Calibration glass electrode

Seawater buffers are useful in calibrating glass electrodes and spectroscopic pH indicators. The buffers are prepared in artificial seawater with the protonated and unprotonated form of a given buffer (0.04 M). Dickson (1993) has determined the pK of the buffers of the total proton scale. Suitable buffers are, for example, AP (aminopyridine), pHsws = 6.77 TRIS = 2-amino-2-hydroxy-methy 1-1.3-propanediol (tris), pHsws = 8.07 and MOR = tetrahydro-1-4-isoxazine (morpholine), pHsws = 8.57. 

Blood and urine pH can be measured easily by means of a calibrated glass electrode, whereas pH measurement inside the metabolizing cells is not easily accomplished. Techniques for estimating intracellular pH include glass electrode measurements on homogenates, calorimetric or fluorometric analysis of intracellular distribution of indicator dyes, and microelectrode methods. 

Three different methods are used in the paper to determine the second dissociation constant, Aia2, of H2Se (1) ordinary potentiometric titration employing KOH(aq) and a glass electrode, (2) addition of small amounts of H2Se(aq) to KOH(aq) and subsequent pH determination with a KOH(aq)-calibrated glass electrode, and (3) the determination of the solubility of Na2Se(cr) in solutions of known initial concentrations of sodium hy-droxide/acetate. For each technique only a very limited number of data (4, 2, and 5, respectively) are presented. 

The Nernst equation shows that the glass electrode potential for a given pH value will be dependent upon the temperature of the solution. A pH meter, therefore, includes a biasing control so that the scale of the meter can be adjusted to correspond to the temperature of the solution under test. This may take the form of a manual control, calibrated in 0 C, and which is set to the temperature of the solution as determined with an ordinary mercury thermometer. In some instruments, arrangements are made for automatic temperature compensation by inserting a temperature probe (a resistance thermometer) into the solution, and the output from this is fed into the pH meter circuit. 

Measurement of pH was performed using a Metrohm model 691 pH meter equipped with a Metrohm combined LL micro pH glass electrode calibrated prior to use with pH = 2 and 9 buffers. The checkers found that adjustment to a lower pH led to product with higher amounts of inorganic impurities. The checkers also found that the use of pH paper results in different pH values as compared to the pH meter. 

A glass electrode, a thin-walled glass bulb containing an electrolyte, is much easier to use than a hydrogen electrode and has a potential that varies linearly with the pH of the solution outside the glass bulb (Fig. 12.11). Often there is a calomel electrode built into the probe that makes contact with the test solution through a miniature salt bridge. A pH meter therefore usually has only one probe, which forms a complete electrochemical cell once it is dipped into a solution. The meter is calibrated with a buffer of known pH, and the measured cell emf is then automatically converted into the pH of the solution, which is displayed. 

Avdeef, A., Budier,).). Accurate measurements of the concentration of hydrogen ions with a glass electrode calibrations using the Prideaux and other universal buffer solutions and a... 

It has become fairly common to adopt the manufacture of combinations of internal reference electrode and its inner electrolyte such that the (inner) potential at the glass electrode lead matches the (outer) potential at the external reference electrode if the glass electrode has been placed in an aqueous solution of pH 7. In fact, each pH glass electrode (single or combined) has its own iso-pH value or isotherm intersection point ideally it equals 0 mV at pH 7 0.5 according to a DIN standard, as is shown in Fig. 2.11 the asymmetry potential can be easily eliminated by calibration with a pH 7.00 0.02 (at 25° C) buffer solution. 

Avdeef, A. Comer, J. E. A. Thomson, S. J., pH-metric logP. 3. Glass electrode calibration in methanol-water, applied to pKa determination of water-insoluble substances, Anal. Chem. 65, 42-49 (1993). 

Dunsmore, H. S. Midgley, D., The calibration of glass electrodes in cells with liquid junction, Anal. Chim. Acta 61, 115-122 (1972). 

For current practice, the described method of pH measurement is too tedious. Moreover, not hydrogen but glass electrodes are used for routine pH measurements (see Section 6.3). Then the expression for the EMF of the cell consisting of the glass and reference electrodes contains a constant term from Eq. (6.3.10), in addition to the terms present in Eq. (3.3.3) this term must be obtained by calibration. Further, a term describing the liquid junction potential between the reference electrode and the measured solution must also be included. 

In practice, the pH is mostly measured with a glass electrode (see Section 6.3), connected with a calomel electrode (see Section 3.2.2). The measuring system is calibrated by using a single standard S, with a pH(S) value lying as close as possible to the pH(X) value. The pH(X) value is then calculated from (S), (X) and pH(S) by Eq. (3.3.4). It is preferable to use two standards Sl and S2, selected so that pH ) is smaller and pH(S2) larger than pH(X) (both the pH(S) values should be as close to pH(X) as possible). The value of pH(X) is then calculated from the usual formula for linear interpolation ... 

It has been emphasized repeatedly that the individual activity coefficients cannot be measured experimentally. However, these values are required for a number of purposes, e.g. for calibration of ion-selective electrodes. Thus, a conventional scale of ionic activities must be defined on the basis of suitably selected standards. In addition, this definition must be consistent with the definition of the conventional activity scale for the oxonium ion, i.e. the definition of the practical pH scale. Similarly, the individual scales for the various ions must be mutually consistent, i.e. they must satisfy the relationship between the experimentally measurable mean activity of the electrolyte and the defined activities of the cation and anion in view of Eq. (1.1.11). Thus, by using galvanic cells without transport, e.g. a sodium-ion-selective glass electrode and a Cl -selective electrode in a NaCl solution, a series of (NaCl) is obtained from which the individual ion activity aNa+ is determined on the basis of the Bates-Guggenheim convention for acr (page 37). Table 6.1 lists three such standard solutions, where pNa = -logflNa+, etc. 

Although a few amperometric pH sensors are reported [32], most pH electrodes are potentiometric sensors. Among various potentiometric pH sensors, conventional glass pH electrodes are widely used and the pH value measured using a glass electrode is often considered as a gold standard in the development and calibration of other novel pH sensors in vivo and in vitro [33], Other pH electrodes, such as metal/metal oxide and ISFETs have received more and more attention in recent years due to their robustness, fast response, all-solid format and capability for miniaturization. Potentiometric microelectrodes for pH measurements will be the focus of this chapter. 

The glass electrode used with so-called pH meters to determine pH (= -log[H ]) responds to log[H ] according to the above relationship. Conunent on the practice of calibrating pH meters at only one [H ]. Why is two-point calibration preferred ... 

This pH definition for non-aqueous and mixed solvent systems is practically the same as that for aqueous solutions (Section 6.2.1). Thus, if a pH standard is available for the solvent or mixed solvent under study, the glass electrode is calibrated with it and then the pH of the sample solution is measured. The pHRVs values for 0.05 mol kg-1 KHPh have been assigned to aqueous mixtures of eight organic solvents (see 5 for pHRVs at 25 °C). Although they are for discrete solvent compositions, the pHRVs in between those compositions can be obtained by use of a multilinear regression equation [14b],... 

If there is a strong acid that completely dissociates, we prepare a solution of the acid of known concentration and use it to calibrate the glass electrode. The pH of the solution is calculated by estimating ym(H+) using the Debye-Hiickel theory. 

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