Humboldt State University ® Department of Chemistry
Refractometry has been a major tool in the chemical laboratory to determine concentrations of solutions and as an aid in identifying unknown substances since the late nineteenth century. In principle every substance has a unique refractive index, n, which could be used in its identification. However, in reality refractive index is of limited value since the values of n for millions of organic compounds are in the range of 1.25 - 1.8, and no instrument has the ability to parse this range to this degree.1
The most common and universal refractometer for laboratory use is the Abbe refractometer and its variations. The Abbe refractometer provides a quick and easy means for determining refractive index and dispersion for liquids and solids. It is used in the examination of organic compounds (oils, solvents, etc.), solutions, food products, and serum protein concentration. The refractive index is measured by aligning the cross hairs in the telescope with the line of total reflection (seen as the edge between light and dark fields in the telescope). This line is moved by rotating the prism assembly with the alidade. Reading at constant temperature is important, thus the prisms are often enclosed in a water jacket which may be connected to a constant temperature bath.
The Carl Zeiss company was the originator and home of the Abbe refractometer for much of its history. Ernst Abbe published his Neue Apparate.... (1874)2 in which he discussed the theory and described instruments he had developed for the measurement of refractive index using prisms and by total reflection. It is here that he first describes the Abbe refractometer he invented in 18693 for determining the refractive index of fluids (n=1.3-1.7). This initial instrument includes Amici prisms and is essentially the same as a modern Abbe refractometer, though without temperature jacketing. Initially, Carl Zeiss produced this instrument for internal use, and for special clients. It wasn't until after 18814 that the Abbe refractometer was offered in Zeiss microscope catalogs, though without illustrations. Carl Zeiss published its first instrument catalog in 1893, a result of the formation of a new optical instruments department under the direction of Dr. C. Pulfrich.5 The Abbe refractometer features prominently in this catalog, both with and without water jacketed prisms, and as the Wolney Butter refractometer.
Up through the destruction of the Zeiss works in WW II Zeiss continued to produce Abbe refractometers recognizably the same as the initial instrument. The timeline at left lists some of the noticeable changes introduced by Zeiss over this 80 year period. Each of the entries on the time-line is also a link to an image of a representative instrument. Although some of the changes are cosmetic, most make the instrument easier and more convenient to use. The earliest instruments followed Abbe's original design as seen in the image of an 1880 instrument held at the University of Nebraska. By 1890, as observed in an instrument held at the Smithsonian Museum of American History, the scale has been placed at an angle and a magnifier added. In 1892 a new specialized Abbe refractometer, the Wolney Butter refractometer was added to the Zeiss line. The limited range (n = 1.418-1.492) of this instrument enabled an enclosed optical scale (a reticle) and fixed prism to substitute for the rotating prism/alidade and engraved scale. By 1893, as seen in an instrument in this exhibit, Zeiss had replaced the lacquer finish of earlier instruments with more chemically resistant nickel plating. Around 1899 a slightly heavier stand was introduced, the post was reversed in orientation, and a larger square mirror was added to provide better illumination. Around 1911 the scale was reoriented for greater convenience, and the stand was redesigned, using a heavy, round, cast iron base with a depression to catch small sample spills. The base, post, sector etc. were also now finished in japanned enamel, with just the telescope assembly and select furnishings nickel plated. This same year another specialized Abbe refractometer, the Sugar refractometer was introduced to satisfy the needs of the sugar industry. Like the Butter refractometer a limited range allows the use of an internal optical scale (a section of a cylinder), but the greater range requires limited motion of the telescope. Zeiss introduced the last redesign of the classic Abbe in 1926, adding a rack and pinion fine motion to the alidade, and placing the mirror on a lever pivoting on the common axis of the prism housing to facilitate improved illumination. Slight cosmetic changes occurred to this instrument over the next 20 years, such as more or less rounding in the castings, differences in the fine control knob knurling, etc., but otherwise it continued to be sold through WWII. In 1926 Zeiss also introduced an innovative new instrument to replace the sugar refractometer (n=1.33-1.54).
After WWII with the split of Germany into east and west, Zeiss was also split. West German Zeiss was left without the records or plant of the Zeiss works (Jena was in East Germany) and so decided to design new instruments "from scratch," initially, in the case of refractometers from 1950-53, operating as Zeiss-Opton.6 In refractometry the result was an innovative and elegant new instrument where the reading and readout were both made through a single eyepiece, making a very convenient instrument. Zeiss ceased making refractometers in 1990,7 but their "new" design continues in manufacture by companies in other countries such as Japan.
Reparations at the end of WW I resulted in Carl Zeiss having to provide plans, rights etc. for refractometers to British and American instrument companies. Adam Hilger in England and Bausch & Lomb in the United States introduced instruments almost immediately, both with obvious relationships to their Zeiss predecessors. The Hilger instrument has an appearance very much like the 1899 Zeiss instrument, though the scale is mounted more like the 1911 Zeiss. The B&L instrument more closely mimics the 1911 Zeiss design. Another American firm, Spencer Lens Co. introduced a somewhat more innovative version of the refractometer a few years later, in 1920. The Spencer instrument had rack & pinion fine control of the alidade, a slide mounting for the mirror to give greater light control (Zeiss introduced similar features in their 1926 design), and a tripod (later triangular) base. AO Spencer later dropped the rack & pinion in favor of a clamp and slow motion screw for fine control.
