Technikatörténeti szemle 19. (1992)

KÖNYVISMERTETÉS - Papers of the First „MINERALKONTOR” International Conference on the History of Chemistry and Chemical Industry (Veszprém, 12-16 August, 1991)

(Figure 4). This roulette wheel improved the comparison by allowing the use of a single eyepiece in which the color of sample and standard were displayed side by side in a single visual field. Rather than preparing and maintaining a series of standard solutions it is possible to use a single standard solution in what is called the ..balancing method." The sample solution is placed in a flat-bottomed graduated tube and the standard solution is run into an identical tube until the color intensities of the two are identical when viewed vertically through the length of the columns of liquids. When thus ..balanced" the concentrations of the two solutions are inversely proportional to their heights. The best known and most widely used type of comparator based on a depthbalancing method was originally designed by the French optician Jules Duboscq in 1854, and modifications based on this principle were still in use in the 1930s (Figure 5). Rather than changing the depth on the standard solution until its color matched that of the sample, illuminated solutions of equal volume are viewed through an eyepiece while the depth of view is changed by raising or lowering one of the solutions until the color intensity matches. Color comparators found wide application in such diverse areas as paints and oils, sugars, sewage, milk, and blood. At best, however, an absolute accuracy of plus or minus two percent was all that could be achieved, and frequently it was only five percent. There were the obvious sources of error, such as eye strain and the physiological perception of color by the operator, compounded by the optical errors, varied light sources, and temperature variations. In 1935 John H. Yoe developed a photoelectric colorimeter in which the human eye was replaced by a photovoltaic cell, and the light source was a six volt automobile headlamp. The light was still passed lengthwise through the vertical tube, and the output of the photovoltaic cell was read by a microammeter. The ratio of the concentration of sample and standard is inversely proportional to the ammeter readings. The Yoe instrument was on the leading edge of a new trend by chemists to include pohotoelectric tubes in optical devices, although it was certainly not new to physicists. But regardless of how the color comparison was done, colorimeters gave no information about the relationship of light absorption and wavelength. Visual spectrophotometry: Using the human eye to measure spectra In 1873 the German physician Karl von Vierordt was the first person to describe an apparatus for determining an absorption spectrum, and suggested the possibility of quantitative analysis through absorption spectroscopy. Unlike colorimeters, spectrophotometers measure transmission of light by a sample at various wavelengths. A method of visual detection was the obvious extension of the colorimeter. The components of such a spectrophotometer are: — a suitable light source — a means of separating the light in two parallel beams returning the split beams into the same field for viewing — a means of changing the intensity ratio of the two beams — suitable absorption cells — isolation of a narrow spectral band in both beams for visual comparison.