Disodium hydrogen phosphite

Phosphorus acid is a moderately strong dibasic acid. It reacts with alkalies forming acid phosphites and normal phosphites. Thus, reaction with sodium hydroxide gives sodium dihydrogen phosphite and disodium hydrogen phosphite, but not sodium phosphate, Na3P04. 

Sodium phosphites.—Disodium hydrogen phosphite, Na2HP03,5H20, is obtained by concentration of a solution of phosphorous acid neutralized with sodium carbonate, and over sulphuric acid changes to the anhydrous salt. The pentahydrate is very deliquescent. The anhydrous salt melts at 53° C., and above this temperature is oxidized to sodium phosphate, with evolution of phosphine.7 The heat of formation from the elements is 285 1 Cal.,8 and from the acid and sodium hydroxide in solution 28 45 Cal.9 The heat of hydration of the anhydride to the pentahydrate is 18 7 Cal. 

The trisodium salt is best made by careful dehydration of an equimolar mixture of hydrated disodium hydrogen phosphate and sodium hydrogen phosphite at 180° ... 

Trisodium phosphate Disodium hydrogen phosphate Monosodium dihydrogen phosphate Disodium phosphite or disodium phosphonate Sodium hydrogen phosphite or sodium hydrogen phosphonate Sodium bisidfate or sodium hydrogen sulfate Sodium bicarbonate or sodium hydrogen carbonate... 

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