Acidimetry is the "measuring of acids," or determining the amount of free acid in an acidulous liquid. It does not indicate the nature of the acid, nor whether more than one acid is present. Three principal methods are available:- (1) The strength of an acid solution may be approximately determined by its boiling-point (see Boiling-points) ; (2) by its specific gravity (see Specific gravity); (3) by the amount of carbonic acid gas evolved from bicarbonate of soda by a measured quantity of the acid liquid. This last is perhaps the simplest process, and that generally in use. The apparatus requiredisshown in Fig. 1, and may be constructed by the operator. It consists of a wide-mouthed flask A, furnished with a tightly-fitting cork,th rough which pass 2 glass tubes cd. The tube c terminates in a bulb B. filled with chloride of calcium; the bent tube d reaches nearly to the bottom of the flask. A carefully weighed quantity of pure bicarbonate of soda is introduced into the flask, and covered with distilled water. This done, a small glass test tube, containing a known volume of the acid to be examined (which must not be sufficient to decompose the whole of the alkali), is carefully lowered into the flask, in the position shown.
The flask is then corked up, and accurately weighed on a delicate balance. After this, the acid in the test-tube is run out upon the alkali by causing the tube to slip into a horizontal position. By this means, a part of the alkali, equivalent to the amount of real acid in the liquid, is decomposed, the carbonic acid gas evolved escaping through the bulb-tube B; any moisture which may be carried upwards mechanically is absorbed by the chloride of calcium, whose affinity for water is well known. When the whole of the acid has been neutralized, and the disengagement of gas has ceased, air is sucked through the tube B in order to withdraw any gas remaining in the flask and tubes. When perfectly cool, the whole apparatus is re-weighed. The difference between the two weighings represents the weight of carbonic acid expelled, and from this the amount of real acid in the volume of liquid operated upon is calculated by multiplying it by the combining weight of the acid and dividing the product by 44, the combining weight of carbonic acid gas.
Thus, suppose the weight of the apparatus before the experiment be 32.355 grm., and after the experiment 31'785 grm., the loss in weight, -570 grm., represents the amount of gas evolved from the bicarbonate of soda by the acid (say sulphuric •570 x 98 acid). Then, 44 = 1.27grm. of real sulphuric acid, the amount contained in the volume of liquid taken for experiment. The same method applies to the estimation of any acid which decomposes carbonates, the combining weight of such acid being substituted for that of sulphuric acid used in the above example.
Another application of the same principle is a method devised by Fre-senius and Will. The apparatus is shown in Fig. 2, and consists of 2 small flasks, A B, A being slightly the larger.
These are furnished with tightly-fitting corks, through each of which pass the glass tubes a b c, arranged as shown. The flask B is half filled with concentrated sulphuric acid, and in the other is placed the acid to be tested, accurately measured, and, if necessary, diluted with water. A test tube is now introduced into the flask A, in the same manner as described in the previous case; this tube contains bicarbonate of soda, in quantity more than sufficient to neutralize the whole of the acid contained in the sample. After carefully weighing the apparatus, the acid and alkali are allowed to mix; carbonic acid is evolved, passes through the sulphuric acid in the other flask, being thereby thoroughly dried, and escapes through the tube a. All effervescence having ceased, air is drawn through the 2 flasks by sucking at the extremity of the tube a, to remove any traces of carbonic acid remaining behind. When quite cool, the apparatus is re-weighed, the loss representing the amount of carbonic acid disengaged frem the alkali.
The calculation to find the total quantity of acid in the volume of liquid employed is, of course, the same as in the preceding example.
Alkalimetry is the determination of the quantity of real alkali in alkaline salts and solutions. As in the case of acidimetry, the determinations may be made either by gravimetric or by volumetric analysis.
Gay-Lu8sac'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 neutralize each other. In order to insure perfect accuracy, larger quantities of the two substances, say 50 or 100 cc.t 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 neutralize 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, and shown in Fig. 1, may be 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 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 & Will's apparatus, shown in Fig. 2, 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 8, 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 6; 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.