Imágenes de páginas
PDF
EPUB
[blocks in formation]

depends upon the amount of carbonate which it contains, a rapid and sufficiently accurate method of analysis of this salt becomes a desideratum. In order to effect this object, the process termed alkalimetry has been invented. In principle it depends upon the determination of the number of divisions of a diluted acid, of definite strength, which 100 grains of the different samples of ash are capable of neutralizing; the neutralization being estimated by the action of the solution upon blue litmus.

The acid solution which is to be employed is measured from a burette or alkalimeter, which is a tube of the form shown in fig. 331. It has an internal diameter of about five-eighths of an inch, and is sufficiently tall to contain rather more than 1000 grains of distilled water. The space occupied by 1000 grains of water at 60° is marked off and indicated as o, and the tube is then subdivided into 100 equal parts, each capable of containing 10 grains of water; opposite every tenth division the number corresponding to it is placed, the numbers increasing from above downwards.

Various plans have been proposed for preparing the diluted acid; the following is substantially the same as that recommended by Faraday (Chemical Manipulation, 3rd ed. p. 281). It has the advantage of being readily applicable to any alkali.

FIG. 331.

10

20

30

50

80

190

A solution of sulphuric acid is prepared by diluting the ordinary commercial acid with 8 times its bulk, or nearly 5 times its weight, of distilled water; when cool, the liquid, which may be termed alkalimetric acid, should have a specific gravity of 11268. In order to ascertain whether the strength of this alkalimetric acid be accurately adjusted, a quantity of crystallized bicarbonate of potassium is fused in a platinum crucible in order to convert it into the carbonate: the fused mass is poured upon a clean iron plate, and 100 grains of it are quickly weighed, and dissolved in about 3 ounces of water in a small evaporating basin. Diluted acid, sufficient to fill 35 divisions, is now to be introduced into the alkalimeter, which consequently is to be filled up to the mark 65 with the diluted acid, and water is to be added until it stands at the mark o: the acid and water are to be thoroughly mixed by closing the tube with the thumb and finger, then inverting and agitating the tube; after which the liquid is added to the solution of carbonate of potassium, which is to be gently warmed in order to expel the carbonic anhydride as it is liberated.

[blocks in formation]

piece of blue litmus-paper, or a small quantity of infusion of litmus, is placed in the basin, and the acid is cautiously added until the litmus is distinctly but permanently reddened. The acid liquid, if properly diluted, ought to contain, in each division, sufficient sulphuric acid to neutralize I grain of carbonate of potassium; and the entire contents of the alkalimeter should therefore exactly produce this effect. If more than 100 divisions of the acid be required, the test acid is too weak; if less than 100 divisions, it is too strong.

Suppose that 95 divisions of the acid were sufficient, the alkalimetric acid from which it was prepared must have contained one-twentieth too much acid; every 95 measures of this acid, therefore, must be diluted with 5 measures of water. If, on the other hand, more acid than 100 divisions be required, say 105 be needed, the acid contains one-twentieth too much water; the quantity of alkalimetric acid used in the experiment requires the addition of one-twentieth more of acid than it originally contained; now the quantity of alkalimetric acid used was 35 divisions, and the one-twentieth part of this is 175, but only onefifth of this is pure acid (H2SO4), so that 0.25 parts by weight of the oil of vitriol originally used, must be added to each 35 parts by weight of alkalimetric acid. This correction, though not mathematically exact, is perfectly sufficient for all practical purposes. The alkalimetric acid, when duly adjusted, is preserved in bottles which are accurately closed.

Having thus prepared a test acid of the proper strength, 100 grains of the sample of pearlash for trial are dissolved in 3 or 4 ounces of water, filtered if necessary, and then tested in the same manner the number of divisions of acid consumed will indicate the per-centage of carbonate of potassium present in the sample.

The same acid may be employed to determine the quantity of soda present in any sample of soda ash; but as a certain weight of soda neutralizes a proportionately larger amount of acid than an equal weight of potash, the alkalimeter must be filled to a higher mark with the acid. For the determination of the quantity of anhydrous potash in any sample, the acid must be poured into the burette till it stands at the division 49, and the tube must be then filled up to o with water. Each division will then contain acid sufficient to neutralize 1 grain of anhydrous potash.

If filled with acid to 65, and then filled up with water, each division will correspond to I grain of carbonate of potassium.

If filled to 546, and then filled up with water, each division will indicate 1 grain of dry carbonate of sodium: and if filled to

METHOD OF WILL AND FRESENIUS.

409

23'4, and then water be added to o, the acid in each division. will neutralize I grain of anhydrous soda.

In cases where greater accuracy is required, the acid solution, instead of being measured from the burette, is weighed; and for this purpose the solution is placed in a light flask of the form shown in par. (939).

In estimating the value of soda ash, which often contains sulphide and hyposulphite of sodium, an error might be occasioned by adopting this method; because both the sulphide and the hyposulphite would be decomposed by the sulphuric acid, and would neutralize it, and thus would be reckoned as carbonate of sodium.

The presence of caustic alkali in any sample is easily ascertained by the action of the solution upon nitrate of silver: the carbonates of the alkaline metals occasion a white precipitate of carbonate of silver; but if they contain any caustic alkali, a brown precipitate of hydrated oxide of silver is produced. The presence of sulphides in the ash is immediately manifested by the odour of sulphuretted hydrogen which is evolved on neutralizing the solution with an acid; if any sulphide be present, it will blacken the salts of silver, and interfere with their application as test for caustic potash or soda.

