Alkaline errors

Departures from the ideal behavior expressed by equation 7 usually are found in alkaline solutions containing alkaH metal ions in appreciable concentration, and often in solutions of strong acids. The supposition that the alkaline error is associated with the development of an imperfect response to alkaH metal ions is substantiated by the successhil design of cation-sensitive electrodes that are used to determine sodium, silver, and other monovalent cations (3). 

The user must be alert to some shortcomings of the glass pH electrode. For example, in solutions of pH 11 or more, the electrode shows a so-called alkaline error in which it responds also to changes in the level of alkali metal ions (particularly sodium) ... 

Alcohol dehydrogenase, 178 Alkaline error, 149 Alkaline phosphatase, 185 Alkanethiols, 46, 123 Alkoxide precursor, 120 Amino acids, 92, 187 Ammonium sensor, 181, 182 Amperometric sensors, 172 Aniline, 35, 39... 

Because soda-glass membranes contain a high proportion of sodium ions, they exhibit a marked response to sodium ions in solution. The effect becomes increasingly significant as the hydrogen ion activity decreases, i.e. at high pH, and it is sometimes referred to as the alkaline error. At pH 12, the error is about 0.3 of a pH unit if the solution is 0.1 M with respect to sodium ions, and 0.7 of a pH unit if the solution is 1 M in sodium ions. Other monovalent cations such as lithium and potassium have a similar but smaller effect. By replacing the sodium in... 

Finally, the measured emf contains a response from ions other than the proton. Of these other ions, the only one that is commonly present is sodium. This error is magnified at very high pH (> 11) when very few protons are in solution, and is known as the alkaline error ... 

Figure 3.12 Demonstration of the extent of the alkaline error, showing plots of the deviations in pH measurement under aqueous alkaline conditions as determined with a standard glass electrode (the Coming 015 ). The figures in parentheses represent the concentrations of the metal hydroxide salt in mol dm". From Christian, G. D., Analytical Chemistry, 5th Edn, Wiley, 1994. Reprinted by permission of John Wiley Sons, Inc.

AAS, 253 Absorbance, 161 Absorption, 53 Accuracy. 386 Adjusted retention time, 13 Adsorption, 5, 53 Alkaline error, 350 AMS, 319 Anisotropy, 140 Anticathode, 240 APCI, 312 API, 311... 

Figure 15-J5 Acid and alkaline errors of some glass electrodes. A Coming 015, H2S04. B. Corning 015, HCI. C Corning 015,1 M Na+...

Sodium error. When [H+] is very low and [Na+] is high, the electrode responds to Na+ and the apparent pH is lower than the true pH. This is called the sodium error or alkaline error (Figure 15-15). 

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... 

The alkaline error , often found in pH electrodes, arises because in very alkaline solution [Na+] or [K+] is normally very high, making a significant extra contribution to the potential as expressed through the Henderson equation (13.10). Minimization of this error is done by using glass of special composition and with very low selectivity coefficients for Na+ and K+. 

The alkaline error can be satisfactorily explained by assuming an exchange equilibrium between the hydrogen ions on the glass surface and the cations in solution. This process is simply the reverse of that shown in Equation 21-6 ... 

The alkaline error. The ordinary glass electrode becomes somewhat sensitive to alkali metal ions and gives low readings at pH values greater than 9. 

Describe the alkaline error in the measurement of pH. Under what circumstances is this error appreciable How are pH data affected by alkaline error ... 

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