Standard Solutions of Acids, Bases, and Salts

STANDARD SOLUTIONS OF ACIDS, BASES, AND SALTS (continued)... 

Standard KCl Solutions for Calibrating Conductivity Cells Molar Conductivity of Aqueous HF, HCl, HBr, and HI Equivalent Conductivity of Electrolytes in Aqueous Solution Ionic Conductivity and Diffusion at Infinite Dilution Activity Coefficients of Acids, Bases, and Salts... 

The electrical potential of tungsten in solutions of different acids, bases, and salts has been measured against certain standard electrodes at 25 C. The tungsten does not behave as an insoluble electrode, but sends ions into the solutions. Under certain specified conditions— for examine, with high-current densities (2 amperes per square decimetre) in aqueous alkalis, but with low-current densities in aqueous solutions of acids and salts—-the tungsten anode becomes passive. The passivity appears to be due to adherent films of hydrated oxides. The electrochemical equivalent of tungsten has been found to be 0-3178 mg. per coulomb," which is in close agreement with the theoretical value. 

It is obvious that the expression enclosed in the brackets by the author of the present book is nothing but the primary medium effect of O2- expressed via the difference in the values of the equilibrium constants of equation (1.3.6) for the media compared the molten equimolar KCl-NaCl mixture, which was chosen as a reference melt, and for which pKHa/H20 was found to be 14 at 700 °C, and the melt studied. As to the physical sense of the common acidity function Cl, this is equal to the pO of the solution in the molten equimolar KCl-NaCl mixture, whose acidic properties (oxide ion activity) are similar to those of the solution studied. Moreover, from equation (1.3.7) it follows that solutions in different melts possess the same acidic properties (f ) if they are in equilibrium with the atmosphere containing HC1 and H20 and Phc/Ph2o — constant. This explanation confirms that the f function is similar to the Hammett function. Therefore, Cl values measured for standard solutions of strong bases in molten salts allow the prediction of the equilibrium constants on the background of other ionic solvents from the known shift of the acidity scales or the f value for the standard solution of a strong Lux base in the solvent in question. According to the assumption made in Refs. [169, 170] this value may be obtained if we know the equilibrium constant of the acid-base reaction (1.3.6) in the solvent studied. 

These arguments are standard for all reversible reactions, and discussion of what happens when reactants and products are mixed, and, in particular, the prediction of the direction of reaction, are individual for each reaction. The basic principles, however, are identical for all cases. Typical reversible reactions for electrolyte solutions involve acid-base equilibria, ion pair, complex and chelate formation, and heterogeneous reactions such as solubility of sparingly soluble salts. 

Macro quantities of selenium can be determined gravimetrically after reduction to the elemental form by various reagents such as tin (II) chloride, potassium iodide, or ascorbic acid (I). Ooba described a technique whereby the element is precipitated from perchloric acid solution with hydrazine (2). Selenium may be titrated with standard solutions of sodium thiosulfate, iodide, and ferrous, chromous, or trivalent titanium salts after oxidation to Se(VI) (I). Photometric and fluorometric methods based on formation of the piaselenol with diaminobenzidine or 2,3-diaminonaphthalene has been used for the determination of selenium (I, 3,4,5). Interfering elements such as As, Co, Cr, Cu, Fe, Hg, and Ni, are masked with EDTA or other chelating agents. 

The polarometric determination of histidine > is based on an ingenious principle. The method utilizes the formation of an anodic wave of the polarographically active bihistidinatocobaltous ion which is produced by the addition of standard solutions of cobaltous salts to buffered histidine solutions. In the presence of histamine, protein hydrolysates and other amino acids, a phosphate pH 8 0 buffer is used when the interferences are at the minimum. In solutions, where only histidine is present, a Britton-Robinson pH 9T buffer is preferred in which the formation of the bihistidinatocobaltous ion is favoured. 

An aqueous solution, which consists of a coupled weak acid and salt or weak base and salt when such a solution is resisting to pH changes upon small addition of add or base, represents an example of the buffer solution. To measnre pH, a set of standard (buffer) solutions should be used to calibrate pH sensor. The compositions of buffer solutions are well known, and they cover a wide range of pH from 1 to 13. 

In the direct method, a solution of the ammonium salt is treated with a solution of a strong base (e.g. sodium hydroxide) and the mixture distilled. Ammonia is quantitatively expelled, and is absorbed in an excess of standard acid. The excess of acid is back-titrated in the presence of methyl red (or methyl orange, methyl orange-indigo carmine, bromophenol blue, or bromocresol green). Each millilitre of 1M monoprotic acid consumed in the reaction is equivalent to 0.017032 g NH3 ... 

Wittwer and Zollinger (1954) determined the neutralization curves of aqueous solutions of diazonium salts under standard conditions of ionic strength, etc., and found that the acidity depended on the degree of neutralization in a manner different to that expected for a dibasic acid. The curve obtained did not exhibit two steps with an intermediate region of a few pH units in which the monobasic acid is stable, as is the case, for instance, with oxalic acid (Fig. 5-1). On the contrary, there was only one step, but it extended over two equivalents of base per diazonium ion. 

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