Determination of the free alkali in detergents and soaps: ISO 456, ISO 4314 and the foundation stone of quality control
The chemical basis for soap production is that fatty acids (triglycerides) with a lye, usually sodium hydroxide (NaOH) or potassium hydroxide (KOH), undergo a saponification reaction. Once this reaction is complete, fatty acid salts (soap) and glycerol are formed. However, due to factors such as dosing the amount of alkali in the production process, the degree of complete saponification and the purity of raw materials, a certain amount of unreacted alkali can remain in the product.
This residue is referred to as free alkali, and the Determination of the Free Alkali in Detergents and Soaps is an important analytical parameter for the safety and quality of the product.
Free alkali increases the pH of the soap and can lead to dryness, irritation and eczema-like skin reactions. In addition, it damages the textile fibers during the washing process and thus leads to color fading and material wear. For this reason, international standards for different product types set permissible maximum values for the content of free alkalinity.
What is free alkali and why does it need to be controlled?
Free alkali occurs in two different chemical forms, and this distinction directly determines the design of the analysis method:
- Free Etchalkali: These are alkalis that have not yet received a reaction and are in the form of hydroxides (NaOH or KOH). This is the most reactive and irritating form.
- Free alkalinity: In addition to the etching alkalis, the free alkalinity also includes alkaline salts such as carbonates (Na₂CO₃, K₂CO₃) and bicarbonates.
The difference between the two forms must be known. Because while the overall alkalinity can be a desired property for the washing power of a product (especially in washing powder, carbonates are used as builders), Free etching alkalke is always an indication of a quality deficiency. For this reason, standards stipulate that when determining the free alkaline substance, carbonates must be separated analytically.
The main reasons for controlling the free lye content are:
- Skin compatibility: A high content of free lye impairs the skin barrier, especially for bath soaps and hand cleaners.
- Fabric protection: Alkali residues in washing soaps lead to chemical damage to the fibers and wear over time.
- Product stability: An excess of alkali decomposes the oils and fragrances in the recipe and thus shortens the shelf life.
- Process control: The content of free lye is a direct indicator of the saponification yield and the lye dosage.
- Compliance with legal regulations: National and international standards set maximum limits depending on the product type.
Norms to Determine Free Alkaline
The most important standards used as a reference for the determination of free alkalinity in detergents and soaps differ depending on the parameter to be measured (free alkalinity or total alkalinity) and the product type. ISO 456 Defines the determination of the free etching acres in soaps; ISO 4314, on the other hand, defines the determination of free alkalinity or free acidity in surfactants using a titrimetric method. ASTM D820, on the other hand, offers a more comprehensive framework for the chemical analysis of soaps containing synthetic detergents and covers all parameters including free alkalinity.
The following table summarizes these standards, their scope and typical limits:
| Standard | Scope and measured parameters | Summary of the method | Typical limit values (depending on product type) |
|---|---|---|---|
| ISO 456 | Determination of free alkalinity in soaps (as NaOH or KOH) | Precipitation of the carbonates with barium chloride, then titration with standard HCl (Phenolphthalein indicator) | Toilet soap: usually ≤ 0.1% (as NaOH) |
| ISO 4314 | Determination of free alkalinity or free acid in surfactants | Dissolution of the sample in a neutralized ethanol-water mixture, titration with standard acid or -base | Varies depending on product specification |
| ASTM D820 | Chemical analysis of soaps with synthetic detergents (including free alkalinity) | Production of the sample solution, separate titrimetric determination of the alkalinity components | Varies depending on soap type and recipe |
| ISO 4316 (Related) | Determination of pH in aqueous solutions of surfactants | Potentiometric Measurement | Supplementary indicator for determining free alkalinity |
GLP compliant services for determining free alkalinity with Nanotest
Determination of free alkalinity forms the basis for product safety and control of the production process for manufacturers of detergents and soaps. NanoTest is a reliable solution partner for the detergent industry with its GLP-compliant analysis services in this area.
NanoTest Service Description
NanoTest offers the service Determination of the content of free alkalinity (free alkali determination) for detergents and soap products. Our analyzes are carried out in full compliance with ISO 456, ISO 4314 and ASTM D820 standards.
Advantages of determining the free alkali in Nanotest:
- Standard compliance: Complete compliance with ISO 456, ISO 4314 and ASTM D820 standards.
- Exact chemical separation: Precipitating the carbonates with barium chloride clearly separates the alkalinity and the overall alkalinity.
- Accredited Infrastructure: Internationally recognized test reports thanks to ISO/IEC 17025 accreditation.
- Broad product range: All product types including solid soap, liquid soap, washing powder and liquid detergent.
With its accredited laboratory infrastructure, its specialist staff and its GLP-compliant procedures, NanoTest fully meets the requirements of detergent and soap manufacturers to determine the free alkali. Collaborate with NanoTest to document the safety of your products and optimize your production process.