It's effect of dose at constant pH, time, stirring rate.

I mean study the effect of pH at different times in the same experiment pH 5 at time 10, 15, 20 min and 100, 200, 300 RPM

At first you must chose basic stirring rate and time. Then do experiments with different doses at these basic rate and time. Then do the same experiments with different rates (time is stable) and experiments with different time (rate is stable).

It's effect of dose at constant pH, time, stirring rate.

I mean study the effect of pH at different times in the same experiment pH 5 at time 10, 15, 20 min and 100, 200, 300 RPM

Hello Ahmed,

try looking up the parameters for pH, rpm and time at a pharmacopeia (USP, Ph.EUr. or else). You will get a good impression on how parameters should be choosen.

Regards