Electrode Calibration Research Articles

Analytical methods and quality assurance

Soil acidification under tropical and sub‐tropical conditions is a well known phenomenon. Because of cost implications in practice it is important that the best available liming materials be used. Various methods have been employed to distinguish between the effectiveness of such materials, most based on the principle of calcium carbonate equivalent. Many of these use relatively strong acids for dissolution of lime, at concentrations far greater than ever actually occur in even the most acid soils. Because contact exchange between the acidified sites and the liming material is considered to be vital, a method in which a slightly acidified cation exchange resin is used was developed (the resin exchange method). This method has been used by several laboratories in South Africa that take part in the quality control scheme run by ALASA. Several problems were experienced in achieving reproducibility between laboratories, that were diagnosed as being due to variations in applications of the method and especially lack of discipline in using a standardized resin. This resulted in a number of independent investigations into reasons for this apparent lack of reproducibility. Reasons for this included inherent particle size distribution in the sub‐samples of liming material, resin properties, water quality and electrode maintenance and calibration. Under standardized and controlled conditions the method proved to be eminently satisfactory. An alternative possibility that is being investigated is the use of an unbuffered acid topsoil to replace the resin. A bulk sample could be homogenized and subdivided to provide an adequate supply.

Application of neural network calibrations to an halide ISE array

The Neural Network (NN) technique was applied to the calibration of an ion selective electrode (ISE) array comprising a bromide selective electrode, two chloride ISEs and one thiocyanate ISE. The measured samples were synthetic mixture solutions of chlorides and bromides in concentration ranges such that interference occurs. The NN method allowed to perform the calibration without estimating the coefficients of the Nikolskii–Eisenman theoretical relation. Only the determination of bromide was detailed. The results obtained using this method were better than those obtained using linear multivariate calibration methods.

Simplifying position-sensitive gas-ionization detectors for heavy ion elastic recoil detection

Heavy ion elastic recoil detection is an effective tool for materials analysis. This has been facilitated by the development of novel detection systems, including large solid-angle gas-ionization detectors with position-sensitivity. With such detectors the exposure of the sample to the beam is reduced, however, data analysis can be complex. This paper reports on a detailed investigation of a detector design commonly used. The results of this work have led to suggestions for design-modifications, which considerably simplify detector operation and data analysis. The modifications have been implemented and tested. With the modified detector, the total energy information can be obtained by adding the signals from the anode electrodes, but also directly from a grid-electrode, obviating the need for the relative calibration of the anode electrodes. The subdivision of the energy loss electrode, together with a carefully chosen position of the entrance window to maintain optimum resolution for two ΔE signals, enable light and heavy ions to be resolved at the same gas pressure. The placement of a sawtooth electrode within the anode gives position information, which is linear and independent of atomic number and ion energy. Protons can be identified simultaneously with heavy ions by combining the information from the grid and residual energy signals, both amplified with high gain.

Calibration of pH glass electrodes by direct strong acid/strong base titrations under dilute conditions

The calibration of an electrode assembly composed of a set of two glass electrodes and a reference electrode, using strong acid/strong base titrations, was performed with an automatic titration system, to which was connected an automatic electrode switcher, controlled by a personal computer. Analysis of the data by the Gran method or by derivative methods reveals that the acquisition of reliable calibration parameters depends on the indicator electrode, particularly so for diluted solutions (1.0 × 10 −3 M) of the strong acid. These discrepancies are attributed to the time of operation of the glass electrodes, but are not a caveat to the use of calibrations with very dilute solutions of strong acid, a practice that must be followed in water mixed media.

GLEE, a new computer program for glass electrode calibration

A new computer program (GLEE, glass electrode evaluation) has been written for the calibration of a glass electrode by means of a strong acid-strong base titration. This program provides an estimate of the carbonate contamination of the base, the pseudo-Nernstian standard potential and slope of the electrode and, optionally, the concentration of the base and p K W. The program runs under the Windows 3.1x, 9x and NT operating systems.

