# Decrease of Bicarbonate to compensate Respiratory alkalosis

I am studying this claim

Compensatory mechanism in respiratory alkalosis is decrease of SB i.e. bicarbonate ions ($\ce{HCO3^-}$).

There are reactions

$\ce{HCO3- \rightarrow H+ + CO3^{2-}}$ (conjugate base)

and

$\ce{HCO3- + H+ \rightarrow H2CO3}$ (conjugate acid)

but since the $pK_a$ for the second reaction is greater

• decrease in $\ce{HCO3-}$ yields more acids

so alleviates respiratory alkalosis.

To claim such a thing, I think you must know that the $pK_a$ for the reaction is greater, so change in bicarbonate ions' content results in bigger change in the second reaction.

How does the decrease of bicarbonate ions lead to decreased respiratory alkalosis?

Bicarbonate ion is amphiprotic, as you've recognized. It can react as both a Brønsted base and as a Brønsted acid in water solution:

As an acid:

$K_a = 4.8 * 10^{-11}$

$\ce{HOCO_2^- + H_2O <=> OH_3^+ + CO_3^{2-}}$

As a base:

$K_b = 2.1 * 10^{-4}$

$\ce{HOCO_2^- + H_2O <=> HO^- + H_2CO_3}$ (not really true carbonic acid).

Through K value inspection, we can see that the bicarbonate ion preferentially reacts as a base in water solution. The magnitude of this difference is massive; $K_b$ is more than a million times greater than $K_a$.

So by decreasing the concentration of bicarbonate ion, we can combat respiratory alkalosis.

• Thank you for your fast answer! I had bad time in recalling this today. – Léo Léopold Hertz 준영 May 12 '14 at 19:44
• It is my pleasure! – Dissenter May 12 '14 at 21:46
• Hmm. I think this situation changes in different pH. How does bicarbonate work in pH 7.4, 7.0 and 7.8? Still as a base? I think as a base at 7.8. But as a acid at 7.8 probably. Probably as a base at 7.4. – Léo Léopold Hertz 준영 May 13 '14 at 14:55
• Why do you say that? How can you support your claims? – Dissenter May 13 '14 at 16:51
• They are hypothesis. I just recall that bicarbonate work differently in different pH. – Léo Léopold Hertz 준영 May 13 '14 at 20:17