Titration Fajans

The Fajans titration of Cl with Ag+ convincingly demonstrates indicator end points in precipitation titrations. Dissolve 0.5 g of NaCl plus 0.15 g of dextrin in 400 mL of water. The purpose of the dextrin is to retard coagulation of the AgCl precipitate. Add 1 mL of dichlorofluorescein indicator solution containing either 1 mg/mL of dichlorofluorescein in 95% aqueous ethanol or 1 mg/mL of the sodium salt in water. Titrate the NaCl solution 

Color Plate 2a shows the yellow color of the indicator in the NaCl solution prior to the titration. Color Plate 2b shows the milky white appearance of the AgCl suspension during titration, before the end point is reached. The pink suspension in Color Plate 2c appears at the end point, when the anionic indicator becomes adsorbed on the cationic particles of precipitate. 

In all argentometric titrations, but especially with adsorption indicators, strong light (such as daylight through a window) should be avoided. Light decomposes silver salts, and adsorbed indicators are especially light sensitive. 

Applications of precipitation titrations are listed in Table 7-1. Whereas the Volhard method is an argentometric titration, the Fajans method has wider applications. Because the Volhard titration is carried out in acidic solution (typically 0.2 M HN03), it avoids certain interferences that affect other titrations. Silver salts of CO -, C20 -, and AsO are soluble in acidic solution, so these anions do not interfere. 

Back titration of Ag left after reaction with BH4  

(a) Why is precipitated AgCl filtered off in the Volhard titration of chloride  

Common ion effect A salt is less soluble in the presence of one of its constituent ions. 

For Reaction 6-1, how many milliliters of 0.165 0 M KMn04 are needed to react with 108.0 mL of 0.165 0 M oxalic acid How many milliliters of 0.165 0 M oxalic acid are required to react with 108.0 mL of 0.165 0 M KMn04  

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Fajans titration adsorption of a colored indicator on the precipitate at the end point... 

In a spectrophotometric titration, absorbance of light is monitored as titrant is added. For many reactions, there is an abrupt change in absorbance when the equivalence point is reached. The Fajans titration is based on the adsorption of a charged indicator onto the charged surface of the precipitate after the equivalence point. The Volhard titration, used to measure Ag+, is based on the reaction of Fe3+ with SCN- after the precipitation of AgSCN is complete. 

Examine the procedure in Table 7-1 for the Fajans titration of Zn2+. Do you expect the charge on the precipitate to be positive or negative after the equivalence point ... 

Fajans titration A precipitation titration in which the end point is signaled... 

Color Plate 2 Fajans Titration of CP with AgN03 Using Dichlorofluorescein (Demonstration 7-1)... 

Fajans titration, which involves the adsorption of a coloured indicator onto the precipitate at the end-point, resulting in a colour change. During this adsorption process a change occurs in the indicator resulting in a change of colour. The indicators used for this are often anionic dyes. 

COLOR PLATE 1 Fajans Titration of G with AgN03, Using Dichlorofluorescein (Demonstration 6-1)... 

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