Hydrogen activities, glass electrode

Ionic Equilibria.. The ion product constant of D2O (see Table 3) is an order of magnitude less than the value for H2O (24,31,32). The relationship pD = pH + 0.41 (molar scale 0.45 molal scale) for pD ia the range 2—9 as measured by a glass electrode standardized ia H2O has been established (33). For many phenomena strongly dependent on hydrogen ion activity, as is the case ia many biological contexts, the difference between pH and pD may have a large effect on the iaterpretation of experiments. 

Assuming that the glass electrode shows an ideal hydrogen electrode response, the emf of the cell still depends on the magnitude of the liquid junction potential j and the activity coefficients y of the ionic species ... 

The concentration of the acid itself is of little significance other than analytical, with the exception of strong acids in dilute aqueous solutions. The concentration of H+ itself is not satisfactory either, because it is solvated diversely and the ability of transferring a proton to another base depends on the nature of the medium. The real physical quantity describing the acidity of a medium is the activity of the proton au. The experimental determination of the activity of the proton requires the measurement of the potential of a hydrogen electrode or a glass electrode in equilibrium with the solution to be tested. The equation is of the following type [Eq. (1.7)], wherein Cis a constant. 

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. 

Since hydrogen ions and metal ions, like Mg2+, are often reactants, it is convenient to define the pHc as — log[H+], where c refers to concentrations, and pMg as —log[Mg2+], However, a glass electrode measures pHa = — log y(H+)[H+] where a refers to activity. Thus... 

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. 

It is appropriate at this point to discuss the "apparent" pH, which results from the sad fact that electrodes do not truly measure hydrogen ion activity. Influences such as the surface chemistry of the glass electrode and liquid junction potential between the reference electrode filling solution and seawater contribute to this complexity (see for example Bates, 1973). Also, commonly used NBS buffer standards have a much lower ionic strength than seawater, which further complicates the problem. One way in which this last problem has been attacked is to make up buffered artificial seawater solutions and very carefully determine the relation between measurements and actual hydrogen ion activities or concentrations. The most widely accepted approach is based on the work of Hansson (1973). pH values measured in seawater on his scale are generally close to 0.15 pH units lower than those based on NBS standards. These two different pH scales also demand their own sets of apparent constants. It is now clear that for very precise work in seawater the Hansson approach is best. 

The hydrophobias are a case where protein nanofibers can play a dual role in creating a biosensor. They can aid in the immobilization of bioactive components within a biosensor and also add further functionality to the transducing element of a biosensor device. Hydrophobins are self-assembling [3-sheet structures observed on the hyphae of filamentous fungi. They are surface active and aid the adhesion of hyphae to hydrophobic surfaces (Corvis et al., 2005). These properties can be used to create hydrophobia layers on glass electrodes. These layers can then facilitate the adsorption of two model enzymes glucose oxidase (GOX) and hydrogen peroxidase (HRP) to the electrode surface. The hydrophobin layer also enhances the electrochemical properties of the electrodes. 

The activity ag has a definite constant value, usually obtained by using a phosphate buffer containing d ions as the solution inside the membrane. An important advantage of the glass electrode is that it can be used under many conditions for which the hydrogen electrode is subject to serious error. 

Been varies only as a function of the test solution pH if the temperature is constant. It is significant to point out that the potential of modern glass electrodes is a linear function of pH (equation 39). By using a test solution of known pH it is possible to relate the cell potential to hydrogen ion activity of a test solution. This standardization must be done each time a pH meter is used because of subtle changes in the various potentials owing to aging of the electrode. Therefore, the accuracy of a pH determination depends on the accuracy of the standard buffer. Table 1-2... 

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