Calibration pH electrodes

Furthermore, pH electrode calibration can be performed in situ by the new method [48], concurrently with the pKj determination. This is a substantial improvement in comparison to the traditional procedure of first doing a blank titration to determine the four Avdeef-Bucher parameters [24]. The traditional cosolvent methods used with sparingly soluble molecules can be considerably limited in the pH<4 region when DMSO-water solutions are used. This is no longer a serious problem, and routine blank titrations are now rarely needed in the new in situ procedure. 

The standard buffer solutions used in pH electrode calibration are designed compositionally to have maximal buffer capacities to assure that their advertised pH values are as constant as possible (cf. Bates 1964 Langmuir 1971a). 

A plot of the Yasuda-Shedlovsky equation generally is a straight line. The fitted coefficients A and B are then used to estimate the pK value in a 100% aqueous solution, for which [H2O] = 55.5 molal and e = 78.3. Successful use of this approach in cosolvent mixtures requires a complex pH electrode calibration procedure [20]. 

Calibrating the electrode presents a third complication since a standard with an accurately known activity for H+ needs to be used. Unfortunately, it is not possible to calculate rigorously the activity of a single ion. For this reason pH electrodes are calibrated using a standard buffer whose composition is chosen such that the defined pH is as close as possible to that given by equation 11.18. Table 11.6 gives pH values for several primary standard buffer solutions accepted by the National Institute of Standards and Technology. 

A pH electrode is normally standardized using two buffers one near a pH of 7 and one that is more acidic or basic depending on the sample s expected pH. The pH electrode is immersed in the first buffer, and the standardize or calibrate control is adjusted until the meter reads the correct pH. The electrode is placed in the second buffer, and the slope or temperature control is adjusted to the-buffer s pH. Some pH meters are equipped with a temperature compensation feature, allowing the pH meter to correct the measured pH for any change in temperature. In this case a thermistor is placed in the sample and connected to the pH meter. The temperature control is set to the solution s temperature, and the pH meter is calibrated using the calibrate and slope controls. If a change in the sample s temperature is indicated by the thermistor, the pH meter adjusts the slope of the calibration based on an assumed Nerstian response of 2.303RT/F. 

Before the pH electrode is used, it should be calibrated using two (or more) buffers of known pH. Many standard buffers are commercially available, with an accuracy of 0.01 pH unit. Calibration must be performed at the same temperature at which the measurement will be made care must be taken to match the temperature of samples and standards. The exact procedure depends on the model of pH meter used. Modem pH meters, such as the one shown in Figure 5-8, are microcomputer controlled, and allow double-point calibration, slope calculation, temperature adjustment, and accuracy to 0.001 pH unit, all with few basic steps. The electrode must... 

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. 

In sugar refinery control operations, pH electrodes should not be, as unfortunately they sometimes are, calibrated with standard buffer solutions, then placed on stream in sugar liquors, and assumed to read equivalent pH. Implicit in this operation is the equivalence of pH electrode response in dilute aqueous buffers at 24°C and in high Brix sugar solutions at elevated temperatures. Such equivalence does not exist. 

Glass pH electrodes are simple to use and maintain. They respond selectively to hydronium ion concentration and provide accurate measurements of pH values between about 0 and 10. They can be small enough to be implanted into blood vessels or even inserted into individual living cells. In precision work, these electrodes are calibrated before each use, because their characteristics change somewhat with time and exposure to solutions. The electrode is dipped into a buffer solution of known pH, and the meter is electronically adjusted until it reads the correct value. 

To establish the operational pH scale [168-170], the pH electrode can be calibrated with a single aqueous pH 7 phosphate buffer, with the ideal Nemst slope assumed. Because the % calculation requires the free hydrogen ion concentration (as described in the preceding section) and because the concentration scale is employed for the ionization constants, an additional electrode standardization step is necessary. That is where the operational scale is converted to the concentration scale pcH (= log [H+]) using the four-parameter equation [116,119,171,172]... 

Since many new substances of interest are very poorly soluble in water, the assessment of the pKa in aqueous solution can be difficult and problematic. Potentiometry can be a quick technique for such assessment, provided the solubility of the substance is at least 100 pM. (Solutions as dilute as 10 pM can still be analyzed, but special attention must be given to electrode calibration, and ambient carbon dioxide must be excluded.) If the substance is soluble to only 1-10 pM and possesses a pH-sensitive UV chromophore, then spectrophotometry can be applied. CE methods may also be useful since very small sample quantities are required, and detection methods are generally quite sensitive. 

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). 

