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 orientation of higher substitution derivatives is determined by considering the diand tri-substitution compounds into which they can be transformed.
The action of chlorine upon diand tri-oxybenzenes has been carefully investigated by Th.
The nitro group has a very important action mainly on account of the readiness with which it can be introduced into the molecule, but its effect is much less than that of the azo group. The colour produced is generally yellow, which, in accordance with a general rule, is intensified with an increase in the number of groups; compare, for example, mono-, diand tri-nitrobenzene.
The reaction proceeds in several stages, mono-, diand finally tri-nitrate being produced, the final stage requiring sulphuric acid as a dehydrator.