The Vohlard method is used to determine halide ions l,, Cl-, Br-. A known excess of silver nitrate standard solution is added to the sample and the Ag+ ions in excess are determined using back-titration with a thiocyanate standard solution.
With the Vohlard method the silver ions are titrated using a standard solution of thiocyanate ions:
Ag+ + SCN- ↔ AgSCN
The iron acts as an indicator. The solution turns red with the first excess of thiocyanate ions (see right).
In this case, the concentration of the indicator is not important.
The reaction occurs in a highly acid environment to avoid the formation of ferric hydroxide. This represents an obvious advantage compared to other methods for analysing halides because there is no interference from ions like carbonate, oxalate and arsenate (which form slightly soluble salts in neutral environments but not in acid ones).
P.M. = P.E. KSCN = 97.18 g/mol AgNO3 = 169.87 g/mol
N = 0.1 g = 9.718 g in 1 L of water.
Imagine we used 4.45 ml of titrant
Vi * Ci = Vf * Cf
Vi = 5 ml
Ci = 0,10 N
Vf = 4.45 ml
Cf = X
Cf = 0.112 N
2. The analytical chemistry laboratory
4. Inorganic qualitative analysis
9. Neutralisation titration - part two
10. Alkalimetry
11. Acidimetry
13. Mohr method
14. Vohlard method
16. Oxidation reduction titration
18. Instrumental Chemical Analysis
19. Optical methods of analysis
20. Chromatography
21. Potentiometry