Humboldt State University ® Department of Chemistry

Robert A. Paselk Scientific Instrument Museum

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Spectronic 20

Bausch & Lomb Co.

Humboldt State College, c. 1960

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Determination of the composition of solutions via the measurement of the absorption of light at particular wavelengths. It can be used to determine what is in a solution by taking a spectrum (the pattern of how much light is absorbed at each particular wavelength vs. wavelength). More commonly it is used to determine the concentration of substances in solution by comparing the amount of light absorbed at a particular wavelength as compared to a standard of known concentration.
The Spectronic 20 was introduced by Bausch & Lomb in 1954.1 It was the first low cost grating based spectrophotometer, providing a single-beam 20 nm bandwidth in the visible light region at a very reasonable cost. It quickly became a standard of the industry and probably the most widely used spectrophotometer in the world. "[It] is eminently familiar to almost anyone who has ever entered an analytical chemistry laboratory, regardless of its size, purpose, or location." 1 A very similar descendant, the Spectronic 20+, is still in production by Spectronic Instruments, who, as Milton-Roy, purchased the Analytical Division of Bausch & Lomb in 1985.
This particular instrument was originally used by the Student Health Service at HSU. It was later transferred to the Chemistry Department as surplus equipment. The Chemistry Department purchased later model Spectronic 20's with larger meters in 1966, and began using them in its instructional programs.


The instrument case is of die-cast metal with a cream-colored enamel finish. It is 14 1/2" x 10 3/4" x 7" high. The trim and control knobs are in a dark green plastic. The 3 3/4" meter has a mirrored scale graduated from 0-100% by 1% and from 0-infinite Optical Density (logarithmic scale in red). The wavelength scale, stamped on a brushed aluminum disk visible through a window below the meter, is graduated from 350-1000 nm by 5 nm with numbered major divisions every 25 nm. There is a yellow decal on the column: STATE OF CALIFORNIA / HUMBOLDT COLLEGE / 19987. The serial number (HB 2.072) and model number (33-29-40) are given on an aluminum tag on the rear of the instrument. With the exception of some staining on the case, the instrument is in very good, operating condition.
The Chemistry department has a number of operating/refernce manuals, a scan of the 1959 edition is linked.


This instrument is shown as item 29302 on pg 384 of Central Scientific Company's Catalog J-300 Instruments for the Laboratory Sciences (1960). 1Jarnutowski, Robert, John R. Ferraro, and David C. Lankin. "Fifty Years of Commercial Instrumentation in Spectroscopy, Part II: Landmark Instruments in UV/Vis." Spectroscopy 7 (September 1992) p 24.

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HSTC (1921-34)
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HSC (1954-1973)
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HSC (1935-1953)

HSC 1956-72 Instrument Collection

© R. Paselk
Last modified 13 August 2010