Potentiometric analysis

Sensitivity The sensitivity of a potentiometric analysis is determined by the term RT/nF or RT/zF in the Nernst equation. Sensitivity is best for smaller values of n or z. 

Selectivity As described earlier, most ion-selective electrodes respond to more than one analyte. For many ion-selective electrodes, however, the selectivity for the analyte is significantly greater than for most interfering ions. Published selectivity coefficients for ion-selective electrodes (representative values are found in Tables 11.1 through 11.3) provide a useful guide in helping the analyst determine whether a potentiometric analysis is feasible for a given sample. 

In such reactions, even though the indicator electrode functions reversibly, the maximum value of AE/AV will not occur exactly at the stoichiometric equivalence point. The resulting titration error (difference between end point and equivalence point) can be calculated or can be determined by experiment and a correction applied. The titration error is small when the potential change at the equivalence point is large. With most of the reactions used in potentiometric analysis, the titration error is usually small enough to be neglected. It is assumed that sufficient time is allowed for the electrodes to reach equilibrium before a reading is recorded. 

The most important non-faradaic methods are conductometric analysis and (normal) potentiometric analysis in the former we have to deal essentially with the ionics and in the latter mainly with the electrodics. Strictly, one should assign a separate position to high-frequency analysis, where not so much the ionic conductance but rather the dielectric and/or diamagnetic properties of the solution are playing a role. Nevertheless, we shall still consider this techniques as a special form of conductometry, because the capacitive and inductive properties of the solution show up versus high-frequency as a kind of AC resistance (impedance) and, therefore, as far as its reciprocal is concerned, as a kind of AC conductance. 

Sylvester DM, Holmes RK, Sander C, et al. 1982. Interference of thiosulfate with potentiometric analysis of cyanide in blood and its elimination. Toxicol Appl Pharmacol 65 116-121. 

The titration error (i.e., difference between end-point and equivalence point) is found to be small when the potential change at the equivalence point is large. Invariably, in most of the reactions employed in potentiometric analysis, the titration error is normally quite small and hence may be neglected. 

Sodium diphosphate is prepared by the following procedure. Disodium hydrogen phosphate dihydrate, Na2HP04 2H20, (7.12 g, 40 mmole) is placed in a 60-mL platinum crucible and heated in air at 400° for 12 hours (Product Merck for analysis K 6580). According to Porthault,2 this method leads to a tetrasodium diphosphate, Na4P207, the purity of which is better than 99.5%. No impurities can be found by chromatographic or potentiometric analysis. 

The extremely low solubility of lead phosphate in water (about 6 x 10 15m) again suggests potentiometric analysis. Selig57,59 determined micro amounts of phosphate by precipitation with lead perchlorate in aqueous medium. The sample was buffered at pH 8.25-8.75 and a lead-selective electrode was used to establish the end-point. The detection limit is about 10 pg of phosphorus. Anions which form insoluble lead salts, such as molybdate, tungstate or chromate, interfere with the procedure. Similar direct potentiometric titrations of phosphate by precipitation as insoluble salts of lanthanum(III), copper(II) or cadmium(II) are suggested, the corresponding ion-selective electrodes being used to detect the end-point. 

Bishop, E. Differential Electrolytic Potentiometry. I. Ultramicro Potentiometric Analysis. Mikrochim. Acta 1956, 619. 

The checker suggests a potentiometric analysis the sample is dissolved in excess 6 N nitric acid and acetone added to enhance the end point determination. The solution is then cooled to 5° and titrated potentiometrically with 0.1 N hydrochloric acid. Anal. Calcd. for [Ag (C6H6N) 2]C104 Ag, 29.52. Found 29.06 28.90. 

Conductivity, Electrical Conductometry and Conductometric Titrations. Electrical conductivity is thequality or ability of a substance to transmit electrical energy. If it deals with the conductivity of an electrolyte in solution, it is then called electrolytic conductivity. Conductometry deals with analyses by measuring electrolytic conductivity, based on the fact that ionic substances in many solvents conduct electricity. Conductometric titrations are quantative analysis based on the fact that with the addn of the titrating agent to a soln being titrated, the specific conductivity (reciprocal of specific resistance in mhos) changes at a different rate before and after the end point (Comp with Potentiometric Analysis) Refs 1 )Kirk Othmer 4 (L 949), 325-33 (Conductometry) 2)W.G.Berl, Edit, "Physical Methods... 

