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The iodine number or iodine value (IV) is an important sum parameter used to assess the quality of edible oils and fats. It provides quantitative information about the presence of unsaturated fats and oils. The higher the amount of unsaturated fatty acids in the sample, the more iodine reacts with these double bonds (which are very reactive), resulting in a higher iodine value. For common fats, oils, and waxes (e.g., sunflower, olive, or linseed oil), the iodine value is well known. Hence, it can be used as a test parameter for counterfeit detection in the fight against food fraud.

For the classical iodine value titration, samples must be kept in darkness for up to two hours after the addition of the reaction solution (Wijs solution). This Application Note describes a modified analysis based on EN ISO 3961, ASTM D5554, AOAC 920.159, AOAC 993.20, AOCS Cd 1d-92, USP<401> Method II, and Ph.Eur. 2.5.4 Method B.

Due to the modification, the reaction time reduces significantly from two hours down to five minutes. This modified analysis therefore allows for much higher lab productivity.

The analysis is demonstrated on canola (rapeseed) oil and olive oil.

No sample preparation is required.

The OMNIS Sample Robot S equipped with an OMNIS Professional Titrator, plus a corresponding amount of OMNIS Dosing Modules to add all necessary solutions, and dPt Titrode for the automated determination of iodine value.
Figure 1. The OMNIS Sample Robot S equipped with an OMNIS Professional Titrator, plus a corresponding amount of OMNIS Dosing Modules to add all necessary solutions, and dPt Titrode for the automated determination of iodine value.

The determinations are carried out on an OMNIS Professional Titrator equipped with a maintenance-free dPt Titrode on an OMNIS Sample Robot S (Figure 1). To avoid manually handling chemicals, all solutions could be automatically added using an OMNIS Dosing Module.

An appropriate amount of sample is weighed into the titration beaker, then the beaker is covered with a lid and placed on the sample rack. Before the titration, glacial acetic acid, Wijs solution (ICl), and magnesium acetate solution are added, and the solution is stirred for five minutes. Afterwards, potassium iodide solution is added, and the solution is titrated with standardized sodium thiosulfate until after the equivalence point.

This method offers very accurate results for iodine value, as displayed in Table 1. One exemplary titration curve of olive oil is shown in Figure 2.

Table 1. Results for the iodine value of canola (rapeseed) oil and olive oil.
Sample (n = 5) Mean iodine value in g I2/g sample SD(rel) in %
Canola oil 109.3 0.1
Olive oil 80.9 0.1
Figure 2. Titration curve of the determination of the iodine value in olive oil with the described OMNIS system.

The standards EN ISO 3961, ASTM D5554, AOAC 920.159, AOAC 993.20, AOCS Cd 1d-92, USP<401> Method II, and Ph.Eur. 2.5.4 Method B describe a procedure which requires a reaction time of two hours.

Titration is a very fast and accurate method that can determine the iodine number of oils and fats within just a few minutes. This significantly enhances sample throughput and reduces the cost per analysis. With an OMNIS system, multiple analyses can even be carried out in parallel so that laboratory throughput can be increased even further. This high-end system offers flexible analyses of the iodine number in fats and oils using powerful and intuitive software.

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Metrohm Portugal

R. Frei Luis de Granada 14G
1500-680 Lisboa

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Internal reference: AW TI CH1-1276-062019