This section is from "Scientific American Supplement Volumes 275, 286, 288, 299, 303, 312, 315, 324, 344 and 358". Also available from Amazon: Scientific American Reference Book.
By Dr. C. Krauch.
Differences obtained in the estimation of nitrogen in the above substances are frequently the source of much annoyance. The cause of these discrepancies is chiefly due to the lack of uniformity in the material, and from its not being in a sufficiently fine state during the combustion. The hair which is found in commerce for the manufacture of fertilizers, is generally mixed with sand and dust. Wool dust often contains old buttons, pieces of wood, shoe pegs, and all sorts of things. The flesh fertilizers are composed of light particles of flesh mixed with the heavier bone dust.
Even after taking all possible precautions to finely comminute these substances by mechanical means, still only imperfect results are obtained, for the impurities, that is to say, the sand, can never be so intimately mixed with the lighter particles that a sample of 0.5 to 0.8 gramme, such as is used in the determination of nitrogen, will correspond to the correct average contents. In substances such as dried blood, pulverization is very tedious. A very good method of overcoming these difficulties, and of obtaining from the most mixed substances a perfectly homogeneous mass, is that recommended by Grandeau1 of decomposing with sulphuric acid - a method which as yet does not seem to be generally known. From a large quantity of the substance to be examined, the coarse stones, etc., are removed by picking or sifting, and the prepared substance, or in cases where the impurities cannot be separated, the original substance, is treated with sulphuric acid; after it is decomposed, the acid is neutralized with calcium carbonate, and the nitrogen is determined in this mass.
In order to operate rapidly, it is best to use as little sulphuric acid as possible. If too much sulphuric acid is used, necessarily a large amount of calcium carbonate is essential to get it into proper condition for pulverizing. Under such circumstances the percentage of nitrogen becomes very low, and a slight error will become correspondingly high.
20 c.c. of concentrated sulphuric acid and 10 c.c. are sufficient for 30 to 40 grammes of material. After the substance and liquid have been thoroughly stirred in a porcelain dish, they are warmed on a water bath and continually stirred until the mass forms a homogeneous liquid. The sirupy liquid thus obtained is then mixed with 80 to 100 grammes of pulverized calcium carbonate (calcspar), dried for fifteen minutes at 40 to 60° C., and after standing for one to two hours the dish and its contents are weighed. From the total weight the weight of the dish is subtracted, which gives the weight of the calcium sulphate and the calcium carbonate, and the known weight of the wool dust, etc. This material is then intimately ground, and 2 to 3 grammes of it are taken for the determination of the nitrogen, which is then calculated for the original substance.
Although the given quantities of water and sulphuric acid hardly appear sufficient for such a large quantity of hair or wool, still in the course of a few minutes to a quarter of an hour, after continual stirring, there is obtained a liquid which, after the addition of the calcium carbonate, is readily converted into a pulverized mass. Frequently a smaller quantity of sulphuric acid will suffice, especially if the material is moist. The chief merit of this process is that in a short time a large quantity of material, having a uniform character, is obtained. Its use is, therefore, recommended for general employment.
When the coarser stones, etc., are weighed, and the purified portion decomposed, absolutely correct results are obtained, and in this way the awkward discrepancies from different analysts may be avoided. - Chemiker Zeitung, v. 7, p. 703.
Handbook d. Agrict. Chem. Analyst., p. 18.