A refractometer is a laboratory or field device for the measurement of an index of refraction.
The index of refraction is calculated from Snell's law and can be calculated from the
composition of the material using the Gladstone–Dale relation.
Types of refractometers There are four main types of refractometers:
traditional handheld refractometers, digital handheld refractometers, laboratory or Abbe
refractometers, and inline process refractometers. There is also the Rayleigh Refractometer used
for measuring the refractive indices of gases. In veterinary medicine, a refractometer is
used to measure the total plasma protein in a blood sample and urine specific gravity.
In drug diagnostics, a refractometer is used to measure the specific gravity of human urine.
In gemology/gemmology, the gemstone refractometer is one the fundamental pieces of equipment
used in a gemological laboratory. Gemstones are transparent minerals and can therefore
be examined using optical methods. Refractive index is a material constant, dependent on
the chemical composition of a substance. The refractometer is used to help identify gem
materials by measuring their refractive index, one of the principal properties used in determining
the type of a gemstone. Due to the dependence of the refractive index on the wavelength
of the light used, the measurement is normally taken at the wavelength of the sodium line
D-line of ~589 nm. This is either filtered out from daylight or generated with a monochromatic
light-emitting diode. Certain stones such as rubies, sapphires, tourmalines and topaz
are optically anisotropic. They demonstrate birefringence based on the polarisation plane
of the light. The two different refractive indexes are classified using a polarisation
filter. Gemstone refractometers are available both as classic optical instruments and as
electronic measurement devices with a digital display.
In marine aquarium keeping, a refractometer is used to measure the salinity and specific
gravity of the water. In Automobile Industry keeping, a refractometer is used to measure
the Coolant Concentration and Ph Value of the Coolant Oils for CNC Machining Process.
In homebrewing, a refractometer is used to measure the specific gravity before fermentation
to determine the amount of fermentable sugars which will potentially be converted to alcohol.
Automatic refractometers
Automatic refractometers automatically measure the refractive index of a sample. The automatic
measurement of the refractive index of the sample is based on the determination of the
critical angle of total reflection. A light source, usually a long-life LED, is focused
onto a prism surface via a lens system. An interference filter guarantees the specified
wavelength. Due to focusing light to a spot at the prism surface, a wide range of different
angles is covered. As shown in the figure "Schematic setup of an automatic refractometer"
the measured sample is in direct contact with the measuring prism. Depending on its refractive
index, the incoming light below the critical angle of total reflection is partly transmitted
into the sample, whereas for higher angles of incidence the light is totally reflected.
This dependence of the reflected light intensity from the incident angle is measured with a
high-resolution sensor array. From the video signal taken with the CCD sensor the refractive
index of the sample can be calculated. This method of detecting the angle of total reflection
is independent on the sample properties. It is even possible to measure the refractive
index of optically dense strongly absorbing samples or samples containing air bubbles
or solid particles . Furthermore, only a few microliters are required and the sample can
be recovered. This determination of the refraction angle is independent of vibrations and other
environmental disturbances. Influence of wavelength
The refractive index of a certain sample varies for nearly all materials for different wavelengths.
This dispersion relation is characteristic for every material. In the visible wavelength
range a decrease of the refractive index and nearly no absorption is observable. In the
infrared wavelength range several absorption maxima and fluctuations in the refractive
index appear. To guarantee a high quality measurement with an accuracy of up to 0.00002
in the refractive index the wavelength has to be determined correctly. Therefore, in
modern refractometers the wavelength is tuned to a bandwidth of +/-0.2 nm to ensure correct
results for samples with different dispersions.
Influence of temperature Temperature has a very important influence
on the refractive index measurement. Therefore, the temperature of the prism and the temperature
of the sample have to be controlled with high precision. There are several subtly-different
designs for controlling the temperature; but there are some key factors common to all,
such as high-precision temperature sensors and Peltier devices to control the temperature
of the sample and the prism. The temperature control accuracy of these devices should be
designed so that the variation in sample temperature is small enough that it will not cause a detectable
refractive-index change. External water baths were used in the past
but are no longer needed.
Extended possibilities of automatic refractometers Automatic refractometers are microprocessor-controlled
electronic devices. This means they can have a high degree of automation and also be combined
with other measuring devices Flow cells
There are different types of sample cells available, ranging from a flow cell for a
few microliters to sample cells with a filling funnel for fast sample exchange without cleaning
the measuring prism in between. The sample cells can also be used for the measurement
of poisonous and toxic samples with minimum exposure to the sample. Micro cells require
only a few microliters volume, assure good recovery of expensive samples and prevent
evaporation of volatile samples or solvents. They can also be used in automated systems
for automatic filling of the sample onto the refractometer prism. For convenient filling
of the sample through a funnel, flow cells with a filling funnel are available. These
are used for fast sample exchange in quality control applications.
Automatic sample feeding
Once an automatic refractometer is equipped with a flow cell, the sample can either be
filled by means of a syringe or by using a peristaltic pump. Modern refractometers have
the option of a built-in peristaltic pump. This is controlled via the instrument‘s
software menu. A peristaltic pump opens the way to monitor batch processes in the laboratory
or perform multiple measurements on one sample without any user interaction. This eliminates
human error and assures a high sample throughput. If an automated measurement of a large number
of samples is required, modern automatic refractometers can be combined with an automatic sample changer.
The sample changer is controlled by the refractometer and assures fully automated measurements of
the samples placed in the vials of the sample changer for measurements.
Multiparameter measurements
Today’s laboratories do not only want to measure the refractive index of samples, but
several additional parameters like density or viscosity to perform efficient quality
control. Due to the microprocessor control and a number of interfaces, automatic refractometers
are able to communicate with computers or other measuring devices, e.g. density meters,
pH meters or viscosity meters, to store refractive index data and density data into one database.
Software features Automatic refractometers do not only measure
the refractive index, but offer a lot of additional software features, like
Instrument settings and configuration via software menu
Automatic data recording into a database User-configurable data output
Export of measuring data into Microsoft Excel data sheets
Statistical functions Predefined methods for different kinds of
applications Automatic checks and adjustments
Check if sufficient amount of sample is on the prism
Data recording only if the results are plausible Pharma documentation and validation
Refractometers are often used in pharmaceutical applications for quality control of raw intermediate
and final products. The manufacturers of pharmaceuticals have to follow several international regulations
like FDA 21 CFR Part 11, GMP, Gamp 5, USP<1058>, which require a lot of documentation work.
The manufacturers of automatic refractometers support these users providing instrument software
fulfills the requirements of 21 CFR Part 11, with user levels, electronic signature and
audit trail. Furthermore, Pharma Validation and Qualification Packages are available containing
Qualification Plan Design Qualification
Risk Analysis Installation Qualification
Operational Qualification Check List 21 CFR Part 11 / SOP
Performance Qualification Scales typically used
Brix Oechsle scale
Plato scale Baumé scale
See also Ernst Abbe
Refractive index Gemology
Must weight Winemaking
Harvest Gravity
High-fructose corn syrup Cutting fluid
German inventors and discoverers High refractive index polymers
References ^ product page of A.KRÜSS Optronic GmbH
Further reading Sella, Andrea. "Abbé's refractometer". Chemistry
World: 67. External links
Refractometer – Gemstone Buzz uses, procedure & limitations.
Rayleigh Refractometer: Operational Principles Refractometers and refractometry explains
how refractometers work. Refractometer products and functions