pH VALUE AND ITS IMPLICATIONS simple "years old" by which people are normally defined. It will be well appreciated that amongst men and women and all animals there is a marked difference in their youthfulness, which is a measure of their biological age. In fact, it could quite easily be said that the biological age of a person can be measured by the extent to which the tissues bind water, and this reflects itself by the extent of the swelling of the tissues when in equilibrium with aqueous solutions of different pH values. It must not be believed that pH value is the only arbiter of the amount of swelling of a protein. A great deal depends on the anion of the acid or the cation of the alkali which is used in order to create the particular pH value. For example, collagen fibres of skin immersed in a solution of hydro- chloric acid of pH 2.4 will swell significantly more than similar fibres placed in a solution of sulphuric acid at pH 2.4. This is due to the valency of the anion which was shown clearly more than 35 years ago by Loeb. In a similar way in the alkaline ranges of pH value, a solution of a divalent base, as, for example, barium hydroxide, will only produce about half the swelling of a col!agen fibre produced by a mono-valent base of the same pH value. Neither will ammonia solutions produce the same sort of swelling at equivalent pH values to those produced by potassium or sodium. But then ammonium hydroxide has characters peculiar to itself. It is because of these peculiarities that ammonium compounds, as, for example, thioglycollates, are used in cold waving solutions. It must not, however, be overlooked that the presence of salts can bring about remarkable changes in the ability of the hydrogen ion concentration to deform protein fibres. This effect is not due to any buffering action. By way of example, it might be mentioned that if a collagen fibre be swollen by dilute hydrochloric acid, the imbibition of water can be completely repressed by the addition of sodium chloride. In the presence of a strong acid and 2 molar sodium chloride the protein will be dehydrated. Such a process has for many decades been employed for preserving unhaired skins, notably sheep skins, the dehaired pelt being soaked in a 1 per cent solution of sulphuric acid in the presence of about 10 per cent of sodium chloride. The pelts in this stage are dehydrated and, provided no water is allowed to come in contact with them to dilute the concentration of sodium chloride, they can be pre- served for long periods of time without any ill-effect. In a similar sort of way, the concentration of salts has to be very carefully controlled in the tanning of leather, as a lack of salts may produce a harsh and unsatisfactory leather, whereas too high concentration of salts will produce too soft a leather. The word "salt" here does not mean only sodium chloride, but salts of other acids and bases, some of which may be organic. The effect of these salts cannot be determined by the pH value alone. In this connection it is also interesting to note that if collagen fibres be treated with solutions of increasing acidity, there is little or no change in 293

JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS the swelling until the pH value is of the order of 4.0. In the particular case of hydrochloric acid it is at this pH value that one can distinguish a faint acid flavour. At pH values of 4-2 and above, the solutions have no acid tang. A solution more acid than pH 4.0, but containing a small quantity of sodium chloride, fails to produce an acid taste. It is the pH value coupled with the presence of the various salts in fruit juices which bring about the variations in sourness which have been noted in different fruits by a number of observers. The concentration of ions derived from the acids and bases shows itself in the swelling of proteins quite apart from the buffering effect of these ions in suppressing the ionisation of the acids or bases. The effects of salts, whether or not they have a common ion with the acid or alkali, are quite pronounced and it shows how careful one must be when buffer solutions are employed. Buffer solutions are very useful when the pH value has to be stabilised. For example, if, to a 0.1N solution of acetic acid, sodium acetate be added so that the concentration of the sodium salt is 0.05N, the pH value will increase from 2.8 to 4.4. This is because the added acetate ions have reduced the hydrogen ion concentration in order to make the product of the con- centration of these two ions constant. A similar result would be produced if the acid had been, say, half neutralised with caustic soda. At this point small additions of acid or alkali may be made without sensibly altering the pH value. Of perhaps still further importance, however, is that such a mixture can be diluted with water without a significant change in pH value. This is because the ratio of non-ionised acid to acid ions is the dominant feature and not the absolute qualities. This fact leads to an important differentiation. A solution of strong acid or base on ten-fold dilution will alter in pH value by 0.9 to 1.0 unit. A weak acid or base on ten-fold dilution will alter in pH value by approxi- mately 0.5 unit. A buffer solution on ten-fold dilution will alter in pH value by less than 0.2 unit. It is often desirable to appreciate quickly the arithmetical equivalent of differences in pH value. For example, consider the difference between pH 5.0 and pH 5.3. The difference 0.3 is the logarithm of 2, so that the higher pH value indicates a solution of half the hydrogen ion concentration of the other. Similarly, a difference of 0-5 in the pH value is approximately equival- ent to a three-fold change in hydrogen ion concentration. A change of 0.6 is indicative of a four-fold increase or decrease in the hydrogen ion con- centration, while a difference of 0.7 signifies a five-fold change. An alteration in pH value of 0.9 is, of course, three times as great as 0.3, which means an increase in concentration of 2 x 2 x 2 times, or eight-fold. With these features in mind one is able to make many valuable implica- tions with regard to pH values. Whereas a simple pH value tells only of the 294