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Concentration determination by means of refractive index

Introduction

Solvent concentration influences the sample in many ways, e.g. with regard to electrical conductivity, pH-value, stability, and even color intensity. That’s why for many products, such as soft drinks, it is necessary to determine the concentration of single components. 

Several measurement principles are used to determine the concentration of binary and quasi-binary solutions. They are based on physical principles like density, refractive index or chemical reaction / interaction, e.g. for titration. Depending on the applicative situation, the best solution can be found by recognizing the advantages of these techniques, e.g. accuracy or measurement cycle speed. 

Refractive index and concentration

Principle

The refractive index is described as the speed of light in a medium compared to the speed of light in the vacuum. To obtain a better picture about its influence, it is possible to assume: The more particles or molecules, the slower light travels. Therefore, a water solution with no particles has a lower refractive index than a solution with 10 g of sugar content inside which light travels more slowly.

For this reason, the refractive index can be used either to determine a substance or to correlate its value with a concentration of particles – so the relationship between refractive index and concentration is crucial. Its influence is already used for a range of applications, e.g. the determination of sugar content or sodium hydroxide in water. Unfortunately, this relation is not linear and depends on the solvent as well as on the particles.

An example: Aqueous nitric acid solution shows a curve in its refractive index and concentration diagram. Non-linear fits, e.g. polynomial functions, allow for use of this correlation, and modern refractometers automatically employ it to determine concentration in binary solutions, which counts as a basic application. Different particles or solvents have different influences on the gradient and shape of the correlation curve.

Concentration determination is also possible with so-called quasi-binary mixtures. They contain two major components and some additional ingredients, which exist in very small concentrations compared to the two main components. Due to their small impact on the main refractive index, these additional ingredients can be ignored.

Temperature, too, significantly influences not the curves’ shape, but the refractive index values. While the number of particles / concentrations remain constant with changing temperature, the density and therefore also the particle density change. Higher temperatures will decrease the refractive index as well as the density and vice versa. Nevertheless, it is possible to integrate the temperature influence in this correlation to obtain a refractive index, temperature, and concentration plot.

Finding the right measurement principle for concentration measurement

Benefits:

  • Fast measurement
  • Low sample volume
  • High accuracy
  • No sample preparation
  • Non-destructive
  • Applicable for virtually all sample properties (from liquid to solid)
  • No moving parts – virtually maintenance-free

Limitations:

  • No powders measureable
  • Only binary and quasi-binary solutions accurately determinable

Applications & industries relevant for concentration measurement

Refractometers for concentration measurement are used in various industries.

Chemical industry:

The refractive index is used in the chemical industry as the concentration parameter for basic chemicals, acids, and bases like sodium hydroxide. It is also used in the production process of polymers, e.g. PVC, to determine solvent concentration and final product quality.

Beverage industry:

In beverages and their raw materials, refractometers are used for brix measurement of syrups like HFCS, juices, smoothies and soft drinks. They are also used as caustic meters in the cleaning process (CIP) for beverage process tanks.

Food industry:

Concentration and purity control of sugar is one of the most common refractometer applications in the food industry. The determination of the iodine value for edible fats and oils is a quantitative parameter for the concentration of unsaturated fatty acids.

Pharmaceutical industry:

Salt or glucose concentration measurements for infusion solutions play an important role in the pharmaceutical industry. 

Abbemat refractometers: Overview of application methods

Abbemat refractometers are used in all industries to measure a wide range of samples, from pharmaceuticals, chemicals, petroleum products, flavors, and fragrances to beverages and food. In close cooperation with customers, Anton Paar continuously collects and develops new methods and applications based on refractive index measurements. Below please find the latest list of methods being installed on the Abbemat refractometers.

s → standard
a → available

Method Substance Standard unit Measuring range Temperature [°C] Accuracy Availability Application report
Abbemat 200/300/350
/450

