• Luca Rota Written by Luca Rota, Application Specialist | April 3, 2024

Validation of a microwave-assisted protocol for the determination of saponification number: a comparison to ISO 3657:2020

Sequential determination of FAMEs, total fat and free fat determination


One of the most commonly used analytical techniques to evaluate the quality of oils and fats is the saponification number (or value). It is a measure of the free and esterified fatty acids present in a lipid-containing matrix. The analysis, based on the ISO 3657:2020 method, consists in titrating the amount of KOH in excess for the complete saponification of one gram of oil or fat. The saponification number is an indirect measure of the length of the fatty acids in the lipid. A high saponification number, such as that of coconut or palm oil, is desirable for soap production. Another important use of the parameter is quality control for possible adulteration.
Although the ISO 3657:2020 reference method is simple and accurate, it is time consuming and requires adequate laboratory space if this analysis is to be performed routinely in the laboratory.
Thus, with the aim of optimizing analysis, Microwave Assisted Extraction is a good candidate as a new time saving and reliable method for determining the saponification value.
In the article, we have developed a simple, accurate and faster method for the determination of the saponification value using ETHOS X together with the fastEX-24 eT rotor, reducing the preparation time and the space required to carry out the analyses.
The results obtained were compared with the ISO 3657:2020 method, firstly on some reference vegetable oils and then on the most widely used oils on the market. Finally, the microwave method is also applied to some widely used cosmetic raw materials, obtaining a comparison also with the official method described by the European Pharmacopoeia.


- Milestone’s ETHOS X (5) with easyTEMP - FastEX-24 eT rotor. - 100-mL disposable glass vials.

Potassium hydroxide (KOH) 0.5 M solution in ethanol and hydrochloric acid (HCl) 0.5 M standard volumetric solutions were used for saponification and titration respectively with phenolphthalein solution (0.1 g / 100 ml of 96 % ethanol) as indicator. All chemicals and solvent used are analytical grade and were purchased from Sigma-Aldrich (Milan, Italy).

The reference vegetable oils used in this study were purchased from BIPEA (Proficiency testing programs Paris – FRANCE) while commercial vegetable oils come from the local market or supplied directly by Associazione Granaria-Milan (Italy); for cosmetics raw materials laboratory samples with certified value were employed.

Sample amount depends on the expected saponification value and it is suggested respectively by the ISO 3657:2020 method for vegetable oils reported on Table 1 and by Pharmacopeia 01/2008:20506 for cosmetic raw materials reported on Table 2:

Sample to be saponified was added to the glass vials of the fastEX-24 rotor and 25 mL of ethanolic KOH 0.5 M solution was added followed by stirring bar. The following temperature method was employed under continuous stirring:

After cooling, the exceeding amount of KOH is directly titrated with HCl 0.5 M into the glass vial, adding also 0.5-1 mL of Phenolphthalein color indicator solution. The equivalence point is reached when the color changes from pink/purple to colorless. While most of the vegetable oils are titrated at room temperature; coconut oil, palm oil and cosmetic raw material should be titrated still warm to avoid sample solidification and erroneous quantification.


The first tests conducted were made on eight different reference materials and a comparison was made between saponification results obtained with ETHOS X and following ISO 3657:2020 method:

The precision of the method was determined by carrying out six analyses under repeatability conditions on three reference materials, where the tests were carried out on the same day and by the same operator. The results of the precision study are presented in Table 5:

The ratio between the relative standard deviation % (RSD%) under intermediate precision and the RSD% calculated by the Horwitz equation is an indicator of the precision of the analysis and is known as the HORRAT value. The HORRAT is usually used to indicate the presence of analytical problems that affect the precision of the analysis: values below 1 indicate good analytical precision, values between 1 and 1.5 are acceptable results, while values above 2 indicate analytical problems. After this validation of the effectiveness of the method, it was applied again, together with ISO 3657:2020, on several commercial oil matrices. The results are shown in Table 6:

Finally, the ETHOS X approach was also tested for saponification on cosmetic ingredients and the results are shown in Table 7:


The results show that the ETHOS X with fastEX-24 eT approach for saponification gives comparable results to both reference material values and the ISO 3657:2020 standard method. Microwave saponification showed clear advantages over the official ISO methods. In fact, the use of microwave as a heating source allows to significantly reduce the time of saponification (20 min compared to 1 h of the official ISO method) and to obtain generally better results in terms of reproducibility (RSD). This observation can be attributed to the better homogenisation provided by the constant stirring of the magnetic stirring rod during both the saponification and the titration processes. In addition, the rotor, which can hold up to 24 samples, reduces the laboratory space and time required for the analysis of several samples. For all these reasons, the ETHOS X has proved to be a successful and innovative approach to oil and fats saponification.

Milestone’s ETHOS X with fastEX-24 eT rotor

Sample to be saponified was added to the glass vials of the fastEX-24 rotor and 25 mL of ethanolic KOH 0.5 M solution was added followed by stirring bar.

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Validation of a microwave-assisted protocol for the determination of saponification number: a comparison to ISO 3657:2020
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