Potentiometric titrations compared

The titration of a mixture ofp-nitrophenol (pfQ = 7.0) and m-nitrophenol pK = 8.3) can be followed spectrophotometrically. Neither acid absorbs at a wavelength of 545 nm, but their respective conjugate bases do absorb at this wavelength. The m-nitrophenolate ion has a greater absorbance than an equimolar solution of the p-nitrophenolate ion. Sketch the spectrophotometric titration curve for a 50.00-mL mixture consisting of 0.0500 M p-nitrophenol and 0.0500 M m-nitrophenol with 0.100 M NaOH, and compare the curve with the expected potentiometric titration curves. 

In systems such as the 2- and 6-hydroxypteridine series, rapid potentiometric or spectrophotometric pA determinations on neutral solutions usually give values near to the acidic pA of the hydrated series. (Exceptions include 2-hydroxy-4,6,7-trimethyl-, 6-hydroxy-7-methyl-, and 4,6-dihydroxy-pteridine, where the neutral solution contains comparable amounts of hydrated and anhydrous species. In such cases, rapid potentiometric titrations show two well-defined and separated curves, one for the hydrated, the other for the anhydrous, species.) Similarly, from solutions of the anion, an approximate pA value for the anhydrous species is obtained. For convenience, the anhydrous molecule is referred to as HX, its anion as X , the hydrated neutral molecule as HY, and its anion as Y, and the two equilibrium constants are defined as follows ... 

The main electroanalytical techniques are electrogravimetry, potentiometry (including potentiometric titrations), conductometry, voltammetry/polarography, coulometry and electrochemical detection. Some electroanalytical techniques have become very widely accepted others, such as polarography/voltammetry, less so. Table 8.74 compares the main electroanalytical methods. 

Conductometric titrations offer several advantages compared with potentiometric titration methods, such as better precision and better differentiation of the basic components in polymers, but they are more laborious. ASTM D 4928-96 is an established KF method for the determination of water in crude oils. 

Again for the titration of Ce(IV) with Fe(II) we shall now consider constant-potential amperometry at one Pt indicator electrode and do so on the basis of the voltammetric curves in Fig. 3.71. One can make a choice from three potentials eu e2 and e3, where the curves are virtually horizontal. Fig. 3.74 shows the current changes concerned during titration at e1 there is no deflection at all as it concerns Fe(III) and Fe(II) only at e2 and e3 there is a deflection at A = 1 but only to an extent determined by the ratio of the it values of the Ce and Fe redox couples. The establishment of the deflection point is easiest at e2 as it simply agrees with the intersection with the zero-current abscissa as being the equivalence point in fact, no deflection is needed in order to determine this intersection point, but if there is a deflection, the amperometric method is not useful compared with the non-faradaic potentiometric titration unless the concentration of analyte is too low. 

Conductometric titrations. Van Meurs and Dahmen25-30,31 showed that these titrations are theoretically of great value in understanding the ionics in non-aqueous solutions (see pp. 250-251) in practice they are of limited application compared with the more selective potentiometric titrations, as a consequence of the low mobilities and the mutually less different equivalent conductivities of the ions in the media concerned. The latter statement is illustrated by Table 4.7108, giving the equivalent conductivities at infinite dilution at 25° C of the H ion and of the other ions (see also Table 2.2 for aqueous solutions). However, in practice conductometric titrations can still be useful, e.g., (i) when a Lewis acid-base titration does not foresee a well defined potential jump at an indicator electrode, or (ii) when precipitations on the indicator electrode hamper its potentiometric functioning. 

The ligand 6,13-dimethyl-l,4,8,ll-tetra-azacyclotetradecane-6,13-diamine coordinates as a hexadentate ligand to zinc in neutral aqueous solution. Potentiometric titrations were used to determine the stability constant for formation. The pXa values were determined for five of the six possible protonation steps of the hexamine (2.9, 5.5, 6.3, 9.9 and 11.0).697 Studies of the syn and anti isomers of 6,13-dimethyl-1,4,8, ll-tetraazacyclotetradecane-6,13-diamine reveal that they offer different shapes for metal binding, which is reflected in the stability constants for 1 1 zinc ligand ratio complexes. The selectivity of binding to the zinc ion compared to the cadmium(II) ion by both isomers is significant.698... 

