Study Of Determination Of Trace Iodine And Ceric-Arsenite Catalytic Dynamics

Iodine has important significance in oceanography and the physiology. Its existence is instable and the concentration is always low, therefore, to establish a fast, sensitive, and accurate method of determination iodine is very significant. At present there are a lot of articles about determining iodine method, but every method has certain flaw. This article plans to use Ceric-Arsenite catalytic method to determine iodine concentration, and to enhance the sensitivity and the determination speed of determining iodine.The theory basis of iodine determination by Ceric-Arsenite catalytic potentiometric method is its pseudo-first order kinetics characteristic, so Ceric-Arsenite catalytic dynamics is studied firstly. To determine iodine in the fresh water, then the detailed experiment technique is researched. The research techniques and the conclusions are as follows:1. The standard method-Ceric-Arsenite Catalytic Spectrophotometric Method comparing with potentiometric method have been used to determine the Ceric-Arsenite catalytic system. The change of reactant Ce⁴⁺ concentration vs. time with series of iodine concentrations has been determined, and the result obtained conforms to the first order kinetics equation: , and then the reaction rate constants are obtained. Comparing the curves of reaction rates calculated vs. iodine concentrations and the curves of absorbency values or potential differences obtained by fixed time method vs. iodine concentration, both have good linearity. Therefore, Ceric-Arsenite Catalytic potentiometric method has good dynamics foundation to determine trace iodine.2. With the concentration of H+ is 1.5mol·L^-1, changing the concentration of Cl- ,and then with the concentration of Cl- is 0.1 mol·L^-1, changing the concentration of H+, the curves of reaction system's potential vs. time with series of iodine concentrations have been determined. The results indicate that when the concentration of chloride ion is more than 0.3 mol·L^-1, the catalytic effect of iodine will be suppressed, and that when the concentration of Cl- is 0.1mol·L^-1, the concentration of hydrogen ion is 1.5mol·L^-1, the determination accuracy and sensitivity are high. The condition is chosen to determine iodine by this method.3. The working electrodes such as gold electrode, platinum plate electrode and glass carbon electrode have been researched to choose a proper one. The change of potential with time of the electrodes are determined separately in a certain concentration of cerium(â…£) solution. Potential stability of the gold electrode is bad, and it's not suitable for the determination. Then the E～t curves of Ceric-Arsenite catalytic system are determined with platinum plate electrode and glass carbon electrode as working electrodes. The curve determined by platinum plate electrode has good linearity, but the changing rate of potential vs. concentrations of iodine curve is almost nonlinear, which can enlarge the determination error. The results obtained by glass carbon electrode are so good that it's suitable to serve as the working electrode by Ceric-Arsenite catalytic potentiometric method to determine iodine.4. The reference electrode is chosen from saturated Ag/AgCl electrode and Saturated Calomel Electrode. The potential of saturated Ag/AgCl electrode vs. SCE has been tested, and it's found that the change of potential of saturated Ag/AgCl electrode with the room temperature in the determination process produces error. Therefore SCE with double salt bridges was chosen as the reference electrode.5. When the concentration of chloride ion is 0.1mol·L^-1, hydrogen ion is 1.5mol·L^-1, the catalytic system with potassium iodide and the potassium iodate as catalyst have been tested separately. The result indicates that iodate can be reduced to iodide completely under this condition, and the contrast experiments indicate that iodate can't be reduced completely with lower acidity. To improve the determination speed, reduction time of potassium iodate was changed, it's found when keeping the reduction time 10min after constant temperature controlled the determination result is best. To optimize the operation, the reduction time is settled 15min including adding in fluid time and controlling constant temperature time.6. After the experimental technique is determined finally, tap water samples are measured. The mean determined concentration of iodine is 4.4μg·L^-1, and the relative standard deviation is 2.7% with 6 parallel samples, indicating the precision is good. The coefficients of recovery of the samples are between 94%¹06%, and the average value is 101%, indicating the accuracy is good. The limit iodine concentration can be determined accurately is 2μgI·L^-1, and the determination range is 2 to 300μgI·L^-1, indicating the determination linear range of this method is wide.