Socius reagents consisting of uranyl acetate and certain divalent metal acetates in dilute acetic acid solution have come into extensive use for the detection and determination of sodium. Unfortunately, all such reagents now in use are more or less sensitive toward lithium, so that they are not satisfactory for the detection or determination of sodium in the presence of appreciable quantities of lithium. An indication was found, however, in an investigation by Caley and Baker (1) that a sodium reagent containing uranyl acetate and cupric acetate is less sensitive toward lithium than other reagents of this general type. The value of such a reagent for the detection and determination of sodium in the presence of lithium was the principal subject of the present investigation.