Many drugs are charged molecules and are weak bases or acids having counterions. Basic drugs may have an inorganic salt or organic acid as counterions, whereas acidic drugs may have a metal cation. Regulatory agencies like the U.S. FDA and European Pharmacopeia, require that pharmaceutical products be tested for composition of their identity, strength, quality, and purity of their active and inactive ingredients. This means that identification of the counterion is an important part of the purity determination of a drug. Here we study phosphate (PO4), a common and interesting molecule in that it has three pKa values (1.97, 6.82, and 12.5) and also has tendencies to interact with the capillary wall.