consumed in neutralising 100 gr., do to the per-centage sought. Thus:—50 gr. of hydrochloric acid take 45 acidimeter divisions to effect neutralisation, what is its real strength?—The equiv. of hydrochloric acid is 36·5: therefore—
100 : 36·5 :: 45 : 16·425%
and, since only 50 gr. (instead of 100 gr.) were examined—
16·425 × 2 = 32·85%
Some operators prefer employing 100 gr. instead of the equivalent weights of the given tests in making their test-solutions, in which case each gr. or 1000th part represents 1⁄10th, and each acidimeter degree 1 gr. of the alkali or carbonate employed; when a similar proportion will obtain to that first above given.
In technical analysis it is more convenient if the number of acidimeter divisions of the ‘test-liquid’ consumed express the per-centage strength of the acid, without further calculation. For this purpose the number of grains of the acid taken for the assay should correspond to the equivalent number of such acid (see Table I, below); or to some convenient aliquot part of it, as the 1⁄2, 1⁄4, 1⁄5, or 1⁄10th; the per-centage answer, in the last case, being doubled, quadrupled, &c., according to the aliquot part taken. The reason of this is obvious.
For the test-solutions, ammonia, and the dry and crystallised carbonates and bicarbonates of potash and soda, are used, and are made by dissolving in water their constituents except ammonia, of which 1000 grains, or one litre, of solution of specific gravity 0·992 contains exactly one equivalent.
53 grains (or grammes) of pure anhydrous carbonate of soda, prepared by gradually heating to redness the crystallised salt, constitute one equivalent (half the atomic weight), and 69 grains (or grammes) of pure dry carbonate of potash. Of the crystallised salt 143 grains of carbonate of soda will be required, and 84 grains (grammes) of the crystallised bicarbonate of soda, and 100 of the crystallised bicarbonate of potash. Occasionally solutions containing in one thousand parts, 50 of pure carbonate of lime or chalk, or 28 of pure caustic lime, are used.
Besides these, a process known as Kiefer’s is practised, and an ammoniacal solution of oxide of copper is employed as the ‘test-liquor,’ and the ‘point of neutralisation’ is known by the turbidity observed as soon as the free acid present is completely saturated.
The normal solution or test-liquor is prepared by adding to an aqueous solution of sulphate of copper, pure ammonia water, until the precipitate, which at first forms, is just redissolved, carefully avoiding excess. Or better, by adding a rather strong solution of sulphate of copper, to a quantity of a rather strong solution of ammonia containing exactly 17 gr., or one equiv. of pure ammonia, as long as the precipitate which forms is redissolved on agitation; the resulting liquid being afterwards diluted with pure distilled water, until it accurately measures 1000 water-grains, or fills 100 divisions of an acidimeter, at 60° Fahr. In either case, the strength of the resulting ‘test-solution’ must be carefully determined by means of standard sulphuric acid, and adjusted, if necessary.