Acidimetry. The estimation of the quantity of an acid contained in any given sample. The methods used are founded chiefly on the capacity of acids to saturate or neutralize alkaline bases; and, in some of the liquid acids, on specific gravity. The accuracy of the tests, when saturation is resorted to, depends greatly on the exact point of neutralization, as already remarked under the head of Acetimetry. The proper point is arrived at when the liquid, after being slightly heated, ceases to redden litmus, or does not alter the color of turmeric paper (see Test Papers) ; if it turns the latter brown, too much test-liquid has been added, and the operation becomes useless. A good method is to tint either the acid sample or the test-liquid with a few drops of litmus (sec No. 71), when the reddish shade will gradually deepen to purple as the point of saturation is approached, and the blue color be restored as soon as that point is reached.

79. To test the strength of an Acid by Saturation

79. To test the strength of an Acid by Saturation. Place in a test tube 100 grains of the acid to bo examined; if the acid be liquid, dilute it - if solid, dissolve it - in 6 or 8 times its weight of distilled water. Then exactly neutralize it with an alkali added drop by drop. The known quantity of alkali consumed for this purpose represents an equivalent quantity of the actual acid contained in the test tube. The common practice is to dissolve 1 equivalent (see No. 80 (Table of Equivalents of Alkalis)) of an alkaline test in water, and to make up the solution to 1000 grains (100 acidimetric divisions). The equivalent value of the test-liquid is then 100; hence, the quantity of the sample tested will bear the same proportion to the equivalent number (see No. 81 (Table of Equivalents of Acids)) of the acid under examination, that the acidimetric divisions of the test-liquid consumed, bear to the percentage of acid sought. For example : Suppose 100 grains of a sample of sulphuric acid require 60 acidimetric divisions (600 grains) of the test-liquid to neutralize them; what is the percentage of the acid ? The equivalent of dry sulphuric acid is 40 (see No. 81); therefore by the rule of proportion, since 100 : 40 : : 60 : 24, the sample contains 24 per cent, of dry sulphuric acid.

In this method the choice of the re-agent must depend on the operator. Some prefer the ammonia test (see No. 76 (Ure's Test of the strength of Acetic Acid)), which is very convenient and easily applied; others give a preference to bicarbonates or carbonates of po-tassa or soda. Whichever be adopted, it must be perfectly pure. A test solution, once carefully prepared of the proper strength, may bo kept unharmed for any length of time in a stoppered bottle, and will be always ready for application.

80. Table of Equivalents of Alkalis

80.    Table of Equivalents of Alkalis.

Grains. Pure ammonia................. 17

Dry carbonate of soda..........53

Crystallized carbonate of soda.. .143 Crystallized bicarbonate of soda.. 84

Dry carbonate of potassa........69

Crystallized carbonate of potassa. 87 Crystallized bicarbonate of potassa 100

Pure or caustic soda............ 31

Pure or caustic potash.......... 47

Sesquicarbonate of soda........ 85

Neutral carbonate of ammonia.. 431/2

Sesquicarbonate of ammonia........59

Bicarbonate of ammonia........79

1000 grain measures of pure water of ammonia of specific gravity .992, contain 17 grains or 1 equivalent of pure gaseous ammonia.

It is understood that all crystals must be perfectly free from attached water, but not in the least effloresced.