Mobility:  The ease with which an element can move within a specific environment.  For example, Cu, Co, Ni, Hg, Ag, Au, and Mo are relatively mobile in most environments (oxidizing or reducing, and acidic to alkaline conditions.  Bi, Sb and Pb are relatively immobile in most environments.

Extrapolate: To estimate from a known set of values.

pH:   The numerical expression of the relative acidity or alkalinity of an aqueous system.  Specifically, the concentration of H+ and OH-, or the negative logarithm (to the base 10) of the hydrogen ion activity.