SH/T 0792-2007 Determination of 2,6- di-tert-butyl-p-cresol and 2,6- di-tert-butylphenol content in electrical insulating oil (Infrared absorption spectrometry)

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Card type: Test method

1. Method number / full title / English name / replacement relationship

• Method number: SH/T 0792—2007
• Chinese full title: Determination of 2,6- di-tert-butyl-p-cresol and 2,6- di-tert-butylphenol content in electrical insulating oil (Infrared absorption spectrometry)
• English name (original title page, source-text extraction has noise): Standard test method for 2,0-di-tert-butyl-p-cresol amd 2,0-di-tert-butyl phcnol in electrical insuliating oil by infrared alsorption
  • Requires confirmation against the source standard (source-text extraction): in the English name, “2,0-” is suspected to be “2,6-”; “amd” is suspected to be “and”; “phcnol” is suspected to be “phenol”; “insuliating” is suspected to be “insulating”; “alsorption” is suspected to be “absorption”.
• Replacement relationship: not provided in the available source (the foreword does not mention replacement of any standard).
• Adoption relationship: This standard is modified adoption of ASTM D2668-02 “Determination of 2,6- di-tert-butyl-p-cresol and 2,6- di-tert-butylphenol content in electrical insulating oil (Infrared absorption spectrometry)”, redrafted according to ASTM D2668-02.
• Publication/implementation: published on 2007-08-01, implemented on 2008-01-01; issuing authority: National Development and Reform Commission of the People's Republic of China.

2. Scope of application and non-applicability

• 1.1 is applicable to the determination of the content of 2,6- di-tert-butyl-p-cresol and 2,6- di-tert-butylphenol in new or used electrical insulating oil; the method determines their absorption at specific wavelengths in the infrared spectrum, and the determined content is not greater than 0.5% (mass fraction).
• 1.2 This standard involves certain hazardous materials, operations and equipment, but does not address all safety issues associated with them; users must establish appropriate safety and protective measures and determine corresponding management rules before using this standard.
• Non-applicable scope: not provided in the available source as an explicit list.

3. Method principle

• Chapter 3 Method summary: Infrared spectrometry is used to record the absorption peaks of 2,6- di-tert-butyl-p-cresol and 2,6- di-tert-butylphenol in new or used electrical insulating oil at specific wavelengths; the absorbance of these absorption peaks is proportional to their concentration, i.e. it conforms to Beer's law, and quantitative calculation is performed using the working curve method.
• 4.2 Absorption characteristics: the absorption of 2,6- di-tert-butyl-p-cresol at 2.74 μm (3650 cm⁻¹) and 11.63 μm (860 cm⁻¹) is significantly enhanced; the absorption of 2,6- di-tert-butylphenol at 2.74 μm (3650 cm⁻¹) and 13.33 μm (750 cm⁻¹) is significantly enhanced.
• 4.3 Recommended scanning range: when analyzing non-deeply oxidized oils or using a double-beam spectrometer, spectra in the range 2.5 μm (4000 cm⁻¹) to 2.9 μm (3450 cm⁻¹) can be conveniently obtained; when testing deeply oxidized oils or using a single-beam instrument, if scanning in the range 10.90 μm (918 cm⁻¹) to 14.00 μm (714 cm⁻¹), good results can also be obtained.
• 4.4 Qualitative identification: when the absorption at 11.63 μm (860 cm⁻¹) or at 13.33 μm (750 cm⁻¹) is enhanced, or both are enhanced simultaneously, the presence of 2,6- di-tert-butyl-p-cresol or 2,6- di-tert-butylphenol antioxidant can be identified respectively.
• Note (4.4 Note): at the same concentration, the ratio of the absorption intensity of 2,6- di-tert-butylphenol at 750 cm⁻¹ to the absorption intensity of 2,6- di-tert-butyl-p-cresol at 860 cm⁻¹ is approximately 2.6:1.
  • Requires confirmation against the source standard (source-text extraction): in the original note, “750km” is suspected to be “750cm⁻¹”.

