Potentiometric calibration curves

In case b) a potentiometrically measured calibration curve for ion M+ would show a sigmoidal form, starting with a sub-Nernstian slope leading to a super-Nemstian... 

The behavior of potentiometric and pulsed galvanostatic polyion sensors can be directly compared. Figure 4.11 shows the time trace for the resulting protamine calibration curve in 0.1 M NaCl, obtained with this method (a) and with a potentiometric protamine membrane electrode (b) analogous to that described in [42, 43], Because of the effective renewal of the electrode surface between measuring pulses, the polyion response in (a) is free of any potential drift, and the signal fully returns to baseline after the calibration run. In contrast, the response of the potentiometric protamine electrode (b) exhibits very strong potential drifts. 

Fig. 18a.7. Typical calibration curve of a potentiometric sensor for measuring monovalent cations. From Ref. [70] with permission.

For higher accuracy in the low polymer concentration range, two different methods were used. In the case of PAA, potentiometric titrations of solutions of PAA were performed with 0.01 N NaOH using a Brinkman model, Westbury, NY, automated titrator. Blank tests indicated no interfering species. Known amounts of PAA were used to prepare a calibration curve immediately after titration of the samples containing unknown amounts of polymer. The starting point of the titration was pH 4.0, and the end point was reached near pH 8. Total volumes of 75 or 100 cc were used for the titrations, and the ionic strength was controlled at 0.01 M NaCl. 

Because each enzyme sensor has its own unique response, it is necessary to construct the calibration curve for each sensor separately. In other words, there is no general theoretical response relationship, in the same sense as the Nernst equation is. As always, the best way to reduce interferences is to use two sensors and measure them differentially. Thus, it is possible to prepare two identical enzyme sensors and either omit or deactivate the enzyme in one of them. This sensor then acts as a reference. If the calibration curve is constructed by plotting the difference of the two outputs as the function of concentration of the substrate, the effects of variations in the composition of the sample as well as temperature and light variations can be substantially reduced. Examples of potentiometric enzyme electrodes are listed in Table 6.5. 

The stability of enzyme electrodes is difficult to define because an enzyme can lose some of its activity. Deterioration of immobilized enzyme in the potentiometric electrodes can be seen by three changes in the response characteristics (a) with age the upper limit will decrease (e.g., from 10-2 to 10 3 moll-1), (b) the slope of the analytical (calibration) curve of potential vs. log [analyte] decrease from 59.2 mV per decade (Nernstian response) to lower value, and (c) the response time of the biosensor will become longer as the enzyme ages [59]. The overall lifetime of the biosensor depends on the frequency with which the biosensor is used and the stability depends on the type of entrapment used, the concentration of enzyme in the tissue or crude extract, the optimum conditions of enzyme, the leaching out of loosely bound cofactor from the active site, a cofactor that is needed for the enzymatic activity and the stability of the base sensor. 

Fig. 2.2. Potentiometric calibration curves obtained for PVC-DOS electrodes based on Cs I and UIC (see text for membrane composition). After the preliminary conditioning steps (0.1-M TMAC1 followed by 0.1-M LiOH), the electrodes were conditioned in 0.01-M NaCl and contained 0.01-M NaCl as the inner filling solution. Data are fitted with Nernstian response slopes for monovalent (solid line) and divalent (dashed line) ions, respectively.

Usually, the analytical chemist needs to determine the concentration of the ion of interest rather than its activity. The obvious approach to converting potentiometric measurements from activity to concentration is to make use of an empirical calibration curve, such as the one shown in Figure 5.3. Electrodes potentials of standard solutions are thus measured and plotted (on a semilog paper) versus the concentration. Since the ionic strength of the sample is seldom known, it is often useful to add a high concentration of an electrolyte to the standards and the sample to maintain approximately the same ionic strength (i.e., the same activity coefficient). The ionic strength adjustor is usually a buffer (since pH control is also desired for most ISEs). The empirical calibration plot thus yields results in terms of concentration. Theoretically,... 

In this equation, E is the potentiometric response, is the standard potential, and is the potentiometric selectivity coefficient [23]. The Nickolsky Eisenman equation is only valid if ions of the same valency are compared [24]. If that is not the case, if a divalent cation is interfering the measurement of a monovalent ion or vice versa, a new selectivity factor AT is recommended [24,25] which more accurately describes the degree of interference log/Tfj is derived graphically from the horizontal distance of the separately measured calibration curves towards the two ions, i and j, of interest (Figure 5) and is formulated as follows ... 

When electrolyte concentrations are not too great, it is often useful to swamp both samples and standards with a measured excess of an inert electrolyte. The added effect of the electrolyte from the sample matrix becomes negligible under these circumstances, and the empirical calibration curve yields results in terms of concentration. This approach has been used, for example, in the potentiometric determination of fluoride ion in drinking water. Both samples and standards are diluted with a solution that contains sodium chloride, an acetate buffer, and a citrate buffer the diluent is sufficiently concentrated so that the samples and standaids have essentially identical ionic strengths. This method provides a rapid means of measuring fluoride concentrations in the part-per-million range with an accuracy of about 5% relative. 

