Polarographic method, measurement

Figure 2.3. The distribution of sample concentrations encountered in the comparisons of the colorimetric method without iodine with (a) the polarographic method and (b) the colorimetric method with iodine water. For convenience the line of the unit gradient is shown in both cases. In each case the precision (95% confidence level) for each measurement is shown for the most concentrated sample. In (b) the broken line divides on shore (A) and offshore ( ) samples. From [80]...

Technetium can be measured by spectrophotmetric methods. It forms two characteristic peaks with absorption maxima at 247 and 285 nm. Also, it can be measured by polarographic methods. All technetium isotopes are radioactive. The element can be identified from its specific activity using a scintillation counter. 

A perfect prototype of an ideally cation-permselective interface is a cathode upon which the cations of a dissolved salt are reduced. Experimental polarization curves measured on metal electrodes fit the theory very closely. Since in dimensional units the limiting current is proportional to the bulk concentration, the polarization measurements on electrodes may serve for determining the former. This is the essence of the electrochemical analytical method named polarography. (For the theory of polarographical methods see [28]—[30].)... 

It will be almost superfluous to mention that such involved procedures are possible nowadays due to the availability of computerized systems [54, 86—90] which organize the measurements as well as the on-line data analysis. On the other hand, many analytical problems may still be attacked by means of the classical d.c. polarogram and, moreover, many other technical or fundamental improvements of polarographic methods have been introduced during the last decade, as can be inferred from recent textbooks [21, 51). In fact, it is lack of knowledge about theoretical backgrounds of (particular) electrode reactions that hampers the reliable application of electroanalytical techniques, especially in more involved practical systems. 

The development of a simple fast-scan polarographic method for the determination of the A4-3-ketosteroid flurandrenolone in pharmaceutical preparations has been reported [134]. The polarographic peak due to the reduction of the carbon-fluorine bond is measured in ointments and creams to determine concentrations as low as 0.01 % w/w. Pulse polarographic procedures have been described for progesterones [135], A4-3-ketosteroids [136], hydrocortisone [137], and flucytosine [138] in pharmaceutical preparations. Recent studies have illus-... 

Polarographic methods have been extremely useful for the determination of the urinary excretion of the 1,4-benzodiazepines. An assay that employs selective solvent extraction and acid hydrolysis of diazepam and its major metabolites, iV-desmethyldiazepam and oxazepam, to their respective benzophe-nones has been employed to measure the urinary excretion of diazepam [183]. A pulse polarographic assay has been reported that will measure the urinary excretion of bromazepam following a single 12-mg dose [184]. The assay employs selective extraction of bromazepam and the 2-amino-5-bromobenzoyl-pyridine metabolite from the deconjugated metabolites, 3-hydroxybromazepam and 2-amino-3-hydroxy-5-bromobenzoylpyridine, into separate diethyl ether fractions. The residues of the respective extracts are dissolved in phosphate buffer (pH 5.4) and analyzed by pulse polarography, which yields two distinct... 

The measurement of an equilibrium constant requires the assumption of a non-thermodynamic measuring technique. Procedures that have been employed to measure K for reaction (15.37) include calorimetric, potentiometric, conductimetric, NMR, light absorption, and polarographic methods. The calorimetric measurements are especially useful, because they give ArH° for the reaction in addition to K. Since... 

The polarographic and potentiometric methods are not HPLC-run. The polarographic method relies upon the measurement of half-wave potentials of various sulfur compounds reacting with a mercury electrode. It is sensitive to submicromolar concentrations (Luther, pers. com.). While sulfide, thiosulfate, polysulfide and polythionates can be measured, the initial sample must be subdivided and pretreated in different ways. The disadvantages are that sample preparation ana analysis are time-consuming and there is no way to preserve samples for later analysis nor to study organic thiols with precision., ... 

Belal et al [15] developed a polarographic method for the determination of EDTA in pharmaceutical dosage forms based on chelation with Eu(III) at pH 4, and followed by measuring the cathodic current. The current-concentration range was found to be linear over the ranges 8-160 pg/mL and over 2-120 pg/mL using direct current (DCt) and differential pulse (DPP) modes of detection, respectively, with minimum detectability of 0.1 pg/mL. 

Reduction potentials for a variety of the 11-atom ligand complexes have been measured either by cyclic voltammetry or by polarographic methods (see Table IV). In certain cases the most stable formal oxidation state for... 

Song et al. [11] developed a polarographic method for the determination of mefenamic acid in tablets, which was based on rapid nitrosation of mefenamic acid with sodium nitrite in acetic acid, and subsequent measurement of the A-nitroso derivative of mefenamic acid by linear-sweep polarography. The method is simple, sensitive, and specific, and was characterized by a detection limit of 2 x 10 7 mol/L. 

The importance of solvent effects can be extracted from correlations of half-wave potentials determined by CV or polarographic methods with other measures of oxidizability, especially gas-phase free energies of ionization and ionization potentials. Within a series of unsubstituted metallocenes that differ only by the metal, the oxidation potentials of the compounds in solution and their gas-phase free energies of ionization vary directly (Fig. 17),154, 60 indicating that the effect of solvent is consistent from one metallocene to another. 

Evidently it is more important to determine the cupric ion concentration than the total copper concentration in natural waters. Polarographic methods have been used to measure the ligand concentration (complexing capacity) (10) and to determine stability constants for some strong chelators (11). 

Hydride transfer rate constants were measured only recently by Stomkowski and the first published values of kn were later confirmed and complemented by other authors It is interesting to note that kn determined spectrophoto-metrically (the rate of disappearance of (C6H5)3C agrees well with the results obtained by the polarographic method elaborated by Plesch ° ... 

The amount of reducible form present in solutions of several aldohexoses and aldopentoses was measured by Cantor and Penis-ton by polarographic methods. Their measurements showed a... 

The pulse polarography of adenine and its nucleosides and nucleotides agree with those obtained with the classical polarographical method. However, at higher pH the pulse-polarographic waves can be better measured than the d.c. polarographic waves [81]. 

Polarographic methods can be used to estimate the equivalence point of a reaction, provided that at least one of the participants or products of the titration is oxidized or reduced at the microelectrode. When the potential applied across the two electrodes is maintained at some constant value, the current may be measured and plotted against the volume of the titrant, thus, the term amperometric titration. In the case of working electrode-reference electrode pair, the potential of the indicator electrode is maintained at a constant value with respect to a reference electrode, measuring a limiting current, which is proportional to the concentration of one or more of the reactants or products of the titration. 

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