ÆäÀÌÁö À̹ÌÁö
PDF
ePub

mate of iron is precipitated, is the same as those already given for chromate of chromic oxide and chromate of alumina, viz. :

FeCl3 +5 (KO CrO3) = 3 (KCl) +2 (KO 2 CrO3) + Fe2O3 CrO3. Until the requisite amount of chromate of potash had been added to the perchloride of iron, the precipitate re-dissolved; when it became permanent, it was drained, pressed, and air-dried for analysis.

[blocks in formation]

From these data may be obtained by the necessary proportions, —

[blocks in formation]
[ocr errors]

Chromate of iron found Fe2O3 + Cr2O+O weighed = 0.3943 Excess of the calculated chromate of iron over the found

= 0.0344

This error would be perfectly corrected by a loss of three milligrammes in the water determination, or as the figures stand without any correction they would lead to the conclusion that the precipitate was a chromate of iron of the formula Fe2O, CrO3, but containing a very small excess of chromic acid.

[blocks in formation]

Chromate of iron found = FeO3 + Cr2O3 + O weighed = 0.3346 Excess of the calculated chromate of iron over the found 0.0251

This error would be corrected by a supposed loss of little more than two milligrammes in the water determination. This second analysis, therefore, confirms the view that the normal composition of this chromate of iron is represented by the formula Fe2O3 CrO3.

It is unnecessary to call attention to the great similarity of the properties of the chromates of chromic oxide, ferric oxide, and alumina; their normal composition is represented by the formula R2O3 CrOз, but they are such feeble compounds as seldom to be found of the exact constitution which the formula indicates.* Of the three, the chromate of chromic oxide is the most stable, and we have been able to prepare it with the exact theoretical composition, from which it so easily varies. It should be observed, that a variation in the composition of these chromates, amounting to even ten per cent of their chromic acid, does not affect in the least their external properties, and it is by no means impossible that they are examples of that variation in the law of definite proportions which has been ably discussed by Professor J. P. Cooke in the Memoirs of this Academy; † but unfortunately, as must often be the case, the limits of error of our methods of analysis are wider than those of the supposed variation, and moreover, on this border-land between the kingdoms of Chemical and of Mechanical Force, there must always be doubt and difference of opinion concerning the cause to which a proved effect is to be ascribed.

3. Chromate of Manganese. The existence of this chromate has been so clearly demonstrated by previous observers that we have thought it unnecessary to add any experiments of our own. On mixing a solution of sulphate of the protoxide of manganese with a solution of chromate of potash, a chocolate-colored precipitate subsides after some time, to which Warington ‡ assigned the formula (MnO), CrO3 2 HO. Bensch § also analyzed a similar precipitate, for which he gives the same formula. In Warington's analysis the amount of protoxide of manganese is larger than the compound which his formula rep

* The monochromate of glucina mentioned by Gmelin (Handbook, IV. 155, Cavendish Soc. Ed.) evidently belongs to this class.

† New Series, V. pp. 348, 352 (1854).

‡ Phil. Mag., [3.] XXI. 380 (1842). Pogg. Ann., LV. 98 (1842).

resents would contain, and it is, moreover, questionable if the protoxide of manganese could exist in contact with chromic acid. The subsequent observations of Fairrie* explain these difficulties. He mixed the solutions of chloride of manganese and chromate of potash, and so obtained a precipitate whose composition by analysis was 3 (Mn2O3 CrO3) + Cr2O3 + 6 HO. Fairrie's criticism on the analyses of Warington and Bensch, that they overlooked the chromic oxide formed by the reaction, is no doubt just. He also remarks, that the precipitate appeared to be formed by the action of seven equivalents of chromate of potash on six equivalents of chloride of manganese, but offers no explanation of the reaction. The decomposition is explained by the formula,

7 (KO CrO3)+6 (MnCl) = 3 MnO3 CrO3) + Cr2O3 + 6 (K Cl) +KO 2 CrO3.

Six equivalents of the protoxide of manganese are oxidized into three equivalents of sesquioxide of manganese, at the expense of the three atoms of oxygen which two equivalents of chromic acid give up in changing into one equivalent of chromic oxide; probably the chromic oxide so formed enters into combination with the chromate of manganese, rendering it basic. Chloride of potassium and bichromate of potash remain in the filtrate.†

* Jour. Chem. Soc., IV. 300 (1852).

† We do not wish to leave the subject of the metallic chromates without noticing the remarkable analyses published by Malaguti and Sarzeau (Ann. de Ch. et de Phys., [3.] IX. 431, 1843) of the chromates of copper, zinc, cadmium, and nickel. The figures of these analyses correspond exactly with the following singular formulæ :

(CuO)4 CrO3 + 5 HO,
(ZnO) CrO3 + 5 HO,

(CdO); (CrO3)2 + 8 HO,
(NiO)4 CrO3 + 6 HO.

