Maximum suppressors, polarographic

Polarographic maxima. Current-voltage curves obtained with the dropping mercury cathode frequently exhibit pronounced maxima, which are reproducible and which can be usually eliminated by the addition of certain appropriate maximum suppressors . These maxima vary in shape from sharp peaks to rounded humps, which gradually decrease to the normal diffusion-current curve as the applied voltage is increased. A typical example is shown in Fig. 16.3. Curve A is that for copper ions in 0.1 M potassium hydrogencitrate solution, and curve B is the same polarogram in the presence of 0.005 per cent acid fuchsine solution. 

Solutions for polarographic analysis should contain a surfactant in low concentration to act as a current maximum suppressor, and all traces of oxygen should be removed. 

The half-wave potentials are dependent on substrate concentration as well as on the drop time of the mercury electrode and the height of the mercury column, and frequently the current (i) versus potential ( ) curves exhibit large polarographic maxima due to adsorption processes. These effects can be minimized by addition of maximum suppressors like gelatine or Triton X-100. A consequence of this non-ideal polarographic behaviour is that results obtained under even slightly different conditions only can be compared in a semi-quantitative way. 

Polarographic measurements were carried out in an acetate buffer (the assumed pH was 3.66), and concentrations of sulphate and acetate were varied in the presence of l.OOx lO M nickel nitrate and 1.09 x 10 M nitrilotriacetate at a constant ionic strength of 0.2 M (maintained with potassium nitrate). A dropping mercury electrode was used, and polyoxyethylene lauryl ether (1-2 x 10 M) was used as a maximum suppressor. Although results are reported for 15, 25 and 35°C, the assumed pH value for the buffer at 15 and 35°C is not stated. The acetate molarities were varied from 0.025 to 0.100 M at 0.03 M sulphate, and the potassium sulphate molarities were varied from 0.01 to 0.05 M at 0.05 M acetate. The results indicated that for the experimental... 

The appearance of maxima may interfere with polarographic determinations maximum suppressors may be used (cautiously) to improve the results. Again, voltammetry is free from this interference (see Chapter 5, Section D and Fig. 61). 

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