On the use of partly neutralised mixtures of acids as hydrion regulators

Abstract

When mixtures of standard (H˙) are prepared, as is usual, from partly neutralised acids, it is well known that the most suitable degree of neutralisation is that at which (H˙) = k , the ionisation constant of the acid. This part of the neutralisation curve (ordinates,—log (H˙) or p H˙ ; abscissæ, equivalents or number of c. c. of alkali) is approximately a straight line inclined at a relatively small angle to the axis of abscissæ. In the case of a dibasic acid there are two such regions corresponding to the first and second constants k 1 and k 2 . Each of these flat parts of the curves (at which the differential increases, reaches a maximum, and then decreases) lie on each side of the points at which (H˙) = √ k 1 k 2 . These inflections on the curves are less suitable, or may be quite unsuitable, for hydrion regulators if the ratio between k 1 and k 2 is about 10 3 or a still higher power. If the constants do not bear a high ratio to one another, as in the case of citric acid ( k 1 = 8·2 × 10 -4 , k 2 = 3·2 × 10 -5 , k 3 = 7 × 10 -7 ), the curve of neutralisation will have no marked inflection throughout its course. The range of regulators obtained from citric acid, with the addition of HCl on the acid side, is unusually great (form p H˙ = 1 to p H˙ = 6). But other regulator mixtures—phosphates, borates, etc.—have marked inflections, and consequently several must be used if a wide range of (H˙) is required. Also there is a small interval, between p H˙ = about 7·5 and 8·3, which cannot be reproduced from any of the standard mixtures with as high an accuracy as can be obtained at all other values of (H·). This interval happens to be that which has a particular importance in the measurement of the acidity of sea-water. These defects in existing standards would appear to be easily remediable by using a selected mixture of acids. For, if in the case of a polybasic acid k 1 bears too high a ratio to k 2 , it should only be necessary to introduce another acid for which k 3 = √ k 1 k 2 in order to flatten out the inflection and make this part of the curve also available. A number of acids could be chosen of which the constants differ by only 1 power of 10— e. g ., salicylic., k = 1 × 10 -3 ; lactic, k = 1·35 × 10 -4 ; acetic, k = 1·8 × 10 -5 . This mixture should give a neutralisation curve which should be almost linear over a limited range.