Alexander Watt

DETERMINING THE REAL VALUE OR PERCENTAGE OF COMMERCIAL SODAS, CHLORIDE OF LIME, ETC.

Examination of Commercial Sodas.—Mohr's Alkalimeter.—Preparation of the Test Acid.—Sampling Alkalies.—The Assay.—Estimation of Chlorine in Bleaching Powder.—Fresenius' Method.—Gay-Lussac's Method.—The Test Liquor.—Testing the Sample.—Estimation of Alumina in Alum Cake, etc.

In a manufacture such as paper-making, which involves the consumption of enormous quantities of materials of variable quality, as soda ash, caustic soda, and bleaching powder, for example, it will be readily seen that some means should be at the command of the consumer who does not avail himself of the services of a practical chemist at his works, by which he can ascertain the actual value of the various substances he uses. An art which, up to a certain point in its progress, is mainly a chemical operation, it would undoubtedly be more safely and economically conducted when supervised by persons well acquainted with chemical principles and reactions, and less dependent upon individual judgment, than is, perhaps, too frequently the case. Under such supervision more perfect uniformity of results—a consideration of the greatest importance in a manufacture of this kind—would be ensured.

Examination of Commercial Sodas.—The methods of determining the percentage of real alkali in the commercial products which have received the name of Alkalimetry are fortunately of a simple character, and such as a person of ordinary intelligence and skill can readily manipulate and render thoroughly reliable by exerting the necessary care. He must, however, be provided with a few indispensable appliances, which will be described, and with these he should make several trials upon various samples until he finds that his results are uniform and his manipulation easy and reliable. He will require a chemical balance,[34] capable of weighing to the tenth of a grain; a few glass "beakers" (Fig. 83) of various sizes, capable of holding from four to eight or ten ounces of fluid; several glass stirrers; a bottle of litmus solution, made by dissolving litmus in hot water; books of litmus and turmeric papers; and several glass flasks (Fig. 84) of various sizes, capable of holding from four to eight ounces. Besides these accessories, certain measuring instruments, termed alkalimeters or burettes, are employed, of which either of the two following may be employed. These instruments are of glass, and hold up to 0 or zero exactly 1,000 grains. The scale is graduated in a hundred divisions, which are again subdivided into tenths. Bink's burette is shown in Fig. 85, and Mohr's burette in Fig. 86. The latter, being provided with a stand, enables the operator to add the test liquor—with, which the burette is charged—drop by drop, when the alkaline solution to be tested is near the point of saturation, without engaging the hands.

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:—

Mr. Arnot recommends the following method for alkali testing: "The sample, which should be a fair average of the drum or cask from which it is drawn, should, in the case of caustic soda, be quickly crushed into small fragments, and returned to the stoppered bottle in which it was collected for testing. It need not be finely ground, and, indeed, should not be, as it very readily attracts moisture from the air. The contents of the drum are usually pretty uniform, and the crushing recommended will give the operator a sample quite fit to work upon. Samples of soda-ash and soda crystals will, of course, be fairly representative of the casks from which they are drawn. One hundred grains of the prepared sample must be weighed out upon a watch-glass or slip of glazed paper, and transferred to a porcelain basin, with at least half a pint of boiling water. The watch-glass is preferable for caustic soda, and the weighing in the case of that agent must be done expeditiously. While the sample is dissolving the burette will be charged with the standard acid. To the soda solution a few drops of solution of litmus, sufficient to colour it distinctly, will be added. The acid will then be run into the blue soda liquor; at first, within reasonable limits, this may be done rapidly, but towards the close of the operation the acid must be added cautiously, and the solution kept well stirred. In the case of caustic, when the blue has distinctly changed to red, the operation may be considered completed, and the measures may be read off the burette; and this is, without calculation, the result required. When the soda in the sample is a carbonate, the blue colour of the litmus will be changed to pink before all the soda is neutralised, owing to a portion of the liberated carbonic acid remaining in the solution; this must be eliminated by placing the basin over a Bunsen burner and boiling the solution. The blue colour will thus be restored, and more acid must be added, repeating the boiling from time to time, until the red colour becomes permanent. It is sometimes necessary to filter the soda solution before testing; this applies specially to recovered soda, and, although in a less degree, to soda-ash." When the soda solution is filtered, it will be necessary to thoroughly wash out the liquor absorbed by the filtering paper, the washings being added to the bulk of the liquor as before. The best plan is to allow the soda solution to stand for some time until all the sediment has deposited, and then to pour off as much of the liquor as possible, and then to wash the sediment into a very small filter, in which it will receive further washing, until no trace of alkali can be detected in the last wash water.