All these precipitates were washed with boiling water till no color came from them, and it is clear that these chromates, like those which we have studied, may be deprived of the greater part of their chromic acid, if not the whole, by prolonged washing; hence the basic character of the substances analyzed. The marvel is, that the analyses corresponded so exactly with such peculiar formulæ ; it is hardly conceivable that the publication of many analyses of each of these substances should not show the existence of a series of compounds, which conform to no definite formulæ.

III. THE BLACK OXIDE OF MANGANESE (3 MnO2 =

Mn2O3 MnO3). Another question now suggests itself,- Would analogy lead us to suppose that there was an oxide of chromium containing two atoms of oxygen? Comparing chromium, as before, with the allied metals, aluminum, iron, and manganese, we meet with no such oxide of aluminum, and we have Fremy's direct statement that he could find no oxide of the formula FeO. Manganese, however, forms a compound with oxygen, stable, insoluble, and natural, which has heretofore always been spoken of as an oxide of manganese and been represented by the formula MnO2. Is not the existence of this compound a strong argument for the oxide CrO2? We propose to adduce the evidence which makes it most probable that this so-called peroxide of manganese is in reality a compound, sometimes definite but oftener indefinite, of manganic acid and manganic oxide, and that its normal composition is to be represented by the formula Mn,O, MnO3.

We shall not expect to resolve this substance into manganic oxide and manganic acid with the same facility with which we analyze the chromate of chromic oxide. This latter body is less stable than either chromic oxide or chromic acid, whereas manganic acid and oxide are both exceedingly unstable substances, obtained with difficulty but easily destroyed. On the other hand, there is no more stable compound of manganese than that called the peroxide. It is not therefore decomposed, as the chromate of chromium is, by boiling water, boiling caustic potash, or a boiling solution of chloride of ammonium. We prepared artificial peroxide of manganese by passing a stream of chlorine through water in which carbonate of protoxide of manganese was diffused, and washing the precipitate, first with dilute nitric acid and then with water. The artificial peroxide thus prepared resisted solutions of chloride of sodium and of caustic potash, even when heated with these liquids in closed tubes to a temperature of 180° by means of a wax bath.

But, notwithstanding the stability of this black oxide of manganese, it is not impossible to obtain from it manganic acid under circumstances which seem to preclude the possibility of any oxidation of the substance during the process by which the manganic acid is exhibited. Mitscherlich † observed and reported the formation of the green manganate of

* Ann. de Ch. et de Phys., [3.] XII. 381.

† Abhandlungen der Akademie der Wiss. zu Berlin, 1831, p. 218. Ann. de Ch. et de Phys., [2.] XLIX. 114. Pogg. Ann., XXV. 287.

potash, when caustic potash is fused in a retort with peroxide of manganese without access of air. Upon such authority this statement must have great weight, although the omission of the details of the experiments renders it impossible to form any opinion of the sufficiency of the evidence upon which it rests. Fortunately the results of Mitscherlich find full confirmation in the recent experiments of Bekétoff.* This observer has shown that, when a mixture of caustic potash and black oxide of manganese is heated, in a closed tube filled with oxygen, to a temperature of 180°, an abundance of the green manganate of potash is formed without the slightest absorption of oxygen. Unless these experimental results can be disproved, it seems impossible to escape the conclusion that the substance called peroxide of manganese contains manganic acid, since it yields manganic acid while the ratio of its manganese to its oxygen remains unchanged.

In 1817 Chevillot and Edwards† published an account of some experiments in which they endeavored to prove that the presence of air or oxygen was absolutely necessary to the formation of chameleon from caustic potash and black oxide of manganese. To demonstrate this, they first heated the mixture of these two substances in a silver crucible, within a glass vessel which was filled with nitrogen, and found that no manganate of potash was produced. The amount of heat applied is not stated, but the nature of the apparatus indicates that a high temperature was used. Secondly, they heated the same mixture of caustic potash and black oxide of manganese in an atmosphere of oxygen, and measured the considerable quantity of oxygen absorbed during the formation of the manganate. These experiments seem at first sight absolutely to contradict those of Mitscherlich and Bekétoff; but it is easy to show that they have no such tendency, and that the conclusions which Chevillot and Edwards drew from them were entirely erroneous, though the experiments themselves were perfectly correct and faithfully reported. It is obvious that an absorption of oxygen is just as necessary in order to convert the substance, whose formula would be written MnO3 MnO3, completely into manganic acid, as it is for the entire conversion of the so-called oxide MnO2 into the same acid. The same amount of oxygen must be absorbed in changing one gramme of the black oxide of manganese into manganic acid, however the rational formula of

*Bulletin de Soc. Chim. de Paris, Séance 13 Mai, 1859, I. 43.

† Ann. de Ch. et de Phys., [2.] IV. 290.

« ÀÌÀü°è¼Ó »