Exercise 40. Hydrogen Phosphate

The compounds may have either the formula H3PO4 (m.p., 40°) or 2H3P04-H20 (m.p., 30°). In a round-bottomed flask of suitable size, place 1 part of white phosphorus and 16 parts of nitric acid of sp. gr. 1.2 together with a few crystals of iodine as a catalyzer. Provide the flask with a reflux condenser, painting the cork with black asphalt paint to protect it from the action of the nitric acid. Heat until all of the phosphorus has dissolved then distill off most of the liquid and pour the residue into an open dish. Evaporate with several additions of concentrated nitric acid to oxidize all lower acids of phosphorus and finally concentrate in a platinum dish until the temperature of the solution reaches 150°. On cooling, if necessary with a freezing mixture, the liquid can be brought to crystallization. The crystals should be dried over phosphorus pentoxide 

References Mellor, VIII, 952 Smith and Menzies, J. Am. Chem. Soc., 31, 1183 (1909). 

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Why is phosphoric acid, H3P04, a stronger acid than disodium hydrogen phosphate, Na2HP04 Hint See exercise 59.)... 

Typical buffers employed commercially are borax, sodium hydrogen phosphate and sodium bicarbonate. Most latices are used at pH greater than 7.5. Where a buffer has been employed to keep the pH acidic during polymerisation, it is often necessary to adjust the final pH upon completion of the polymerisation. Ammonia is frequently added on completion of polymerisation to ensure the fimd latex is alkaline, but care must be exercised in its use since destabilisation of the latex can result. 

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