A third American firm introduced a refractometer shortly after WW I, but followed a different, independent path. Warren Valentine apparently wanted a Zeiss refractometer in 1917, but due to the war was unable to obtain one. He thus studied the Zeiss instrument with the purpose of reproducing it. The lack of available skilled labor and optics delayed completion of his project until 1919. The process inspired him to undertake redesigning the Abbe to make it more stable and accurate, resulting in the creation of Valentine's Precision refractometer and Improved Precision refractometer (treated below with high precision instruments).
Of the companies noted above, Bausch & Lomb holds a special place for its on-going efforts to refine and improve the Abbe refractometer, developing a high precision instrument (discussed below under High Precision...) and evolving an elegant, convenient, and popular instrument, the Abbe AL. The initial B&L refractometer remained nearly unchanged for over twenty years with the exception of the addition of a slow motion fine adjustment screw in the mid 1920's. Then around 1949 B&L introduced the Abbe 56. The Abbe 56 is basically a classic Abbe enclosed in a heavy cast aluminum case, and with a scale on a glass arc on the alidade replacing the traditional engraved metal scale and alidade. The somewhat clunky Abbe 56 was soon replaced, in 1956, by the more elegant Abbe 3L. The AL is a much more innovative instrument: the prism is now held fixed with its surface horizontal and directly in front of the operator; a complex optical system enables the index interface and the readout to be seen through a single eyepiece with the push of a button, and fine and course controls allow rapid and precise setting of the interface. The Abbe 3L is still sold today with little change after nearly 50 years of production.
A history of the Abbe refractometer is available on-line: The Evolution of the Abbe Refractometer.
The standard Abbe-type refractometer was not able to fulfill all of the needs for precision measurements, so a number of attempts were made to improve the Abbe design. The most well known, the Dipping or Immersion refractometer by Zeiss (1899/1902, various after WW I: a very narrow range Abbe-type instrument with no illuminating prism), the Improved Precision Abbe by Valentine (1919), and the Precision Abbe by Bausch & Lomb (1938) are represented in the exhibit. Each of these instruments is designed to give readings good to 2-3 units in the fifth decimal place of refractive index, as opposed to the one unit in the fourth place typical of the standard Abbe.
Two non-Abbe type instruments important to chemical studies are also exhibited: the Pulfrich refractometer (Zeiss 1895, Hilger 1916) and a Brice-Phoenix Differential refractometer. The Pulfrich refractometer is actually only as accurate as a standard Abbe (one unit in the fourth decimal place) in absolute determinations. However, it can give measurements to 2-3 units in the fifth-place for differential measurements and for dispersion determinations. The Brice-Phoenix Differential refractometer is of narrower range, and can give two units in the sixth-place, and is less sensitive to temperature since the sample and reference are compared simultaneously in the same chamber.
Finally, the exhibit includes two refractometer types developed for low precision routine work: the hand refractometer and the Fisher refractometer.
Some original descriptions of the features and use of the refractometer are provided below:1
Additional literature in the form of instruction manuals and descriptive leaflets for specific instrument may be accessed via individual instrument pages.
1 Detailed treatments of refractometry and refractometers are given in: Tilton, Leroy W. and John K. Taylor. "Refractive Index Measurement." in Physical Methods in Chemical Analysis Vol. 1, 2nd ed. ; Walter G. Berl, editor, Academic Press, New York.(1961) pp 41162, and Bauer, N., K. Fajans, and S. Z. Lewin. "Refractometry." in Physical Methods of Organic Chemistry Vol. I, Pt II, 3rd ed.; Arnold Weissberger, editor, Interscience publishers, Inc., New York. (1960) pp 1139-1281. Both articles provide details on the theory of refractometry and the instruments used. The article by Bauer et. al. also gives an extensive discussion of the types of physical and molecular information obtainable by refractometry.
2 Abbe, E. Neue Apparate zur Bestimmung des Brechungs - und Zerstreuungsvermögens fester und flüssiger Körper. Mauke's Verlag, Jena (1874) Taffel: Fig. 5-7.
3 Wittig, Joachim. Ernst Abbe. BSB B. G. Teubner Verlagsgesellschaft, Leipzig (1989) p 60.
4 Personal communication (1998), Dr. Helga Beez, Carl Zeiss Optical Museum, Jena.
5 This catalog is now available on-line from the
Smithsonian Institution libraries at (8/24/2010):
6 According to the Carl Zeiss company history, the West German Zeiss company operated as Zeiss-Opton beginning in October 1946 and was taken over by the reformed Carl Zeiss in October 1953 (http://www.zeiss.com/C12567A100537AB9/Contents-Frame/63125BAC5434D55DC1256DB900446D61) downloaded 8/24/2010.
7 Personal communication (2003), Dr. Wolfgang Wimmer, Archivar, Carl Zeiss Jena GMBH.