Polyprotic acids buffer calculations

D. W. King and D. R. Kester, A General Approach for Calculating Polyprotic Acid Specification and Buffer Capacity, J. Chem. Ed., 67, 932 (1990). 

Another problem of interest is the calculation of the concentration of the divalent ion (2 ) in a solution of a weak polyprotic acid, when the total [H+] is essentially due to a stronger acid present in the solution or to a buffer. In such a case, the concentration of the divalent ion can best be calculated by multiplying the expressions for and K2. Again, illustrating with H2S, we find the following ... 

The common amino acids are simply weak polyprotic acids. Calculations of pH, buffer preparation, and capacity, and so on, are done exacdy as shown in the preceding sections. Neutral amino acids (e.g., glycine, alanine, threonine) are treated as diprotic acids (Table l-l). Acidic amino acids (e.g., aspanic. acid, glutamic acid) and basic amino acids (e.g., lysine, histidine, arginine) are treated as triprotic acids, exactly as shown earlier for phosphoric acid. 

Titration and buffer calculations involving peptides are done exactly as shown earlier for polyprotic acids. We must remember that the amino acid carboxyl groups used in forming the peptide bond are no longer available for titration. The structure of the fully protonated glutamylserylglutamylvaline is shown below. It is assumed that the ct-COOH group retains a pKa of — 2.5... 

Where is the total concentration of acid species and A, and K2 are the first and second stepwise dissociation constants of the acids. This equation can be used to compute the buffer index of a polyprotic acid as long as successive dissociation constants differ by at least 20 times (this assures a calculation error of 5% or less). In other words, for a diprotic acid K2fK should be less than 0.05 (cf. Butler 1964). Thus, for example, Eq. (5.114) may be used to compute the buffer index due to species of carbonic acid, for which A = 10 and K2 = 10" °, or/ for species of silicic acid, for which a , = lO- and K, = 10... 

The acidity or basicity of a solution is frequently an important factor in chemical reactions. The use of buffers of a given pH to maintain the solution pH at a desired level is very important. In addition, fundamental acid-base equihbria are important in understanding acid-base titrations and the effects of acids on chemical species and reactions, for example, the effects of complexation or precipitation. In Chapter 6, we described the fundamental concept of equilibrium constants. In this chapter, we consider in more detail various acid-base equilibrium calculations, including weak acids and bases, hydrolysis, of salts of weak acids and bases, buffers, polyprotic acids and their salts, and physiological buffers. Acid-base theories and the basic pH concept are reviewed first. 

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