Isohydric

This anomalous pH behavior results from the presence of polyborates, which dissociate into B(OH)2 and B(OH) as the solutions are diluted. Below pH of about 9 the solution pH increases on dilution the inverse is tme above pH 9. This is probably because of the combined effects of a shift in the equihbrium concentration of polymeric and monomeric species and their relative acidities. At a Na20 B202 mol ratio equal to 0.41 at pH 8.91, or K20 B202 mol ratio equal to 0.405 at pH 9 the pH is independent of concentration. This ratio and the pH associated with it have been termed the isohydric point of borate solutions (62). 

Isohydric 385 localizing 378 mixed solvents strength 465 isoeluotropic 467 properties 460 solubility parameters 460 solvatochromic parameters 461 solvent triangle classification 463 types 458... 

Figure 3.1 Four-step construction of the Bjerrum difference plot for a three-pi molecule, whose constants are obscured in the simple titration curve (see text) (a) titration curves (b) isohydric volume differences (c) rotated difference plot (d) Bjerrum plot. [Avdeef, A., Curr. Topics Med. Chem., 1, 277-351 (2001). Reproduced with permission from Bentham Science Publishers, Ltd.]...

The isohydric concept and the MCS are equivalent in adjusting and guarantee the constancy of the water concentration of the eluent., ... 

I lolling II column wiih an isohydric eluent, an equilibrium is established between the water in the eluent and that on the adsorbent, thereby yielding a constant and reproducible covering of the surface by water. 

The studies of Pauli he. cit.) and his co-workers, however, have revealed the fact that isohydric solutions of different acids do not effect equal combination with the isoelectric protein relatively more acetic acid for example being combined than hydrochloric acid in isohydric solutions. Again, both the actual position of these maxima as well as the magnitudes of the viscosities observed vary much with the nature of the acid employed. Thus the relatively weak oxalic acid appears to be a much stronger acid than sulphuric acid, whilst trichloracetic acid does not differ appreciably from acetic acid in its effect on the viscosity of albumin. It is probable that the degree of solvation of the protein molecules and of the protein salts must not be regarded as constant but that they vary both with the nature of the salt and in the presence of neutral salts which exert like alcohol a desolvating action more or less complete on the solvated isoelectric protein as well as on the undissociated protein salts. 

The logarithms of the times elapsing before appearance of turbidity are proportional to the concentration of the hydrogen ions in the mineral acids added, so that for isohydric solutions the time of reaction is independent of the acid employed.4 When hydrochloric acid is added slowly to a boiling solution of sodium thiosulphate, the sulphur is oxidized to sulphur dioxide 5... 

Errors in Measurements with Indicators.—Three chief sources of error in connection with pll determinations by means of indicators may be mentioned. In the first place, if the test solution is not buffered, eg., solutions of very weak acids or Imsos or of neutral salts of strong acids and bases, the addition of the indicator may produce an appreciable change of pH this source of error may be minimized by employing small amounts of indicator which have been previously adjusted, as a result of preliminary experiments, to have approximately the same pH as the test solution. Such indicator solutions are said to be isohydric vith the test solution. 

The two solutions are therefore not isohydric, and a displacement of equilibrium will occur, producing an effect on the conductivity which may be calculated, and tested for the case of equal volumes of each component. The total concentration of the acids is then... 

The solution is imagined divided into two parts of volume Vj and on the one side is the acid Zj H, on the other that Z2H, and the base is divided between the two in such a way that two isohydric solutionsare obtained—solutions, that is, in which the concentration of the hydrogen ions Oh is the same. According to p. 121 the equilibrium of the two solutions is not upset by mixing them, and the base is divided between the two acids in the same proportion after the mixture as before. 

Figure 46-9 Scheme demonstrating the isohydric and chloride shift.The encircled numbers refer to the reactions described in the text. For details, see text. 

The transformations described, so far (Figure 46-9, reactions 1 through 5) are referred to as the isohydric shift (i.e., a shift in which the hydrogen ion concentration remains unchanged). 

The acid-base error of indicators. Isohydric indicators. The measurement of pH in unbuffered or slightly buffered solutions. 

Isohydric indicator solutions. Reliable information concerning unbuffered solutions is obtained only when the pH of the liquid is unaffected by the added indicator. Actually the indicator solution should have a composition such that its pH equals that of the solution being investigated. Indicator solutions of this kind have been called isohydric by E. H. Fawcett and S. F. Agree. ... 

The indicator mixture with the [HI ] [I ] ratio equal to 10 5.5 is isohydric with water. v... 

Preparation of suitable indicator solutions. The prepar on of indicator solutions for use in the colorimetric pH determination has already been described in detail in Chapter Five ( 3). These solutions, however, were suited only for measurements of buffered solutions. Isohydric indicator solutions have to e prepared in another manner. H. T. Stern titrates the indicator with sodium hydroxide and measures the pH during neutralization with the quinhydrone electrode. A similar procedure has been described by Pierre and Fxjdge. Fawcett and Acree keep in stock a large series of neutralized indicator solutions and determine their pH approximately by colorimetric means. 

The reliability of the method is illustrated by results taken from the report of Kolthofp and Kameda. It is clear from the table that a 100 55 bromthymol mixture and a 100 10 phenol red mixture are isohydric with pure water. 

Results of measurements of the pH of slightly hydrolyzed zinc sulfate solutions are contained in the accompan3dng table. The determinations were made both with the hydrogen electrode and with isohydric methyl red solutions. ... 

Molar Concentration OP ZnS04 pH Ha-ELBCTRODB pH Isohydric Mbthyl Red... 

A value between 5.7 and 5.8 actually was found with isohydric methyl red. 

Ordinary distilled water is apt to contain more carbon dioxide than corresponds to equilibrium with air, frequently as much as 2.5 X 10 molar. If water is allowed to stand exposed to air but protected against dust for longer than a week, equilibrium is praetically attained. This condition is realized more quickly by bubbling through the water for ten hours air washed by acid and water. If the water so obtained shows a pH less than 5.7 with isohydric methyl red, acid impurities other than carbonic acid are responsible. A pH greater than 5.9 indicates the presence of basic contamination. 

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