Reference solution detergency determination is a core laboratory procedure for evaluating the soil-removal performance of detergents against a standardized benchmark solution. This testing approach, governed primarily by GB/T 13174 and related international protocols, provides objective, reproducible data that manufacturers, regulators, and accredited third-party laboratories rely on for product development, quality control, and compliance. This article explains the technical principles, sample requirements, testing procedures, calculation methods, and practical applications of reference solution detergency testing.
Technical Overview
Reference solution detergency determination refers to the quantitative evaluation of cleaning performance by comparing the soil-removal capability of a test detergent against a standardized reference solution under strictly controlled laboratory conditions. The reference solution, typically prepared from a standard reference detergent with a defined composition, serves as a calibration benchmark that ensures comparability of results across different laboratories, instruments, and test batches.
The fundamental principle involves washing standardized soiled test cloths in a controlled washing device—such as a Terg-o-tometer or bottle-type detergency machine—using both the reference solution and the test detergent solution under identical conditions of temperature, time, agitation, water hardness, and bath ratio. Detergency is quantified by measuring the reflectance (whiteness) of the soiled cloth before and after washing using a reflectance spectrophotometer, and the results are expressed as a detergency ratio (R-value) relative to the reference solution.
This methodology is established in the Chinese national standard GB/T 13174, "Determination of detergency and anti-soil-deposition property of laundry detergents," and shares conceptual alignment with international practices described in ISO 2267 (assessment of reference detergents), ISO 4319, and ASTM D3050-related washing performance evaluations. The use of a reference solution eliminates variability caused by differences in water quality, cloth batch, and machine operation, making it the recognized basis for performance claims in the detergent industry.
Testing Samples
Standard Soiled Test Cloths
The core test specimen is a standardized soiled cloth, typically cut into circular or square pieces of specified dimensions (commonly 6 cm × 6 cm or diameter 10 cm discs). Chinese standard practice defines several soiled cloth types, each designed to represent a specific soil category:
- JB-01: Carbon black/sebum oily soil cloth for evaluating general detergency
- JB-02: Protein-soiled cloth for assessing removal of proteinaceous stains
- JB-03: Sebaceous soil cloth simulating body-oil staining
- JB-04 and JB-05: Additional soil matrices for specialized performance evaluation such as greasy particulate soils
Each soiled cloth batch must be characterized by its reflectance value range, and only cloth pieces falling within the specified uniformity limits are qualified for testing. Cloth must be conditioned at constant temperature and humidity before measurement.
Reference Solution
The reference solution is prepared from a standard reference detergent whose formulation is defined in the normative annex of GB/T 13174. The detergent is dissolved at a specified concentration in standardized hard water (typically 250 mg/kg as calcium carbonate, with a defined calcium-to-magnesium ratio) at a controlled temperature. The solution must be freshly prepared and used within a defined time window to prevent degradation or precipitation.
Test Detergent Solutions
The detergent under evaluation is dissolved under the same concentration and hardness conditions as the reference solution. Special test requirements—such as low-temperature washing, high-hardness water, or concentrated dosing—may be applied depending on the product positioning and the applicable performance standard.
Standard Hard Water
Artificial hard water of defined hardness is prepared from calcium chloride and magnesium sulfate stock solutions. Water hardness, typically 250 mg/kg (as CaCO₃) with a Ca²⁺:Mg²⁺ molar ratio of 6:4, must be verified before each test series because hardness significantly influences surfactant performance.
Testing Items
The key parameters determined in reference solution detergency testing include:
- Detergency ratio (R): the relative cleaning power of the test detergent versus the reference solution, calculated from reflectance measurements
- Whiteness difference (ΔW): the change in reflectance of soiled cloth after washing
- Average detergency value of the reference solution itself, used to verify the validity of each test run
- Anti-soil-deposition performance: the tendency of the wash liquor to redeposit soil onto clean white cloth, often measured in parallel
- Comparison ratio against standard thresholds: for example, an R-value ≥ 1.0 indicates performance equivalent to or better than the reference detergent, while specific product standards may require ratios such as ≥ 1.2 for premium performance claims
- Repeatability and reproducibility checks across parallel specimens to confirm statistical validity
Testing Methods
Sample Preparation and Pre-Test Verification
Soiled cloth pieces are selected, numbered, and their initial reflectance (F₁) is measured using a calibrated reflectance spectrophotometer at a defined wavelength (typically 546 nm under GB/T 13174 conditions). The reference solution and test detergent solutions are prepared with standardized hard water at the specified concentration, and their temperature is equilibrated to the test condition—commonly 30 °C ± 1 °C for standard evaluation, with 20 °C or 25 °C variants for low-temperature claims.
