The pKa values of oxalic acid ($\mathrm{HOOC}-\mathrm{COOH}$) in water are $\mathrm{p}{K}_{a1}=1.25$ and $\mathrm{p}{K}_{a2}=4.27$. The second dissociation is much weaker because

1. Athe second carboxyl group is less electronegative than the first
2. Boxalate ion cannot be stabilised by resonance, while the hydrogenoxalate ion can
3. CH+ must be removed from an anion, which is harder because of the negative charge already present
4. Dthe -I effect of the carboxyl group is lost after the first ionisation, so the molecule becomes a stronger electron donor to carbon