Potentiometry direct

The dependence between the main ionic activity coefficient (y ) and the ionic strength (/) is usually given by one of the Debye-Htickel forms  

The reliability of the standard addition method can be increased if several Vq standard values are applied and several E2 voltages are taken. Of course, care must be taken avoiding too much change in the slope or in ionic strength. 

There is an interesting standard addition method variety that can be used in cases when the slope of the ISE is not known. So according to the method called standard addition, slope by dilution three cell voltage values are considered. E is measured in the sample of the known small volume, U . E2 is obtained after the addition of a small known volume (Vc) of standard solution of known (Q) concentration. Furthermore, for obtaining the slope (S) valid in the actual concentration range of the analysis, we add the Vg -l- volume of distilled water and take the (E3) cell voltage value. The second addition causes 1 2 dilution after taking E2 into consideration. So we can write 

Knowing 5, from AE = E2 — E, we can get Cj using the above given equation. 

When thinking about the accuracy and precision of direct potentiometry, it must be kept in mind that 0.5 mV difference in cell voltage translates to almost 2% difference in sample concentration for univalent ions. For divalent ions, it is close to 4%, while for a trivalent ion, in which case S 20mV/decade, it is close to 6%. Therefore, for high accuracy, very well controlled measuring parameters such as cell temperature, junction potential, electrode selectivity, and so on, are needed. 

When replacing a reference electrode by a second ion-selective electrode, one must be especially aware of the fact, that with very large electrical resistances ( 1 kS2) an amplifier with two high ohmic inputs (differential amplifier) must be used  

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Redox Electrodes Electrodes of the first and second kind develop a potential as the result of a redox reaction in which the metallic electrode undergoes a change in its oxidation state. Metallic electrodes also can serve simply as a source of, or a sink for, electrons in other redox reactions. Such electrodes are called redox electrodes. The Pt cathode in Example 11.1 is an example of a redox electrode because its potential is determined by the concentrations of Ee + and Ee + in the indicator half-cell. Note that the potential of a redox electrode generally responds to the concentration of more than one ion, limiting their usefulness for direct potentiometry. 

CrP" -selective and Ni " -selective electrodes have been used to detenuine the copper and nickel ions in aqueous solutions, both by direct potentiometry and by potentiometric titration with EDTA. They have also been used for detenuining the CiT and Ni " ions in indushial waters by direct potentiomehy. 

This procedure of using a single measurement of electrode potential to determine the concentration of an ionic species in solution is referred to as direct potentiometry. The electrode whose potential is dependent upon the concentration of the ion to be determined is termed the indicator electrode, and when, as in the case above, the ion to be determined is directly involved in the electrode reaction, we are said to be dealing with an electrode of the first kind . 

It is also possible in appropriate cases to measure by direct potentiometry the concentration of an ion which is not directly concerned in the electrode reaction. This involves the use of an electrode of the second kind , an example of which is the silver-silver chloride electrode which is formed by coating a silver wire with silver chloride this electrode can be used to measure the concentration of chloride ions in solution. 

In the Nernst equation the term RT/nF involves known constants, and introducing the factor for converting natural logarithms to logarithms to base 10, the term has a value at a temperature of 25 °C of 0.0591 V when n is equal to 1. Hence, for an ion M+, a ten-fold change in ionic activity will alter the electrode potential by about 60 millivolts, whilst for an ion M2 +, a similar change in activity will alter the electrode potential by approximately 30 millivolts, and it follows that to achieve an accuracy of 1 per cent in the value determined for the ionic concentration by direct potentiometry, the electrode potential must be capable of measurement to within 0.26 mV for the ion M+, and to within 0.13 mV for the ion M2 +. ... 

In view of the problems referred to above in connection with direct potentiometry, much attention has been directed to the procedure of potentio-metric titration as an analytical method. As the name implies, it is a titrimetric procedure in which potentiometric measurements are carried out in order to fix the end point. In this procedure we are concerned with changes in electrode potential rather than in an accurate value for the electrode potential with a given solution, and under these circumstances the effect of the liquid junction potential may be ignored. In such a titration, the change in cell e.m.f. occurs most rapidly in the neighbourhood of the end point, and as will be explained later (Section 15.18), various methods can be used to ascertain the point at which the rate of potential change is at a maximum this is at the end point of the titration. 

In the present chapter consideration is given to various types of indicator and reference electrodes, to the procedures and instrumentation for measuring cell e.m.f., to some selected examples of determinations carried out by direct potentiometry, and to some typical examples of potentiometric titrations. 

The use of a pH meter or an ion activity meter to measure the concentration of hydrogen ions or of some other ionic species in a solution is clearly an example of direct potentiometry. In view of the discussion in the preceding sections the procedure involved will be evident, and two examples will suffice to illustrate the experimental method. 

Ion-selective electrodes allow the measurement of ionic activity in diluted or undiluted whole blood, plasma or rum. The direct (undiluted) measurement may be preferred, since no sample pretreatment is necessary and the assay values are independent of hematocrit and amount of solids present. However, direct potentiometry by its very nature does not provide total concentration values similar to those obtained by flame photometry and indirect (diluted) potentiometry... 

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. 

