PK‘, determination

Avdeef, A., Comer, J. E. A., Thomson, S. J. pH-metric log P. 3. Glass eledrode calibration in methanol-water, applied to pK determination of water-insoluble substances. Anal. Chem. 1993, 65, 42-49. 

A., Potentiometric pK determination of water-insoluble compounds validation study in methanol/water mixtures, Int.J. Pharm. 1997, 353, 235-248. 

The titration process has been automated so that batches of samples can be titrated non-manually and the data processed and reported via printouts and screens. One such instrument is the Metrohm 670 titroprocessor. This incorporates a built-in control unit and sample changer so that up to nine samples can be automatically titrated. The 670 titroprocessor offers incremental titrations with variable or constant-volume steps (dynamic or monotonic titration). The measured value transfer in these titrations is either drift controlled (equilibrium titration) or effected after a fixed waiting time pK determinations and fixed end points (e.g. for specified standard procedures) are naturally included. End-point titrations can also be carried out. 

The reactions discussed in the following sections take place in aprotic solvents, and reference to known or estimated thermodynamic basicities will relate to DM SO unless otherwise noted, since DM SO is the polar aprotic solvent in which most thermodynamic acidities have been measured [55-58]. Values of pK determined in DM SO can usually be assumed to parallel values in DMF [59, 60], MeCN, and other polar aprotic solvents whereas pK values (and relative pK values) related to water and other hydroxylic solvents can be very different. 

The fundamental principles of pK determination by CE rely upon measuring the ionic effective mobility of the solute as a function of pH. Consequently, the pK value is obtained by fitting an equilibrium equation to the effective mobility and pH data with a non-linear regression technique (Figure 1). Depending on the chemical character of the compound... 

To some extent these effects seem unavoidable and great caution has to be exercised when fine tuning the influence of small structural changes on pKs determined by acidity functions. On the other hand, these effects may well be small in many cases. Evidence supporting this contention originates in the existence of some excellent linear free energy relationships between gas phase and solution acidities and basicities (83MI2). 

Within families of compounds, good linear relationships often exist between pKs determined in water or other solvents and the logarithms of the equilibrium constants for the formation of 1 1 HB complexes in inert solvents (76RGS691 83JOC2877). The data reported in Table IV follow this pattern as shown in Figs. 2a and 2b. 

The impurity interacts with a substance participating in the process to be measured. An example is met when we determine the dissociation constant (pfQ of a weak acid HA by conductimetry. In the determination, we dissolve HA in the pure solvent and measure the conductivity of the solution containing dilute H+ and A formed by dissociation. If a basic impurity B is contained in the solution, it disturbs the measurement, producing ionic species by the reaction B+HA — BH++A . Because trace amounts of basic impurity are contained even in a purified solvent, conductimetric pK determination is practically impossible for HA with pfCa>7.5. 

For discussions of pK determinations for the conjugate acids of ketones, sec Bagno Lucchini Scorrano Bull. Soc. Chim. Fr. 1987, 563 Toullec Tetrahedron Lett. 1988, 29, 5541. 

The results of all our pK determinations are summarized graphically in Figures 1-8. In these plots we show our results as ... 

The second protonation of dimethylaminopyridines and their N-oxides, all weak bases whose pK determination involves acidity function theory, yield o values in good agreement with previous values, affording thereby a check on the validity of the acidity function procedure. 

Plotting the same graph at wavelengths equal to or higher than 400 nm yields a pK equal to 6.7. At these wavelengths, the pH domain in which Equation (6.21) can be applied is restricted to two or three pH (6-8) (Figure 6.7). This result indicates that at wavelengths where absorption is dominated mainly by one species, the pK determined by this method would be more dependent on the form of the dye. 

Finally, pK determination is also dependent on the ionic strength of the solvent used. In the literature, the pKa of 4-methyl-2-nitrophenol is calculated to be equal to 7.11. Instructors and students can refer to Aptula etal. (2002). 

A kinetic method for pK determination in aqueous DMSO has recently been developed (Earls et al., 1974 Cockerill et al., 1974). This is based on rates of isotopic exchange and is applicable to the p a range ca. 15-25. The method has the advantage that it does not depend on spectral changes occurring on ionization. However, the majority of p/sfa values of weak acids have been obtained by the equilibrium technique, using acidity functions (AF) (Hammett, 1940 Bowden, 1966 Rochester, 1970). 

In this equation, pK° is the pK of the unsubstituted molecule, o refers to the constant assigned to a respective substituent and p is a constant assigned to the particular ionizable group. The approach was later refined by Taft et al extending the theory and equation to aliphatic and other systems. The principles and methodologies of a classic experimental pK determination are well known. Ionization constants for an n-protic weak acid may be represented by the following generic definitions ... 

In conclusion, theoretically, the HPLC method can provide a tool for pK determination of compounds but requires measuring the retention at various pHs. However, the theory is not very well understood when various concentrations of oiganic additives are used in the mobile phase as these change both the pH and the pA"a of the compounds. For example. Rived et al. [Ill] give a compilation of the di.ssociation constants of neutral and charged acids in methyl alcohol and compared the pA"a values in water and in methyl alcohol. 

Acidity usually decreases down a periodic column, e.g., the acidity of h CjHj-M(CO)jH decreases Cr > Mo > W HjOs(CO) is a weaker acid than H2Fe(CO) and HRe(CO)j cannot be deprotonated under the conditions employed for the titration and pK determination of HMn(CO)j. However, there is little difference in pK between the Co and Rh dimethylglyoxime complexes (see Table 1), and IR observations... 

Figure 7 Values of pK — pK determined from the measurements of Lee et al. (1996) as a function of pco ...

An alternative approach to pK determination for very weak hydrocarbon acids is the electrochemical method of Breslow " . This method is thermodynamic in origin employing voltammetric reduction/oxidation of the cation (or anion) to radical thence to anion (or cation) and comparing the energetics of these steps to the triphenylmethyl system (including bond dissociation energies of the respective hydrocarbons). Values of p aS obtained for some weak carbon acids by this method are given in Table 1 for comparison with the Streitwieser results. 

Since many purines exist in lactam and lactim forms (c/. Section 4.09.2.1) the position and intensity of the peaks are often affected by pH and this can be used for pK determinations. In addition the position of substituents affects the spectra by increasing the intensity and causing bathochromic shifts usually in the order 6 < 8 < 2 and of increasing hypsochromic effect 2 > 6 > 8 for the longer wavelength band. The effects of the substituents in polysub-stituted derivatives are not, however, additive. 

Chou PP, Jaynes PK. Determination of urinary 5-hydroxyindole-3-acetic acid using solid-phase extraction and reversed-phase high-performance liquid chromatography with electrochemical detection. J Chromatogr 1985 341 167-71. 

Abbreviations MS, metabolic stability DDI, drug—drug interaction screening XSP, cross-species comparison and PK, determination of pharmacokinetic parameters. 

Recent AMI MO studies agree with earlier results (based on pK determinations and rates of solvolysis of imidazolyl-2-carboxylates) that 5-substituent effects follow and 4-substituent effects follow (Tm". The substituent effects are not readily transmitted through the —NH— system and the. resonance effects of a 5-substituent are more important than those of the same group at C-4 <93JOC7336, 94UP 302-02>. 

For a comprehensive compilation of the dissociation constants of organic acids (including phenols) in aqueous solution, as well as summary of the methods for pK determinations, see G Kortum, W Vogel and K Andrussow, m Pure and Applied Chemistry, Vo 1, Butterworths, London, 1961, pp 190-536... 

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