Negative pH and Extremely Acidic Mine Waters from Iron Mountain, California

Extremely acidic mine waters with pH values as low as −3.6, total dissolved metal concentrations as high as 200 g/L, and sulfate concentrations as high as 760 g/L, have been encountered underground in the Richmond Mine at Iron Mountain, CA. These are the most acidic waters known. The pH measurements were obtained by using the Pitzer method to define pH for calibration of glass membrane electrodes. The calibration of pH below 0.5 with glass membrane electrodes becomes strongly nonlinear but is reproducible to a pH as low as −4. Numerous efflorescent minerals were found forming from these acid waters. These extreme acid waters were formed primarily by pyrite oxidation and concentration by evaporation with minor effects from aqueous ferrous iron oxidation and efflorescent mineral formation.

Resolving uncertainty in chemical speciation determinations

Speciation determinations involve uncertainty in system definition and experimentation. Identification of appropriate metals and ligands from basic chemical principles, analytical window considerations, types of species and checking for consistency in equilibrium calculations are considered in system definition uncertainty. A systematic approach to system definition limits uncertainty in speciation investigations. Experimental uncertainty is discussed with an example of proton interactions with Suwannee River fulvic acid (SRFA). A Monte Carlo approach was used to estimate uncertainty in experimental data, resulting from the propagation of uncertainties in electrode calibration parameters and experimental data points. Monte Carlo simulations revealed large uncertainties present at high (>9–10) and low (<4) −log[H+], which result from larger instabilities in the proton balance function in these regions. Uncertainties in speciation parameters were compared for uniresponse fitting (linear programming and least-squares) and multiresponse fitting. Linear programming and least-squares approaches both fit the observed data, but suggest different mixtures of monoprotic ligands. Least-squares fit the data with 21 sites, whereas linear programming fit the data equally well with 9 sites. Multiresponse fitting, involving simultaneous fluorescence and pH measurements, improved model discrimination. Deconvolution of the excitation versus emission fluorescence surface for SRFA establishes a minimum of five sites. Diprotic sites are also required for the five fluorescent sites, and one non-fluorescent monoprotic site was added to accommodate the pH data. Consistent with greater complexity, the multiresponse method had broader confidence limits than the uniresponse methods, but corresponded better with the accepted total carboxylic content for SRFA. Overall there was a 40% standard deviation in total carboxylic content for the multiresponse fitting, versus 10% and 1% for least-squares and linear programming, respectively.

PH-metric log P 11. pKa determination of water-insoluble drugs in organic solvent–water mixtures

The apparent acid dissociation constants (psKa) of two water-insoluble drugs, ibuprofen and quinine, were determined pH-metrically in acetonitrile–water, dimethylformamide–water, dimethylsulfoxide–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=α+SpcH+jH[H+]+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 (psKa+log[H2O]=A/ε+B) was used to derive acid dissociation constants in aqueous solution (pKa). It has been demonstrated that the pKa 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.

Voltammetric measurements on MexOy (Me=Bi, Cu, V) compounds and comparison with results for BICUVOX.10

Voltammetric measurements on cells of arrangement Pt,Ar|sample|YSZ|Ar,Pt with a very small sample mass (`microsamples') turned out to be a very convenient tool for the determination of thermodynamic properties of oxides. Measurements were made on Bi2O3, CuO and V2O5 microsamples and on nickel and copper metal between 550 and 700°C. The measurements on nickel were used for the calibration of the reference electrode. The measurements on CuO showed the advantage of the use of microsamples. Comparison of the results for CuO with the standard potential, calculated from thermodynamic data in the literature, revealed the high accuracy of the method. In the case of measurements on V2O5 a possible reaction path was identified. The vanadium system is very complicated because of the large number of stable oxidation states for vanadium in VxOy compounds. The measurements on Bi2O3 showed a large deviation of about 100–150 mV from the calculated equilibrium potentials. BICUVOX.10 microsamples are reduced irreversibly below −570 mV against a reversible oxygen electrode in 1 atm O2 at 600°C in the first negative sweep. The voltammograms for BICUVOX.10 are compared with measurements on pure MexOy compounds and with calculated data.