Washing Procedure
The washing trial is conducted in a detergency testing machine (Terg-o-tometer type or bottle-type washer) under fixed parameters:
- Wash temperature: typically 30 °C ± 1 °C (or other specified temperature)
- Wash time: typically 20 minutes per cycle
- Agitation speed: standardized stirring or rotation rate defined by the equipment standard
- Bath ratio: defined mass ratio of wash solution to cloth load
- Number of soiled cloth pieces per bath: commonly three pieces per test beaker, with parallel runs for reference and test solutions
After washing, the cloth pieces are rinsed in prepared hard water, dried in a controlled environment (oven or air-drying under specified conditions), and conditioned before final reflectance measurement.
Reflectance Measurement and Calculation
The reflectance of each washed cloth piece (F₂) is measured at the same wavelength and geometry as the initial reading. The whiteness increase and detergency ratio are calculated using the standard formula:
Detergency (R) for each cloth is computed as (F₂ − F₁) / (F₀ − F₁), where F₀ is the average reflectance of the unsoiled white control cloth, F₁ is the reflectance of the soiled cloth before washing, and F₂ is the reflectance after washing. The mean detergency of the test detergent (Rₜₑₛₜ) is divided by the mean detergency of the reference solution (Rᵣₑf) obtained in the same run to yield the comparison ratio. A ratio greater than 1.0 demonstrates that the test product outperforms the reference solution under the declared conditions.
Quality Control of Test Runs
Each analytical run is considered valid only when the reference solution produces detergency results within the acceptance range established for the soiled cloth batch and laboratory control charts. If the reference value falls outside control limits, the entire run is rejected and repeated. Statistical evaluation—using the range of parallel determinations and the specified repeatability limits—confirms the reliability of the reported ratio.
Reported Results
The final test report states the soiled cloth types used, washing conditions (temperature, time, hardness, dosage), individual and mean detergency values for the reference solution and test detergent, the calculated detergency ratio, and a conclusion regarding the performance grade claimed by the product standard.
Applications
Reference solution detergency determination is applied across the full life cycle of detergent products and related regulatory activities:
- Product development: formulators screen surfactant systems, builders, enzymes, and polymers by measuring detergency ratios against the reference solution at each optimization stage
- Quality control: routine batch release testing verifies that production lots maintain the declared performance level
- Standard compliance: products seeking certification under GB/T 13174-based product standards—including laundry powders, liquids, and concentrated formulations—must demonstrate detergency ratios meeting the required thresholds
- Performance labeling and marketing claims: claims such as "twice the cleaning power of standard detergent" or low-temperature efficacy must be substantiated by reference-based detergency data
- Regulatory supervision: market surveillance authorities use reference detergency testing to verify that marketed products fulfill mandatory or recommended performance requirements
- Comparative benchmarking: accredited third-party laboratories conduct side-by-side evaluations of competing formulations under identical, controlled washing conditions
- Research applications: academic and industrial laboratories use the method to study soil removal mechanisms, surfactant synergy, and the influence of water hardness, temperature, and wash cycle variables
FAQ
What does a detergency ratio of 1.0 mean?
A detergency ratio (Rₜₑₛₜ/Rᵣₑf) of 1.0 means the test detergent removes soil at the same level as the standard reference solution under identical washing conditions. Values above 1.0 indicate superior performance; product standards commonly set thresholds such as 1.1 or 1.2 for enhanced-performance classifications.
Why is a reference solution necessary instead of testing detergency alone?
Absolute detergency values vary with soiled cloth batch, water hardness, equipment condition, and operator technique. Running the reference solution in parallel normalizes these variables, enabling valid comparisons between laboratories, over time, and between products.
Which soiled cloth should be used for testing?
The selection depends on the performance claim being evaluated. JB-01 carbon black-oily soil cloth is used for general detergency, JB-02 for protein stain removal, and JB-03 for sebum-type soil. Comprehensive evaluations often combine multiple cloth types to characterize performance across soil categories.
What washing temperature is used in the standard method?
GB/T 13174 specifies 30 °C ± 1 °C as the standard evaluation temperature for most detergency comparisons. Lower temperatures such as 20 °C or 25 °C are used when the product claims cold-water washing performance, and the test report must state the temperature applied.
How is water hardness controlled during testing?
Standardized hard water is prepared gravimetrically from calcium and magnesium salt stock solutions to achieve the target hardness, typically 250 mg/kg expressed as calcium carbonate with a defined Ca:Mg ratio. Hardness is verified before each test series because uncontrolled hardness can distort surfactant performance and invalidate results.
Can reference solution detergency testing support international certification?
Yes. While GB/T 13174 governs the Chinese reference method, the underlying comparative principle is recognized internationally through ISO 2267 and related performance-testing frameworks. Accredited third-party laboratories routinely conduct reference-based detergency determinations to support product registration, eco-labeling, and export compliance documentation.
How long does a typical detergency determination take?
A complete test cycle—including cloth selection, pre-measurement of reflectance, solution preparation, washing, drying, post-measurement, and calculation—typically requires one to two working days. Replicate runs and multi-cloth evaluation programs extend the timeline accordingly to ensure statistical robustness.