Potentiometric titrations constant solvents

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. 

Thus, the dissociation equilibrium is affected by the ionic strength, temperature and dielectric constant of the solvent as well as by the parameter h (involved in AGf,). On the other hand, the term dG /dn does not depend on the degree of polymerization (except for very small values of n). The degree of polymerization does not affect, for example, the course of the potentiometric titration of a poly acid. 

The single-electron reduction and oxidation of Co(salen) is solvent dependent as a result of the available coordination sites perpendicular to the CoN202 plane.1220 Furthermore, substituents on the phenyl rings modulate the observed redox potentials and subsequently the 02 binding constants. Hammett correlations are obtained.1221 Potentiometric titrations were performed to determine the 02 binding constants and species distribution as a function of pH for a variety of Schiff base Co complexes.1222... 

To conduct meaningful mechanistic and kinetic studies in alcohol media reliable and simple measurement and control of the solution jjpH is essential. Potentiometric titration is the method of choice for obtaining acid dissociation constants or metal ion complex stability constants and in favorable cases the speciation of mixtures of metal-ion-containing complexes in solution can be proposed.20 Titrations in non-aqueous solvents are not nearly as widely reported as those in aqueous media, particularly in cases with metal ions21 and determination of pH in a non-aqueous solvent referenced to that solvent is complicated due to the lack of a way to relate the electrode EMF readings to absolute jjpH (see footnote and ref. 6) so non-aqueous solvents are generally inconvenient to use22 for detailed studies of reaction mechanisms where pH control is required. 

The basicity of the amide bond, when the substituents are changed, remains almost constant or increases slightly with increasing size of the substituents. These results were obtained from infrared studies and potentiometric titrations of N,N-disubstituted amides in several organic solvents (I) and corroborated by the magnitude of the experimentally obtained electrical moments (12). 

Cox et al. and Manahan et al. performed potentiometric titrations and calculated the equilibrium constants for the complexes formed between Ag or Cu and some ligand molecules (solvent B). Several solvents (A) were used as reaction medium. 

Fig. 2.14. (A) Potentiometric titration curves of a polymer imprinted with L-PA (PLPA), a polymer imprinted with benzylamine (PBA), a blank non-imprinted polymer (PBL) and acetic acid in MeCN/O.lM NaCl 70/30 (v/v). The NaOH equivalents (x-axis) are calculated based on the theoretical amount of carboxylic acid groups present in the polymer. In (B) is seen the calculated distribution as a function of the degree of ionization (a ). The polymer swelling (ml/ml) in this solvent system was constant in the pH interval 3-12 and was for PLPA 1.32 and for PBL 1.26. From Sellergren and Shea [67].

Computerized method provides convenient and accurate determination of the ionization constant in aqueous solution and of the apparent ionization constant in the presence of octanol. From these parameters, partition coefficients and apparent partition coefficients are easily calculated and agree with data reported using the shaker technique or HPLC. The curve-fitting method has been applied to the differential titration technique in which the solvent curve is subtracted from the solution curve before calculations are begun. This method has been applied to the potentiometric titration of aqueous solutions of the salts of bases with very low solubility in water. 

The relative insolubility of chloroBiiazide in most common solvents has made die determination of ionization constants difficult. Several groups have used aqueous potentiometric titration to obtain pJC data, with the general consensus being that pKgi = 6.85 and pXa2 = 9.45."... 

A potentiometric titration meOiod is proposed in which only one titration is necessary to obtain dissociation constants for different solvent compositions. The mettiod allows the results to be... 

Valenti determined the ionization constants. .. of nystatin in a. .. solvent system composed of methanol, 2-medioxyethanol and water by potentiometric titration and established the following apparent pJCa values. .. pJCi = 5.72 p 2 = 8.64.. .. The isoelectric point for nystatin in diis system, calculated from die average of pKi and pJC2/ was found to be at pH 7.18.. .. There is, as yet, no experimental evidence to establish whether nystatin exists at the isoelectric point as a zwitterion or as an unionized molecule. Resolution of diis question requires the examination of singly charged derivation of the antibiotic, such as an ester and / or suitable salt. 

