# What is the Pka for potassium phosphate?

Jan 6, 2016

$p {K}_{a}$ $=$ $- {\log}_{10} {K}_{a}$

#### Explanation:

${H}_{3} P {O}_{4} + {H}_{2} O \rightarrow {H}_{2} P {O}_{4}^{-} + {H}_{3} {O}^{+}$ ${K}_{a 1} = 7.25 \times {10}^{- 3}$

${H}_{2} P {O}_{4}^{-} + {H}_{2} O \rightarrow H P {O}_{4}^{2 -} + {H}_{3} {O}^{+}$ ${K}_{a 2} = 6.31 \times {10}^{- 8}$

$H P {O}_{4}^{2 -} + {H}_{2} O \rightarrow P {O}_{4}^{3 -} + {H}_{3} {O}^{+}$ ${K}_{a 3} = 4.8 \times {10}^{- 13}$

I assume the one you want is for potassium dihydrogen phosphate, K^+""^(-)OP(=O)(OH)_2, i.e. ${H}_{2} P {O}_{4}^{2 -}$.

$p {K}_{a 2}$ $=$ $7.20$. See this link for a discussion of this acid-base behaviour.