Electrode calibration

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. 

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. 

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... 

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. 

Potentiometric measurements with ISEs can be approached by direct potentiometry, standard addition and titrations. The determination of an ionic species by direct potentiometry is rapid and simple since it only requires pretreatment and electrode calibration. Here, the ion-selective and reference electrodes are placed in the sample solution and the change in the cell potential is plotted against the activity of the target ion. This method requires that the matrix of the calibration solutions and sample solutions be well matched so that the only changing parameter allowed is the activity of the target ion. 

This chapter deals with ISE construction, their characteristic properties such as selectivity coefficient, response time, temperature coefficient and drift, as well as electrode calibration and composite sensors containing ISEs. 

In collaboration with Jon Belisle, octanol pKa values were measured for a series of benzoic acids and phenols. A coupled electrode calibrated in aqueous buffers was used. The haIf-neutralization potential was measured since the Renderson-Hasselbalch equations would not apply. The titrant was 0.1 sodium hydroxide in isopropanol methanol 4 1. The titrant was only 6% of the total volume at half-neutralization, so the medium was essentially octanol-like. The results are listed in Table I and some benzoic acid values are plotted in Figure 6. 

Additional Sample Measurement Requirements. The sample measurement should ideally be made under the same conditions as the electrode calibration. 

Hg2 ion-selective electrode calibration curve from J. A. Shatkin, H. S. Brown, and S. licht, Composite Graphite Ion Selective Electrode Array Potentiometry for the Detection of Mercury and Other Relevant Ions in Aquatic Systems, Anal. Chem. 1995, 67,1147. It was not stated in the paper, but we presume that all solutions had the same ionic strength. 

Early on, it was anticipated that many repetitive calibrations and EMF measurements would be carried out in the evaluation of a large quantity of electrodes. The first microcomputer-based automated titration system utilizing high level software (CONVERS) hastened these studies (10), as did a more recently constructed minicomputer system (Figure 1). Typical results are shown in Figure 2, where a set of five protriptyline CWEs were calibrated simultaneously (11). Graphic side-by-side comparison of different electrode calibrations was also useful in establishing structure-selectivity relationships. 

EDTA was determined using an amperometric flow injection method conducted in 0.1 M H2SO4, and which used a glassy carbon electrode held at 1.25 V vs. a saturated calomel electrode. Calibration curves were found to be linear over the range of 0.01-10 pg/mL EDTA [18]. 

Allen RI, Box KJ, Comer J et al. (1998) Multiwavelength spec-trophotometric determination of acid dissociation constants of ionizable drugs. 17(4-5) 699-712 Avdeef A, Bucher JJ (1978) Accurate measurements of the concentration of hydrogen ions with a glass electrode calibrations using the Prideaux and other universal buffer solutions and a computer-controlled automatic titrator. Anal Chem 50 2137-2142... 

Finally, the measurement of pH(X) of unknown solutions, the actual subject of pH measurement, is carried out in practical cells with transference containing -> glass electrodes. Calibration procedures for such practical cells are recommended so that the unknown pH(X) is traced back to pH(SS), pH(PS) and to the defined pH. 

Electrode Calibration Pipet 50 mL of the Buffer Solution into a plastic beaker. Place the fluoride ion and reference electrodes (or a combination fluoride electrode) into the plastic beaker and stir. At 5-min intervals, add 100 pL and 1000 pi. of the 1000 mg/kg Fluoride Standard and read the potential, in millivolts, after each addition. The difference between the two readings is the slope of the fluoride electrode and should typically be in the range of 54 to 60 mV at 25°. If the difference in potential is not within this range, check, and, if necessary, replace the electrode, instmment, or solutions. 

The nitrate ion activities in the aqueous phase were measured with a nitrate ion selective electrode taking into account the presence of high hydrogen ion concentration by calibration of the nitrate electrode with nitric acid. The nitrate ion concentration in the organic phase owing to the extraction of neodymium complexes by HDEHP was determined by back-extraction of the organic phase with 3M sulfuric acid, dilution, and analysis with a nitrate ion electrode calibrated for different nitrate and sulfate concentrations. The amount of the nitrate species extracted into the organic phase increases as the initial neodymium nitrate concentration increases. 

The pH of the protein solution in 0.01 M KC1 (5-7 mg/ml) is adjusted to the desired value by addition of KOH or KCl (0.01 M). Ethylene glycol, at any concentration up to 50% by volume, containing 0.01 M KCl, is then added. With the electrode calibrated in mixed solvent, the paH variation can be observed relative to the previous pH measurement in pure water. With the pH stat, a sufficient amount of titrant in mixed solvent is added to the sample to obtain a paH similar... 

The electrode-calibration method is also referred to as the method of external standards, which is described in some detail in Section 8C-2. 

In the electrode-calibration method, K in Equations 21-22 and 21-23 is determined by measuring (-eii for ono or more standard solutions of known pX or pA. The assumption is then made that K is unchanged when the standard is replaced by the analyte solution. The calibration is ordinarily performed at the time that pX or pA for the unknown is determined. With membrane electrodes, recalibration may be required if measurements extend over several hours because of slow changes in the asymmetry potential. 

Inherent Error in the Electrode-Calibration Procedure A serious disadvantage of the electrode-calibration method is the inherent error that results from the assumption that K in Equations 21 -22 and 21 -23 remains constant after calibration. This assumption can seldom, if ever, be exactly true because... 

The process depicted for phenol in equations 5 consists of an enzyme-catalyzed oxidation to a quinone, and a reduction process taking place at the electrode these reactions may serve for electrode calibration. The development of AMD biosensors for detection of phenols in environmental waters has been described for phenoloxidases such as tyrosinases and laccases and less specific oxidases such as peroxidases. Such biosensors may be part of a FIA system for direct determination of phenols or may serve as detectors for LC °. 

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