Question

What is the dominant form of the phosphate ion in (a) a lake at pH=5.0, and (b) seawater with pH=8.0, and what is the ration of this form to the next most abundant form? Ka1=10-2.17 , Ka2=10-7.31 , Ka2=10-12.36

Answer 1

a)

in a lake, with pH = 5.0

use the henderson hasselbach equation for the main ratios

pKa1 = 2.17 , pKA2 = 7.21 pKa3 = 12.36

clearly, the nearest point is pKA2, with 7.21 vs. 5.0

so

pH = pKa2 + log(HPO4-2/H2PO4-)

5.0 =7.21 + log(HPO4-2/H2PO4-)

HPO4-2/H2PO4- = 10^(5-7.21) = 0.006165

HPO4-2 = 0.006165*H2PO4-

Clearly, H2PO4- is the LARGEST species in equilibrium

b)

for pH = 8.0

apply same logic

pH = pKa2 + log(HPO4-2/H2PO4-)

8=7.21 + log(HPO4-2/H2PO4-)

HPO4-2/H2PO4- = 10^(8-7.21) = 6.165950

HPO4-2 = 6.165950*H2PO4-

clearly

HPO4-2 is largest species