What is a digital refractometer and how does it work?

Select your Question

1. What is a digital refractometer??
2. How does a digital refractometer work?
3. Which instruments are used to measure refractive index of liquids?
4. What is the refractive index of water?
5. How much sample volume do you need for a digital refractometer?
6. How much time do you need to measure a sample with a digital refractometer?
7. How do you adjust a digital refractometer?
8. Which samples can be measured with digital refractometers?

What is a digital refractometer?

A digital refractometer uses the total reflection method to measure the refractive index or related values of a liquid sample. This measurement is done digitally, reducing operator influence considerably. Accuracy of up to 0.00002 or 0.01% Brix can be reached, with small sample volume (0.5 to 1 mL) and short measuring time (a few seconds).

How does a digital refractometer work?

Digital refractometers use a light-emitting diode, a sapphire prism and a high-resolution optical sensor. The sample is put on the prism and the measurement started. From a certain angle of incidence – the so-called critical angle – the ray of light no longer penetrates the sample but is fully reflected from its surface and is detected by the optical sensor. The refractive index is calculated from this critical angle. In addition, benchtop digital refractometers use a built-in Peltier thermostat to control the temperature of the sample.

Learn more: have a look at our interactive animation

Which instruments are used to measure refractive index of liquids?

The refractive index of a solution can be measured manually or digitally.

Manual methods like portable optical refractometers and ABBE optical refractometer are widely used for the determination of refractive index and related values, e.g. BRIX°, alcohol %, Baumé, Plato, etc. Although these methods are easy to use and quite inexpensive, they require expertise in every step of the manual operation protocols, which often affects the accuracy and reliability of the results.

Digital Refractometers are available as benchtop and handheld instruments. They measure liquid samples digitally, reducing operator influence considerably.

Compare digital refractometers with manual methods: Comparison of different measuring techniques

What is the refractive index of water?

The refractive index of water is 1.33299 at 20°C. The refractive index of water changes with the temperature, decreasing with increasing temperature (e.g. 1.33061 at 40°C). Note that this is the commonly used refractive index nD, measured with a light of wavelength of 589.3 nm (D-line of sodium).

How much sample volume do you need for a digital refractometer?

For most bench-top refractometers the typical volume is around 0.5 mL for samples with high surface tension and 1mL for samples with low surface tension. As only a few drops are required, the temperatures of the sample and the measuring cell equalize fast and the analysis therefore takes only a short time.

How much time do you need to measure a sample with a digital refractometer?

This depends on the temperature difference between the sample and the measuring cell, as well as on the sample type and the required accuracy of the measurement. This measuring time is typically a few seconds.

How do you adjust a digital refractometer?

Usually refractometers are adjusted with water at a specific measuring temperature. This temperature is controlled with a Peltier element, which can cool down or heat up the measuring cell to a specific temperature (e.g. 20°C). Sometimes, Brix standards are used to adjust a refractometer for the measurement of sugar concentrations. These Brix standards are often produced onsite. Smart BRIX Standard Preparation can be done using a balance and a refractometer connected to the LabX PC software, increasing the accuracy and traceability and eliminating transcription errors.

Which samples can be measured with digital refractometers?

Digital refractometers were originally designed for measuring homogeneous liquids. In reality, many other samples can successfully be measured, such as pasty samples or even foils. These specific samples require some additional accessories, like a press or a foil stamp. Special care has to be taken for volatile samples (where a flow cell is recommended) or non-homogenous samples where the sample must be stirred/homogenized prior to the measurement. This can be done automatically with the in-beaker homogenizing of the InMotion autosampler.