Structure and Bonding:  Diprotic Acids and Amino Acids 

Let's consider the dissociation of diprotic acids and amino acids.  Here are the possible species produced from carbonic acid, H2CO3, an important buffer in blood.  The dissociation reactions are successive step-wise reactions.  The pKa is -logKa.  Why is Ka2 smaller than Ka1?

H2CO3   =    H+  +    HCO3      Ka1 =  4.45 x 10-7       pKa1 = 6.35

HCO3-   =   H+  +    CO3-2        Ka2 =  4.69 x 10-11      pKa2 = 10.33

 

H2CO3                                HCO3-                                    CO3-2

   

How is the speciation controlled by pH?  Click here to get an interactive Excel spreadsheet that will allow you to model the distribution of the various species.  You have seen this for monoprotic acids earlier.  This distribution diagram requires two pKa's be entered into the yellow boxes and shows an orange vertical line at a pH = 7.4, which is physiological pH.  Physiological pH is the pH found in normal blood and cellular fluids.

For the carbonic acid system, describe the distribution of species (fraction) at physiological pH.

        H2CO3  ________                HCO3-  ___________           CO3-2 _________

Can the carbonate ion occur when undissociated carbonic acid is present?

The pKa's values above are for carbonic acid in pure water.  Consider the values below which are for blood, essentially a 0.15M NaCl solution.  How do the acid dissociation constants change compared to water?

Ka1 =  1.10 x 10-6      pKa1 =  5.96                  Ka2 =  2.88 x 10-10      pKa2 =  9.54

For the carbonic acid system using the values in blood, describe the distribution of species (fraction) at physiological pH.

        H2CO3  ________          HCO3-  _________            CO3-2 _________

How did the distribution shift?  

How would carbonic acid behave in seawater, which is a 0.50 M NaCl solution?

Here are the various species of glycine.  View the structures and write their formulas and do not forget the charges.

   

___________                    ___________                    ___________

Which species of glycine above occurs at high pH?         at low pH?

Which species is "neutral" in charge?  Explain.

In amino acids remember we treat the -NH3+ as an acid, so here are the pKa's values for glycine.  Determine the distribution of species for glycine at physiological pH.

pKa1 =  2.35   for -COOH           pKa2 =  9.78     for -NH3+

H2A+  ________           HA  _________              A- _________

Most amino acids, except the ones with a third acid (or base) group (these need to be treated like triprotic acids), behave similar to glycine.  The pKa1's are between 2-3 and pKa2's are between 9-10.  What is the typical form of glycine and most amino acids at physiological pH?

The dissociation constants for acids, or bases for that matter, increase as the salt content or ionic strength increases.  This is known as the salt effect and is due to increased interactions between the ions in solution.

Data from C.G. Enke, The Art and Science of Chemical Analysis, John Wiley and Sons, Inc (2001).

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