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GRIFFIN, BOHN AND CO., WINE OFFICE COURT, FLEET STREET, E.C.

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THE CHEMICAL NEWS.

VOL. V. No. 109.-January 4, 1862.

SCIENTIFIC AND ANALYTICAL CHEMISTRY.

Researches on the Relations existing between Atomic Weights, by M. J. S. STAS, Member of the Royal Academy of Belgium.

(Continued from page 337.)

Determination of the Oxygen in Chlorate of Potash by means of the Action of Hydrochloric Acid. To effect this operation, and the weighing of the vessel containing the substances, I took a hard glass globe, of a litre and a-half capacity. I caused to be ground with emery into its neck a hard glass stopper, pierced with two holes, in which two hard glass tubes could be tighty fitted. Exteriorly the neck of the hard glass globe was roughened to the depth of six centimètres; on this ground portion I fitted a cap of natural caoutchouc furnished with a stopcock, taking all the precautions pointed out when speaking of the means employed to effect the weighings in vacuo. After having first weighed the globe free from air, and then free from air but containing the dried chlorate of potash counterpoised in each case with a closed globe of the same capacity, I remove the caoutchouc cap and preserve it in vacuo with the flattened metallic wire which binds it round.

On the other hand, I arrange an apparatus furnishing pure hydrochloric acid; for this purpose I disengage gaseous hydrochloric acid, by means of sulphuric acid acting upon concentrated liquid hydrochloric acid free from sulphurous acid. To make more certain, I wash the gas by passing it through two Woulf's bottles containing pure hydrochloric acid. I then place the apparatus for the disengagement of hydrochloric acid in communication with the globe, in which I have poured before putting in the stopper a hundred cubic centimètres of absolutely pure water. With this object I arrange through one of the holes in the glass stopper a long hard glass tube, descending into the body of the flask, which is inclined until the neck is almost horizontal.

This tube is connected at its opposite end with a caoutchouc pipe adapted to the disengagement tube of the hydrochloric acid. Through the other opening of the stopper I fix a tube which extends half-way down the neck of the flask. The portion of this latter tube which extends outside the flask, is doubly curved, and has two strong bulbs blown in it which act as washingbottles. All the gas which is evolved from the globe should traverse the liquid contained in the bulbs before arriving at a Woulf's bottle containing pure water, and intended to retain the traces of chloride, which the first system of condensation might have allowed to escape. To this Woulf's bottle was adapted a tube through which the gases were conducted into a good chimney.

After having surrounded the body of the flask with

ice, I disengaged the hydrochloric acid. The chlorate of potash, covered with water, is immediately attacked with disengagement of chlorine and even of chlorous acid, which goes out of the flask through the second tube traversing the liquid contained in the bulbs with which it is furnished. If the disengagement of hydrochloric acid is rapid, the temperature rises too much in the globe in spite of the ice which surrounds it, and the chlorous acid which is formed detonates in presence of the hydrochloric acid. This accident happened to me on one occasion and destroyed the apparatus. If, on the contrary, the hydrochloric acid enters the globe too slowly, hydrate of chlorine is formed which surrounds the chlorate and renders the experiment almost interminable. To avoid, at the same time both these inconveniences, I have, in the last two analyses, effected the decomposition of the chlorate on the previously fused salt. The salt had been weighed before fusion; I weighed it afterwards, and am enabled to state that chlorate of potash can be fused and kept in fusion without losing oxygen. The chlorate of potash, fused and covered with water, is decomposed by hydrochloric acid always kept in excess without sensible production of chlorous acid gas. I have been able to decompose, in five or six hours, from 100 to 150 grammes of chlorate without perceiving in the globe the white vapours which are always produced in the simultaneous presence of hydrochloric and chlorous acid. When all the chlorate of potash is decomposed, which may be immediately known by the decolouration of the atmosphere of the globe, I interrupt the current, remove the ice-water, and replace it by water made gradually warmer; finally, I remove the stopper from the globe and pour into it all the liquid which is contained in the bulbs of the disengagement tube. I then proceed to evaporate all the liquid in the flask. In none of the three experiments which I have made by this method, has the quantity of liquid been sufficient to dissolve, at o°, the half of the chloride of potassium produced.

The globe being conveniently inclined, I fix its neck in a flask of hard glass and heat it to a temperature near to, but always below, the boiling-point of the liquid contained. The evaporation of the liquid and the desiccation of the chloride being effected, the residuary chloride retains hydrochloric acid, which cannot be removed from it without fusing the chloride,—an operation which it is impossible to perform in a glass vessel, even when refractory. M. Penny has already shown the presence of hydrochloric acid in this case.

