Soap is used extensively in everyday life. Industrial production of this product is a complicated process that first begins with saponification and then purification to eliminate by-products and impurities of saponification. In the next step, the soap is dried and compacted into noodles or pellets, which are utilized for soap finishing processes like changing into a soap bar.
The quality of the used soap noodles decides the specification of the end product. Following are the standard quality parameters that have to be established:
Total Fatty Matter (TFM)
Total fatty matter (TFM) is the total amount of fatty matter, which can be isolated from a soap following reaction with a mineral acid. Soaps containing low TFM content strip water from the skin, causing it to become dry. Titration is often used to determine this parameter.
Iodine Number or Value (IV)
This parameter refers to the amount of iodine in grams taken up by 100 g of sample. Low IV means fewer amounts of unsaturated fatty acids are present in the soap and therefore the soap bar will tend to be harder. Titration is commonly used to determine this parameter.
C8-C14 content is a total parameter of different 8 to 14 carbon chain fatty acids, which function as surfactants. Chromatographic techniques can be used to determine this parameter.
Vis-NIR spectroscopy is an analytical technique that provides an alternative means for the quality control analysis of soap noodles. The benefit of this technique is that all the aforementioned quality parameters can be determined at the same time in less than one minute without any need for sample preparation, as demonstrated in this article.
For the experiment, a set of customer samples were used which included 46 samples of soap noodles. Using Metrohm XDS SmartProbe Analyzer in reflection mode, near-infrared spectra were obtained (Table 1 and Figure 1).
Table 1. Used equipment and software
||Metrohm order code
|NIRS XDS SmartProbe Analyzer – 2 m Fiber
|Vision Air 2.0 Complete
Figure 1. The NIRS XDS SmartProbe Analyzer was used for spectral data acquisition over the full range from 400 nm to 2500 nm.
After the probe is directly placed on the sample ziplock bag, spectra were subsequently collected over the full Vis-NIR wavelength range of 400 to 2500 nm. For the development of the quantification techniques, data acquisition, and data management, the software package called Vision Air 2.0 Complete was utilized.
A 2nd derivative was used to pre-treat the spectra, and a Partial Least Squares Regression (PLS) was carried out over the particular spectral regions. Figure 2 shows the typical pretreated spectra.
Figure 2. 2nd derivative spectra of the soap noodle samples.
Total Fatty Matter
TFM’s concentration range was 78.5 to 80%. When the model was fitted with 5 factors, it yielded a standard error of cross validation (SECV) of 0.13% and a standard error of calibration (SEC) of 0.10%, as shown in Figure 3.
Figure 3. Correlation plot of the total fatty matter content determined by titration and by NIRS.
With regards to the iodine value, its calibration range was 36–40. When the model was fitted with 6 factors, it produced a standard error of calibration (SEC) and a standard error of cross validation (SECV) of 0.39 and 0.45, respectively as shown in Figure 4.
Figure 4. Correlation plot of the iodine value determined by titration and by NIRS.
C8-C14 content has a concentration range of 1 to 17%. When the model was fitted with 5 factors, it yielded a Standard Error of Cross Validation (SECV) of 0.21% and a Standard Error of Calibration (SEC) of 0.17%, as shown in Figure 5.
Figure 5. Correlation plot of the C8–C14 content determined by chromatography and by NIRS. These results show that NIRS is excellently suited to reliably determine different quality parameters in soap noodles and, thus, it can be used in quality control.
The iodine value, C8–C14 content, and total fatty matter content in soap noodles were determined with a Metrohm NIRS XDS SmartProbe Analyzer. Results with acceptable accuracy and standard errors were provided by the calibration models of the Vis-NIR method. Vis-NIR spectroscopy has been shown to be ideal for the high throughput quality control analysis technique of soap production steps. This method saves both time and cost for routine analysis of soap noodles, and when combined with the intuitive Vision Air Software, it can be effectively performed even by novice operators. Moreover, additional quality parameters in soap and soap noodles like active detergents, moisture, and acid value can also be determined by Vis-NIR.
At Metrohm is one of the world’s most trusted manufacturers of high-precision instruments for chemical analysis. Metrohm was founded in 1943 by engineer Bertold Suhner in Herisau, Switzerland. Today, Metrohm is represented in 120 countries by subsidiaries and exclusive distributors. The global Metrohm Group also includes the Dutch companies Metrohm Applikon and Metrohm Autolab, manufacturers of online analyzers and instruments for electrochemical research, respectively. Recently, the Metrohm Group was joined by Metrohm Raman, a leading manufacturer of handheld Raman spectrometers.
Metrohm is the global market leader in analytical instruments for titration. Instruments for ion chromatography, voltammetry, conductivity, and stability measurement make the Metrohm portfolio for ion analysis complete. Instruments for Near-infrared and Raman spectroscopy are another, strongly growing segment of the Metrohm portfolio.
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