Fajans method

Fajans method The titration of Cl" with Ag using fluorescein as an adsorption indicator. At the end point the precipitate becomes red. 

Faience Failure analysis Fair-Plus Fajans method Falcon... 

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. 

In what respect is the Fajans method superior to the Volhard method for the titration of chloride ion ... 

Table 11.1 lists some adsorption indicators. Fluorescein can be used as an indicator for any of the halides at pH 7 because it will not displace any of them. Dichlorofluoroscein will displace Cl" at pH 7 but not at pH 4. Hence, results tend to be low when titrations are performed at pH 7. Titration of chloride using these indicators is called Fajans method. Fluorescein was the original indicator described by Fajans, but dichlorofluoroscein is now preferred. Eosin cannot be used for the titration of chloride at any pH because it is too strongly adsorbed. 

As silver nitrate solution is added to the sodium chloride, silver chloride precipitates. As long as CT ions are in excess, they adsorb on the precipitate particles. At the end point, no CT ions are left in solution and negative fluorescein ions are then adsorbed, giving a pink colour to the precipitate. The technique is sometimes known as Fajans method. 

Gravimetric method such as AgCl or fire assay is used. Gay-Lussac method involves the titration of Ag with NaCl and/or KBr in acid solution. The endpoint is taken as the point at which no further precipitation of silver halide is observed upon the addition of more precipitant. Fajans method is based on the precipitation of Ag with chloride or bromide ions, using an adsorption indicator to detect the endpoint. Rhodamine 6G or dichlorofluorescein is used as the adsorption indicators. Volhard method is based on the titration of Ag with potassium or sodium thiocyanate to form insoluble silver thiocyanate. This titration is... 

In this chapter, we study some titration curves involving a precipitation from a theoretical standpoint. This chapter is an introduction to the so-called Charpentier-Volhard, Mohr, and Fajans methods, which all involve standard solutions of silver nitrate. Liebig-Denigds method, which also involves such silver nitrate solutions, will be considered in the next chapter. 

Argentometry in Weakly Acidic or Neutral Medium Fajans Method... 

Like Mohr s method, Fajans method is a direct titration method. It is based on the use of adsorption indicators. A small volume of an acetic acid solution and an adsorption indicator are added to the solution containing the halide to be determined. Then a silver nitrate solution is added. At the equivalence point, the precipitate suddenly turns colored and separates completely from the solution. 

Mittenberger and Gross " determined chlorine in PVC by fusion with sodium peroxide followed by silver nitrate titration of the chloride produced. Tanaka and Morikawa described a semimicro technique for the determination of total chlorine in PVC using a semimicro method based on the Schoniger oxygen flask and Fajans method. 

---