Reduction polarographic analysis

The shift of the half-wave potentials of metal ions by complexation is of value in polarographic analysis to eliminate the interfering effect of one metal upon another, and to promote sufficient separation of the waves of metals in mixtures to make possible their simultaneous determination. Thus, in the analysis of copper-base alloys for nickel, lead, etc., the reduction wave of copper(II) ions in most supporting electrolytes precedes that of the other metals and swamps those of the other metals present by using a cyanide supporting electrolyte, the copper is converted into the difficultly reducible cyanocuprate(I) ion and, in such a medium, nickel, lead, etc., can be determined. 

In polarographic analysis it gives two breaks in the curve, indicating a two electron reduction probably by the eqn ClCH2N02+2e = CH2N02 +CJ- (Ref 5)... 

Dichloronitromethane. This article supplements the material in Vol 5, D1213-R under Dichloro-methane and Derivatives. In polarographic analysis it shows a complex curve indicating two 2-electron reductions giving finally NMe and two Cl ions (Ref 1). Its pKa at 25° was found to be 5.99 (Ref 2)... 

Bromonitromethane. This article supplements the material in Vol 2, B312-R under Bromo-rnethane and Derivatives CA Registry No 563-70-2. The IR and UV spectra are given in Ref 3. In polarographic analysis a two electron reduction to CH2N02 and Br is indicated (Ref 4). It can be quanty detd by treatment with coned sul-... 

Many handbooks like the CRC Handbook of Chemistry and Physics provide, on behalf of electrochemistry investigation, values of standard reduction potentials, listed either in alphabetical order and/or in potential order. These must be considered as potentials of completely reversible redox systems. In current analytical practice one is interested in half-wave potentials of voltammetric, mostly polarographic analysis in various specific media, also in the case of irreversible systems. Apart from data such as those recently provided by Rach and Seiler (Spurenanalyse mit Polarographischen und Voltammetrischen Methoden, Hiithig, Heidelberg, 1984), these half-wave potentials are given in the following table (Application Note N-l, EG G Princeton Applied Research, Princeton, NJ, 1980). 

The ac polarograms of the mixture at varying frequencies (Fig. 19) shows four ac summit peaks corresponding to reduction of T1(I), In(III), Cd(II), and Zn(II). The summit peaks for In(III) and Cd(II) are very close and so their ac waves are not very sharp. The first summit peak corresponding to T1(I) appears to be due to the combined reduction of lead and thallium ions, as is evident from the summit peak height. Hence, ac polarographic analysis only enables the identification of four metal ions out of seven and... 

The ESR data for several silaacenaphthene and silaphenalene radical anions (Table VI) obtained by electrolytic reduction (101), are similar to the results for trimethylsilyl-substituted naphthalenes (Table V) (49). The polarographic reduction potentials were 0.2-0.4 volt more positive than that of naphthalene (101). Similarly, some silaphenalene derivatives of the type X[R = H(R ,R = H,H H,Me Me,Me) and R = Ph R1 = R2 = Me] were shown by polarographic analysis to reduce more easily than silylnaphthalene or naphthalene and the unassigned ESR spectra of the radical anions were reported (99). 

Irradiation at 254 nm of an aqueous neutral solution of the reduction product of wave I led to stepwise appearance of a new band in the UV with 305 nm, with a quantum yield of 0.08 (Fig. 5), with on overall yield of 80%. Chromatography and polarographic analysis demonstrated that the photoproduct was the parent l-methyl-l,2-dihydro-2-iminopyrimidine... 

Polarographic analysis of a series of substituted phenylarsonic acids shows that the reduction process is electrochemically irreversible giving linear plots of log [(L — i)/i] vs E from which a value of an = 0.49 was determined according to equation 22 for all members of the series except for the p-Cl derivative . The cm value was independent of pH... 

