Methods of oxide solubility determination

The complete set of constants describing the equilibria taking place in the saturated solutions of oxides can be obtained if we know at least two parameters of the three presented in equation (3.6.8). 

Since there are no well-developed and generally accepted methods for the determination of the activity coefficients and of their dependence on the concentration of the corresponding particles in molten salts, practical investigations of solubility usually result in the corresponding parameters being obtained based on the concentration, but not on the ion activities. 

Investigations of the solubilities of metal oxides in oxygen-containing and oxygen-free ionic melts are performed with saturated solutions obtained in different ways. During the past few decades two methods for the solubility studies have been examined and introduced into practice. They are the methods of isothermal saturation and of potentiometric titration. 

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Part 6. Characteristics of Oxide Solubilities and Methods of Their Determination... 

Thus, in the present part we have considered the key features of the main methods used for the determination of oxide solubilities in molten salts. Such an analysis of their advantages and drawbacks will be very helpful for the consideration and understanding of the data on metal-oxide solubility in alkali-metal halide melts which will be discussed in Part 7. 

As a rule, the investigations of oxide solubility are performed within the temperature range 500-800 °C. At these temperatures the choice of substances suitable for this purpose is essentially limited by their chemical and thermal stability. All the above-described methods of solubility determination imply the existence of the following equilibrium in the melt ... 

In certain cases, the results obtained by different authors are in good agreement, which can be explained by the similar experimental conditions. They are the method of solubility determination, the donor of oxide ion used and the initial molality of the cation forming the oxide being studied. However, the scatter of the solubility data is usually too large. Such a scatter of the data has not been explained. Naturally, the impossibility of systematising of the obtained results has not favoured the development of oxide-solubility investigations in molten salts. 

Instrumental Quantitative Analysis. Methods such as x-ray spectroscopy, oaes, and naa do not necessarily require pretreatment of samples to soluble forms. Only reUable and verified standards are needed. Other instmmental methods that can be used to determine a wide range of chromium concentrations are atomic absorption spectroscopy (aas), flame photometry, icap-aes, and direct current plasma—atomic emission spectroscopy (dcp-aes). These methods caimot distinguish the oxidation states of chromium, and speciation at trace levels usually requires a previous wet-chemical separation. However, the instmmental methods are preferred over (3)-diphenylcarbazide for trace chromium concentrations, because of the difficulty of oxidizing very small quantities of Cr(III). 

For the rapid determination of Tc in a mixture of uranium fission products. Love and Greendale have used the method of amalgam polarography. It consists in a selective reduction of technetium at a dropping mercury electrode at a potential of —1.55 V vs. SCE in a medium of 1 M sodium citrate and 0.1 M NaOH. Under these conditions, technetium is reduced to an oxidation state which is soluble in mercury. The amalgam is removed from the solution of fission fragments and the amount of Tc determined in nitric acid solution of the amalgam by a y count. For Tc the measurement accuracy is within 1 %, and the decontamination factor from other fission products 10 . 

Phenan thro line (182) can be used instead of thiocyanate to form a complex with Fe(III) ions resulting from the oxidation of Fe(II), and the measurement is made at 500 to 510 nm. The use of 182 has the advantage of stability in the presence of air and also of allowing the use of hydrocarbon solvents for increased solubility of certain analytes. The method was applied for determination of hydroperoxides in natural rubber and synthetic elastomers, in the range of 10 to 20 ppm active oxygen. The sensitivity can be improved to less than 1 ppm, depending on the color of the sample solution. ... 

A method to measure soluble organic carbon in seawater includes oxidation of the organic materials to C02 with K2S2Og, followed by gravimetric determination of the C02 trapped by a column of NaOH-coated asbestos. A water sample weighing 6.234 g produced 2.378 mg of CO, (FM 44.010). Calculate the ppm carbon in the seawater. 

Measurements of alkali solubles in these coal samples—conventionally accepted as indices of humic acid concentrations—were initially performed by using Kreulen s method (7). However, even when the most stringent precautions were taken to exclude air, this method yielded markedly time-dependent results (presumably owing to oxidation of the coal by the relatively strong alkali solution), and a more satisfactory colorimetric technique (by J. F. Fryer) was therefore employed. This entailed extracting the coal sample with 0.1 N aqueous sodium hydroxide for 16-20 hours in an inert atmosphere and subsequent photoelectric scanning of the extract solutions. Actual humic acid concentrations were then obtained from specially constructed reference curves which related optical density (at an appropriate wavelength) to humic acid contents. The inherent error in this determination is estimated at less than 10%. 

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