Acetimetry. The art of deter-mining the strength of acetic acid and vinegar. Several methods arc employed for the purpose, based on—the quantity of acid required for saturation;- the specific gravity alter the liquid has been neutralized with hydrate of lime;- and the simple specific gravity. In all these methods, account should be taken of any mineral acid which may have been added, as is common with vinegars, to impart artificial strength.

70. To find the Comparative Weights of Dry and Glacial Acetic Acid

70.    To find the Comparative Weights of Dry and Glacial Acetic Acid. As both dry and glacial (or hydrated) acetic acid are referred to in many places, in speaking of strengths, it may be convenient to know that 51 parts of dry acetic acid are equal to 60 parts of glacial. (See No. 81 (Table of Equivalents of Acids).) Hence the weight of glacial acid multiplied by .8512, gives the weight of dry acid; and the weight of dry acid, multiplied by 1.1748 gives a very close approximation to the weight of glacial acid.

71. Precautions in Testing Acids

71.    Precautions in Testing Acids. It is essential to success, in testing acetic or other acids by saturation, to hit the exact point of neutralization. It will be found greatly to simplify matters to tint with litmus (see No. 78) either the sample under examination, or the test liquid; but when litmus is used, it is advisable to apply a gentle heat to the test tube when saturation appears nearly reached; the heat will expel from the liquor the free carbonic acid, which itself has the property of reddening litmus. A glass or wooden rod should be used for stirring, and the test liquid added drop by drop.

72. To find the strength of Acetic Acid by its Saturating Power

72.    To find the strength of Acetic Acid by its Saturating Power. Dissolve 1961/2 grains pure crystallized bicarbonate of potassa in a little water; add to the solution sufficient water to make up exactly 1000 minims, or the 100 divisions of an acidimeter, a graduated glass tube of 100 divisions, each division representing 10 minims. (Seeillustra-tions, No. 82.) A solution is thus formed, which, when added by degrees to 100 minims of the acetic acid or vinegar under examination, until the latter is exactly saturated, indicates the exact amount of acid present in the sample. Each minim of the alkaline solution thus employed represents 1 per cent, of dry acetic acid. The test liquid must be added a drop at a time to avoid the risk of loss by excessive effervescence.

73. To find the strength of strong Acetic Acid

73.    To find the strength of strong Acetic Acid. If strong acetic acid be under inspection, it will be found convenient, previously to testing it, to dilute it with from 2 to 8 times its weight of distilled water, according to its degree of concentration. Dilute acid and vinegar require no further dilution.

Instead of 1961/2 grains crystallized bicarbonate of potassa, may be used either 135 grains dry (see No. 12 (Desiccation)) carbonate of potassa, 281 grains crystallized carbonate of soda, or 104 grains dry carbonate of soda. (See No. 80 (Table of Equivalents of Alkalis).)

By using 981/4 grains (half the quantity) of the bicarbonate of potassa, we obtain a still more delicate test liquid; as each minim used for saturating a sample of acid will represent only 1/2 of 1 per cent, of dry acid.