The larger part of this book has been devoted to studies related to electrochemical measurements away from equilibrium. As demonstrated, these permit the determination of kinetic and thermodynamic parameters of the electrode processes, whereas measurements at equilibrium furnish only thermodynamic data. So, whilst potentiometric analysis is a powerful tool in the determination of activities or concentrations, the specificity arising from the electrode material, amperometric or voltammetric analysis permits other parameters besides these to be obtined. 

Apparatus. The following apparatus was utilized for this study a PRT 2000 type potentiostat (Tacussel) for the electrolysis a Graphi-spectral spectrophotometer (Jouan) for the absorption measurements a TS 6 type millivoltmeter (Tacussel) for potentiometric analysis a 2ir windowless flow gas counter (S.A.I.P.) for measuring the soft beta from and a bell type Geiger counter for Ge and Ge beta counting. 

E87 Paquet, J. (1982). Principles of multilayer film analysis. Potentiometric analysis sli<. In Kaiser, E., GabI, F., Muller, M.M. and Bayer, P.M. (Eds.), Proc. Xlth Int. Congr. Clin. Chem. Vienna (1981). Walter de Gruyter, Berlin, New York, 1167-1171. 

EXPERIMENT 2.5 ANALYSIS OF COBALT COORDINATION COMPOUNDS III POTENTIOMETRIC ANALYSIS OF IONIC HALIDE (CHLORIDE/BROMIDE)1-5... 

Result Summary for Potentiometric Analysis of Ionic Halide Calculation of percentage halide ... 

In Feature 18-3, we showed that absolute values for individual half-cell potentials cannot be determined in the laboratory. That is, only relative cell potentials can be measured experimentally. Figure 21-1 shows a typical cell for potentiometric analysis. This cell can be represented as... 

Awad, M. 1. and Ohsaka, T., Potentiometric analysis of peroxyacetic acid in the presence of a large excess of hydrogen peroxide, J. Electroanal. Chem., 544, 35-40, 2003. 

Noble, M.A., A.W. Munro, S.L. Rivers, L. Robledo, S.N. Daff, L.J. Yellowlees et al. (1999). Potentiometric analysis of the flavin cofactors of neuronal nitric oxide synthase. Biochemistry 38, 16413-16418. 

A complete potentiometric analysis of water gas (cf. Section 25.2.4) is very complicated [11]. In some cases, eg, during the firing of porcelain, in which iron (existing in the ceramic... 

It is safe to predict that most of the emphasis on analytical research on methods foy trace concentrations of anions in water during the next several years will continue to be on spectrophotometric chemical methods, and that interest in ion-selective electrodes for potentiometric analysis probably will continue to grow. It remains to be seen whether the scope of the latter method is limited. If these electrodes prove to be as simple and effective as reported, they may revolutionize the monitoring of water supplies and replace many chemical methods in situations where large numbers of repetitive analyses must be made. 

Zheng and coworkers have employed a similar methodology and have demonstrated that PVC membranes doped with indium tetraphenylporphyrin show high relative sensitivity to nitrite ions.- Manganese and indium porphyrins immobilized in silicone by Paeng et al. have been used for the potentiometric analysis of serum chloride levels." ... 

Sensors based on CoTM-3,4pyPz or CoTAPc coated HOPG electrode and further coated with Nafion ion exchange membranes or FePc modified carbon paste electrode " were employed for potentiometric analysis of sulfide ion and 2-mercaptoethanol and 2-pyridinethiol. The sensors showed a faster response time than commercial potentiometric sulfide indicators . The overpotential of the analytes were lowered by more than 400 mV in the case of FePc modified carbon paste electrodes ". ... 

The need to measure fluorinated and perfluorinated molecules arose already in the 1960s, when Taves discovered two forms of fluorine in human serum by ashing and subsequent potentiometric analysis with a fluoride-selective electrode [41]. During that time, however, no powerful tool that allows for sensitive and selective detection of these compounds was available. GC-MS, which was already available at that time, did not meet these criteria, mostly due to the ionic structure of the majority of PFC, which disallows volatilization needed for GC-MS. 

No special skill or experience of the laboratory personals are needed for making the potentiometric analysis. 

The signal used in potentiometric analysis is generated at the boundary between the electrode and the sample solution. When no current passes through the cell then, the charge separation process is in dynamic equilibrium. For equilibrium, we can write... 

In potentiometric analysis, the indicator and the reference electrodes are in the measurement cell and the AE cell voltage is measured between them. In most of the cases, the reference electrode with a constant potential is selected. The potential of the indicating electrode depends on the concentration of the analyte. In a favorable case, the applied... 

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