Refractometry
Refractometers are widely used analytical devices in quality assurance. From light to dark, from alkaline to acidic, from low to high viscosity – our refractometers can measure almost all samples.
The devices determine the refractive index of liquid or solid substances, from which their identity and quality as well as the concentration in binary (mixture of two different components) or quasi-binary mixtures can be derived. For example, if sugar is added to pure water, the refractive index changes depending on the amount added.
Example: If you add salt to water, the refractive index also changes, but in a different ratio to the concentration. This means that if water at 20°C does not have a refractive index of 1.3330 nD, it is not pure water.
⇒ The measurement of the refractive index (nD) depends on the temperature of the medium and the wavelength of the light used.
Temperature compensation
A temperature control of the measurement sample is not required when sugary beverages and confectionary products are measured. A conversion table published by ICUMSA, which shows the influence of temperature on the refractive index of sucrose, glucose, fructose and invert sugar solutions, makes this possible. Since the influence of the temperature on the measured value is known, it is possible to take measurements at any ambient temperature and to automatically convert them to the desired reference temperature – often 20 °C. Temperature differences will also be automatically compensated.
Influencing factor temperature
If the temperature of a sample varies by 1 °C, this will then become already apparent with the fourth decimal place of the measured value. A precise temperature control is therefore extremely important in order to ensure a high accuracy and reproducibility of the measurement results. This is realised either through a builtin Peltier element or through external thermostats. This allows for standard-compliant measurements at various temperatures, for example, at 20 °C (Ph. Eur) or at 25 °C (USP). The refractive index (nD) of distilled water at 20 °C (589 nm) is exactly 1.33299. This Medium is ideal for calibration and adjustment.
Scales
In the measurement technology, we speak generally in terms of scales whenever, for example, the refractive index is referred to in an appropriate substance-specific dimension. The most common scale is the BRIX scale. It is used to determine the sugar concentration in aqueous solutions. It is used, for example, to measure the original gravity of beer. The OECHSLE scale is intended for measuring the sugar content of wine must. The scale is based on the assumption that the density (specific gravity) of the must is greater than the density of the water. The BAUMÉ scale (a scale for measuring the density of liquids) is widely used in French-speaking countries. It is mainly used to measure the sugar content of grape juice and other fruit juices.
What does the refractometer measure?
Refractometer – total internal reflection and refraction
What does the refractometer measure? The critical angle of total internal reflection is decisive for the measurement results of the refractometer. Refractometers use it to measure the refractive index of a sample.
If light hits an interface of media with different refractive indices, refraction and reflection of the light usually occurs.
From a certain angle of incidence, which depends on the material, the light is no longer refracted at the interface, but only reflected. There is total internal reflection of the light.
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 does not always move at the same speed. The speed of light depends on the medium in which it spreads out. The greater the refractive index of a medium, the lower the speed of the light. In a vacuum, for example, light travels at a speed of around 300,000 kilometers per second. In water, on the other hand, the speed is around 225,000 kilometers per second, i.e. 25% slower.
Light refraction also has the effect that objects that are completely or partially under water may appear to be bent. This is due to the different refractive indices of the media (air | water).
The video shows another example: If a cup containing air (refractive index nD 1.0003) is filled with water (refractive index nD 1.3330), the contents can be seen more clearly due to the refraction of light.
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More InformationOpen by flipping and see the measuring applications that are fullfilled by different refractometers
Digital refractometer: Measurement in reflection
Digital refractometers measure the critical angle of total internal reflection in reflection. The light source and photodetector are located on the same side of the sample. The light source, prism and photodetector are aligned with each other in a manner which means that total internal reflection occurs over the entire surface of the prism, provided there is no sample on the prism. If there is a sample on the prism, total internal reflection no longer takes place over the entire surface of the prism. 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 honey sample (water content 18%) the critical angle of total internal reflection is 57°.
Measurement of refractive index with photodetector
Digital refractometers work with reflected light, which means the light does not have to traverse the sample. During the measurement An illuminated and an unilluminated area appear on the photodetector. The angle, from which the total internal reflection arises, noticeably separates the two areas, through which the refractive index can be determined; see CCD line.. The refractive index provides information about the purity of a substance, but gives not exact information about its consistency.
Sample measurements in practice
When using the measurement function with reflected light, the color, texture or composition of the sample will only have a minor influence on the measurement.
Jam measurement: Even supposedly challenging samples, such as sugar syrup, fruit jam and molasses, can be precisely and quickly measured with a digital refractometer using reflected light measurement.
Honey measurement: In the case of 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 smaller the refractive index.
Abbe refractometer function
The Abbe refractometer has become widely used as it provides a simple and fast measurement of the refractive index. The operation of the device can be learned quickly. Handheld refractometers and Abbe refractometers use transmitted light: The light beam goes through two right-angled prisms, an illumination prism and a measurement prism. Both prisms are made from glass with the same refractive index and are pressed against each other. The sample is placed in the space between these two prisms.
Correct measurement and read-out Abbe refractometer
The inside of the illumination prism, which is in contact with the sample, is roughened. As a result, incident light in diffuse rays can penetrate the sample at all possible angles. At the interface “sample – measurement prism” these light rays are refracted and travel through the interface at different angles.