Measurements of the electrical conductivity and solubility of difficultly soluble salts in sodium tetrametaphosphate solutions 62, 151, 196, 197) and potentiometric titrations lead to the assumption of the existence of complexes of the type of Na+(P40i2)3 Ba"1-1", Sr, Ca++, Mg++, Mn++, Ni++, Cu++(PiO]2) - La(P40i2) Cu(P40i2)26- and Ni(P40i2)26. No comparable dissociation constants for these have so far been given, though in any case they will be smaller than for the corresponding ion pairs of the trimetaphosphate anion 145). 

Potentiometry and potentiometric titrations are widely used in studying various types of reactions and equilibria in non-aqueous systems (Sections 6.3.1-6.3.4). They also provide a convenient method of solvent characterization (Section 6.3.5). Moreover, if the electrode potentials in different solvents can accurately be compared, potentiometry is a powerful method of studying ion solvation (Section 6.3.6). 

Table 5), and several are now being used, or are potentially useful, for measuring key ocean elements. The most common use of direct potentiometry (as compared with potentiometric titrations) is for measurement of pH (Culberson, 1981). Most other cation electrodes are subject to some degree of interference from other major ions. Electrodes for sodium, potassium, calcium, and magnesium have been used successfully. Copper, cadmium, and lead electrodes in seawater have been tested, with variable success. Anion-selective electrodes for chloride, bromide, fluoride, sulfate, sulfide, and silver ions have also been tested but have not yet found wide application. 

The redox chemistry of PQQ has been investigated by a number of research groups. Duine et al. [14,15] performed potentiometric titrations of PQQH2 at several pHs and measured the redox potential of PQQ/PQQH2. Eckert et al. [16,17] compared the redox properties of PQQ with those of o-phenanthroline quinones. Kano et al. [18] performed cyclic voltammetry at acidic pH. Bergethon [19] investigated the amperometric detection of PQQ as a tool for HPLC. From pulse radiolysis experiments, McWhirter and Klapper [20] derived a value of -122 mV (NHE) for Em PQQ/PQQH at pH 7, as compared to the value of - 218 mV calculated from mediator-linked potentiometric titrations [15],... 

NMR titrations (of anion into ligand at fixed pH) and pH-potentiometric titrations (of pH at fixed anion ligand ratios) provide comparable values of the stability constants for binding of mononegative oxoanions by protonated R3Bm, R3F, and R3P hosts [15,20,21] Table 2. The weak complexation at hexaprotonated levels for tetrahedral monoanionic oxoanions makes it difficult to obtain reliable data for protonation levels below 5. This has however been achieved for nitrate with the cleft binding host R3P as well as for Re O4 with the most basic cryptand R3Bm. 

After our discovery of the metal carbonyl hydrides, other authors (32) pointed out their acidic character in aqueous solution. Potentiometric titrations by Reppe and later by us, showed that in water HCo(CO)4 possesses an acidity (pWa l) comparable to that of nitric acid. The first ionization stage for H2Fe(CO)4 corresponds approximately to that of acetic acid (33), whereas the pentacarbonyl hydrides HM(CO)5 (M = Mn or Re) (VII, 11, 26) are hardly acidic at all. The redox potentials of the cobalt and iron carbonyl hydrides were also measured (33). 

Tab. 4.1 Partitioning of ionizable drugs (see pKa) into small unilamellar vesicles (SUVs) of DOPC and into octanol for uncharged and charged species, determined by the potentiometric titration technique, compared with their intestinal absorption (%) at the doses indicated. (Adapted from Tab. 2 of ref. 16)... 

Table 1 tabulates literature values for acidity constants of seven amine-Ptn complexes with notations on the temperature, ionic strength, total Ptn concentration, method employed, conditions and other remarks, and the reference number. At least six factors enter into comparing determinations of a single complex. First is the purity of the complex under investigation. Because they rely on chemical shifts of an individual species, NMR methods are less dependent on purity than potentiometric titrations, which are interpreted on the basis of equivalents of added base. Rarely is the raw titration data published, but in one case it is evident from a plot of the data that the titration curve reveals up to about 10% impurity [7], Without knowing whether the impurities are acidic, basic, inert, or even forming during... 

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