Potentiometric techniques

Ion Selective Electrodes Technique. Ion selective (ISE) methods, based on a direct potentiometric technique (7) (see Electroanalytical techniques), are routinely used in clinical chemistry to measure pH, sodium, potassium, carbon dioxide, calcium, lithium, and chloride levels in biological fluids. 

The remarkable selectivity that is inherent in the reaction of an antibody with the antigen or hapten against which it was raised is the basis for the extensive use of immunoassay for the rapid analysis of samples in clinical chemistry. Immunochemical reactions offer a means by which the applicability of potentiometric techniques can be broadened. A number of strategies for incorporating immunoassay into the methodology of potentiometry have been explored... 

The limitations of simple conductivity measurement can be overcome using a combined impedimetric and potentiometric technique [75]. 

The potentiometric technique has proved to be of great significance and utility for determining endpoints of titrations in a non-aqueous media. The mV scale rather than the pH scale of the potentiometer must be used for obvious reasons, namely ... 

Diagnostic Criteria to Identify an Irreversible Process. In order to characterize an irreversible process it would be necessary to be able to calculate either the thermodynamic parameter E° or the kinetic parameters a. and k°. Unfortunately, we will see below that k° can only be calculated if E0/ is known, and E0 cannot be calculated by voltammetric techniques. Thus, either one knows Eel (for example, by using potentiometric techniques in solutions containing both Ox and Red), or one is limited to give simply the peak potential of the electrode process at a certain rate (usually at 0.1 V s-1 or at 0.2 V s-1). 

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. 

Kono et al. [92] made use of this polyelectrolyte complexation between PAA and PEI in the formation of pH sensitive capsules that could release the contents contained in their aqueous centers as a function of pH. In this work, PAA-PEI complex capsule membranes were produced which contained 43.7 % PAA and 56.3% PEI (confirmed by elementary analysis). These capsules showed a minimum degree of swelling between a pH of 3.5 and 7.0 due to complex formation. Consequently, the permeability of phenylethylene glycol through the capsule membrane was lowest between pH 3.5 and 7.0 and increased above 7.0 and below 3.5. These investigators also used turbidimetric and potentiometric techniques to confirm the presence of these polyelectrolyte complexes. 

A potentiometric technique was used in Ref 50 to measure composition, found to be Cui.gsS. In this deposition, stirring the deposition solution resulted in nonuniform and poor-quality films, while good films were obtained in unstirred films. The bandgap was measured to be direct, with a value of 2.58 eV. This is a particularly high value. Examination of the transmittance spectrum showed a sharp drop in transmission at ca. 1.7 eV, which is more likely to be the true bandgap. 

Measurement of pH is a potentiometric technique frequently used for measuring the degree of the deterioration of materials that are subjected to natural aging. The determination of pH levels is commonly carried out on ethnographic objects manufactured with parchment or leather, and it is especially relevant in altered paper due to the formation of acidic compounds from the decomposition of the woodpulps and other raw materials, which can induce the hydrolysis of the cellulose and then decrease the resistance and mechanical properties of the document [29]. 

Potentiometric techniques have been used to study autonomous reaction rate oscillations over catalysts and carbon monoxide oxidation on platinum has received a considerable amount of attention43,48,58 Possible explanations for reaction rate oscillations over platinum for carbon monoxide oxidation include, (i) strong dependence of activation energy or heat of adsorption on coverage, (ii) surface temperature oscillations, (iii) shift between multiple steady states due to adsorption or desorption of inert species, (iv) periodic oxidation or reduction of the surface. The work of Sales, Turner and Maple has indicated that the most... 

Propene to acrolein. Hildenbrand and Lintz87,88 have used solid electrolyte potentiometry to study the effect of the phase composition of a copper oxide catalyst on the selectivity and yield of acrolein during the partial oxidation of propene in the temperature range of 420-510°C. Potentiometric techniques were used to determine the catalyst oxygen activity, and hence the stable copper phase, under working conditions. Hildenbrand and Lintz used kinetic measurements to confirm that the thermodynamically stable phase had been formed (it is known that propene is totally oxidised over CuO but partially oxidised over ). 

A great number of anionic complexes containing four, five or six halide ions have been characterized. However, in aqueous solutions containing V111 ions and F-, the potentiometric technique using a fluoride selective electrode has shown no indication of any complex other than VF2+ (stability constant log / = 5.00 0.03 at 25 °C in 1.0 M NaC104 solution).295... 

