The address of the Retiring President to the British Society, 1955 pH VALUE AND ITS IMPLICATIONS R. H. MARRIOTT, D.Sc., F.R.I.C.* Dr. Marriott gives an easily understood explanation of "t•H," discussing strong and weak acids and buffer action. The effects of varying the/•H value on l•rotein structures are used as examl•les of its iml•ortance to cosmeticians. A siml•le method of assessing the extent of buffering action is given and the behaviour of indicators and their selection for l•articular l•url•oses is described. The l•recise understanding of the meaning of the/•H value is stressed through- out. THIn SUBJECT of this address was chosen as it appeared to me that the occasion of the President's address was a suitable one on which to talk on a matter which was tending more and more to become misused. It is quite clear that the notation of pH value is slowly losing its precise significance and is becoming contracted to the simple term "pH," which is often quite meaningless and unreal. There is no such thing as "pH," but there is a significant figure known as the pH value which is of prime importance to all engaged in industrial chemistry. The pH value is a mode of expressing the hydrogen ion concentration of aqueous liquids. The hydrogen ion con- centrations, which are obtained when acids or alkalies are added to water, range over concentrations from one to one hundred million millions, and are expressed in negative powers of the base 10, i.e., they are reciprocals, and are not easy to manipulate. The scheme put forward by S6rensen, known as the pH scale, simplifies the matter. The pH scale is, however, simply a mode of expression. It is necessary for me to give a very simple and brief outline of the basic theory underlying this conception. Historically, the notion that all chemical reactions depended on the dissociation of substances into charged atoms dates back to the middle of the nineteenth century, but the hypothesis proved to be full of complications. It can be said, however, that clarification, at any rate in respect of reactions in solutions, dates back to Arrhenius who, in 1887, laid the foundations of the concept of hydrogen ion concentration. This breakdown or dissociation of molecules is electrolytic in character, the charged units being called "ions" which can be either single atoms or small groups of atoms (radicals) or larger co-ordinated groups (complexes). The term "ionisation" should, perhaps, be employed to describe this process in contrast with dissociation, which is the simple breaking down of a compound into two or more components which may or may not be electrically charged. * County Laboratories Ltd., Startmore, Middx. 289

JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS In the early days, and even in relatively recent times, many objections were made against this idea, notably by the H. E. Armstrong school, but as time went on and knowledge increased the reality of ionisation became more and more established. The discovery of the electron by J. J. Thomson in 1897 laid the foundations of modem atomic structure and gave reality to the charged particle. Following the isolation of radium by the Curies and the outstanding work of Moseley, Rutherford and, later, Bohr, were enabled to indicate the principle on which the atom is built. As everyone now knows, the atom consists of a positively charged nucleus surrounded by a dynamic field of electrons. The structure of the field of electrons is known with considerably certainty and the build-up enables the chemistry of the elements to be forecast and, in particular, what happens when the atoms ionise. The loss or gain of an electron produces a positive or a negative charge and the modern conception of chemistry, especially in the organic field, is based on this phenomenon. To understand the basic theory of hydrogen ion concentration, it is necessary to consider the physical chemistry of water. It is known that pure water, although a poor conductor, nevertheless conducts electricity and that the passage of an electric current produces hydrogen at the cathode and oxygen at the anode. The breakdown of water into hydrogen and oxygen is facilitated by adding an electrolyte of suitable character so that the conductivity, or the number of ions per c.c., is significantly increased. From the law of mass action of Guldberg & Waage, it can be deduced that the ionisation of water, HaO •H + q- OH - can be expressed bythe simple equation [H +] x [OH-] ---- k, (a constant), [H•O] the brackets meaning concentration in moles. per htre. It is known that the ionisation of water is so small that the concentration of un-ionised water is virtually constant and the equation can now be written [H +] x [OH-] = k• x ks. k, x ks is also a constant and can be written K w. Thus, we have the equation that K w ---- [H +] x [OH-]. K, is the ionisation constant of water. From conductivity measurements, it is known that at 22 ø C. the K•. of pure water is 10-'4 the value varying, of course, with the temperature. For the purpose of this talk, the figure K, x 10 -'4 will be used. Bearing in mind that for every hydrogen ion there must be a hydroxyl ion, [H +] = [OH-], and, therefore, the concentration of hydrogen ions in pure water is N x 10-', as is also the concentration of hydroxyl ions. When an acid is dissolved in the water the hydrogen ions produced by the acid increase the concentration of the hydrogen ions, and because K•. must be constant there is a corresponding decrease in the concentration of OH ions to compensate for the increased acidity. Apart from the mere concentration of the added acid, the change in the ratio of hydrogen to 290