Prop. Insoluble in water and in dilute acids, except the oxalic, in solutions of which it dissolves freely when pure; oil of vitriol dissolves it to a white pasty mass, which is again precipitated of the usual blue colour by water; alkalies instantly decompose it, and so do red oxide of mercury and some other oxides when boiled with it; it burns in the air like tinder, leaving an ash of oxide of iron. It is not poisonous.
Pur., &c. The quality of Prussian blue may be estimated by the richness of its colour, and by the quantity of potassa or soda required to destroy this. If it effervesces with acids, it contains chalk; and if it forms a paste with boiling water, it is adulterated with starch. It is pure if, “after being boiled with dilute hydrochloric acid, ammonia throws down nothing from the filtered liquid.” (Ph. L. 1836.) It is distinguished from indigo by exhibiting a coppery tint when broken, but which is removed by rubbing with the nail.
Concluding Remarks. The commercial Prussian blue is not pure ferrocyanide of iron, but a mixture of this salt with varying proportions of the ferricyanide of iron and potassium, which also has a fine deep blue colour. The object in employing alum is to prevent or lessen the precipitation of oxide of iron by the free alkali in the blood lye, but a portion of alumina is in consequence thrown down with the blue, and tends to render it paler and increase the product. The quantity of alum employed may be varied according to the shades of the intended blue. Samples containing this contamination must not be employed medicinally. (See page 324.)
Prussian Blue, Sol′uble. Prep. 1. (Basic Prussian blue.) By adding a solution of protosulphate of iron to a solution of ferrocyanide of potassium; a bluish-white precipitate, turning dark blue by free exposure, is formed, which, after it has acquired this colour, is washed until it begins to dissolve in the water, and colour it blue; it is then either collected and dried, or is at once dissolved in pure water. This variety is not precipitated from its solution by alcohol.
2. (Ferrocyanide of potassium and iron.) By precipitating a solution of a sesquisalt or persalt of iron (as the persulphate, pernitrate, peracetate, or sesquichloride) with a stronger solution of ferrocyanide of potassium, so that the latter may be in considerable excess. A blue precipitate is formed, which is treated
as before. This variety is precipitated by alcohol. Both are freely soluble in pure water, but not in water which has the slightest saline contamination. Hence it is that lengthened exposure to the atmosphere and the use of the common steel pen causes the gradual precipitation of this substance from its solution when used as ink. See Writing fluids.
PRU′′SSIC ACID. See Hydrocyanic acid.
PSEU′DO-MOR′PHIA. A substance of little importance, occasionally found in opium. It differs from morphine chiefly in not decomposing iodic acid. It is said to contain nitrogen.
PTIS′AN. Syn. Ptisana, L. A decoction made of pearl barley, liquorice, raisins, and other like vegetable matters, either alone or so slightly medicated as to be taken as a common drink in fevers, catarrhs, &c. Those retained in English pharmacy have been already noticed. The French physicians often employ this form of medicine. The ’tisanes’ of the P. Cod. are numerous. See Decoction, Infusion, Julep, Tisane, &c.
PTY′ALIN. A peculiar animal matter, analogous to diastase, obtained from the saliva. It is soluble in water, but insoluble in alcohol.