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:
- The precipitate must be practically insoluble.
- Rapid precipitation.
- Ease of detection of the endpoint.
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.
