A method for determining the total concentration of a metal ion originally present in a solution, ctot, and the ratio of total to free metal ion, ZM, is proposed. It consists of putting different aliquots of the sample (V) in contact with the resin (w) in batch, by keeping the acidity constant, and determining the concentration of the sorbed metal ion c. Then 1/c is plotted against V/w, and ctot and ZM are calculated from the straight line obtained. The chemical model that allows evaluation of the ratio of total to free metal ion is illustrated and discussed. Some synthetic systems are examined to demonstrate the validity of the procedure.