The buffer solution is used to control the pH to insure that it does not become too high because excessively basic solutions cou
ld cause the corresponding hydroxides of hard metal ions (such as Ca(OH)2 and Mg(OH)2) to precipitate. Using the calcium ion as a typical representative, just how high a pH do you think could be considered as "too high" for a solution with a hardness of about 250 ppm CaCO3? Ksp for Ca(OH)2 is 6.5 x 10-6.
It would depend, because the independent variable is what you are measuring, the dependent variable is what you are changing, and the controlled variable is the things that stay the same. So it would all depend on the experiment.