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SECTION III.

ON THE COMPOSITION AND PROPERTIES OF THE FORMS AND COMBINATIONS IN WHICH SUBSTANCES ARE SEPARATED FROM OTHERS, OR IN WHICH THEIR WEIGHT IS DETERMINED.

§ 41.

THE quantitative analysis of a compound substance requires, as the first and most indispensable condition, a correct and accurate knowledge of the composition and properties of the new combinations into which it is intended to convert its several individual constituents for the purpose of separating them from one another and determining their weight. Respecting the properties of these new compounds and their deportment, we have to inquire more particularly, in the first place, how they comport themselves with solvents; secondly, what is their deportment in the air; and, thirdly, what phenomena do they manifest when exposed to the action of a red heat? It may be laid down as a general rule that compounds are the better adapted for quantitative determination the more absolutely insoluble they are, and the less alteration they undergo upon exposure to the air, or to a high temperature.

The composition of bodies is expressed either in per cent. figures, or in a stachiometrical or symbolic formula; by means of the latter, the constitution of the more frequently recurring compounds may be more easily committed to memory. With respect to its composition, a compound is the better adapted for quantitative determination the less it contains relatively of the substance which it is intended to determine, since the less the relative proportion of the latter, the less influence will any error or loss

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of substance that may occur in the course of the analytical process, exercise upon the accuracy of the results. Chloride of platinum and ammonium is, consequently, in this respect, better adapted for the determination of nitrogen than sal ammoniac, since 100 parts of the former contain only 6.28 of that element, whilst 100 parts of sal ammoniac contains 26.2 of it.

Suppose we have to analyse a nitrogenous substance: - we convert its nitrogen into chloride of platinum and ammonium, conducting the process with absolute accuracy, we obtain from 0.300 grammes of the analysed body, 1.000 gram. of chloride of platinum and ammonium: 100 parts of this double chloride contain 6.28 parts of nitrogen, 1.000 contains therefore 0.0628 of that element. These 0.0628 have been derived from 0.300 of substance; 100 parts of the analysed body, consequently, contain 20.93 of nitrogen.

We make a second analysis of the substance, in which we convert the nitrogen of the substance to be analysed into sal ammoniac, instead of chloride of platinum and ammonium; we conduct the process with absolute accuracy, and obtain from 0.300 of the substance under examination, 0.2396 of sal ammoniac, corresponding to 0.0628 of nitrogen, or 20.93 per

cent.

Now, let us assume a loss of 10 milligrammes to have occurred in both processes;-this will alter the result, in the first instance, from 1.000 to 0.990 of chloride of platinum and ammonium, corresponding to 0.062172 of nitrogen, or 20.72 per cent.; the loss of nitrogen will therefore be 20.93 minus 20.72=0.21.

In the second instance the result will be altered from 0.2396 to 0.2296 of sal ammoniac, corresponding to 0.0601 of nitrogen, or 20.05 per cent. The loss in this case will consequently amount to 0.88.

We see here that the same error occasions, in the one case, a loss of 0.21 per cent., with respect to the amount of nitrogen, whilst, in the other case, the loss amounts to 0.88 per cent.

We will now proceed to enumerate and examine all those

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combinations which are best adapted for the quantitative determination of every individual substance. The description of the external form and appearance of the new compounds relates to the state in which they are obtained in our analyses. With regard to the properties of the new compounds, we shall confine ourselves to enumerate those which bear upon the special object we have here more immediately in view.

A. FORMS AND COMBINATIONS, IN WHICH THE VARIOUS BASES ARE SEPARATED FROM OTHER BODIES, OR IN WHICH THEIR WEIGHT AND RELATIVE PROPORTION IS DETERMINED.

BASES OF THE FIRST GROUP.

$42.

1. POTASS.

The combinations best adapted for the weighing of potass are, SULPHATE OF POTASS, NITRATE OF POTASS, CHLORIDE OF POTASSIUM, CHLORIDE OF PLATINUM AND POTASSIUM.

a. Sulphate of potass (which, when its crystallization proceeds undisturbed, forms usually small, hard, oblique, four-sided prisms, or double pyramids of six faces) is obtained in analysis as a white crystalline mass. It dissolves pretty readily in water; it is nearly altogether insoluble in pure alcohol, but in a slight degree more soluble in alcohol containing sulphuric acid in admixture. (Experiment No. 2.) It does not alter vegetable colors; it is unalterable in the air. The crystals decrepitate strongly when heated, yielding at the same time a little water (which is mechanically confined between their plates). The decrepitation of crystals that have been kept long drying is less marked.

At a

strong red heat, sulphate of potass fuses unaltered and without volatilizing.

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The acid sulphate of potass, (hydrated bi-sulphate of potass,) (K O, S 0, +HO, S O,) which is uniformly produced when a solution of neutral sulphate of potass, impregnated with free sulphuric acid, is evaporated to dryness, is fusible even at a moderate heat. At a red heat, it loses half its sulphuric acid, together with the basic water, but not readily; the complete re-conversion of the acid into the neutral salt requiring the long-continued application of an intense red-heat. If heated, however, in an atmosphere of carbonate of ammonia,(this may be readily procured by throwing a fragment of pure carbonate of ammonia into a crucible when heated to feeble redness, and putting the cover on,)—the acid salt changes readily and quickly into the neutral sulphate. This transmutation may be considered complete as soon as the salt, although kept simply at a feeble red-heat, has perfectly re-assumed the solid state.

b. Nitrate of potass crystallizes generally in long striated prisms. In analysis it is obtained as a white crystalline mass; it is readily soluble in water, nearly altogether insoluble in absolute alcohol, and scantily soluble in spirits of wine. It does not alter vegetable colors, and is unalterable in the air. On being exposed to a gentle heat, far below redness, it fuses unaltered and without diminution of weight; upon the application of a stronger heat, it changes into nitrite of potass, with evolution of oxygen, and if the heat be increased to intense redness, it becomes converted into caustic potass, and peroxide of potassium, with evolution of oxygen, and of nitrogen.

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c. Chloride of potassium crystallizes usually in cubes and rectangular prisms, rarely in octahedrons; in analysis we obtain it either in the former shape or as an amorphous mass. It is readily soluble in water, nearly insoluble in absolute alcohol, and but scantily soluble in spirits of wine. It does not change vegetable colors, and is unalterable in the air. When heated, it decrepitates, (if it has not been kept long drying,) yielding a little water (which is mechanically confined in it). At a feeble red-heat it fuses unaltered, and without diminution of weight, but when exposed to a higher temperature, it volatilizes in white vapors; and this volatilization proceeds the more slowly and difficultly, the more effectually and completely the access of air is prevented. (Experiments No. 3.)

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d. Chloride of platinum and potassium presents either small reddish-yellow octahedrons, or a lemon-yellow powder. It is difficultly soluble in cold, but more readily in hot water; nearly insoluble in absolute alcohol, and but scantily soluble in spirits of wine, 1 part requiring for its solution 12083 parts of absolute alcohol,-3775 parts of spirits of wine of 76 per cent,-1053 parts of spirits of wine of 55 per cent. (Experiment No. 4 a.) Presence of free hydrochloric acid sensibly increases its solubility.

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