This paper describes a tri-colour light emitting diode (LED)-photoresistor (PR)-based in situ spectrometer with geometry that differs from the previously presented and allows for small volumes of test solution (350mul) and effective homogenisation of reagents. The emission maximums of the tri-colour LED (TC-LED) are at 470, 565 and 660nm. The basic measuring characteristics of the prototype were evaluated. The prototype was tested for determination of calcium in natural waters with the o-cresolphtalein complexon and was proved to be useful for real life applications. The in situ spectrometer with microreaction chamber enables the rapid optimisation of experimental conditions, as was demonstrated on the example of oxidation of alcohols with potassium dichromate. The drop-based experimental approach to the ruggedness test is fast, economic and gives reliable results with the minimum waste production. The prototype, connected to the computer, functions as a kinetic microreactor suitable also for following rapid changes. In spite of the manual addition of the initialising reagent, the data are lost only during the first 3s.