ALKALIM′ETRY. Syn. Alkalime′tria, L.; Alcalimétrie, Fr. In chemistry, the estimation of the strength of the commercial alkalies; the art or process of determining the quantity or proportion of pure caustic alkali, or of its carbonate, in any given sample or simple solution. It is the reverse of ‘acidimetry,’ and it should be understood that it does not apply to alkalies occurring under any other form or condition than those just mentioned. Alkalimetric assays are now also frequently and conveniently extended to the estimation of the alkaline earths and their carbonates, as hereafter noticed.
Alkalimetrical processes. These, like those
of ‘acidimetry,’ are for the most part founded on—the capacity of the bases to saturate acids—the estimation of the quantity of dry carbonic acid liberated from a given weight of an alkaline carbonate under the influence of a stronger acid; and, in the case of the pure alkalies, the sp. gr. of their solutions. From any one of these results the exact amount of alkali, or of alkaline carbonate, present in a sample, is easily found or calculated. These processes are, indeed, precisely similar to those described under Acidimetry; but here the unknown quantity sought is the alkali, instead of the acid.
Assay. The SAMPLE is drawn from as near the centre of the cask containing the alkali as possible, and at once placed in a wide-mouthed bottle, which is then closely corked up and numbered. Before proceeding to the assay, the contents of the bottle are thrown on a piece of dry paper, the lumps crushed small, and the whole reduced to coarse powder as rapidly as possible. The number of grains required for the trial are then at once weighed, placed in a phial or small glass tube, and agitated with about 1⁄2 oz. of hot water. After a short time allowed for repose, the clear liquid is poured off into a beaker-glass or other vessel in which the trial is to be made. This process is repeated with a second and a third quantity of water, or until nothing soluble remains, shown by the last washings not affecting the colour of turmeric paper. The greatest care must here be taken not to waste the smallest portion of the liquid, which would render the results inaccurate.
To the solution in the beaker-glass a little solution of litmus is added, unless the acid is tinted with it when it is unnecessary. The solution is now heated until near its boiling point, and a piece of white paper or porcelain put behind it, to better show up the changes of colour. The alkaline solution is now treated with the standard test-acid, which is poured carefully from an alkalimeter or Mohr’s burette, until the solution, after turning a purple red, suddenly assumes a pink colour. Neutralisation being thus effected, the operator allows the sides of the alkalimeter or burette to drain, and then either ‘reads off’ the number of divisions which have been consumed, or (if using the test-acid by weight) determines the quantity by again weighing the alkalimeter. The common practice is to allow two drops (= 1⁄5th of an alkalimetrical division by VOLUME, or 2 gr. by WEIGHT) for over-saturation, which is, therefore, deducted from the ‘observed quantity’ of the test-liquor employed.
In testing solutions of the PURE or CAUSTIC ALKALIES, the colour, on neutralisation, suddenly changes from blue to pink or red, without any intermediate vinous or purple colour being produced.
The quantity of test-acid used gives the absolute or per-centage composition of the sample examined, according to the constitution of the test-acid used.
Standard Acids. The various test-acids in use as described below, each being used by different operators as they think best.
The most convenient test-acid, or normal solution, both for commercial and chemical assays, is perhaps dilute sulphuric acid, which, when intended to be used VOLUMETRICALLY, has the sp. gr. 1·032 at 60° Fahr., and contains in 100 alkalimetrical divisions 1000 water-grains measure, or 1 litre, exactly 49 gr. (or grammes) of sulphuric acid; and when intended to be used GRAVIMETRICALLY, or by weight, has the sp. gr. 1·033, and contains in 1000 gr. (or grammes) weight exactly 49 gr. (grammes) of sulphuric acid; and, in both cases, consequently corresponds to 1 equiv. of every other base. These dilute acids are easily prepared by mixing 1 part of the concentrated acid with 11 or 12 parts of distilled water; the precise quantity depending on the strength of the acid employed, and must be so arranged that 1000 grains shall exactly neutralise 1000 grains of water containing 53 grains of pure anhydrous sodium carbonate.
This acid (as well as all those hereafter mentioned) may be kept faintly tinged with litmus, which is often more convenient than tinging the alkaline solution at the time of making the assay.