Comparative colorimeter, 2000

Museum Boerhaave, Leiden, Netherlands. Comparative colorimeter, c. 1885, Hamburg, A. Kruss. In this type of colorimeter, a solution of known concentration is placed in the left-hand compartment; the unknown solution in the right. The optics (at the top) produce a split circle image. As the flat-bottom, closed-end, glass cylinder is lowered into the left-hand sample compartment, light passes through a successively shorter length of solution. Eventually, the color intensity of the two half-circles match, at which point the product of concentration (C) times path length (L) is equal for each solution, standard (s) and unknown (u), that is, C(u) L(u) = C(s) L(s). As both path lengths are known as well as the concentration of the standard, the concentration of the unknown, C(u), can be calculated.