Fig. 86. Fig. 87.
Mohr's Alkalimeter.—This useful instrument (Fig. 86) and the method of using it is thus described by Mohr:—"I have succeeded in substituting for expensive glass stop-cocks an arrangement which may be constructed by any person with ease, which remains absolutely air and water-tight for an indefinite period, which may be opened and regulated at will by the pressure of the fingers, and which costs almost nothing. It consists of a small piece of vulcanized indiarubber tube, which is closed by a clamp of brass wire (Fig. 87). The ends of this clamp, which I call a pressure-cock, are bent laterally at right angles in opposite directions and furnished with knobs, so that when both ends are pressed the clamp is opened, and a single drop or a continuous current of liquid may be allowed to escape at pleasure. The measuring-tube is a straight glass cylinder, as uniform as possible, graduated to 0·2 or 0·1 cubic centimètres, and somewhat contracted at its lower end, so as to fit into the indiarubber tube. A small piece of glass tube inserted below the pressure-cock forms the spout. The pressure-cock has the advantage of not leaking, for it closes itself when the pressure of the fingers is removed. The measure, furnished with the pressure-cock, is fastened upon an appropriate stand, which can be placed at any required height. When used, it is filled above the zero point with test liquor, the cock opened for an instant, so as to let the air escape from the spout, and the level of the solution is then adjusted. This is done by bringing the eye level with the zero point, and applying a gentle pressure to the cock until the liquid has sunk so low that the inferior curve of the liquid touches the graduation like the circle of a tangent; the cock is then closed, and at the same moment the liquid remains at zero, and continues to do so for weeks if evaporation is prevented. The test-measure being normally filled, the experiment may be commenced; this is done sitting, while the filling of the measure is done standing.
"The weighed sample of alkali is first placed in a beaker-glass, and the test-liquor is allowed to flow into it by gently pressing the cock. Both hands are set at liberty, for when the pressure-cock is released it closes of itself. The volumetric[35] operation may be interrupted at pleasure, in order to heat the liquid, shake it, or do whatever else may be required. The quantity of liquid used may be read off at any moment, and in repeating an experiment, the limit of the quantity used before may be approached so near that the further addition of liquid may be made drop by drop." The test-acid to be used volumetrically—that is, with the alkalimeter, has a specific gravity of 1·032 at 60° F., and 1,000 grains by measure contain exactly 40 grains of real or anhydrous (that is, without water) sulphuric acid.
The chemical principles involved in the process of alkali-testing may be thus briefly stated:—According to the laws of chemical combination defined by the atomic theory of Dalton, all substances combine in definite proportions or "equivalents"; thus, 1 part by weight of hydrogen combines with 8 parts by weight of oxygen to form water. The equivalent number of hydrogen, therefore, is 1, and of oxygen 8, and that of water 9. Again, 3 equivalents of oxygen combine with 1 equivalent of sulphur (16) to form sulphuric acid; thus, sulphur 16, oxygen 24, equals anhydrous sulphuric acid 40; therefore 40 is the equivalent or combining number of this acid, and it cannot be made to unite with alkalies or other bases in any other proportion. For example, 40 grains by weight of pure sulphuric acid will neutralise exactly 53 grains of dried carbonate of soda, 31 grains of pure anhydrous soda, or 40 grains of hydrate of soda (caustic soda). This being so, it is only necessary to have exactly 40 grains of real sulphuric acid in 1,000 grains of water to form a test-acid, which, when employed to neutralise an alkaline solution, will show, by the proportion of dilute acid used to saturate the alkali, the absolute percentage present in the sample.
Preparation of the Test-Acid or Standard Solution.—As there is some trouble involved in the preparation of the test-liquor, it is advisable to prepare a sufficient quantity at a time to last for many operations. It may be readily made by mixing 1 part of concentrated sulphuric acid with 11 or 12 parts of distilled water, the mixture being made in what is termed a "Winchester" bottle, which holds rather more than half a gallon, and is provided with a glass stopper. The acid solution must be adjusted or brought to the proper strength after it has cooled down to 60° F.; and it should be faintly tinged with litmus, which will give it a pinkish hue. The acid, to be of the proper strength, should exactly neutralise 53 grains of pure carbonate of soda, previously calcined at a red heat, or 31 grains of pure anhydrous soda. To prepare the anhydrous carbonate of soda, a few crystals of carbonate of soda are placed in a Berlin porcelain crucible, and this must be heated over a spirit-lamp or Bunsen burner. When all the water of crystallisation has become expelled, the calcination is continued until the mass is at a bright red heat, when the vessel may be allowed to cool. 53 grains of the calcined carbonate are now to be carefully weighed, and next dissolved in a glass beaker, in about 2 ounces of distilled water. The alkalimeter is now to be charged with the test-acid to the level of zero, and (if Mohr's burette be used) the beaker containing the alkaline solution is to be placed upon the stand immediately beneath the exit-tube. Now press the knobs of the pressure-cock, and allow a portion of the liquor to flow into the beaker. When the effervescence which immediately sets up subsides, make further additions of the test-liquor from time to time, until the effervescence becomes sluggish, at which period the acid must be added with greater caution. When the solution approaches saturation it acquires a purplish tint (due to the litmus with which the acid is tinged), which it retains until the point of saturation is reached, when it suddenly changes to a pink colour. After each addition of the acid the solution should be stirred with a thin and clean glass rod; and before the final change from purple to pink, the end of the glass rod should be applied to a strip of blue litmus paper, when, if the moistened spot touched assumes a red colour, the saturation is complete; if, on the contrary, the paper is unchanged, or has a violet or reddish hue, add the test-liquor, one or two drops at a time, with continued stirring, until a drop of the solution applied with a glass rod reddens litmus paper, when the saturation is finished. If any test-liquor remain in the burette, this indicates that there is excess of acid in the test-liquor; consequently more distilled water must be added to the bulk, the burette emptied and refilled with the reduced liquor, and another 53 grains of anhydrous carbonate of soda treated as before, until 1,000 grains of the acid liquor exactly neutralise the solution. Should the whole contents of the burette in the first trial be used before saturation is complete, a little more sulphuric acid must be put into the Winchester or test-acid bottle, and a 53-grain solution of carbonate of soda treated as before. A very little practice will enable the operator to adjust his test-liquor with perfect accuracy; and, to prevent mistakes, the bottle should be labelled "Test-acid," and always be kept closed by its stopper.
