Potentiometric methods

This section deals with the fabrication of potentiometric probes and their use in SECM studies. Potentiometric probes (see Chapter 7) can detect many non-electroactive species not accessible to amperometric techniques. They are highly selective and have found widespread application in clinical chemistry, in environmental studies and the food industry. A general review of potentiometric probe fabrication has been presented previously, and several publications have demonstrated the utility of potentiometric probes in SECM studies (55). This section will provide the reader with a highlight of potentiometric probe fabrication techniques taken from the literature. The section will also include a discussion of the basic concepts, fabrication steps, necessary equipment, and characterization of ion-selective micropipettes applied in SECM studies. 

Potentiometric detection is based on the detection of a membrane potential across the membrane of a probe electrode. Potentiometric probes measure a membrane potential linearly dependent on the logarithm of the activity of a primary ion. Experimentally, a typical setup would be 

Outer-reference 11 Sample I Monbrane I Internal Solution I Internal-Reference 

The membranes used in ion-selective electrodes separate two different electrolytes and are not equally permeable to all kinds of ions. At the interface between the two electrolytes, different events contribnte to the measured membrane potential. First, a diffusion potential arises from differences in mobility and concentration of ions in contact at the interface, as seen in liquid junctions. Second, a Donnan potential arises when the membrane completely prevents the diffnsion of at least one species from solution to the other. Third, the exchange equilibria between the electrolyte and the membrane interface must also be considered to adequately describe the membrane potential of ion-selective electrodes with solid or liquid electrolyte manbranes. 

When no current is flowing, the measured cell potential ( ceii) the sum of all potential contributions in the cell such that 

The plot of pH against titrant volume added is called a potentiometric titration curve. The latter curve is usually transformed into a Bjerrum plot [8, 24, 27], for better visual indication of overlapping pKiS or for pffjS below 3 or above 10. The actual values of pKa are determined by weighted nonlinear regression analysis [25-27]. 

After precipitation of the halogen ions with AgN03, ( otassium is precipitated with sodium tetraphenylborate as described. The excess of tetraphenylborate is then determined by AgNO 3 titration, using a silver electrode for endpoint detection. The method has been applied to seawater samples as small as 1 mL as well as for interstitial waters, with a precision of better than 1 %. The procedure saves the tedious filtration and drying process but offers a somewhat lower precision than the gravimetric method. A detailed description is presented by Marquis and Lebel (1981). 

A metal is regarded as an assembly of metal ions of free electrons. When the metal is in contact with water, some metal ions enter into the liquid due to a tendency in the metal, called by Nemst as electrolytic solution tension . As some metal ions leave the solid, the solid becomes negatively charged and the solution positively charged. In consequence, due to electrostatic force, any further transference of the metal ions is prevented and the ions attracted by the negatively charged metal, remain near the metal surface forming a double layer. If the metal is 

It is to be remembered that reduction potential of an electrode is same as its oxidation potential with the sign changed. Usually anode of a cell is written in the left and cathode in the right. It is also a common convention that current in external circuit flows from cathode to anode although the electrons are flowing in the opposite direction through the wire. 

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Thus, for example, an analysis using coloured solutions can be carried out, where an indicator cannot be used. Moreover, it is not easy to find a redox indicator which will change colour at the right point. Potentiometric methods can fairly readily be made automatic. 

Quantitative Analysis Using the Method of Standard Additions Because of the difficulty of maintaining a constant matrix for samples and standards, many quantitative potentiometric methods use the method of standard additions. A sample of volume, Vx) and analyte concentration, Cx, is transferred to a sample cell, and the potential, (ficell)x) measured. A standard addition is made by adding a small volume, Vs) of a standard containing a known concentration of analyte, Cs, to the sample, and the potential, (ficell)s) measured. Provided that Vs is significantly smaller than Vx, the change in sample matrix is ignored, and the analyte s activity coefficient remains constant. Example 11.7 shows how a one-point standard addition can be used to determine the concentration of an analyte. 

The effect of an uncertainty in potential on the accuracy of a potentiometric method of analysis is evaluated using a propagation of uncertainty. For a membrane ion-selective electrode the general expression for potential is given as... 

