Acid errors

FIGURE 5-7 The alkaline and acid errors of several glass pH electrodes. A, Corning 015/H2SO4 B, Corning 015/HC1 C, Coming 015/1 M Na+ D, Beckman-GP/1 M Na+ E, L N BlackDot/lM Na+ F, Beckman E/1M Na+ G, Ross electrode. (Reproduced with permission from reference 16.)... 

To use this method, the sample is dissolved in a system containing two phases (e.g., water and octanol) such that the solution is at least about 5 x 10-4 M. The solution is acidified (or basified) and titrated with base (or acid) under controlled conditions. The shape of the ensuing titration curve is compared with the shape of a simulated curve, which is created in silico. The estimated p0Ka values (together with other variables used to construct the simulated curve such as substance concentration factor, CO2 content of the solution and acidity error) are allowed to vary systematically until the simulated curve fits as closely as possible to the experimental curve. The p0Ka values required to achieve the best fit are assumed to be the correct measured p0Ka values. This computerized calculation technique is called refinement , and is described elsewhere [14, 15]. 

Cabrera et al. [95] determined total dissolved and suspended phosphorus in natural waters by a method involving digestion with hydrogen peroxide and sulphuric acid, errors may be caused by adsorption of phosphorus on hydrous iron and aluminium oxides formed during neutralization prior to filtration. It is proposed that this can be prevented by adding extra sulphuric acid after neutralization, to dissolve such oxides and release the adsorbed phosphorus into solution. 

Acid error. In strong acid, the measured pH is higher than the actual pH, perhaps because the glass is saturated with H+ and cannot be further protonated (Figure 15-15). 

Figure 13 The change in the transition temperature (Jm) for the second lipid endotherm (near 70°C) in porcine stratum comeum as a function of the vehicle ethanol concentration. All vehicles contained 0.25% (v/v) oleic acid. Error bars depict SE (n = 3) for those points with no error bars, n = 2. (From Ref. 126.)...

Electronic instruments are subject to instrumental systematic errors. These can have many sources. For example, errors may emerge as the voltage of a battery-operated power supply decreases with use. Errors can also occur if instruments are not calibrated frequently or calibrated incorrectly. The experimenter may also use an instrument under conditions in which errors are large. For example, a pH meter used in strongly acidic media is prone to an acid error, as discussed in Chapter 20. Temperature changes cause variation in many electronic components, which can lead to drifts and errors. Some instruments are susceptible to noise induced from the alternating current (ac) power lines, and this noise may influence precision and accuracy. In many cases, errors of these types are detectable and correctable. 

The acid error. Values registered by the glass electrode tend to be somewhat high when the pH is less than about 0.5. 

Uncertainties include (1) the acid error in highly acidic solutions, (2) the alkaline error in strongly basic solutions,... 

Acid error The tendency of a glass electrode to register anomalously high pH response in highly acidic media. 

Erroneous results are obtained also in the colorimetric determination of the pH of solutions of very weak acids (or bases). When an indicator acid is employed, the acid error of the indicator itself must be taken into account and when the indicator salt is used, allowance must be made for the following reaction between the salt and the acid present ... 

C, L. Brightman, M. R. Meachem, and S. F. Agree have measured the salt error of phenol red spectrophotometrically. Their results do not agree with the measurements of the author, especially at a low salt content. They appear to have neglected the acid error of the indicator. 

The ability of other ions (Na+, etc.) to interfere increases with increasing solution pH, becoming more important at pH values in excess of 9. This effect, called "alkahne error", was investigated by Eisenmann [7]. In addition, the electrode suffers from what is known as "acid" error in solutions of about 0 pH and less. This situation was reported by Dole [8]. 

What is the alkaline error and the acid error of a glass membrane pH electrode ... 

A. Asuero, M. Herrador and A. Camean, Spectrophotometric evaluation of acidity constants of diprotic acids Errors involved as a consequence of an erroneous choice of the limit absorbances. Anal. Lett, 1986,19,1867-1880. 

NB Several computational approaches were used to deconvolute the overlapping pKa values. Mean ionic activity coefficients were estimated with a Guntelberg correction. Another paper by the same authors reported an almost identical pKa2 value of 4.88 for isonicotinic acid (Asuero AG, Herrador MA and Camean AM, Spectrophotometric evaluation of acidity constants of diprotic acids Errors involved as a consequence of an erroneous choice of the limit absorbances. Analytical Letters, 19,1867-1880 (1986))... 

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