Sampling Alkalies.—Soda-ash of commerce is usually packed in wooden casks, and in order to obtain a fair average sample from a large number of these casks, which may represent one consignment, it is important to take small samples, as near the centre of each cask as possible, from as many of the casks as time will permit. Each sample, as drawn from the cask, should be at once placed in a rather wide-mouthed bottle furnished with a well-fitting cork. Each sample should be numbered and marked with the brand which distinguishes the cask from which it was taken. The duty of sampling should be placed in the hands of a person of known integrity and intelligence.
When about to test a sample of soda-ash, the contents of the bottle should first be emptied upon a sheet of dry paper, and the larger lumps then crushed to reduce the whole to a coarse powder, and this must be done as quickly as possible to prevent absorption of moisture from the atmosphere. 100 grains of the alkali must now be accurately weighed and put into a glass flask (Fig. 84), and the remainder of the alkali returned to the bottle and the vessel securely corked. About half an ounce of distilled water is then to be put into the flask and gentle heat applied, with an occasional shaking, until the alkali is all dissolved. The flask is then to be set aside for a few minutes, until any insoluble matter present has subsided, when the clear liquor is to be carefully poured into a beaker glass; the sediment must be washed several times with small quantities of distilled water, and the washings added to the solution in the beaker. This washing is of great importance and must be performed several times, or until the last washing liquor produces no effect upon yellow turmeric paper, which even slight traces of alkali will turn a brown colour. So long as this brown tint is given to the turmeric paper the presence of alkali is assured, and the washing must be continued. It is important, after each washing, to pour off the last drop of the liquor above the sediment, by which the operation is more effectual, and is effected with less water than when this precaution is not observed. In order to ensure perfect accuracy in the result, every particle of the washings must be added to the contents of the beaker-glass in which the assay is to be made.
The Assay.—The alkalimeter is first to be filled with the test-acid exactly to the line 0 or zero of the scale as described, and the beaker containing the solution to be tested then placed immediately beneath the dropping tube of the instrument; a thin glass rod should be placed in the beaker as a stirrer. The acid liquor is then allowed to flow gradually into the alkaline solution (which should be repeatedly stirred with the glass rod), by pressing the knobs of the pressure-cock, until the solution assumes a purple tint, which it will retain until the exact point of saturation has been arrived at, when, as before stated, it will suddenly change to a pink colour. Before the latter stage is reached the beaker should be placed over a spirit lamp or Bunsen burner, and the liquid heated to expel the carbonic acid which is evolved, and partly absorbed by the solution during the process of saturation. When the neutralisation is complete, the alkalimeter is allowed to repose for a few moments, so that the acid liquor may drain from the interior of the glass tube into the bulk of the fluid, and the quantity of test-acid used is then determined by reading off the number of divisions of the alkalimeter that have been exhausted, every one of which represents 1/100th part, or 1 per cent. of alkali, whenever the equivalent weight is taken for assay. Every 1/10th part of an alkalimeter division represents 1/10th of 1 per cent., and the result is thus obtained without the necessity of any calculation. The following table shows the equivalent or combining proportions of soda with 40 grains of real (that is, anhydrous) sulphuric acid:—
| 40 grains of sulphuric acid | } | Are equiv- alent to | 31 grains soda (anhydrous). |
| 1,000 grains of dilute sulphuric acid (sp. gr. 1·033) | 40 grains hydrate of soda (pure caustic soda). | ||
| } | 53 grains carbonate of soda (anhydrous). | ||
| 1,000 grains of dilute sulphuric acid (water-grain measure) sp. gr. 1·032 | 143 grains crystallized carbonate of soda. |