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Ammonia, a white precipitate soluble in excess, reprecipitated by sulphuretted hydrogen; ferrocyanide of potassium, a white precipitate; sulphuretted hydrogen, a white precipitate in pure and neutral solutions. Nitrate of baryta will show the presence of sulphuric acid, and nitrate of silver of hydrochloric acid. =Silver.= Nitrate of silver is a powerful irritant. Tests.
The sulphocyanide of potassium, KCsy, is prepared by fusing ferrocyanide of potassium, deprived of its water of crystallization, intimately mixed with half its weight of sulphur and 17 parts of carbonate of potassa.
Iron. 50 c.c. of the solution should not immediately be coloured blue by 0·5 c.c. of potassium ferrocyanide solution. Soap. Soap is a salt in the chemical sense, and this leads to a wider definition of the term "salt" or "saline" compound.
This is known also as the 'biologic, or Uhlenhuth's test. =Rust Stains.= These are yellowish-red in colour, and do not stiffen the cloth. The iron may be dissolved by placing the stain in a dilute solution of hydrochloric acid, when, on adding ferrocyanide of potassium, Prussian blue is produced. =Fruit Stains= are seldom so dark as blood-stains.
This amalgam you may apply with a cloth to any metal, even iron, though it be a rusty bar, and you have it neatly silvered over. This is the latest method, and that now in use. To a solution of sulphate of copper, add a solution of ferrocyanide of pottasium, so long as a precipitate continues to be formed.
The ferri or ferrid cyanide of potassium discovered by Gmelin is often, but improperly, termed red prussiate of potash. It is formed by passing a current of chlorine gas through a solution of ferrocyanide of potassium until the liquid ceases to give a precipitate with a salt of sesquioxide of iron, and acquires a deep, reddish-green color.
The molten mass, after having cooled, is exhausted with water, the solution evaporated to dryness, and extracted with alcohol, from which the crystals of the salt are separated by evaporation. It is also made by melting the ferrocyanide of potassium with sulphide of potassium.
The ferrocyanide of potassium is a chemical reagent of great value, giving rise to precipitates with the neutral or slightly acid solutions of metals, like the beautiful brown ferrocyanide of copper, and that of lead. When a ferrocyanide is added to a solution of a sesquioxide of iron, Prussian blue or ferrocyanide of iron is produced.
If the solution of cyanide of potassium that is used is strong, the greater portion of the ferrocyanide of potassium crystalises in the solution, and may be collected and preserved for use again. If the solution of cyanide of potassium used to dissolve the precipitate is dilute, it will be necessary to condense the liquor by evaporation to obtain the yellow prussiate in crystals.
Ferrocyanide of potassium is in large beautiful transparent four-sided tabular crystals, of a lemon-yellow color, soluble in four parts of cold and two of boiling water, insoluble in alcohol. Exposed to heat it loses three eq. of water, and becomes anhydrous; at a high temperature it yields cyanide of potassium, carbide of iron, and various gases.
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