Question

Which Indicator Is Used In Mohr’s Method?

Answer 1

Hint: The silver(I) ion is used in argentometry, which is a kind of titration in analytical chemistry. It's often used to figure out how much chloride is in a sample. The sample solution is titrated against a known concentration of silver nitrate solution. Chloride ions react with silver(I) ions to form silver chloride, which is insoluble:
\[A{g^ + }\;\left( {aq} \right){\text{ }} + {\text{ }}C{l^ - }\;\left( {aq} \right){\text{ }} \to {\text{ }}AgCl{\text{ }}\left( s \right){\text{ }}(K\; = {\text{ }}5.88{\text{ }} \times {\text{ }}{10^9})\]

Complete answer:
Potassium chromate is used as an indicator in the Mohr technique, which produces red silver chromate when all chloride ions have interacted.
\[2A{g^ + }\;\left( {aq} \right){\text{ }} + {\text{ }}Cr{O^{2 - }}_4\;\left( {aq} \right){\text{ }} \to {\text{ }}A{g_2}Cr{O_4}\;\left( s \right){\text{ }}({K_{sp}}\; = {\text{ }}1.1{\text{ }} \times {\text{ }}{10^{ - 12}})\]
Because silver hydroxide forms at high pH whereas chromate creates H2CrO4 at low pH, the solution must be near neutral to reduce the concentration of chromate ions and delay the development of the precipitate. Carbonates and phosphates precipitate with silver and must be avoided in order to avoid erroneous findings.
By burning a sample with calcium and subsequently ferric acetate, the Mohr technique may be used to estimate the total chlorine concentration of a sample. Free chlorine is "fixed" by calcium acetate, which precipitates carbonates and neutralises the resulting solution. Phosphates are removed by ferric acetate. The residue is dissolved and titrated to remove any chlorides.

Note:
Typically, dichlorofluorescein is employed as an indicator in the Fajans technique; the end-point is identified by the green suspension becoming pink. Chloride ions are still in excess prior to the titration's completion. They adsorb on the AgCl surface, giving the particles a negative charge. Excess silver(I) ions attach on the AgCl surface after the equivalence point, imparting a positive charge. Anionic dyes like dichlorofluorescein are attracted to the particles and change colour as they adsorb, indicating the end-point. Eosin gives a sharper end-point than dichlorofluorescein when titrating against bromide, iodide, and thiocyanate anions.