Conductometric Measurement

End Point Determination Adding a mediator solves the problem of maintaining 100% current efficiency, but does not solve the problem of determining when the analyte s electrolysis is complete. Using the same example, once all the Fe + has been oxidized current continues to flow as a result of the oxidation of Ce + and, eventually, the oxidation of 1T20. What is needed is a means of indicating when the oxidation of Fe + is complete. In this respect it is convenient to treat a controlled-current coulometric analysis as if electrolysis of the analyte occurs only as a result of its reaction with the mediator. A reaction between an analyte and a mediator, such as that shown in reaction 11.31, is identical to that encountered in a redox titration. Thus, the same end points that are used in redox titrimetry (see Chapter 9), such as visual indicators, and potentiometric and conductometric measurements, may be used to signal the end point of a controlled-current coulometric analysis. For example, ferroin may be used to provide a visual end point for the Ce -mediated coulometric analysis for Fe +. 

Dr. T. Nagy s help in performing conductometric measurement is gratefully aknowledged... 

For polymer/additive analysis, electrogravimetry, potentiometry, conductometry and voltammetry have never played a major role. Because of many complications, which can arise by the use of conductometry for complicated matrices (such as most polymeric compounds), the technique is not extensively applied in this field. Conductometric measurements are mostly... 

During the conductometric measurements it was also observed that for various acids the equivalent conductivity became constant at higher concentrations, which can be explained67 by the formation of their homoconjugate ions HX2 = X" H+ X" for a number of acids we estimated by means of the method of French and Roe68 the formation constants concerned ... 

Conductometric measurements of SeF4 in liquid hydrogen fluoride prove it to be a weak base (25) [Eq. (2)] ... 

It has long been recognized that ferric iron is a moderately strong acid. As early as 1896, Goodwin (5) concluded from conductometric measurements that simple dilution of ferric chloride solutions led to the formation of FeOH2+. The insolubility of ferric hydroxide has of course been appreciated even longer. The best current estimate of the solubility product constant for Fe OH)s at 25° (in 3 M NaC104) is (d). 

The first viewpoint contradicts the autocatalytic character of the reaction, conductometric measurements in the polymerization system and some other facts (see below). Scheme (33) can be considered as completely experimentally substantiated. The following important proofs were obtained A direct experimental discovery of a quaternary ammonium alcoholate in the reaction system, 42) a full agreement of the nature of the active propagating site with all the existing kinetic and structural data l4,149 153 157 I58) establishment of the ionic behaviour of the propagating sites by comparison of the kinetic curves of the process with the character of the electric... 

In this chapter we take a careful look at the phenomenon of electrical conductivity of materials, particularly electrolytic solutions. In the first section, the nature of electrical conductivity and its relation to the electrolyte composition and temperature is developed. The first section and the second (which deals with the direct-current contact methods for measuring conductance) introduce the basic considerations and techniques of conductance measurement. This introduction to conductance measurements is useful to the scientist, not only for electrolytic conductance, but also for understanding the applications of common resistive indicator devices such as thermistors for temperature, photoconductors for light, and strain gauges for mechanical distortion. The third section of this chapter describes the special techniques that are used to minimize the effects of electrode phenomena on the measurement of electrolytic conductance. In that section you will encounter the most recent solutions to the problems of conductometric measurements, the solutions that have sparked the resurgent interest in analytical conductometry. 

Conductometric measurements were used in a recent study of the hydrolysis of alkyldi- and trichlorosilanes in polar solvents by Shaffer and Flanigen (66). A first order dependence on water concentration and a second order dependence on chlorosilane was observed for the hydrolysis of trichlorosilanes. The dichlorosilanes evinced a highly anamolous negative order. For the trichlorosilanes the data are consistent with a mechanism that assumes the following steps ... 

The kinetics of alkoxysilane hydrolysis have not yet been resolved successfully. Reuther (56) published a preliminary report a few years ago on conductometric measurements on some alkoxysilanes, but kinetic expressions were not devised. Shaffer and Flanigen (66) also found that conductometric data obtained on hydrolysis of alkoxysilanes were difficult to interpret. 

The bromination of dibenzoazepine 63 in 1,2-dichloroethane gives the /raw.v-dibromide 64 as the only product. The reaction was monitored spectrophotometrically and found to exhibit a third-order kinetics (second-order in Br2). A significant conductivity has also been found during the course of bromination. Both spectrophotometric and conductometric measurements are consistent with the presence of Br3- salt intermediates at a maximum concentration of ca 2% of that of the initial reactants. The X-ray structure of dibromide 64 shows a considerable strain at carbons bearing bromine atoms. The strain appears to be responsible for an easy, spontaneous debromination of 64, as well as for high barrier for the formation of 64 from the bromonium-tribromide intermediate. That makes possible the cumulation of the intermediate itself during the bromination of 63119. 

