The Hellige Duboscq Colorimeter has a black bakelite cover shaped like a microscope. It consists of two flat-bottomed tubes* containing smaller tubes*. The bigger tubes are placed on wedges that can be adjusted (up and down) by two knobs on each side of the cover. Two reading-meters, and their calibrating screws, can be found just in front of the wedges. The unit is in mm and may give 0.1 mm uncertainty at maximum 50 mm. The smaller tubes are attached to the ceiling of the inner space, just below Fresnel rhomb mirrors. A lamp*, situated in isolated space under the tubes, is used as a light source. On the top, there is a telescope where observer can see two half-images. There is also a filter holder on the back and a front cover* to prevent other light disturbing the measurement.
The Duboscq calorimeter was invented by Jules Duboscq in 1870. The basic principle of Duboscq colorimeter is based on Beer’s Law2, which is expressed as: A = ebc, where A is the absorbance, e is the molar absorptivity of a solution, b is the path length, and c is the molar concentration. Therefore we can compare path length and concentration of two solutions. The Duboscq colorimeter is used to determine concentration of an unknown coloured solution (sample) by comparing its visual appearances (hue, purity, and brightness1) with a standard solution.
The simple procedure in measuring a sample concentration is as follows. The sample and the standard are placed in two bigger tubes respectively. The smaller tubes inserted into the bigger tubes and they are attached to the inner ceiling. A calibration may be needed by adjusting the two screws (for sample and standard solutions). Observer can see, with one eye, images of both solutions using telescope, where the image of each solution is half visualised, so one can see two half-images. Both knobs are adjusted so that observer can make the two half-images are identical. Therefore the concentration of the sample is calculated by1: b1c1 = b2c2, where b is the distance between underside of the bottom of the smaller tube and the inner of the bottom of the bigger tube as can be seen on the reading-meters directly, and the indexes represent sample and standard solution.
This simple technical tool was widely used by scientists and engineers, before digitalized colorimeter was introduced. Many ancient spectroscopy equipments were developed based on Duboscq colorimeter. One may modify Duboscq colorimeter to become pH-meter2, nephelometer3, and hemoglobinometer4. In the later used, some scientist used a filter*, placed on the back of this colorimeter, to make more precise images, as a filter only pass through a certain wavelength5.
[1]. R. Bud, D.J. Warner, Instruments of science : an historical encyclopedia, New York : Science Museum, London, and National Museum of American History, Smithsonian Institution, in association with Garland Pub., 1998, pp.123-125.
[2]. J.T. Stock, J. Chem. Edu. 71(11), 1994, pp.967-970.
[3]. J. Grant, J.H. Booth, J. Sci. Instrum. 10 106-110.
[4]. L.E. Locke, 50 Years ‘In The Service of Science, Available online : http://www.labasia.net/pdf/las/2003/11/las200311article-1.pdf
[5]. J.M. Vaughan, M.F. Saifi, Journal of Pathology and Bacteriology 49(1) 1939 pp.69-82.
* Missing parts.
