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IEC 62021-2:2007 – Determination of Acidity in Insulating Liquids by Colourimetric Titration
A practical guide to measuring acidity in mineral insulating oils using the Alkali Blue 6B colourimetric method
1. Understanding the Importance of Acidity Testing in Insulating Oils
Mineral insulating oils serve as both a dielectric medium and a coolant in high-voltage electrical equipment such as power transformers, bushings, and switchgear. Over time, these oils undergo oxidative degradation due to thermal stress, exposure to oxygen, and catalytic effects from metal surfaces. One of the most reliable indicators of oil degradation is the increase in acidity — the accumulation of organic acids, phenolic compounds, oxidation products, resins, organometallic salts, and certain additives that exhibit acidic characteristics.
IEC 62021-2:2007, prepared by IEC Technical Committee 10 (Fluids for electrotechnical applications), provides a standardized colourimetric titration method for determining the acidity of both unused and used mineral insulating oils. Unlike the potentiometric method described in IEC 62021-1 which requires specialized pH electrodes and instrumentation, the colourimetric method offers a simpler, visually intuitive alternative that many laboratories worldwide still prefer. The method relies on the Alkali Blue 6B indicator, which changes colour at approximately pH 9.5, signaling the neutralization point of the acidic components in the oil.
The acidity test measures the quantity of base (KOH) required to neutralize acidic constituents in the oil. Results are expressed in milligrams of KOH per gram of oil (mg KOH/g). However, this test alone cannot predict the corrosiveness of an oil under service conditions, as the variety of oxidation products varies widely in their corrosion properties.
The standard acknowledges an important caveat: results obtained by colourimetric titration may differ slightly from those obtained by potentiometric methods. The potentiometric method uses an endpoint at pH 11.3 to ensure complete titration of all species, whereas the colourimetric endpoint occurs at approximately pH 9.5. This can yield slightly higher results for oils with acidity above 0.3 mg KOH/g when using the potentiometric approach. Understanding this distinction is critical when comparing historical data across different testing methodologies.
| Item | Specification | Purpose |
|---|---|---|
| Alcoholic KOH solution | 0.05 mol/l in 2-propanol | Titration reagent (standardized every 2 weeks) |
| Alkali Blue 6B indicator | 2% (w/v) in 2-propanol with HCl | Colour change indicator (blue to red at pH ~9.5) |
| Titration solvent | 2-propanol (isopropanol), pure | Dissolves oil sample for titration |
| Potassium hydrogen phthalate | Primary standard, dried 2h at 105 C | Standardization of KOH solution |
| Cobalt nitrate solution | 10% Co(NO3)2.6H2O in water | Reference colour for indicator check |
| Burette or syringe | 0.001 ml aliquots | Precise dispensing of titrant |
2. The Colourimetric Titration Procedure According to IEC 62021-2
The procedure described in IEC 62021-2 follows a methodical sequence designed to ensure repeatable and reproducible results across different laboratories. The test portion is dissolved in a specified solvent (pure 2-propanol) and titrated colourimetrically with alcoholic potassium hydroxide using Alkali Blue 6B as the indicator.
2.1 Reagent Preparation
The titration reagent is prepared by dissolving 3.0 g of potassium hydroxide in 1 litre of 2-propanol, boiling gently for 10 minutes, then cooling and allowing the solution to stand in darkness for 2 days. The supernatant liquid is filtered through a 5-micrometre membrane filter and stored in an amber glass bottle. The solution must be protected from atmospheric carbon dioxide, which would otherwise react with KOH and alter the effective concentration. Standardization against potassium hydrogen phthalate is required at least every two weeks.
2.2 Blank and Sample Titration
A blank titration on the solvent alone must be performed daily and whenever a fresh batch of solvent is used. For the sample titration, approximately 20 g of unused oil (or 5 g for heavily used oil) is weighed into the titration vessel, dissolved in the solvent, and titrated until the colour change from blue to red matches the cobalt nitrate reference solution. The endpoint volume is recorded, and the acidity is calculated using the difference between sample and blank titration volumes.
For high-throughput laboratories, Annex A of the standard describes an alternative photometric titration method using para-naphtolbenzein indicator at 660 nm. This approach automates endpoint detection and can be integrated with automatic samplers for routine analysis, significantly improving laboratory productivity.
