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D1768-89 – Standard Test Method Technical Guide
🔬 Test Method Overview and Scope
This test method, ASTM D1768 −89 (Reapproved 2024), covers the determination of Sodium Alkylbenzene Sulfonate in synthetic detergents by ultraviolet absorption. It is applicable to commercial synthetic detergents and built detergent formulations provided they do not contain organic additives such as amides. Optical dyes and materials normally found in formulated detergents do not interfere; however, materials other than sulfonates that possess strong ultraviolet absorptions must be absent. The indicated sample size and aliquoting scheme is based on samples containing 30 % to 40 % of active ingredient.
⚠️ Safety Compliance: This standard does not purport to address all safety concerns. Material Safety Data Sheets are available for reagents and materials. Review them for hazards prior to usage and establish appropriate safety, health, and environmental practices.
⚙️ Apparatus Requirements and Reagents
The precision of the instrumentation is critical for reliable results. A spectrophotometer equipped for liquid samples in the ultraviolet region is required, along with matched absorption cells. The following table outlines the critical specifications:
| 🟦 Equipment Parameter | 📏 Required Specification |
|---|---|
| Spectral Region | 220 nm to 270 nm |
| Absorbance Repeatability | ±0.5 % at the 0.4 absorbance level |
| Wavelength Accuracy | ±0.5 nm |
| Wavelength Repeatability | ±0.2 nm |
| Optical Cells | Silica, matched pairs, 1.000 cm ± 0.005 cm path length |
The specified reagent for dissolving the sample is 95 % Ethyl Alcohol, conforming to Formula No. 3A or No. 30 of the U.S. Bureau of Internal Revenue.
📊 Analytical Procedure and Key Parameters
The core of the test method involves measuring the ultraviolet absorbance of the sample at the maximum absorbance point for Sodium Alkylbenzene Sulfonate and at a selected background point to correct for matrix effects. The percentage of active ingredient is calculated from these absorbance values and the data on the pure active ingredient.
| 🎯 Measurement Parameter | ⚡ Specification / Value |
|---|---|
| Analyte Max Absorbance Wavelength | 224 nm |
| Background Correction Wavelength | 270 nm |
| Typical Sample Activity Range | 30 % to 40 % active ingredient |
💡 Method Guidance: The background measurement at 270 nm is critical for correcting non-specific absorbance and baseline drift. It is recommended to calibrate and validate the performance of the spectrophotometer in accordance with Practice E275 (Describing and Measuring Performance of Ultraviolet and Visible Spectrophotometers).
❓ Frequently Asked Questions
🔍 What types of detergent samples are suitable for this test method?
This method covers commercial synthetic detergents and built formulations that do not contain organic additives such as amides. Optical dyes and materials normally found in formulated detergents do not interfere, but non-sulfonate materials possessing strong ultraviolet absorptions must be absent.
💡 Why is a background correction at 270 nm necessary?
The measurement at 270 nm serves as a background point to compensate for any non-specific UV absorbance, baseline drift, or scattering from the sample matrix, allowing the calculation to isolate the specific absorbance of the active ingredient at 224 nm.
⚡ What are the critical spectrophotometer specifications required?
The spectrophotometer must measure absorbance with a repeatability of ±0.5 % or better at the 0.4 absorbance level. Spectral positions must be accurate to ±0.5 nm and repeatable to ±0.2 nm. Absorption cells must be silica with a path length of 1.000 cm ± 0.005 cm.
📌 What reagent is specified for sample preparation?
The standard specifies the use of 95 % Ethyl Alcohol, conforming to either Formula No. 3A or No. 30 of the U.S. Bureau of Internal Revenue.