The chief use of this salt is in calico printing and dyeing, where it takes the place of stannate of soda to some extent. Its cheapness is, however, the chief recommendation. Some of the tungsten compounds give good colours, but they are exceedingly unstable. It has also received some appli cation in rendering ladies' dresses and other fibrous materials uninflammable. J. L.
is the name given to the various processes by which the quantity of real alkali in alkaline salts and solutions is determined. As in the case of acidimetry, these determinations may either be made by gravimetrio or by volumetric analysis.
Gay-Lussac's alkalimetric method is based upon a titrated solution of carbonate of soda with a corresponding solution of sulphuric acid. Instead of the carbonate, it is preferable to use caustic soda, in order to avoid the objectionable interference caused by the presence of carbonic acid. The indicator employed is a solution of litmus, made by digesting about 10 grammes of litmus in half a litre of distilled water for a few hours ; the clear liquid is decanted and kept in a small, tightly corked wash-bottle, from which a few drops can be expelled when required. A very small quantity of dilute nitric acid may bo advantageously added to the solution, in order to produce a violet colour, which increases the sensibility of the indicator.
The standard solution of sulphuric acid contains 49 grm. of real sulphuric acid per litre, and may be made in the following way :—Thirty cc. of the pure acid, of sp. gr. 1.840, is diluted with water iu a beaker, and tho mixture is left to stand ; when perfectly cool, it is washed into a litre flask, and dilutol to the containiug-mark. Tho solution is next to be tested with a standard solution of car bonate of soda, e,outaining 53 grm. of the pure carbonate to the litre, carefully weighed and measured. Ten cc. of this latter solution is placed in a beaker with a little distilled ater and a few drops of the litmus solution, and the acid is run in carefully and slowly until the point of satu ration im reached. If more than 10 cc. be required, the solution is too weak ; if less, it is too strong, and it must either be strengthened or diluted, as the case may be, until 10 cc. of each solution exactly neutralize each other. In order to ensure perfect accuracy, larger quantities of the two substances, say 50 or 100 cc., may be employed, when the difference. if any, will be more readily detected.
If it be preferred to use caustic soda instead of carbonate., about 42 grm. is to be dissolved in water (about 800 cc.); the above test is applied, and small quantities of water are added until equal volumes exactly correspond. All these solutions are kept in tightly-stoppered bottles.
The method of procedure is as follows :—The necessary quantity of alkali being weighed or measured, as the case may be, it is diluted with distilled water in a flask, and enough litmus is added to produce a dietinet, but not too deep, blue c,olour. The acid from the burette is then run in until the contents of the flask have been changed to a bright red colour. In order to expel the carbonic acid, the flask is boiled until the blue colour reappears ; the acid solution must now be run in, a few drops at a time, with continued boiling, until, by the addition of a single drop, a distinct pink colour is produced. In order to obtain a very accurate result, it is well to run in an excess of acid, boil tho liquid well, and then add, drop by drop, the standard alkaline solution until the liquid suddenly changes from pink to violet blue. The quantity of the alkaline solution required to effect this change is subtracted from the volume of aeid originally run in, and the exact volume of standard acid required to neutmlizo the amount of alkali previously taken from analysis is thus determined at once.
It will be readily seen that the converse of this process may be .applied to the estimation of the amount of acid contained in acid liquids or mixtures (see Acidimetry).
Mohr recommends the use of oxalio acid instead of sulphuric or hydrochloric, because it is more readily weighed than a liquid, and because its solution may be kept for a much longer period than these without undergoing change in strength. The weight required is 63 grin. per litre of water.
In making determinations of the quantity of alkali c,ontained aamples of crude carbonate of potash and soda by gravimetric or weight analysis, the apparatus used in acidimetry, and shown in Fig. 2, may he employed. The weighed carbonate is dissolved in warm water in the flask A, and a quantity of acid, more than sufficient to neutralize the alkali, is placed in the short tube in the interior. The apparatus is then weighed, and the tube d closed by a plug of wax ; the flask is tilted gently, so as to cause the aeid to flow into the flask upon the carbonate. Carbonic acid is thus evolved, and the apparatus should be gently warmed until the evolution of gas completely ceases. When this is the case, the plug is removed, air is drawn through, and the whole is again weighed. The loss indicates the quantity of carbonic acid evolved, from which the amount of real carbonate contained in the sample may be calculated at once. The acidity of the solution, at the conclusion of the test, ahould be determined by adding a drop of litmus solution; if it be not acid, more add must be added, and the operation repeated.