The present invention relates to a method of optical measurement of parameters of a froth flotation substance (10) involved in a froth flotation process, the froth flotation substance (10) with collector molecules, the method comprising the steps of: adding in a predetermined amount indicator molecules to the froth flotation substance (10), wherein the indicator molecules are adapted for specifically binding to the collector molecules, said binding involving a detectable change of an optical absorption spectrum of said indicator molecules; hereinafter measuring by means of measurement light said optical absorption spectrum; and hereinafter determining from the measured part of the optical absorption spectrum of said indicator molecules an amount or a concentration of residual collector molecules in the froth flotation substance (10). Moreover, the invention relates to the froth flotation substance (10) itself, to a system (1) for executing said method, and to a use of the method and the system for controlling the froth flotation process, e.g., by addition of collector molecules to the froth flotation substance (10).