Dear Dr Atta, You can find some data in

M. Voelker and M. Hamme, Dissolution and pharmacokinetics of a novel micronized aspirin formulation, Inflammopharmacology. Aug 2012; 20(4): 225–231 (doi:10.1007/s10787-011-0099-z)

http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3398252/

 and

http://www.dissolutiontech.com/DTresour/201Art/DTFeb01_Aspirin_art.pdf

In water (25 deg C) 3.3 mg/ml (Analytical Profiles of Drug Substances)

Regards

http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3398252/

The best way to find out would likely be to prepare the buffer solutions, and obtain empirical results to the resolution with which you need. Might take a few days, but typically solubility data does not pertain to such specific conditions.