pH value is one of the most fundamental quality and safety parameters for chemical products, and Chinese national standards (GB) define unified potentiometric methods for its accurate determination. This article explains the key GB standards for chemical pH testing, including GB/T 9724, GB/T 23769, and GB/T 6368, together with applicable sample types, test procedures, precision requirements, and practical application scenarios. The information helps manufacturers, traders, and quality engineers select the correct standard and obtain reliable, internationally accepted results from accredited third-party laboratories.
Technical Overview
The pH value characterizes the acidity or alkalinity of an aqueous medium and is defined as the negative decimal logarithm of the hydrogen ion activity. For chemical products, pH governs reaction behavior, product stability, corrosion risk, packaging compatibility, and downstream process performance, which makes it a mandatory indicator in the majority of Chinese product specifications.
Chinese national standards establish a harmonized measurement framework based on the potentiometric glass electrode principle. The principal standards include:
- GB/T 9724-2007, Chemical reagents - General rule for the determination of pH value, the core method for laboratory and fine chemicals, technically aligned with ISO 6353-1
- GB/T 23769-2009, Inorganic chemical products for industrial use - General method for the determination of pH value
- GB/T 6368-2008, Surface active agents - Determination of pH of aqueous solutions - Potentiometric method, aligned with ISO 4316
- GB/T 7573-2009, Textiles - Determination of pH of aqueous extract, aligned with ISO 3071
- GB/T 1545-2008, Paper, board and pulp - Determination of pH of extracts, aligned with ISO 6588
- GB 6920-86, Water quality - Determination of pH value - Glass electrode method
Where a product-specific GB standard defines its own pH limit and test concentration, that product standard takes precedence, while the general methods above govern the measurement technique itself.
Testing Samples
Aqueous Chemical Solutions
Acids, alkalis, salt solutions, and process liquors are measured directly as supplied or after dilution to the concentration stated in the product specification. The sample must be homogeneous, free from oil layers or suspended matter that could foul the electrode, and protected from carbon dioxide absorption during handling.
Solid and Powdered Chemicals
Water-soluble solids are dissolved in carbon dioxide-free water to prepare a test solution, commonly 10 g/L or a 10 percent mass fraction, as specified. Poorly soluble products are characterized by the pH of a saturated solution or an aqueous extract prepared at a defined solid-to-liquid ratio.
Viscous Products, Pastes, and Emulsions
Adhesives, creams, and emulsified formulations are dispersed or diluted with water under standardized conditions, for example a 1:10 dilution for cosmetics under the general cosmetics test method series, before the measurement is taken.
Surfactants and Detergent Formulations
Surface active agents are measured as aqueous solutions at agreed concentrations such as 1, 5, or 10 g/L following GB/T 6368, because the pH of surfactant solutions often varies with the dilution level.
Textiles, Paper, and Solid Finished Goods
These materials are cut or milled and extracted in water or potassium chloride solution at a defined liquor ratio, and the pH of the resulting extract is reported as the characteristic value.
Testing Items
- pH value of the neat liquid product as supplied
- pH of an aqueous solution or extract at a specified concentration, such as 10 g/L or 10 percent
- pH of aqueous extracts from solids, textiles, paper, and pulp
- Measurement temperature, typically 25 °C, reported together with the result
- Verification of conformity with the pH limits stated in the applicable product standard
Testing Methods
Potentiometric Method with Glass Electrode
All major GB pH methods use the potentiometric principle. A glass electrode sensitive to hydrogen ion activity and a reference electrode, typically silver-silver chloride or saturated calomel, are immersed in the test solution. The measured potential difference follows the Nernst equation and is converted by the pH meter into a direct pH reading.
Instrument Calibration and Buffer Solutions
The meter is calibrated before use with certified standard buffer solutions traceable to national metrology standards. Routine two-point calibration brackets the expected pH using, for example, pH 4.00 potassium hydrogen phthalate, pH 6.86 mixed phosphate, and pH 9.18 borax buffers at 25 °C. The electrode slope should fall within approximately 95 to 105 percent of the theoretical value; otherwise the electrode is cleaned or replaced.
Sample Preparation and Measurement
Test solutions are prepared with carbon dioxide-free distilled or deionized water meeting reagent water requirements. The solution temperature is controlled, normally at 25 °C plus or minus 1 °C, and the instrument applies automatic or manual temperature compensation. The electrode is rinsed and blotted between solutions, immersed in the test sample, and the reading is recorded once it stabilizes. Duplicate determinations are performed and the mean is reported when the difference meets the repeatability limit, commonly 0.05 pH units or less.
Precision and Quality Control
Benchtop pH meters with a resolution of 0.01 pH units are recommended. Quality control includes regular calibration verification, buffer expiry management, proper electrode storage, and participation in interlaboratory comparisons. ISO/IEC 17025-accredited third-party laboratories apply these controls systematically and issue results with full measurement traceability.
Applications
pH testing to GB standards supports the full chemical product life cycle:
- Incoming inspection of raw materials such as acids, alkalis, salts, and surfactants against purchase specifications
- In-process quality control and batch release in reagent, fine chemical, and commodity chemical production
- Compliance demonstration for industrial inorganic chemicals, water treatment chemicals, and surface active agents
- Textile, paper, cosmetics, and adhesive product evaluation for ecological and consumer safety requirements
- Wastewater and environmental monitoring referenced to discharge limits
- Export documentation and customs clearance supported by reports from accredited third-party laboratories
FAQ
Which GB standard applies to my product?
Chemical reagents follow GB/T 9724, industrial inorganic chemicals follow GB/T 23769, surfactants follow GB/T 6368, and textiles follow GB/T 7573. If a product-specific GB standard states its own pH requirement and test conditions, that specification governs the sample preparation while the general method governs the measurement itself.
How much sample is required?
About 50 to 100 grams or milliliters is normally sufficient for duplicate testing, buffer checks, and laboratory retention. Highly heterogeneous materials may require larger quantities or composite sampling.
What is the typical turnaround time?
The pH determination itself takes less than one hour. Including registration, sample preparation, and report issuance, accredited third-party laboratories usually complete the service within three to seven working days.
Why is temperature control so important?
Both the electrode response and the true pH of a solution depend on temperature, and the assigned values of buffer solutions change with temperature. GB methods therefore specify measurement at 25 °C with temperature compensation, ensuring results are comparable between laboratories.
Can pH be measured for non-aqueous or oily products?
pH is strictly defined for aqueous media. For oily or non-aqueous products, laboratories report the pH of a defined aqueous extract or dilution, or use special electrodes to obtain an operational pH value agreed with the client.
Are the test reports recognized internationally?
Yes. Reports issued by laboratories accredited to ISO/IEC 17025 are widely accepted by regulators, buyers, and certification bodies worldwide for domestic compliance and international trade.