Conductimetric titrations

Conductimetric measurements can also be used to ascertain the end-point in many titrations, but such use is limited to comparatively simple systems in which there are no excessive amounts of reagents present. Thus, many oxidation titrations which require the presence of relatively large amounts of acid are not suited to conductimetric titration. Conductimetric titrations have been largely superseded by potentiometric procedures (see Chapter 15), but there are occasions when the conductimetric method can be advantageous.14... 

Attention is directed to the importance of temperature control in conductance measurements. While the use of a thermostat is not essential in conductimetric titrations, constancy of temperature is required but it is usually only necessary to place the conductivity cell in a large vessel of water at the laboratory temperature. 

A conductivity cell for conductimetric titrations may be of any kind that lends... 

Some typical conductimetric titration curves are collected in Fig. 13.2 (a)-(d). 

Clearly for titration purposes, it is low-dielectric constant conducting solutions which will be important, and addition of a suitable reagent to such a solution permits the plotting of a titration curve from which the end point can be deduced as described in Section 13.7. It should be noted that in view of the enhanced conductance in the high-frequency field, the maximum concentration of reagents is much smaller than with normal conductimetric titrations, and the maximum concentration will depend on the frequency chosen. It is found that... 

Figure 4.18 Conductimetric titration curves. As an acid is titrated with an alkali, so the ionic composition of the mixture changes and is reflected in the conductivity of the solution, (a) A strong acid and a strong base, (b) A strong acid and a weak base, (c) A weak acid and a weak base, (d) A weak acid and a strong base.

Conductimetric titrations, particularly when they are automated, are useful if either of the reactants is deeply coloured, so preventing visual monitoring of the titration using indicators, or if both reactants are very dilute. 

Adsorption and desorption reactions of protons on iron oxides have been measured by the pressure jump relaxation method using conductimetric titration and found to be fast (Tab. 10.3). The desorption rate constant appears to be related to the acidity of the surface hydroxyl groups (Astumian et al., 1981). Proton adsorption on iron oxides is exothermic potentiometric calorimetric titration measurements indicated that the enthalpy of proton adsorption is -25 to -38 kj mol (Tab. 10.3). For hematite, the enthalpy of proton adsorption is -36.6 kJ mol and the free energy of adsorption, -48.8 kJ mol (Lyklema, 1987). 

Fig. 7.8 Conductimetric titration of 3,5-dinitro-benzoic acid (HA) with triethylamine. (I)...

Fig. 10.3 Conductimetric titration curve of an amine impurity with a very weak acid.

The final increase in particle size, shown in figure 7, is pro bably caused by limited flocculation,since particle coverage by emulsifier is very limited. Conductimetric titration of emulsifier shows that only a part of it is used for stabilizing particles Typi cal results are shown in figure 9. 

A stable and sensitive conductivity cell has been described that measures the DC current that passes through a small volume of solution confined in a small capillary tube. The very simple apparatus is illustrated in Figure 6.19. When filled with 0.1 M KC1, the cell exhibits an Ohm s law behavior with an applied voltage from 5 to 100 V (approximately 5-100 mA of current). The cell also can be used for conductimetric titrations, with the titration carried out in vessel B. Because some liquid flows through the capillary during a titration, a correction factor is used to obtain the most accurate results (although corrections usually are <0.2%). 

Soil or soil-mineral titrations are often used to establish surface acidity composition and acid-base behavior. Soil or soil-mineral surfaces are complex in nature owing to their large variation in functional group content and behavior. For example, the data in Figure 3.33 show that soil surface acidity is made up mostly by A1 and a smaller quantity of H+. The titration behavior of such soil would depend on amount of A1 present, affinity by which this A1 is adsorbed by the surface, degree of surface A1 hydroxylation, and finally the pKg values of the surface-associated H+. Commonly, two types of titrations are employed to evaluate soil or soil-mineral surfaces (1) conductimetric titration and (2) potentiometric titration. 

Conductimetric titration denotes change in specific conductance of any given clay or soil suspension as a function of base or acid added. An ideal conductimetric titration curve is shown in Figure 3.34. The first slope of the curve (left-hand side) is attributed to easily dissociated hydrogen (very low pKa surface-functional groups) (Kissel et al.. 

The second and third slopes are attributed to Al3+ and interlayer hydroxy-aluminum (Kissel and Thomas, 1969 Rich, 1970). Conductimetric titrations of two soil samples at three initial pH values are shown in Figure 3.35. None of the soil samples contain dissociated protons. This was concluded because the lines with the... 

Some soils produce conductimetric titration lines that parallel the x axis they represent adsorbed Al3+. When the slopes of the titration lines become positive, they represent surface-adsorbed Al-hydroxy species (Kissel and Thomas, 1969, Kissel et al., 1971, and Rich, 1970). For example, at all three initial pH values, the Nicholson soil (Fig. 3.35) exhibits titration lines that nearly parallel the x axis. On the other hand, the Eden soil exhibits a line nearly parallel to the x axis only for the sample with an initial pH of 5.78. One expects this behavior to be exhibited by the sample with initial pH of 4.3. For the samples with initial pH 4.3 or 7.3 the titration lines exhibit positive slopes, suggesting neutralization of surface-adsorbed Al-hydroxy species. The overall data in Figure 3.35 show that most surface acidity of the soils is dominated by aluminum under various degrees of hydroxylation. 

Kissel, D. E. and G. W. Thomas. 1969. Conductimetric titrations with CaOH2 to estimate the neutral salt replacability and total soil acidity. Soil Sci. 108 177-179. 

Total Carboxyl and Phenolic Hydroxyl Content Conductimetric titrations employed were modifications of the procedure described by Sarkanen and Schuerch (11) for lignin total phenolic content. Spectroscopic determinations on the phenolics and neutrals fractions were carried out using the JEOL FX-900 Fourier Transform NMR spectrometer and the Nicolet 5SXC Fourier Transform Infrared Spectrometer. In addition, the solid state CP/MAS 13C-NMR spectra were obtained by the Regional NMR Center at Colorado State University using conditions described in Bryson et al. (12). 

The low temperatures required and the short liquid range are limitations, but conductimetric titrations are readily made. 

The capacity of the cell is 450 cc. It is constructed of silica, the adsorption of water on borosilicate glass being too troublesome. The electrodes are made of platinized platinum and have been calibrated with an aqueous solution of potassium chloride O.OIN at 0°C. Introduction of alkali metal and ammonia is made in a similar way to that used in kinetic studies. The formation reaction of the alkali amides is catalyzed by the electrodes. Various concentrations are studied with the same preparation of alkali amide. After ammonia has been removed, the alkali amide is hydrolyzed in the cell. The amounts of alkali ammonium hydroxides are determined by conductimetric titration. 

Potentiometric titration and assumed 1 1 metal/fulvate mole ratio in the complex. "Potentiometric and acid-base conductimetric titrations and assumed 1 1 metal/fulvate mole ratio in the complex. 

The twentieth century saw the evolution of instrumental techniques. Steven Popoff s second edition of Quantitative Analysis in 1927 included electroanalysis, conductimetric titrations, and colorimetric melliods. Today, of course, analytical technology has progressed to include sophisticated and powerful computer-controlled instrumentation and the ability to perform highly complex analyses and measurements at extremely low concentrations. 

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