The potassium iodide-rhoda mine B spectrophotometric method was applied to estimate the cadmium content in waste water. The ternary complex can be formed by Cd2+, potassium iodide and rhoda mine B in acidic conditions. The influences of acidity, developer dosage, developing time and temperature on absorbance were studied. The optimum conditions are the amount of 0.5 mol/L sulphuric acid solution was 5 mL, the concentration of potassium iodide was 6.0 mL, the developing time was 50 min and the temperature was at the room-temperature. Under the optimum conditions, the relative standard deviations of the test results are＜5%, and the recoveries of standard addition are in the range of 89.6%～95.7%. For its convenience, low cost and accuracy, this method can be used to estimate the trace cadmium in wastewater.
