Refractometry in the laboratory, quality assurance and process control
Refractometry provides a fast and accurate way to determine quality and concentration in laboratories, production environments and quality assurance.
It measures the refractive index of liquid or solid substances. The measurement of the refractive index (nD) is influenced by the temperature of the sample and by the wavelength of the light used.
This measurement value can be used to calculate concentrations in binary or quasi‑binary mixtures. These are, for example, solutions containing sugar, saline solutions, oils or solids materials.
Different refractometers use different measurement methods. A basic understanding of the measurement principle helps in selecting the most suitable instrument.
Contents
Basic principle of refractometry
What does the refractometer measure?
The key factor in refractometric measurement is the critical angle of total internal reflection. This angle is used to determine the refractive index of a sample.
When light strikes the boundary between two media with different refractive indices, part of the light is refracted and part is reflected. Above a material-specific angle of incidence, however, refraction no longer occurs and the light is reflected entirely — this is known as total internal reflection.
Example: With Plexiglas, the critical angle of total internal reflection is 42°, resulting in a refractive index of 1.49 nD, measured at 20°C.
Why is light refracted?
Light propagates at different speeds depending on the medium. The higher the refractive index, the lower the speed of light. In a vacuum, the speed of light is approximately 300,000 km/s, whereas in water it is reduced to around 225,000 km/s.
For example, the difference in how light is refracted makes objects under water appear bent. This effect is caused by the different refractive indices of air and water.
A video example: If an air-filled cup (nD 1.0003) is filled with water (nD 1.3330), the contents become much more clearly visible due to the stronger refraction of light.
https://www.youtube.com/watch?v=7nxDgTr-1nM&t=4s
Scales & influencing factors
Temperature compensation
When measuring sugary drinks, confectionery, jams or honey with automatic temperature compensation (ATC), there is no need for temperature control of the sample. This is based on ICUMSA tables that describe the influence of temperature on the refractive index of sucrose, glucose, fructose and invert sugar solutions.
Since this influence is known, the measured refractive index can be automatically converted to a defined reference temperature – usually 20 °C – irrespective of the prevailing ambient temperature. Temperature differentials are reliably compensated to maintain consistent and accurate measurement results.
Device with temperature compensation
Influencing factor temperature
Even a temperature change of 1 °C has a measurable effect on the refractive index to the fourth decimal place. Precise temperature control is crucial for accurate, repeatable and reproducible measurements.
This is achieved either via an integrated Peltier element or via an external thermostat. With clearly defined temperatures, such as 20 °C (Ph. Eur.) or 25 °C (USP), standard-compliant measurements can be carried out.
The refractive index of distilled water at 20 °C and 589 nm is exactly nD = 1.33299, making it a suitable standard for calibration and adjustment procedures.
Device with Peltier temperature control
Scales
In measurement technology, we speak of scales when the refractive index is converted into a substance-specific unit. The most common used is the BRIX scale for determining the sugar concentration in aqueous solutions, for example to measure the original gravity of beer.
The OECHSLE scale is used to determine the sugar content of wine must and is based on its higher density compared to water.
The BAUMÉ scale is mainly used in French-speaking countries and measures the density of liquids, in particular the sugar content in grape and fruit juices.
Device with Oechsle measuring scale
If you want to achieve valid measurement results, you should be familiar with the relevant scales, influencing factors such as temperature, and the different instrument models and measurement methods — so that you can choose the right measurement system for each application.
- Why buy an expensive laboratory device when a simple analog device will do?
- Or vice versa: Why work with a simple model when accuracy, measurement range and resolution are crucial or even 21 CFR Part 11 and EU Annex 11 compliance is required?
Device types and measurement technology
Digital precision measurement in detail
The critical angle of total internal reflection is measured in reflection. The light source and photodetector are on the same side of the sample. The light source, prism and photodetector are aligned in such a way that, when no sample is placed on the prism, total internal reflection occurs across the entire prism surface. If a sample is placed on the prism, total internal reflection no longer occurs across the entire prism surface. Two examples measured at 20 °C:
For a water sample on a sapphire prism, the critical angle of total internal reflection is 49°.
