Principles and practice of agricultural analysis. Volume 2 (of 3), Fertilizers

by Harvey Washington Wiley

294. Analysis of Wood Ashes.—The only kinds of ashes used extensively for manurial purposes are those derived from the burning of hard woods. The ash of soft woods, such as the pine, is too poor in plant foods to warrant its transportation to any great distance for manurial purposes. The methods of incineration of organic bodies for the purpose of obtaining and estimating their mineral contents will be fully discussed in the third volume of this work.

It is important in ash analysis to know whether there be enough of iron present to combine with all the phosphoric acid. For manurial purposes it will be found sufficient to determine the percentages of potash and phosphoric acid alone. For hygienic purposes it is advisable to examine the ash qualitatively and, if necessary, quantitatively for zinc, lead, copper, boric acid, and other bodies of a similar character which may be naturally present in the ash, or may have been added to the organic substance from which it was prepared for preservation or other purposes. The methods of making these special investigations will be discussed in the succeeding volume. At present will be given, however, not only the methods for detecting phosphoric acid and potash, but also for a complete analysis of an ash in so far as its usual constituents are concerned.

295. Carbon, Sand, and Silica.—The official agricultural chemists have recommended the following procedure for the determination of the unburned carbon, and the sand and silica.[250]

Five grams of the ash are treated in a beaker, covered with a watch glass with fifty cubic centimeters of hydrochloric acid of 1.115 specific gravity, and digested on the water-bath until all effervescence has ceased. The cover is then removed and the liquid evaporated to complete dryness to render the silica insoluble. The residue is moistened with two or three cubic centimeters of hydrochloric acid and taken up with about fifty cubic centimeters of water, allowed to stand on the water-bath a few minutes, filtered, and thoroughly washed. The filtrate and washings are made up to a quarter of a liter for analysis. The residue is washed from the filter into a platinum dish and boiled about five minutes with twenty cubic centimeters of a saturated solution of pure sodium carbonate; afterwards a few drops of pure sodium hydroxid solution are added and the liquid allowed to settle, and it is then decanted through a tared gooch. The residue is boiled with sodium carbonate solution and decanted as before, a second and a third time, and finally brought upon the felt and thoroughly washed, first with hot water, then with a little dilute hydrochloric acid, and finally with hot water until free of chlorids. The residue in the gooch is dried at 110° to constant weight, giving the carbon and sand. It is then incinerated and the weight of the sand determined, the difference giving the carbon. It is advisable to examine the sand with a microscope to determine if it be pure. The alkaline filtrate and washings from the carbon and sand are acidified with hydrochloric, evaporated to dryness, and the silica separated and determined in the usual way.

Instead of determining soluble silica directly from the sodium carbonate solution, as above, another portion of the ash may be treated with hydrochloric acid and evaporated to dryness as before described, filtered on an ordinary filter, washed, burned, and weighed, giving the weight of silica plus sand, from which the weight of sand is deducted to obtain soluble silica. It is inadmissible to separate the soluble silica from the residue after it has been ignited.

Instead of limiting the quantity of hydrochloric acid used for moistening the dried residue, as suggested above by the official chemists, enough should be employed to fully saturate the mass. The weight of pure ash is obtained by subtracting from the weight of the sample taken the sum of the weights of carbon, sand, and carbon dioxid.

296. Ferric Phosphate and the Alkaline Earths.—The ferric phosphate, lime, magnesia, and manganese are determined in an aliquot part of the first hydrochloric acid solution and washings obtained above. Fifty or one hundred cubic centimeters may be used, corresponding to one or two grams of the original ash. The accurately measured quantity of the solution is carefully treated with ammonia until the precipitate formed on its addition becomes permanent on shaking. Ammonium acetate and acetic acid are then added until the mixture has assumed a strongly acid reaction. The separation of the ferric phosphate precipitate is promoted by gentle warming, and it is separated by filtration without unnecessary delay. If the precipitate be not large the sample contains no manganese and alumina in weighable quantities, and if the filtrate be not red the precipitate be washed with hot water containing a little ammonium nitrate. It is then ignited and weighed as Fe₂P₂O₈ and the quantity of ferric oxid computed therefrom. If, however, the precipitate be large it is well washed as above and then dissolved in as small a quantity as possible of hydrochloric acid, and the solution is again precipitated as above by the addition of ammonia, ammonium acetate, and acetic acid. The ferric phosphate obtained by the second precipitation is treated exactly as above described.

In case, however, any weighable quantities of manganese or alumina are present it will not do to weigh the precipitate of ferric phosphate directly even after a second precipitation. Also if the filtrate at first obtained have a red color the precipitate may contain basic ferric phosphate. In this latter case it should be ignited and weighed, then dissolved in hydrochloric acid and the ferric oxid estimated in the solution and from the difference the quantity of combined phosphoric acid calculated.

The separation of the iron from the phosphoric acid may be accomplished by adding tartaric acid to the hydrochloric acid solution of the iron phosphate above obtained and then ammonium chlorid and ammonia. The mixture is placed in a flask and ammonium sulfid added. The flask is closed, placed in a warm place, and allowed to stand until the supernatant liquid is clear and of a pure yellow color without a trace of green. The iron is separated by filtration, washed, dissolved, and estimated in the usual way.

If manganese and alumina be present the iron and manganese are separated from the phosphoric acid and alumina by the processes just given for the separation of iron from phosphoric acid. In the filtrate the alumina and phosphoric acid are separated as follows: The filtrate is evaporated in a platinum dish after the addition of an excess of pure sodium carbonate until no ammonia is set free by a further addition of the carbonate. Some nitric acid is then added and the evaporation continued to dryness. The residue is fused and after cooling softened with water, washed into a small beaker, some hydrochloric acid added, warmed, and filtered. Ammonia is next added until the reaction is alkaline. If no precipitate be produced no alumina is present. In this case more nitric acid is added, the solution again evaporated, and the phosphoric acid determined by the usual methods.