Automated determination of carbon dioxide, oxygen, and nitrogen partial pressures in surface waters

An automated system capable of accurate determinations of 0,, CO,, and N, partial pressures (p) in surface waters is described. The system is composed of a programmable data logger-controller, an infrared CO, analyzer, a polarographic 0, electrode, and a barometer linked to a submersed gas exchanger made of thin-walled (100 pm) silicone elastomer tubing. Oxygen electrode calibration and drift problems are eliminated by continuously referencing p0, measurements to atmospheric readings. pN, is calculated by subtracting p0, + pH,O + pC0, from total dissolved gas pressure. Complete (>99%) equilibration of internal gases with water pC0, occurs within 4 min. Equilibration half-times for p0, range from 7.1 min at 21°C to 9.4 min at 05°C. The equilibration half-time for pN, is 15.1 min at 15*C. Observations conducted in a small oligotrophic Shield Lake show that the epilimnion is usually supersaturated with both CO, (50-150 patm) and 0, (500-4,000 patm), with both gases exhibiting marked daily variations. The epilimnetic supersaturation (up to 2%) with respect to atmospheric 0, indicates that primary production slightly exceeds community respiration in the epilimnion. The system appears adequate for metabolic and gas flux studies, even in the most oligotrophic waters. Measurements of dissolved gas concentrations or partial pressures in surface waters are required in several fields of investigation in limnology and oceanography. Frequent needs for such measurements include the estimation of carbon fluxes across the air-water interface (Broecker and Peng 1984; Wanninkhof et al. 1987), the assessment of net ecosystem production (autotrophy) or respiration (heterotrophy) in lakes and rivers (Schurr and Ruchti 1977; Cole et al. 1994), and the monitoring of gas supersaturation and associated risks to aquatic life below reservoir spillways (Weitkamp and Katz 1980). These needs have stimulated a continuing interest in the design of automated systems for measuring dissolved gases in aquatic environments (Kelly et al. 1974; D’Aoust and Clarke 1980; DeGrandpre et al. 1995; Sellers et al. 1995). Determinations of the partial pressures, concentrations, and fluxes of CO, and 0, in surface waters can be used to document the production-respiration balance of lakes, rivers, and oceans at the ecosystem level. Gas fluxes (F) across the air-water interface can be estimated from differences in dissolved gas concentrations between surface water (C,) and air-saturated water (C,Y) using a relationship of the form F = K(C, - C,),

A Thermodynamic Approach to the Calibration of Ion Selective Membrane Electrodes in the Presence of Interfering Species

The applicability of ion selective membrane electrodes is often limited by interference effects from species other than that one wishes to monitor. Such effects can be described in terms of an interference potential, E+, which is the difference in the measured electrode response in the solution of interest minus that which would be observed in a solution where the activity of the species to be studied is the same but which contains no interfering species. Previous work, based on thermodynamic arguments, that identifies the variables that determine E+ is amplified. This leads to calibration procedures that enable E+ in an unknown solution to be determined from appropriate measurements using known solutions and also to criteria that must be met for these procedures to apply. In contrast with previous work, no models for equilibria and transport processes in the membrane interior are required. The method is extended to species involved in multiple dissociation equilibria such as polyacids and polyamines.

New approach for elucidating the oxygen sensitivity and calibration of the antimony electrode.