(578) Alkalimetry; Process of Will and Fresenius.-The proportion of carbonic anhydride in any sample of alkali is readily ascertained by means of the apparatus employed for the purpose by Will and Fresenius, represented in fig. 332: b is a light flask, of about 3 ounces capacity, in which 100 grains of the alkali are placed with about I ounce of water: d is a similar flask, in which about an ounce

FIG. 332.

[graphic]

and a half by measure of oil of vitriol is placed. A sound cork is fitted into the neck of each flask, and is pierced with two apertures for the reception of the tubes, a, c, and e, all of which are open at both ends: the tube, a, is sufficiently long to dip into the liquid in the flask; c is a bent tube, the longer limb of which passes into the acid in the flask, d. The outer extremity of a is closed during the experiment, by a plug of wax or of soft cement. The apparatus is charged in the manner already described, and is accurately weighed after it has been connected together. A partial vacuum is now made by applying the mouth to the tube, e, and exhausting a portion of the air; on ceasing to exhaust, the

410

ALKALIMETRY-BICARBONATE OF POTASSIUM.

acid rises in the tube, c, and passes over into b, to supply the place of the air which has been withdrawn; effervescence is occasioned by the escape of the carbonic anhydride, which passes off through the tube, c, and is dried as it bubbles up through the oil of vitriol in the flask, d. As soon as the effervescence has ceased, a fresh portion of acid is forced over from d into b by again partially exhausting the air: and this process is repeated until no further effervescence is occasioned by the fresh acid. The plug of wax is now withdrawn from the tube a, and a current of air is forced through the apparatus by exhausting with the mouth at e, and the carbonic anhydride is thus completely displaced. The plug is now replaced in the tube a, and the apparatus is weighed a second time. The difference between this weight and that obtained on the first occasion, indicates the amount of carbonic anhydride which has been expelled.

If any sulphide or sulphite of the alkaline metal be present, the error which it might occasion by loss of sulphuretted hydrogen, or of sulphurous anhydride, in the gaseous state, and which would be reckoned as carbonic anhydride, is prevented by mixing from 20 to 30 grains of neutral chromate of potassium with the sample under trial: the chromic acid which is liberated by the subsequent action of the sulphuric acid upon the chromate, imparts oxygen to the sulphuretted hydrogen or sulphurous acid, and converts both into sulphuric acid, which would be retained, and would in no way interfere with the result.

(579) Acid Carbonate or Bicarbonate of Potassium (KHEÐ ̧, or KO,HO, 2 CO2=100); Sp. Gr. 2.052.—By passing a current of carbonic acid through a strong solution of the carbonate of potassium, crystals of the carbonate are deposited in the form of right rhombic prisms; they are permanent in the air, and require about 4 parts of cold water for solution. The solution of the acid carbonate, if exposed to the atmosphere, gradually loses one-fourth of its carbonic acid, forming a sesquicarbonate; and if boiled, the same change occurs much more quickly. The acid carbonate is converted into the normal carbonate when fused by means of heat. The acid carbonate of potassium has no alkaline reaction upon turmeric. It may be employed for procuring the compounds of potassium in great purity, since, if well crystallized, it is almost absolutely pure, and may be obtained in this state with less difficulty than any other salt of potassium. It is consumed medicinally in considerable quantities, for making effervescing draughts by the addition of citric or tartaric acid to its solution in water.

The Silicates of Potassium are important compounds in con

TESTS FOR SALTS OF POTASSIUM.

411

nexion with the manufacture of glass: they will be noticed in treating this subject (593 et seq.).

(580) CHARACTERS OF THE SALTS OF POTASSIUM.-The salts of potassium, with a colourless acid, are all colourless. They seldom contain any water of crystallization, yet many of them are deliquescent; the carbonate and acetate offer striking instances of this peculiarity, and furnish in this respect a marked contrast to the corresponding salts of sodium. The salts of potassium, when pure, if introduced upon a platinum wire into the reducing flame of the blowpipe, communicate to it a violet tint; the presence, however, of a small quantity of a salt of sodium masks this effect, in consequence of the strong yellow flame occasioned in similar circumstances by the compounds of sodium; by means of the spectroscope, however, the potassium is distinctly recognizable, though the sodium salt may be in very large excess. The light emitted by a

salt of potassium, in the flame of a Bunsen burner, consists of a feeble continuous spectrum, terminated at one end by a bright line in the red, and at the other by a feebler bright line in the violet (K, fig. 82, page 172, Part I.). Solutions of the salts of potassium yield no precipitate with solutions of the carbonates of the alkaline metals, with ferrocyanide of potassium, or with sulphide of ammonium. The presence of potassium in solution is recognized, after the absence of every metal but sodium has been ascertained, by the following characters: if moderately concentrated, a solution of tartaric acid in excess causes, upon brisk stirring, a white crystalline precipitate of acid tartrate of potassium, which is readily dissolved upon adding an alkali. Perchlorate or carbazotate of sodium has also sometimes been employed as a test for potassium, since both the perchloric and carbazotic acids form potassium salts of sparing solubility. These compounds, however, are all soluble to a considerable extent in cold water, and unless tolerably strong solutions are employed, they do not immediately subside. With silicofluoric acid they yield a transparent gelatinous silicofluoride, which forms a white powder on drying. The most conclusive reaction, however, is produced with the perchloride of platinum; upon mixing a strong solution of this salt with a concentrated one of a salt of potassium, a yellow double salt, consisting of (2 KCl,PtCl) is separated in crystals; it is quite insoluble in alcohol and ether, but is slightly taken up by cold water. It is therefore best for analytical purposes to acidulate the solution suspected to contain potassium with a little hydrochloric acid, and having added a slight excess of the

« AnteriorContinuar »