Weigh out exactly the amount of acid crystals your experiment requires using an electronic balance. An acid's pH--and thus the extent to which adding it to another reactant can alter the reaction's pH--is a function of its molarity, or the number of moles (a figure chemists use to measure large numbers of molecules) per liter of solution. Adding too many or too few acid crystals will almost certainly render your results inaccurate (see Tips). Place your acid crystals in a beaker.
Add distilled water to a measuring cylinder until it reaches precisely the level your experiment requires. As is the case with the acid crystals, adding the wrong quantity of water--remember, molarity measures moles per liter--can hamper your reaction's accuracy (see Tips).
Add a small amount of the water to the beaker and stir the acid around until it is fully dissolved. Add the rest of the water to this fully-mixed solution, which will limit the amount of further mixing, and, therefore, the possibility that any will spill over the sides of the beaker and affect the acid's molarity.