Potentiometric titrations direct method

The acid hydrolysis of the 2-aryl-1,3,4-oxadiazoles can be used for their analytical determination. The method used is either to break down the compound by heating with hydrochloric acid under reflux to give the acid hydrazide and then to titrate the hydrazide with iodide in bicarbonate solution,68, 69 or to titrate potentiometrically directly with sodium nitrite in a hydrochloric acid medium.128 In this way the acid hydrazide is formed in the first reaction step and is then converted into the insoluble azide by the sodium nitrite. 

Measurement of Reaction Rates by Titrimetry. The rates of cerium(IV) consumption by each of Cr(C204)3 3, m-Cr(OH2)2(C204)2 and Cr(0H2)4C204+ were also measured by a direct titrimetric method. Solutions were prepared and mixed as for the spectrophotometric procedure. At appropriate times aliquots of the reactant solutions were quenched with known volumes of standard ferrous sulfate, and the excess ferrous ion was titrated potentiometrically with standard potassium dichromate,... 

An apparatus designed by J. H. Hildebrand,1 for the potentiometric titration of acids and bases, illustrates the principles involved in the direct method. The apparatus is shown in Fig. 1. The solution to... 

All the routines described for the determination of the thermodynamic (concentration) parameters in metal oxide solutions include some indirectly obtained values. For example, the equilibrium concentration of metal cations is calculated proceeding from the quantity of the oxide-ion donor consumed for titration (precipitation). Direct determination of the concentration of metal cations in the melt (if it is possible) allows one to obtain more correctly the obtained solubility product values. Our paper [332] reports a method for correction of the solubility product values for oxides on the basis of the potentiometric titration data. The modification of the standard routine consists of the simultaneous use of two indicator electrodes, one of which is the membrane oxygen electrode and the other is a metal electrode, reversible to the cations the oxide consists of. This routine was used to estimate the solubility products of copper(I) and nickel(II) oxides in the molten KCl-NaCl equimolar mixture at 700 °C. Investigation of Cu20 by the proposed method is of considerable importance since, as will be shown further, the process of dissociation/dissolution of copper(I) oxide in molten alkali-metal halides differs from the generally accepted one which was considered, e.g. in Ref. [119]. 

Michaelis et al. 16, 17) have estimated Ei and > for several molecules by analyzing the potentiometric titration curve. This method can be applied only to redox systems which give a large equilibrium concentration of semiquinone, usually for systems for which > 0.01. Direct estimation of semiquinone concentration by ESR (electron spin resonance) is about 10 times more sensitive for the Kg estimation than analysis of titration curves. Some values are shown in Table I. The Kg for NADH (reduced form of nicotineamide adenine dinucleotide) is too small to be measured even by ESR, and only an approximate upper limit is given. 

The analytical techniques used for single element analysis can be divided into three groups. The first group consists of titration techniques using various means to detect the end point of titration, such as volumetric titration, fluorophotometric titration, potentiometric titration, and spectrophotometric titration. The second group includes direct detection techniques such as direct ternary inclusion compound fluorescent spectrophotometry and the use of ISEs. The third group is that of separation methods such as IC and HPLC, which are used in complicated sample matrices to reduce the sample matrix interference. 

Titrimetric methods with potentiometric end point location can be applied when an electrode with the needed selectivity is not available. The precision and accuracy of potentiometric titrations are superior comparing it with the properties of direct potentiometry. However, the concentration range where potentiometric titration can be used effectively is narrower. A solution with analyte concentration below 1 mM seldom is determined by potentiometric titrations. Potentiometric end point location is most often employed in the case of acid-, base-, precipitate-, redox-, or complexometric titrations. 

Potentiometry also is a direct means to evaluate the standard potential for half reactions (E°) and has been applied for appropriate reversible systems. Such measurements require corrections for activity coefficients or extrapolation of the data to infinite dilution. Again, direct measurements in which equal molar concentrations of the oxidized or reduced species are introduced into the system provide a simple approach to such evaluations and are as precise as those obtained by less direct methods. However, E° values also can be extracted from potentiometric titration data. For example, in the titration of Fe (OH2)g+ ion with Ce (OH2) + ion, theFei (OH2)6+ ion concentration equals the Fe (OH2)5" concentration... 

