, and the air bubbles rising in the Schliff apparatus are removed, from time to time, by carefully opening _h_ and raising _g_. When bubbles no longer come over, the cock at _e_ is closed and at _b_ opened, and the steam issuing at _a_ is conducted through a mixture of ferrous chlorid and strong hydrochloric acid to free it, as far as possible, from air. When the contents of the flask have been evaporated to about five cubic centimeters, _b_ is closed and the lamp at once removed.

By carefully opening _b_ about ten cubic centimeters of a mixture of ferrous chlorid and hydrochloric acid are allowed to enter the flask, when _b_ is closed and the flask slowly heated until the positive pressure is restored. The pinch-cock _e_ is then opened and the contents of the flask evaporated nearly to dryness. The cock _e_ is again closed and the flame removed. Another quantity (fifteen cubic centimeters) of ferrous chlorid and hydrochloric acid solution is sucked into the flask and the process of distillation repeated, whereby the whole of the nitric oxid is collected in _h_. The nitric oxid evolved is measured in the usual way and calculated to nitric acid, one cubic centimeter of nitrogen dioxid being equal to 2.417 milligrams of nitric acid.

=469. Merits of the Ferrous Chlorid Process.=—The possibility of an accurate determination of nitrates; by decomposition with a ferrous salt in presence of an excess of acid, has been established by many years of experience and by the testimony of many analysts. The method is applicable especially where the quantity of nitrate is not too small and when organic matter is present. In the case of minute quantities of nitrate, however, the process is inapplicable and must give way to some of the colorimetric methods to be hereafter described.

In respect of the apparatus modern practice has led to the preference of that form which does not require the use of carbon dioxid for displacing the air. Steam appears to be quite as effective as carbon dioxid and is much more easily employed. That form of apparatus should be used which is the simplest in construction and has the least cubical content.

The measurement of the evolved gas is most simply made by collecting over lye in an azotometer, reading the volume, noting the reading of the barometer and thermometer and then reducing to standard conditions of pressure and temperature by the customary calculations. Where a very strong lye is used the tension of the aqueous vapor may be neglected. While every analyst should have a thorough knowledge of the ferrous chlorid method and the principles on which it is based it can not be compared in simplicity to the later methods with pure nitrates which are based on the conversion of the nitric acid into ammonia by the action of nascent hydrogen. In accuracy, moreover, it does not appear to have any marked advantage over the reduction methods.

=470. Mercury and Sulfuric Acid Method.=—This simple and accurate method of determining nitric acid in the absence of organic matter is known as the Crum-Frankland process.[301]

The method rests on the principle of converting nitric acid into nitric oxid by the action of mercury in the presence of sulfuric acid. The operation as at first described is conducted in a glass jar eight inches long by one and a half inches in diameter filled with mercury and inverted in a trough containing the same liquid. The nitrate to be examined, in a solid form, is passed into the tube together with three cubic centimeters of water and five of sulfuric acid. With occasional shaking, two hours are allowed for the disengagement of the gas, which is then measured.

=471. Warington’s Modification.=—A graduated shaking tube is employed which allows the nitrate solution and oil of vitriol to be brought to a definite volume. The nitrate solution, with rinsings, is always two cubic centimeters and enough sulfuric acid is added to increase the volume to five cubic centimeters. The sulfuric acid should give no gas when shaken with distilled water. Any gas given off in the apparatus before shaking, is not expelled but is included in the final result. The persistent froth sometimes noticed where some kinds of organic matter are present, is reduced by the addition of a few drops of hot water through the stop-cock of the apparatus. The nitric oxid is finally measured in Frankland’s modification of Regnault’s apparatus.

This method, accurate for pure nitrates, unfortunately fails in the presence of any considerable amount of organic matter.

According to Warington’s observations the presence of chlorids is no hindrance to the accurate determination of both nitric and nitrous acids by the mercury method. This simplifies the operation as carried on by Frankland who directs that any chlorin present, be removed before the determination of the nitric acid is commenced.

