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A refractometer is a laboratory or field device for the measurement of an index of refraction (refractometry). 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 (typically) 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 in human urine.

In gemology, a refractometer is used to help identify gem materials by measuring their refractive index.

In marine aquarium keeping, a refractometer is used to measure the salinity and specific gravity of the water.

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

Schematic setup of an automatic refractometer: An LED light source is imaged under a wide range of angles onto a prism surface which is in contact with a sample. Depending on the difference in the refractive index between prism material and sample the light is partly transmitted or totally reflected. The critical angle of total reflection is determined by measuring the reflected light intensity as a function of the incident angle - Source of image: Anton Paar GmbH,

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 optical dens 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.

Modern Automatic Refractometers - Source of image: Anton Paar GmbH,

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.

- Flow cell with filling funnel for an automatic refractometer assures fast sample exchange, e.g. in quality control - Source of image: Anton Paar GmbH,

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

Automatic refractometer with sample changer for automatic measurement of a large number of samples - Source of image: Anton Paar GmbH,

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

Measuring combination of an automatic refractometer and a density meter as widely used in the flavors and fragrances industry - Source of image: Anton Paar GmbH,

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 (and other parameters) into one database.

Software Features

Automatic refractometers do not only measure the refractive index, but offer a lot of additional software features, like

Pharma Documentation and Validation

Typical Pharma Validation and Qualification Folder - Source of image: Anton Paar GmbH,

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

Scales typically used

See also

Further reading

External links