4. Reagents and materials (purity / preparation)

• 6.1 Petroleum ether: chemically pure, 30 °C to 60 °C.
• 6.2 2,6- di-tert-butyl-p-cresol or 2,6- di-tert-butylphenol: chemically pure.
• Base oil: a suitable base oil known by the manufacturer may be used to prepare standard samples; when using a base oil of unknown origin, it must meet the requirements specified in SH 0040 (see 8.1).
• Preparation of standard samples (Appendix B, informative appendix):
  • B.1 Weigh 2,6- di-tert-butyl-p-cresol or 2,6- di-tert-butylphenol 1 g (accurate to ±0.0002 g), dissolve in 199 g base oil; this is a 0.5% (mass fraction) standard stock solution.
  • B.2 Respectively weigh 0.5% (mass fraction) standard stock solution 2 g, 4 g, 8 g, 12 g, 16 g and dissolve in 18 g, 16 g, 12 g, 8 g, 4 g base oil; in order, the antioxidant contents are 0.05%, 0.1%, 0.2%, 0.3%, 0.4% (mass fraction). Store protected from light in brown bottles until use.
  • Requires confirmation against the source standard (source-text extraction): in the original text of B.2, “2 number, 4g, 8g, 12g, 16g”, “2 number” is suspected to be “2 g”; “18 number” is suspected to be “18 g”.

5. Instruments and equipment

• 5.1 Dispersive or Fourier transform rapid-scanning infrared spectrometer with a wavenumber range of 400 cm⁻¹ to 4000 cm⁻¹.
• 5.2 Absorption cell: potassium bromide (KBr) or sodium chloride (NaCl) liquid absorption cell with a path length in the range 0.3 mm to 1.0 mm.
• 5.3 Syringe: 5 mL syringe or other suitable injection device.
• 8.2 It is recommended to use a liquid absorption cell with a standard path length in the range 0.3 mm to 1.0 mm; for different instruments or special wavelengths, absorption cells with other path lengths may be more suitable; if the instrument sensitivity is adjustable, absorption cells with other path lengths may also be used.
• Note (8.2 Note 3): transmission spectra are required in the test; intensity measurement requires special scales and graphical methods or logarithmic tables; the instrument software should be able to record all spectrometer settings during measurement.

6. Recommended operating conditions

• Scanning wavenumber range: 400 cm⁻¹ to 4000 cm⁻¹ (5.1).
• Quantitative bands: at specific wavelengths such as 2.74 μm (3650 cm⁻¹), 11.63 μm (860 cm⁻¹), 13.33 μm (750 cm⁻¹) (4.2, 4.4).
• Absorption cell path length: 0.3 mm to 1.0 mm (5.2, 8.2).
• Number of scans: scan each standard sample at least three times repeatedly (8.2); scan each test sample at least three times (9.3).
• Routine testing: as long as the instrument can meet the repeatability requirements specified in 10.2.1, a single scan may also be used instead of multiple scans (9.4).
• Arbitration testing: it is necessary to use a manual mode, scan the test sample once, and take the average value; or use an automatic mode, scan the test sample with the same number of scans as when establishing the calibration curve, and take the average value (9.4).
• Instrument setting record: when obtaining the corresponding spectrum, the instrument must record all spectrometer setting parameters (8.2).
• Note (8.2 Note 1): it is desirable to use an abscissa with a nonlinear wavelength scale.
• Note (8.2 Note 2): the precision of this method is established based on results measured by the manual method, i.e. results obtained by accurately performing one scan repeatedly for each standard sample; the latest instruments can perform automatic scanning faster, and it is recommended to increase the number of scans to statistically compensate for any outlier points with large differences; the laboratory needs to determine the minimum number of scans used for instrument standardization and sample analysis to meet testing needs.
  • The source term is unclear; its interpretation as compensation remains unverified. Check the original standard.

7. Sampling and sample preparation

• Chapter 7 Sampling: sampling shall be carried out in accordance with the requirements of GB 7597.
• 9.1 Thoroughly rinse the absorption cell with petroleum ether, dry the solvent, and place it in a desiccator for later use.
• 9.2 Select the absorption cell according to clause 8.2, use a syringe cleaned with petroleum ether to inject part of the test sample into a clean sample cell for rinsing, then inject the test sample into the sample cell, and there must be no bubbles; otherwise, reload the sample. Clean and inject a suitable antioxidant-free base oil into the reference cell.
• For preparation of standard samples, see Section 4 (Appendix B).

