I want to test the effect of ph level on enzyme activity?

how can i go about doing this (changing the ph level) but still have just one control variable.I want to test the effect of ph level on enzyme activity?
Use three test tubes, put equal amout of enzymes in all and put the substance they digest in both (equal amounts of course).



take a test tube(A) and put 2cm3 of Hydrochloric acid.

Take test tube (B) and put 2cm3 of Pure water(pH7).

Take test tube (C) and put 2cm3 of Sodium Hydroxide.



shake them all a little just as you start the test.



Leave them for half an hour at their optimum temperature and other factors.Make sure all factors among them are equal.



After that, Test for the substance you put in (e.g test for starch using iodine if you put starch in).



The one with the most negative result is the optimum pH.