Our analysis details two specific impurities in different source materials that were relatively abundant, positively identifiable, and successfully removed by aqueous recrystallization to measure an effect on devices. We found that iodide substoichiometry (I:Pb ratio < 2) was largely due to substitution of iodide by oxygen-containing anions including hydroxide (OH -), proposed previously , as well as acetate (OAc) unambiguously identified in the lowest quality PbI2 examined here. Oxidic impurities are notoriously difficult to remove from Pb halides, but reducing their concentration has been shown to have a positive effect on perovskite devices. In contrast, the high quality PbI2 sources investigated displayed relatively low concentrations of oxidic anion impurities. One PbI2 reagent, in particular, simultaneously indicated elevated levels of potassium (K) and superstoichiometry (I:Pb > 2) suggesting KI as a plausible impurity. When oxidic anion concentrations are low, the beneficial effect of even small amounts of alkali iodide impurities like KI may become visible, helping to explain this PbI2's high material quality and device performance. Simply put, the beneficial effects of K can be easily overshadowed by other impurities. This was substantiated by purifying the as-received PbI2 sources via aqueous recrystallization which reduced both OAc and adventitious K, among other impurities, and amended stoichiometries nearer to 2. Finally, we compared devices fabricated from all ten as-received and recrystallized PbI2 sources which, in combination with the analytical characterization, provides insight regarding dominant impurities and rationalizes expected outcomes in devices. Our work demonstrates that a competition can and does exist between beneficial (e.g. KI) and detrimental (e.g. oxidic anions) impurities which influence the overall performance of the materials at the device level.