Acid Strength Depends on Molecular Structure

Many acids of biologic interest possess more than one dissociating group. The presence of adjacent negative charge hinders the release of a proton from a nearby group, raising its pKa. This is apparent from the pKa values for the three dissociating groups of phosphoric acid and citric acid (Table 2-2). The effect of adjacent charge decreases with distance. The second pKa for suc-cinic acid, which has two methylene groups between its carboxyl groups, is 5.6, whereas the second pK^ for glu-

Table 2-2. Relative strengths of selected acids of biologic significance. Tabulated values are the pKa values (-log of the dissociation constant) of selected monoprotic, diprotic, and triprotic acids.

Notice that the change in pH per milliequivalent of OH- added depends on the initial pH. The solution resists changes in pH most effectively at pH values close

Table 2-2. Relative strengths of selected acids of biologic significance. Tabulated values are the pKa values (-log of the dissociation constant) of selected monoprotic, diprotic, and triprotic acids.

Monoprotic Acids

Formic

pK

3.75

Lactic

pK

3.86

Acetic

pK

4.76

Ammonium ion

pK

9.25

Diprotic Acids

Carbonic

pK,

6.37

pK2

10.25

Succinic

pK1

4.21

pK2

5.64

Glutaric

pK!

4.34

pK2

5.41

Triprotic Acids

Phosphoric

pK1

2.15

pK2

6.82

pK3

12.38

Citric

pK1

3.08

pK2

4.74

pK3

5.40

taric acid, which has one additional methylene group, is 5.4.

Diabetes 2

Diabetes 2

Diabetes is a disease that affects the way your body uses food. Normally, your body converts sugars, starches and other foods into a form of sugar called glucose. Your body uses glucose for fuel. The cells receive the glucose through the bloodstream. They then use insulin a hormone made by the pancreas to absorb the glucose, convert it into energy, and either use it or store it for later use. Learn more...

Get My Free Ebook


Post a comment