Dissociation constant potentiometric determination

Generally, the results of the measurements indicated that, where dissociation constants were determined by conductometry and also potentiometric titration, they were in agreement with each other further, KHX is low, e.g., about 10 4-10 6moll 1 for aromatic sulphonic acids and 10 13-10 16 moll-1 for carboxylic acids, Xhx2 is high, e.g., 102-104, and KBis low again, e.g., 10 5-10 6 for aliphatic amines and 10 10 for aromatic amines. 

The acid dissociation constant of the free ligand, when the proton attached to the nitrogen dissociates, is approximately 10 14 B, and the effect of metal chelation is to increase this dissociation constant by a factor of 10B to 108. The values of Ka, and Ka the stepwise dissociation constants, were determined spectrophotometrically and potentiometrically, and the acid-strengthening effect on the NH group was found to depend markedly on the chelated metal ion (95). The order of increasing acidity is Mn(II) < Cd(II) < Zn(II) < Ni(II) < Fe(II). The values of pKa, and pKat for any one metal were found to be within 1.3. It therefore appears that these... 

Considering importance of solutions with citrate ions, their dissociation constants were determined many times (Tables 3.1, 3.2 and 3.3). However, there is a number of dissociation constants which can be considered as by-products because they are coming indirectly from study of complexation reactions. Dissociation constants of citric acid were mostly evaluated using various variants of potentiometric... 

Partanen, J. I. Karki, M. H. Determination of the Thermodynamic Dissociation Constant of a Weak Acid by Potentiometric Acid-Base Titration, /. Chem. Educ. 1994,... 

Directions are provided in this experiment for determining the dissociation constant for a weak acid. Potentiometric titration data are analyzed by a modified Gran plot. The experiment is carried out at a variety of ionic strengths and the thermodynamic dissociation constant determined by extrapolating to zero ionic strength. 

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. 

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. 

Potentiometric titration curves The procedure involves the addition of a salt of a weak acid to the resin and the determination of the pH of the equilibrated solution. Table 9 shows the pK values of the OH groups and dissociation constants of the studied resin. The first ionization occurs at a pH slightly higher than that of sul-... 

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. 

Among the possible analytical methods for alkalinity determination, Gran-type potentiometric titration [2] combined with a curve-fitting algorithm is considered a suitable method in seawaters because it does not require a priori knowledge of thermodynamic parameters such as activity coefficients and dissociation constants, which must be known when other analytical methods for alkalinity determination are applied [3-6],... 

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. 

Our method at present is not based upon theoretical models or departures from ideal behavior. It consists in the use of potentiometric determinations and literature values of activity coefficients, starting with HCl-HC10 electrolyte mixtures and with the assumption that HClOi, is completely dissociated since the association constant pK = 7 is extremely small in this case. 

Drug dissociation constants are experimentally determined by manual or automated potentiometric titration or by spectrophotometric methods.40 Current methods allow determination of pXa values with drug concentrations as low as 10 to 100 pM. For highly insoluble compounds (concentration <1 to 10 pM), the Yesuda-Shedlovsky method41 is commonly used where organic cosolvents (i.e., methanol) are employed to improve solubility. The method takes three or more titrations at different cosolvent concentrations, and the result is then extrapolated to pure aqueous system. The dissociation constant can also be determined with less accuracy from the pH-solubility profile using the following modification of Henderson-Hasselbach equation ... 

The dissociation constants of brinzolamide were determined by potentiometric titration of the hydrochloride salt with 0.05N NaOH, with two ionizations being observed. The first pKa, attributed to the deprotonation of secondary ammonium moiety, was observed at 5.8 8. The second pKa, attributed to deprotonation of the sulfonamide group, was at 8.48. 

Kust (4) has potentiometrically determined the equilibrium constant for carbonate dissociation in fused sodium-potassium nitrate eutectic ... 

G. Papanastasiou and I. Ziogas, Simultaneous Determination of Equivalence Volumes and Acid Dissociation Constants from Potentiometric Titration Data, Talanta 1995,42, 827. 

Dissociation constants of ionizable components can be determined using various methods such as potentiometric titrations [85] CE, NMR, [86] and UV spectrophotometric methods [87]. Potentiometric methods have been used in aqueous and hydro-organic systems however, these methods usually require a large quantity of pure compound and solubility could be a problem. Potentiometric methods are not selective because if the ionizable impurities in an impure sample of the analyte have a pK similar to that of the analyte, this could interfere with determining the titration endpoint. If the titration endpoint is confounded, then these may lead to erroneous values for the target analyte pKa. 

