Information on the most widely used ASTM standards within the materials testing industry
ISO 304:1985 Surface active agents — Determination of surface tension by drawing up liquid films
ISO 304 specify a method for determining the surface tension of aqueous or organic solutions of surface active agents (surfactants) and mixtures containing them, as well as pure liquids .
The maximum force is measured which is necessary to act vertically on a stirrup or a ring, in contact with the surface of the liquid being examined placed in a measuring cup, in order to separate it from this surface, or on a plate with an edge in contact with the surface, in order to draw up the film that has formed. The surface tension of pure liquids or other solutions can also be measured by this method.
Test Principle
The test measures the maximum vertical force required to either:
Separate a platinum-based plate, stirrup, or ring from the liquid surface;
Draw up the stable liquid film formed between the measuring unit and the liquid. Surface tension (unit: mN/m, 1 mN/m = 1 dyn/cm) is calculated using this force and the geometric perimeter/radius of the measuring unit.

ISO 304 Specific Test Methods
Three interchangeable liquid-film-drawing methods are defined, all using platinum‑iridium components for chemical inertness:
| Plate method | Uses a roughened rectangular platinum plate | ![]() |
| Stirrup method | Uses a thin platinum‑iridium wire stirrup | ![]() |
| Ring method | Uses a circular platinum‑iridium ring (most widely used) |
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ISO 304 Test Equipment Required:
Recommend UnitedTest Brand Tensiometer, include:
*A horizontal platform movable vertically by a micrometer screw (estimating movements to 0.1 mm) .
*A dynamometer to continuously measure force with an accuracy of at least 0.1 mN/m .
*A recording/indicating device.
Measuring Unit (One of the following):
Plate: A 0.1 mm thick rectangular platinum sheet (approx. 30x20 mm), roughened perpendicular to the submerged side, welded to a suspension rod .
Stirrup: Platinum-iridium wire (≤0.1 mm diameter) with a 20-40 mm horizontal arm and 10 mm vertical branches ending in small platinum balls .
Ring: Platinum-iridium wire (0.3 mm diameter) with a circumference between 40 and 60 mm, fixed to a suspending rod .
Measuring Cup: A small glass tank (at least 8 cm in diameter/width). Watch-glasses are strictly prohibited
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| Plate surface tension test | Du Nouy Ring surface tension test |
Test Specimen Information
Material Type: The test applies to aqueous or organic solutions of surface active agents, mixtures of surfactants, pure liquids, or other solutions .
Preparation: Solutions must be prepared with extreme care. If testing aqueous surfactant solutions, the water used must be fresh double-distilled water, verified by checking its own surface tension .
Contamination Sensitivity: Specimens are highly sensitive to atmospheric dust and solvent vapors. The test portion should be sucked from the center of the liquid mass using a pipette to avoid surface contaminants.
Key Test Parameters:
Temperature control: The temperature of the solutions must be strictly maintained to within ±0.5 °C . Measurements near critical solubility points (like the Krafft temperature or cloud point) should be avoided or conducted at specific temperatures above/below these points .
Measurement unit: millinewton per meter (mN/m)
Accuracy: 0.1 mN/m for pure, fully platinum-wetting liquids
Geometric parameters: Perimeter of plate/stirrup, radius of ring
Equilibrium state: Stable surface tension value after time-dependent variation
Surface Age: Since surface tension varies with time as the surfactant adsorbs at the interface, multiple measurements over time are recommended to determine the equilibrium state .
Atmospheric Pressure: Implicitly standard laboratory conditions, but the environment must be protected from vibrations and drafts.
Test Stipluations:
No volatile chemicals allowed in the test room; apparatus protected by a balance bell.
No finger contact with measuring units or inner cup surfaces.
Platinum plates must not be dried by flame.
Strict cleaning: Use sulfochromic acid, phosphoric acid, or special solvents (toluene, perchloroethylene) for stubborn impurities; rinse to neutral with fresh double-distilled water.
Calibration required before each test (with known-mass riders or pure liquids of certified surface tension).
ISO 304 Test Procedures:
| Solution preparation | Prepare samples with ISO 2456-compliant water; control temperature and avoid contamination. |
| Cleaning | Thoroughly clean the measuring cup and plate/stirrup/ring; rinse repeatedly. |
| Levelling | Place a liquid level on the platform and adjust the apparatus base screws until the platform is perfectly horizontal. |
| Calibration | Calibrate the tensiometer according to the specific geometry used (plate, stirrup, or ring) to ensure readings are expressed directly in mN/m |
| Placement | Place the measuring cup containing the specimen liquid on the platform. Raise the platform until the measuring unit (plate, stirrup, or ring) contacts the liquid surface |
| Equilibration | Allow a few minutes for the liquid interface to stabilize after immersion, as disturbing the surface layer affects the initial reading |
| Measurement | Gently lower the platform (or the unit) while maintaining the dynamometer's equilibrium. Record the maximum force applied to the dynamometer at the exact moment the liquid "film" breaks or separates from the measuring unit. |
| Repetition | Perform several successive measurements under the same conditions to ensure reproducibility. |
Test Application (Industry Fields)
This test is fundamental to any industry that utilizes liquids, polymers, or chemical formulations where interfacial properties are critical. Key fields include:
Chemical Manufacturing: Production and quality control of industrial and household surfactants, detergents, and emulsifiers.
