Polyprotic systems

For our first example of a polyprotic case, let us look at the important amino acid glycine. It is a zwitterion molecule. 

Step 1. Initial species are HGly and H2O. Set equal the new species formed by gain and loss of protons (this is also the charge balance) 

Step 2. Substitute from the K expressions above to get an equation in terms of H, [HGly], and the K %  

Since ionic strength should be low, we try the K° values and [HGly] of 0.1 M in the solved H equation. 

[HGly] remains effectively at 0.10 M and / is below 10 M. Note that slightly more glycinate forms than glycinium and the solution is acidic Glycine is a better acid than it is a base. The old 

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We now introduce polyprotic systems—acids or bases that can donate or accept more than one proton. After we have studied diprotic systems (with two acidic or basic sites), the extension to three or more acidic sites is straightforward. Then we step back and take a qualitative look at the big picture and think about which species are dominant at any given pH. 

The treatment of diprotic acids and bases can be extended to polyprotic systems. By way of review, let s write the pertinent equilibria for a triprotic system. 

The principal species of a monoprotic or polyprotic system is found by comparing the pH with the various pKa values. For PH < pKh the fully protonated species, H A, is the predominant form. For pA) < pH < pK2, the form H , A is favored and, at each successive pK value, the next deprotonated species becomes principal. Finally, at pH values higher than the highest pK, the fully basic form (A"-) is dominant. The fractional composition of a solution is expressed by a, given in Equations 10-17 and 10-18 for a monoprotic system and Equations 10-19 through 10-21 for a diprotic system. 

Point-by-point plotting of equations (2.15) and (2.16) produces the curves for the nonionized, 2,4-DB[COOH], and ionized, 2,4-DB[COO ], species in Figure 2.8. This approach can be expanded to generate master variable diagrams of more complex polyprotic systems (Figure 2.9) such as phosphoric... 

Borkovec, M., Jonsson, B., and Koper, G.J.M., Ionization processes and proton binding in polyprotic systems Small molecules, proteins, interfaces, and polyelectrolytes, Surface Colloid Sci., 16, 99, 2001. 

Polyprotic Systems In the same fashion as equation 91 has been derived, expressions for the buffer intensity of polyprotic acid-base systems can be developed. In Table 3.8, the buffer intensity of a diprotic acid-base system is derived. A polyprotic acid can be treated the same way as a mixture of indi-... 

The four possible situations which occur in polyprotic systems may be simply described in terms of the ratio of available acidity to the total anion, C /C. Methods of approaching pH problems for all these types are given in this chapter and in Chapter 6. 

Species fractions, a s, and ratios between the members of any pair of species in equilibrium in mononuclear polyprotic systems have been shown to depend only upon H and the K values. Uses of plots of these functions have been illustrated in problem solving. The n function related to a s is useful because it can be determined experimentally from analytical concentrations and a pH measurement. It also provides a fundamentally complete and rigorous approach to pH calculations. 

Example 6. Compare the two contrasting polyprotic systems, 0.010 M triammonium citrate and triammonium phosphate (a hypothetical compound, as we shall see). At ionic strength 0.1 M at 25°, we have the following pK values ... 

In each case, the other possible forms of a polyprotic system can be ignored, provided the pK , values are not too close together. 

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