《分析化学》课程教学课件（PPT讲稿，英文）Chapter 6 Oxidation-reduction titration

G归东理工大 Analytical chemistry Chapter 6 Oxidation-reduction titration Learning objectives >To master the fundamental principles and relevant calculations about oxidation-reduction equilibrium; >To master the calculation of titration curve,the end point and the titration and the titration break; Toknow about the application of oxidatioction titration. 2025/4/6

Analytical chemistry 2025/4/6 Chapter 6 Oxidation-reduction titration Learning objectives ➢To master the fundamental principles and relevant calculations about oxidation-reduction equilibrium; ➢To master the calculation of titration curve, the end point and the titration and the titration break; ➢To know about the application of oxidation-reduction titration

G山东第王大军 Analytical chemistry The word redox is frequently applied to oxidation- reduction reactions.A redox titration is based on an oxidation-reduction reaction between analyte and titrant. Even though many of the oxidizing agents can be used as titrants,few are primary standards.Most reducing agents react with oxygen and require protection from air to be used as titrant. 2025/4/6

Analytical chemistry 2025/4/6 The word redox is frequently applied to oxidation￾reduction reactions. A redox titration is based on an oxidation-reduction reaction between analyte and titrant. Even though many of the oxidizing agents can be used as titrants, few are primary standards. Most reducing agents react with oxygen and require protection from air to be used as titrant

归东工大军 Analytical chemistry 6.1 Redox reaction 1.Conditional potentials A redox reaction often can be arranged as a cell,such as: Zn ZnSO(x molL)CuSO(y mol/L)Cu At anode,Zn is oxidized Zn-2eZn2+ At cathode,Cu2+is reduced Cu2++2e→Cu 2025/4/6

Analytical chemistry 2025/4/6 6.1 Redox reaction 1. Conditional potentials A redox reaction often can be arranged as a cell, such as: Zn | ZnSO4 (x mol/L) ||CuSO4 (y mol/L) |Cu  2+ Zn - 2e Zn At anode, Zn is oxidized At cathode, Cu2+ is reduced Cu 2e Cu 2+ + 

山东理工大 Analytical chemistry The electromotive force of the cell can be obtained from the difference between the potentials of the two electrodes E=p,-0.=9au2+10w-9an2*a） The potentials of the electrodes is obtained by Nernst equation nFa°red So the potentials of the electrodes is determined by the activities of the oxidized and reduced forms.At the unit activities,it is the standard potential. 2025/4/6

Analytical chemistry 2025/4/6 The electromotive force of the cell can be obtained from the difference between the potentials of the two electrodes (Cu /Cu) (Zn /Zn) - E =+ - = 2+ − 2+ The potentials of the electrodes is obtained by Nernst equation Red 0 OX ln b a a a nF RT  = + So the potentials of the electrodes is determined by the activities of the oxidized and reduced forms. At the unit activities, it is the standard potential

归东王大军 Analytical chemistry In practice,however,some factors that affect the potential should be taken into account.For instance,consider the potential of the Fe3+/Fe2+couple in the presence of HCI. D=0'+0059g0 =p°+0.059 YraIF3] Yre2.[Fe2+] In the HCI solution,ferric ion complex with H2O and Cl as follows 2025/4/6

Analytical chemistry 2025/4/6 In practice, however, some factors that affect the potential should be taken into account. For instance, consider the potential of the Fe3+/Fe2+ couple in the presence of HCl. + + = + 2 3 Fe 0 Fe 0.059lg a a   [Fe ] [Fe ] 0.059lg 2 Fe 3 0 Fe 2 3 + + + + = +    In the HCl solution, ferric ion complex with H2O and Cl- as follows :

0山东龙工大 Analytical chemistry Fe*+H,OFe(OH)]2++H* Fe*+CI [FeCl]2+ Therefore,the total concentration of Fe()in the solution may be expressed as c(Fe3)=Fe3*+Fe(OH]2++[FeCI]2++. [Fe3+] Where a is the side reaction coefficient of Fe3+,similarly,to the Fe2+ 2025/4/6

