Preparation of silver hypophosphate
50g of good quality bleaching powder with a high content of available chlorine are mixed with 400ml water until no more dissolves. The insoluble residue is removed by suction-filtration and the filtrate cooled to 15-20°C by means of a little added ice. 0.5g of red phosphorus is now added to the mechanically stirred liquid and then a 25% solution of glacial acetic acid and water is added in dropwise from a burette. A turbidity, followed by the appearance of a white precipitate, indicates the onset of oxidation of the phosphorus. About 10ml of the acid mixture will usually be needed. The vessel containing the reaction mixture is now surrounded by an ice bath in order to maintain a temperature range of 15-25°C throughout the oxidation. Stirring is continued while 5g more of the phosphorus is added in 0.5g portions at such intervals that the solution stays within the temperature limits prescribed. Care should be taken to add each amount of the phosphorus quickly so that the chance of ignition due to escaping chlorine is minimized. The initial precipitate of calcium hypophosphate redissolves during the reaction as the solution becomes increasingly acidic. When all the phosphorus has been added an inert violet residue remains in suspension through the stirred liquid, and this is agitated for ten minutes more. The mixture is suction-filtered, warmed to 40 °C, and again repeatedly stirred while solid sodium carbonate is added in small portions until the liquid is neutral to litmus paper. About 18g of carbonate should be required. A heavy white precipitate of calcium hypophosphate, orthophosphate, and carbonate forms during the neutralization. The whole mixture is stirred for 30 minutes just below the boiling point and then suction-filtered. The residue is washed free of chloride ion with water, rinsed with acetone, and dried at 100°C. Yield about 15g (containing about 15% of calcium hypophosphate) The mixed calcium salt is dissolved in 3N nitric acid, using 15ml of the latter for every 2g of the material. The clear solution is diluted to double its volume with distilled water and 5g of silver nitrate (for each 2g portion of calcium salt) is dissolved in the liquid. The mixture is stirred until the beige precipitate settles out. This is filtered, the silver hypophosphate rinsed with acetone, and the product is dried in warm air. It is only slightly photosensitive and may be stored in clear bottles in artificial light for extended periods of time.
Inorganic laboratory preparations, by G. G. Schlessinger, 49, 1962