Favaro and Fiorani [34] used an electrode, prepared by doping conductive C cement with 5% cobalt phthalocyanine, in LC systems to detect the pharmaceutical thiols, captopril, thiopronine, and penicillamine. FIA determinations were performed with pH 2 phosphate buffer as the carrier stream (1 mL/min), an injection volume of 20 pL, and an applied potential of 0.6 V versus Ag/AgCl (stainless steel counter electrode). Calibration curves were developed for 5-100 pM of each analyte, and the dynamic linear range was up to approximately 20 pM. The detection limits were 76, 73, and 88 nM for captopril, thiopronine, and penicillamine, respectively. LC determinations were performed using a 5-pm Bio-Sil C18 HL 90-5S column (15 cm x 4.6 mm i.d.) with 1 mM sodium 1-octanesulfonate in 0.01 M phosphate buffer/acetonitrile as the mobile phase (1 mL/min) and gradient elution from 9 1 (held for 5 min) to 7 3 (held for 10 min) in 5 min. The working electrode was maintained at 0.6 V versus Ag/AgCl, and the injection volume was 20 pL. For thiopronine, penicillamine, and captopril, the retention times were 3.1, 5.0, and 11.3 min, and the detection limits were 0.71, 1.0, and 2.5 pM, respectively. 

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. 

It is clear from the Nemst equation that the temperature of the solution affects the response slope (2.303A7//0 of the calibration curve. The electrode voltage changes linearly in relationship to changes in temperature at a given pH therefore, the pH of any solution is a function of its temperature. For example, the electrode response slope increases from 59.2mV/pH at 25°C to 61.5 mV/pH at a body temperature of 37°C. For modem pH sensing systems, a temperature probe is normally combined with the pH electrode. The pH meter with an automatic temperature compensation (ATC) function automatically corrects the pH value based on the temperature of the solution detected with the temperature probe. 

The wireless pH capsule (Medtronic Inc.) is oblong in shape and contains an antimony pH electrode, a reference electrode at its distal tip, a battery, and a RF transmitter. The whole device is encapsulated in epoxy. The capsule is introduced into the esophagus on a catheter through the nose or mouth and is attached to the lining of the esophagus with a clip. The probe monitors the pH in the esophagus and transmits the information via RF telemetry at a rate of 6 per second (0.17 Hz) to a pager-sized receiver that is worn by the patient on a belt. Prior to implantation, the capsule is calibrated with its receiver in pH buffer solutions of pH 1.07 and pH 7.01 [168],... 

If recently calibrated, the GE and pH electrodes give an accurate response... 

The pH meter is standardized (calibrated) with the use of buffer solutions. Usually, two buffer solutions are used for maximum accuracy. The pH values for these solutions should bracket the pH value expected for the sample. For example, if the pH of a sample to be measured is expected to be 9.0, buffers of pH = 7.0 and pH = 10.0 should be used. Buffers with pH values of 4.0,7.0, and 10.0 are available commercially specifically for pH meter standardization. Alternatively, of course, homemade buffer solutions (see Chapter 5) may be used. In either case, when the pH electrode and reference electrode are immersed in the buffer solution being measured and the electrode leads are connected to the pH meter, the meter reading is electronically adjusted (refer to manufacturer s literature for specifics) to read the pH of this soluiton. The electrodes can then be immersed into the solution being tested and the pH directly determined. 

We will first calibrate the pH electrode with a buffer solution, i.e. to obtain K. Inserting the appropriate values into equation (3.16) gives the following ... 

Once the electrodes have been prepared for a given aqueous-organic solvent, the pan determinations can be made at each temperature, either graphically or by direct reading on commercial pH meters calibrated for poH measurements. In this procedure the pH meter is used as a milli-voltmeter. A solution A (10 M HCl in the aqueous-organic solvent considered) is selected as the standard reference solution, its pan being calculated for any temperature according to the Debye-Hilckel formula. After the electrodes have been immersed in this solution, the... 

The dependence of precision on different parameters has already been discussed. Precision is strongly dependent on the constancy of migration data. Thus, the stability of the EOF is most important. Buffer recipes describe clearly the preparation and avoid errors caused by, e.g., a poorly calibrated pH electrode. 

The use of buffers for the calibration of a pH electrode is a critical step in any pH measurement. Buffer solutions of different pH values can be prepared or are commercially available. They should be traceable to national standards with 0.01... 

Electrode. Your pH meter user s guide is a valuable source of information on how to calibrate the pH electrode, since specific instructions or steps may be required to obtain reliable pH data. A two-point or multipoint calibration, using buffers is required, before the pH is measured. Daily calibration before use is necessary to determine the slope of the electrode. This serves the dual purpose of determining if the electrode is working properly and storing the slope value in the instrument s memory. A calibration of at least two points is needed to determine the slope of the electrode. 

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