Potentiometric, cryoscopic, and conductometric measurements of the reaction between Fe3+ and B40 have revealed the presence of FeB4Of (373), and in oxalate media, the cationic complexes FeB02+ and Fe(B02)2 were assumed to be present (370). The instability constant for FeBOi+ was estimated as 3.12 x 10 by spectrophotometric techniques (68). The appearance of IR absorption bands at 1260,1105, 860, and 815 cm 1 was attributed to the formation of an iron borate complex during oxidation of elemental iron in borate solutions (118). 

As with amperometric titrations, to have straight-line portions of the titration curve dilution corrections must be made because the response is directly dependent on the concentration of the ionic species. Also, the important data are taken before and after the equivalence point rather than precisely at the equivalence point. The general conditions for effective conductometric measurements of solutions are discussed in Chapter 5 and are directly applicable when the system is used as the endpoint detection method. A particularly complete review of the subject has been presented.8... 

Another method widely used is based on conductometric measurements at short times after an irradiation pulse. This method determines the electrical charge of the species studied and is thus useful for radicals where several dissociations can take place. This technique can be complicated by buffering effects if the parent compound itself undergoes acid-base equilibria in the region of interest for the study of the radical (see e.g. Bhatia and Schuler, 1973b). It is therefore, a prerequisite to know all the other p/ -values which may be involved. Another limitation of this technique lies in the fact that only p f-values between about 2 and 12 can be studied and high background conductivity decreases the sensitivity of the measurements. 

In conductometric measurements of two-component system where HN03 was one of the components, Usanovich and his co-workers [50-50c] confirmed the view that nitric acid has an amphoteric character-it can be mi acid or a base depending on the other component. Thus, it acts as a base towMds sulphuric acid and as a strong acid towards water. It also acts as an acid towards CH3OOH Mid CCl3COOH and combines with these compounds, but behaves as a base with CH2ClCOOH and CHCl2COOH. I... 

The objective of Usanovich s work, mentioned above [50] was to elucidate the structure of mixtures of nitric acid with acetic or chloroacetic acid by means of conductometric measurements. 

Zwitter ions — Ions that have opposite charges at different sites of the molecule, e.g., amino acids in the zwit-terionic form +H3N-CH(R)-COO . Zwitter means in German hermaphrodite. Zwitter ions do not move in an electric field and thus conductometric measurements do not allow detecting their hermaphroditic character. - Kolthoff and - Furman [i] used the term hybrid ion however, that term is now obsolete. 

Foams from Triton-X-100 solutions (10 3 mol dm"3), containing NaCl (0.4 mol dm"3) were studied. The foam volume was 40 cm3. Foam expansion ratio was regulated by changing either the time for foam drying or the pressure Ap in the solution, contacting the porous plates. Conductometric measurements of foam expansion ratio were performed for comparison. 

The rate constants of nearly all of the elementary reactions in trityl-initi-ated polymerizations of cyclopentadiene [216], p-methoxystyrene [186], vinyl ethers [217], and a-methylstyrene [218] were determined by kinetic measurements, sometimes combined with conductometric measurements. Monomer conversion was followed by either dilatometry, spectroscopy, or calorimetry. Initiation was followed by the decrease in the 410-nm absorption of the trityl carbenium ions (e = 36,000 mol- L em-1), caused by their reaction with monomer by either direct addition or hydride abstraction. The initiator was assumed not to be consumed in any other reactions. The reaction orders (usually first order in each reagent) and rate constants of initiation were then determined by plotting the rate of initiation versus the initial monomer and initiator concentrations according to Eq. (52). 

Ionic mobilities under consideration here are based on the results of conductometric measurements carried out on diluted solutions (10"3 c 7.10"3 mol/L) of Bu4NC1, Bu4NBr, Bu4NI, BruNClO iAm3BuNI (TABI), NaBPn4, and Nal in water-TMS at 30°C (2,12). Experimental... 

The failure of the Fuoss equation to reproduce experimental data appears particularly evident by conductometric measurements of hydrochloric acid (18) and by spectrophotometric measurements of picric acid (21) which we have carried out in TMS at 35° and 30°C, respectively. 

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