For a sample of honey (water content 18%), the critical angle of total internal reflection is 57°.
Measure refractive index with photodetector (CCD sensor)
Digital device types operate using reflected light, meaning that the light does not need to pass through the sample. During the measurement, an illuminated and a non‑illuminated area is created on the photodetector. The angle at which total internal reflection occurs distinctly delineates the two regions (see CCD sensor). This can be used to determine the refractive index. The refractive index provides information about the purity of a substance, but not about its exact composition.
Example measurements with digital devices
When measuring in reflection, the color, texture or condition of the sample have only a minor influence on the measurement.
Even difficult‑to‑measure samples, such as sugar syrup, jams or molasses, can be measured precisely and quickly using a digital refractometer.
With honey, the refraction of light depends on the water content. The higher the water content, the faster the speed of light in honey and the lower the refractive index.
Measuring the refractive index with the transmitted light method
The Abbe refractometer has become widely used because it enables simple and rapid measurement of the refractive index. In this method, the refractive index is determined using transmitted light: the light beam strikes two parallel prisms, a lighting prism and a measuring prism. Both prisms are made of glass with the same refractive index and are pressed against each other. The sample is placed in the space between these two prisms.
Reading the refractive index
The inner surface of the illumination prism that is in contact with the sample is roughened (matt‑finished). This allows incident light to enter the sample as diffuse rays at all possible angles. At the “sample-measurement prism” interface, these light rays are refracted and cross the interface at different angles.
Only rays incident at angles below the critical angle of total internal reflection pass through the measurement prism. This creates a light-dark area on a screen. When the point of intersection of the reticle is brought into alignment, the measurement value can be read from the scale.
Analog handheld devices in use
The measurement principle is the same as that used in Abbe refractometers. This is how the measurement works:
After applying the sample, close the device and look through the eyepiece. Ambient light illuminates the inner scale. At this point, the boundary line becomes visible, indicating the percentage value of the solution. For example, the sugar content of beverages or the water content of honey can be determined. Oils, greases, radiator fluids and lubricants can also be analyzed.
Analog handheld refractometers differ mainly in the selectable scales (depending on the application), e.g. for determining the salt content, water content in honey, serum protein content, Oechsle, Brix and potential alcohol content as well as ethylene and propylene glycol content.
Many digital models have automatic temperature compensation (ATC) for the BRIX scale. This function (based on the ICUMSA conversion table) supports the measurement of sugary drinks and confectionery.
Typical areas of application & applications
Once the measurement methods have been technically described, it is worth taking a look at the many areas of application and possible uses of refractometers, the importance of which is central to process and quality assurance.
- They are used to determine the purity and concentration of pharmaceutical ingredients. In these highly regulated areas, the requirements of 21 CFR Part 11 and EU Annex 11 often apply.
- The refractive index measurement is also important for analyzing the sugar content and the analysis of petroleum products.
- They are also an indispensable tool in the quality control of operating fluids for machines and engines – including AdBlue.
- They also enable the determination of the water content in emulsions such as drilling emulsions, cutting oils or cooling lubricants.
Standards & guidelines
It is equally important to know the applicable standards, as they define terms, measurement methods and units of measurement and form a reliable basis for selecting suitable devices.
- However, standards also regulate networking by standardizing interfaces, communication and secure data transmission and data integrity.
- They enable a quality agreement for standards and measurement tolerances. Although not legally binding, normative action is associated with a high degree of legal certainty for the user. It facilitates device selection and thus safeguards the company’s own processes and quality assurance.
- Standards regulate environmental conditions and sample preparation as well as measurement tolerances, properties and equipment of devices or calibration media under the aspect of “best practice” (good manufacturing practice).
If you use your device in accordance with standard specifications, you can guarantee that measurements are carried out correctly and according to reproducible procedures.
