0.2mM were stable and sufficiently concentrated to give quantitative results. The thiocyanate concentration was found to be important in driving the reaction to completion and affected the rate of complex formation. Low absorbances observed at concentrations less than about 0.2M might, in part, result from the decomposition of I(SCN)zto form the non-absorbing ISCN (IO).Hydrogen ion concentrations were about 1M in all cases.Lower concentrations caused a slight decrease in stability and higher concentrations (above 2-3M) led to the decomposition of thiocyanic acid (9).Absorbance measurements on thirty separately prepared solutions are shown in Table I. Precision of replicate measurements is good and the overall accuracy has a range of about f3%. A linear least squares fit to the data gives a slope of 41100 f 200 1. mol-' cm-l and a y-intercept of Possible interferences from extraneous ions were examined. Table I1 summarizes these observations. In each case, two solutions of identical iodide concentrations were compared with an extraneous ion added to one. These were carried through the procedure in parallel.Clearly, chloride presents no serious interference at the levels examined and bromide lowers the absorbance only a few percent. The fact that the chloride interference does not change with chloride concentration suggests that the 2% difference in absorbance may be a measurement error rather than an interference. Sulfate, a frequent interference in iodide analyses, causes a small decrease in absorbance.The transition metal ions interfered to varying extents. Since thiocyanate complexes form with these cations, interferences are to be expected.-0.003.