The biomedical use of the polycrystalline antimony pH electrode is hampered by oxygen dependence and low pH resolution (0.1 pH). Although, the pH resolution is improved by using highly purified crystallographically oriented monocrystalline antimony (COMA), the oxygen effect on the electrode remains. In an in vitro examination of the COMA electrode it was found that the oxygen dependence is similar to that of polycrystalline antimony, i.e. nonlinear with an increase in sensitivity below 10 kPa. These findings imply that the oxygen tension in the vicinity of a COMA pH electrode, has to be known or controllable for the pH reading to be at all valid especially at low oxygen levels, such as is often seen in biomedical applications. A calibration procedure using a zero oxygen solution is used in order to obtain absolute pH readings in a measurement solution of pre-set or known oxygen levels. Furthermore, the nonlinear oxygen sensitivity of the antimony electrode suggests that the underlying electrochemical reactions are other than those described by corrosion theory.

Calibration and field trial of a dissolved-oxygen membrane electrode: Long-term automatic analyser

Calibration and in-field determination of dissolved oxygen (DO) concentrations were carried out with an automatic water-quality monitoring system known as QUALTEL. A calibrated Orion oxygen membrane electrode and Winkler method were employed to measure DO in water with different chlorinities and compared with similar measurements using a Danfoss electrode. From the regression equations, a current of 2.97 mA at 25°C for water-saturated air was determined as being equivalent to a value of 8.68 mg DO L −1 for the Danfoss sensor. From a regression of log ϕ (μA mg −1 L −1) versus t (°C), a constant ( m) and intercept ( b) for water-saturated air was obtained, the values being 0.0263 and −0.759, respectively. A linear relationship with a high correlation coefficient for the salt content of the samples at different temperatures was obtained. The field measurements of DO showed that this system was reliable for the unattended monitoring of DO for 70 days.

Flow injection analysis with amperometric detection of cocaine in confiscated samples

Flow injection analysis (FIA) with amperometric detection is applied to the determination of cocaine. Flow parameters were optimised prior to electrode calibration: a detection potential of +1 V vs. Ag/AgCl/sat. KCl and a flow rate of 2.5 ml min −1 resulted adequate. A relative standard deviation of 1.7% was calculated for a concentration level of 6.2 × 10 −6 ( n = 12) without carrying out a carbon paste electrode pretreatment. Calibration curves were found to be linear between 2 × 10 −7 and 10 −5 M, and the method has a detection limit of 2 × 10 −7 M. A simple and reproducible procedure is proposed for the determination of cocaine in illegal dosage forms confiscated by police authorities. The method compared favourably with liquid chromatography with UV detection.

Carbon fibre micro-electrode and in vitro or in brain slices voltammetric measurement of ascorbate, catechol and indole oxidation signals: influence of temperature and physiological media

Carbon fibre micro-electrodes have been used to determine the influence of temperature and physiological media on the oxidation potential value of three carboxylic acids of physiological interest such as ascorbate (AA), 3,4-dihydroxyphenylacetic acid (DOPAC) and 5-hydroxyindolacetic acid (5HIAA). Standard calibrations at room temperature (18–20°C) in phosphate buffered saline (PBS, pH 7·4), in Krebs (pH 7.4) or in artificial cerebral spinal fluid (ACSF, pH 7.4) have been compared with calibrations performed at 37°C under 95% oxygen, 5% carbon dioxide. Ex vivo experiments were then performed with the electrode inserted in the striatum of rat brain slices maintained in ACSF at 37°C under 95% oxygen, 5% carbon dioxide. The results obtained from both in vitro and ex vivo experimentation indicate that the oxidation potential of peak 2 (DOPAC) is highly sensitive to changes in temperature and medium. Therefore the extrapolation from in vitro electrode calibrations performed in PBS at room temperature to ex vivo (brain slices) and possibly in vivo measurements of DOPAC oxidation should be reconsidered.

Derivative Analysis of Potentiometric Titration Data To Obtain Protonation Constants

A methodology is described to calibrate glass electrodes and to analyze potentiometric titration data to calculate protonation constants. The analysis uses the variation of dV/dpH with titrant addition in terms of two physical parameters which involve the concentrations of H+, OH-, and HmA. The data for titration of acetic acid and 8-hydroxyquinoline in 0.10−5.0 m NaCl media are analyzed by this method to obtain the stoichiometric protonation constants of the acids, the ionization constants of water, and the parameters s and b in the pH electrode calibration equation, pcH = spHm + b, where pcH = −log [H+], and pHm is the pH meter reading.