8. Analytical procedure

• Chapter 8 Calibration:
  • 8.1 A suitable base oil known by the manufacturer may be used to prepare standard samples. For base oils of unknown origin, when used they must meet the requirements specified in SH 0040. Some base oils have better matching properties, so it is worthwhile to screen some suitable base oils.
  • 8.2 Dissolve 2,6- di-tert-butyl-p-cresol or 2,6- di-tert-butylphenol in base oil containing no antioxidant to prepare standard samples from 0.05% (mass fraction) to 0.5% (mass fraction) (for preparation of standard samples, see Appendix B). For each standard sample, perform infrared scanning within a suitable spectral band range to obtain a spectrum. It is recommended to use a liquid absorption cell with a standard path length in the range of 0.3 mm ~ 1.0 mm. For different instruments or special wavelengths, absorption cells with other path lengths may be more suitable. If the sensitivity of the instrument can be adjusted, absorption cells with other path lengths may also be used. The absorption of the additive-containing oil sample at the critical wavelength or frequency should show a clear increase on the working curve. Scan each standard sample at least three times. When obtaining the corresponding spectrum, the instrument shall record all spectrometer setting parameters.
    • Requires confirmation against the source standard (source-text extraction): "additive-containing oil sample" is suspected to be "additive-containing oil sample"; "on the lower working curve" is suspected to be "on the working curve".
  • 8.3 Quantitative determination follows Beer's law, see formula (1): absorbance value = A − A₀; where A₀ is the absorbance value of the base oil; A is the absorbance value of the oil containing 2,6- di-tert-butyl-p-cresol or 2,6- di-tert-butylphenol.
    • Requires confirmation against the source standard (source-text extraction): the original formula (1) is typeset as "absorbance value=A A.", suspected to be "A − A₀".
  • 8.4 Manual drawing of calibration curve: designate the maximum absorption point in the specified quantitative determination band as point A, draw a tangent at the bottom of this absorption peak, and draw a vertical line through A toward the absorption line, as shown in Figure 1, define the intersection of these two lines as point A₀. From the spectrum of each scan performed on each sample, read the absorbance values of these points, accurate to 0.001 (a magnifying glass may be used), and subtract the value of A₀ from the value of A. Plot the average of the three data obtained after analyzing the standard samples against the content to obtain a straight line. Determine the best straight line passing through the calibration data points by linear regression analysis. This straight line is the calibration curve required for determining the unknown concentration of 2,6- di-tert-butyl-p-cresol or 2,6- di-tert-butylphenol in the sample, as shown in Figure 2. Figure 3 and Figure 4 show differential scanning spectra.
    • Requires confirmation against the source standard (source-text extraction): "manually drawn" is suspected to be "manually drawn"; "each scan" is suspected to be "each scan"; "using the value of A" is suspected to be "using the value of A"; "two differential scanning spectra" is suspected to be "two differential scanning spectra".
  • 8.4.1 Automatic drawing of calibration curve: for instruments capable of automatically generating a standard calibration curve, follow the instrument manufacturer's recommendations. For each standard sample in each calibration range, scan at least once (see note 2 after clause 8.2). The correlation coefficient of the established calibration curve should not be less than 0.99, so as to ensure the linearity of the calibration curve.
  • 8.5 Since the electronic components in the spectrometer change over time, if daily work requires high-frequency testing, it is recommended to use one or more standard samples for periodic checks. If the difference between the absorption of the standard sample and the calibration curve is greater than the limit given in 10.2.1, then a new calibration curve must be redrawn. Some laboratories have established stricter limits than those described in 10.2.1. Whenever the test conditions change, such as light source, scanning speed, etc., a new calibration curve must be redrawn.
    • Note (8.5 note): In order to quickly check the characteristics of the spectrometer used to establish the calibration curve, store the solution of known concentration in an amber bottle for no more than one year.
• Chapter 9 Test Procedure:
  • 9.1 Thoroughly rinse the absorption cell with petroleum ether, dry the solvent, and place it in a desiccator for later use.
  • 9.2 Referring to clause 8.2, select the absorption cell, use a syringe cleaned with petroleum ether to inject part of the sample into a clean sample cell for rinsing, then inject the sample into the sample cell without bubbles; otherwise, reload the sample. Clean and inject the reference cell with suitable base oil containing no antioxidant.
  • 9.3 For a Fourier transform rapid scanning infrared spectrometer, insert the sample cell and reference cell into the sample optical path successively (for a dispersive infrared spectrometer, insert the sample cell and reference cell into the sample optical path and reference optical path respectively for synchronous measurement), use the quantitative scanning mode at the determined band, determine the content of 2,6- di-tert-butyl-p-cresol or 2,6- di-tert-butylphenol in the sample, and scan each sample at least three times. The absorption cell used in the specific analysis must be the same absorption cell used in the test for drawing the calibration curve. The spectrometer used must meet the requirements described in clause 5.1, and the same instrument must be used for testing samples and drawing the standard curve.
  • 9.4 For routine test analysis, as long as the instrument can meet the repeatability requirements specified in 10.2.1, a single scan may be used instead of multiple scans. For arbitration tests, it is necessary to use the manual mode, scan the sample once, and take the average value; or use the automatic mode, scan the sample with the same number of scans as when establishing the calibration curve, and take the average value.
  • 9.5 Manual measurement results: read the absorbance values of point A and point A₀ in the three spectra, accurate to 0.001 absorbance value, calculate the difference and average value, and determine the content from the calibration curve using the average value.
  • 9.6 Automatic measurement results: use the instrument's automatic measurement mode, test the sample with the same number of scans as when establishing the calibration curve, obtain the absorbance values of point A and point A₀, calculate the average value of the difference between the two, and use the calibration curve to determine the content in the sample.

