Mohr’s method indicator

Chromate ions (CrO42-) are typically used as the indicator in Mohr’s titration, which argentometrically determines the bromide, chloride, and cyanide ions by giving a red-brown color at the endpoint.

Precipitation titration is a type of titration that involves the formation of a precipitate during the titration process. Based on the application type and endpoint detection technique precipitation titration is classified into three methods, Mohr, Volhard, and Fajan's method.

Precipitation titration is based on chemical reactions that yield ionic compounds of limited solubility. Silver nitrate (AgNO3) is often considered the most important precipitating reagent. Titration methods that are based on the use of silver nitrate are commonly referred to as argentometric methods.

Which indicator is used in the Mohr method?

Potassium chromate can be used as an indicator for the argentometric detection of chloride, bromide, and cyanide ions that react with silver ions to produce a brick-red silver chromate precipitate in the region of the equivalence point.

When determining the concentration of chloride ions in a solution containing a silver nitrate standard, the Mohr method employs the use of chromate ions as indicators. Once all the chloride has precipitated as white silver chloride, the first excess of titrant will form a silver chromate precipitate, indicating that the process has reached its endpoint.

The following are the reactions:

Ag+ Cl- ↔ AgCl(s)

2Ag + CrO42-↔ Ag2CrO4 (s)

The amount of chloride in an unknown sample can be calculated by knowing the stoichiometry and moles consumed at the endpoint. The test determines the concentration of chloride in a solid sample.

List of indicators:

Method	Name of indicator	Endpoint color
Mohr's titration	Potassium chromate (K2CrO4)	Red-silver chromate
Volhard’s titration	Ferric ion (Fe3+)	Red-brown
Fajan’s titration	Dye Dichlorofluorescein (C20H10Cl2O5)	Green suspension turn pink

Commonly asked questions on the Mohr’s indicator are as follows.

Why is potassium chromate used as an indicator in the Mohr method?

The silver ions react with chromate ions when the potassium chromate (K2CrO4) is used as an indicator after the chloride ion has been extinction and when the solubility product of silver chromate is exceeded, a reddish-brown precipitate starts to form.

What solution is used in the Mohr method?

The Mohr method is a kind of precipitation titration in which the concentration of chloride ions in a given solution is measured by titrating it with a solution of silver nitrate.

What is used as a standard solution in Mohr’s method of titration?

The silver nitrate (AgNO3) solution is used as a standard in the titration of Mohr’s method.

What is the color in Mohr's method?

In Mohr's method, once all the chloride ions have interacted, red silver chromate is formed.

Why is it called Mohr's method?

German chemist Karl Friedrich Mohr was given the method. Thus, this process is known as Mohr's method. It is a method of direct titration.

Keywords:

Titrant, principle, medium, acid, analysis, ferric acetate, volumetric, compound, potassium thiocyanate (KSCN), potassium chloride (KCl), sodium chloride(NaCl) chemistry, class, and students, etc.