2.3 Photometric Titration Alternative (Annex A)
The informative Annex A provides a photometric titration method for those seeking greater precision and automation. The sample is dissolved in a mixed solvent of 50% toluene, 49.5% 2-propanol, and 0.5% demineralized water. Para-naphtolbenzein indicator replaces Alkali Blue 6B, and a photometric sensor operating at 660 nm detects the colour change. This method eliminates subjective interpretation of colour endpoints, particularly valuable for heavily oxidized oils where the colour change may be less distinct.
| Parameter | Unused Oils | Used Oils |
|---|---|---|
| Repeatability (same operator, same lab) | 13% of mean value | 7% of mean value |
| Reproducibility (different operators, different labs) | 35% of mean value (above 0.01 mg KOH/g) | 20% of mean value |
| Quantification limit | 0.01 mg KOH/g oil | — |
Safety is paramount when performing this test. The mineral oils and solvents involved require careful handling. Direct contact with eyes may cause irritation, requiring irrigation with copious clean water and medical attention. All waste — used oils, chemicals, and sample containers — must be disposed of in accordance with national environmental legislation.
3. Precision, Reporting, and Practical Application
The standard establishes clear precision criteria based on an inter-laboratory study. For unused oils, the repeatability is 13% of the mean value, while for used oils it improves to 7%. This difference reflects the typically higher acidity levels found in service-aged oils, where the relative measurement uncertainty is lower. Reproducibility between laboratories is 35% for unused oils and 20% for used oils, underscoring the importance of careful procedural adherence when comparing results across different testing facilities.
3.1 Engineering Insights for Maintenance Programs
Acidity trending is one of the most cost-effective tools in transformer condition assessment. A sudden increase in acidity rate often signals accelerated oxidation, which may be triggered by overheating, moisture ingress, or depletion of oxidation inhibitors. Modern transformer maintenance programs typically establish baseline acidity values for each unit and track changes over time. The colourimetric method remains popular for field laboratories due to its minimal instrument requirements and straightforward procedure.
When properly implemented as part of a comprehensive oil analysis program — including dissolved gas analysis (DGA), moisture content, dielectric strength, and interfacial tension — the IEC 62021-2 acidity test provides invaluable early warning of insulation system degradation, enabling proactive maintenance before costly failures occur.
3.2 Report Requirements
The standard mandates that the test report include: the type and identification of the product tested, a reference to IEC 62021-2, the acidity result expressed to the nearest 0.01 mg KOH/g, any deviations from the specified procedure, and the date of the test. This standardized reporting framework ensures traceability and comparability across the industry.
Q1: What is the difference between IEC 62021-1 and IEC 62021-2?
IEC 62021-1 uses potentiometric titration with pH electrode detection (endpoint pH 11.3), while IEC 62021-2 uses colourimetric titration with Alkali Blue 6B indicator (endpoint ~pH 9.5). The colourimetric method generally yields slightly lower results for oils with acidity above 0.3 mg KOH/g.
IEC 62021-1 uses potentiometric titration with pH electrode detection (endpoint pH 11.3), while IEC 62021-2 uses colourimetric titration with Alkali Blue 6B indicator (endpoint ~pH 9.5). The colourimetric method generally yields slightly lower results for oils with acidity above 0.3 mg KOH/g.
Q2: How often should the KOH solution be standardized?
The standard requires standardization at least every two weeks against potassium hydrogen phthalate or certified standard acid. The solution may also be checked monthly using the photometric method described in Annex A.
The standard requires standardization at least every two weeks against potassium hydrogen phthalate or certified standard acid. The solution may also be checked monthly using the photometric method described in Annex A.
Q3: Can this method be used for all types of insulating liquids?
The scope of IEC 62021-2 is specifically mineral insulating oils. For other insulating liquids such as synthetic esters or silicone fluids, different test methods may be more appropriate.
The scope of IEC 62021-2 is specifically mineral insulating oils. For other insulating liquids such as synthetic esters or silicone fluids, different test methods may be more appropriate.
Q4: Why perform a blank titration daily?
The blank titration accounts for any acidity present in the solvent itself and any CO2 absorbed during handling. Daily blank correction ensures accuracy by compensating for day-to-day variations in solvent condition and environmental exposure.
The blank titration accounts for any acidity present in the solvent itself and any CO2 absorbed during handling. Daily blank correction ensures accuracy by compensating for day-to-day variations in solvent condition and environmental exposure.
