The amount of sulphides may be estimated by titrating the hot soda solution, to which ammonia has been added, with an ammoniacal silver nitrate solution, 1 c.c. of which corresponds to 0.005 gramme Na2S. As the titration proceeds, the precipitate is filtered off, and the addition of ammoniacal silver solution to the filtrate continued until a drop produces only a slight opacity. The presence of chloride, sulphate, hydrate, or carbonate does not interfere with the accuracy of this method. The ammoniacal silver nitrate solution is prepared by dissolving 13.345 grammes of pure silver in pure nitric acid, adding 250 c.c. liquor ammoniæ fortis, and diluting to 1 litre.
Carbonate of Potash (Pearl Ash).—The total or available alkali may be estimated by taking 6.9 grammes of the sample, and titrating with N/1 acid directly, or adding 100 c.c. N/1 sulphuric acid, boiling for a few minutes, and titrating the excess of acid with N/1 caustic soda solution, using litmus as indicator. In this case each c.c. N/1 acid required, is equivalent, in the absence of Na2CO3, to 1 per cent. K2CO3.
Carbonate of potash may be further examined for the following:—
Moisture.—From 2-3 grammes are heated for thirty minutes in a crucible over a gas flame, and weighed when cold, the loss in weight representing the moisture.
Insoluble residue, remaining after solution in water, filtering and well washing.
Potassium may be determined by precipitation as potassium platino-chloride thus:—Dissolve 0.5 gramme in a small quantity (say 10 c.c.) of water, and carefully acidulate with hydrochloric acid, evaporate the resultant liquor to dryness in a tared platinum basin, and heat the residue gradually to dull redness. Cool in a desicator, weigh, and express the result as "mixed chlorides," i.e. chlorides of soda and potash. To the mixed chlorides add 10 c.c. water, and platinic chloride in excess (the quantity may be three times the amount of the mixed chlorides) and evaporate nearly to dryness; add 15 c.c. alcohol and allow to stand three hours covered with a watch-glass, giving the dish a gentle rotatory movement occasionally. The clear liquid is decanted through a tared filter, and the precipitate well washed with alcohol by decantation, and finally transferred to the filter, dried and weighed. From the weight of potassium platino-chloride, K2PtCl6, is calculated the amount of potassium oxide K2O by the use of the factor 94/488.2 or 0.19254.
Chlorides, determined with N/10 silver nitrate solution, and calculated to KCl.
Sulphates, estimated as barium sulphate, and calculated to K2SO4.
Sodium Carbonate, found by deducting the K2CO3 corresponding to the actual potassium as determined above, from the total alkali.
Iron, precipitated with excess of ammonia, filtered, ignited, and weighed as Fe2O3.