Page 149 - Pharmaceutical_Analytical_Chemistry_1_Theoretical_Notes_Level_1 2024-2025

Mansoura National University
Pharm D-Clinical Pharmacy Program Level 1 Pharm. Anal. Chem. 1 (PC 101)

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A Problem in Cl estimation:

Silver thiocyanate can replace chloride in the precipitated AgCl:

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AgCl(s) + SCN ↔ AgSCN(s) + Cl

• This will take place before reaction occurs with the iron (III) ions in the solution, and

there will consequently be a considerable titration error.

• It is therefore absolutely necessary to prevent the reaction between thiocyanate and

silver chloride.

➢ This reaction may be prevented in two ways, the first one is the most reliable:

1. The silver chloride ppt is filtered off before back-titrating.

2. An immiscible liquid (as nitrobenzene, chloroform or carbon tetrachloride) is

added to 'coat' the silver chloride particles and thereby protect them from

interaction with the thiocyanate. The most successful liquid is nitrobenzene.

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• This problem does not exist during the determination of I or Br , as both AgBr and

AgI have much lower solubilities (lower K ) than AgSCN.
sp

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