To eliminate this acid I had recourse to the following artifice:-I added to the chloride a small quantity of chlorate of potash, carefully dried and weighed (one to five grammes), and then I poured absolutely pure water on the whole. I evaporated the liquid again. During the evaporation, a sensible odour of chlorine was disengaged. The flask was then placed in a bath of magnesia and raised to a high temperature. The chlorate contained in the chloride was destroyed, with disengage

2

Chemical Researches on the Assay of Silver.

ment of oxygen, and there remained chloride of potassium, pure, colourless, and perfectly neutral.

While the globe was in the magnesia bath, I adapted to it the caoutchouc stopper, and, after it was thoroughly cool, made a vacuum in it.

The washing-water contained in the Woulfe's bottle was evaporated over the water-bath in a small porcelain capsule, and left a residue, which, being dried, weighed 0.0038, consisting of a yellow matter, which heat and air completely destroyed. There was, therefore, no hloride carried over.

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{CHEMICAL NEWS,

Jan. 4, 1862.

The liquid obtained by evaporation from the chloride in the flask having been evaporated to dryness, did not leave, in either of the three experiments, any trace of chloride. Thus, whatever may have been said, I am convinced, that solutions of alkaline chlorides may be evaporated down without the slightest trace of these compounds being carried over by the vapour of water. The three analyses made by this method agree very well among them selves, and the result is identical with that obtained by the calcination of chlorate of potash.

Analysis of Chlorate of Potash.

Second Series.

Amount of Oxygen disengaged by the action of Hydrochloric Acid.

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Chemical Researches on the Assay of Silver by the Moist Way, by G. J. MULDER, Math. Mag. Phil. Nat. Medicine, Artis Obstetricia et Pharmaceutica Doctor, Professor of Chemistry at the University of Utrecht, Grand Cross of the Order of the Netherlands Lion; of the Luxembourg Order of the Couronne de Chene; of the Swedish Orders of Wasa and Polar Star, &c., &c.

(Continued from page 322.)

4. Determination of the Completion of the Analysis.-There exist three methods of finishing an assay experiment.

1. At the end just so much of the decimal solution of salt is added that a trace of precipitate is seen, but a precipitate only of a somewhat peculiar nature and of such peculiar character, that by experience it is already known to be final, while, at the same time, one is quite sure that if more salt solution had been added nothing more would have been seen. It is called the confirmative precipitate. As soon as this precipitate is seen, the quantities of salt used-i.e., the quantities of standard soli tion and decimal solution used-are added together and calculated as equivalent to the quantity of silver.

2. It is a usual practice, also, to add repeatedly small portions of decimal solution of salt to the solution under experiment, until no more precipitate is seen. The half only of the last added portion is taken from the portion added just previous to the last. This, however, readily becomes a source of error, unless care be taken to add very small portions of salt (decimal solution).

3. Another method, again, is to find the neutral point at which in the two half portions of the liquid an equally large precipitate is produced both by addition of decimal solution of salt and by addition of decimal solution of

silver.

While we have only given here the principles of the

three methods, and while we shall have to treat more particularly of them hereafter, we must not neglect to take into consideration some scientific points which may duly aid us in estimating the divers influences which affect all three, or one of the three, methods just alluded to.

The first question which arises in every one of these methods is, whether or not all the silver is precipitated, and if not, whether the result will not be hereby affected. When the experiment has been well executed, all the silver is precipitated according to the first method, and it is needless to say that this is sure to be the case according to the second method, for if the liquid is filtered off from the precipitate and tested with sulphide of ammonium, after having saturated the nitric acid with ammonia, no trace even of colouration of the clear liquids, obtained by the first and second methods can be perceived. (The expression "all the silver precipitated" means that no re-agent will detect any of that metal in any of the liquids.) It has already been proved that if we work according to the third method, silver, indeed, is left in the assay liquid, for common salt produces a precipitate, and by treating the clear liquid as indicated with ammonia and sulphide of ammonium there will be seen in the liquid, brought to the neutral point, a very perceptible colouration occasioned by the formation of sulphide of silver. How much silver is dissolved, and does not this render the experiment incorrect? are two questions which must now be answered. As far as the cuntity of silver kept in solution is concerned, this depends on the temperature of the test bottle. I merely remark here that at from 15° to 17° C. the quantity is 0.5 milligramme, less at lower, and more at higher temperatires (see page 321).

The question now arises, whether the deficiency is just the half of 0.54 milligramme-ie., o 27 milligramme salt, equivalent to o'5 milligramme silver?

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