Ionic EquilibriaNandita G. Das, PhD, RPhLearning Objectives:Review pH, pKa and the Henderson Hasselbalch equationPredict resultant pH (acidic or basic) of solutions when pharmaceutical salts are added to waterDefine buffers and buffering actionUnderstand buffer capacity and appropriate application of the same in pharmacy practiceUnderstand the buffering action in our blood systemLearn about some commonly used buffer systems in pharmacy practiceIonizationrefers to the complete separation of the ions of a crystal lattice when a salt is dissolved. The ions in solution may be associated by inter-ionic attraction and may not exist as individual ions; the separation of these ions in solution is known as dissociation.Sorensen's pH scaleSorensen established the term pH, which was originally written as pH+, to represent the hydrogen ion potential, and he defined it as the common logarithm of the reciprocal of the hydrogen ion concentration:1pH=log[H+]and since the logarithm of 1 is zero,pH=- log [H+]pH0 1 2 3 4 5 6 78 9 10 11 12 13 14Acidic(Neutral)BasicThe “common ion effect” and the Henderson and Hasselbalch equationHenderson and Hasselbalch derived an equation by which the pH of a buffer solution consisting of a weak acid and its salt may be calculated. This is based on the consideration of the effect thatthe common ion of the salt has on the dissociation of the weak acid. [salt]
pH=pKa+ log [acid]A comparable relation may be derived for buffers composed of weak bases and the corresponding salt of a strong acid,[base]pH=pKw- pKa+ log [salt]Determination of pKapKa values are generally determined by titration.pH of an aqueous salt solutionWhen a strong acid and a strong base react to form a salt, it is completely ionized. When the salt is dissolved it dissociates into anions and cations, and as no change occurs in hydrogen or hydroxyl ions, the solution is neutral.