Answers

2015-11-29T08:43:21+05:30
t is not always possible to use Mohr method to determine concentration of chlorides. For example, Mohr method requires neutral solution, but in many cases solution has to be acidic, to prevent precipitation of metal hydroxides (like in the presence of Fe3+). In such cases we can use Volhard method, which is not sensitive to low pH.In the Volhard method chlorides are first precipitated with excess silver nitrate, then excess silver is titrated with potassium (or sodium) thiocyanate. To detect end point we use Fe3+ cations, which easily react with the thiocyanate, creating distinct wine red complex.There is a problem though. Silver thiocyanate solutility is slightly lower than solubility of silver chloride, and during titration thiocyanate can replace chlorides in the existing precipitate:AgCl(s) + SCN- → AgSCN(s) + Cl-To avoid problems we can filtrate precipitated AgCl before titration. However, there exist much simpler and easier procedure that gives the same result. Before titration we add some small volume of a heavy organic liquid that is not miscible with water (like nitrobenzene, chloroform or carbon tetrachloride). These liquids are better at wetting precipitate than water. Once the precipitate is covered with non polar liquid, it is separated from the water and unable to dissolve.Precipitate solubility is not a problem during determination of I- and Br-, as both AgBr and AgI have much lower solubilities than AgSCN.reaction
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