Potentiometric titrations acidity constants from

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 ... 

In the practice of potentiometric titration there are two aspects to be dealt with first the shape of the titration curve, i.e., its qualitative aspect, and second the titration end-point, i.e., its quantitative aspect. In relation to these aspects, an answer should also be given to the questions of analogy and/or mutual differences between the potentiometric curves of the acid-base, precipitation, complex-formation and redox reactions during titration. Excellent guidance is given by the Nernst equation, while the acid-base titration may serve as a basic model. Further, for convenience we start from the following fairly approximate assumptions (1) as titrations usually take place in dilute (0.1 M) solutions we use ion concentrations in the Nernst equation, etc., instead of ion activities and (2) during titration the volume of the reaction solution is considered to remain constant. 

The potentiometric titration curves of gels, which relate the pH of the exterior solution to the degree of ionization of the gel, resemble the titration curves of monofunctional acids or bases. However, the dissociation constants differ, often by two orders of magnitude, from the expected value for the functional group, and the slope of the curves is not the usual one. Addition of neutral salt changes the picture markedly and brings the curves closer to expectation. In the case of weak or medium... 

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

The procedure for potentiometric titration is presented in Chapter 1.6. In this titration, a standard acid titrant is added to a measured volume of sample aliquot in small increments of 0.5 mL or less, that would cause a change in pH of 0.2 unit or less per increment. The solution is stirred after each addition and the pH is recorded when a constant reading is obtained. A titration curve is constructed, plotting pH vs. cumulative volume titrant added. The volume of titrant required to produce the specific pH is read from the titration curve. 

From the potentiometric titration data (Si02 - H+/OH") Sidorova et al.72) calculated the value of the surface potential ip0 and the dissociation constant of the surface silicic acid Ko for the reaction ... 

Oxygen Donors. The formation and stability constants oi complexes between Pt (Pd, Rh, Ir, Os, Ru) and o-coumaric acid have been determined by pH titration.31 The results indicate that a 1 2 complex is formed with Pt. Acid-base properties of aquo-complexes formed from [Pt(X)2(OH)2(NH3)(MeNH2)] (X = Cl, Br or N02) in aqueous solutions have been examined using potentiometric titration experiments.186 The Ka of co-ordinated water was lower for (X)2 = (H20)2 than for (X)2 = (H20) (OH-). 

A related form of an automatic potentiometric titrator is instrumentation that permits the maintenance of the acidity or basicity of a solution over a period of time. Such devices are known as pH-stats, and find application in kinetic studies of hydrolysis reactions. The general approach is (by either manual or automatic means) to add either acid or base such that the pH in the solution is maintained constant over a period of time. Normally the amount of acid or base added as a function of time is sought in order that kinetic measurements may be made for the system. In its simplest form the acidity of the solution is monitored with a pH meter and controlled at a preselected value by the addition of acid or base from a burette the quantity delivered as a function of time is recorded in a notebook. Obviously for the fast reactions this becomes difficult and dependent on the dexterity of the individual. 

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... 

The degree of induction of the dissociation of PMAA in the presence of various polycations is estimated by its apparent dissociation constant (pKJ calculated from the potentiometric titration results as shown in Fig. 6, using the Henderson-Hasselbach equation (see Table 5). In the presence of polycations, pKa and one of the interaction parameters relative to adjacent ionic sites, n, decreases. This confirms the induction of dissociation. Furthermore, the difference in the composition of the polyelectrolyte complex composed of one kind of polycation and different poly(carboxylic acid)s, may be affected by pKa and the conformation of them. 

The data for the determination of the intrinsic equilibrium constants for Na and Cl are shown in Figures 5 and 6. For Na (Figure 5), the acidity quotients, pH-log (a /l - a ), are plotted as a function of the fractional ionization, a, and the log of the electrolyte concentration. The concentration term is multiplied by an arbitrary constant in order to separate the curves. The acidity quotients calculated from the potentiometric titration data as a function of a - 0.05 log [Na" ] are represented by the filled circles. For each ionic strength the points are extrapolated to a = 0. These extrapolated points are designated by open squares. These extrapolated points are then further extrapolated to 1 M electrolyte concentration. The open circle is the value for P K at zero charge and 1 M electrolyte concentration. 

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].

Dickson A. G. and Riley J. P. (1979a) The estimation of acid dissociation constants in seawater from potentiometric titrations with strong base I. The ion product of water— K. Mar. Chem. 7, 89-99. 

Potentiometric acid-base titrations are particularly useful for the analysis of mixtures of acids or poly-protic acids (or bases) because often, discrimination between the endpoints can be made. An approximate numerical value for the dissociation constant of a weak acid or base can be estimated from potentiometric titration curves. In theory, this quantity can be obtained from any point along the curve, but it is most easily derived from the pH at the point of halfneutralization. 

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... 

It follows from Tables 17 and 18 that tetrakis(dimethylamino)naphthalenes 35 and especially 65 possess higher basicity in comparison with parent compound 1. It was hoped that a further increase in the number of NMe2 groups would increase the basicity even higher. Unfortunately, the first representatives of such compounds, namely hexa- and heptakis(dialkylamino)naphthalenes (Section II.B.2), turned out to be scarcely soluble in MeCN, DMSO, EtOH and H2O. Yet, their pK, values were measured in 80% aqueous dioxane by a potentiometric titration technique166. The acid ionization constants are summarized in Table 19 together with the pXa values of tetrakis(dimethylamino)naphthalenes 35 and 65 determined under the same conditions. 

Potentiometric Methods. - Potassium trithiocarbonate has been used as a reductant (-S-S- reduction cleavage) for the potentiometric (and spectrophoto-metric) determination of the disulfides of dithio-phosphinic acids in DMF-H2O medium at millimolar levels.The protonation equilibria for N,N -diethylami-nomethylenephosphonic acid (88) were elucidated from both potentiometric titration and determination of the pH dependence of the NMR chemical shift (83 ip), and protonation constants for aminoalkanephosphonates RCH(NH2)P(0)(0Et)2 and the acidity constant of di(2-ethylhexyl)thiopho-sphoric acid (111) have also been obtained potentiometrically. 

---