Refractometry Definition – Measuring Refractive Index
Refractive Index Measurement or Refractometry is the method of measuring substances refractive index and assess their composition or purity. Refractometry is a technique that measures how light is refracted when it passes through a given substance. Snell’s Law which is also known as The Law of Refraction describes the formula behind the refraction:
The formula for Snell’s Law is below. It is often used in refractometry to describe the relationship angles of incidence and light.
Since the late nineteenth century refractometry was one of the main techniques used in chemical analysis. Refractometry was used to determine concentrations of solutions and as an aid in the identification of unknown substances.
Refractive index is the amount by which light is
refracted. The ratio of the speed of light in a
vacuum to the speed of light in another substance
is defined as the refractive index for the
substance. The speed of light in a vacuum
is always the same, but when light moves
through any other medium it travels more
slowly since it is constantly being
absorbed and re-emitted by the atoms in
the material. The refractive index is a
quantity which is a constant for a pure
substance under standard conditions of
temperature and pressure.
Instrumentation for Refractometry
A Refractometer is the instrument used to measure refractive index (RI). A refractometer measures the extent to which light is bent when it moves from air into a sample and is typically used to determine the refractive index of a liquid sample.
Types of Refractometers
There are four main types of refractometers, traditional handheld refractometers, digital handheld refractometers, laboratory or Abbe refractometer and incline process refractometers. The Abbe refractometer named for it’s inventor, Ernst Abbé, designed his refractometer in 1869. This was the first refractometer to be offered commercially.
While the general principle is still the same today, the accuracy of measurements has increased. In the 1890s jacketing was added to the refractometer prism. This allowed an operator to study
the change of reading with temperature (dndc) as well as providing far better accuracy than a room temperature instrument offers.
Automatic shadow line reading in the 1980s enabled refractometers to move from a laboratory instrument operated by technically trained users to a factory control tool used by line operators.
The current generation of instruments combine all these advances with built in peltier temperature control meaning laboratory accuracy and reproducibility without the necessity of an
additional water bath.
Traditional Handheld Refractometer
A traditional handheld refractometer is an analog
instrument for measuring a liquid’s refractive
index. Handhelds are compact and easy to use, the
handheld refractometers are ideal for laboratory or
production environment measurements. Simply immerse
the sensor in the sample or drop the sample
on top of the sensor prism. These units
are best used for calculating the Brix value
of fruit juices, soft drinks, sauces, soups,
water-soluble cutting oils, washing liquids,
industrial liquids, and coolants. Thus, many
of these handheld refractometers are
rated for water resistance that enhances
durability and simplifies cleanup procedures.
A Traditional Handheld Refractometer does not
require batteries or a power source. This
allows the handheld refractometer to
be used anywhere, at least in daylight.
A digital refractometer can offer more than double the
resolution of a traditional style refractometer
producing a far more accurate reading. A digital
refractometer has automatic temperature compensation
based on sugar and calculates the current temperature
into the final result. A digital refractometer not only
offers a wider scale but also a much better resolution.
Ultimate accuracy is limited by the sugar temperature
compensation but so long as the sample is primarily sugar
and the sample is fairly close to room temperature then
these types of refractometers will give a good result.
Laboratory / Abbe Refractometer
Abbe Refractometer are highly reliable classic instruments
which measure Refractive Index. Compared with other types of
refractometers, the Abbe refractometer are capable of
measuring liquid and solid samples such as plate glasses,
plastic sheets, and other solid films. They have sapphire
prisms and sealed optics. A computer interface is usually
a standard feature for printing and data download. Productivity
features include flow through modules and temperature
stabilization accessories. Therefore, this style of instrument can
give good accuracy on most sample types, not just sugar.
Inline Process Refractometer
Inline Process Refractometers are designed for inserting
into pipelines or vessels. Inline Refractometers monitor a process variable
(typically dissolved solids) which is refractive
index dependent. The refractometer produces a
digital and analog output ready for transmission
to a PLC (Programmable logic controller) or other
Refractometers for measuring Refractive Index from Rudolph Research
Using Refractive Index Measurement in Industries
Refractometry for the Pharmaceutical Industry
- Total Parenteral Nutrition (TPN)
- Toxicology Testing (Urine SG)
- Pharmacy compounding and drug diversion
- USP <831>
- EP 2.2.6
Refractometry for Flavor, Fragrance, and Essential Oil Industry
- Skin cleansers
- Lemon, lime, orange
- Natural oils
Refractometry for Food and Beverage Industry Applications
- Seed oils
- Soybean oils
- Dairy products
- Edible oils
- Coffee extracts
- Juice concentrates
- Fruit products
- Hydrolysis products
- Vegetable products
- Soft drink
Measuring Refractive Index in the Sugar Industry
- Cane sugar
- Beet sugar
- Invert sugar
- Liquid sugar
- Confectionery sugar
- Brown sugar
Refractometry in the Petroleum Industry
- Oil based paints
- Fine chemicals