Ka and pKa for Polyprotic Acids
Acid Name    K pKa
Carbonic, H2CO 1st 4.5 x 10-7  6.35

2nd 4.7 x 10-11 10.33 
Hydrogen sulphide, H2 1st 9.5 x 10-8 7.02

2nd 1.0 x 10-19 19.0
Phosphoric acid, H3PO 1st  7.1 x 10-3  2.15

2nd  6.3 x 10-8 7.20

3rd  4.5 x 10-13  12.35 
Sulphuric acid, H2SO 1st  large

2nd 1.2 x 10-2  1.92
Telluric acid, H6TeO6 1st  2.0 x 10-8  7.7

2nd  1.0 x 10-11 11.0

3rd  3.0 x 10-15 14.5
Sulphurous acid, H2SO3 1st  1.2 x 10-2  1.92

2nd 6.6 x 10-8 7.18
Selenous acid, H2SeO3 1st  3.5 x 10-3  2.46

2nd  5.0 x 10-8  7.3
Tellurous acid, H2TeO3 1st  3.0 x 10-3  2.5

2nd  2.0 x 10-8  7.7
Ascorbic acid, H2C6H6O 1st 7.9 x 10-5 4.10

2nd  1.6 x 10-12 11.79
Citric acid, H3C6H5O7 1st  7.1 x 10-4  3.15

2nd 1.7 x 10-5 4.77

3rd  6.4 x 10-6  5.19