In general, oxygen acid, into the acid elements of the higher valence,phosphorous acid the stronger the acid.
The oxygen-containing phosphorus acid is the opposite happens.
Phosphorous Acid large.
Phosphorous acid: H3PO3, ka == 6.12 × 10 ^ -2
Phosphoric acid: H3PO4, monopotassium phosphite ka == 7.013 × 10-3
It can be seen, diammonium phosphite more than phosphate phosphorous Ka ka
There is a conclusion: the greater the acid ka, the stronger the acid.
Phosphorous can be seen more than the strong acid phosphatase
However, like high acid, polyphosphoric acid low acid stronger than the relatively rare case.

In addition, phosphorus pentoxide I read an authoritative information: Songtian You made Inorganic Chemistry (P494) (Fourth Edition), Jilin University, Wuhan University, HFC-227ea Nankai University (Higher Education Press).
H3PO2 (hypophosphorous acid)> H3PO3 (phosphorous acid)> H3PO4 (phosphoric acid)
Oxygen-containing phosphorus acid and the central atom oxidation number (valence) HFC-134a the higher the stronger the acid does not match the law.