Calibration potentiometry

The trade-offs between direct calibration and standard addition are treated in Ref 103. The same recovery as is found for the native analyte has to be obtained for the spiked analyte (see Section 3.2). The application of spiking to potentiometry is reviewed in Refs. 104 and 105. A worked example for the application of standard addition methodology to FIA/AAS is found in Ref 106. Reference 70 discusses the optimization of the standard addition method. 

Most measurements include the determination of ions in aqueous solution, but electrodes that employ selective membranes also allow the determination of molecules. The sensitivity is high for certain ions. When specificity causes a problem, more precise complexometric or titri-metric measurements must replace direct potentiometry. According to the Nernst equation, the measured potential difference is a measure of the activity (rather than concentration) of certain ions. Since the concentration is related to the activity through an appropriate activity coefficient, calibration of the electrode with known solution(s) should be carried out under conditions of reasonable agreement of ionic strengths. For quantitation, the standard addition method is used. 

In electroanalysis, the techniques are pre-eminently based on processes that take place when two separate poles, the so-called electrodes, are in contact with a liquid electrolyte, which usually is a solution of the substance to be analysed, the analyte. By means of electrometry, i.e., by measuring the electrochemical phenomena occurring or intentionally generated, one obtains signals from which chemical-analytical data can be derived through calibration. Often electrometry (e.g., potentiometry) is applied in order to follow a reaction that goes to completion (e.g., a titration), which essentially represents a stoichiometric method, so that the electrometry merely acts as an end-point indicator of the reaction (which means a potentiometric titration). The electrochemical phenomena in electroanalysis, whether they take place in the solution or at the electrodes, are often complicated and their explanation requires a systematic treatment of electroanalysis. 

Another galvanic cell of highly practical and theoretical importance is the so-called standard cell (see Section 2.2.2), use of which has to be made as a calibration standard in non-faradaic potentiometry. For this purpose, the saturated Weston cell is the most accepted as its emf is reproducible, precisely known, only slightly temperature dependent in the region around 25° C (1.01832 V) and insensitive to unexpected current flows, if any. 

This method is primarily concerned with the phenomena that occur at electrode surfaces (electrodics) in a solution from which, as an absolute method, through previous calibration a component concentration can be derived. If desirable the technique can be used to follow the progress of a chemical reaction, e.g., in kinetic analysis. Mostly, however, potentiometry is applied to reactions that go to completion (e.g. a titration) merely in order to indicate the end-point (a potentiometric titration in this instance) and so do not need calibration. The overwhelming importance of potentiometry in general and of potentiometric titration in particular is due to the selectivity of its indication, the simplicity of the technique and the ample choice of electrodes. 

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. 

In analytical practice, some methods using definitive measurements, in principle, are also calibrated by indirect reference measurements using least squares estimating to provide reliable estimates of b (spectrophotometry, potentiometry, ISE, polarography). 

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. 

In contrast to potentiometry with ISEs, the drain current is measured with the ISFET and not the voltage. As the drain current depends only approximately linearly on A 0 and as the aK (>ri) value depends on the properties of the membrane surface (for example, on the adsorption of surfactants), measurement of activities using an ISFET requires careful calibration. The response time depends on the membrane properties and is not affected by the components of the solid-phase sensor [162]. 

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. 

Coulometry comprises a set of techniques in which the total charge required (not the current, as in potentiometry) to oxidize or reduce the chemical species of interest is measured. The prime virtue of coulometric techniques is that they link the quantity of substance determined directly to the quantity of electrical charge, and thus expensive and often difficult procedures for standardization or calibration can be minimized or eliminated. 

Determine the unknown concentration from the calibration graph, as described above (direct potentiometry). 

In some cases, potentiometry can also be performed with electrodes at which a -> mixed potential is measured. In these cases careful calibration is needed, or the electrodes are only used to monitor the equivalence point of titration by providing a certain characteristic inlection point f. 

For titrations to a fixed potential, the calibration slope is not needed, but the correct potential must be chosen by some means of calibration. In pH titrations, for example, this can be by means of a buffer solution. The errors involved are usually much smaller than those in direct potentiometry. 

Potentiometry is the measurement of an electrical potential difference between two electrodes (half-ceUs) in an electrochemical cell (Figure 4-1) when the cell current is zero (galvanic cell). Such a cell consists of two electrodes (electron or metallic conductors) that are connected by an electrolyte solution (ion conductor). An electrode, or half-cell, consists of a single metallic conductor that is in contact with an electrolyte solution. The ion conductors can be composed of one or more phases that are either in direct contact with each other or separated by membranes permeable only to specific cations or anions (see Figure 4-1). One of the electrolyte solutions is the unknown or test solution this solution may be replaced by an appropriate reference solution for calibration purposes. By convention, the cell notation is shown so that the left electrode (Mi,) is the reference electrode the right electrode (Mr) is the indicator (measuring) electrode (see later equation 3). ... 

TABLE 4-2 Example of Two-Level Calibrating Solutions for Measurement of pH and Electrolytes by Direct Potentiometry ... 

Analytical Chemistry the group of Prof Vasil D. Simeonov is performing research in analytical chemistry, chemometrics, environmetrics, multivariate calibration classification, interpretation and modelling of environmental data sets evaluation and optimization of analytical procedures potentiometry with ion selective electrodes atmospheric and marine chemistry. A very wide network of international collaboration is associated with the group. 

Figure 19.6 Typical ccdibration curve of an ISE by direct potentiometry. The calibration curve of the specific electrode for the chloride ion has almost an ideal slope value. The range of linear response for the different ISE extends over 4 to 6 orders of magnitude depending on the ion. Expression 19.5 leads to the estimation that an uncertainty of 0.2 mV on E leads to an inexactness of 0.8 per cent in the concentration (for a monovalent ion). Here, the TISAB consists of NaCl 1 M for adjusting the ionic force, a complexing agent for metals and a hufler mixture of acetic acid/sodium acetate.

Compare this equation with Eqs. (15.7) and (15.15). By convention, the reference electrode is connected to the negative terminal of the potentiometer (the readout device). The common reference electrodes used in potentiometry are the SCE and the silver/silver chloride electrode, which have been described. Their potentials are fixed and known over a wide temperature range. Some values for these electrode potentials are given in Table 15.3. The total cell potential is measured experimentally, the reference potential is known, and therefore the variable indicator electrode potential can be calculated and related to the concentration of the analyte through the Nemst equation. In practice, the concentration of the unknown analyte is determined after calibration of the potentiometer with suitable standard solutions. The choice of reference electrode depends on the application. For example, the Ag/AgCl electrode cannot be used in solutions containing species such as halides or sulfides that will precipitate or otherwise react with silver. 

Differential potentiometry is a concentration cell technique involving the use of a matched pair of electrodes whose liquid junction potentials become negligible when a sufficiently large excess of an inert electrolyte is used [14]. The unknown solution is placed in one half-cell and a standard solution in the other. The potential difference is related to the ion concentration by a calibration curve. 

HCI as a titrant Potentiometry No calibration required Determination of partial and intermediate alkalinity... 

A theoretically predicted positive bias of 6.7% for direct potentiometry (total ion molality in plasma water) versus indirect assay (total ion concentration in the whole sample volume) is generally accepted for normal situations and is compensated by calibration. 

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. 

Standardization of Potentials. The increasing interest in potentiometry stressed the need for standard values of electrode potentials. In 1890, Ostwald introduced the calomel electrode, calibrating this against a dropping mercury electrode. Nernst chose the normal hydrogen electrode, assigning to it a potential of zero. The upshot was a confrontation between the two scientists as the 19th century ended. 

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