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Zn + X =ZnX H Cd CdI i H i X CdX

Zn + X =ZnX H Cd CdI i H i X CdX. I -. 在 pH 为 5.5 时以 2×10 -2 mol·L -1 HEDTA 滴定浓度均为 2×10 -2 mol·L -1 的 Zn 2+ 、 Cd 2+ 混合液中的 Zn 2+ ，以 KI 掩蔽 Cd 2+ 。计算化学计量点时的 lg K (ZnX) , 能否准确滴定 ? 指示剂是什么 ?[Zn 2+ ] 、 [CdX] 、∑ [CdI i ] 各是多少？

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Zn + X =ZnX H Cd CdI i H i X CdX

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  1. Zn + X =ZnX H Cd CdIi HiX CdX I- 在pH为5.5时以2×10-2 mol·L-1 HEDTA滴定浓度均为2×10-2 mol·L-1的Zn2+、Cd2+混合液中的Zn2+，以KI掩蔽Cd2+。计算化学计量点时的lgK(ZnX), 能否准确滴定?指示剂是什么?[Zn2+]、[CdX]、∑[CdIi] 各是多少？ 已知: lgK(ZnX)=14.5, lg(CdX)=13.0, lgX(H)=4.6, lgCd (I)=5.1

  2. 解: [Cd]= X(Cd)=1+10-7.1+13.0=105.9> X(H)=104.6 lg X=5.9 lgK(ZnX)=14.5-5.9=8.6>8, 能准确滴定, 指示剂:XO pZn=1/2(lgK +pc(Zn))=1/2(8.6+2.0)=5.3 [Zn2+]=10-5.3 mol/L,  Cd(I)={[Cd]+∑[CdIi]}/[Cd] ∑[CdIi] =105.1-7.1-10-7.1=10-2.0mol/L

  3. ∵X(Cd) > X(H) ∴[CdX]=[X]= 10-5.3mol/L [CdX]=[Cd][X]K(CdX)=10-7.1(10-5.3/105.9)1013.0=10-5.3mol/L [CdX]=105.9-11.2-10-11.2= 10-5.3mol/L

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