Citric acid, being tribasic, forms either acid monometallic, acid dimetallic or neutral trimetallic salts; thus, mono-, diand tri-potassium and sodium citrates are known.
The first set may be called monometallic, the second dimetallic, and the third trimetallic salts.
The soluble trimetallic salts are decomposed by carbonic acid into a dimetallic salt and an acid carbonate.
The three series also differ when heated: the trimetallic salts, containing fixed bases are unaltered, whilst the monoand dimetallic salts yield metaand pyrophosphates respectively.
If the heating be with charcoal, the trimetallic salts of the alkalis and alkaline earths are unaltered, whilst the monoand di-salts give free phosphorus and a trimetallic salt.
Microcosmic salt: MH 2 PO 4 = MP0 3 +H 2 0, (NH 4) NaHPO 4 = NaP03+NH3+H20; they may also be obtained by acting with phosphorus pentoxide on trimetallic orthophosphates: Na3P04+P205 = 3NaP0 3.
On boiling their solutions they yield orthophosphates, whilst those of the heavy metals on boiling with water give a trimetallic orthophosphate and orthophosphoric acid: 3AgP0 3 +3H 2 O=Ag 3 PO 4 +2H 3 PO 4.
On heating with an oxide or carbonate they yield a trimetallic orthophosphate, carbon dioxide being evolved in the latter case.