AN-NIR-150
2026-08
Monitoring the production of stearic acid and its salts with NIR spectroscopy
Acid number, iodine value, granulometry, moisture, and more
Summary
Stearic acid and its derivatives (e.g., magnesium, calcium, and zinc stearates) are widely used in the lubricants, pharmaceutical, cosmetics, plastics, and food industries. Producing these materials involves several steps: clarification, hydrogenation, cleavage, and distillation, followed by converting stearic acid into its respective stearates. This Application Note demonstrates how the OMNIS NIR Analyzer can be used to monitor the stearic acid iodine value, acid value, moisture, ash content, melting point, and granulometry during the production process and in the final product. Near-infrared (NIR) spectroscopy is a rapid, nondestructive analytical technique that is fast becoming a preferred method over other slower, more costly alternatives.
Experimental equipment
Quality control during stearic acid production is essential to guarantee consistency and compliance with industry standards. Each stage of the process, from initial raw material preparation to the final conversion into stearates, requires analytical checks. The samples used in this study include:
Pre-entry (raw material):
Acid number/iodine value – 44/40 samples
Hydrogenation:
Acid number – 17 samples
Cleavage:
Acid number – 99 samples
Mg, Ca, and Zn stearates:
Ash, granulometry, melting point, and moisture – 69 samples
During stearic acid production, samples occur in two forms depending on the process stage. Early steps involve liquid samples (a viscous mixture of saturated fatty acids and/or animal fat). After purification, the product solidifies into flakes, powder, or pellets, representing the final stearic acids and their salts. Since quality control is required at different stages of production, both liquid and solid sample types can be measured with the OMNIS NIR Analyzer Liquid/Solid (Figure 1). This allows an easy transition from monitoring intermediate products to performing quality control of final stearic acids.
The OMNIS Software (Table 1) was used for all data acquisition, and prediction model development was done with the help of the OMD (OMNIS Model Developer).
Liquid samples (pre-entry, hydrogenation, cleavage):
- Each sample was measured in transmission mode using an 8 mm disposable vial.
- Spectra were collected at 75 °C using the vessel temperature control mode.
Solid samples (Mg, Ca, and Zn stearates):
- Each sample was measured by reflectance mode in a 28 mm disposable vial.
| Equipment | Article Number |
|---|---|
| OMNIS NIR Analyzer Liquid/Solid | 2.1072.0010 |
| Disposable vials, 28 mm, reflection | 6.7402.140 |
| Flexible holder OMNIS NIR | 6.07402.300 |
| Disposable vial, 8 mm, transmission | 6.7402.240 |
| Holder OMNIS NIR, vial, 8 mm | 6.07401.070 |
| OMNIS Stand-Alone license | 6.06003.010 |
| Software license Quant Development | 6.06008.002 |
Result
NIR spectroscopy was successfully applied to monitor both process samples and final stearate products. Figure 2 shows NIR spectra collected from cleavage samples, illustrating the spectral information used for model development. Quantitative models were developed for stearic acid's acid value, iodine value, moisture, ash content, melting point, and granulometry. The reference values used were obtained using the routine quality control methods employed during stearic acid and stearate production. Figures 3–9 present the correlation plots and corresponding figures of merit (FOMs) for each parameter. The data points along the regression line indicate good correlation between the NIR model and the reference method. The FOMs provide additional information about model performance, including calibration accuracy and robustness.
Overall, the developed models demonstrated good correlation with the laboratory reference values, indicating that the selected NIR spectral range contains sufficient information for reliable quality control throughout the production process.
Result acid number in pre-entry samples
| R2 | SEC | SECV | SEP |
|---|---|---|---|
| 0.994 | 0.16 mg KOH/g | 0.21 mg KOH/g | 0.12 mg KOH/g |
Result iodine value in pre-entry samples
| R2 | SEC | SECV | SEP |
|---|---|---|---|
| 0.962 | 0.40 g l2/100 g | 0.46 g l2/100 g | 0.52 g l2/100 g |
Result acid number in the hydrogenation process
| R2 | SEC | SECV | SEP |
|---|---|---|---|
| 0.921 | 0.11 mg KOH/g | 0.40 mg KOH/g | 0.17 mg KOH/g |
Result ash in stearates
| R2 | SEC | SECV | SEP |
|---|---|---|---|
| 0.971 | 0.29% | 0.33% | 0.36% |
Result moisture in stearates
| R2 | SEC | SECV | SEP |
|---|---|---|---|
| 0.992 | 0.14% | 0.14% | 0.15% |
Result melting point in stearates
| R2 | SEC | SECV | SEP |
|---|---|---|---|
| 0.996 | 1.05 °C | 1.40 °C | 1.25 °C |
Result granulometry in stearates
| R2 | SEC | SECV | SEP |
|---|---|---|---|
| 0.862 | 0.09 mm | 0.12 mm | 0.15 mm |
Conclusion
The results obtained in this study demonstrate the strong applicability of NIR spectroscopy for quantitative analysis of both process samples and final products across multiple parameters during the production of stearic acid and its salts.
The OMNIS NIR Analyzer offers significant operational advantages by combining liquid and solid measurement modules in a single instrument, making sample analysis simple and efficient. The liquid module’s ability to heat samples up to 80 °C with precise temperature control ensures complete melting of samples with higher melting points, which improves measurement accuracy and reproducibility.
The final Mg, Ca, and Zn stearates are widely used in pharmaceuticals (as tablet lubricants and excipients), cosmetics (in creams, lotions, and soaps), food (as emulsifiers and coating agents), lubricants (as thickeners and stabilizers), and other sectors such as plastics and rubber (as stabilizers and release agents). This versatility shows the importance of robust analytical solutions like NIRS for quality control across diverse industries.
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