Analytical chemistry 2025/4/6  Fe Cl [FeCl] Fe H O [Fe(OH)] H 3 2 2 2 3 + − + + + + +  +  + c(Fe3+ ) = Fe3+ +[Fe(OH)]2+ +[FeCl]2+ + Therefore, the total concentration of Fe(Ⅲ) in the solution may be expressed as + + = + 3 3 3 Fe Fe [Fe ]  c Where α is the side reaction coefficient of Fe3+ , similarly, to the Fe2+

©山东我卫大罩 Analytical chemistry [Fe2+] Taking the activity coefficient and the side reaction coefficient into account,we have 0=p°+0.059g"elFe3] ye2+[Fe2+] =0°+0.059g1e8e9e 2025/4/6

Analytical chemistry 2025/4/6 + + = + 2 2 2 Fe Fe [Fe ]  c [Fe ] [Fe ] 0.059lg 2 Fe 3 0 Fe 2 3 + + + + = +     Taking the activity coefficient and the side reaction coefficient into account, we have + + + + + + = + 2 3 2 3 2 3 Fe Fe Fe 0 Fe Fe Fe 0.059lg c c     

G山东茂子大图 Analytical chemistry When c(Fe3+)=c(fe2+)=1 mol/L The equation may be expressed as p=p°+0.0591g =0 Where oo'is called the conditional potential Obviously,the conditional potential of a couple is just the potential that is measured when the total concentration of both the oxidized and reduced forms are equal to 1 mol/L. Some factors may affect the conditional potential. 2025/4/6

Analytical chemistry 2025/4/6 When c(Fe3+ )=c(fe2+ )=1 mol/L The equation may be expressed as 0.059lg ' 0 Fe Fe 0 Fe Fe 2 3 3 2       = + = + + + + Where φ 0 ' is called the conditional potential Obviously, the conditional potential of a couple is just the potential that is measured when the total concentration of both the oxidized and reduced forms are equal to 1 mol/L. Some factors may affect the conditional potential

G归东我工大军 Analytical chemistry (1).Ionic strength effect For a solution with high ionic strength,the activity coefficient is much less than 1.in this case,significant error will be introduced if the concentration is used in place of the activity for calculation involving Nernst equation.Because the effect of the ionic strength is much less significant than the side reaction,and it is more difficult to correct,it can be neglected in approximate estimation. (2).Side reaction effect Precipitation and complex formed reaction are usually used to change the concentration of the oxidized or reduced form in a redox reaction,so as to change the conditional potential of the couBA

Analytical chemistry 2025/4/6 (1). Ionic strength effect For a solution with high ionic strength, the activity coefficient is much less than 1. in this case, significant error will be introduced if the concentration is used in place of the activity for calculation involving Nernst equation. Because the effect of the ionic strength is much less significant than the side reaction, and it is more difficultto correct, it can be neglected in approximate estimation. (2). Side reaction effect Precipitation and complex formed reaction are usually used to change the concentration of the oxidized or reduced form in a redox reaction, so as to change the conditional potential of the couple

归东理大 Analytical chemistry When a precipitating agent that can precipitate the oxidized form is added,the conditional potential of the couple decreases.If the reduced form precipitates,however,the conditional potential will increase. 2.Equilibrium constant The equilibrium constant indicates the ability of a redox reaction to proceed.If the conditional potential is employed,then we obtain a conditional equilibrium constant. n2OX +n Red2 =n2Red+n OX2 2025/4/6

Analytical chemistry 2025/4/6 When a precipitating agent that can precipitate the oxidized form is added, the conditional potential of the couple decreases. If the reduced form precipitates, however, the conditional potential will increase. 2. Equilibrium constant The equilibrium constant indicates the ability of a redox reaction to proceed. If the conditional potential is employed, then we obtain a conditional equilibrium constant. n2 OX1 + n1 Red2 = n2 Red1 + n1 OX2