Estimation of Chlorine in Bleaching Powder.—It is desirable that the manager or foreman of a paper-mill should have at his command some ready means by which he may test the percentage of chlorine in samples of bleaching powder, or chloride of lime, delivered at the mill, not alone to enable him to determine the proportions to be used in making up his bleaching liquors, but also to ensure his employers against possible loss in case of inferior qualities being delivered at the mill. Bleaching powders being purchased according to percentage, it is absolutely necessary that the purchaser should have this determined to his own satisfaction before either using or paying for the material. Good chloride of lime should contain 35 per cent. of available chlorine, but the powder should not be accepted which contains less than 32 per cent. There are several methods of estimating the percentage of chlorine in bleaching powder, which is composed of hypochlorite of lime, chloride of calcium, and hydrate of lime, the latter substances being of no service in the bleaching process.

According to Fresenius, in freshly prepared and perfectly normal chloride of lime, the quantities of hypochlorite of lime and chloride of calcium present stand to each other in the proportion of their equivalents. When such chloride of lime is brought into contact with dilute sulphuric acid, the whole of the chlorine it contains is liberated in the elementary form. On keeping chloride of lime, however, the proportion between hypochlorite of lime and chloride of calcium gradually changes: the former decreases, the latter increases. Hence from this cause alone, to say nothing of original difference, the commercial article is not of uniform quality, and on treatment with acid gives sometimes more, and sometimes less, chlorine. As the value of bleaching powder depends entirely upon the amount of chlorine set free on treatment with acids, chemists have devised very simple methods of determining the available amount of chlorine in any given sample, these methods having received the name of chlorimetry. The method of Fresenius is generally considered both practicable and reliable.

Fresenius' Method of preparing the solution of bleaching powder to be tested is as follows:—Carefully weigh out 10 grains of the sample, and finely triturate it in a mortar with a little cold water, gradually adding more water; next allow the liquor to settle, then pour the liquid into a litre flask, and triturate the residue again with a little water, and rinse the contents of the mortar carefully into the flask, which should then be filled with water up to the graduated mark. Now shake the milky fluid and proceed to examine it while in the turbid state; and each time, before measuring off a fresh portion, the vessel must be again shaken to prevent the material from depositing. The results obtained with the solution in its turbid condition are considered more accurate and reliable than when the clear liquid alone is treated, even though the deposit be frequently washed. This may be proved, Fresenius says, by making two separate experiments, one with the decanted clear liquor, and another with the residuary turbid mixture. In an experiment made in his own laboratory the decanted clear fluid gives 22·6 of chlorine, the residuary mixture 25·0, and the uniformly mixed turbid solution 24·5. One cubic centimÈtre of the solution of chloride of lime so prepared corresponds to 0·01 gramme of chloride of lime.

Gay-Lussac's Method.—This method, which is known as the arsenious acid process, has been much adopted for the determination of chlorine in bleaching powders, and is conducted as follows:—

The Test-liquor.—This is prepared by dissolving 100 grains of pure arsenious acid in about 4 ounces of pure hydrochloric acid, and the solution is to be diluted with water until, on being poured into a graduated 10,000 grains measure-glass, it occupies the volume of 700 grains measure marked on the scale. Each 1,000 grains measure of this liquid now contains 14·29 grains of arsenious acid, corresponding to 10 grains of chlorine, or 1/10 grain of chlorine for every division or degree of the scale of the chlorimeter, for which purpose a Mohr's burette of the above capacity may be used, or a graduated tube of the form shown in Fig. 85 may be employed.