The formation of 1 1 complexes between ethylenediamine-N, N1 -diacetic acid (edda) and Zn11 or Cd11 have been studied by calorimetric (AH) and potentiometric techniques.389 Earlier studies had omitted to allow for protonation reactions of the ethanoato groups in the ligand. Thermodynamic parameters for edda lie between those of nitrilotriacetate and triethylenetetra-mine. Edda appears to undergo a slow metal-ion-catalyzed hydrolysis in aqueous acid solution. 

The ninhydrin reaction (see Basic Protocol 1), the TNBS reaction (see Alternate Protocol 1), the fluorescamine reaction (see Alternate Protocol 2), and formol titration (see Alternate Protocol 3) all evaluate released amino groups by comparing the amounts of free amino groups before and after hydrolysis. The first three methods are spectro-photometric techniques, whereas the fourth is a potentiometric technique. The first and second are chromogenic techniques, whereas the third is fluorometric. These techniques are usually performed as time-course experiments. As the hydrolysis reaction proceeds, aliquots (samples) of the reaction are taken periodically and treated with a test reagent. Products of this reaction are proportional to the amount of free amino groups at each time point. 

EPMEs based on antibiotics are proposed for the enantioanalysis of L-vesamicol [51]. A carbon paste was modified with antibiotics (vancomycin, teicoplanin and teicoplanin modified with acetonitrile), as chiral selectors. The EPMEs based on antibiotics were reliably used for enantiopurity tests of L-vesamicol using the direct potentiometric technique. The following linear concentration ranges 1.0 x 10-6 - 1.0 x 10 4,... 

It is seen that the amperometric technique is more used for heavy-metal determination in comparison to potentiometric techniques. The research published during the last 5 years concerning heavy-metal determination has shown the applications especially for water samples (Table 14.1). 

It is based on the addition of Mn2+ solution, followed by die addition of a strong alkali to die sample in a glass-stoppered bottle. Dissolved 02 rapidly oxidizes an equivalent amount of the dispersed divalent manganous hydroxide precipitate to hydroxides of higher valence states. In die presence of iodide ions in an acidic solution, the oxidized manganese reverts to die divalent state, with die liberation of a quantity of iodine equivalent to die original dissolved 02 content. The iodine is then titrated with a standard solution of thiosulfate. The titration end point can be detected visually with a starch indicator, or by potentiometric techniques. The liberated iodine can be determined colorimetrically. 

MethodA The potentiometric technique ASTM D-2986 (Annual Book, 1985), 0.1 HC104 in glacial acetic acid was used as the titrant and chlorobenzene-glacial acetic acid (2 l,v/v) was used as the titration medium. Curve of two zones. 

Most polymers belonging to this class are water-soluble only in their protonated form. As a consequence, most potentiometric titrations of these polymers have been performed with OH starting from their hydrochlorides, either in water or in mixed solvents, such as water/dioxane, water/sulfolane, water/acetonitrile 64). Many poly[thio-l-(N,N-dialkyl-aminomethyl)ethylene]s are optically active, and in these cases ORD and CD techniques may be used to study their protonation behaviour, in addition to potentiometric techniques. 

A considerable amount of data on the protonation, and complex formation with metal ions of polymeric amines have been reported. A critical insight leads to the conclusion that much has to be done in order to reach a clear vision of the chemical properties of many polymers of this kind. Most protonation studies deal with the determination of basicity constants with potentiometric techniques, which alone give little information on the protonation mechanism only few studies have been substantiated by spectroscopic (nmr) and calorimetric measurements. 

D-4782 Paladium in Molecular Sieve Catalyst Potentiometric technique for determining about 0.5 weight percent palladium in molecular sieve containing fresh catalyst. 

As described earlier, excipients can contact the electrodes of the system. Therefore, excipients must be screened for their compatibility with the electrodes. Sacriflcial electrodes (e.g., Ag and AgCl) are particularly reactive. If inherently non-reactive electrodes are used (e.g., platinum or carbon), then the excipient can be exposed to a relatively large electric potential at the electrode/reservoir interface during system use. In such cases, excipients that are inherently stable should be selected. Excipients are typically evaluated for their electrochemical stability using standard potentiometric techniques (e.g., cyclic voltammetry) before being selected for use in an IDDS formulation. 

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