Phosphates Orthophosphates European Pharmacopoeia

Phosphorus is capable of making many different oxoacids of which some of most important are orthophosphoric acid, Ff3P04. diphosphoric acid, 4 207, which is a dimeric example of the series of orthophosphoric acid anhydrates called pyrophosphoric acid. Metaphosphoric acid is a polymeric form of phosphorous acid, HPOg. Since these substances, and the other oxoacids, are unstable and fairly easily convert into orthophosphoric acid is by far the most widely encountered, and for the same reason it is normally referred to as just phosphoric acid without the prefix ortho. 

Orthophosphates can be regarded as a series of corresponding salts of phosphoric acid. 

With the exception of lithium and ammonium all alkali metals and the primary salts of the alkali earth metals form water-soluble salts. All other salts are water insoluble. 

This monograph contains two tests. Test (a) is a silver salt precipitation of limited selectivity, and test (b) is a far more specific test that is based on a precipitation with molybdate and vanadate. 

In the first step of the analysis, 5 ml of the test solution is, following neutralization, added to 5 ml of silver nitrate solution Rl. This gives a yellow precipitate of silver phosphate. 

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