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708 Transactions of S.M.P.E., Vol XI, No. 32, 1927
mined either by the use of a hydrogen electrode by electromotive force measurements, or more conveniently in most cases by the use of indicators ; that is, of dyes which change color at certain definite jpH values.
I must pass by here any reference to methods of exact control of the p^ of solutions, merely noting that the pu value is only definite in solutions which are said to be "buffered." By "buffering" a solution is meant the adjustment of the chemical equilibrium in the solution such that the concentration of a certain component will be maintained nearly constant even when the solution is considerably diluted or the equilibrium disturbed in other ways. As an example, if we have a solution containing acetic acid together with sodium acetate, the hydrogen ion concentration will remain relatively constant either on dilution or on the addition of considerable amounts of alkali or acid. The sodium acetate being the salt of a weak acid with a strong base dissociates giving certain amounts of acetic acid and a certain amount of caustic alkali. Therefore, in the presence of excess of sodium acetate, the addition of alkali merely removes a certain amount of free nondissociated acetic acid, leaving the sodium acetate equilibrium nearly at the same point. It is important to note that statements concerning the relation of a given effect to the p^ value do not have full significance unless they refer to "buffered" systems. It so happens that gelatin is a substance having a considerable "buffering" action, so that we may treat gelatin with not too strong acid or alkaline solutions which are thus "buffered," and obtain a system which is in equilibrium.
The swelling of gelatin in aqueous solutions is very markedly affected by the hydrogen ion concentration. Fig. 1 gives a picture of the behavior of gelatin in solutions at different ps values. It will be seen that a definite minimum of swelling occurs at a point not far from a pi? of 5. It can be shown by methods which I cannot delay to specify now that this point of minimum swelling coincides with what is termed the iso-electric point of gelatin. This means that gelatin behaves in solutions of higher yu than 5 as though it were negatively charged, and in solutions oi pn lower than 5 as though it were positively charged. Another expression of this relation is that in solutions of pn greater than 5 gelatin tends to combine with alkalies, and in solutions of pn lower than 5 it tends to combine with acids. This ambiguous behavior is partly due to the fact that gelatin behaves in many ways Hke an amino-acid, such as glycin(NIl2CH2