The final step in the partitioning cycle is the uranium stripping operation (Fig. 6.2, U strip contactor) to recover the U from the organic phase prior to solvent cleanup and recycle. At this point in the process, uranium is the predominant metal present in the organic phase. Because of the strong affinity of TBP for the uranyl ion, U back extraction is somewhat more difficult than for many of the other extracted metals. The uranium stripping operation is universal and relatively straightforward in PUREX processing: the metal laden organic is contacted with a low-acidity (0.01 M HNO3) aqueous phase, at elevated temperature (50 °C). The aqueous-to-organic phase ratio (O/A) is slightly higher than unity to help maintain a large concentration profile between phases.