Abstract: A pump colorimetric analyzer is described including: (a) a sample cell through which a light beam is transmitted, (b) a light source, (c) a photodetector, and (d) a piston which reciprocates between first and second positions in the cell. The piston includes a sample chamber which permits the light beam to be transmitted therethrough when the piston is in its second position. When the piston is in its first position the light beam is transmitted through the sample cell. The novel analyzer uses a single light source, a single sample cell and a single photodetector.