Testing the Sample.—100 grains of the chloride of lime to be tested are next dissolved in water, and poured into a tube graduated up to 2,000 grains measure. The whole must be well shaken in order to obtain a uniformly turbid solution, and half of it (1,000 grains measure) transferred to a graduated chlorimeter, which is, therefore, thus filled up to 0°, or the zero of the scale, and contains exactly 50 grains of the chloride of lime under examination, whilst each degree or division of the scale contains only ½ grain. 1,000 grains measure of the arsenious acid test-liquor are now poured into a glass beaker, and a few drops of a solution of sulphate of indigo added, in order to impart a faint, but distinct, blue colour to it; the glass is then to be shaken so as to give a circular movement to the liquid, and whilst it is whirling round the chloride of lime solution from the chlorimeter is gradually and cautiously added until the blue tinge given to the arsenious acid test-liquor is destroyed, care being taken to stir the mixture well with a glass rod during the whole process, and to stop as soon as the decoloration is complete. We will assume that in order to destroy the blue colour of 1,000 grains measure of the arsenious acid test-liquor 90 divisions or degrees of the chloride of lime solution have been employed. These 90 divisions, therefore, contained the 10 grains of chlorine required to destroy the colour of the test solution; and since each division represents ½ grain of chloride of lime, 45 grains of chloride of lime (10 grains of chlorine) were present in the 90 divisions so employed, from which the percentage strength may be ascertained:—

For 45 : 10 :: 100 : 22·22.

The chloride of lime examined, therefore, contained 22¼ per cent. (nearly) of chlorine. This method is extremely simple and trustworthy when properly employed, but to ensure accuracy certain precautions must be adopted. Instead of pouring the test liquor into the solution of the sample (as in alkalimetry), the solution of the sample must be poured into the test-liquor. If the contrary plan were adopted the hydrochloric acid of the test-liquor would liberate chlorine gas so fast that much would be lost, and the result rendered incorrect. By pouring, on the contrary, the chloride of lime solution into the arsenious acid solution the chlorine is disengaged in small portions at a time, and meets with an abundance of arsenious acid to react on. The mixture of chloride of lime should also be employed turbid.

Estimation of Alumina in Alum Cake, etc.—Mr. Rowland Williams, F.C.S., in a paper read before the Chemical Society in June, 1888, describes a method of estimating the alumina in alums, alum cakes, and sulphate of alumina, by which he obtained more accurate results than are obtained by the ordinary ammonia method of estimation. After pointing out several objections to the method of precipitating the alumina by ammonia, he proceeds:—"There is another method for the estimation of alumina which is not so well known as the above. This is by means of sodium thiosulphate. Having had a very extensive and successful experience of this process, I can recommend it with confidence. Considerable practice is, however, necessary in order to secure good results, as certain conditions must be carefully attended to, otherwise the precipitation will be incomplete. The estimation is made in a moderately dilute solution. In the case of alum cake and sulphate of alumina I dissolve 400 grains in water, filter, dilute to 10,000 grains. I use 1,000 grains of this solution (equal to 40 grains of the sample) for estimating the alumina. If any free acid is present it is neutralised by a few drops of carbonate of soda solution, and the whole diluted to about 8 ounces measure. A large quantity of crystallized thiosulphate of soda is then added, and the liquid boiled for at least half-an-hour, constantly replacing the water lost by evaporation. By the end of that time all the alumina will be precipitated in a finely-divided form, along with more or less free sulphur. The precipitate is then filtered off and washed well with boiling water. The filtration and washing take place very rapidly, and may generally be accomplished in about twenty minutes, this being a great saving of time in comparison with the long and tedious washing by decantation, which is necessary in the case of gelatinous alumina. Before filtration, it is advisable to add a drop or two of carbonate of soda solution, lest the liquid should have become slightly acid during boiling."