3200/3100/
3000
Abbemat 500/550/
650
Abbemat 200/300/
500
Abbemat 350/550 Abbemat 450/650 Abbbemat 3000 Abbemat 3100 Abbemat 3200
Acids
Acetic acid 0.0 - 67.0 %a) Acetic acid content in aqueous solution g/100 g 0.0 - 67.0 20 0.27 0.05 s s a - a a Link
Acetic acid 67.0 - 85.0 %a) Acetic acid content in aqueous solution g/100 g 67.0 - 85.0 20 0.88 0.28 s s a - a a Link
Acetic acid 86.0 - 99.8 %a) Acetic acid content in aqueous solution g/100 g 86.0 - 99.8 20 1.80 0.66 s s a - a a Link
Citric acid 0.0 - 30.0 %d) Citric acid content in aqueous solution g/100 g 0.0 - 30.0 20 0.1 0.05 a a a - a a Link
Formic acid 0 - 68 %d) Formic acid content in aqueous solution g/100 g 0.0 - 68.0 20 0.4 0.16 a a a - a a Link
Hydrochloric acida) Hydrochloric acid content in aqueous solution g/100 g 0.0 - 37.0 20 0.048 0.009 a a a - - - Link
Lactic acid 0 - 80 %d) Lactic acid content in aqueous solution g/100 g 0.0 - 80.0 20 0.15 0.07 a a a - a a
Nitric acid 0 - 50 %a) Nitric acid content in aqueous solution g/100 g 0.0 - 50.0 20 0.12 0.02 a a a - - - Link
Nitric acid 50 - 68.1 %a) Nitric acid content in aqueous solution g/100 g 50.0 - 68.1 20 50-68.1%: 1.15 50-61%: 0.28 61-65%: 0.80 65-68.1%: 1.15 50-68.1%: 0.25 50-61%: 0.06 61-65%: 0.15 65-68.1%: 0.25 a a a - - - Link
Oxalic acid 0 - 8 %d) Ocalic acid content in aqueous solution g/100 g 0.0 - 8.0 20 0.13 0.054 a a a - a a
Phosphoric acid 0.0 - 40.0 %d) Phosphoric acid content in aqueous solution g/100 g 0.0 - 40.0 20 0.16 0.07 a a a - a a
Sulfuric acid 0.0 - 84.5 %a) Sulfuric acid content in aqueous solution g/100 g 0.0 - 84.5 20 0.142 0.028 s s a - a a Link
Sulfuric acid 87.0 - 98.0 %a) Sulfuric acid content in aqueous solution g/100 g 87.0 - 98.0 20 0.29 0.06 s s a - a a Link
Tartaric acid 0.0 - 58.0 %d) Tartaric acid content in aqueous solution g/100 g 0.0 - 56.0 20 0.08 0.02 a a a - a a
Trichloracetic acid 0.0 - 48.0 %d) Trichloracetic acid content in aqueous solution g/100 g 0.0 - 48.0 20 0.18 0.11 a a a - a a

Alternative methods

Titration

Titration (also known as volumetry, titrimetry, or volumetric analysis) is a quantitative analytic method using a standard solution which reacts with the sample and shows a transition point at a defined ratio. Direct titration is the best-known form of titration which is used to determine, e.g., the amount of acids or bases. 

Finding the right measurement principle for concentration measurement

Benefits:

  • High accuracy
  • Selective concentration determination in multi-component mixtures

Limitations:

  • Sample preparation necessary
  • Accuracy depends on mass solution
  • Destructive measurement
  • Chemical waste
  • Slow measurement cycle

Chromatography

Chromatography, e.g. HPLC (high-pressure liquid chromatography) and GC (gas chromatography), is a quantitatively and qualitatively analytical method for separation and concentration measurement of substances. The principle is based on molecular interaction between the dissolved analyte (mobile phase) and the environment (stationary phase). A stronger interaction with the stationary phase leads to a longer retention period and results in a separation of substances. 

Finding the right measurement principle for concentration measurement

Benefits:

  • High accuracy
  • Selective concentration determination in multi-component mixtures

Limitations:

  • Sample preparation necessary
  • Complex setup and experienced users necessary
  • Chemical waste
  •  Slow measurement cycle