Adrien Albert

Determination of ionization constants by spectrophotometry

The determination of ionization constants by ultraviolet or visible spectrophotometry is more time-consuming than by potentiometry. However, spectrometry is an ideal method when a substance is too insoluble for potentiometry or when its pKa value is particularly low or high (e.g. less than 2 or more than 11). The method depends upon the direct determination of the ratio of molecular species (neutral molecule) to ionized species in a series of non-absorbing buffer solutions (whose pH values are either known or measured). For this purpose, the spectrum of the molecular species must first be obtained in a buffer solution whose pH is so chosen that the compound to be measured is present wholly as this species. This spectrum is compared with that of the pure ionized species similarly isolated at another suitable pH. A wavelength is chosen at which the greatest difference between the absorbances of the two species is observed. This is termed the ‘analytical wavelength’. Fig. 4.1, in which the base 2-amino- pyridine is used as an example, shows how these two pH values can be found.

Metal-binding substances

Heavy metals, in traces, are essential for all forms of life. They are taken up by the living cell as cations, and their uptake is strictly regulated because most (or all) of them are toxic in excess. A remarkable specificity has been found: seldom can an excess of one essential metal prevent the damage caused by deficiency of another. In fact, such an excess often increases the injurious effect of the deficiency.

Determination of ionization constants by Potentiometric titration using a glass electrode

Potentiometry, because it is most economical of time, is usually the best choice for determination of ionization constants. From Sorensen’s announcement, in 1909, of his concept and definition of pH, the means for determining this quantity have steadily improved. All the work described in this chapter can be done with the now universally available potentiometer-electrode assembly, commonly referred to as an Ion Activity Meter or pH set. If used with care, this apparatus can be made to give acceptable results. An outline of the theory of pH is given in Appendix VI (p. 203).

The ionization constants of typical acids and bases

In order to select the most appropriate method for the determination of an unknown ionization constant, the experimenter should be able to forecast an approximate value for the expected pKa . For uncomplicated aromatic substances, this can be done from Hammett’s Free Energy Equation, using its sigma and rho values (Hammett, 1940, 1970). In all cases, help can be had from a book on the prediction of pKa for organic acids and bases (Perrin et al 1981).

The covalent bond in selective toxicity

Although the majority of biologically active substances combine only loosely with receptors and are easily released by washing, a few agents combine by covalent bonds which are of a more durable character (see Section 7.1). Covalent bonds involving carbon can be broken by great heat and also by powerful chemicals, but few can be ruptured by mild reagents at temperatures compatible with life.

Some other methods for the determination of ionization constants

As traditionally practised, Raman technique required aqueous solutions to be so highly concentrated that activity corrections became meaningless. Introduction of the laser source, about 1970, permitted examination of more dilute solutions. Nevertheless, the method has not been much used, probably because only small molecules give spectra that are simple enough for meaningful analysis.

The behaviour of foreign substances in the human body

Abstract Oxygen and ethanol admirably illustrate the Paracelsian doctrine that foods, drugs and poisons are much the same and that the type of effect obtained depends mainly on the dose. The present review compares and contrasts these three categories of xenobiotics. The uncomfortable fact that good, pure, wholesome food has insidiously toxic properties has been brushed under the carpet for too many centuries, but can now be seen emerging. Adrien Albert discusses the main aspects of malnutrition (too much, too little, and the wrong mix) and indicates opportunities for wider reading. A clear separation of medicines from poisons, Paracelsus notwithstanding, is taking place at last, in our own times, through implementation of the principles of selectivity, a concept that flows from Paul Ehrlichs discovery of the Chemotherapeutic Index in 1911.

Chelation and the stability constants of metal complexes

The link between this chapter and the preceding ones is the fact that those ionic species which can bind a hydrogen cation have an affinity, also, for the cations of metals. Usually this affinity is greatest when an organic molecule is so designed that it can form at least two bonds to the metal, thus creating a ring. This ringformation is called chelation, from the Greek word for the lobster’s claw, and the complexing species is known as a ligand. A complexant (chelating agent) may have two or more ionic species, but often only one of these is the ligand.

Surface chemistry. The modification of membranes by surface-active agents

The intense cytological studies of the last two decades have shown that cells, as well as most of the organelles within them, are covered with lipoprotein membranes (see Section 4.1). It is now widely thought that life is possible only because of the presence of these biphasic membranes (lipophilic within, and hydrophilic on both outer surfaces), which provide for the separation of reactants. They also impose, on the sequence of reactions, an order that would otherwise be almost impossible to arrange. Often in, but sometimes apart from, these membranes, enzymes offer surfaces of the very greatest biological importance.

Some numerical assistance

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