Here is one case: I want to perform X + Y in the presence of Z catalyst in a 50 mL milling jar with 8 12 mm milling balls and investigate whether the the amount of catalyst affect the yield of the product. So, I change the amount of catalyst from 1 molar equivalent to 3 molar equivalent as compared to X. However, this will change the total volume of powder as well. And as we all know, when the volume changes, the kinetic energy delivered from the ball to the material will not longer be the same. Less energy will be delivered to the powder and the kinetics of the reaction will be affected by this.
I've read that the milling load in a mechanochemical reaction is defined as the total powder mass divided by the void volume in the jar (jar total volume - volume of balls)
In the case I mentioned before, should I (1) keep the amount of everything the same and increase the amount of the catalyst as what it should be, basically ignoring the effect of volume and just do the experiment or (2) Readjusting the mass of all samples to keep the total mass the same as before to maintain the same milling load as before (as a result, it will decrease the mass of the reactant) ???
One of the most important parameters in any mechanochemical reaction (ball milling) is the ball to powder ration (BPR). What I personally do and I find to give consistent results is keeping the powder volume constant (which means if I have to change the ration of 2 or 3 components I readjust it in order to keep the same BPR).
In case the amount of your catalyst is negligible compared to the total volume (mass) of the powder you can ignore it.