Naproxen has two pKa about 6 and 11. Protonated and deprotonated forms have different chemical properties. 

Thanks Dr Yurii for your answer but Naproxen has one only pka at  4.2. 

Please, explain what are the "the constant rates" and how they are measured

I studied the degradation kinetics of Naproxen by photolysis under various pH from 3 to 11. The degradation kinetics followed pseudo first order and I traced the curve of the constatnts (Kapp) in function of pH. The degradation rate increases from 3 to 6 and then a decrease was observed until pH=11. The decrease in the kapp remains unexplainable as the Naproxen is in his anionic form which is more reactive than the molecular form (pka = 4.2) Thanks

Did you use  buffers to keep pH constant? Did you measure pH before and after photolysis?

No it was just initial pH adjusted with HCL  and NaOH

If pH is not constant in the course of the reaction you can't make any conclusion on the dependence of the reaction rate on pH

i undrestand. thank you Pr. Yurii for your help

Generally, after   irradiation of  organic solution, the initial pH decreases .By products of naproxen may play an important  role in decreasing of pH .Plz chech the article:

Degradation Kinetics and By-Products of Naproxen in Aqueous Solutions by Gamma Irradiation,Environmental Engineering Science 29(6):386-391 · June 2012