Below is an overview of all standards and directives known to us.
Let our experts advise you in a personal consultation on the specifications that the various KRÜSS Optronic models fulfill.
Overview of samples & nD measured values
Based on normative specifications, the focus shifts to the sample itself. Almost every sample is measurable. It is important to know the refractive index (nᴅ) of the sample. It defines the range in which the device must measure and decides on the selection of a suitable device that covers exactly this range. This offers advantages:
-
High measurement reliability because the device is optimally matched to the sample. Measurement range, measurement accuracy and resolution are spot on.
-
Cost efficiency, as devices with a smaller measurement range are usually significantly cheaper than models that cover a large measurement range. In this way, measurement accuracy and budget can be optimally combined.
An overview with measurement samples and their refractive indices is available for download here. Most samples were measured at standard measuring conditions (20°C, λ = 589 nm), deviations are indicated. The measured values given are not specifications.
We will be happy to discuss which models are best suited to your measurement requirements in a personal consultation. Please feel free to contact us.
Cleaning tips
Many samples leave residues, and only thorough cleaning ensures precise measurement results and extends the service life:
- A soft, lint-free cellulose cloth is sufficient to absorb the sample and wipe the measurement prism with a moistened cloth.
- Water is usually suitable as a solvent, ethanol is recommended for oily samples; cleaning with acetone is also possible depending on the compatibility of the device.
- The measurement prism should be cleaned immediately after each measurement to prevent samples from drying out. If the substances vary greatly, a control measurement with water or a suitable reference is recommended.
- As the measurement prism is not alkali-resistant, alkaline or aggressive samples must be removed particularly quickly. Short contact times protect the device and ensure measurement quality.
Complete overview of cleaning agents & tips
Open by flipping and see what are the best cleaning agents:
| Sample | Cleaning agent 1 | Cleaning agent 2 |
|---|---|---|
| Sector: Food, beverages and spirits | ||
| Fruit juices | Water | Ethanol |
| Soft drinks | Water | Ethanol |
| Sugar solutions, salt solutions, honey | Water | – |
| Beer, beer wort | Water | Ethanol |
| Spirits, distillates | Water | – |
| Sector: Chemicals, cosmetics, pharmaceutical industry | ||
| Aromas, fragrances, after shave, perfume | Ethanol, isopropanol | _ |
| AdBlue | Water | _ |
| Cleaning agent | Water | Ethanol |
| Ethylene glycol, propylene glycol | Water | Ethanol |
| Polyamides, polymers | Cresol | – |
| Paraffinic substances | Toluene | Xylene, white spirit |
|
Wood preservative on Turpentine base |
White spirit | Ethanol |
| Water-based wood preservative | Water | Ethanol |
| Industry: Petrochemicals | ||
| Engine oil, lubricating oil | White spirit | Acetone |
| Mineral oils | Isopropanol | _ |
| Diesel, kerosene and heating oil | White spirit, petroleum ether | Acetone |
Automated solutions
Automated rinsing, cleaning and drying increase efficiency – especially with high sample volumes
- Fully automatic: The DR6000-TF models work automatically with a flow-through cell, drying unit and peristaltic pump. Individual cleaning programs and a powerful rinsing process reliably remove residues.
- Semi-automatic: Sample and cleaning agent are automatically drawn in with the flow-through cell, drying unit DS7060 and peristaltic pump DS7070. The integrated 3/2-way valve allows you to quickly switch between sample supply, cleaning and drying – without having to reconnect.
How to use – practical videos from experts
Our devices are developed in collaboration with leading industry and research partners and are designed to fit effortlessly into modern laboratory processes.
In the practical videos, we show you how our devices perform in a real working environment – from typical measurement processes to a particularly popular topic: the precise measurement and adjustment of handheld refractometers. Benefit from real expert knowledge and see how easy it is to achieve precise results.
Measure & adjust
4 Videos
0:16
0:16
0:16
0:16