One of the most fruitful uses of potentiometry in analytical chemistry is its application to titrimetry. Prior to this application, most titrations were carried out using colour-change indicators to signal the titration endpoint. A potentiometric titration (or indirect potentiometry) involves measurement of the potential of a suitable indicator electrode as a function of titrant volume. The information provided by a potentiometric titration is not the same as that obtained from a direct potentiometric measurement. As pointed out by Dick [473], there are advantages to potentiometric titration over direct potentiometry, despite the fact that the two techniques very often use the same type of electrodes. Potentiometric titrations provide data that are more reliable than data from titrations that use chemical indicators, but potentiometric titrations are more time-consuming. 

Sodium valproate has been determined in pharmaceuticals using a valproate selective electrode [13,14]. The electroactive material was a valproate-methyl-tris (tetra-decyl)ammonium ion-pair complex in decanol. Silver-silver chloride electrode was used as the reference electrode. The electrode life span was >1 month. Determination of 90-1500 pg/mL in aqueous solution by direct potentiometry gave an average recovery of 100.0% and a response time of 1 min. 

Direct particle interception, in depth filtration theory, 11 339 Direct potentiometry, 9 582—585 Direct printing, 9 218 Direct process, silicone synthesis via,... 

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. 

Direct Potentiometry The procedure adopted of employing a single measurement of electrode potential to determine the concentration of an ionic species in a solution is usually termed as direct potentiometry. 

Disadvantages Direct potentiometry has the following two serious disadvantages namely ... 

Keeping in view the above serious anomalies commonly encountered with direct potentiometry, such as an element of uncertainty triggered by liquid junction potential (E.) and high degree of sensitivity required to measure electrode potential (E), it promptly gave birth to the phenomenon of potentiometric titrations,... 

The ISEs described in this section are useful primarily for determination of halide ions by direct potentiometry, where the silver halide in the membrane is identical with the determinand. As follows from the discussion on p. 48 an electrode made of a less soluble silver halide X can be used to determine other halide Y" if the condition... 

Analytical determinations with the fluoride ion-selective electrode These are based either on direct potentiometry of fluorides [37, 84, 85, 88, 430] or on titration determinations of either fluorides or of other ions and also on titrations with fluoride ions as indicator. The advantages of potentiometry with an ISE over other analytical methods for determining fluorides were pointed out by Crosby etai [67], Further comparison studies [42, 56, 191, 433] came to the same conclusions, confirmed also by a study of 16 methods [365]. Fluoride ions are titrated either with La (for concentrations greater than 1 mM) or Th (in the concentration range 0.2-1 mM F ) [13, 102, 103, 113,233, 234]. Titration with fluoride ions can be used for the determination of Al with formation of the AIF4 complex up to nanomolar concentrations, especially in ethanol-water mixtures [25] (see also [267,384]). Precipitation titrations can also be used to determine La, Th and UOJ [241, 384] as well as Li in... 

The fluoride ion selective electrode is the most popular means of fluoride ion determination after sample destruction by any method but it does have limitations. It can be used either directly to measure the fluoride potential6 or as an end-point detector in a potentiometric titration with a lanthanum(l II) reagent as titrant.4,7 Problems can be experienced with potential drift in direct potentiometry, especially at low fluoride ion concentrations. Titration methods often yield sluggish end points unless water miscible solvents are used to decrease solubilities and increase potentia 1 breaks and sulfate and phosphate can interfere. End-point determination can be facilitated by using a computerized Gran plotting procedure.4... 

Figure 18.6—Example of a measurement by direct potentiometry. The slope for the chloride selective electrode has almost an ideal value. The dynamic range for most selective electrodes spans 4 to 6 orders of magnitude, depending on the ion.

Table 5), and several are now being used, or are potentially useful, for measuring key ocean elements. The most common use of direct potentiometry (as compared with potentiometric titrations) is for measurement of pH (Culberson, 1981). Most other cation electrodes are subject to some degree of interference from other major ions. Electrodes for sodium, potassium, calcium, and magnesium have been used successfully. Copper, cadmium, and lead electrodes in seawater have been tested, with variable success. Anion-selective electrodes for chloride, bromide, fluoride, sulfate, sulfide, and silver ions have also been tested but have not yet found wide application. 

Culberson, C.H. 1981. Direct potentiometry, in Marine Electrochemistry A Practical Introduction, M. Whitfield and D. Jagner (eds.)., John Wiley Sons, New York, pp. 187-261. 

Direct Potentiometry and Ion-Selective Electrodes. The reference electrode, Ag-AgCl, illustrated in Figure 2 may be taken as... 

The counterion activity of DADMAC polymers has been determined by direct potentiometry using chloride ion selective electrodes. If the polyelectrolyte con-... 

Maltodextrins (dextrose equivalent (DE) 4.0-7.0, 13.0-17.0 and 16.5-19.5) are proposed as novel chiral selectors for the construction of EPMEs for S-captopril assay [36]. The EPMEs can be used reliably for the assay of S-captopril as raw material and from pharmaceutical formulations as Novocaptopril tablets, using direct potentiometry. The best response was obtained when maltodextrin with higher DE was used for the electrode s construction. The best enantioselectivity and stability in time was achieved for the lower DE maltodextrin. L-Proline was found to be the main interferent for all proposed electrodes. The surface of the electrodes can be regenerated by simply polishing, obtaining a fresh surface ready to be used in a new assay. 

Determination of Cs+ in natural water samples using direct potentiometry... 

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

Akaiwa et al. [324] have used ion exchange chromatography on hydrous zirconium oxide, combined with detection based on direct potentiometry with an ion selective electrode, for the simultaneous determination of chloride and bromide in non saline waters. 

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