Sensitivity polarography

Bromo-2-pyridyla2o)-5-diethylamiQophenol (5-Br-PADAP) is a very sensitive reagent for certain metals and methods for cobalt have been developed (23). Nitroso-naphthol is an effective precipitant for cobalt(III) and is used in its gravimetric determination (24,25). Atomic absorption spectroscopy (26,27), x-ray fluorescence, polarography, and atomic emission spectroscopy are specific and sensitive methods for trace level cobalt analysis (see... 

Raki, a Turkish alcoholic drink was also analyzed by differential pulse polarography and copper, iron and zinc could be determined. For the arsenic content in beer a more sensitive method had to be applied. For this method a new catalytic method is established and the arsenic content was determined by using this new method. 

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... 

Because of these advantages, polarography became very popular immediately after its inception in 1922. For the development of this method, Heyrovsky was awarded a Nobel prize in 1959. Over the period from 1922 to 1960, several tens of thousands of papers concerned with the use and improvement of polarography were published. However, interest in this method declined markedly in the 1960s, due primarily to a drastic increase in the requirements to be met by methods of chemical analysis. With the production of new superpure materials and increasing awareness for ecological problems, it became necessary to develop much more sensitive methods of analysis able to detect the different impurities down to a level of 10 M. 

The major defects of classical polarography are as follows (1) it is not possible to increase the sensitivity drastically, owing to interference from DME charging currents ... 

Under optimum conditions LPS voltammetry is an order of magnitude more sensitive than polarography (i.e., the detection limit is about 10 M). As in classical polarography, somewhat higher sensitivity and selectivity can be attained when using a differential version (i.e., when recording, as a function of potential, not the current but its derivative with respect to potential). 

The ability of differential pulse polarography to resolve multicomponent systems and evaluate concentrations with excellent sensitivity has made this technique an attractive candidate for simultaneous measurement of CO, O, and some inhalation anesthetics Though very preliminary, the results appear promising and will likely lead to more intensive investigation of the approach. 

A hanging electrolyte drop has also been applied to determine ionic species in solution using differential-pulse-stripping voltammetry procedures [69]. Particular emphasis was given to assessing the selectivity and sensitivity of the method. The technique of current-scan polarography has also been applied in the study of electron-transfer [70] and coupled electron-transfer-ion-transfer [71,72] reactions at the ITIES in this configuration. 

Specifications for modem detectors in HPLC are given by Hanai [538] and comprise spectroscopic detectors (UV, F, FUR, Raman, RID, ICP, AAS, AES), electrochemical detectors (polarography, coulometry, (pulsed) amperometry, conductivity), mass spectromet-ric and other devices (FID, ECD, ELSD, ESR, NMR). None of these detectors meets all the requirement criteria of Table 4.40. The four most commonly used HPLC detectors are UV (80%), electrochemical, fluorescence and refractive index detectors. As these detectors are several orders of magnitude less sensitive than their GC counterparts, sensor contamination is not so severe, and... 

The key factor in voltammetry (and polarography) is that the applied potential is varied over the course of the measurement. The voltammogram, which is a current-applied potential curve, / = /( ), corresponds to a voltage scan over a range that induces oxidation or reduction of the analytes. This plot allows identification and measurement of the concentration of each species. Several metals can be determined. The limiting currents in the redox processes can be used for quantitative analysis this is the basis of voltammetric analysis [489]. The methods are based on the direct proportionality between the current and the concentration of the electroactive species, and exploit the ease and precision of measuring electric currents. Voltammetry is suitable for concentrations at or above ppm level. The sensitivity is often much higher than can be obtained with classical titrations. The sensitivity of voltammetric... 

Classical polarography is not optimised for analytical sensitivity. This inefficiency has been remedied in pulse polarography. Instead of applying a steadily increasing voltage on the mercury droplet, in pulse... 

The considerable gain in sensitivity with NPP compared with sampled DC polarography can be shown still more clearly on a calculative basis. Thus for a reversible process A 4- ne B and for a solution with ox only, we obtain from eqn. 3.50 by substituting P = exp(nF/RT) (E - El) the equation... 

Fig. 3.4037 illustrates well the character of the curves and the gain in sensitivity on going from conventional DC via sampled DC and normal pulse to differential pulse polarography. It should be realized that, although the DPP curve AijAE) might seem an approximation of a derivative curve, we cannot speak of derivative polarography. 

It has been shown that the charging current if leads to the potential Eac by 90° (cf., eqns. 3.62 and 3.61) and the faraday current if by 45° (cf., eqn. 3.63) these phenomena can be used to discriminate if from if and vice versa by means of phase-sensitive AC polarography. 

A method for the determination of organotin compounds by anodic stripping polarography has been published [121]. It has yet to be applied to seawater. Since the sensitivity permits the measurement of 0.01 ppm of the tin compounds, it is likely to be not quite sensitive enough for seawater and would require a preconcentration step. 

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