Sodium monofluorophosphate (I) was prepared by the reaction of sodium fluoride with various sodium phosphates, the reaction conditions and difference in the amount of I according to the starting material used were examined, and the results were as follows : (1) Although I is not formed by the reaction of NaF with sodium ortho-, pyro-, or tripoly- phospates, it can be produced from these phosphates when P
2O
5 was added, even at such a low temperature as 200°.(2) In the case of chain polyphosphates, the amount of I produced increases with the chain length, but its yield becomes almost constant when n^
- is longer than 12.(3) In case of ring phosphates, I is readily produced, and sodium tetrametaphosphate can give I in a better yield than sodium trimetaphosphate. (4) The reaction of a mixture of NaF, P
2O
5, and Na
2CO
3 gives I in a good yield, and a mixture of NaF and P
2O
5 also gives I but contaminated with impurities. These results show that the yield of I varies according to the phosphates used as the starting material, heating temperature, and heating time. The optimum condition for preparing I seems to be to heat for a short time (6 min) at a high temperature (700-800°) when long-chain (n^
-≥12) or ring phosphates are used, and to add P
2O
5 and heat for a long time (60 min) at a low temperature (300-500° in case of the orthophosphate, 600° in case of pyro- and tripoly-phosphates) when short-chain polyphosphates are used. Further, I can also be prepared by heating a mixture of P
2O
5, Na
2CO
3, and NaF.
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