9. Qualitative and Quantitative Determination and Calculation Formulas

• Quantitative basis: Beer's law, absorbance value = A − A₀ (formula 1); A₀ is the absorbance value of the base oil; A is the absorbance value of the oil containing 2,6- di-tert-butyl-p-cresol or 2,6- di-tert-butylphenol.
• Quantitative method: working curve method (calibration curve method).
• Calibration curve drawing: see 8.4 (manual) and 8.4.1 (automatic); the correlation coefficient of the automatic method should not be less than 0.99.
• Qualitative identification: see 4.4 (enhanced absorption at 860 cm⁻¹ or 750 cm⁻¹).
• Calculation formula: the original text only gives the calculation of absorbance difference in formula (1), and no other calculation formulas are given.

10. Precision

• 10.1 Limit of the check range (A−A₀): after scanning three or more times to obtain spectra, read the absorbance values of point A and point A₀ from the spectra, calculate the difference of A−A₀, determine the maximum and minimum values among these differences, and calculate the difference between the maximum and minimum values; if the result is greater than 0.010 absorbance value, then discard these results and redo the test.
• 10.2 Precision: judge the reliability of the test results according to the following provisions (95% confidence level).
  • 10.2.1 Repeatability (r): the difference between two results obtained by the same operator using the same instrument under the same operating conditions on the same sample shall not exceed 0.04% (mass fraction).
  • 10.2.2 Reproducibility (R): the difference between two results obtained by different operators in different laboratories on the same sample shall not exceed 0.04% (mass fraction).
• 10.3 Bias: because no bias determination work has been carried out, this method has no bias specification for the determination of 2,6- di-tert-butyl-p-cresol or 2,6- di-tert-butylphenol.
  • Requires confirmation against the source standard (source-text extraction): "bias" is suspected to be "bias".

11. Report

• No independent "Report" chapter is provided in the available source. Relevant recording requirements are scattered in: 8.2 (the instrument shall record all spectrometer setting parameters), 8.2 note 3 (the instrument software should be able to record all spectrometer settings during measurement), 9.5/9.6 (read absorbance values and calculate, determine content from the calibration curve).

12. Reference Documents

• GB 7597 Sampling method for electric power oil (transformer oil, turbine oil)
• SH 0040 Ultra-high voltage transformer oil
• ASTM D2668-02 (modified adoption of the original standard)
• ASTM D2144 "Test Method for Electrical Insulating Oil by Infrared Absorption Spectroscopy" (mentioned in the foreword, the substantive content is placed in this standard 5.2, 5.3, 6.1, 6.2, 9.1, 9.2, 9.3 respectively)

13. Source and verification

This page is an AI translation of the available Chinese method card. Human verification is pending. It is not an official translation of the standard. Check the source standard and product certificate before use.

14. Quick Reference for Questions and Answers