Hydrogen-ion activity in solution

The value of corresponding to values of Ka2 and Ka2 determined from Method I would be relatively small, reflecting the predominance of the neutral XOH group. (ii) The hydrogen ion activity in solution that yields zero protonic surface charge is defined from a combination of Equations 1 and 2 with nx+ set equal to nx- ... 

At the other side of the membrane, a similar hydrogen-ion exchange is obtained (not shown in Fig.3.2). At both interfaces, a potential is obtained (E2 and Eu respectively), and the potential difference over the entire membrane is defined as AE=E2-E1 (Equation3.1). This potential difference, which is related to the hydrogen-ion acitivity in solution (see also Equation 3.3), can be used for analytical purposes to measure the hydrogen-ion activity in solution and thus to determine the pH. The relation between pH and hydrogen-ion solution is given by ... 

The quinhydrone electrode has been adapted for pH measurements in non-aqueous media, such as alcohols, acetone, formic acid, benzene and liquid ammonia. For the determination of hydrogen ion activities in solutions in pure acetic acid a form of quinhydrone electrode involving tetrachloroquinone (chloranil) and its hydroquinone has been used. ... 

The dependence of mineral weathering on hydrogen ion activity in solution can be demonstrated using surface coordination chemistry as in... 

Figure 11 is a plot of H+ ion consumption versus pH for batch pH-stat experiments at pH 2.7, 3.0, 3.5, and 4.0 (Etringer, 1989). The pH was controlled with dilute sulfuric acid. Rates were determined after 150 h in the linear portion of the weathering experiment. Fractional order dissolution rates with respect to bulk hydrogen ion activity in solution were demonstrated for these B-horizon soils. Fractional order weathering with hydrogen ion activity is consistent with the proposed surface controlled dissolution reaction for chemical weathering. 

Logarithmic scale devised by Sorensen for expressing acidity or alkalinity of a solution. It is expressed numerically as the logarithm to the base 10 of the reciprocal of the hydrogen ions activity (in moles per litre). Volume 2(3). 

The method is based on determining the potential difference between an electrode pair, consisting of a glass electrode sensitive to the difference in the hydrogen ion activity in the sample solution and the internal filling solution, and a reference electrode, which is supposed to have a constant potential independent of the immersing solution. 

Such a solution is e. g. one with an exact concentration of 1,18 N — HC1 the IN — HC1 solution does not give the standard electrode since the hydrogen ion activity in it is only Oh+ = 0,811. On defining tho standard hydrogen electrode the nature of the acid and its concentration need not be stated. This is justified by the fact, that for calculations of potentials we need the definition of such an electrode only, and its design is less important. 

Figure 12. The hydrogen ion activity in the Na(NOs)s-HNOs-HtO electrolyte solution as a function of ionic strength fraction of Nd(NOs)s...

The experimental method by which S0rensen proposed to measure pH did not, however, actually provide an unequivocal value for the hydrogen ion concentration in solutions of unknown composition. Introduction of the concept of activity (a) and the activity coefficient (y, concentration scales or y, molality scale) led to a modified definition (10) for which a modified symbol pan was first suggested ... 

Figure 18-6 shows how a hydrogen electrode is constructed. The metal conductor is a piece of platinum that has been coated, or platinized, with finely divided platinum (platinum black) to increase its specific surface area. This electrode is immersed in an aqueous acid solution of known, constant hydrogen ion activity. The solution is kept saturated with hydrogen by bubbling the gas at constant pressure over the surface of the electrode. The platinum does not take part in the electrochemical reaction and serves only as the site where electrons are transferred. The half-reaction responsible for the potential that develops at this electrode is... 

Equation (8.7.34) shows that the compensation potential depends on the hydrogen ion activity in the electrolyte solution. Strictly speaking, single ion activities are not subject to experimental determination. However, if the solution is sufficiently dilute, the single ion activity a + may be estimated by the mean activity of the electrolyte to a good approximation. 

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