Cosmetics & Personal Care: Formulating shampoos, lotions, and creams where foam stability and spreading coefficients are vital.
Pharmaceuticals: Developing liquid drug formulations, emulsions, and understanding wetting properties of coatings.
Petroleum & Oil: Analyzing drilling fluids, evaluating oil recovery processes, and studying crude oil emulsions.
Paints & Coatings: Assessing the wetting and leveling characteristics of paints, inks, and protective coatings on various substrates.
Related test standard:
| EN 14370 | Surface active agents - Determination of surface tension |
| ASTM D1331 | Standard Test Methods for Surface and Interfacial Tension of Solutions of Paints, Solvents, Solutions of Surface-Active Agents, and Related Materials |
| ASTM D1173 | Standard Test Method for Foaming Properties of Surface-Active Agents |
| ASTM D971 | Standard Test Method for Interfacial Tension of Insulating Liquids Against Water by the Ring Method |
| ISO 6889 | Surface active agents; Determination of interfacial tension by drawing up liquid films |
| ISO 6295 | Petroleum products; Mineral oils; Determination of interfacial tension of oil against water; Ring method |
| DIN 53914 | Testing of surface active agents - Determination of surface tension |
| NF T73-060*NF EN 14370 | Surface active agents - Determination of surface tension |
| EN 14210 | Surface active agents - Determination of interfacial tension of solutions of surface active agents by the stirrup or ring method |
| ISO 1409 | Plastics/rubber — Polymer dispersions and rubber latices (natural and synthetic) — Determination of surface tension |
| ASTM D1417 | Standard Test Methods for Rubber Latices—Synthetic |
Related products and device
FAQs about ISO 304 Surface Tension Test
Q1: What is the ISO 304 test and what does it measure?
A: ISO 304:1985 specifies a method for determining the surface tension of aqueous or organic solutions of surface active agents (surfactants), mixtures containing them, and pure liquids. It measures the maximum vertical force required to separate a plate, stirrup, or ring from the liquid surface, or to draw up a liquid film.
Q2: Why is surface tension measurement important for surfactants?
A: Surface tension is a fundamental physical property of liquids. While the standard notes that it does not directly predict practical performance (like detergency or foaming), it is essential for quality control, research and development of new formulations, and understanding interfacial behavior in many industrial applications.
Q3: What types of specimens can be tested using ISO 304?
A: The test applies to aqueous or organic solutions of surface active agents, mixtures of surfactants, pure liquids, or other solutions. Specimens must be prepared with fresh double‑distilled water (for aqueous solutions) and protected from atmospheric dust and solvent vapors.
Q4: How is the tensiometer calibrated?
A: Calibration can be done in two ways: (a) using riders of known mass placed on the measuring unit, or (b) using pure organic liquids with accurately known surface tensions (listed in Annex A of the standard). This ensures the instrument reads directly in millinewtons per metre (mN/m).
Q5: Why must measuring units use platinum‑iridium materials?
A: Platinum‑iridium is chemically inert, resists corrosion, ensures complete liquid wetting, and avoids contamination or reaction with test liquids—critical for accurate results.
Q6: What water is required for sample preparation, and which standard applies?
A: Fresh double‑distilled water complying with ISO 2456 (Surface active agents — Water used as a solvent for tests). Cork/rubber stoppers are forbidden to avoid contamination.
Q7: Why wait for “equilibrium” before taking measurements?
A: Surface tension of surfactant solutions changes over time due to adsorption. Measurements are only valid when the value stabilizes (the flat portion of the time‑surface‑tension curve).
Q8: What are the most common causes of inaccurate results?
A:Surface contamination (dust, volatile solvents in the lab).
Poor cleaning of measuring units/cups.
Finger contact with test components.
Incorrect zero‑setting or calibration.
Ignoring temperature control or equilibrium time.
Using watch‑glasses instead of standard measuring cups.
Q9: Do I need a correction factor for all three methods?
A: Only the ring method requires a correction factor (f) to account for meniscus asymmetry; plate and stirrup methods use direct force‑per‑perimeter calculation.
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