《分析化学》课程教学课件（PPT讲稿，英文）Chapter 7 precipitation titration

归东觅子大军 Analytical chemistry Chapter 7 precipitation titration The formation of a precipitate can be used as the basis of titration,provided that there is a suitable way of determining when a stoichiometric amount of titrant has been added.In this section we briefly introduce argenometry,a type of the precipitation titrations. 2025/4/3

Analytical chemistry 2025/4/3 Chapter 7 precipitation titration The formation of a precipitate can be used as the basis of titration, provided that there is a suitable way of determining when a stoichiometric amount of titrant has been added. In this section we briefly introduce argenometry, a type of the precipitation titrations

山东理子大写 Analytical chemistry 7.1 precipitation reactions The titrations of chloride,bromide,iodide,and other anions with silver (I are based on the formation of insoluble silver salts, such as: Ag+CI→AgCI↓ Ag+SCN→AgSCN↓ These procedures are termed argenometry

Analytical chemistry 7.1 precipitation reactions These procedures are termed argenometry. + →  + →  + + Ag SCN AgSCN Ag Cl AgCl - - The titrations of chloride, bromide, iodide, and other anions with silver (Ⅰ) are based on the formation of insoluble silver salts, such as:

G山东理王大子 Analytical chemistry Take the formation of AgCl procipitate as an example,its sulubility produce constant is Kp=ang da =[Ag']CI lYAgYa The solubility of AgCl precipitate is very small,and y are all approximate to 1,so K'sp =[Ag'][CI-] For the insoluble salt AB,if we take the side reactions into account,then 2025/4/3

Analytical chemistry 2025/4/3 Take the formation of AgCl procipitate as an example, its sulubility produce constant is + − + − + − = = A g Cl A g Cl s p K a a [Ag ][Cl ]  The solubility of AgCl precipitate is very small, and γ are all approximate to 1, so [Ag ][Cl ] sp + − K = For the insoluble salt AB, if we take the side reactions into account, then

归东我子大军 Analytical chemistry [A'] [B'] 二0B [A] [B] Where a is the side reactions coefficient,so the conditional solubility product is introduced: Kp'=[A'][B'] 2025/4/3

Analytical chemistry 2025/4/3 A B [B] [B'] , [A] [A'] = α = α Where α is the side reactions coefficient, so the conditional solubility product is introduced: ' [A'][B'] Ks p =

归东龙卫大写 Analytical chemistry 7.2 Titration curves Consider the titration of 0.1 mol/L KCI with 0.1 mol/L AgNO3 solution.A titration curve can be prepared by plotting pCl ys the volume of AgNO3. At the beginning of the titration,pCl=1 As the titration continues,part of Cl is removed from solution by precipitation as AgCl,and the pCl is determined by the concentration of the remaining Cl;the contribution of Cl from dissociation of the precipitate is negligible,except near the equivalence point. 2025/4/3

Analytical chemistry 2025/4/3 7.2 Titration curves Consider the titration of 0.1 mol/L KCl with 0.1 mol/L AgNO3 solution. A titration curve can be prepared by plotting pCl vs the volume of AgNO3 . At the beginning of the titration, pCl=1 As the titration continues, part of Cl￾is removed from solution by precipitation as AgCl, and the pCl is determined by the concentration of the remaining Cl- ; the contribution of Cl￾from dissociation of the precipitate is negligible, except near the equivalence point

G山东我子大 Analytical chemistry Before the stoichiometric point, [C]=G V)c-(cV)Ag V At the equivalence point,we have a saturated solution of AgCL. K'sp=[Ag*][CI-] [CI-]=[Ag']=Ksp =105mol/L pCl=5 2025/4/3

Analytical chemistry 2025/4/3 t cV cV V ( ) -( ) [Cl ] Cl Ag - + = − Before the stoichiometric point, At the equivalence point, we have a saturated solution of AgCl. [Ag ][Cl ] sp + − K = [Cl ] [Ag ] K 10 mol / L 5 s p − + − = = = pCl=5

©归东理大军 Analytical chemistry After the equivalence point,there is excess Ag+and the Cl concentration is determined from the concentration of Ag and Ksp [C1]= Ksp [Ag"] The figure below shows the complete titration curve. pM % 2025/4/3

Analytical chemistry 2025/4/3 After the equivalence point, there is excess Ag+ and the Cl￾concentration is determined from the concentration of Ag+ and Ksp. [Ag ] [Cl ] sp + − = K The figure below shows the complete titration curve

归东我子大？ Analytical chemistry 7.3 End-point detection Three techniques are commonly employed to detect the end point in precipitation titrations:potentiometric methods, indicator methods and light-scattering methods. 1.Mohr titration The Mohr titration for chloride was published more than a hundred years ago and is still used.A colored precipitate at the end point is formed in Mohr titration.In the Mohr titration, Cl is titrated with Ag+in the presence of CrO2. Titration reaction 2025/4/3

Analytical chemistry 2025/4/3 7.3 End-point detection Three techniques are commonly employed to detect the end point in precipitation titrations: potentiometric methods, indicator methods and light-scattering methods. 1. Mohr titration The Mohr titration for chloride was published more than a hundred years ago and is still used. A colored precipitate at the end point is formed in Mohr titration. In the Mohr titration, Cl￾is titrated with Ag+ in the presence of CrO4 2- . Titration reaction

©归东我工大军 Analytical chemistry Ag*+CI→AgCl↓(white) End-point reaction 2Ag*+CrO>Ag2 CrO(red) The AgCl precipitates before Ag,CrO.The color of AgCl is white,dissolved CrO2 is yellow,and AgCrO is red.The end point is indicated by the first appearance of red. For the Mohr titration,the pH should be controlled in the range6.5-10.5. If the pH is too low,the concentration of CrO2-is reduced by the equilibrium 2025/4/3

Analytical chemistry 2025/4/3Ag Cl AgCl white) + + - → （ End-point reaction 2Ag CrO Ag CrO red) 2 4 2- + + 4 → （ The AgCl precipitates before Ag2CrO4 . The color of AgCl is white, dissolved CrO4 2- is yellow, and Ag2CrO4 is red. The end point is indicated by the first appearance of red. For the Mohr titration, the pH should be controlled in the range 6.5-10.5. If the pH is too low, the concentration of CrO4 2- is reduced by the equilibrium

©山东我王大 Analytical chemistry 2CrO}+2H+→2HCr04→Cr0号+H,0 If the pH is too high,the concentration of AgOH may precipitate.About pH8 is ideal for the titration. The Mohr method is useful for Cl and Br,but not for I or SCN. 2.Volhard titration The Vohhard titration is actually a procedure for the titration of Ag".To determine Cl,a back titration is necessary.First,the Cl is precipitated by a known,excess quantity of standard AgNO3. 2025/4/3

Analytical chemistry 2025/4/3 2CrO 2H 2HCrO Cr O H2 O 2- 2 7 - 4 2- 4 + → → + + If the pH is too high, the concentration of AgOH may precipitate. About pH8 is ideal for the titration. The Mohr method is useful for Cl- and Br- , but not for I - or SCN- . 2. Volhard titration The Vohhard titration is actually a procedure for the titration of Ag+ . To determine Cl- , a back titration is necessary. First, the Cl￾is precipitated by a known, excess quantity of standard AgNO3