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D1747-09 – Standard Test Method Technical Guide
ASTM D1747-09 (Reapproved 2019) outlines the standard test method for determining the refractive index of transparent, light-colored viscous hydrocarbon liquids and melted solids. This test method is critical for material identification, purity assessment, and quality control in petroleum and chemical processes.
🔬 Scope and Applicability
This test method covers the measurement of refractive indexes of viscous materials with an accuracy of two units in the fourth decimal place. The applicable refractive index range spans from 1.33 to 1.60, and the standard test temperature range is between 80 °C and 100 °C. Lower temperatures may be employed provided the sample’s melting point is at least 10 °C below the test temperature.
The method is restricted to transparent and light-colored liquids. Samples with an ASTM Color greater than No. 4, as determined by Test Method D1500, are excluded from this procedure due to potential accuracy degradation. Similarly, liquids that readily smoke or vaporize at the test temperature are not suitable.
| 📐 Parameter | 📏 Specification / Requirement | 🎯 Reference |
|---|---|---|
| Refractive Index Range | 1.33 to 1.60 | Use > 1.60 possible, accuracy unevaluated |
| Test Temperature Range | 80 °C to 100 °C | Lower temp allowed if MP is 10 °C below test temp |
| Stated Accuracy | ± 0.0002 | Two units in the fourth decimal place |
| Maximum Sample Color | ≤ ASTM Color No. 4 | As defined by Test Method D1500 |
⚙️ Apparatus and Safety Considerations
The test relies on a refractometer capable of maintaining precise temperature control. Thermometers must conform to Specification E1 and should be periodically verified following Test Method E77. Toluene, used as a solvent, must meet the specifications of Specification D841 (Nitration Grade).
⚠️ Safety Warning: This method historically references instruments containing mercury, which the EPA has designated as a hazardous material capable of causing central nervous system, kidney, and liver damage. Extreme caution is required when handling mercury or mercury-containing products. Users should consult the MSDS and applicable state laws, as the sale of mercury may be prohibited.
📊 Key Terminology and Measurement
The standard defines refractive index as the ratio of the velocity of light of a specified wavelength in air to its velocity in the substance under examination. It is determined as the sine of the angle of incidence divided by the sine of the angle of refraction.
| 🟦 Term | 📏 Definition / Formula | ⚡ Notes |
|---|---|---|
| Refractive Index (n) | n = sin(i) / sin(r) | Relative index (air to substance) |
| Absolute Refractive Index | n_abs = n × 1.00027 | To convert to vacuum reference |
| Wavelength Dependence | n varies inversely with wavelength | Typically measured at the sodium D line |
| Quality Assurance | Practice D6299 | Statistical control charting for performance |
📋 Test Procedure and Quality Control
The accuracy of the test depends on strict temperature control. The user must establish appropriate safety, health, and environmental practices prior to use. Effective application of statistical quality assurance techniques, such as those described in Practice D6299, is essential for evaluating the analytical measurement system performance.
💡 Technical Note: While the instrument can be successfully used for refractive indices above 1.60, certified liquid standards for ranges above this value are not yet available. Consequently, the accuracy of measurements under these extended conditions has not been officially evaluated. Data generated outside the 1.33 to 1.60 range should be interpreted with caution regarding absolute accuracy.
❓ Frequently Asked Questions
🔍 What materials are suitable for testing under ASTM D1747?
This standard applies to transparent and light-colored viscous hydrocarbon liquids and melted solids. The sample must not be darker than ASTM Color No. 4 (D1500), must not smoke or vaporize readily at the test temperature, and must have a melting point at least 10 °C below the test temperature.
💡 What is the specified accuracy and temperature range?
The test method provides accuracy to two units in the fourth decimal place (e.g., 1.5000 ± 0.0002). The specified standard test temperature range is 80 °C to 100 °C, though lower temperatures are acceptable if the melting point condition is satisfied.
⚡ What does the refractive index value represent?
The refractive index is the ratio of the velocity of light in air to its velocity in the substance. It is practically calculated as the sine of the angle of incidence over the sine of the angle of refraction. The absolute refractive index (referred to a vacuum) is obtained by multiplying the standard value by 1.00027.
📌 Why is the 10 °C differential between melting point and test temperature required?
This differential ensures the sample remains completely molten and free of crystal nuclei during measurement. Preventing crystallization or solidification within the instrument is critical, as any solid matter would scatter the light beam and produce inaccurate refractive index readings.