In most nonaqueous solvents, precise explanation or prediction of the shape of a potentiometric titration curve, or the possible utility of a specific titration, is usually not possible because of the lack of complete thermodynamic equilibrium constants for the numerous possible processes. In general, the shapes of curves must be determined experimentally, and the behavior of substances in diverse solvents must be considered empirically. 

NH3 H2O-acetone (30-70) trans-[PtH(NH2R)- (PPh3)2] (R = H or Me) potentiometric titration with acid Measured instability constants for removal of NH2R by solvent. The presence of hydride decreases strength of trans-Pt NH2R bond a... 

The precise interpretation of potentiometric titration curves for base titrations in acetonitrile and the calculation of the pH changes near the equivalence point require only knowledge of the dissociation constants, J hb+5 of protonated bases to be titrated, because in solvents of relatively high relative permittivity (such as acetonitrile) perchloric acid and perchlorate salts can be considered to be completely dissociated. The well-known expression commonly used is... 

Aqueous titrations of amines are amply discussed elsewhere d. Nonaqueous titrations. Three main purposes may be served by carrying out titrations in nonaqueous solvents increased solubility, change of the pH scale, and resolution of mixtures. The prediction of a potentiometric titration curve in an arbitrary solvent is a difficult task, in which many factors intervene, such as dielectric constant, definition of acid and base in relation to the solvent, electrodes, actual structure of conjugate acids and bases, etc. Acetic acid, sulphuric acid, acetonitrile, and alcohol-water mixtures have been extensively studied and were reviewed elsewhere Some solvents will be treated here briefly ... 

In Tables 8.8 and 8.9, the following abbreviations are used Spec, spectrophotometry Sol, solubility SX, solvent extraction IX, ion exchange Relax, relaxation EM, electromigration PT, potentiometric titration and PEP, paper electrophoresis. The constants kj and are defined for the reaction of a cation M with a ligand L as follows ... 

Considering increasing use of non-aqueous or water-organic solvent media in analytical chemistry (for example in potentiometric titrations and in the high-performance liquid chromatography), dissociation constants of citric acid in few such systems were also determined. Evidently, they are less abundant than those in water. These dissociation constants are actually the concentration quotients, because activity coefficients and aetivities of water were always ignored. 

The energy of the N B interaction has been calculated from the stepwise formation constants of potentiometric titrations. Based on the relative stabilities of ternary phosphate complexes, it was calculated that the upper and lower limits of the N-B interaction must be bound between 15 and 25 kJ mol in A-methyl-o-(phenylboronic acid)-A-benzylamine. This value is in good agreement with computational data, which estimated the N-B interaction to be 13 kJ mol or less in the absence of solvent. To qualify this in terms of familiar... 

Potentiometric titrations in nunaqueous solvents can be divided into two types, the thermodynamic titration, where a plot of emf versns quantity of titrant added is used to measure equilibrium constants, and the analytical titration in which interest is focussed on the position of the quantitative end point. A representative sample of the thermodynamic titrations have been tabulated in (a)—(c) of this section. 

The protonation equilibria for nine hydroxamic acids in solutions have been studied pH-potentiometrically via a modified Irving and Rossotti technique. The dissociation constants (p/fa values) of hydroxamic acids and the thermodynamic functions (AG°, AH°, AS°, and 5) for the successive and overall protonation processes of hydroxamic acids have been derived at different temperatures in water and in three different mixtures of water and dioxane (the mole fractions of dioxane were 0.083, 0.174, and 0.33). Titrations were also carried out in water ionic strengths of (0.15, 0.20, and 0.25) mol dm NaNOg, and the resulting dissociation constants are reported. A detailed thermodynamic analysis of the effects of organic solvent (dioxane), temperature, and ionic strength on the protonation processes of hydroxamic acids is presented and discussed to determine the factors which control these processes. 

The determination of partition caefcients by the potentiometric method is part of the function of the Sirus PCA101 (Avdeef, 1993). Typically, a preacidiLed solution of a weak acid is alkalimetrically titrated to some appropriately high pH the partition solvent such as octanol is then added, and the dual-solvent mixture is acidimetrically titrated back to the starting pH. Analysis of the two titration curves will yield two fKgS pl< and pol, where pdQ, is the apparent constant derived from... 

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