Standard potentiometry

Resistivity measurements are done on meander shaped samples by the standard potentiometrie method in a stirred bath of liquid nitrogen relative to a dummy speeimen For this proeedure of residual re istometry the ultra-high measuring aeeuraey of 3.10 results below 300°C and of 3.10 for annealing treatments at higher temperatures. 

Definite Method. No definitive method exists for ionized calcium. To date there is no international consensus agreement on a reference method and no certified reference materials available for standardization. Potentiometry with ion-selective electrodes has been selected as the method to be used. The International Federation of Clinical Chemists (IFCC) Working Group on Selective Electrodes is currently preparing a draft document on the reference method and testing a prototype reference cell [20]. 

Wang and Taha described an interesting application of potentiometry called batch injection. As shown in the following figure, an ion-selective electrode is placed in an inverted position in a large-volume tank, and a fixed volume of a sample or standard solution is injected toward the electrode s surface using a micropipet. 

Other detection methods are based on optical transmittance [228-231], Alcohol sulfates have been determined by spectrophotometric titration with barium chloride in aqueous acetone at pH 3 and an indicator [232] or by titration with Septonex (carbethoxypentadecyltrimethylammonium bromide) and neutral red as indicator at pH 8.2-8.4 and 540 nm [233]. In a modified two-phase back-titration method, the anionic surfactant solution is treated with hyamine solution, methylene blue, and chloroform and then titrated with standard sodium dodecyl sulfate. The chloroform passing through a porous PTFE membrane is circulated through a spectrometer and the surfactant is analyzed by determining the absorbance at 655 nm [234]. The use of a stirred titration vessel combined with spectrophotometric measurement has also been suggested [235]. Alternative endpoint detections are based on physical methods, such as stalag-mometry [236] and nonfaradaic potentiometry [237]. 

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. 

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. 

Potentiometry is used in the determination of various physicochemical quantities and for quantitative analysis based on measurements of the EMF of galvanic cells. By means of the potentiometric method it is possible to determine activity coefficients, pH values, dissociation constants and solubility products, the standard affinities of chemical reactions, in simple cases transport numbers, etc. In analytical chemistry, potentiometry is used for titrations or for direct determination of ion activities. 

Potentiometry is the most widely used electroanalytical technique. It involves the measurement of the potential of a galvanic cell, usually under conditions of zero current, for which purpose potentiometers are used. Measurements may be direct whereby the response of samples and standards are compared, or the change in cell potential during a titration can be monitored. 

In addition, sodium valproate can be potentiometri-cally titrated with standardized 0.1 N perchloric acid using a modified glass-calomel electrode system, in which 0.1 N lithium perchlorate in acetic acid has been substituted for potassium chloride, and employing glacial acetic acid as the sample solvent. 

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. 

Some of the typical parameters or properties utilized for NIR detection are potentiometry,(5) absorbance,(52 54) refractometry/18,19) or fluorescence spectros-copy.(55) Of these, has proven to be the most valuable detection method in fiber optic applications/2,56) In standard spectroscopic techniques, the detection limits of a method are greatly determined by the instrument and by the chemical method used for the analysis. However, in OFCD research the detection limits are governed by a series of other variables including the dye, the matrix, and the instrument. By optimizing these variables, low detection limits can be obtained with this technique. 

In this present book, we will look at the analytical use of two fundamentally different types of electrochemical technique, namely potentiometry and amper-ometry. The distinctions between the two are outlined in some detail in Chapter 2. For now, we will anticipate and say that a potentiometric technique determines the potential of electrochemical cells - usually at zero current. The potential of the electrode of interest responds (with respect to a standard reference electrode) to changes in the concentration of the species under study. The most common potentiometric methods used by the analyst employ voltmeters, potentiometers or pH meters. Such measurements are generally relatively cheap to perform, but can be slow and tedious unless automated. 