Polarographic analysis of the first wave in 0.1 M HCl showed that a plot of log(i,j — i) vs was linear, which is consistent with a reversible reduction to an insoluble product ". Application of DPP showed a change in the width of the first peak when pH was changed from 2 to 8 which suggested that the electrode process changed from a reversible to an irreversible process . At the same time it was concluded that the product of the first reduction process adsorbed to the electrode surface , and from CV it appears that this adsorbed product could be oxidized at 0.06 V vs NHE (pH 4.7). When the sweep is reversed after the second double peak, a new oxidation peak at — 0.30 V vs NHE arises in addition to the oxidation peak at 0.06 V vs NHE (pH 4.7) . 

Y(III) and Sc(III) yield double reduction currents representing the reduction to the bivalent form and subsequently to the metallic state, although there is little reported polarographic analysis for elements from group IIIB. 

As ammonium ions give a reduction wave in tetra-alkyl-ammonium hydroxide solutions, a method for the determination of nitrogen was devised. The method consists of mineralization by the conventional Kjeldahl method used in the semimicro or micro modification. Because waves at very negative potentials (e.g. of ammonium ions) are usually less reproducible than waves at more positive potentials, the accuracy of this method is somewhat lower than that for ordinary polarographic analysis. Other types of polarographic determinations of ammonium ions could be applied, e.g. using the anodic waves corresponding to the formation of salts with mercury or the condensation with phthalic dialdehyde (p. 142). 

Addition methods were used for the determination of m-saturated compounds and olefins. The methods are based on the additions of bromine to unsaturated bonds, and the waves for the brominated compounds corresponding to the reduction of o, j8-dibromides (involving elimination) are measured. Their heights are proportional to the concentration of the unsaturated compound. Thus vinylchloride and 1,2-dichloroethylene were transformed into l-chloro-l,2-dibromoethane and l,2-dichloro-l,2-dibromoethane, by the action of a 3 M solution of bromine in methanol saturated with sodium bromide. The excess of bromine was removed with ammonia and the polarographic analysis was performed with sodium sulphite or lithium chloride as a supporting electrolyte. On the other hand, acetylene, vinyl-chloride, 1,2-dichloroethylene and 1,1,2-trichloroethylene were determined ) after a 24 hr reaction with bromine in glacial acetic acid (1 1). The excess bromine was removed with a stream of nitrogen or carbon dioxide. An aliquot portion is diluted (1 10) with a 3 M solution of sodium acetate in 80 per cent acetic acid and after deaeration the curve is recorded. 

For this example a modification of the existing analytical method which employed titrimetry, was used. After the conversion, which is carried out essentially in the same way as in the titri-metric method, the solution (instead of extraction, oxidation-reduction treatment and final titration) is directly subjected to polarographic analysis in a pH 8-8 ammonia and ammonium chloride buffer. 

Defect concentrations of Fe in Fei xO are usually measured by chemical analysis. It is impossible to determine the compositions of non-stoichiometric compounds, because the error of an ordinary quantitative analysis is about 10, while the deviation of a crystal with intrinsic defect from its stoichiometric composition is about <10. Nevertheless, it is possible for chemical analysis to determine if the metal atoms in non-stoichiometric compounds are excessive or less. Because a non-stoichiometric compound, in common, is a multi-component solid solution in which the different components have different valences, e.g., Fei xO can be viewed as a solid solution which consists of Fe +O and Fe2" 03. Deviation of those types of compounds can directly be determined by measuring of the concentration of an atom that shows an abnormal valence in it. For example, it forms the solutions containing large amounts of Fe + ion and less amounts of Fe + ion, when Fei xO (catalyst) is solved by hydrochloric acid solution under conditions with the absence of air or oxygen. Among these ions, the contents of both Fe + and Fe + can be determined by a titration of EDTA, but the Fe + needs to be oxidated to Fe + by ammonium persulfate prior to titration. The volume ratio of EDTA solutions that are consumed by Fe + and Fe + ions respectively is the ratio of Fe + and Fe + in sample, namely defect concentration of Fe, i.e., x = 1/(3 - - 2Fe +/Fe +). There is also Kulun titration or polarographic analysis except for oxidation-reduction titration which can be used for such measurement of an ion with abnormal valence in the solution of a solid sample. 

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