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Good Refractometry Practice Brochure - How to Achieve the Best Results in Day-to-Day Refractive Index Measurement |
Download a free copy of the refractive index measurement guide to learn how to avoid errors when measuring the refractive index, BRIX, HFCS or concentration of liquids and how to achieve the best results in refractive index measurement, especially with digital refractometers.
This refractive index measurement guide explains what precautions should be taken to avoid errors when measuring the refractive index, BRIX, HFCS or concentration of liquids. Following these recommendations will allow you to achieve the best results in refractive index measurement, especially with digital refractometers. Following topics are covered:
- Test and Adjustments: What is best, regular tests or regular adjustments? How often should the refractometer be tested or adjusted, with which substance? Which tolerance should be applied?
- Samples: Which types of sample can be measured with a digital refractometer? Which are the difficult samples and how to measure them? What can be the effect on the measurement?
- Sampling: Which types of syringe should be used? How to avoid air cushions? How much sample should be applied?
- Cleaning: How should the cell be cleaned, depending on the sample? Which solvents should be used?
- Result verification and documentation: How to detect errors for critical/problematic samples? How to convert the result in other units or concentrations? How to automatically verify if the result is within product specification (Quality Control)?
1. Test and Adjustments
The commonly held opinion that frequent adjustment of the instrument guarantees accurate results is not true. Any adjustment operation results in changes being made to the internal instrument settings.
If the adjustment is not properly performed, all the measurements performed afterwards will be wrong.
Instead of frequent adjustment, it is better to regularly verify the measurement accuracy of the system by measuring a sample of accurately known density (e.g. distilled water or a standard) which is called test, calibration or check. Then the measured refractive index is compared to the known nominal value of the sample.
Get more information in the Refractive Index Measurement Guide
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One Click Test with Standards - Video
See how to run a test with a standard to check if your density meter or refractometer is still working properly.
Test (Calibration)
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Adjustment
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2. Samples
Pasty samples
Pasty samples, for instance tomato puree, bear the risk of air cushions between the prism and the sample. Make sure that the sample is in full contact with the prism by “pressing” it down.
METTLER TOLEDO RM Refractometers can be equipped with an easy mountable presser. When the lid is being closed, the sample is automati- cally pressed to the prism.
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Get more information in the Refractive Index Measurement Guide
Sticky / viscous samples
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Aggressive samples
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Volatile samples
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Non-homogeneous samples/suspensions
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3. Sampling
With a syringe
Use plastic syringes with luer tip, preferably 3-component syringes (with rubber O-ring) as they allow a much better speed control than cheap 2-component syringes.
Get more information in the Refractive Index Measurement Guide
Avoid air cushions
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Add Enough Sample
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Automatic filling
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4. Cleaning
Procedure for manual use of refractometer
Remove old sample
To remove the sample (and the solvents) from the refractometer cell, it is suggested to use a syringe. This “waste syringe” can be used over and over again (tip: mark this syringe, for instance with black tape). Using a syringe saves a lot of soft tissue cleaning wipes and reduces waste.
Rinse
Clean with an ideal solvent a few times. The solvent must be able to quickly dissolve the sample.
- Add the solvent
- Stir with the “waste syringe”
- Remove all with the “waste syringe”
- A second solvent which allows quick drying (e.g. Acetone) of ten bears the risk for contamination!
Dry
Wipe the prism/cell dry with a soft tissue. Wait 10 seconds, before adding next sample
Get more information in the Refractive Index Measurement Guide
Cleaning with automation devices
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5. Result verification and documentation
Automatic result conversion
Often the result has to be converted using a table. Looking up in or interpolating from a table is error-prone and time-consuming. Automatic conversion using built-in tables (e.g. alcohol, Brix, temperature compensation according to API) prevents reading or calculation errors and saves time. A digital refractometer of the latest generation allows the use of built-in conversion tables to show the result directly in the desired unit. METTLER TOLEDO RM Refractometers have to following built-in result units / concentration tables:
- nD, Zeiss (14.45), Zeiss (15.00)
- Brix, HFCS 42/55, Invert Sugar, Oechsle
- Up to 30 user -defined concentration tables (can be entered as tables or formulas)
Get more information in the Refractive Index Measurement Guide
Error detection
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Result limits
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Proper Documentation
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