Only rays which radiate at an angle smaller than the critical angle of the total internal reflection pass through the measuring prism. This creates a light-dark area on a screen. The refractive index can be read on a running scale when adjusted to the cut-off line.
Handheld refractometer application
After applying the sample, close the device and hold it in front of a light source. Now you can see the boundary line, which shows on the scale the percentage value of the solution. This is used, for example, to determine the sugar content in drinks or the water content in honey. Furthermore, oils, greases, radiator fluids and lubricants can be analyzed.
Accordingly, hand refractometers differ essentially 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 of our handheld refractometers are equipped with automatic temperature compensation for the Brix scale. This function (based on the ICUMSA published conversion table) is useful for measuring sugary drinks and confectionery.
Standards and guidelines
Refractometers are designed and used in accordance with standards
Standards in metrology describe terms, measurement methods and units of measurement. However, standards also regulate networking by standardizing interfaces, communication and secure data transmission. They enable a quality agreement for standards and tolerances. Although not legally binding, normative action is associated with a high degree of legal certainty for the user. It facilitates the selection of devices and thus ensures the company’s own processes and product quality.
Standards in refractometry regulate the environmental conditions and sample preparation as well as measurement tolerances, properties and equipment of a device or calibration medium under the aspect of “best practice”. Whoever uses refractometers according to normative specifications can guarantee that measurements are correct and work according to reproducible procedures.
In order to make it easier for you to choose the right refractometer for your application, you will find below an overview of all standards and guidelines that refer to refractometers. Note:This is a general overview of the standards. Which normative specifications the different refractometer models from A.KRÜSS fulfill your requirements, we will be happy to discuss in a personal consultation. Please feel free to contact us.
Samples and measured values
Almost all samples are measurable
To select the right refractometer, it is helpful to define the required measurement range. There is real potential for savings when purchasing a new refractometer, as refractometers with a small measurement range are usually more cost-effective compared to measuring devices that cover the entire measurement range.
Find out more: We have compiled a comprehensive overview of measurement samples with the specific refractive index in our table. Most samples were measured at standard measuring conditions (20°C, λ = 589 nm), deviations are indicated. The measured values given are not specifications.
Note: This overview is intended as a summary. Which refractometer models are best suited to your measurement requirements is a subject we would be happy to discuss in a personal consultation. Please feel free to contact us.
Typical areas of application
Refractometers used in many industries
Refractometers are used in a wide range of applications: from determining the purity and concentration of drug ingredients, to measuring the sugar content in food and beverages, to petroleum analysis. The dilution of sunflower oil with cheap oils can also be detected with this device. They are also the means of choice for the quality control of operating fluids for machines and engines as well as AdBlue, the synthetic urea solution for treating diesel engine exhaust gases. Refractometers can also be used to determine the water content of emulsions such as drilling emulsions.
You can also obtain very precise handheld refractometers from us, e.g. for measuring the extract content in fruit juice, grape must, sugar syrup, fruit products, ice cream and honey. The selection of the suitable scale for these devices depends on the sample to be measured and the desired dimension and accuracy in which the measurement result is to be displayed.
Below you will find an overview of the various A.KRÜSS refractometers with examples of applications. For the handheld refractometers, we have created an overview of the device models with the available scales. This information should make it easier for you to select a suitable refractometer. We are also happy to discuss details in a personal consultation. Please feel free to contact us.
Cleaning tips
Accurate cleaning guarantees precise measurement results
- Cleaning is normally easy. As a rule, it is sufficient to use a soft, lint-free and absorbent cellulose cloth to clean the measuring prism. Pick up the sample with the tissue, then wipe with a damp one.
- Water is often a suitable solvent. Ethanol is recommended for oily samples. If the refractometer is compatible, it can also be cleaned with acetone.
- Clean the measurement prism immediately after each use. This ensures that your sample does not dry or harden on the prism.
- Between the measurement of very different substances and samples, it is helpful to check the cleaning success with the test measurement of water or another suitable reference.
- The sensitive component of every refractometer is the measurement prism, which is not alkali-resistant. Therefore, please clean the measurement prism quickly after measuring alkaline or aggressive samples and check the cleaning afterwards. A quick measurement with these samples shortens the contact time and protects the refractometer.
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 |
Maximum efficiency through automation
Automated rinsing, cleaning and drying of our semi-automatic and fully automatic refractometers increase effectiveness not only in routine analysis with high sample volumes.
The DR6000-TF models with flow-through cell enable fully automatic operation in conjunction with drying unit, peristaltic pump and autosampler. You can create any number of individual cleaning processes via the user interface of the DR6000-TF models. The cleaning fluid flows through the system at high speed and with a lot of compressed air, flushing all contamination and sample residues out of the measurement chamber.
For semi-automatic operation, devices with flow-through function, flow-through cell, drying unit (model DS7060) and peristaltic pump (model DS7070) are required. These suck the sample or the cleaning agent into the flow-through cell. The drying unit has a 3/2-way valve, so it is not necessary to reconnect the hoses for the cleaning process when changing from sample supply or cleaning to drying.
How to Use
Measuring with A.KRÜSS refractometers – tips from the experts
In close cooperation with renowned partners from industry and research, we have developed refractometers that can be easily integrated into existing laboratory workflows. Here you can see refractometers in practical application. Frequently requested by many of our customers: the measurement and adjustment of handheld refractometers.
