Colloidal Arsenic

Arsenic sols may be prepared by the reduction of arsenic compounds under suitable conditions. For example, a mixture containing 1 gram of arsenious oxide and 8 c.c. of hydrochloric acid (D = 1.182) is shaken until dissolution is complete, and then poured into 50 c.c. of warm Paal's liquid acidified with acetic acid. A slight excess of hypophosphorous acid is added and the mixture kept on a water-bath for about 15 minutes until reduction is complete. After cooling, sodium carbonate is added to cause complete precipitation, and the precipitate separated by centrifuging. It is then redissolved in 20 per cent, aqueous sodium hydroxide, and on evaporation a water- soluble product containing 23 to 27 per cent, of arsenic remains. The hydrosol is purified by dialysis; it is negatively charged and fairly stable in air.

Stable hydrosols may be obtained similarly by reduction of arsenious oxide, dissolved in aqueous sodium hydroxide containing some other protective colloid such as gelatin or egg-albumin, by means of alkaline pyrogallol. Salts of metallic acids, such as sodium antimonate or calcium plumbate, with or without the addition of protalbic acid, may also be employed as protective colloids.

Colloidal arsenic is also formed by electrolytic reduction of a cold alkaline solution of arsenious oxide using a platinum cathode and a mercury anode, a small current density being employed; a trace of protective colloid should be present and the solution purified by dialysis. Hydrosols may also be obtained by subjecting dilute aqueous solutions of arsenious oxide to a silent electric discharge in a current of hydrogen, or of a mixture of hydrogen and arsine. Alcosols of arsenic have also been obtained by the latter method, replacing water by ethyl, butyl or amyl alcohol.

The hydrosol is generally brown in colour, and has little toxicity for animals. The element may be precipitated by addition of alcohol or acetone. A sol containing a protective colloid is not affected appreciably by the addition of hydrochloric or sulphuric acid or of sodium or barium chloride; sodium hydroxide or carbonate causes the colour to change to light yellow. Sols having a yellow to bluish-violet colour have been prepared by passing pure arsine through water exposed to light of short wavelengths; the particles are negatively charged and are little affected by electrolytes.

Stable sols of arsenic in glacial acetic acid have been prepared by dissolving yellow arsenic (0.12 g.) in carbon disulphide (333 ml.) and pouring the solution into pure anhydrous glacial acetic acid (667 ml.). The stability of the sols, which are yellow in colour, is decreased on addition of a protective colloid (0.1 g. per 1.), and also on attempting to remove carbon disulphide by dialysis or by heating.