Electrochemical methods covered in this chapter include poten-tiometry, coulometry, and voltammetry. Potentiometric methods are based on the measurement of an electrochemical cell s potential when only a negligible current is allowed to flow, fn principle the Nernst equation can be used to calculate the concentration of species in the electrochemical cell by measuring its potential and solving the Nernst equation the presence of liquid junction potentials, however, necessitates the use of an external standardization or the use of standard additions. 

Quantitative analytical methods using FIA have been developed for cationic, anionic, and molecular pollutants in wastewater, fresh waters, groundwaters, and marine waters, several examples of which were described in the previous section. Table 13.2 provides a partial listing of other analytes that have been determined using FIA, many of which are modifications of conventional standard spectropho-tometric and potentiometric methods. An additional advantage of FIA for environmental analysis is its ability to provide for the continuous, in situ monitoring of pollutants in the field. ... 

There are several comprehensive reviews of analytical methods for vitamin K (19,20). Owiag to the preseace of a aaphthoquiaoae aucleus, the majority of analytical methods use this stmctural feature as a basis for analysis. Several identity tests such as its reaction with sodium bisulfite or its uv spectmm exploit this characteristic. Although not specific, titrimetric, polarographic, and potentiometric methods have also been used (20). 

Samples were deformed by rolling at room temperature after disordering and quenching (40% and 80% reduction in thickness) and after ordering by a 19h annealing at 300°C (30% reduction). Completely recrystallized samples were prepared by annealing 48h at 600°C. Resistivity measurement was done by the potentiometric method in liquid N2 relative to a dummy specimen (accuracy 3x10" ). 

Thus an almost complete separation is theoretically possible. The separation is feasible in practice if the point at which the iodide precipitation is complete can be detected. This may be done (a) by the use of an adsorption indicator (see Section 10.75(c)), or (b) by a potentiometric method with a silver electrode (see Chapter 15). 

Dagnall and West8 have described the formation and extraction of a blue ternary complex, Ag(I)-l,10-phenanthroline-bromopyrogallol red (BPR), as the basis of a highly sensitive spectrophotometric procedure for the determination of traces of silver (Section 6.16). The reaction mechanism for the formation of the blue complex in aqueous solution was investigated by photometric and potentiometric methods and these studies led to the conclusion that the complex is an ion association system, (Ag(phen)2)2BPR2, i.e. involving a cationic chelate complex of a metal ion (Ag + ) associated with an anionic counter ion derived from the dyestuff (BPR). Ternary complexes have been reviewed by Babko.9... 

A. Direct titration. The solution containing the metal ion to be determined is buffered to the desired pH (e.g. to PH = 10 with NH4-aq. NH3) and titrated directly with the standard EDTA solution. It may be necessary to prevent precipitation of the hydroxide of the metal (or a basic salt) by the addition of some auxiliary complexing agent, such as tartrate or citrate or triethanolamine. At the equivalence point the magnitude of the concentration of the metal ion being determined decreases abruptly. This is generally determined by the change in colour of a metal indicator or by amperometric, spectrophotometric, or potentiometric methods. 

C. Potentiometric methods. This is a procedure which depends upon measurement of the e.m.f. between a reference electrode and an indicator (redox) electrode at suitable intervals during the titration, i.e. a potentiometric titration is carried out. The procedure is discussed fully in Chapter 15 let it suffice at this stage to point out that the procedure is applicable not only to those cases where suitable indicators are available, but also to those cases, e.g. coloured or very dilute solutions, where the indicator method is inapplicable, or of limited accuracy. 

In electro-gravimetric analysis the element to be determined is deposited electroly tically upon a suitable electrode. Filtration is not required, and provided the experimental conditions are carefully controlled, the co-deposition of two metals can often be avoided. Although this procedure has to a large extent been superseded by potentiometric methods based upon the use of ion-selective electrodes (see Chapter 15), the method, when applicable has many advantages. The theory of the process is briefly discussed below in order to understand how and when it may be applied for a more detailed treatment see Refs 1-9. 

