Mohr’s Method of Precipitation Titration

MOHR’S METHOD OF DETERMINATION OF CHLORIDES BY TITRATION WITH SILVER NITRATE

Chem Pharma

is one of the oldest titration methods still in use – it was researched and published by Karl Friedrich Mohr in 1856.The idea behind this titration is very simple – chlorides are titrated with the silver nitrate solution in the presence of chromate anions. End point is signaled by the appearance of the red silver chromate.

Intense yellow color of chromate may make detection of first signs of formation of red silver chromate precipitation difficult. As some excess of silver must be added before precipitate starts to form, if concentration of titrant is below 0.1M, we may expect significant positive error. To correct for this error we can determine a blank, titrating a solution of the indicator potassium chromate with standard silver nitrate solution. To make result more realistic we can add small amount of chloride free calcium carbonate to the solution to imitate the white silver precipitate.
Solution during titration should be close to neutral. In low pH silver chromate solubility grows due to the protonation of chromate anions, in high pH silver starts to react with hydroxide anions, precipitating in form of AgOH and Ag2O. Both processes interfere with the determination accuracy.
Exactly the same approach can be used for determination of bromides. Other halides and pseudo halides, like I- and SCN-, behave very similarly in the solution, but their precipitate tends to adsorb chromate anions making end point detection difficult.

Reaction

Reaction taking place during titration is
Ag + Cl¯  →   AgCl (s)

Sample size

Assuming 0.1M titrant concentration and 50 mL burette, aliquot taken for titration should contain about 0.12-0.16 g chloride anion (3.5-4.5 millimoles).

End point detection

Before titration small amount of sodium or potassium chromate is added to the solution, making its slightly yellow in color. During titration, as long as chlorides are present, concentration of Ag is too low for silver chromate formation. Near equivalence point concentration of silver cations rapidly grows, allowing precipitation of intensively red silver chromate which signalls end point. See precipitation titration end point detection page for more detailed, quantitative discussion.

Solutions used

To perform titration we will need titrant – 0.1 M silver nitrate solution, indicator – potassium chromate solution, and some amount of distilled water to dilute sample.

Procedure

Pipette aliquot of chlorides solution into 250mL Erlenmeyer flask. Dilute with distilled water to about 100 mL. Add 1 mL of 5% potassium chromate solution. Titrate with silver nitrate solution till the first color change.

Result calculation

According to the reaction equation

Ag Cl → AgCl

silver nitrate reacts with chloride anion on the 1:1 basis. That makes calculation especially easy – when we calculate number of moles of AgNO3 used it will be already number of moles of Cl titrated. 

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