Liquid chromatography has also been widely used for the determination of dissociation constants [88-92] since it only requires small quantity of compounds, compounds do not need to be pure, and solubility is not a serious concern. However, the effect of an organic eluent modifier on the analyte ionization needs to also be considered. It has been shown that increase of the organic content in hydro-organic mixture leads to suppression of the basic analyte pKa and leads to an increase in the acidic analyte pK compared to their potentiometric pKa values determined in pure water [74]. 

The free xanthic acids are unstable, colodess, or yellow oils, and may decompose with explosive violence. They are soluble in the common organic solvents and are slightly soluble in water methyl xanthic acid at 0 0, 0.05 mol/L ethji xanthic acid at 0 0, 0.02 mol/L and -butyl xanthic acid at 0 0,0.0008 mol/L. Values for the dissociation constant for ethyl xanthic acid are (2.0—3.0) xl0 (5). Potentiometric determinations for Cg xanthic acids show a decreasing acid strength with increasing molecular weight (6,7). The values for the ethji derivative are (1.82—3.4) x 10 . Similar values determined by the same method are reported in Reference 8 for a series in dimethylformamide. Some piC values for alkyl xanthic acids are given in Table 2. 

For an acid titrated halfway to its equivalent point, pH = pKa. For mixtures of acids and bases, and hence for carbons having functional groups of different acid or basic strength, this holds true as well. For weak acid and base groups, the effect of water dissociation is significant around pH = 7. Therefore, a simple potentiometric titration can give information about the dissociation constants and neutralization equivalence of the carbon. In several cases these indications can be sufficient to determine the nature of the functional groups and provide a comprehensive description of the behavior of carbon in terms of acidity and basicity. A differential plot of the titration curve can be considered in the same way as a conventional absorption spectrum of the sample. Acidity or basicity constants are then calculated at half-titration, as pH = pKw — pKb for a base and pH = pKa for an acid. 

The content of narcotoline in poppy capsules (0.1-0.2%) is useful for preparative purposes (212, 221-224). In connection with this problem, colorimetric determination (223), paper chromatography (92, 94, 103, 107, 108, 112, 189), potentiometric determination (225), and thin-layer chromatography (161) were utilized. Indirect determination of narcotoline is possible via cotarnoline (149), especially polarographically (226, 227). The dissociation constant of narcotoline has been determined (228). 

The second group of values came from studies where it was assumed that polymerization reactions occurred, such as the formation of H5As206 (aq>, in addition to the deprotonation reaction. For chemical and mathematical reasons, the dissociation constant calculated from a set of measurements becomes smaller as one introduces polymeric anions into the model. The differences of the models chosen, at first appearance, could serve to explain the differences of the equilibrium constants given in the previous table. Unfortunately, the situation, from the perspective of data evaluation, is more complex. In principle, there should be a sufficient dilution of arsenious acid for which one would not expect the formation of a significant proportion of species like HsAsaOe caq) upon addition of base. For such a condition, the equilibrium constant determined assuming that only the monomer exists, should approach that determined for the multi-species model. Britton and Jackson (1934) performed potentiometric titration at two concentrations of arsenious acid (0.0170 and 0.0914 molar) and obtained essentially the same... 

F. M. Cray and G. M. Westrip have determined the influence of acetone upon the transformation interval of a number of indicators. They worked with acetone containing 10% by volume of water. They prepared various buffer mixtures, the pH s of which were measured potentiometrically by means of the quinhydrone electrode. The following table shows how large an effect acetone has upon the magnitude of the dissociation constant. 

It is generally assumed that measurements with the hydrogen electrode yield the hydrogen ion activity. That this is only approximately true can be seen from the discussion of the potentiometric method of determining dissociation constants (cf. pages 75 and 76). 

Roman L, Mirel S, Plorean E, Oprean R. The potentiometric and spectrophotometric determination of dissociation constants for same 2-mercapto-5-R-amino-l,3,4-thiadiazole derivatives. J Pharm Biomed Anal 1998 Oct 18(1-2) 137-144. 

Three different methods are used in the paper to determine the second dissociation constant, Aia2, of H2Se (1) ordinary potentiometric titration employing KOH(aq) and a glass electrode, (2) addition of small amounts of H2Se(aq) to KOH(aq) and subsequent pH determination with a KOH(aq)-calibrated glass electrode, and (3) the determination of the solubility of Na2Se(cr) in solutions of known initial concentrations of sodium hy-droxide/acetate. For each technique only a very limited number of data (4, 2, and 5, respectively) are presented. 

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