It will at once be seen that every alkalimeter division of the first of the above acids, and every 10 gr. of the second, represent the 1⁄100th part, or 1% of alkali whenever the equivalent weight[18] of the latter is taken for the assay. Every 1-10th part of an alkalimeter-division (or every drop), and every grain weight (when a Schüster’s alkalimeter is employed) then respectively represents the 1⁄10 of 1%; and the result sought is obtained without the necessity of any calculation.
[18] See Table II, at the end of this article.
This is obvious—for if the equivalent of a pure alkali or of its carbonate (i. e. one of 100%) requires an equiv. (100 alkalimeter-divisions, or 1000 gr.) of test-acid to saturate it, an alkali or alkaline carbonate of 75%, 50%, or 25%, will respectively require only 75, 50, or 25 divisions, or 750, 500, or 250 gr.; and so of other strengths in proportion. The only precaution necessary is always to take the standard weight for the assay answering to the equiv. of the denomination of the per-centage result sought. Thus, in testing a carbonate of potash, we may either wish to determine its per-centage richness in ‘dry carbonate,’ or in ‘pure potassa,’ the latter being usually the case. To obtain the first, we must take 69 gr. for the assay; and to obtain the second, 47 gr. With CAUSTIC ALKALIES, or mixtures containing them, the weight, in grains, taken for the assay, must always correspond to the equiv. of the pure base. See Table II, at the end of this article.
In commercial assays, when 100 gr. (or some
aliquot part thereof) are taken for trial, the per-centage result is obtained from the number of alkalimeter-divisions, or the number of grains, of the test-acid consumed, by the common Rule of Proportion. Thus:—A crude sample of potash having taken 90 alkalimeter-divisions of test-acid to neutralise it, would contain—
100 : 47 :: 90 : 42·30%
or nearly 421⁄3 per cent. of pure potassa. If only 50, 25, or 20 gr. are tested, the result must, of course, be double, quadruple, &c., as the case may be. Or the third term of the proportion may be multiplied by the denominator of the fraction representing the aliquot part. This, in the case of 50 gr. (repeating the above example), would be—
10 : 47 :: 45 × 2 : 42·30%
as before; but even these easy calculations may be simplified, as is shown below.
One of the advantages, and not the least, attending the use of test-acids corresponding to equivalents, is, that by means of the simple Rule of Three, the per-centage quantity of alkali may be found whether 100 or any other number of grains have been submitted to trial. For—The weight of the sample tested (in grains) bears the same relation to the equivalent weight of the alkali under examination, that the number of alkalimeter-divisions or of the grains of test-acid consumed do to the per-centage of alkali sought. Thus, with a sample of 33 gr. of pearlash taking 35 alkalimeter-divisions or 350 grains (every 10 gr. being = 1%) of test-acid for neutralisation, this would be—