The steady state values for the X-rays-on part of the cycle, given by a + b -c, and the steady state values for the X-rays-off part of the cycle, given by g, are larger for pH 4.5 samples than for those at pH 3.0. This represents the expected enhancement of bound Nd 3+ over H + to DHDP at higher pH. This is consistent with the measured effective surface pK (= 1.9) of trivalent lanthanum binding to a monolayer of DHDP and known trends in liquid-liquid extraction of lanthanides with a similar extractant, bis(2-ethylhexyl) phosphoric acid (HDEHP). dependence in which the equilibrium is shifted to the extracted complex at higher pH. The DTPA and the extractant compete generally between pH 2 and 4 but the distribution of Nd begins to increase past pH 4.5, indicating greater binding to the extractant at higher pH. 30