Polarographic current, measurement

Radiopolarography offers highly increased sensitivity and selectivity over polarographic current measurement, without interference from major components of the solution. It measures the amount of labeled ions deposited in single drops in a dropping mercury electrode as a function of potential [85]. 

Further, the operator must be able to choose the drop lifetime and the scan parameters, viz., the starting potential, direction (cathodic or anodic), rate and end potential, together with the sensitivity of the current measurement and the amplification in the ohmic cell resistance compensation circuit. Convenient additional facilities are (a) display of the polarogram on an oscilloscope, (b) delivery of hard copy of the polarograms on a chart recorder and (c) repeated recording of the polarographic curve for the same sample. 

Fig. 5.8 Schematic diagram of polarographic (or voltammetric) circuits for two-electrode (a) and three-electrode (b) systems. WE(DME) indicator or working electrode (dropping mercury electrode in the case of polarography) RE reference electrode CE counter electrode DC voltage (V) DC voltage source Current (/) current measuring device.

Thiocyanate ion frequently reacts in oxidation with a rate-determining step reminiscent of those with the halides, and this oxidation is no exception. A study based on measurements of the limiting polarographic currents for thiocyanate in the presence of hydrochloric acid and bromate gives clear evidence of first-order dependence on bromate and on thiocyanate ion concentrations and of second-order dependence on hydrogen ion concentration. The authors report the Arrhenius parameters as, AH = 11.4 kcal.mole and AS = 28.0 caLmole . deg . One strange, and unexplained, anomaly is present. The experimental first-order coefficient for bromate reaction appears to be constant over about one half-life for equimolar initial thiocyanate and bromate concentrations. Yet these rate coefficients depend linearly on thiocyanate ion concentration for [SCN ] > [BrOj ], but non-linearly below. It seems improbable that the postulated mechanism with first steps (1 )-(3), is capable of explaining this, viz. 

In pulse radiolysis experiments the radicals are produced homogeneously in the solution. The polarographic current, /, is determined by the concentration of radicals at the electrode surface, the rate at which they are oxidized or reduced and the rate at which they are replaced by other radicals diffusing to the surface from the bulk solution account also has to be taken of reactions of the radicals in the bulk solution. By measuring the current, at a fixed time after the pulse, as a function of the potential applied, one can obtain a polarogram which is characteristic of the redox behavior of the radical and so can be used to identify it. Information about the rate of electron transfer can be extracted from measurements of the time dependence of / at a fixed potential. For radicals which undergo self-reaction in the bulk solution, the appropriate relationship is given by Eq. 71, provided the time is shorter than the first half-life of the radical [140],... 

Radiometer pOz electrode, type E 5046 consists of a platinum cathode (20 /am diameter) and silver-silver chloride reference electrode placed in an electrochemical solution behind a 20 /am thick polypropylene membrane. A polarizing voltage of about 650 mV is applied. The polarographic current is about 10 " A per mm Hg of oxygen tension at 38°C. Zero current is lower than 10 A, response time less than 60 sec at 38°C 99% of full deflection. The PO2 electrode is used with the pH-Meter 27 GM or the Astrup Micro-Equipment, in conjunction with the Oxygen Monitor. The scale can be calibrated to the range 0-100 mm Hg p02. Thermostated cells provide measurements at constant temperature of volumes down to 70 /al. The small volume makes this cell useful to measure the PO2 of capillary blood. The cell is supplied with accessories for blood sampling. 

The measurement of polarographic currents is carried out by various techniques according to the rate of the reaction involved. Whereas for reactions with a half-time greater than about 15 sec the measurement of mean currents in the classical polarographic arrangement is more useful, for faster reactions it is necessary to use special equipment, as will be discussed separately. 

The improvements in this method yield detection limits near 10 M, perhaps slightly lower than those of conventional polarography. Since tast measurements are only sampled-current presentations of conventional polarographic currents, all conclusions about the shapes of waves and all diagnostics developed for conventional measurements of maximum currents apply to the tast technique. 

A simple polarograph can be constructed from a potentiometer and a sensitive current-measuring instrument according to the scheme in Figure 3.1. Manual recording of polarographic curves with such an apparatus is, however, very time consuming and cannot be recommended for practical analyses. 

When analysing a two-component system, the maximum accuracy wiU be achieved when measuring the differential pulse polarographic current at potentials where the difference between the values of the constant k will be as large as possible. 

Figure 1 Fundamental scheme of a three-electrode polarographic circuit for /Ffdrop correction. RG, ramp generator A, current measurement device V, voltmeter WE, working electrode RE, reference electrode AE, auxiliary electrode.

---