"As to what relates to chemical types, which M. Dumas says I do not admit, I dare hope that the Academy, and all enlightened men will not participate in this opinion; for it will without doubt be allowed that he who first classed chemical types, must necessarily have admitted them, even before M. Dumas had acquired any notion of them, as his memoir appears to show, since he makes this notion only go back to his experiments on chloracetic acid, the discovery of which is posterior to my thesis."

M. Dumas replies, that his memoir is conceived in such terms, that it should have spared the Academy all the reclamations of which it has been the subject. In a historical note which he intends soon communicating to the Academy, he will show in what the views which are represented as identical differ, and to whom belongs the discovery of each of the principal points of the theory.

XXVII. On the Ferrosesquicyanuret of Potassium. By ALFRED SMEE, Esq., Surgeon.*

THE action of chlorine upon the ferrocyanate of potassium

is a subject of much interest to the chemist, and has not been examined to any extent in this country. It therefore has been my endeavour to investigate this action carefully, and to see under what circumstances the change from the ferrocyanate into the ferrosesquicyanuret takes place; and the methods which are here detailed to obtain this latter salt uncontaminated with impurities, will be found free from the difficulties and uncertainties attending on the present mode of preparing it.

When a current of chlorine is passed through a solution of ferrocyanate of potassa, or an aqueous solution of that gas is added to it in certain quantities, the persalts of iron are not precipitated. This solution has no smell of chlorine, and is changed from a yellow colour to a dark red, and deposits on evaporation red crystals. A similar change takes place when bromine is added to the ferrocyanate, and in both cases the weight of the entire red mass is equal to that of the yellow ferrocyanate, plus the weight of the chlorine or bromine used, but minus the quantity of water which the yellow crystals are known to contain. This indicates, first, that the red crystals are anhydrous; and secondly, that the chlorine or bromine is actually absorbed by the salt. The former fact is con

* Read before the Royal Society, June 18, 1840; and now communicated by the Author. Phil, Mag. S. 3. Vol. 17. No. 109. Sept. 1840.

firmed by heating the red precipitate in a test tube, when no water is given off; and the latter fact is also proved by the evolution of chlorine or bromine, on the addition of two or three drops of strong heated sulphuric acid to a few grains of red salt.

When heated alcohol is added to this red mass a small portion is dissolved, which is again deposited when the spirit is evaporated. This salt by its characters is known to be either the bromide or the chloride of potassium. By this method the red ferrocyanate of potassa, which is insoluble in alcohol, becomes purified; but this is a troublesome and expensive process, as the bromide or chloride is but little soluble in the spirit, and therefore a large quantity must be used.

About half an equivalent of chlorine or bromine is required to effect this change, and great care must be employed to prevent excess of these substances, as they are apt to react upon a portion of the salt. The liquid in this case contains Prussian blue dissolved, which materially discolours the salts, and it can only be precipitated from the solution by the addition of neutral salts, as sulphate of soda, which renders the red ferrocyanate impure. In a similar manner, chloride of soda, as might be expected, forms the red ferrocyanate of po

tassa.

From the foregoing details a knowledge is obtained of the action of chlorine and bromine upon the ferrocyanate, for we have seen that chloride and bromide of potassium is formed, and that one half an equivalent of these substances is necessary for this change. Now it is manifest that half an equivalent of potassium is removed from the ferrocyanate, so that the new salt, instead of consisting of iron one equivalent, potassium two equivalents, cyanogen three equivalents, contains iron one equivalent, potassium one and a half equivalent, cyanogen three equivalents; and therefore it is rightly named the ferrosesquicyanuret of potassium: that half an equivalent of potassium has been removed from the salt, two or three experiments have verified.

The acids as a class will not effect a similar change, because as they combine not with potassium but with potassa, water must be decomposed, the oxygen uniting with the metal, and the hydrogen passing to the ferrocyanate, forming hydroferrocyanic acid.

A question naturally arises whether the potassium may not be removed from the ferrocyanuret by other processes, and we are led to try the action of the anions, and of these I attempted to add oxygen to the salts by the use of nitric acid. This acid, when added in small quantities to the yellow ferrocyanate, acts as the other acids by liberating hydroferrocyanic acid, which is speedily decomposed into a pale blueish cyanuret of iron. When, however, further additions of this acid are made, the potassium takes oxygen, forms potassa, deutoxide of nitrogen is evolved, and the solution becomes dark coloured. This liquor, when neutralized with potassa, is found to give no precipitate with the persalts of iron, but forms Prussian blue with the protosalts of that metal. The rapidity of this change depends upon the heat of the solution, for when warm the effect takes place immediately, whilst on the contrary, two or three days are required at a low temperature. When evaporated, a large quantity of nitrate of potassa is deposited; and lastly some red crystals are formed. When acid is more used, the ferrocyanate is totally decomposed; the black mass which is the result has at first a sweet, but afterwards leaves a disagreeable metallic taste upon the palate. This process can never be used advantageously to form the ferrosesquicyanuret, from the quantity of acid which is required, the degree of nicety which must be employed to effect the change, and the impurity of the salt when obtained.

The next highly oxygenated acid which we have to examine is the iodic; this when added to ferrocyanate of potash becomes decomposed, the oxygen passes to the potassium to form potassa, free iodine is evolved, and the potassa passes to another portion of iodic acid, and is precipitated as the iodate of potassa. The free iodine can be readily removed by agitation with a little æther, and in this way a tolerably pure ferrosesquicyanuret of potassium can be extemporaneously obtained, for the solution contains but little iodate of potassa from its insolubility*.

