Iodine can be determined in water supplies by utilizing its ability to catalyze the reduction of
ceric ions by arsenious acid, the affect being proportional, but not linearly, to the amount of iodine present.
Photometric determination of the loss of ceric ion color directly without recording device, since the color fades rapidly while it is being read in the photometer. If the reaction is stopped after a specific time interval by the addition of ferrous
ammonium sulfate, the resulting ferric ions, which are directly proportional to the remaining ceric ions, develop a color complex with potassium thiocyanate that is relatively stable. This
method has the advantages of requiring only a small untreated water samples, eliminating distillation procedures, minimizing certain interferences, and giving stable color for spectrophotometric determination.
Apparatus Water bath capable of temperature control to 30°±0.5°C Spectrophotometer Filter photometer Test tube ( 2x15 cm ) Stopwatch Reagents Distilled water Sodium chloride solution – dissolves 200.0 g NaCl in distilled water and dilute to 1L. Arsenious acid 0.1 N. Dissolve 4.946 g arsenious oxide, As2O3, in distilled water, add 0.20
ml conc. H2SO4. Ceric ammonium sulfate 0.02 N – dissolve 13.38g Ce(NH4)4(SO4)4.4H2O in distilled water, add 44 ml conc. H2SO4, and make up to 1 L. Ferrous ammonium sulfate reagent. Dissolve 1.50 g Fe(NH4)2(SO4)2.6H2O I 100 ml distilled water containing 0.6 ml conc. H2SO4 Potassium thiocyanate sol''n dissolves 4.00 g KSCN in 100 ml distilled water Stock
iodide sol''n dissolve 0.2616 g anhydrous potassium iodide in distilled water and dilute to 1 L Intermediate iodide sol''ñ dilute 20.0 ml stock iodide sol''ñ to 1 L distilled water Standard iodide sol''ñ dilute 25.0 ml intermidiate iodide solution to 1 L with distilled water.
PROCEDURE 1. Add 10.00 ml water sample, or an aliquot made up to 10.0 ml with iodine-free distilled water, to a 2 x 15 cm test tube. If possible, keep the iodine content of the diluted sample in the range of 0.2 – 0.6 µg. use the scrupulously clean glassware and apparatus. 2. add reagents to the sample in the ff. order: 1.00 ml NaCl solution, 0.50 ml arsenious acid solution and 0.50 ml conc. H2SO4 3. place the reaction mixture and the ceric ammonium sulfate solution in the 30°C water bath and allow to come to temperature equilibrium. Add 1.00 ml ceric ammonium sulfate solution, mix the contents of the test tube by inversion, and start the stop watch to time the reaction. Use an inert, clean test tube stopper when mixing. After 15 ± 0.1 min remove the sample from the water bath and add immediately 1.00 ml ferrous ammonium sulfate reagent with mixing, whereupon the yellow ceric ion should disappear;. Then add, with mixing, 1.00 ml potassium thiocyanate solution. Replace the sample in the water bath. Within 1 hr after the thiocyanate addition, read the red color as percent transmittance in the photometric instrument. Maintain the temperature of the solution and the cell compartment at 30° ± 0.5°C until the transmittance is determined. If several samples are run, start the reaction at 1 min intervals, to allow time for addition s of ferrous ammonium sulfate and thiocyanate. ( if the temperature control of the cell compartment is not possible, allow the final solution to come to room temperature and measure the transmittance with the cell compartment at room temperature) 4. treat standards containing 0.0, 0.2, 0.4, 0.6, 0.8 µg I per 10.00 ml of solutions. Run with each set of samples for the purpose of establishing a calibration curve.
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