Refractive index of liquid

Measuring the refractive index of a liquid is a fundamental optical analysis used to determine how light propagates in a sample. This index, specific to each substance, depends on the chemical composition, purity and concentration of the liquid.
Performed by temperature-controlled refractometry , this measurement is a rapid and reliable indicator of the quality, authenticity and identity of a product.

Refractive index is commonly used in the cosmetic, food, chemical and nutraceutical to control the conformity of formulations, verify the concentration of ingredients or detect alterations in a batch.

Principle and method of analysis

Refractometry is based on the principle of light bending as it passes through two media of different optical densities. The refractometer measures the angle of refraction between the incident light and the light refracted at the surface of the liquid.
The refractive index (n) is then calculated according to Snell-Descartes' law , usually at a standard wavelength (589 nm) and at a controlled temperature (20 °C or 25 °C).

The samples analyzed can be:

• essential or vegetable oils , to check their purity and origin;
• hydroalcoholic extracts or aqueous solutions , to assess the concentration of solutes;
• cosmetic formulations or liquid supplements , to check the stability or conformity of batches.

For additional analyses, also discover:

• solid refractometry for transparent materials;
• UV-Visible spectrophotometry to measure optical transmission and absorbance;
• or HPLC chromatography to characterize the composition of solutions.

Industrial applications

The measurement of refractive index is used in many fields:

• Cosmetics : quality control of oils, gels and solutions, verification of the purity of ingredients and detection of formulation variations.
• Food and beverages : measurement of the concentration of sugars, fats or dry extracts in syrups, juices, oils or flavorings.
• Nutraceuticals : monitoring the stability and concentration of active ingredient solutions or plant extracts.
• Materials and packaging : control of the optical or chemical compatibility of liquids used in transparent polymers.
• Chemistry and environment : identification of technical solutions, solvents or aqueous effluents.

This analysis is particularly useful for validating batch consistency , ensuring the traceability of raw materials and guaranteeing the stability of liquid formulations .

Analytical interest and complementarity

The refractive index can be correlated with other physicochemical parameters for a complete characterization of the product:

• Density and bulk density to relate composition to optical behavior;
• Viscosity to study the fluidity and consistency of formulations;
• Acid number and peroxide number to assess the oxidation of oils;
• Phenolic assay to relate antioxidant activity to stability of natural extracts;
• FTIR infrared spectroscopy to identify the chemical components responsible for index variations.

These combined analyses provide a global view of the optical, chemical and physical properties of a liquid product.

YesWeLab Services

YesWeLab provides you with a network of ISO 17025 and/or COFRAC , specialized in refractometry, physical chemistry and liquid quality control .
Our experts will work with you to define the measurement conditions adapted to your matrix (temperature, wavelength, concentration) and interpret the results according to the standards applicable to your sector.

Thanks to the YesWeLab digital platform , you can centralize your analysis requests, track your samples and access your results online. Since 2020, YesWeLab has been supporting manufacturers in the cosmetics, food and nutraceutical with their characterization and quality control needs.

To get a quote or speak with our scientific team, contact us today .