128 DR. L. PLAYFAIR ON FERRICPANIDE OF POTASSIUM,

VI.-Note on a ATew Xeethod of N a k i y Ferricynnide of Potcissium and a Pumcyccnogen Conapouizd.

By L Y O W PLAYFAIB, C.B., F.R.S. THIS note refers to two processes ~ l t i c h only deserve description because they are of an izniisnal kind, and may possibly be em- ployed for other purposes.

I.-Ferricyanide of Potassiu?nn.--’CVlien yellow prussicle of po- tassium is precipitated by salts of lime or bar>-ta, well-known sparingly soluble precipitates are obtained, containing 2 equivalents of barium or of calcium, instead of 2 equivalents of potassium. If R represent either of these metals, the formula of the precipitate is Fe,Cy,, K,R,. When these precipitates are treated with car- bonate of ammonia, the earthy carbonates fall, and a solution is obtained of a salt Fe,Cy,, 2K, 2NH,.

When this latter yellow salt containing ammonia is boiled with peroxide of manganese finely divided, ferricyanicle of potassium is altundaiitly formed ; and after the ebullition has been continued for some time, a portion of the solution filtered will be found to give no further precipitate with perchloride of iron, being wholly converted into a ferricyanide. During the boiling with the peroxide of manganese, ammonia is freely given off. The salt formed by this process in solution has all the properties of a ferricyanide, and would obviously be Fe,Cy,, K2NH4, in which 1 equivalent of am- monium replaces 1 equiv. of potassium in the red prussicie. Rut this salt, by prolonged boiling with peroxide of manganese, is further decomposed, a green-coloured precipitate occurring, while the salt in solution still retains the ordinary cliaracters of a ferri- cyanide, but contains less ammonium, more potassium, and gene- rally an excess of iron. A salt of silver made from the red solution, and possessing the ordinary characters of the ferricyanide of siiver, gave on analysis 10.89 per cent. of iron, and 60.28 per cent. of silver : calculation gives 10.44 per cent. of iron, and 6045 per cent. of silver. This analysis leaves no doubt as to the cliaracter of the variable red salt in solution, though it contains, like the fatter, an excess of iron.

The formation of the ferricyanide is clearly owing to an oxida- tion of the ammonium.

*

Fc2Cy,2K, 2NH, + 2;54110, = Fe,Cy$K,NH, + NII, + TI0 + &4n203.

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AND A PARACYANOGEW COMPOUND. 129

It is well known that when ferricyanide of potassium is boiled with ammonia, a ferrocyanide is produced, 1 equiv. of hydrogen replacing 1 equiv. of potassium in the ordinary yellow salt. The presence of an excess of peroxide of manganese in the reaction now described appears to prevent this decomposition, as ammonia is given off during the whole oxidation.

I do not describe this formation of ferricyanide as an economical process for procuring the salt, but as an instance of an unusual process of oxidation which may be applied to other purposes."

11.-Paracyanogen Compound.--In making the nitroprussides, I had found that, when cyanogen was in the nascent state, it combined with 2 equivalents of water, and produced oxamide :

Cy + 2HO = 2CO,NH,. Either this substance or paracyanogen might, therefore, be expected from hydrocyanic acid if the hydrogen were subjected to oxidation. For this purpose, 1 equiv. of ferricyanide of potassium was mixed with an equivalent of potash; and to this mixture was added an equivalent of hydrocyanic acid, the purpose being to liberate the cyanogen under favourable circumstances. The reaction expected was as follows :-

Fe,Cy,, 3K + KO + HCy = Fe,Cy&K + HO + Cy. On adding the hydrocyanic acid to the mixture of red prusside

and potash, the solution becomes yellow in colour, but soon passes to a dark red,- or even black colour,- a bulky precipitate of a similar colour falling dov\7n, During this action, a small quantity of gas is observed to escape. The formation of the precipitate is much favoured by the solutions being warm : when they are dilute, it is even necessary to raise them nearly to the boiling point.

The bulky reddish-black precipitate is collected on B filter, and washed with cold water, in which it is scarcely soluble. I t dis- solves readily in water containing caustic soda or potash, and may

* Ferricyanide of potassium may also be procured under the following circumstance, which deserves inquiry, as the reaction must be somewhat complicated. Common prusside of copper has the formula Fe2Cy,3CuK. When this salt is treated with Caustic ammonia in the cold, a very pale red-coloured solution is obtained, which, boiled with peroxide of manganese, gives ordinary ferricyanide of potassium Analysis gave the following results :-

I. 12-26 grains gave 3 00 peroxide of iron, and 9'605 sulphate of potash. 14.225 grains gave 3.49 peroxide of iron, and 11'19 sulphate of potash. 11.

I. 11. Calculated. Iron . . . 17.12 17.11 17.02 Potassium . . 35'11 3 5'2 6 35.56

VOL. IX.-NO. XXXIV. H

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130 DR. L. PLAYFAIR OX' FERRICPANIDE OF POTASSIUM, &C.

be purified in this way from a small quantity of oxide of iron which may fall down with it: it is again precipitated from the alkaline solution by the addition of an acid.

I t loses about 10% per cent. of water at 212O. Specimens obtained at dif- ferent times did not give constant results.

The precipitate contracts much in bulk daring drying.

First Preparation. Second Preparation. -- Third I. 11. I. 11, Preparation.

Carbon . 40*050 40.240 42.053 42.415 38.918

Hydrogen 3.400 3.393 3.324 3-318 3.252 Nitrogen . 41.618 - 42.772 - _.

- Oxygen . 14.932 - 11.851 - - -- 1 oo*ooo 1 oo*ooo

From these analyses it is obvious that the compound is not simply paracyanogen. Not only is the carbon in too large proportion to the nitrogen, but the hydrogen is more than sufficient to convert the oxygen into water. In the preparation of the substance, a little gas is always observed to escape,-due, probably, to the oxidation of a portion of the paracyanogen, The substance as prepared is therefore impure, and requires proper inquiry."

Not being able, without interfering with another investigation, to pursue the subject further, in the very limited time which I have at my disposal for scientific pursuits, Baron L i e b i g has kindly placed it in the hands of one of his pupils for investigation.

* I find that the same substance can be procured by passing chlorine through cyanide of potassium. Purified by solution in soda, precipitation by hydrochloric acid, and washing, two preparations dried at 212O gave thc following results on analysis :-

First Preparation. ----..

35.200 uarbon . . 36.22 Nitrogen. . - 42.300 Hydrogen . 3'150 3.000 Oxygen . . - 19'500

Pi

Second Preparation.

35'200

3.030 - 100'000

In this case, the carbon and nitrogen are nearly in the proportion to form cyanogen, but the hydrogen is in excess to-the oxygen 0; the supposition that it m s present as water. It should bc iixntioncd that, on trcating this black compound with nitric acid, n yc1low sobstancc is obtained, mlliclt, as I liave o~ i ly mndc one nndysis, 1: (lo not vcn t nre to (t escri bc.

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