Weak acids functional groups

All amino acids possess at least two weakly acidic functional groups, R—and R—COOH. Many also possess additional weakly acidic functional groups such as —OH, —SH, guanidino, or imidazole groups. 

The increase in log with pH is typical of copper binding to weak acid functional groups. For the reaction of copper with a protonated binding site HpL,... 

Tyr possesses a weakly acidic functional group in its aromatic side chain with pK, = 10.1. The phenolic group forms hydrogen bond and the phenolic ring of Tyr is relatively reactive in electrophilic substitution reactions. 

Generally, the interaction of polar analytes with the packing is rather weak due to the hydrophilic polyhydroxy functions on the surface of the packing. However, small amounts of acidic functional groups are present on the surface of the packing. The influence of these functional groups can be suppressed easily with the use of salts in the mobile phase. 

One fact to keep in mind with such phases is that weak acid cation-exchange materials based on carboxylic acid functional groups are subject to esterification in the presence of alcohol containing eluents. Even thongh typical eluent conditions (i.e., weakly acidic aqneous eluents containing alcohol) do not favor ester formation, such stationary phases typically exhibit slowly declining capacity when operated in the presence of alcohol-containing eluents. Consequently, such columns are normally operated with acetonitrile, tetrahydrofuran or acetone rather than with methanol, in order to avoid this problem. 

The surface of the silica is covered with weakly acidic silanol groups and it is assumed that the pK, value is in the range of 5 to 7. The maximum surface concentration of silanol functions is about 8 /imol/m. As the binding of ihe organic funclioitK lakes place via Ihe accessible silanol groups it is desirable to maximize their surface concentralion and to pre... 

Anion-exchange sorbents for SPE contain weakly basic functional groups such as primary or secondary amines which are charged under low-pH conditions or strongly basic quaternary ammonium groups which are charged at all pHs. Cation-exchange sorbents for SPE contain weakly acidic functional... 

The acidic functional groups can be characterized by their neutralization reactions with a selection of bases differing in their pK values. The usual reagents are sodium hydroxide, sodium carbonate, sodium bicarbonate and, for very weak acids, sodium ethoxide in alcohol. An additional dimension of information can be obtained by pretreating the carbon prior to neutralization analysis [156]. 

The Interest In the chemical interrelationships of phytate Initially centered around Its complexatlon with calcium and subsequent effect on the availability of calcium (33). This led to the classical studies by Hoff-Jorgenson (34). He determined eight of the twelve dissociation constants of phytic acid and using these data calculated thj solubilij products of penta-calclum phytate as between 10 and 10. A decade later with the use of neutralization curves and conductivity measurements it was concluded that phytic acid contained 6 strong acid groups which were completely dissociated in solution (pk = 1.84), 2 weak acid functions (pk = 6.3) and 4 very weak acid protons (pk = 9.7) (M). 

The effects of solution acidity and basicity on luminescence spectra result from the dissociation of acidic functional groups or protonation of basic functional groups associated with the aromatic portions of fluorescing and phosphorescing molecules. Protonation or dissociation can alter the natures and rates of non-radiative processes competing with luminescence and, thereby, affect the quantum yields of emission. For example, the antimalarial mefloquine fluoresces very weakly but phosphoresces well in neutral aqueous media. However, at pH < 1, its protonated form fluoresces intensely, and its phosphorescence is very weak. 

In conclusion, weakly basic exchangers in the free base form are only usefully functional at low pH when the hydrogen ion concentration is sufficiently high to protonate the resin. Therefore although conceptually useful, it is somewhat fallacious to propose the initially ionized hydroxide form to explain the behaviour of weak base functional groups. Instead their reaction is better regarded as initially one of acid salt formation by a direct addition reaction and then where appropriate subsequent ion exchange, as depicted by equations 4.18-4.20. 

Weak-acid resins generally extract metal ions only at a pH value of 3 or more. At high pH values, where the carboxylic acid functional group is predominantly ionized, the metal-extraction behaviour of a weak-acid resin parallels that of a strong-acid resin. The weak-acid resins tend to be more selective than strong-acid resins, however, and practical separations of metals are possible. In certain interactions between metal ions and weak-acid resins it is difficult to determine whether the interaction is purely electrostatic or whether chemical bonds are formed. In particular, there is strong evidence that Ag" and Cu + form complexes with carboxylic acid groups. Copper, in... 

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