Syrupus Ferri Iodidi

It is remarkable how many changes and reactions are met with in dispensing this preparation, which is universally 'official,' but differs in ferrous-iodide strength in different Pharmacopoeias. The following are a few prescriptions which have come under our notice:

Made with tap-water ferrous hydroxide is precipitated, and with any water containing oxygen ferric oxyiodide is thrown out slightly.

A reddish-coloured separation of ferric hydroxide rises to the surface and is dissolved by a drop or two of dilute hydrochloric acid.

Make this by dissolving the bromide in 2 ounces of simple syrup by the aid of heat and allowing to cool; then dissolving the calcium chloride in 1/2 ounce of water and 1/2 ounce of syrup, with enough dilute hydrochloric acid to produce a non-alkaline solution. Mix these solutions, and finally add sufficient concentrated liq. ferri iodidi (made with hypophosphorous acid as a preservative) to correspond with 4 ounces of syrup of iodide of iron, making up to 4 ounces with simple syrup.

This mixture darkens in colour, becomes red, and precipitates ferric oxyiodide (Fe2OI4) owing to absorption of oxygen. This must be prevented, otherwise free iodine will ultimately be liberated. Acid, hypophos. mxv. prevents the change.

Ferrous arsenate is precipitated, but not if there is a trace of hypo-phosphorous acid in the syrup.

It is obvious from these examples that the neutral ferrous-iodide syrup of the Pharmacopoeias is a mistake, for the changes referred to do not occur when the mixtures are compounded with syrup prepared from liquor ferri iodidi, a preparation which usually contains hypophosphorous acid to preserve it. Without this the ferrous iodide in the syrup is on the brink of precipitation. The following series of equations, suggested by Mr. D. B. Kidd, explain the precipitation in the first instance and re-solution by hypophosphorous acid:

(1) 2FeI2 + O = Fe,OI4.

(2) Fe2OI, + O2 = Fe,O2 + 2I2.

(3) 4I2 + 2H,PO3+ 4H2O = 2H3PO4 + 8HI.

(4) 2Fe2O3 + 4H3PO4 = 4FePO4 + 6H2O.

A point which students should note is that this cycle of reactions does not go on ad infinitum; it is exceedingly limited, for a small amount of hypophosphorous acid delays precipitation so much that its inhibitory action must be attributed to something else than its deoxidising power. One very important combination with syr. ferri iodidi (viz., with potassium chlorate) is referred to on page 376, as this incompatible is so serious that it should not be dispensed.