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D2907-97 – Standard Test Method Technical Guide
📋 Scope and Significance
The ASTM D2907-97 standard provides test methods for determining microquantities of uranium in water in the concentration range from 0.005 to 50 mg/L. These methods are referenced in the National Interim Primary Drinking Water Regulations as approved methods for analysis. Uranium occurs naturally in three isotopic forms: U-238, U-235, and U-234, with approximate mass percentages of 99.3%, 0.7%, and 0.0057%, respectively. However, due to different decay rates, their alpha particle activities vary, which must be considered when interpreting results.
| 🟦 Isotope | 📏 Mass Percentage | ⚡ Alpha Activity (Bq/mg) |
|---|---|---|
| U-238 | 99.3% | 12.21 |
| U-235 | 0.7% | 0.55 |
| U-234 | 0.0057% | 13.02 |
⚠️ Warning: Mass measurement methods like fluorometry should not be used to determine uranium alpha particle activity due to variable isotopic ratios in ground water.
⚙️ Test Methods and Procedures
Two test methods are specified: Test Method A—Direct Fluorometric for concentrations from 0.005 to 2 mg/L, and Test Method B—Extraction for concentrations from 0.04 to 50 mg/L. The extraction method is required when interfering ions are present, as uranium fluorescence can be quenched by many cations and some anions.
| 🟦 Test Method | 📏 Concentration Range (mg/L) | 📐 Sections |
|---|---|---|
| Direct Fluorometric | 0.005 to 2 | 7 to 15 |
| Extraction | 0.04 to 50 | 16 to 24 |
💡 Tip: If the sample contains interfering ions, use the extraction method for accurate results. Refer to the standard for detailed procedures.
📊 Key Measured Properties and Limitations
The fluorescence measurement is sensitive to interferences; many cations and some anions can quench fluorescence, while a few cations enhance it. Therefore, the direct method is only suitable for samples without such interferences. Additionally, the standard notes that the uranium alpha activity cannot be inferred from mass measurements due to isotopic imbalances in ground water systems.
❓ Frequently Asked Questions
🔍 What is the concentration range for the direct fluorometric method?
The direct fluorometric method covers uranium concentrations from 0.005 to 2 mg/L.
💡 When is the extraction method necessary?
The extraction method is necessary when the sample contains interfering ions that could quench or enhance the uranium fluorescence, which would render direct measurement inaccurate.
⚡ What are the safety hazards associated with these test methods?
Users should refer to Note 1 in the standard for specific hazards. It is the user’s responsibility to establish safety practices and comply with regulations.
📌 Why should fluorometry not be used for alpha activity determination?
Uranium isotopic ratios in water can be out of equilibrium, meaning the mass-to-activity conversion is not consistent. Alpha spectrometry is required for accurate alpha activity measurement.