There seems to be a missunderstanding. Diphenylmethyl pottassium is not the salt of the diphenyl methylalcohol, but the reaction product of diphenylmethane and pottassium.
Fopr preparation, use a high vacuum line.Prepare a potassium mirror, by heating (sublimating) in vacuum a small amount of potassium.
Add your solution of diphenylmethane in dry THF to the K-mirror. You should obtain a red color uponn formation of the DPM-anion after some stirring. Transfer the solution into a ampoulle through a frit to remove excess of pottassium.
The activity of the solution must be determined in separate experiments (e.g. polymerization of PMMA and comparing the expected with the true number average molar mass) .
All reactions must be carried out under rigorous exclusion of moisture and air.
Sorry, I just found one reference (in German, http://www.chemie.uni-bayreuth.de/mcii/de/forschung/html/26845th.pdf).