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.
(K,Na,NH 4)H 5 (PO 4)2, are also known; these may be regarded as composed of a monometallic phosphate with phosphoric acid, thus M'H 2 PO 4 H 3 PO 4.
The monometallic salts are strongly acid, the dimetallic are neutral or faintly alkaline, whilst the soluble trimetallic salts are strongly alkaline.
The monometallic salts of the alkalis and alkaline earths may be obtained in crystal form, but those of the heavy metals are only stable when in solution.
They may be obtained by heating a monometallic orthophosphate of a fixed base, or a dimetallic orthophosphate of one fixed and one volatile base, e.g.