Polarographic method, measurement compounds

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., ... 

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. 

If the sample is a complicated mixture, it depends on the nature and ratio of the single constituents whether or not a polarographic method is preferable. It is a simple problem in polarography to determine a trace constituent that is reduced at more positive potentials (A) than an excess of a reducible compound (B) (Fig. 22, curve 1). Similarly a small anodic wave at more negative potentials (C ) can be measured exactly in the presence of excess of a substance (D ) which gives a large anodic wave at more positive potentials (Fig. 22, curve 4). 

Thermodynamic functions (entropy, heat capacity, enthalpy and free energy functions) have not been reported for 1,2,4-thiadiazoles. The ionization constants of a number of 1,2,4-thiadiazoIes have been determined potentiometrically or by Hammett s method (65AHC(5)ll9). Polarographic measurements of a series of methylated 5-amino-l,2,4-thiadiazoles show that thiadiazoles are not reducible in methanolic lithium chloride solution, while thiadiazolines are uniformly reduced at E0.s = -1.6 0.02 V. This technique has been used to assign structures to compounds which may exist theoretically as either thiadiazoles or thiadiazolines (65AHC(5)119). 

In addition to this comprehensive scheme, other determinations may be carried out by utilizing specific properties of the sulfur compound types (54, 72). Sulfur may be determined by removal with elemental mercury 51, 70), or by reaction with a mercaptan. In the latter case the sulfur determination is made by measurement of the excess mercaptan after the sample has been treated with litharge, sodium hydroxide, and a known excess of butyl mercaptan 78,81). A qualitative method of detecting elemental sulfur in gasoline is by the addition of sodium hydroxide to a pyridine solution of the gasoline which turns blue if sulfur is present 67). Free sulfur can also be determined polarographically 60). 

Kalvoda has reviewed different electrochemical methods suppression of polarographic maxima, eletrocapillary measurements, Kalousek commutator technique, differential pulse polarography and tensammetric methods for the determination of oily substances in various types of waters. These compounds are neither reducible nor oxidizable but are surface active. The study of oil fractions has shown that the Kalousek technique responds only to higher levels of dissolved petroleum fractions (0.02 to 100 mg/1) found only in pollution sources such as harbors and refinery effluents.Recent results indicate that the water pollution with petroleum can be determined by differential pulse tensammetry. Good calibration graphs were obtained with Diesel oil at a concentration from 30-50 /xg to 0.5 mg/1 and with Saratov petroleum from 0.1-0.2 to 3.3 mg/1. ... 

The lower detection limit is improved when a thinner spacer is used, and 1x10" M can be achieved easily. The method is very simple and even simple and inexpensive dc polarographs can be used. The selectivity of the method is the same as for normal dc polarography. A high excess of an easily oxidized compound prevents the measurement of another compound oxidized at a higher positive potential. 

Since its introduction, the technique has been associated with important applications in many branches of chemistry a principal example is in analytical chemistry, including that of organic substances. The method provides a simple technique for the detection and estimation of electroreducible ions and compounds present in solution at very small concentrations. The sensitivity of conventional polarography allows the measurement of concentrations of various ions to approximately 10 mol liter. Several procedures have been developed to extend the analytical range, notably the compensation method and the square wave polarographic technique introduced by Barker and Jenkins. " ... 

A sensitive method for the estimation of different surface active compounds (tetraalkylammonium salts, dextrans, crude oil components) in water is based on the measurements of the depression of the electrocapillary curve under conditions where the transport of the compound to the dropping Hg-electrode is accelerated by stirring of the solution being examined[12] The detection limit is in the range of 10-100 jjg 1 More details on "adsorptive polarographic analysis" are given in[13]. 

Although the direct application of polarographic waves as a measure of the functional group content is limited to certain groups of compounds, there is a whole field to be explored in which polarography can be used to evaluate the existing chemical methods of functional analysis. It should be, for example, possible to determine the total amount of oximes, or the decrease of the... 

In indirect methods, as mentioned in Chapter IV, polarographic-ally inactive substances are transformed into compounds showing waves on polarographic curves, or concentration changes of a polarographically active substance which reacts with the electro-inactive compound to be determined, are measured. Finally, polarometric (amperometric) titrations can be included in this group of analytical methods. 

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. 

The materials tested, and the half-wave potentials measured, are given in Table 4. If the final polarographic analysis of the pyrolysis products and the corresponding nitro-compounds is carried out in three different buffered solutions, this interesting method would gain additional selectivity. 

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