pH of the Aqueous Phase:

If we refer to the Eqs. 3.16 and 3.15 as given below.

pH ½  = 1/n log K*´

log D = log E - log (100 - E) = n (pH - pH ½ )

The separation factor, β, may be expressed as

log β = log D₁ - log D₂ = n₁  pH ½ - n ₂  pH ½

If we rake the criteria of a successful single stage separation by pH control, a 99% extraction of one (D₁ = 99) with a maximum of 1% extraction of the other (D₂ = 0.01) gives a minimum value of β is 10⁴ (log β = 4). Using the above equation for bivalent metals, a difference of two pH units in pH1/2 values would be necessary. For tetravalent metal ions, the necessary difference is less. Figure depicting the extraction of dithizonates, shows a pH ½ differences of at least two units for Hg², Bi³⁺ Sn²⁺ Pb²⁺ and Cd²⁺. Thus, it should be possible to employ a systematic separation of these ions by simply pH adjustment. For a separation of Hg²⁺ from other four ions, the pH should be adjusted to 1.0 so that Hg²⁺ will be quantitatively extracted while the others will remain in the aqueous phase. If the extraction of Cd²⁺ is desired, the pH should be just under 10 at which all the other metal ions will be extracted leaving behind Cd²⁺ in solution. If the metal ion of interest is neither the most nor the least extractable of the mixuture, as is the case of Sn²⁺, the pH should be adjusted to about 6 which will lead to the extraction of Hg²⁺, Bi³⁺ and Sn²⁺ leaving Pb²⁺ and Cd²⁺ in the solution. The organic phase is now back extracted with a fresh aqueous phase of pH~ 3 when Sn²⁺ returns to the aqueous phase leaving behind Hg²⁺ and Bi³⁺ in the organic phase.

The pH1/2 values may be altered by the use of masking agents. The pH ½  values of Hg²⁺, Cu^2+, Ag⁺ Zn²⁺ and Cd²⁺ are raised to much higher values by the addition of cyanide. EDTA also moves the pH ½ values of most of the metals to the right, Ag+ being an exception. Cu²⁺ is more affected than Hg²⁺ so that the latter is separated from the former.