Abstract: An apparatus according to which a sample and a reagent are mixed together at a given temperature in a colorimetric cell to enable a property of the sample to be determined in an instrument such as a photoelectric colorimeter or a spectrophotometer. The sample and reagent are initially situated in a pair of containers one of which can be enclosed entirely within the other with the inner container extending above the level of the contents in the outer container. A colorimetric cell is connected in a fluid-tight manner at its open end to an upper open top of the outer container, and then after the entire assembly and the contents of the containers are brought to a given temperature, this assembly is inverted to discharge the contents of both containers into the colorimetric cell.