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

归东理工大军 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; To know about the application of oxidatioton titration. 2022/11/19

Analytical chemistry Chapter 6 Oxidation-reduction titration Learning objectives ØTo master the fundamental principles and relevant calculations about oxidation-reduction equilibrium; ØTo master the calculation of titration curve, the end point and the titration and the titration break; ØTo know about the application of oxidation-reduction titration. 2022/11/19

©归东我子大 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. 2022/11/19

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. 2022/11/19

归东理工大军 Analytical chemistry 6.1 Redox reaction 1.Conditional potentials A redox reaction often can be arranged as a cell,such as: Zn ZnSO (x mol/L)CusO(y mol/L)Cu At anode,Zn is oxidized Zn-2eU Zn2 At cathode,Cu2+is reduced Cu2 +2eU Cu 2022/11/19

Analytical chemistry 6.1 Redox reaction 1. Conditional potentials A redox reaction often can be arranged as a cell, such as: At anode, Zn is oxidized At cathode, Cu2+ is reduced 2022/11/19

G山东我子大写 Analytical chemistry The electromotive force of the cell can be obtained from the difference between the potentials of the two electrodes E=j.j(cuicu)-(ngn) The potentials of the electrodes is obtained by Nernst equation i=i+ RIIn ox nF a 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. 2022/11/19

Analytical chemistry The electromotive force of the cell can be obtained from the difference between the potentials of the two electrodes The potentials of the electrodes is obtained by Nernst equation 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. 2022/11/19

G山东子大图 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 HCl. /-/0+0.0591g4e -j°+0.0591 g Se1 8e[Fe2+] In the HCI solution,ferric ion complex with H,O and Cl as follows 2022/11/19

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 HCl. In the HCl solution, ferric ion complex with H2O and Cl- as follows : 2022/11/19

G山东我子大写 Analytical chemistry Fe3+H,OO [Fe(OH)]2 +H" Fe3+CI0 [FeCI]2 M Therefore,the total concentration of Fe()in the solution may be expressed as c(Fe)=Fe +[Fe(OH)+[FeCl]+L [Fe*] Where a is the side reaction coefficient of Fe3+,similarly,to the Fe2+ 2022/11/19

Analytical chemistry Therefore, the total concentration of Fe(Ⅲ) in the solution may be expressed as Where α is the side reaction coefficient of Fe3+ , similarly, to the Fe2+ 2022/11/19

归东理工大军 Analytical chemistry CFe [Fe2] =2 Taking the activity coefficient and the side reaction coefficient into account,we have i=j9+0.0591g e[Fe] 8e[Fe2+] =j9+0.0591g9 Fe2-a 2022/11/19

Analytical chemistry Taking the activity coefficient and the side reaction coefficient into account, we have 2022/11/19

G归东我工大图 Analytical chemistry When c(Fe3+)=c(fe2+)=1 mol/L The equation may be expressed as 7=j+0.0591g3:0心=j Where 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. 2022/11/19

Analytical chemistry When c(Fe3+)=c(fe2+)=1 mol/L The equation may be expressed as Where φ0 ' 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. 2022/11/19

山东工大军 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 coupeng

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 couple. 2022/11/19

©山东我工大 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 2022/11/19

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. 2022/11/19