A technique has been devised for Ihe location of fluid of a predetermined concentration. It is based on (he colour change of a pH sensitive dye which is present in both a host fluid and an introduced pollutant. The acid and base levels in both liquids are controlled to produce the colour change at a given dilution of the pollutant. The technique is demonstrated in the mixing produced by a fountain in a container. A turbulent jet of fresh water is directed downward into a tank of salt water, ll descends a short distance, stops and Hows upward surrounding the downward Mow like a drinking fountain. When the llow returns to the top of the container it spreads out to form a mixed layer, the bottom of which advances downward as a front. The continuously changing density flowing from the mixed layer produces the density profile which expands slowly to till the container. The pi I technique is used to track the location of a surface of specified concentration and density.