Polarography Using Double Complexing Agents as Supporting Electrolyte

Abstract

Recent polarographic works using double complexing agents are reported. When double complexing agents are used as supporting electrolyte, the following three cases occur. (1) A negatively reducible complex ion of smaller dissociation constant is predominantly formed in the solution. (2) Two complex ions are formed in a mixed base solution, and the negatively reducible complex ion is transfered to the positively reducible complex ion at the electrode surface as the reduction proceeds. (3) A new complex ion of mixed ligand is formed in a mixed base solution. Examples of (1) are as follows : (i) Simultaneous determination of Fe and Ni using the supporting electrolyte composed of 1M NH4OH-1M NH4Cl and excess 5-sulfosalicylic acid, or 1 M NH4Cl-0.5M pyridine and excess 5-sulfosalicylic acid are reported. (ii) Polarographic determination of trace of Bi in the presence of a large amount of Cu was scucessfully carried out by using double complexing agent consisting of 0.1M EDTA, 0.1M KCN and 10% ethanol at pH 9.7 (Fig. 1). Example of (2) is as follows: Polarographic behaviour of UO22+ in each supporting electrolyte is shown in Fig. 2. Polarogram of UO22+ using 0.05M EDTA, 0.05M Na4P2O7, 0.3M KCl and 0.01 % gelatin are separated into two waves(Fig. 2b). Some part of the pyrophosphate complex ion is transfered to the EDTA complex ion at the electrode surface as the reduction of EDTA complex ion proceeds. Examples of (3) are as follows : (i) The half-wave potentials of Cu, Bi and Pb are separated by the use of 0.5M Na citrate and 0.01MEDTA (pH 4.5) to -0.09, -0.64 and -1.1V. (vs. SCE), respectively (Fig. 3). (ii) A supporting electrolyte composed of 0.1M EDTA and 0.02M sodium citrate (pH 6.0) is used for the simultaneous determination of Ti and V (Fig. 4). A new Ti EDTA-cit-rate complex ion having mixed ligand is formed in this medium. Both ions give well defined reduction waves, and the half-wave potentials are -0.59 for Ti and -1.3 V. (vs. SCE) for V at pH 6.0 (Table 1). (iii) The base solution composed of 0.1 N HNO3, 0.005M nitrilotriacetic acid arid 0.04% sodium carboxy methylcellulose is used for the simultaneous determination of Cu and Bi in Pb. As Bi forms a stable complex with nitrilotriacetic acid in this medium, the reduction step of Bi complex ion can be shifted from the copper step cvcn in the prescnce of large amounts of lead. The half-wave potentials of Cu and Bi are -0.004 V. and -0.14 V. (vs. Hg pool), respectively (Table 2). (iv) Simultaneous determination of Sn and Pb using the supporting electrolyte cornposed of 3MNaBr, 0.05M EDTA and 0.008% gelatin (pH 1.3-1.7) are reported (Fig. 5). A new Sn mixed complex using EDTA and Br as a ligand is formed in this medium, On the other hand, Pb is present as a bromide complex ion in this condition (pH 1.3-1.7). The half-wave potentials of Pb and Sn are -0.48 and -0.71V. (vs. SCE), respectively. (v) Sn (IV) in 1 MNaBr, 0.2M oxalic acid and 0.004% gelatin as a supporting electrolyte forms a well defined four electron reduction wave. Polarograms of Sn (IV) using oxalic acid and alkali halide (KCI, NaBr, KI) as double complexing agents are presented in Fig. 6.