In potentiometry with ISEs, however, the second Gran method [46] has found especially wide use, not only in titrations, but also in multiple addition methods in general. In these methods, the concentration of the test substance is plotted against the volume of the titrant or of the standard solution and thus the curve is linearized. The end-point in the titration or the determinand concentration in a multiple addition method is found as the intercept of the straight line with the volume axis. Linearization is attained by taking the antilogarithm of the Nernst equation ... 

The characteristics of redox reactions in non-aqueous solutions were discussed in Chapter 4. Potentiometry is a powerful tool for studying redox reactions, although polarography and voltammetry are more popular. The indicator electrode is a platinum wire or other inert electrode. We can accurately determine the standard potential of a redox couple by measuring the electrode potential in the solution containing both the reduced and the oxidized forms of known concentrations. Poten-tiometric redox titrations are also useful to elucidate redox reaction mechanisms and to obtain standard redox potentials. In some solvents, the measurable potential range is much wider than in aqueous solutions and various redox reactions that are impossible in aqueous solutions are possible. 

With very few exceptions, quantitative epoxide assay techniques currently in use are derived from the reeotion of ethylene oxides with halogen adds, notably hydrochloric acid and hydrobromio add, in a variety of solvents. Acid uptake may be determined by any of several reliable procedures. These include titration with standard base8 nr back-titration with standard acid.744 The end-point may be detected visually in the presence of suitable acid-base indicators, or by the more precise technique of potontionaetry.447.4 -470 A useful alternative, applicable in the presence of easily hydrolysed substances or of amines that buffer the end-point, is the technique of argentiometry. In this procedure excess of halide ion is titrated with silver nitrate in tV presence of ferric thiocyanate indicator,470 1884 or potentiometri-cally.188 ... 

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. 

In addition to its obvious routine importance, UV spectrophotometry has been used as an assay method the berbamine content of crude ethanolic extracts of 22 Berberis species was determined, with standard deviation 3.83%, by measuring the absorption at 282 nm (171). Thalicarpine was similarly assayed at 280 nm and also by TLC densitometry and titrimetry or potentiometry (308). Tetrandrine was determined in drug preparations by UV spectrophotometry of a dilute HC1 solution at 280 nm (standard deviation 0.40%) (309). 

Complexation constants for binding of mono- and di-negative oxoanions by the cryptands can be determined by pH-potentiometry. For the more weakly-binding mononegative systems, NMR shifts can provide complementary information. For these studies we use the large size-excluded tosylate anion as the supporting electrolyte and all log K values are thus relative to this notional standard [19],... 

Lead tetraacetate consumption is measured conveniently by iodometry.4 The reaction mixture is added to excess potassium iodide solution, usually in the presence of sodium acetate,6 and the iodine liberated is then titrated with standard thiosulfate. Oxidation may also be measured potentiometri-cally,78 210 211 a procedure especially useful for fast glycol groups,78 or with redox indicators.211... 

Laboratory investigations play an essential role in medicine. Laboratory results are taken into consideration in about two thirds of all medical decisions in medical systems of industrialized countries today. The vast majority of clinical chemistry analyses are based on few analytical principles including photometry, ligand binding assays and potentiometry. For these standard methods complete automation has been achieved and multi-channel, random access analyzers realize several hundred analyses per instrument and hour on a very high level of user-friendliness. Consequently, clinical chemistry is very cost efficient today typically clinical chemistry analyses contribute less than 5 % of all costs of tertiary care hospitals. 

Potentiometry has found extensive application over the past half-century as a means to evaluate various thermodynamic parameters. Although this is not the major application of the technique today, it still provides one of the most convenient and reliable approaches to the evaluation of thermodynamic quantities. In particular, the activity coefficients of electroactive species can be evaluated directly through the use of the Nemst equation (for species that give a reversible electrochemical response). Thus, if an electrochemical system is used without a junction potential and with a reference electrode that has a well-established potential, then potentiometric measurement of the constituent species at a known concentration provides a direct measure of its activity. This provides a direct means for evaluation of the activity coefficient (assuming that the standard potential is known accurately for the constituent half-reaction). If the standard half-reaction potential is not available, it must be evaluated under conditions where the activity coefficient can be determined by the Debye-Hiickel equation. 

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