Hydroxamic acids as colorimetric reagents

Thirty-three hydroxamic acids and three N-substituted hydroxamic acids or structurally similar compounds have been studied as possible colorimetric reagents for metal ions. They were tested with 78 ions under varying conditions of acidity and basicity. Aliphatic, substituted aliphatic, aromatic, substituted aromatic, and heterocyclic hydroxamic acids were represented in the compounds studied. An attempt was made to correlate the activity towards metal ions with variations in the molecular structure of the hydroxamic acids. The studies with C-substituted hydroxamic acids indicate that the preferential formation of a colour or a precipitate depends on pH, the solvent, and reagent concentration, and is not a function of the presence or absence of a substituent on the nitrogen atom. A number of the compounds offer promise of being useful colorimetric reagents under proper reaction conditions.