Maximum suppressors

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. 

As a precautionary measure to prevent the appearance of maxima, sufficient gelatin to give a final concentration of 0.005 per cent should be added. The gelatin should preferably be prepared fresh each day bacterial action usually appears after a few days. Other maximum suppressors (e.g. Triton X-100 and methyl cellulose) are sometimes used. 

Maximum suppressors. Gelatin is widely used as a maximum suppressor in spite of the fact that its aqueous solution deteriorates fairly rapidly, and must therefore be prepared afresh every few days as needed. Usually a 0.2 per cent stock solution is prepared as follows. Allow 0.2 g of pure powdered gelatin (the grade sold for bacteriological work is very satisfactory) to stand in 100 mL of boiled-out distilled water for about 30 minutes with occasional swirling warm the flask containing the mixture to about 70 °C on a water bath for about 15 minutes or until all the solid has dissolved. The solution must not be boiled or heated with a free flame. Stopper the flask firmly. This solution does not usually keep for more than about 48 hours. Its stability may be increased to a few days by adding a few drops of sulphur-free toluene or a small crystal of thymol, but the addition is rarely worth while and is not recommended. 

Figure 6.11 Polarogratn of a solution containing three analytes, showing three different waves . The half-wave potential, 1/2, for each is characteristic of the respective analyte couples, while the wave heights reflect the relative concentrations of each ion. The trace has been smoothed to remove the sawtoothed effects seen in Figures 6.7 and 6.8. The solution also contained KCl (0.1 mol dm ) as a swamping ionic electrolyte, and Triton X-lOO (a non-ionic surfactant) as a current maximum suppressor.

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. 

Bhatt et al. have described a method based on the complex formation between chlorpromazine and K3Fe(CN)5 [151]. The latter substance yielded a reduction wave at zero applied potential, and addition of chlorpromazine decreased the wave height in an amount directly proportional to the amount added. Optimum conditions for the determination were reported to be pH 7.4-6.2, use of 0.1 M KCl as the supporting electrolyte, and 0.001% methyl red solution as the maximum suppressor. In this system, chlorpromazine can be determined up to concentrations of 1.4 pg/mL. 

This column describes the composition of the supporting electrolyte. Concentrations are given In moles per liter wherever possible the entry "KOH 0.1" denotes 0.1 F potassium hydroxide. BrItton-RobInson, Mcllvaine, and other buffers, both mixed and simple, are identified by means of abbreviations. The entry "buffer" means that the solution was said to be buffered at the pH quoted in Column 9 but that no Information was given about the composition of the buffer employed. Maximum suppressors are identified In this column and their concentrations are given in weight/volume per cent. 

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. 

Iodonium and sulfonium salts undergo irreversible one electron electrochemical or chemical reduction [51,52], Reduction of diaryliodonium salts in water exhibit two to four waves in the polarogram depending upon the concentration of iodonium salt, type of electrode, nature and concentration of the supporting electrolyte, and the maximum suppressor [51,53-56]. Reductive electrolysis of diphenyliodonium salts in water at mercury yields mixtures of diphenylmercury, iodobenzene, and benzene, depending upon the potential used during the electrolysis [51,53-55]. Reduction at platinum or glassy carbon electrodes occurs without appearance of the first wave (see below) [54,56,57]. The mechanism shown in Scheme 1 was proposed for aqueous electrolysis of diphenyliodonium salts [51a] ... 

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 half wave potential (E1/2 versus standard calomel electrode) of the acetone derivative of phenelzine was reported (7) as -1.33V in pH 5.9 phosphate buffer (0.1 M KH2PO1, Na2HPO 2H2O) employing a 0.01% gelatin solution as a maximum suppressor. The diffusion current constant (I ) was approximately 2.5. Four electrons were transferred for the reduction of the hydrazone derivative. Waveheight was proportional to concentration in the range of 2.5 x 10 to 2.5 X 10 M in pH 5.9 phosphate buffer. 

Differential pulse polarography of water containing trace amounts of Ag(I) and Au(III), in the presence of 0.1 M ammonium tartrate as supporting electrolyte and 0.001% gelahn as maximum suppressor, gave a unique peak for both ions at = —0.08 V vs standard calomel electrode. On addition of EDTA the signal could be resolved into two peaks, one at —0.04 V and the other at —0.2 V for silver and gold, respechvely. A piezoelectric method based on electrolytic deposition was described for Ag(I). ... 

Maxima have been shown to be associated with streaming at the mercury drop surface, perhaps because the potential at some part of the surface differs from that at the other part. However, the reason for this streaming and its cessation in the presence of maximum suppressors has not been well explained. 

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). 

Volume of vessel (free volume V) Shape of vessel (area and aspect ratio) Type of dust cloud distribution (ISO method/pneumatic-loading method) Dust explosihility characteristics Maximum explosion overpressure P ax Maximum explosion constant K ax Minimum ignition temperature MIT Type of explosion suppressant and its suppression efficiency Type of HRD suppressors number and free volume of HRD suppressors and the outlet diameter and valve opening time Suppressant charge and propelling agent pressure Fittings elbow and/or stub pipe and type of nozzle Type of explosion detector(s) dynamic or threshold pressure, UV or IR radiation, effective system activation overpressure Hardware deployment location of HRD suppressor(s) on vessel... 

In this procedure up to 70 ml seawater, 10 ml 0.01 M lead nitrate, 20 ml 95% ethanol, 0.2 ml of 0.1% methyl red maximum current suppressor, and two drops of 3 M nitric acid are added to a 100 ml volumetric flask, the contents of which are then diluted to 100 ml with distilled water. 

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