PH and pK values

An alternative model for ESI was developed by the group of Siu [24-25]. They correlated the ion envelope of a protein with the predicted distribution of charge states of the protein in solution. The latter depends on pH and pK values of the acidic and basic amino acids in the protein. From the good correlation obtained, they conclude that the ion envelope nicely reflects the abundances of the preformed ions of the protein in solution. Based on further experiments, they postulate that the droplet evaporation is not that important and that the ESI-MS spectrum results from ions directly emitted from the Taylor cone [25-26]. 

Be It Resolved that the Board of Trustees consider publishing in the Pharmacopeia, or in a companion publication, information on such attributes as solubilities, pH and pK values, spectra and spectrophotometric constants, and stability data, pertaining to pharmacopeial drugs. 

Make sure that you know how to use your calculator to evaluate logarithms, and how to turn pH and pK, values into [H ] and K, values. 

Thus, to prepare a buffer solution, we work backward. First we choose a weak acid whose pATa is close to the desired pH. Next, we substitute the pH and pK values in Equation 12.2 to obtain the ratio [conjugate base]/[acid]. This ratio can then be converted to molar quantities for the preparation of the buffer solution. Example 12.4 shows this approach. 

Global LSER calculations have also been applied to the study of the retention of ioniz-able analyses in RP-HPLC. While the retention of neutral analyses does not depend on the pH of the mobile phase the retention of analyses with one or more ionizable substructures considerably depends on the pH even at the same concentration of organic modifier in the eluent. The relationship between the retention and pH of the mobile phase and pK value of the analyte can be described by... 

Using Eq. 11 -34 (Box 11.1) and the goethite surface site density, its pA i and pK values, and the ionic strength of the solution, find the intensity of surface charging of the goethite at the three pH values of interest (note that [Pg.446]

Creatine Kinase. Six of the sixteen imidazole C-2 proton resonances and one imidazole C-4 proton resonance per subunit of this dimeric enzyme (Mr 82,000) were detected. Titrations measuring their chemical shifts as a function of pH yielded pK values of 7.0, 7.1, 5.9, and 5.2 for his(2), (3), (4) and (6), respectively, and permitted the assignment of the C-4 resonance to his(3) (7). The pK of his(2) was unaffected by saturation of the enzyme with creatine but was increased by 0.6 - 0.7 units on saturation with the phosphorylated substrates phosphocreatine or MgATP, in... 

If these measurements are made at a pH where only the neutral molecules are present, the ratio is equal to the constant K. This restriction about pH is necessary because and k are ordinarily composite, their relationship to the equilibrium constants and pK values being given by the relevant equations, ... 

The pH of solutions of isoelectric aspartic acid may be calculated from the pKa, and pK, values (the pif values on either side of isoelectric aspartic acid in its titration curve). 

The log P, log D al pH 7, and pK values are from Chemical Absiracis Service. American Chemical Society, Columbus. OH. 2003, and were calculated by using Advanced Chemistry Development (ACD) Software Solaris V4.67. The pK v ues are for the most acidic HA acid and most weakly acidic BH groups. The latter represent the most basic nitrogen. Keep in mind that pK, values for HA acids that exceed 10 to 11 mean that there will be little, if any, anionic contri-... 

Hase (H16) studied the effect of pH on the hydrolysis of acetylcholine by horse serum cholinesterase, and his results have been reanalyzed by Laidler (L5) and extensively discussed by Dixon and Webb (D21). The relationship between pH and the rate of hydrolysis of acetylcholine has been used to obtain information on the structure of the active site of the enzyme (B19, W28). Acetylcholine is a particularly suitable substrate for these studies since it does not change its charge in the pH range studied. Similar pH-activity curves have been obtained using other substrates for cholinesterase (H23, S20, P19). Moreover the pH dependence of enzymic activity varies with the buffer system (K3). By investigating the effect of pH and sodium chloride concentration on the rate of hydrolysis of ben-zoylcholine by human plasma cholinesterase, Kalow (K6) deduced that for this substrate, each enzyme molecule contains at least two binding sites which differ in their dependence on pH. Michaelis constants and maximum hydrolysis velocities were measured for each of the two binding sites, and pK values of the enzyme-substrate complexes were found to be 5.2, 6.7, and 9.2 for one site, and 5.2, 7.0, 8.4, and 8.8 for the other. 

Figure 6. Reduction potentials of the four reduction steps of 02. (a) pH dependence calculated form standard E° data with pQl = 1 atm as reference state and pK values of the intermediates. References Biclski et al. (1985), Sawyer and Valentine (1981), George (1965). Superoxide is a strong reducing agent. (b) Reaction path diagram for the reduction of Oa. The the formation of radicals in the first and third step is endergonic.

The equilibrium pHpp, values and pK values for all 4 bile salts as functions of bile salt concentration (HjO, 37°C) are plotted in Fig. 8 (from ref. 35). For comparable bile salt concentrations, the HA species of UDC and GUDC precipitate at higher pH values than the HA species of CDC and GCDC, despite similar pA values. The physical-chemical reason why the pHpp, of the UDC and GUDC species... 

Ampholytes represent complex mixtures of homologous polyamino polycarboxylic or polysulfonic acids with close pi and pK values [135,136]. The differences in pi values represent inter-species differences, while the differences in pA values are related to a single species (intra-species differences). Usually the following ranges of pH are covered 3.5-10.0, 2.0-4.0, 3.5-5.0, 4.0-6.0, 5.0-7.0, 5.0-8.0, 6.0-8.0,... 

The pH range of the dyes is narrow (3 pH units). The best performance (accuracy <0.01) is obtained in a narrow zone, generally 0.02 units around the pK, . The accuracy decreases sharply with increasing distance from the pK (the relative intensity curves f(pH) are nonlinear). In the outer zones ( 1.5 pK units), the accuracy hardly exceeds 0.1 pH units. The use of several p J, values with the same indicators requires differentiated spectral responses for the two species present [12, 14, 36). New dyes are under development [58, 59] to broaden the pH range with a single indicator for several pIC values and for wider spectra. Also, the pIC is different in solution and for a semi-active optode as presented in Figure 17-6 [35]. A summary of theoretical pK and pK values for some optodes is given in Thble 17-3. 

Temperature dependence equations for pK and pK values show the importance of good temperature control during pH meter calibration as well as in measurements. To a greater or lesser extent, all acid-base reactions vary with temperature. The effect of temperature changes for equilibrium reactions are closely described by the Valentiner-Lannung equation [80-82] ... 

When buffer solutions were not used and the pH was not reported, we calculated the pH using the solution concentration and pK values for all dissociation reactions and assuming that the pH was 7.0 prior to solute addition. A general treatment of simultaneous equilibrium involving equations for all linearly independent reactions, the water dissociation reaction (K = 1.0 x lO" " ), a molecular balance on the active species, and an equation requiring solution electroneutrafity is required to calculate the natural pH (Brescia et al., 1975). A more detailed discussion of the adjustment for ionization and associated calculations is presented in Vecchia and Bimge (2002b). 

The first table ever to list the values for percentage ionized, as calculated from pH and pK, appeared in the first edition of Selective Toxicity (1951), and is reproduced below. For acids it is calculated as follows ... 

---