Conductance electrodes calibration

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. 

Rinse twice and fill the polyethylene decanters with stream water, place the electrodes in water and measure the pH and conductivity with calibrated meter (mark with permanent drawing ink marker two decanters for pH and conductivity). 

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 determination of riser and downcomer [/l is often accomplished using a tracer technique or specially calibrated flow meters and mathematical relationships to convert the measurable Vl to I/l- The tracer techniques commonly used to determine Vl are based on determining the time it takes for a given tracer to travel a set distance. For example, a potassium chloride salt tracer and conductivity electrodes are commonly used to measure the time it takes an injection of the salt solution to travel past two fixed locations from which Vl is calculated (Bello et al.. 

Conductometric Analysis Solutions of elec trolytes in ionizing solvents (e.g., water) conduct current when an electrical potential is applied across electrodes immersed in the solution. Conductance is a function of ion concentration, ionic charge, and ion mobility. Conductance measurements are ideally suited tor measurement of the concentration of a single strong elec trolyte in dilute solutions. At higher concentrations, conduc tance becomes a complex, nonlinear func tion of concentration requiring suitable calibration for quantitative measurements. 

To measure the conductivity of a solution it is placed in a cell carrying a pair of platinum electrodes which are firmly fixed in position. It is usually very difficult to measure precisely the area of the electrodes and their distance apart, and so if accurate conductivity values are to be determined, the cell constant must be evaluated by calibration with a solution of accurately known conductivity,... 

In a typical study of conductivity. Cook (1982) used a cell consisting of two platinum disc electrodes, 12 mm in diameter and 1-5 mm apart. The setting AB cement was examined in this cell which had been calibrated using a standard solution of0 02 M potassium chloride. Plots were recorded of spedfic conductance against time for each of the setting cements. For zinc polycarboxylate there was found to be a rapid drop in spedfic conductance about 10 minutes after the start of mixing. This behaviour was consistent with the replacement of relatively mobile protons by significantly less mobile zinc ions in the polycarboxylate chain. Con-... 

Table 8.76 shows the main characteristics of voltammetry. Trace-element analysis by electrochemical methods is attractive due to the low limits of detection that can be achieved at relatively low cost. The advantage of using standard addition as a means of calibration and quantification is that matrix effects in the sample are taken into consideration. Analytical responses in voltammetry sometimes lack the predictability of techniques such as optical spectrometry, mostly because interactions at electrode/solution interfaces can be extremely complex. The role of the electrolyte and additional solutions in voltammetry are crucial. Many determinations are pH dependent, and the electrolyte can increase both the conductivity and selectivity of the solution. Voltammetry offers some advantages over atomic absorption. It allows the determination of an element under different oxidation states (e.g. Fe2+/Fe3+). 

Conductivity was measured as described (10) using a conductivity meter (Radiometer, Copenhagen, Denmark) type CDM3 equipped with a CDC 304 immersion electrode with manual temperature compensator type CDA 100. The instrument was calibrated as specified by the manufacturer. The determination of the (NH4)2S04 concentration from the conductivity measurements was done at constant temperature (4°C) using a calibration curve, in the range of 0.016 mM to 120 mM AS in glucose or sucrose (total... 

The difference in the conductivity of the calibration buffers and sample can cause a very large error on the sample measurement, due to junction potentials in different environments. Solid samples should be dissolved in purified water. It is necessary that the water be carbon dioxide-free. The presence of dissolved carbon dioxide will cause significant bias in the measurement of samples with low buffering capacity. For pH measurements with an accuracy of 0.01 to 0.1 pH unit, the limiting factor is often the electrochemical system (i.e., the characteristics of the electrodes and the solution in which they are immersed). 

The experimental protocol used to conduct measurements is based on the following principle a series of standard solutions is prepared by successive dilution of a stock solution and an excess but constant volume of buffer (ISAB or TISAB) is added at each step. Sample solutions are prepared in the same fashion. For each of the standards, the potential across the electrodes is measured and a semi-logarithmic calibration curve E — f(q) is obtained (Fig. 18.6). Using this curve and the potential difference obtained for each of the sample solutions, the concentration of species i can be obtained. 

Figure 15-14 Solid colored circles show the drift in apparent pH of a low-conductivity industrial water supply measured continuously by a single electrode. Individual measurements with a freshly calibrated electrode (black circles) demonstrate that the pH is not drifting. Drift is attributed to slow clogging of the electrode s porous plug with AgCI(s). When a cation-exchange resin was placed inside the reference electrode near the porous plug, Ag(l) was bound by the resin and did not precipitate. This electrode gave the drift-free, continuous reading shown by open diamonds. [From S. Ho, H. Hachlya. K. Baba. Y. Asano. and H. Wada, Improvement of the Ag I AgCt Reference Electrode and Its Application to pH Measurement," talonta 1995,42.1685.]...

In order to evaluate the electrode configuration, similar experiments were performed as described in the previous section, and the results were compared in order to determine whether the electrodes behave identically in the absence and presence of cotton. As expected, similar results were obtained concerning the relationships Equation 10.1 is also valid for electrodes immersed in cotton that act as an immobilising substance for the electrolyte, but the value of k is different. Indeed, all experimentally obtained curves are shifted towards higher resistive behaviour, which can be explained by the fact that the presence of cotton forms a barrier for the conductivity of ions through the electrolyte solution. However, as explained in the previous section, k can be obtained by calibrating the electrode setup, so calibration in the presence of cotton circumvents the problem of different results in the absence and presence of cotton. 

As many physicochemical characteristics as possible of reactants, possible intermediates and products should be considered. These might include spectral features (IR, UV-vis, NMR), ion conductivities (if ions participate in the reaction), optical activity, etc. If such data are not available in the literature, they should be investigated early on as they may lead to a monitoring procedure for the kinetic study. Although physicochemical properties which are directly proportional to concentration are most convenient, others such as pH or electrode potentials may be used as their relationship to concentration is well understood. When the relationship between an observed property or measured signal and concentration of a component in the reaction mixture is not theoretically derived, e.g. GLC signals from analysed samples of the reaction mixture, calibration curves may be used. These are constructed by analysis of standard solutions of a reaction component (see Chapter 2). 

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