VALUE AND ITS IMPLICATIONS hydroxyl ions depends on the ionisation constant of the added acid, so that the concentration of hydrogen ions in a tenth normal solution of a strong acid, hydrochloric acid, will be different from that in a tenth normal solution of a weak acid, acetic acid. In the case of hydrochloric acid at 0'IN, the concen- tration of hydrogen ions will be approximately N x 10-•, whereas in the case of acetic acid the hydrogen ion concentration will be approximately 2N x 10 -•. This is because weak acids, such as acetic, do not ionise to the same extent as strong acids, and in this lies the difference. As everybody knows, the two acid solutions, if neutralised by means of added alkali, require pre- cisely equal amounts since titration measures the total reservoir of acid, whereas the hydrogen ion concentration measures what might be called the "effective acidity." The concentration of hydrogen ions can be expressed as a pressure, for it is easy to see that if in one solution of acid in water the concentration of the hydrogen ions is 10-•N and in another it is 10-'N, the number of collisions in unit time with, say, the walls of the container in the one will be ten thou- sand times that of the other. This view corresponds with the kinetic theory of gases. This application of the kinetic theory clearly indicates that hydrogen ion concentration is covered by the gas laws. Indeed, the first and, incident- ally, the standard method of determining the hydrogen ion concentration was to take a strip of noble metal, e.g., platinum, coat it with platinum black and saturate this surface with hydrogen. When this coated metal is placed in a liquid, it behaves as though the electrode is hydrogen and it will give a pressure of hydrogen ions. Depending on how far the pressure of hydrogen ions in the water can combat the hydrogen pressure due to the hydrogen sorbed in the platinum black, so an E.M.F. is obtained. From this one can, by using the electrical equivalents, calculate the hydrogen ion pressure using the simple gas law formula PV = RT as the basis. It must be emphasised that the gas laws are based on Boyle's and Charles' laws, and are only accurate when applied to low pressures, i.e., equivalent to dilute solutions. The values obtained, however, relate to molar solutions and are so small that they have to be expressed in terms of negative powers of the base 10. They prove very clumsy when large numbers of solutions are being dealt with or if the values are to be plotted. S6rensen's scheme, put forward in 1909, was to consider all the values in terms of reciprocals of the logarithms to the base 10. That is to say, the logarithm with the sign changed. This notation enables one to see quickly differences in effective acidity, bearing in mind that as one proceeds from one unit of pH to another the effective concentration of hydrogen ions has changed ten-fold. Each whole step is ten-fold and mere plotting of pH values in uniform intervals does not in any way alter the ten-fold steps. It is merely a convenience. 291

JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS The •bH scale led, of course, to a range of values from 0 to 14. (In actual fact the •bH range is not exactly 0 to 14: it would be nearer the truth to suggest that it was from -- 3 to • 14.5.) This range, however, should not be looked on as being a precise reality when the •bH values are less than 1 or more than 13, for it must be remembered that the gas laws only hold for ideal gases and when the pressure of a gas is increased (i.e., the concentration), the equation ceases to be accurate for reasons which need not, perhaps, be gone into here. Nevertheless, the point is that the •bH notation and, indeed, the theory of hydrogen ion concentration can hold only in dilute solution, the limit of accuracy being probably when the concentrations are not greater than N/100, unless the "activity" be used as a correction factor. The need for such a factor indicates the need for caution when very low or very high •bH values are being handled. The hydrogen ion concentration concept via the •bH scale has, of course, been extensively applied to industry. It is interesting to note, however, that the first use was described by Wood, Sands and Law in 1911. They used a very primitive platinum electrode set-up on tanning liquors. As a result of this, the leather industry can be claimed to be the initiators of its industrial application. Its application to protein research was quickly appreciated, and since 1918 many papers have been published on the swelling and the acid and base combinations of fibrous and non-fibrous proteins. If the •bH value measures the hydrogen ion pressure or alternatively the hydroxyl pressure, it is quite clearly implied that the reaction between acids, alkalies and proteins must be a direct function of the •bH value. In the case of hair, the swelling of the protein is never very great, except when the value is of the order of 10 or more, but even in the case of hair the hydrogen ion concentration does play a part in the water uptake. The proteins collagen, muscle, and, of course, the degradation product from collagen, gelatin, are very susceptible to changes in hydrogen ion concentration. When gelatin is immersed in water containing hydrochloric acid at a •bH value of 2.4, the swelling can be two or three thousand per cent on the dry weight. With the fibrous proteins the swelling is not so great, although it varies with the age of the animal from which the fibre has been taken. The significance of the small degree of swelling of hair and epidermis in acid and mild alkaline solutions is strongly indicative of the usefulness of this particular protein as a protective surface for the living body. The living proteins are sensitive to variation in •bH value and this fact is almost the key to the metabolic processes which take place. Biologically active proteins not only bind large amounts of water, but their sensitivity to swelling with changes in •bH value is also great. The swelling of fibrous protein is a very good measure of the biological age of the animal. I use the word "biological" to differentiate between the 292