Experimental techniques logarithmic method

The potentiometric titration method possesses some advantages characteristic of potentiometry. The measured e.m.f. values are dependent on the logarithm of the activity (concentration) of the potential-defining ion and this considerably widens the range of concentrations which can be detected. The experimental techniques and routine are rather simple and the obtained results are well reproducible. Owing to the stable activity coefficients of metal cations the measurements can be performed in sufficiently concentrated solutions of the corresponding metal halides. Performing the measurement does not imply any interference in the acid-base processes in the melt studied and the experiment is faster than the isothermal saturation technique discussed above. 

This phenomenon has been studied by different combined electrochemical techniques such as -> spectroelec-trochemistry, radioactive -> tracer method, -> electrochemical quartz crystal microbalance, conductivity etc. by varying the experimental parameters, e.g., film thickness, the composition and concentration of the electrolyte solutions, the wait-time at different waiting potentials, and temperature [iii-x]. Several interpretations have been developed beside the ESCR model. The linear dependence of the anodic peak potential on the logarithm of the time of cathodic electrolysis (wait-time) -when the polymer in its reduced state is an insulator -has been interpreted by using the concept of electric percolation [ix]. Other effects have also been taken into account such as incomplete reduction [vii], slow sorp-tion/desorption of ions and solvent molecules [iii-vi], variation of the equilibrium constants of -+polarons and - bipolarons [viii], dimerization [xi], heterogeneous effects [xii], etc. 

The automation of the torsion pendulum utilizing a desktop computer eliminates the tedious data analysis previously associated with that technique. Any one of four data reduction methods can be used the experimental conditions will determine which is the optimum one to employ. The torsion pendulum technique provides quantitative values of shear modulus and logarithmic decrement and in the torsion braid mode provides a qualitative analysis of materia i, especially in the liquid-to-solid transition region. 

In this simple technique, the metal to be determined, in the form of a solution of a suitable compound, is sprayed into a flame. As in atomic absorption, when the solvent evaporates in the flame, the solid obtained is atomised and a gaseous metal ion is excited to a higher electronic level. When this drops to a lower level, a line spectrum is emitted and its intensity is measured. Flame photometers rely on the use of filters to isolate the line emitted, which is detected by a photocell and its output is measured by a calibrated galvanometer. The method is applicable to 16 metals. Reliable results are only obtainable by careful control of the experimental conditions. These depend on temperature (i.e. the type and rate of flow of the flammable gas and the oxidant which is usually air), the rate of flow of the solution to the flame as well as the compound tested and solvent used. A method used to minimise the effects of these variables is to add a known constant amount of an internal standard of a compound of a metal other than the metal to be determined but with similar excitation characteristics. Ihe ratio of the intensities of the standard and the test sample is determined. A calibration plot of the logarithm of the intensity ratio and the logarithm of the concentration of the test element is drawn. The concentration of an unknown is found by interpolation of the calibration plot. Alternatively, the standard additions method as in Sec.2.4.3 is used. In all cases, allowance should be made for any dilution effects. 

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