=472. Noyes’ Method.=—In the analyses made by Noyes for the National Board of Health, the Crum-Frankland method was employed.[302] The apparatus used was essentially that which is now known as Lunge’s nitrometer and it will be described in the next paragraph. No correction is made by Noyes for the tension of aqueous vapor in the measurement of the nitric oxid because of the moderate dilution of the sulfuric acid by the liquid holding the nitric compounds in solution. The chlorin was not removed from the dry residue of the evaporated water as its presence in moderate quantity does not interfere with the accuracy of the process. In order to obtain the amount of nitrogen in the form of nitrates, the total volume of nitric oxid must be diminished by that due to nitrites present, which must be determined in a separate analysis. The method of manipulation is given in the following paragraph.

FIGURE 82. LUNGE’S NITROMETER.
]

=473. Lunge’s Nitrometer.=—The apparatus employed by Noyes, in a somewhat more elaborate form, is known as Lunge’s nitrometer.[303] This apparatus is shown in Fig. 82. It consists of a burette, _a_, divided into one-fifth cubic centimeters. At its upper end it is expanded into a cup-shaped funnel attached by a three-way glass stop-cock. Below, the burette is joined to a plain tube, _b_, of similar size, by means of rubber tubing. The apparatus is first filled with mercury through the tube _b_, the stop-cock being so adjusted as to allow the mercury to fill the cup at the top of _a_. The cock is then turned until the mercury in the cup flows out through the side tube carrying the rubber tube and clamp. The three-way cock is closed, and the solution containing the nitrate placed in the cup. By lowering the tube _b_ and opening the cock the liquid is carefully passed into _a_, being careful to close the cock before all the liquid has passed out of the cup. By repeated rinsings with pure concentrated sulfuric acid, every particle of the nitric compound is finally introduced into _a_, together with a large excess of sulfuric acid. The total volume of the introduced liquid should not exceed ten cubic centimeters. The mixture of the mercury, nitric compound, and sulfuric acid is effected by detaching _a_ from its support, compressing the rubber connection between _a_ and _b_, placing _a_ nearly in a horizontal position, and quickly bringing it into a vertical position with vigorous shaking.

After about five minutes the reaction is complete, and the level of the liquids in the two tubes is so adjusted as to compensate for the difference in specific gravity between the acid mixture in _a_ and the mercury in _b_; in other words, the mercury column in _b_ should stand above the mercury column in _a_ one-seventh of the length of the acid mixture in _a_. This secures atmospheric pressure on the nitric oxid which has been collected in _a_. The measured volume of nitric oxid should be reduced to 0° and 760 millimeters barometric pressure. Each cubic centimeter of nitric oxid thus obtained corresponds to 1.343 milligrams NO; 2.417 milligrams N₂O₅; 4.521 milligrams KNO₃; 1.701 milligrams N₂O₃; 2.820 milligrams HNO₃; and 3.805 milligrams NaNO₃.

=474. Lunge’s Improved Apparatus.=—Lunge has lately improved his apparatus for generating and measuring gases and extended its applicability.[304] The part of it designed to measure the volume of a gas is the same in all cases. For generating the gas, the apparatus varies according to the character of the substance under examination.

The measuring apparatus is shown in Fig. 83. It is composed essentially of three tubes, conveniently mounted on a wooden holder with a box base for securing any spilled mercury. The support is not shown in the illustration.

The tubes A, B, C, are mutually connected by means of a three-way tube and rubber tubing with very thick walls to safely hold the mercury without expansion. In the middle of the measuring tube A, is found a bulb of seventy cubic centimeters capacity. Above and below the bulb the tube is divided into tenths of a cubic centimeter, and its diameter is such, _viz._, 11.3 millimeters, that each cubic centimeter occupies a length of one centimeter. The upper end of A is closed with a glass cock with two oblique perforations, by means of which communication can be established at will, either through _e_ with the apparatus for generating the gas, or through _d_ with the absorption apparatus, or the opening be completely closed.

FIGURE. 83.

LUNGE’S IMPROVED APPARATUS.
]