Study On The Mechanism Of Indirect Iodine Kinetic Spectrophotometry And Its Application

Vitamin C(Vctim C),also known as L-ascorbic acid(L-ascorbic acid),has enol hexose acid lactone structure.The two enediol(ene glycol)groups of the molecular can release the H+ very easily,which makes vitamin C has reducing action.Vitamin C is a water-soluble vitamin and it can be decomposed easily in water or alkaline condition but done slowly in weak acid solution.It is widely found in plant cells,participating in plant antioxidant,photoprotection,being cofactor of reductases and increasing stress resistance,etc.Plant Vitamin C is the main source of vitamin C in the human body.Due to the instability of vitamin C,rapid analysis is needed in the determination of plant vitamin C to improve the efficiency of analysis and reduce the analysis error.Recent main determination methods of plant vitamins C are titration,electrochemical proces,chromatographic analysis,spectrophotometry,and kinetic analysis,etc.,but due to the various type of plant products,these methods have their own limitations,which can not be used to analysis a variety of plant vitamins C content.In this study,by vitamin C,potassium iodate,sulfuric acid,starch composition reaction system by Spectrophotometry of kinetic spectrophotometric analysis at 600nm,the chemical reaction mechanism of the reaction system is studied.Through the study on the reaction mechanism,the reaction process of the system of vitamin C and the related measurement method of quantitative analysis are established and it can be applied in the study on the vitamin C content of plants.The main research contents and results are as follows:1.The reaction process of the kinetic reaction system which consists of Vitamin C,potassium iodate,sulfuric acid,starch is divided into three stages(with A,B,Csaid).Stage A is divided into three steps(indicated by ???),?(?)(?).It is the step to determine the rate,which contains three double electron transfer process:(?)H2O(Determine the rate),(?)+ 3H2O,it contains the reaction pathways:21-+ IO3-+ 2H+ ?2IOH + IO-(slow),IO-+H+?IOH(quick),I-+IOH + H+?I2+H2O(quick),?(?).The kinetic study shows that the reaction rate equation of stage A is v=k·c1.04(KIO3)·c1.06(VC)·c1.05(H+).When vitamin C is completly consumed,the stage A ends and comes into the process of the stage B which contains the reaction pathways:2I-+ IO3-+ 2H+?2IOH + IO-(slow),IO-+ H+?IOH(quick),I-+ IOH + H+?I2+ H2O(quick).It also shows that the reaction rate equation of stage B is v=k·c1(IO3-)·c2(I-)·c2(H+).The C stage reaction is I2+H2O?HIO + H+ + I+,the Iodine-starch clathrate compound disintegrates.2.The stoichiometrie equation of the A stage reaction process of the kinetic reaction system consisted by Vitamin C,potassium iodate,sulfuric acid,starch is,stoichiomettric relationship between vitamin C and I-is 1/3nvc=n1-.The stoichiometric equation of the stage B reaction process is IO3-+51-+6H+?3I2+3H2O.The stoichiomettric relationship between the I-and I2 in this stage is nI-=5/3nl2.There is an Amax value at the end of the stage B,Amax?c(VC).It is concluded that Amax has a definite function relation with VC,which has the quantitative analysis significance of vitamin C.3.In this study,the concentration parameters of the reactants in the quantitative analysis of vitamin C are optimized.It shows the suitable concentration of each reaction was:the concentration of oxalic acid was 0.04%,the concentration of sulfuric acid was 0.075 mol/L,the concentration range of potassium iodate was 0.075-0.187 mol/L,and the concentration of vitamin C was 1.38 × 10-4-4.82 ×10-4 mol/L.4.The method of indirect iodometric kinetic spectrophotometric for the determination of vitamin C was established,which shows that the linear relationship between Amax and vitamin C concentration was r>0.999,and the linear range was 1.38×10-4-4.82×10-4 mol/L.5.By measure the vitamin C in the plant with the help of Indirect iodometric kinetic spectrophotometric method,it indicates the dynamic process of analysis samples of vitamin C in plants consistent with the standard solution of vitamin C in this experimental research and no effect on the determination for many other substances in plants.Through the,comparative study of different methods,the accuracy of iodometric kinetic spectrophotometric method is better than 2,6-dichloroindophenol titration method,indicating that the method can be used for the determination of plant vitamin C.The method is applied to other areas and the results are equally applicable.