Polarograms

The limiting current was 5.67 tA. Verify that the reduction reaction is reversible, and determine values for n and 1/2. The half-wave potentials for the normal pulse polarograms of Pb + in the presence of several different concentrations of OH are shown in the following table. 

The method, as so far developed, is limited by the condition that the hydration-dehydration reaction must proceed at a rate that is slow compared with the time needed to obtain a polarogram. In principle, the method is capable of much wider application to covalent-hydration studies if use is made of oscillographic polarographic techniques or of chronopotentiometry. These refinements are currently being investigated. 

The methods dependent upon measurement of an electrical property, and those based upon determination of the extent to which radiation is absorbed or upon assessment of the intensity of emitted radiation, all require the use of a suitable instrument, e.g. polarograph, spectrophotometer, etc., and in consequence such methods are referred to as instrumental methods . Instrumental methods are usually much faster than purely chemical procedures, they are normally applicable at concentrations far too small to be amenable to determination by classical methods, and they find wide application in industry. In most cases a microcomputer can be interfaced to the instrument so that absorption curves, polarograms, titration curves, etc., can be plotted automatically, and in fact, by the incorporation of appropriate servo-mechanisms, the whole analytical process may, in suitable cases, be completely automated. 

Controlled-potential separation of many metals can be effected with the aid of the mercury cathode. This is because the optimum control potential and the most favourable solution conditions for a given separation can be deduced from polarograms recorded with the dropping mercury electrode see Chapter 16. 

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. 

Half-wave potentials. The salient features of a typical current-applied voltage curve (polarogram) are shown in Fig. 16.4. 

If the reaction at the indicator electrode involves complex ions, satisfactory polarograms can be obtained only if the dissociation of the complex ion is very rapid as compared with the diffusion rate, so that the concentration of the simple ion is maintained constant at the electrode interface. Consider the general case of the dissociation of a complex ion ... 

Method of standard addition. The polarogram of the unknown solution is first recorded, after which a known volume of a standard solution of the same ion is added to the cell and a second polarogram is taken. From the magnitude of the heights of the two waves, the known concentration of ion added, and the volume of the solution after the addition, the concentration of the unknown... 

Commercial polarographs are also available in which the voltage scan is carried out automatically while a chart recorder plots the current-voltage curve. A counter-current control is incorporated which applies a small opposing current to the cell which can be adjusted to compensate for the residual current this leads to polarograms which are better defined. Most of these instruments also incorporate circuits which permit the performance of alternative, more sensitive types of polarography as discussed in Section 16.9... 

A typical conventional polarogram for 0.003 M-cadmium sulphate in 1M potassium chloride in the presence of 0.001 per cent gelatin, and the corresponding derivative curve, are shown in Fig. 16.7 (/max is the maximum current recorded in the derivative mode). 

If the values of the a.c. current are plotted against the potential applied by the potentiometer, a series of peaks are obtained as illustrated in Fig. 16.11(a) the normal d.c. polarogram of the same solution is also shown (b). 

The following experiments (Section 16.14-16.16), which can well be performed with a manual polarograph, serve to illustrate the general procedure to be followed in d.c. polarography it is advantageous to use a chart recorder to produce the polarogram. 

Method of standard addition. The polarogram of the unknown solution will have been determined under (1). A new polarogram must now be recorded after the addition of a known volume of a standard solution containing the same ion, care being taken that in the resulting solution the concentrations of the supporting electrolyte and the suppressor are maintained constant. 

Finally, determine the polarogram of the aniline solution, and from the wave height deduce the nitrobenzene content of the sample E1/2 ca —0.4 V. 

If the current-voltage curves of the reagent and of the substance being titrated are not known, the polarograms must first be determined in the supporting... 

FIGURE 3-2 Polarograms for 1 M hydrochloric acid (Curve A) and 4 x 10 4 M Cd+2 in 1 M hydrochloric acid B. id represents the limiting current is the half-wave potential. 

To determine die diffusion current, it is necessary to subtract the residual current. This can be achieved by extrapolating the residual current prior to the wave or by recording die response of the deaerated supporting electrolyte (blank) solution. Addition of a standard or a calibration curve are often used for quantitation. Polarograms to be compared for this purpose must be recorded in the same way. 

FIGURE 3-6 Differential pulse (a) and DC (b) polarograms for a 1.3 x 10 5 M chloramphenicol solution. (Reproduced with permission from reference 7.)... 

FIGURE 3-7 Normal-pulse (curve A) and differential-pulse (curve B) polarograms for a mixture of 1 mg L-1 cadmium and lead ions. The electrolyte is 0.1 M HNOj. 

Example 3-5 Polarogram A was obtained for a lOmL lead-containing sample. The limiting current increased (to B) after adding 100 pL of a 0.10 m lead standard to the 10 ml sample. Calculate the original lead concentration in the sample. 

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