211. Commercial Forms of Chile Saltpeter.—The Chile saltpeter of commerce may reach the farmer or analyst in the lumpy state in which it is shipped or as finely ground and ready for application to the fields. Unless the farmer is provided with means for grinding, the latter condition is much to be preferred. It permits of a more even distribution of the salt and thus encourages economy in its use. For the chemist also it is advantageous to have the finely ground material, which condition permits more easily a perfect sampling, a process which, with the unground salt, is attended with no little difficulty.
212. Percentage of Nitrogen in Chile Saltpeter.—Chemically pure sodium nitrate contains 16.49 per cent of nitrogen. The salt of commerce is never pure. It contains moisture, potash, magnesia, lime, sulfur, chlorin, iodin, silica and insoluble materials, and traces of other bodies. The value of the salt depends, therefore, not only on the market value of nitrogen at the time of sale, but also on its content of nitrogen. The nitrate of commerce varies greatly in its nitrogen content and is sold on a guaranty of its purity. The best grades range in nitrogen from fifteen to sixteen per cent. The content of nitrogen has long been estimated in the trade by determining the other constituents and counting the rest as nitrogen. This practice arose in former times when no convenient method was at hand for determining nitric nitrogen. The process is tiresome and unreliable because all errors of every kind are accumulated in the nitrogen content, but inasmuch as the method is still required by many merchants, the analyst should be acquainted with it, and it is therefore given further along. The usual methods for determining nitric nitrogen may be applied in all cases where samples of sodium nitrate are under examination, but some special processes are added for convenience.
213. Adulteration of Chile Saltpeter.—The analyst is the only protector of the farmer in guarding against the practice of adulteration of sodium nitrate aside from the honesty of the dealer. Even the honest dealer is compelled to protect himself against fraud, and therefore, the world over, commerce in this fertilizer is now conducted solely on the analyst’s certificate. Happily, therefore, adulteration is almost unknown because it is certain to be detected. Formerly, the saltpeter was adulterated with common salt, or low grade salts from the potash mines; but it is an extremely rare thing now to find any impurities in the salts other than those naturally present.
In every case the analyst may grow suspicious when he finds the content of nitrogen in a sample to fall below thirteen per cent. It must not be forgotten, however, that some potassium nitrate may be present in the sample, and since that salt contains only 13.87 per cent of nitrogen its presence would tend to lower the value of the fertilizer; but although the potash itself is a fertilizer of value it is not worth more than one-third as much as nitrogen. In all cases of suspected adulteration, it is advisable to make a complete analysis. The results of this work will, as a rule, lead the analyst to a correct judgment.
Figure. 15.
Halle Nitric Acid Apparatus.
214. The Halle Zinc-Iron Method.—For determining the nitrogen in Chile saltpeter the reduction method is conducted at the Halle Station as follows:[179] Ten grams of the nitrate are dissolved in one liter and fifty cubic centimeters of the solution corresponding to half a gram of the sample, taken for each determination. The apparatus employed is shown in [Fig. 15]. A mixture of five grams of zinc dust and an equal weight of iron filings is employed as the source of hydrogen. The reduction takes place in an alkaline medium secured by adding to the other materials mentioned, eighty cubic centimeters of soda-lye of 1.30 specific gravity. The respective quantities of iron and zinc may be measured instead of weighed, as exact proportions are not required. After the addition of all the materials the flask is allowed to stand for an hour at room temperature. The distillation is then commenced and continued until at least 100 cubic centimeters of distillate have been collected. The receiving flasks are ordinary erlenmeyers, each of which contains twenty cubic centimeters of set sulfuric acid, as in the usual kjeldahl process. The flasks are sealed with a few drops of water by the device [shown in the figure]. After the end of the operation the water in each one is washed back into its proper flask with freshly boiled water. During the vigorous evolution of hydrogen, at the beginning of the operation, some kind of a safety arrangement is necessary to prevent the particles of soda-lye being carried over by the bubbles of that gas. The siphon bulb shown in the figure is found effective for this purpose. In this operation better results are obtained by condensing the escaping steam, and for this reason the block tin tubes are conducted through a tank supplied with a current of cold water. The ends of the tubes should not dip below the surface of the liquid in the receivers. When the condensed liquid collects in considerable quantities in the safety tube the lamp should be extinguished under the flask, which permits the return of the liquid to the flask by means of the siphon. This should be done two or three times during the progress of the distillation to prevent a too high concentration of the soda-lye, thus endangering the flask. The excess of the acid in the receiver is determined by titration, as in the regular kjeldahl method. Blank determinations should be made, from time to time, and corrections made in harmony therewith.
215. Method of the French Sugar Chemists.—The nitrogen in Chile saltpeter is estimated by the French chemists according to the method of Schlösing, described in the first volume, page 500. In order to avoid the trouble of calculating the results from the volume of nitric oxid obtained, a determination is first made with a pure salt, sodium or potassium nitrate. The volume of gas obtained is read directly without correction and taken for direct comparison. The comparison is made as follows:
The solutions of the pure salts and of the sample to be analyzed are made of such a strength as to contain sixty-six grams of sodium nitrate, or eighty grams of potassium nitrate, in a liter. Five cubic centimeters of such a solution will yield a little less than 100 cubic centimeters of nitric oxid under usual conditions. Let the volume of gas obtained with the pure salt be v; and that with the sample be v′. The calculation is then made from the equation: