Chapter 9: Polyprotic Acid-Base Equilibrium

Zwitterions

a neutral molecule with a postive charge localized in one position and a negative charge localized at another

Amphiprotic

can donate and accept a proton

Diprotic

diprotic acids (based) can donate (accept) more than one proton

Amino acids are amphiprotic and can form zwitterions

TRUE

Low pH: amino group protonated - H3N+CHRCOOH

 High pH: carboxylic acid deprotonated - H2NCHRCOO-

Intermediate pH (medium): exists as zwitterion - H3N+CHRCOO-

Monoprotic

only one proton is donated or accepted

Triprotic acid

formalism for diprotic acids and bases extendable to systems with > 2 protons

Fractional Composition

shows which species dominates at different pH

when pH = pKa then acid and conjugate base are equal

for diprotic/triprotic acids, three (or more) species appear on diagram

Isoionic point

the pH of pure, neutral polyprotic acid dissolved in water (no added acid/base)

• at this point, [H2L+] and [L-] are not equal

• at the isoelectric point, [H2L+]=[L-]

• important for protein separation 

Isoelectric point

the pH where the average charge of a polyprotic acid is zero