Try the sigma alkaline phosphatase yellow pNPP liquid substrate system for elisa. Cat No. P7998-100ml. The protocol I do is simple. incubate SaOS2 with treatment and use RA as positive control. if using 96well plate, wash cells twice with PBS, add 50ul of the sigma solution directly on the cells, incubate at 37 in incubator for 45 minutes, read with spectrophotometer