Chloric acid operates in the same way as iodic acid, but is more difficult of decomposition, and it requires the action of heat before the smell of chlorine is exhaled and the red ferrocyanate formed.

If chlorate of potassa be added to the ferrocyanate, and dilute sulphuric acid be dropped into the solution, red ferrocyanate of potash will also be formed.

Bromic acid will not act upon the ferrocyanate with the production of the ferrosesquicyanuret, but acts as other acids in forming Prussian blue.

A great variety of other oxyacids have been tried, but none were found to part with their oxygen.

When a large quantity of peroxide of manganese in fine • This elegant process can be employed with advantage when a small quantity of the salt is suddenly wanted, as it scarcely requires a minute to effect.

powder is added to a solution of the ferrocyanate of potash, and the mixture digested for a considerable time, the ferrocyanate becomes converted into the ferrosesquicyanuret, and on evaporation crystals of the most beautiful ruby red are obtained. The salt thus procured appears to be very pure.

If a little dilute sulphuric acid be added to the solution in conjunction with the peroxide of manganese, the action takes place more quickly, but sulphate of potassa is formed, which is a great disadvantage.

The last process in which nascent oxygen contributes to the formation of ferrosesquicyanuret of potassium, is, perhaps, one of the most elegant, efficient, and simple processes in the whole range of chemistry. This mode I was induced to follow from the consideration, that as nascent oxygen effects a change of the yellow to the red ferrocyanate of potassa, a similar change must be produced by its being subjected to a galvanic current. Accordingly some solution of the salt was placed in a tube bent like a syphon, and at the bottom a piece of tow was thrust, in order that a separation might so far be effected, that the solution on one side could not readily pass to the solution on the other. Having thus completed the arrangement, a galvanic circuit was passed through the fluid; when at the cathode, hydrogen was evolved, and at the anode no oxygen, on the contrary, was given off, but the solution became of a dark colour. The dark solution was found to precipitate only the protosalts of iron, and on evaporation deposited red crystals of the ferrosesquicyanuret, but at the cathode potash was discovered. The rationale of this change may be deduced from circumstances attending slight alterations of arrangement; for if on the zinc side of the bent tube a saturated solution of the ferrocyanate be placed, and on the platinum side distilled water, and then the galvanic circuit be completed, potash will appear at the platinode, and red ferrocyanate at the zincode. On the contrary, if the distilled water is placed at the zinc side and the ferrocyanate at the platinum side, potash is left at the platinode, whilst at the zincode no red ferrocyanate is found, but a substance which does not redden litmus paper, and which speedily decomposes into Prussian blue; this is probably ferrocyanogen. Thus it appears that one equivalent of the yellow ferrocyanate is decomposed, the free potash travels one way and the hydroferrocyanic acid the other; the oxygen unites with the hydrogen of the acid and sets ferrocyanogen at liberty; this again unites with an equivalent of ferrocyanuret of potassium to form the ferrosesquicyanuret.

Various other attempts were made to form the red ferrocyanate by oxygen, such as heating it with nitrate of potassa, but the mixture exploded at a temperature below redness.

When a mixture of powdered ferrocyanate and peroxide of manganese were heated together no ferrosesquicyanuret was formed. Several other oxides, as those of mercury, silver, tin, iron, &c. &c., were digested with ferrocyanate of potassa, but none that were tried, except the peroxide of manganese, formed the red ferrocyanate; many of them were converted into cyanurets.

A current of oxygen gas passed through the solution of the salt produces no alteratiou, showing that the gas must be in a nascent state to cause the change.

The next substance we have to examine is phosphorus, and its action is somewhat remarkable; for little or no change is effected by the addition of an alcoholic or ætherial solution of phosphorus. When a piece of phosphorus is also placed in a solution of the ferrocyanate, or when phosphorus is heated with powdered ferrocyanate, the sesquicyanuret is not produced; but if a stick of phosphorus is placed in a bottle containing a solution of the salt, and only a portion of it is covered with the liquor, the phosphorus gradually burns away, the solution becomes sour and red, and ceases to precipitate the persalts of iron. This change takes place with a rapidity exactly proportionate to the wasting of the phosphorus; for if the temperature is below 45°, but little action takes place, but above 60° the reddening is very speedily produced. The red solution is not to be tested with the salt of iron whilst it is acid, for in that case a copious greenish-white precipitate is produced of phosphate of iron; but after it has been neutralized with potassa a solution of baryta is to be added, to throw down the phosphate, and a drop of dilute sulphuric acid may then be added to remove any excess of baryta.

The solution will now be found not to precipitate persalts of iron, but, on the contrary, a large quantity of Prussian blue is produced with the protosalts. The actual combustion of the phosphorus seems essential to this change; for if the water in which phosphorus has been allowed to burn, be added to the solution of the ferrocyanate, a similar change will not be produced. The cause of this change appears paradoxical, for phosphorus has in other instances a deoxidizing agency, so that a piece placed in a solution of either gold, silver, platinum, or copper, has the metal precipitated upon it. Perhaps it depends upon decomposition of water and the formation of phosphuretted hydrogen; for a narrow bottle, to which air has but limited access, is more favourable to the change

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