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D1767-89 – Standard Test Method Technical Guide
🧪 Scope and Reagent Preparation
This test method, designated ASTM D1767-89 (Reapproved 2017), covers the determination of ethylene diamine tetraacetate (EDTA) in soaps and synthetic detergents. Throughout this standard, “EDTA” represents the hydrated disodium salt containing 2 moles of H₂O, chemically abbreviated as Na₂H₂Y·4H₂O (or Na₂H₂Y·2H₂O). The values stated in SI units are to be regarded as standard.
⚠️ Reagent Purity: All reagents must conform to the specifications of the Committee on Analytical Reagents of the American Chemical Society (Section 3.1). Water used in the procedure must meet ASTM D1193 Specification for Reagent Water (Section 3.2). Review Material Safety Data Sheets for all reagents prior to use.
The key reagents required for this titration are outlined below:
| 🟦 Reagent | 📝 Composition | 🎯 Concentration / Note |
|---|---|---|
| Copper Sulfate Solution | 2.497 g CuSO₄·5H₂O / L | 1 mL = 0.0037 g EDTA disodium salt (equivalent must be verified empirically) |
| PAN Indicator Solution | 0.1 g 1-(2-pyridylazo)-2-naphthol in 100 mL methyl alcohol | Use 4 drops per titration |
⚙️ Test Procedure and Titration
The procedure requires careful sample preparation and controlled titration conditions. A 3 to 5 g portion of the sample, estimated to contain 1% EDTA disodium salt, is weighed to the nearest 0.001 g into a 250 mL beaker. The sample is diluted to approximately 75 mL with water, and the pH is adjusted to approximately 5.0 by the addition of acetic acid.
💡 Tip for Improved Endpoint: For samples containing soaps, the fatty acids may interfere with the visual endpoint. Filter the solution through a wet filter paper prior to titration to sharpen the color transition from yellow to red (See Note 1 in the standard).
After pH adjustment, 4 drops of PAN indicator solution are added. The sample is then titrated with the standardized Copper Sulfate solution. The endpoint is detected visually by a sharp transition in color from yellow to red.
| 📏 Parameter | ⚡ Specification / Value |
|---|---|
| Sample Weight | 3 to 5 g |
| Weighing Precision | 0.001 g |
| EDTA Content Estimate | ~1% |
| Dilution Volume | ~75 mL |
| Titration pH | ~5.0 |
| Endpoint Color Change | Yellow to Red |
📊 Calculation of Results
The percentage of EDTA disodium salt in the sample is calculated using the following formula derived from Section 6.1 of the standard:
EDTA, weight % = (A × B / C) × 100
Where:
- A = millilitres of CuSO₄ solution required for titration of the sample,
- B = grams of EDTA disodium salt equivalent to 1 mL of the CuSO₄ solution,
- C = grams of sample used.
✅ Key Quality Note: The actual EDTA equivalent of the Copper Sulfate solution (Factor B) must be determined empirically by titrating a known weight of EDTA disodium salt (Section 4.1). Always ensure the sample size is varied appropriately depending on the actual amount of EDTA present.
❓ Frequently Asked Questions
🔍 What specific form of EDTA does this test method measure?
This test method specifically measures the hydrated disodium salt of EDTA containing 2 moles of H₂O. In the chemical literature and throughout this standard, this is represented as Na₂H₂Y·2H₂O (or Na₂H₂Y·4H₂O).
💡 What indicator is used in the titration and how is it prepared?
The indicator used is 1-(2-pyridylazo)-2-naphthol (PAN). The PAN indicator solution is prepared by dissolving 0.1 g of PAN in 100 mL of methyl alcohol. Exactly 4 drops of this solution are added to the sample prior to titration.
⚡ What sample weight should be used for the analysis?
The standard specifies weighing a 3 to 5 g portion of the sample to the nearest 0.001 g. The portion selected should be estimated to contain approximately 1% of EDTA disodium salt. The amount can be varied by the analyst depending on the actual EDTA concentration in the detergent sample.
📌 How can the endpoint of the titration be sharpened for soap-containing samples?
According to Note 1 in the standard, the removal of soap fatty acids by filtration through a wet filter paper prior to the titration will significantly sharpen the visual endpoint, making the transition from yellow to red more distinct and accurate.