Formation Reactions in Non-Aqueous Solutions

Many potentiometric studies have been carried out to obtain information about the compositions and formation constants of metal complexes in non-aqueous solutions. Ion-selective electrodes and metal (or metal amalgam) electrodes are used as indicator electrodes. 

For example, the formation constants of 1 1 complexes of alkali metal ions with crown ethers or cryptands have been determined by Procedures 1 or 2 below [24]  

Procedure 1 Cell (VI) is constructed using a univalent cation-sensitive glass electrode (Ag electrode for Ag+) and the emf is measured by titrating metal ion M+ with ligand L  

The emf is converted to p M (=-loga(M+)) and the values of p M are plotted against the amount of L added. If M+ and L react in a 1 1 ratio, the formation constant can be obtained by the following relation at twice the equivalence point log K(ML) =p M+log yML- If the complex is very stable (log K(ML) 10), the concentration of the free M+ after the equivalence point may be too low for the glass electrode to get the Nernstian response. In this case, an indirect method as in Procedure 2 is used. 

Procedure 2 Cell (VII) is constructed using silver electrodes on the two sides. After the equilibrium of reaction (6.8 b) is reached in the solution on the left of the junction, the emf of the cell is measured and the equilibrium constant K,.x [Eq. (6.8 c)] is obtained by calculation.10 If K(AgL) is known, we can get the value of JC(ML). 

---