The elements of materia medica and therapeutics: Zinci oxydum. - Oxide of zinc.

The elements of materia medica and therapeutics
by Jonathan Pereira, M.D. F.R.S. & L.S.
Fourth Edition, enlarged and improved, including notices of themost of the medicinal substances in use in the civilized world, and forming an Encyclopædia of Materia Medica.
Vol. I.
London: printed for Longman, Brown, Green, and Longmans, Paternoster Row.
Formula ZnO. Equivalent Weight 40.5.

History. — The oxide was first prepared by Hellot in 1735. When obtained by burning metal in the air it has received various names, some of them of a fantastic nature; as nihil album, philosopher's wool or lana philosophica, pompholyx, and flowers or calx of zinc (flores seu calx zinci).

Natural History. — Oxide of zinc is found in America, mixed or combined with the sesquioxide of manganese, and constituting the red oxide of zinc of the mineralogist. It is also found in various localities, in combination with carbonic, sulphuric, or silicic acid.

Preparation. — All the British Colleges give directions for the preparation of this compound.

The London College orders of Sulphate of Zinc, lb. j.; Sesquicarbonate of Ammonia, ξviss.; Distilled Water, Cong. iij. Dissolve the Sulphate of Zinc and Sesquicarbonate of Ammonia separately, in twelve parts of the distilled water, and strain; then mix. Wash what is precipitated frequently with water; and, lastly, burn it for two hours in a strong fire.

The Edinburgh College employs of Sulphate of Zinc, ξxij.; Carbonate of Ammonia, ξvj. The process is otherwise the same as that of the London College.

By the mutual reaction of sulphate of zinc and sesquicarbonate of ammonia, sulphate of ammonia is formed in solution, and the basic carbonate of zinc 3(ZnO,HO) + 3(ZnO,CO2) precipitated. By the subsequent ignition the carbonic acid and water are expelled,

The Dublin College directs Oxide of Zinc to be prepared as follows: — Take of Carbonate of Zinc any convenient quantity. Place it in a clay crucible furnished with a cover, and expose it to a very low red heat, until a portion of the contents of the crucible, takes from its centre, ceases to effervesce on being dropped into dilute sulphuric acid.

In this process the carbonic acid is simply expelled.

A manufacturing chemist who prepares oxide of zinc (so called), stated that he obtained it from a solution of chloride of zinc, which he procureed from the workers of palladium. This liquid is boiled with small pieces of zinc and some caustic potash, to get rid of the iron; and to the clear liquid is then added a solution of carbonate of soda (soda ash), by which the white basic carbonate of zinc is precipitated. This is washed, dried, and sold as oxide of zinc

1 Pharm. Journ. 1849, p. 319.[Mr. Midgley1 has constructed an apparatus for the preparation of oxide of zinc by combustion, whereby he has overcome the difficulties previously experienced in carrying ou this processs. The apparatus consists of a large muffle heated to redness in a furnace: a tube passes from the top of the muffle through water in which that portion of the metal which is oxidised and carried into the air is detained. This gentleman states that the oxide thus produced is purer and more efficacious than that made by any of the processes ordered by the Pharmacopoeias. — Ed.]

Properties. — The form of the crystallised native oxide of zinc (containing the oxides of iron and manganese) is a right rhombic prism.

The artificial oxide of the Pharmacopoeia is a white, or, when ignited, yellowish-white, tasteless, odourless powder. It is fusible, forming a yellow glass, and at a white heat is volatilised. When heated with charcoal it is readily reduced. It is insoluble in water, but is dissolved by most acids and [in the hydrated state it is soluble in alkalies and in a strong solution of ses quicarbonate of ammonia. — Ed.] It forms two classes of salt: one (the zincic salts), in which it is the base; a second, [zincates) in which it acts the part of an acid.

Characteristics. — It dissolves in dilute sulphuric acid. The characteristics of the solution have been already detailed (see ante, p. 762).

Composition. — Oxide of zinc has the following composition:
 AtomsEq. Wt. Per Cent. Proust. Berzelius.
Zinc 1 32.5 84.25 80 80.1
Oxygen 1 8 19.75 20 19.9
Oxide of Zinc 1 40.5 100.00 100 100.0

Purity. — Pure oxide of zinc is completely and readily soluble in diluted sulphuric, nitric, or hydrochloric acid, without effervescence. The substance met with in the shops under the name of oxide of zinc is in reality a carbonate of this metal containing a variable quantity of oxide, and, therefore, it effer vesces on the addition of an acid. The solution obtained by dissolving the oxide in any of the above acids yields a precipitate, on the addition of caustic ammonia or potash, which should be completely soluble in an excess of the precipitant. The oxide of zinc of the shops sometimes yields traces of sulphuric acid when its solution in nitric acid is tested with a salt of baryta.

1 Thomson, Hut. of Chem. ii. 219.The oxide of lead or cadmium is sometimes found in it, and the oxide of cadmium was once mistaken for arsenious acid.1 Iron and manganese may be present in oxide of zinc, and communicate to it a yellow tinge The oxide is, —

A yellowish white powder soluble in ammonia, potash, and hydrochloric acid. — Ph. Lond.

White: tasteless: entirely soluble in diluted nitric acid without effervescence: this solution is not affected by nitrate of baryta, but gives with ammonia a white precipitate entirely soluble in an excess of the test. — Ph. Ed.

[When the oxide of lead or cadmium is present the precipitate first formed by sesquicarbonate of ammonia in the acid solution of the oxide is not entirely redissolved by an excess of the precipitant. If oxide of cadmium be the impurity, sulphuretted hydrogen water gives a yellow tint: if oxide of lead be present, this gas blackens the solution. It should oe remembered that the latter impurity may really be present in the sesquicarbonate, and not in the oxide of zinc, since leaden receivers are employed in the manufacture of the sesquicarbonate of ammonia. — Ed.]


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