Refractometers are instruments that quantify and identify chemicals and materials based on their refractive index (RI). A unique property related to the speed of light as it passes through a substance, RI is defined as the ratio of the speed of light in a vacuum relative to its speed in the test material.

Since light also changes direction as it passes from one medium to another, RI is conveniently measured as a function of this unique “angle of refraction.” Everyone is familiar with how a partially submerged stick seems to “bend” as it enters water. Refractometers quantify RI by measuring the angle formed by light as it leaves air and enters the test object.

Refractometers range in size and capability, from handheld units costing a few hundred dollars to fullfeatured instruments in the $10,000 to $15,000 range. Handheld models may be of “traditional” or digital design; higher-end instruments operate on a benchtop or, in some industries, in-line to monitor manufacturing processes. Refractometers are priced to fit the job and the value of what is being analyzed.

Refractometry is one of the most versatile analytical techniques. Pure substances that transmit light have unique RIs that change as the concentration of additives changes. For example, the RI rises with increasing sugar concentration since sugar molecules dissolving in water slow light down as it passes through.

As a “universal concentration meter,” refractometry serves a range of industries: water treatment, chemistry, biology, foods, beverages, brewing and viticulture, paints, lubricants, personal care products, pharmaceuticals, and many others. Manufacturers routinely use RI to check incoming raw materials as well as complex products used in manufacturing, research, and development.

Refractometers in constant use must be maintained, primarily by cleaning the prism. If this is not done after every test, fluid from the previous run will evaporate, leaving the solute on the prism and distorting the next measurement. “Crosscontamination between samples is another source of error,” observes Richard Spanier, sales and marketing director at Rudolph Research Analytical (Hackettstown, NJ).

Protecting the sample during measurement is another precaution. Volatile liquids like alcohol evaporate, which distorts the concentration, while hydroscopic liquids like glycerol can dilute if they pick up water. “You also want to make sure there is no temperature gradient across the sample,” Mr. Spanier warns, since RI is a function of temperature.

Unlike spectroscopy, RI is not diagnostic or conclusive for a particular ingredient—a sugar-water solution may have the same RI as a saltwater solution (albeit at different concentrations). RI is most appropriately applied when analytes are known, or when the RI of a finished product has been previously established. Noah Radford, a technical specialist at Atago U.S.A. (Bellevue, WA), says users should carefully consider the composition and value of what they seek to measure, the measurement accuracy required, and the cost of an incorrect analysis. “Also consider the testing environment, temperature and humidity conditions, and whether you’re measuring in a controlled laboratory setting or on a manufacturing floor.”


Abbemat Series

  • Sophisticated measuring principle guarantees maximum accuracy
  • Low sample volume, highest efficiency
  • Simple to adjust
  • User-friendly, quick and easy
  • Performance, Performance Plus, Heavy Duty, and WR-MW models available

Anton Paar
www.anton-paar.com


PRM-100a

  • Easily incorporated in piping of manufacturing plants, liquid mixing devices, and washing apparatuses to measure the concentration of various liquids
  • Measurement accuracy: Brix ±0.05%, nD ±0.00010
  • Prism is specially designed to prevent solid materials from collecting on the surface, even after extended periods of use

ATAGO U.S.A.
www.atago.net


HI 96811 Digital Wine Refractometer

  • Rugged portable, water resistant device that is easy to use and calibrate
  • Employs the measurement of refractive index to display the sugar content in grape samples
  • Within 1.5 seconds, the instrument measures the refractive index of the grape sample and converts it to % Brix concentration units

Hanna Instruments
www.hannainst.com/usa


Brix/RI-Chek Digital Pocket-Sized

  • Multi-scale unit measures % Brix Temperature Compensated, Refractive Index Temperature Compensated, and Refractive Index at actual temperature
  • Weighs 100 grams and can fit into a shirt pocket
  • Includes a detachable neck lanyard, non-slip side grips for ease of handling, and a durable storage case

Reichert
www.reichert.com