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D2505-88 – Standard Test Method Technical Guide
📐 Overview and Scope
This test method, designated D2505-88 (Reapproved 2015), covers the determination of carbon dioxide, methane, ethane, acetylene, and other hydrocarbons in high-purity ethylene. It is applicable for impurities in the range of 1 to 500 parts per million volume (ppmV). The method also references Test Method D2504 for noncondensable gases such as hydrogen, nitrogen, oxygen, and carbon monoxide. The ethylene purity is derived by subtracting the sum of all detected impurities from 100%.
⚠️ Safety: This standard does not address all safety concerns. Users must establish appropriate safety and health practices and determine the applicability of regulatory limitations before use. See Section 6 for specific hazard statements.
⚙️ Test Procedure and Column Configuration
The sample is separated in a gas chromatograph system using four packed chromatographic columns with helium as the carrier gas. The columns are specifically chosen for different compounds:
| 🟦 Column Type | 📏 Packing Material | 🔍 Target Compounds |
|---|---|---|
| Silica Gel | Silica gel | Methane, Ethane |
| HMPA | Hexamethylphosphoramide | Propylene, Heavier Hydrocarbons |
| Hexadecane + Squalane | Hexadecane and Squalane in series | Acetylene |
| Activated Charcoal | Activated charcoal impregnated with silver nitrate in β,β’-oxydipropionitrile | Carbon Dioxide |
The apparatus must have an overall sensitivity sufficient to detect 2 ppmV or less of the listed compounds with a peak height of at least 2 mm without loss of resolution. Thermal conductivity detectors are standard, but for carbon dioxide determination with a flame ionization detector, a methanation reactor is required.
💡 Tip: For carbon dioxide detection with a flame ionization detector, insert a methanation reactor between the column and detector, and add hydrogen as a reduction gas. Refer to Test Method D2504 for preparation instructions.
📊 Key Measured Properties and Calculations
Calibration data are obtained using standard samples with impurities in the expected range. For carbon dioxide, methane, and ethane, calculations are carried out by the peak-height measurement method. Acetylene content is calculated based on the ratio of the peak area of acetylene to the peak area of a known amount of methane, assuming equal response on a weight basis. The final ethylene purity is obtained by subtracting the sum of the percentages of all hydrocarbon and nonhydrocarbon impurities from 100%. This method is suitable for setting specifications, internal quality control, and research work.
❓ Frequently Asked Questions
🔍 What is the detection limit of the D2505-88 test method?
The method requires an overall sensitivity sufficient to detect 2 ppmV or less of the target compounds with a peak height of at least 2 mm.
💡 How is acetylene content calculated?
Acetylene is calculated by comparing its peak area to that of a known amount of methane, assuming equal peak area response on a weight basis.
⚡ What columns are used for methane and ethane?
A silica gel column is used for the separation and quantification of methane and ethane.
📌 What is the applicable impurity range?
The test method is applicable over the range of impurities from 1 to 500 parts per million volume (ppmV).