Electrode calibration methods

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 standard-addition method has been applied to the determination of chloride and fluoride in samples of commercial phosphors. In this application, solid-state indicator electrtKies for chloride and fluoride were used in conjunction with a reference electrode the added standard contained known quantities of the two anions. I he relative standard deviation for the measurement of replicate standard samples was 0.7% for fluoride and 0.4% for chloride. In routine use with real samples, the standard-addition method yielded relative standard deviations of l.l k) for fluoride and 0.8% for chloride. When similar samples were ana-Ivred bv the usual electrode calibration methods, the... 

The direct electrode calibration method offers the advantages of simplicity, speed, and applicability to the continuous monitoring of pX or pA. The method has two important disadvantages, however. One of these is that the accuracy of a measurement obtained by this procedure is limited by the inherent uncertainty caused by the junction potential and unfortunately, this uncertainly can never be totally eliminated. The second disadvantage of this procedure is that results of an analysis are activities rather than concentrations (for some applications this is an advantage rather than a disadvantage). 

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. 

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. 

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. 

The simplest but also the least reliable calibration method is the use of a single standard solution. The electrode response is assumed to be Nernstian. The slope of the potential versus concentration dependence can also be determined experimentally, by using two standard solutions with different concentrations. To avoid large errors, the standard concentration should be as close as possible to the sample concentration in calibration with a single standard solution. 

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]. 

The apparent acid dissociation constants (p s)Ka) of two water-insoluble drugs, ibuprofen and quinine, were determined pH-metrically in ACN water, dimethyl-formamide water, DMSO water, 1,4-dioxane-water, ethanol water, ethylene glycol-water, methanol water, and tetrahydrofuran water mixtures. A glass electrode calibration procedure based on a four-parameter equation (pH = alpha-i- SpcH -i-jH[H+] -i-jOH[OH ]) was used to obtain pH readings based on the concentration scale (pcH). We have called this four-parameter method the Four-Plus technique. The Yasuda Shedlovsky extrapolation p s)K a + log [H2O] = A/epsllon -1- B) was used to derive acid dissociation constants in aqueous solution (pKa). It has been demonstrated that the pK a values extrapolated from such solvent-water mixtures are consistent with each other and with previously reported measurements. The suggested method has also been applied with success to determine the pKa values of two pyridine derivatives of pharmaceutical Interest. Spectrometric, ultraviolet (UV) ... 

Note that the precision of the measurement of - logio [H ] and pH is virtually the same, in very good experiments, + 0.001. However, the accuracy is generally considerably poorer, depending in the case of glass electrodes largely on the response of the electrode (linearity, age, pH range, etc.), and to a lesser extent on the calibration method employed. 

The utilization of ion-selective membrane electrodes involves their prior calibration. It is necessary to make a regression to obtain good reliability of the analytical information. Because a linear relation between the independent and dependent variables cannot forced, sometimes the nonlinear calibration method is successful. The ion-selective membrane electrode linearization and subsequent calibration use multiple linear regression (MLR) and/or partial least-squares (PLS) when limited calibration data are available.219... 

The behaviour of a Pt(02)lYSZ membrane oxygen electrode at 700 °C was studied in Refs. [162, 189]. The calibration E-pO plots obtained, one of which is presented in Fig. 2.4.8, clearly demonstrate the drawbacks of the direct calibration method at low initial molalities of oxide ions. 

Rafols C, Roses M and Bosch E, Dissociation constants of several nonsteroidal anti-inflammatory drugs in isopropyl alcohol /water mixtures. Anal. Chim. Acta, 350,249-255 (1997). NB Potentiometric method using i-PrOH/water mixtures in the range 10/90 to 90/10 v/v. Metrohm autotitrator careful electrode calibration. Activity corrections applied and extrapolated to zero [i-PrOH]. Alternative data handling procedure gave pKa = 4.64 0.04. 

The polyacrylamide sample is extracted with aqueous methanol and tetra n-butyl ammonium hydroxide added as base electrolyte. This solution is polarographed and the peak height at -2V of the polyacrylamide peak evaluated using a platinum wire anode electrode and a saturated calomel reference electrode. The method is calibrated against standard aqueous methanolic solutions of acrylamide. 

Description of Method. The amount of chlorpromazine in a pharmaceutical formulation is determined voltammetrically at a graphite working electrode in a nonstirred solution. Calibration is achieved using the method of standard additions. 

The purity of a sample of K3Fe(CN)6 was determined using linear-potential scan hydrodynamic voltammetry at a glassy carbon electrode using the method of external standards. The following data were obtained for a set of calibration standards. 

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