As indicated in Section 2.4 the strength of an acid (and of a base) is dependent upon the solvent in which it has been dissolved, and in Sections 10.19-10.21 it has been shown how this modification of strength can be used to carry out titrations in non-aqueous solvents which are impossible to perform in aqueous solution. Potentiometric methods can be used to determine the end point of such non-aqueous titrations, which are mainly of the acid-base type and offer very valuable methods for the determination of many organic compounds. 

ISO 4316 1977, Surface active agents Determination of pH of aqueous solutions—Potentiometric method. 

ISO 4323 1977, Soaps Determination of chlorides content—Potentiometric method. 

Pectins in the acidic form were dissolved either in pure water or in 0.1 M NaNOs and put to pH -7.2 by adding 0.05 M NaOH. Various amounts of metal ions were added to pectins solutions at two different concentrations, for 2 h under stirring at 25.0 0.1°C. Concentration of metal ions in solution at equilibrium was determined either by a potentiometric method using ion-selective electrodes for Cu2+ and Pb2+ or by a spectrophotometric method using tetramethylmurexide dye (Kwak Joshi, 1981) for Ni2+, Tsfi and Ca2+. 

The fluorine titration of chromium oxide was carried out at the Elf-Atochem Research Center, Pieire-Benite.The catalyst mineralization was carried out in a Parr bomb by reaction with sodium peroxyde. Fluorine ions were then titrated by a potentiometric method with a specific fluoride electrode. 

Among potentiometric methods of analysis that are important for ecological applications, the one most widely used is that of pH measurements with an indicator electrode whose potential is a function of the hydrogen ion concentration. More recently, ion-selective electrodes reversible to other cations such as those of heavy metals have become available. 

Potentiometric methods are based on the measurement of the potential of an electrochemical cell consisting of two electrodes immersed in a solution. Since the cell potential is measured under the condition of zero cmrent, usually with a pH/mV meter, potentiometry is an equilibrium method. One electrode, the indicator electrode, is chosen to respond to a particular species in solution whose activity or concentration is to be measured. The other electrode is a reference electrode whose half-cell potential is invariant. 

Recent Improvements in the Potentiometric Method Applied to Sparingly Soluble Drugs... 

Slater, B., McCormack, A., Avdeef A., Comer, J. E. pH-metric logP. 4. Comparison of partition coefficients determined by HPLC and potentiometric methods to literature values. J. Pharm. 

Takacs-Novak, K., Avdeef, A. Interlaboratory study of log P determination by shake-flask and potentiometric methods. J. Pharm. 

In fact, any type of titration can be carried out potentiometrically provided that an indicator electrode is applied whose potential changes markedly at the equivalence point. As the potential is a selective property of both reactants (titrand and titrant), notwithstanding an appreciable influence by the titration medium [aqueous or non-aqueous, with or without an ISA (ionic strength adjuster) or pH buffer, etc.] on that property, potentiometric titration is far more important than conductometric titration. Moreover, the potentiometric method has greater applicability because it is used not only for acid-base, precipitation, complex-formation and displacement titrations, but also for redox titrations. 

However, for m-cresol purple, thymol blue and o-nitroaniline, many estimated K, values yielded straight lines with eqn. 4.77, so that with these indicators the spectrophotometric method failed. This result led Bos to check some of the above results by means of the potentiometric method of Tanaka and Nakagawa64 applied to titrations in glacial acetic acid also he obtained the following data ... 

The transfer of an automated analysis from the laboratory to the plant will often require special precautions for instance, while turbidities in a process stream can cause a loss of selective absorptivity in a spectrophotometric measurement, in potentiometric methods fouling of the electrodes, potential leakage in metal containers or tubing and loss of signal in remote control may occur (see later). 

Considering the related Mettler DL 40 and DL 40 RC MemoTitrators, the DL 40 can be used for ten different volumetric and potentiometric methods titration to a pre-selected absolute (EPA) or relative (EPR) end-point equilibrium titration (EQU)-recording titration (REC)-incremental titration (INC) Karl Fischer water determination (KF)-controlled dispensing (DOSE) pH and pX measurements (pX/E) multi-level titrations and back-titrations with automatic calculation (CALC) and manual titration (MAN)-automatic calibration of electrodes (CAL). 

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