CHAPTER XIV.

POTASH, SODA, AND LIME SALTS.

POTASH.

■p OTASH, according to the commercial term, but chemically carbonate ofpotash, K 0 ,C 0 2 , occurs in the inorganic and in the organic kingdoms.In the inorganic state its compounds are found in feldspar, in deposits withrock salt, the mother-lye of saline springs, and in sea-water. But the organicsources are also very important, and include the ash of plants, and the ash ofother organic substances. Potash is obtained by elutriation of the ashesof plants, the evaporation of the lye, and the calcination of the residue;this residue, or commercial carbonate of potash, is more or less rich in purecarbonate of potash. The qualities of good potash are that it should be hard,porous, of a white, geen, blue, gray, or red shade, possessing a caustic taste,and the quality of absorbing atmospheric moisture or of deliquescing, beingreadily soluble in water. The value of potash depends upon the quantity ofpotash which as carbonate of potash or as caustic potash it contains. Theproportion of carbonate of potash can be estimated by various methodsgiving more or less accurate results. A useful test is the following:—ioo grainsof well-dried potash are weighed out and dissolved in a beaker. Into another300 grains of diluted chemically pure sulphuric acid are introduced in theproportion of 100 grains of concentrated sulphuric acid to 200 grains of purewater. The sulphuric acid sets free the carbonic acid of the carbonate,which can be measured by the corresponding loss of weight. Multiplying thenumber indicating this loss of weight by 100 and dividing by 32 will give thepercentage of pure carbonate of potash contained in the sample. It will benecessary, should not the solution of potash be clear, to first filter it, to washthe residue, and to add the wash-water to the filtrate.

The alkalimetric method is grounded upon the fadt that caustic potash andcarbonate of potash are converted into sulphate upon the addition of sulphuricacid. 100 parts by weight of anhydrous potash will saturate 104 parts byweight of sulphuric acid of 66° B. It becomes then only necessary to add104 parts by weight of sulphuric acid (66° B.) to 100 c.c. of water in a burette,so that the fluid reaches to o°. The potash is weighed out in 100 parts,dissolved in water, and the clear solution coloured blue with litmus-solution.The acid is added in small quantities, in order that the mixture may notbecome much heated. The solution presently acquires a wine-red colour, andthe number of c.c. of acid solution required to effedt this change will give thepercentage of anhydrous potash. The chief impurities of commercial potashare sand, silica, and metallic salts. The sand can in great part be removedby dissolving the potash, and filtering. The silica occurs chiefly as silicate