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D5355-95 – Standard Test Method Technical Guide
🧪 Overview, Scope, and Key Definitions
ASTM D5355-95 (Reapproved 2021) establishes a standard gravimetric procedure for measuring the specific gravity of oils and liquid fats. This test method is primarily utilized for quality control of softening agents employed in the leather industry. The specific gravity is determined by calculating the ratio of the weight of a unit volume of the sample to the weight of a unit volume of water at a precisely controlled temperature of 25 °C.
The standard clarifies the definitions of its test materials (Section 2.1). Understanding these classifications is essential for applying the correct measurement procedure and interpreting the results.
| 📖 Term | 🟦 Definition (Section 2.1) |
|---|---|
| Density | Weight per unit volume, expressed as grams per cubic centimeter (g/cm³). |
| Specific Gravity | The ratio of the density of a substance to the density of the reference substance (water at 25 °C). |
| Oils | Substances fluid at 25 °C, consisting of glycerides of fatty acids, aliphatic hydrocarbons, and aromatic hydrocarbons. |
| Liquid Fats | Fats that melt below 58 °C. |
✅ Significance and Use: This test method falls under the jurisdiction of ASTM Committee D31 on Leather. The specific gravity value serves as a simple yet effective parameter for monitoring the quality and consistency of oils and fats used to soften leather (Section 3.1). The values stated in SI units are regarded as the standard.
⚙️ Apparatus Requirements and Test Procedure
The accuracy of this test method relies heavily on precisely calibrated equipment and strict adherence to temperature control. The table below details the required apparatus and their specified tolerances as defined in Section 4.
| 🟦 Apparatus | 📏 Specification | 🎯 Tolerance |
|---|---|---|
| Specific Gravity Bottles | ~50 mL capacity, ground glass joints | N/A |
| Water Bath | Maintained at 25 °C | ± 0.2 °C |
| Water Bath | Maintained at 60 °C | ± 0.2 °C |
| Laboratory Thermometer | Range −20 °C to +102 °C | Subdivisions of 0.1 or 0.2 °C |
Calibration and Measurement Procedure
The procedure begins with the calibration of the specific gravity bottle (Section 5.1). It is thoroughly cleaned, dried, and filled with freshly boiled and cooled distilled water (20–23 °C) while carefully held on its side to prevent air entrapment. The stoppered bottle is then immersed in a 25 °C ± 0.2 °C water bath for 30 minutes. Any water exuded from the capillary is removed before external drying and weighing.
🚨 Avoiding Air Bubbles: A common source of error is the entrapment of air bubbles during the filling process. The standard explicitly states the bottle must be held on its side while filling to ensure the liquid pushes all air out of the bottle (Sections 5.1 and 5.2.2). All samples must be completely dry and filtered through filter paper before measurement to remove impurities and the last traces of moisture.
For the sample measurement (Section 5.2), the oil or melted fat is prepared by filtering to remove impurities and moisture. It is cooled to 20–23 °C and used to fill the bottle using the same anti-bubble technique. The filled bottle is conditioned in the 25 °C bath for 30 minutes, allowing the volume to stabilize at the test temperature. After wiping off any excess oil from the capillary opening, the bottle is dried and weighed. The specific gravity at 25/25 °C is calculated from the weight of the sample divided by the weight of the water determined during calibration.
The standard also provides a procedure for measurements at 60/