It doesn't matter which hydrate of sodium phosphate you use to make the buffer because the salts will be dissolved in water anyway. The only things that matter are the pH and the concentration. When you decide on the concentration (in molarity), make two solutions at that concentration: one of monobasic sodium phosphate and one of dibasic sodium phosphate. Then mix the two solutions together in the appropriate proportion to get the desired pH. Use a calibrated pH meter if you want an accurate pH, or pH indicator strips if you are content with +/- 0.5 pH units. There are also tables online that you can use to get the right proportions for any pH in the accessible range of phosphate buffer.

Dear, try to use Na2HPO4.12H2O=5.462 g/500 mL, NaH2PO4.2H2O= 1.521 g/500 mL and 500 mM NaCl dissolve these chemicals in 400 mL pure water, adjust the pH=7.0 and make volume up to 500 mL this is your binding buffer. For elution buffer add 300-500 mM imadazole in above mentioned buffer.

If you want to make Tris-HCl buffer then please consider this paper. It will help you a lot.

Thanks Adam B Shapiro and Muhammad Waheed Iqbal .

Your advice was very helpful.