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alkalimetry


Значение термина alkalimetry в knolik


alkalimetry - alkalimetry
alkalimetry - Alkalimetry is the determination of the quantity of real alkali in alkaline salts and solutions. As in the case of acidimetry, p. 6, the determinations may be made either by gravimetric or by volumetric analysis.

Gay-Lussac's 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 grm. of litmus in £ 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 be 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: 30 cc. of the pure acid, 1'840 sp. gr., is diluted with water in a beaker, and the mixture is left to stand; when perfectly cool, it is washed into a litre flask, and diluted to the containing-mark. The solution is next tested with a standard solution of carbonate of soda, containing 53 grm. of pure carbonate to the litre, carefully weighed and measured; 10 cc. of this latter solution is placed in a beaker with a little distilled water and a few drops of the litmus solution, and the acid is run in carefully and slowly until the point of saturation is 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 neutralise each other. In order to insure perfect accuracy, large quantities of the two substances, say 50 or 100 cc., may be employed, when the difference, if any, will be more readily detected. If caustic soda be used 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 distinct, but not too deep, blue colour. 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 the 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 acid originally run in, and the exact volume of standard acid required to neutralise the amount of alkali previously taken from analysis is thus determined at once.

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 oxalic 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 grm. per litre of water.

In making determinations of the quantity of alkali contained in samples of crude carbonate of potash and soda by gravimetric [weight] analysis, the apparatus used in acidimetry, maybe employed. The weighed carbonate is dissolved in warm water in the flask A, and a quantity of acid more than sufficient to neutralise 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 acid 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, should be determined by adding a drop of litmus solution; if it be not acid, more acid must be added and the operation repeated.

Fresenius and Will's apparatus, shown in Fig., may also be employed in making alkalimetrical estimations the same as in acidimetry. The alkali to be tested is carefully weighed, and dissolved in water in the flask A; concentrated sulphuric acid is placed in the flask B, and the apparatus is accurately weighed. After closing the end of the tube c, suction is applied to the tube a, so as to draw over a small quantity of air from A into B through the tube b; on withdrawing the lips, the pressure of air forces a little of the acid over into A, by which means the alkali is decomposed. This is continued until the evolution of carbonic acid ceases, when heat is applied gently for a few moments. Air is then drawn through, and the apparatus is cooled and weighed. The loss in weight gives the amount of carbonic acid evolved, as in the previous case.

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