ADSORPTION INDICATORS

 ADSORPTION INDICATORS:

Titration is a method of determining the concentration of an unknown solute using a known concentration solution. In which a titrant is usually applied from a burette to a known compound volume until the reaction is complete, the endpoint is detected using an indicator.

Ex: Acid-base titration, redox titration, precipitation titration, and complexometric titration.

 What is precipitation titration? 

Precipitation titration is a type of titration in which the titration reaction formed a precipitate. The titrant reacts with the compound to produce an insoluble substance (precipitate). The basic principle of precipitation titrations is that the amount of precipitating reagent or precipitant added is equivalent to the substance being precipitated. 

Amount of added precipitating agent = the amount of a compound that is precipitated 

What is Fajan’s method in chemistry?  

The Fajans method is an analytical technique that uses adsorption to detect halide content. It is commonly used to quantitatively analysis of halide ions or thiocyanate ions.  This method uses a dichlorofluorescein indicator. 

Cl– ions titration using AgN03 in the presence of an adsorption indicator, in which, AgN03 is filled in a burette, and Cl– ion solution with indicator is added in a conical flask. The titration reaction would be

Ag+ + Cl− ⇌ AgCl(s)

Colloidal particles are formed by silver chloride. Because of the adsorption of excess Cl- to the surface of the precipitant particles before the equivalence point, the surface of the precipitant particles will be negatively charged.

The dye dichlorofluorescein can be used as an indicator to determine chloride in Fajan's titration, by which the endpoint is detected by the green suspension turning pink. The dye dichlorofluorescein can be used as an indicator to determine chloride in Fajan's titration, by which the endpoint is detected by the green suspension turning pink.

The mechanism of indicators action:

 The best–known adsorption indicator is fluorescein, which is used to indicate the equivalence point in the titration of Cl- with Ag+ . Fluorescein is a weak acid, which partially dissociates in water to form fluoresceinate anion.

The fluoresceinate anion has a yellow–green colour in solution.  When Cl is titrated with Ag+ in the presence of fluorescein, the negatively charged fluoresceinate anions are initially repelled by the negatively charged AgCl colloidal particles, with their primary adsorption layer of Cl ions.  Thus the fluorescein remains in a yellow–green colour prior to the equivalence point.  At the equivalence point, the colloidal AgCl particles undergo an abrupt change from a negative charge to a positive charge by virtue of Ag+ ions adsorbed in the primary adsorption layer.  The fluoresceinate ions are strongly adsorbed in the counter–ion layer of the AgCl colloids, giving these particles a red colour and providing an end point colour change from yellow–green to red or pink.