Iodide buffer systems for the calibration of iodide-selective electrodes

A new approach to the creation of anion buffer systems is proposed, which extends the linear range. The new iodide buffer systems, containing (1) HgI 2 precipitate, CaEDTA 2−, Ca 2+, HgEDTA 2−, and (2) HgI 2 precipitate, chloride complexes of Hg 2+ and excess of Cl − ions, are proposed for the calibration of iodide-selective electrodes at p I > 5. It is shown with the help of these buffers, that a solid-state Orion 94-53 iodide-selective electrode keeps linear Nernstian response up to p I = 7.5. Iodide buffer systems are used for the estimation of the reliability of literature data on selectivity coefficients of iodide-selective electrodes with respect to Cl − ions.

Modelling complex solution equilibria: III. Error-robust calculation of equilibrium constants from pH or potentiometric titration data

In calculating unknown equilibrium constants by the least-squares technique from pH (or potentiometric) titration data, errors in the chemical model (initial volume, reactant concentrations, carbonate or other impurities, p K w and other known equilibrium constants) and measurement errors (electrode calibration, drifts in ionic strength, non-ideal titrant mixing or temperature control) can strongly influence the results. Most of these errors will produce non-randomly distributed pH (or e.m.f.) residuals over some regions of data and thus violate the assumptions of the least-squares method. Simulations and deliberate errors in real data show that fitting the point-to-point changes in pH is less sensitive to such errors than is fitting the raw pH data, producing more trustworthy and reliable refinements of unknown equilibrium constants and more randomly distributed residuals. The refinement results are also insensitive to errors in calibration and more robust with regard to the range of pH spanned by the data. When applied to titrations of glycine-proton and Ni 2+-glycine-proton mixtures from one laboratory, the results more closely matched the averages from several laboratories. Further, this approach will actually signal the presence of unsuspected errors.

Approximal Plaque pH following Topical Applications of Standard Buffers in vivo

In this study, the null hypothesis was tested that topical application of standard buffer solutions, pH 7.00 and pH 4.01, to 4-day-old plaque deposits accumulated in situ causes the plaque to attain the pH values of the buffer solutions applied. Following a 4-day abstention from all oral hygiene procedures, the plaque pH of four interdental sites in each of 5 volunteers was measured at resting state and following topical applications of buffers pH 7 and pH 4. Later the same day plaque pH was measured following rinses with the buffers. Topical application of buffer pH 7 caused a plaque pH increased from a mean value of 6.40 to 6.59 within 30 s, while buffer pH 4 caused plaque pH to drop to a mean value of 5.11. Rinsing with buffers caused plaque pH to increase to 6.50 (buffer pH 7) and to drop to 4.92 (buffer pH 4). Statistical analysis of the results led to rejection of the null hypothesis. The results thus indicated that the validity of the in situ approach of calibration of plaque-covered indwelling electrodes may be questioned.

Behavior and calibration of the copper(II) ion-selective electrode in high chloride media and marine waters

The response of the solid-state Cu(II) ion-selective electrode in high chloride media was investigated and found to be Nernstian under normal laboratory and field measurement conditions. The previously reported «non-Nernstian» behavior has been identified as a combination of electrode fouling, due to specific storage and measurement conditions, and the action of stoichiometric levels of a Lewis base on the standard addition method of electrode calibration. Electrode calibration by external Cu(II) activity buffers in artificial sea-water is described. The copper activity of the buffers is calculated from an ion-interaction model, and the electrode response in these buffers is shown to be consistent with that in chloride solutions over 10 orders of magnitude copper concentration and from pH 8 to 2