280

ELEMENTS OF MATERIA MEDICA.

potassium or with some other metal (sodium or magnesium) it is impossi-ble to say with certainty.

Preparation.— There are two principal methods of procuring thissalt.

(a.) Process of the Pharmacopoeia , —- This consists in adding twoounces of iron filings to six ounces of iodine mixed with four pints ofwater, stirring them frequently with a spatula for half an hour. Apply agentle heat, and, when a greenish colour appears, add four ounces of car-bonate of potash, first dissolved in two pints of water, and strain. Washthe residue with two pints of boiling distilled water, and again strain.Let the mixed liquor be evaporated, that crystals may be formed.

The theory of this process is as follows:—One equivalent or 126 partsof iodine combine with one equivalent or 28 parts of iron. The resultingiodide of iron is decomposed by one equivalent or 70 parts of carbonateof potash, by which one equivalent or 166 parts of iodide of potassiumand one equivalent or 58 parts of protocarbonate of iron are procured.

REAGENTS.

PRODUCTS.

1 eq. IodideIron 154

1 eq.Carb te .Potash 70

\

1 eq. Iodine . 1261 eq.Iron . . 28

1 eq.Potassium 401 eq. Oxygen 81 eq.Carbon.A d .22.

1 eq. Piottnids Iron 36

1 eq. IodidePotas m . 166

1 eq. Carb te . Iron . 58

(b.) Dr. Turner's process .—This is the simplest, and I believe the mosteligible method. It consists in adding to a hot solution of pure potashas much iodine as the liquid will dissolve, by which means a reddish-brown fluid is obtained. Then pass hydrosulphuric acid through theliquid until it becomes colourless. Apply a gentle heat, to expel anyexcess of the acid ; filter to get rid of the free sulphur, and exactlyneutralize the free acid present, with potash ; then crystallize. Whenthe potash comes in contact with iodine two salts are formed, iodide ofpotassium and iodate of potash : the latter is decomposed by the hydro-sulphuric acid, the hydrogen of which forms water, by combining withthe oxygen of the iodate; sulphur is precipitated, and iodide of potassiumremains in solution. This process has been subsequently modified thus :Iodate of potash is decomposed by exposing it to a red heat in a platinumcrucible, instead o£iiydrosulphuric acid.

Properties. —This salt occurs in white, somewhat shining, transpa-rent, or semi-opaque cubes, or octahedrons. Its taste is acrid saline,somewhat similar to common salt: it is without odour. It is readilyfusible, and at a red heat volatilizes unchanged. It decrepitates whenheated, in consequence of the water which is mechanically lodged be-tween the plates of the crystal. Both water and alcohol readily dissolve it:it requires only two-thirds of its weight of water to dissolve it at 60° F.Iodine is readily dissolved by an aqueous solution of this salt.

Characteristics. —This salt is known to be an iodide by the follow-ing characters:— {a.) With a solution of bichloride of mercury it occa-sions a vermilion-red precipitate of the biniodide of mercury, which isvery soluble in excess of iodide of potassium, (b.) With acetate of leadit forms a yellow precipitate of the iodide of lead. ( c .) With the nitrateof silver, a yellow precipitate of the iodide of silver, (d.) With the pro-tonitrate of mercury or with calomel a greenish yellow precipitate of the