The potentiometric SECM experiment yields the potential of the tip electrode, E, as a function of the tip position. To establish the correspondence between these data and the above theory, one needs to use a calibration curve, i.e, a Nemstian E vs. c plot. Using such a calibration, one can transform the experimental results to the c vs. (z, r) dependence and fit them to the theory in order to find Js and establish the distance scale. 

As is seen from Fig. 3.7.1 the pO values at the excess of the studied cation are sufficiently low, and owing to the enhanced acidic properties of the KCl-LiCl melt, the increase in the melt acidity results in a considerable increase in the oxide solubility, so that CoO, which is practically insoluble in the molten KCl-NaCl equimolar mixture, becomes appreciably soluble with a sharp pronounced section of the unsaturated solution (see Fig. 3.7.1, curve 3). The existence of the said section allows us to calculate the dissociation constant of CoO in the molten KCl-LiCl eutectic at 700 °C using the potentiometric data for three initial points of the calibration curve (the corresponding treatment results are collected in Table 3.7.2), and its average value is presented in Table 3.7.3. The fourth point of the titration curve is the boundary one between the saturated and unsaturated solution, and therefore, it is available for calculations of the values of both the dissociation constant and the solubility product. 

The calcium and sodium activity coefficients were determined at 25.0 - 0.1 C with an Orion electrode (model 92-32) and a Radiometer electrode (model G502 Na), respectively. A saturated calomel electrode was used as the reference. Calibration curves were obtained using CaCl or NaCl solutions before and after each measurement. The CaCl2 and NaCl concentrations were measured by potentiometric determinations of the chlorides with silver nitrate and with a silver electrode. 

Figure 7.11 A theoretical potentiometric enzyme electrode calibration curve based on external diffusion control of the reaction a plot of the logarithm of the product concentration in the enzyme layer versus the logarithm of the bulk substrate concentration. K = 10" the value of kaEV/PsE is given on the curve [24],...

The influence of chloride ions on the behaviour of copper(II) sulphide electrode was studied by using potentiometric and voltammetric methods in four different media 1X10 " M HNO3, 1x10-2 M HCl, 1x10-2 M HCl + 1 M KCl and 1 M HCl. The potentiometric calibration curves obtained in diluted acids are similar to those reported above (Fig.4, curves 1,2), indicating no difference in the mechanism of copper(II) sulphide electrode action. 

In the conductivity, potentiometric, and voltam-metric measurements the response is correlated to concentration or activity of the analyte usually by using calibration curves. In coulometry, however, the charge measured gives directly the amount of substance and therefore no calibration is needed. However, in coulometry the sample is consumed in the measurements and the problem is that the method requires 100% current efficiency to be reliable. Conductimetry and potentiometry are sample nonconsuming methods. In voltammetry, only an insignificant amount of the sample is consumed and therefore the measurement can be repeated. Only in voltammetric stripping methods of very low concentrations of the analyte the amount consumed at the electrode reaction has to be considered if repeated measurements are to be done. 

Potentiometric pH Sensors at Ambient Tempera- beverages [18] (Reprinted from Analytica Chunica Acta, ture. Fig. 2 (Left) Potentiometric calibration curve of 1997. 351(1-3) p. 143-149 with permission from screen-printed RUO2 in universal buffer and (right) com- Elsevier) parison to a commercial glass electrode in some drink and... 

Potentiometric pH Sensors at Ambient Temperature, Fig. 5 SEM and calibration curves of PPy, PPPD, PAM, and PEI on Pt electrode [29] (Reprinted from Polymer, 2005. 46(26) p. 12233-12239 with permission from Elsevier)... 

The usual calibration curve is constructed by plotting peak height against concentration or mass in most analytical methodologies (Figure 2.1) or vs the logarithmic concentration in potentiometric determinations. 

Organophosphate pesticides studied in this work were the model low-toxic OPC trichlorfon, and some common organophosphate pesticides malathion, parathion, dichlorvos, and diazinon (Table I). Calibration curves for these pesticides (dependences of the sensor inhibition response on the analyte concentration) were obtained for all of these OPCs. These calibration curves were obtained under conditions (time of inhibition, pH and temperature) optimize with the model analyte trichlorfon. All of the pesticide calibration curves are similar and Fig. 4 illustrate the method by the example of malathion. The lowest concentration of pesticide samples assayed with 10 min. of incubation of the electrode in inhibitor containing solution was 5 ppb. This resulted in approximately 10 % of the relative inhibition signal. Fig. 4 predicts much better performance of our system compared with the literature data. For example, trichlorfon detection by means of ISFET had a reported limit of detection of ca 250 ppb (5), while conductometric sensor assay registered trichlorfon at ca. 25 ppb (5), still an order of magnitude higher than the described sensor. An amperometric sensor was used to detect dichlorvos with a limit of detection of 350 ppb (2J) and a potentiometric (pH-sensitive) sensor was shown to detect parathion at 39 ppm and diazinon at 35 ppb (9). 

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