Physical Address
304 North Cardinal St.
Dorchester Center, MA 02124
D2668-07 – Standard Test Method Technical Guide
ASTM D2668 −07 (Reapproved 2021) provides a standardized infrared absorption methodology for the quantitative determination of two key oxidation inhibitors—2,6-di-tert-butyl-p-cresol (DBPC) and 2,6-di-tert-butyl phenol (DBP)—in electrical insulating mineral oils. The method is effective for concentrations up to 0.5 % by weight and is applicable to both new and in-service oils.
📖 Scope and Significance
This test method measures the amount of inhibitor present in new oil as a safeguard against oxidation and evaluates the residual concentration in used oil, offering critical data for condition monitoring and remaining life assessment. The test is also applicable for manufacturing quality control and specification acceptance of insulating oils.
✅ Key Application: Tracking the depletion of DBPC and DBP inhibitors in service allows operators to determine when oil reclamation or replacement is necessary to maintain the dielectric and cooling performance of electrical equipment.
📊 Key Absorbance Criteria and Quantification
The infrared spectrum of an inhibited oil shows distinct increases in absorbance at characteristic wavelengths. DBPC produces pronounced increases at 2.72 µm (3650 cm⁻¹) and 11.63 µm (860 cm⁻¹), while DBP produces increases at 2.72 µm (3650 cm⁻¹) and 13.42 µm (745 cm⁻¹). The following table summarizes the specific identification bands.
| 🟦 Inhibitor Compound | 📏 Identification Band (µm) | 📐 Wavenumber (cm⁻¹) | 🎯 Primary Fingerprint Band |
|---|---|---|---|
| DBPC (2,6-di-tert-butyl-p-cresol) | 2.72 / 11.63 | 3650 / 860 | 11.63 µm (860 cm⁻¹) |
| DBP (2,6-di-tert-butyl phenol) | 2.72 / 13.42 | 3650 / 745 | 13.42 µm (745 cm⁻¹) |
An increase in absorbance at 860 cm⁻¹ identifies DBPC, while an increase at 745 cm⁻¹ identifies DBP. A crucial quantitative detail provided in Note 1 is the absorbance ratio: for equal concentrations, the absorbance of DBP at 745 cm⁻¹ is approximately 2.6 times greater than that of DBPC at 860 cm⁻¹. This ratio is an essential reference for method validation.
⚠️ Technical Guidance: The choice of scan range depends on the instrument and oil condition. For double-beam spectrophotometers testing low-oxidation oils, scan between 2.5–2.9 µm (4000–3450 cm⁻¹) to minimize moisture interference. For single-beam instruments or highly oxidized oils, the fingerprint region (10.90–14.00 µm / 918–714 cm⁻¹) yields superior results.
| ⚡ Condition / Instrument Type | 📏 Recommended Scan Range (µm) | 📐 Wavenumber Range (cm⁻¹) |
|---|---|---|
| Double-beam spectrophotometer, low oxidation | 2.5 – 2.9 | 4000 – 3450 |
| Single-beam instrument / Highly oxidized oil | 10.90 – 14.00 | 918 – 714 |
⚙️ Apparatus and Sampling Requirements
Per Section 4 of the standard, the equipment must comply with Practices D2144. This reference explicitly permits the use of modern Fourier-transform rapid scan infrared (FTIR) spectrophotometers. The validity of the test also depends heavily on the sample quality; sampling must be performed strictly in accordance with Practices D923 to ensure a representative specimen of the bulk insulating liquid is obtained.
💡 Compliance Note: When calibrating, verify that the spectrophotometer resolution and cell path length meet the tolerances defined in D2144 to maintain sensitivity and accuracy at low concentrations.
❓ Frequently Asked Questions
🔍 What is the maximum concentration range for this test?
ASTM D2668 is designed for the determination of DBPC and DBP in electrical insulating oils at concentrations up to 0.5 % by weight.
💡 How do I specifically identify DBPC versus DBP in a sample?
Identification is based on the presence of specific absorption bands. DBPC is identified by increased absorbance at 11.63 µm (860 cm⁻¹), while DBP is identified by increased absorbance at 13.42 µm (745 cm⁻¹).
⚡ Why is the 2.6:1 absorbance ratio important?
This ratio, established in Note 1, describes the relative response of DBP at 745 cm⁻¹ to DBPC at 860 cm⁻¹ for equal concentrations. It is a critical factor for ensuring the correct absorption coefficient is applied when calculating concentrations of mixed inhibitors.
📌 Is FTIR technology allowed under this standard?
Yes. The standard directly permits the use of Fourier-transform rapid scan infrared (FTIR) spectrophotometers by reference to Practices D2144.