Determination of cadmium in fertilizers by flow-injection spectrophotometry

A flow-injection procedure for spectrophotometric determination of cadmium in fertilizers based on formation of the blue ternary complex between cadmium, iodide and Malachite Green (MG) is proposed. Addition order and concentrations of reagents, interaction time between iodide and MG, flow rates, pH and temperature for ternary complex formation, surfactant addition, and presence of potential interferent ions were studied. A 30-s interaction between iodide and MG, a temperature of 20 °C and use of picolinic acid for copper masking were selected. Linearity of the calibration graph was improved with cadmium addition. For selectivity enhancement, masking agents, kinetic discrimination, and ion-exchange separation were investigated and a Chelex-100 resin minicolumn (50–100 mesh) was employed for in-line interferent removal. The proposed system is very stable and handles about 60 samples per hour (10.0–100.0 μg Cd l−1) corresponding to 0.105 mg MG and 166 mg KI consumed per determination. Linearity of the calibration plot is observed up to 200 μg Cd l−1, the detection limit being 2 μg Cd l−1. Results are precise (R.S.D. = 1.3% and 4.0% for 93.0 and 35.3 μg Cd l−1) and in agreement with tungsten coil atomic absorption spectrometry. Accurate results are also obtained for lobster and soya bean samples.