Fajans Titration Method for Silver Nitrate Determination

Theoretical Background / Context

• Precipitimetry: 

It is a volumetric method of analysis that involves the formation of a practically insoluble salt using a precipitating agent.

• Solubility Product (Ksp): 

In a saturated solution of sparingly soluble electrolytes, the product of the molar concentration of ions each raised to a power equal to the number of ions produced is constant at room temperature and pressure.

Ksp of AnBm =[A]n[B]m

N.B: Substance with low Ksp precipitate first.

Conditions Required for a Preciptimetric Reactions:

1. The precipitate must be practically insoluble.
2. Rapid precipitation.
3. Ease of detection of the endpoint.

Principle Work of Fajans Titration

Adsorption Indicator: 

• Adsorption indicators are acidic or basic dyes that change their color upon adsorption on the precipitate at the equivalence point. 
• It has a color when not adsorbed and another colour when adsorbed on ppt.

Types:

• Weak acid: e.g. Eosin & Fluorescein.
• Weak base: e.g. Rhodamine-6-G.

For Successful Use of Adsorption Indicator,  Precipitate Must Be:

• Colloidal à surface area increases à Adsorption increase.
• Precipitate strongly adsorb its own ion.

 Fajans Method Indicator Must Be:

• Opposite in charge to titrant.
• Its adsorption power is not higher than the ion to be determined (Adsorbed after complete precipitation).
• A suitable concentration of indicator to precipitate after complete precipitation of ions (not exceeding the ksp of its silver salt during the titration).

The Medium Must Be:

• Suitable for the ionization of indicators.
• Acid indicator acts in alkaline media.
• Basic indicator acts in acid media.
• Fajans titration method involves the use of an adsorption indicator.
• Titration of NaCl standard with AgNO3 solution using adsorption indicator.