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1、Chapter Chapter Potentiometric Analysis.Potentiometric Analysis.1.7 1.7 Method for Determining the End Point of Method for Determining the End Point of potentiometric titration potentiometric titration Compared with traditional chemical titration,potentiometric titration does not require an indicato
2、r.Often used in chemical equilibrium constant is small,titrimetric jump is not obvious or colored test fluid,turbidity and so on.It is a titration analysis method to determine the end point of titration based on the jump of electromotive force during titration.There are usually three ways.DC millivo
3、ltmeterReference electrodesIndicator electrodeBurette Titration vesselElectromagnetic agitatorAgitator.E-V curve methodThe E(ordinate)and V(transverse)of titrant volume for the measured electromotive force jump range of the working battery were plotted,and the curve was E-V curve.The inflection poin
4、t of the curve is the volume corresponding to the jump site(the maximum slope of the curve)and the titrating volume Vep at the end of the titration.In mathematics,the inflection point of a curve(the intersection of the upper concave and the lower concave).Features:This method is simple,but easy to p
5、roduce human error,poor accuracy.E/V-V curve method(first order micro quotient method)In this method,the first order derivative curve is obtained by calculating the first order derivative E/V value of corresponding points in E-V curve,taking E/V as the longitudinal coordinate and V as the transverse
6、 coordinate.How to find E/V and V values?E ratio of the difference of the electromotive force of the working cell,which is measured twice,to the corresponding of the two titrant,is E/V,and the average of the V of the two titrates,which is adjacent to each other.The volume corresponding to the maximu
7、m value of the curve in the diagram is the terminal titration volume,Vep.The inflection point property:the first derivative reaches the maximum value.Features:Compared with the E-V curve method,the end point obtained by this method is more accurate.By calculating the second order quotient of the E-V
8、 curve,the second order quotient 2E/V2(vertical coordinate)is plotted against the titration volume V(horizontal coordinate)to obtain the second order quotient curve,and the intersection of the line between the highest and lowest points of the curve and the horizontal coordinate axis(the second-order
9、 quotient is zero)is the end point of the titration,and the corresponding volume is the end point titration volume Vep.Advantages:avoiding the trouble of graphing,by calculating the titration volume corresponding to the second order quotient equal to zero is the titration end point volume.In case of
10、 positive or negative sign change in the value of the second-order quotient,there must be such a volume between the two corresponding volumes,i.e.,the titration volume Vep,which can be calculated by the interpolation method.The second-order microquotient is calculated by:Examples:A standard solution
11、 of 0.1000 mol/L AgNO3 was prepared by using a silver electrode as the indicator electrode and a double-liquid saturated mercury electrode(SCE)as the reference electrode.For titration with Cl-test solution,the following original data are shown in the table below.The volume of AgNO3 standard solution
12、 consumed at the end point of titration was calculated by two-stage micro-quotient method.Drop volume(mL)Electric potential(V)Positive and negative mutation 2-point linear interpolationA two-point definite linear equation that calculates the value of x at y=0Solution:The original data are processed
13、by the second-order microquotient method as follows.(1)Calculate the first-order quotient and the second-order quotient E/V is obtained from the ratio of the difference between two adjacent titration cell potentials,E and the difference between the corresponding two titration volumes V,V can be obta
14、ined as the average of the two adjacent titration volumes;2E/V2 is obtained from the ratio of the difference between two adjacent E/V and the corresponding V in the first-order quotient method,and the corresponding V is the average of the two adjacent E/V corresponding V.(2)Calculation method to find the end-point titration volumeWhat is V when the positive and negative suddenly become 0?The explanation is over,thank The explanation is over,thank you!you!