The measurement of pK for bases as weak as thiazoles can be undertaken in two ways by potentiometric titration and by absorption spectrophotometry. In the cases of thiazoles, the second method has been used (140, 148-150). A certain number of anomalies in the results obtained by potentiometry in aqueous medium using Henderson s classical equation directly have led to the development of an indirect method of treatment of the experimental results, while keeping the Henderson equation (144). 

Although not commonly used, thermometric titrations have one distinct advantage over methods based on the direct or indirect monitoring of plT. As discussed earlier, visual indicators and potentiometric titration curves are limited by the magnitude of the relevant equilibrium constants. For example, the titration of boric acid, ITaBOa, for which is 5.8 X 10 °, yields a poorly defined equivalence point (Figure 9.15a). The enthalpy of neutralization for boric acid with NaOlT, however, is only 23% less than that for a strong acid (-42.7 kj/mol... 

The free maleic acid content in maleic anhydride is determined by direct potentiometric titration (166). The procedure involves the use of a tertiary amine, A/-ethylpipetidine [766-09-6J, as a titrant. A tertiary amine is chosen as a titrant since it is nonreactive with anhydrides (166,167). The titration is conducted in an anhydrous solvent system. Only one of the carboxyhc acid groups is titrated by this procedure. The second hydrogen s dissociation constant is too weak to titrate (166). This test method is not only used to determine the latent acid content in refined maleic acid, but also as a measure of the sample exposure to moisture during shipping. 

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. 

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. 

Applications Potentiometry finds widespread use for direct and selective measurement of analyte concentrations, mainly in routine analyses, and for endpoint determinations of titrations. Direct potentiometric measurements provide a rapid and convenient method for determining the activity of a variety of cations and anions. The most frequently determined ion in water is the hydrogen ion (pH measurement). Ion chromatography combined with potentiometric detection techniques using ISEs allows the selective quantification of selected analytes, even in complex matrices. The sensitivity of the electrodes allows sub-ppm concentrations to be measured. 

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. 

Complex stability constants are often determined by pH-potentiometric titration of the ligand in the presence and absence of the metal ion (129). This method works well when equilibrium is reached rapidly (within few minutes), which is usually the case for linear ligands. For macrocyclic compounds, such as DOTA and its derivatives, complex formation is slow, especially at pH-s where the formation is not yet complete, therefore a batch method is used instead of direct titration (130,131). A few representative examples of stability constant data mainly collected from Ref. (132), on MRI relevant Gdm complexes are presented in Table IV. 

Chloride can also be estimated by potentiometric titration using standard silver nitrate [27]. The results are recorded directly and evaluated by means of a computer program based on the Gran extrapolation method. The determinations have a precision of 0.02% and since many samples can be titrated simultaneously, the time for a single determination can be reduced to less than 5 min. 

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. 

The first electrochemical studies of Mb were reported for the horse heart protein in 1942 (94) and subsequently for sperm whale Mb (e.g., 95) through use of potentiometric titrations employing a mediator to achieve efficient equilibriation of the protein with the electrode (96). More recently, spectroelectrochemical measurements have also been employed (97, 98). The alternative methods of direct electrochemistry (99-102) that are used widely for other heme proteins (e.g., cytochrome c, cytochrome bs) have not been as readily applied to the study of myoglobin because coupling the oxidation-reduction eqiulibrium of this protein to a modified working electrode surface has been more difficult to achieve. As a result, most published electrochemical studies of wild-type and variant myoglobins have involved measurements at eqiulibrium rather than dynamic techniques. 

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... 

Potentiometric titration is a commonly used technique determination (Albert and Serjeant, 1984). The PCA101 chemical analyzer, developed and manufactured by Sirius Analytical Instruments Ltd., is the rst commercial instrument designed specilly to determine ionization constants (Avdeef, 1993). The theory and applications of the method have been discussed byAvdeef (Avdeef, 1993), and the reader is directed to that excellent review for a detailed discussion. 

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