Process of producing Maddrell salt

In a process of producing Maddrell salt, including the steps of heating a starting material selected from the group consisting of mono-sodium orthophosphate, di-sodium pyrophosphate, and mixtures thereof, to a temperature between about 250.degree. C. and about 450.degree. C., whereby said starting material is converted to Maddrell salt, and recovering the Maddrell salt formed, the improvement which comprises carrying out said heating step with an admixture of said starting material and a phosphoric acid salt of a nitogen-containing base in a catalytically effective amount of between about 0.25% and about 5.0%, by weight, of the starting material, whereby the Maddrell salt containing less than about 2% water-soluble impurities is formed.