Activity, of hydrogen ion

Griess has observed crevice corrosion of titanium in hot concentrated solutions of Cl , SOj I ions, and considers that the formation of acid within the crevice is the major factor in the mechanism. He points out that at room temperature Ti(OH)3 precipitates at pH 3, and Ti(OH)4 at pH 0-7, and that at elevated temperatures and at the high concentrations of Cl ions that prevail within a crevice the activity of hydrogen ions could be even greater than that indicated by the equilibrium pH values at ambient temperatures. Alloys that remain passive in acid solutions of the same pH as that developed within a crevice should be more immune to crevice attack than pure titanium, and this appears to be the case with alloys containing 0-2% Pd, 2% Mo or 2[Pg.169]

Hydrogen Electrode an electrode at which the equilibrium (aq.) + jHj, is established. By definition, at unit activity of hydrogen ions and unit fugacity of hydrogen gas the potential of the standard hydrogen electrode h+/y//2 =... 

Nevertheless, Ta5+ and Nb5+ interact with aqueous media containing fluorine ions, such as solutions of hydrofluoric acid. On the other hand, as was clearly shown by Majima et al. [448 - 450], the increased hydrogen ion activity can also significantly enhance the dissolution rate of oxides. The activity of hydrogen ions can be increased by the addition of mineral salts or mineral acids to the solution. 

Let us now express the activities of the main ionic species in solution as a function of the activity of hydrogen ions and f co2(gy Combining equations 8.82 to 8.86, we obtain... 

The definition of pH represents the measure of the activity of hydrogen ions in a solution at a given temperature. It is derived from a combination of p for the word power and H for the symbol for the element hydrogen. Mathematically, pH is the negative log of the activity of hydrogen ions. This relationship is illustrated in the formula... 

For example, the output of a glass electrode (in mV) plotted against the antilog of activity of hydrogen ion yields a linear pH scale. It is the simplest form of performing analysis. This simplicity comes with a price, however. If the sample is contaminated by an unknown impurity, or if the response function 91 changes for whatever reason, an undetectable error accrues. Therefore, the first-order analysis relies on the invariability of the experimental conditions. 

In practice, the trick to obtain the pH is to use a membrane that is on one side exposed to a solution of known and constant hydrogen-ion activity (e.g. a buffer solution) and on the other side is exposed to the solution of unknown pH (Fig.3.3). On the first side of the membrane, an exchange of H+ occurs, but because of the constant composition of the buffer solution 1, this will result in a constant potential, Ex. On the other side of the membrane, a potential, E2, is established which is determined by the activity of hydrogen ions in the solution of unknown pH. Therefore, AE is dependent only on E2, and thus the pH of this solution can be determined using a glass membrane. 

C) which he derived from the ionic product of water (Kw = 10 14 mol x dm 3). Some years later, Lewis introduced the concept of activity, and in 1923 Debye and Hiickel published their theory for strong electrolyte solutions. On the basis of this knowledge, Soerensen and Linderstroem-Lang [2] suggested a new pH definition in terms of the relative activity of hydrogen ions in solution ... 

Thus, online measurements of composition are usually limited to some overall property. A typical example is pH, defined as the absolute value of the logarithm of the molar concentration (or, more exactly, activity) of hydrogen ion pH can be measured by exploiting the electric potential established between two proper electrodes immersed in the sample fluid, usually a glass membrane electrode and a reference electrode [15], Notwithstanding the temperature dependence and the alkaline error (at high pH, a marked sensitivity to the effect of Na+ and of other monovalent... 

According to the International Union of Pure and Applied Chemistry (IUPAC), pH is defined as the negative logarithm (base 10) of the activity of hydrogen ions (see Equation [16.1]), and it is... 

From the equation (V-52) it is apparent that the activity of hydrogen ions is inversely proportional to the activity of hydroxyl ions. Following the sugges-... 

This equation is of great importance in electrochemistry because by it the activity of hydrogen ions or the pH of the solution can be computed from the measured EMF of a suitably combined cell. 

Thus the potential of the hydrogen electrode depends on the hydrogen-ion concentration in the solution and on the pressure of hydrogen gas above the solution. The standard potential, E9, can be measured in a system where the activity of hydrogen ions is unity in the solution and the pressure of hydrogen gas over the solution is 1 atm. We have seen that the standard potential, B, of such an electrode is by definition 0. 

For example, a mixed acidity constant is frequently used where pH has been measured according to the lUPAC convention as the activity of hydrogen ions but the concentrations of the conjugate acid-base pair are used. 

There is a simple way to avoid these problems. One can define a pH scale in a completely arbitrary manner relative to the emf of a suitable cell. One can then relate the pH on this scale to an arbitrarily defined activity of hydrogen ions, simply be setting pH = —log an+. The dissociation constants of model compounds can then be determined in terms of this arbitrary scale. This method has been used by Donovan et ai. (1959) for protein titrations in concentrated aqueous solutions of guanidine hydrochloride and of urea, and by Sage and Singer (1962) for titrations in ethylene glycol. 

By adopting the convention that the potential of the standard hydrogen electrode, i.e., with ideal gas at 1 atm. pressure in a solution of unit activity of hydrogen ions shall be zero in each solvent, and using methods essentially similar to those described above, the standard potentials of a number of electrodes have been evaluated in methyl alcohol, ethyl alcohol and liquid ammonia. These values represent therefore, in each case, the e.m.f. of the cell... 

The Sorensen pH unit, often designated psH, is neither the negative logarithm of concentration pcn nor the activity of hydrogen ion pan- It does, however, closely resemble modem operational pH scales. 

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