NB/SH/T 0207-2010 Determination of water content in insulating liquids by Karl Fischer coulometric titration

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Card type: Test method

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

• Standard number: NB/SH/T 0207-2010
• Chinese title: Determination of water content in insulating liquids by Karl Fischer coulometric titration
• English title: Standard test method for water in insulating liquids by coulometric Karl Fischer titration
• Replacement relationship: replaces SH/T 0207—1992 "Determination of water content in insulating oil (Karl Fischer method)"
• Adoption relationship: modified adoption of ASTM D1533-00 (2005)
• Publication/implementation: published on 2010-05-01, implemented on 2010-10-01
• Custodian: National Technical Committee for Standardization of Petroleum Products and Lubricants, Subcommittee on Petroleum Fuels and Lubricants (SAC/TC280/SC1)
• Drafting organization: Lubricant Research and Development (Shanghai) Center, China Petroleum & Chemical Corporation
• Main drafters: Lü Wenji, Lin Bin

2. Scope of application and non-applicability

• Applicable to insulating liquids (1.1) with a relative water saturation in oil below 100%.
• The coulometric method has high sensitivity and can usually detect 10 μg water (1.1).
• It is recommended to use commercially available Karl Fischer coulometric titration instruments and reagents (1.2).
• Uses International System of Units (SI) units (1.3).
• Not applicable: the source text does not explicitly list a non-applicable scope; Annex A points out that certain high-viscosity oils and silicone oils may give erroneous results, which can be resolved by using an alternative solvent system (A.1).

3. Method principle

• Based on the traditional Karl Fischer reaction in which iodine in the reagent reacts with water and iodine is consumed (3.1).
• The reaction mechanism is as shown in formula (1) and formula (2) (original formulas (1), (2) source-text extraction contain noise, requires confirmation against the source standard (source-text extraction)).
• When iodine has completely reacted with the water in the sample, the platinum electrode indicates the endpoint by detecting a sudden change in the impedance of the titration cell via coulometric analysis (3.1).
• An automatic titrator and commercially available titration reagents are used; the titrator first converts iodide in the Karl Fischer reagent into iodine by the coulometric method. After the sample is injected into the titration cell, the iodine consumed by reaction with water generates iodide, which is oxidized at the anode and electrolyzed to iodine. The platinum electrode indicates the reaction endpoint, and the coulometric quantity of current consumed by reducing iodine is converted into the water content in the sample by Faraday's equation (3.2).
• Titration cell: the coulometric titration cell consists of a sealed cell and an anode and cathode separated by a diaphragm within the cell, or a sealed cell and an anode and cathode not separated by a diaphragm within the cell; the anode compartment is filled with an electrolyte composed of sulfur dioxide, iodine, and a solvent containing methanol/chloroform or methanol/long-chain alcohol and an amine dissolved therein; in a cell with a diaphragm, the cathode compartment is filled with a similar reagent capable of optimal reduction reaction (3.3).

4. Reagents and materials (purity / preparation)

• Unless otherwise specified, all reagents are of analytical grade (Chapter 7).
• Coulometric Karl Fischer reagent: commercially available reagent; the appendix provides information on alternative solvent systems (7.1).
  • Anode reagent: commonly called the electrolytic cell solution (7.1.1).
  • Cathode reagent: commonly called the electrolytic solution (7.1.2).
• Water: meets the grade three water requirements of GB/T 6682 (7.2).
• Verification solution: commercially available verification solutions exist, and they can also be prepared with long-chain alcohols (7.3).
  • Water content of water-saturated n-octanol solution: 39.2 mg/mL ± 0.85 mg/mL (7.3.1).
  • Preparation of water-saturated n-octanol: add water to 25℃ n-octanol until the mixture forms two phases, with the depth of the lower aqueous phase not less than 2 cm; after thorough mixing, let stand at room temperature for more than one day to reach complete equilibrium (7.3.2).
  • Note: to ensure accuracy, the solution should not be shaken or the two phases mixed again after precise sampling. Pipette the upper layer solution and immediately inject it into the titration cell. Changes in water saturation at the time of sampling due to temperature 1% (7.3.2 note, source-text extraction contains noise, requires confirmation against the source standard (source-text extraction)).
  • Verify the instrument response; the response value is 1000 μg~2000 μg, and the precision is within the range specified by the instrument (7.3.3, source-text extraction contains noise, requires confirmation against the source standard (source-text extraction)).
• Safety precautions (Chapter 8): pyridine has traditionally been used as the organic amine in Karl Fischer reagents, but pyridine-free formulations are now commercially available. Pyridine-free reagents titrate faster, are less toxic, have a lighter odor, and are more stable than pyridine-containing types. The reagents used in this standard may contain hazardous chemicals such as iodine, pyridine, sulfur dioxide, methanol, chloroform, chlorinated hydrocarbons, or other organic materials. Therefore, when removing solution from the titration cell and mixing reagents, wear chemical-resistant protective gloves and operate carefully to avoid unnecessary inhalation of reagent vapors or direct contact of skin or eyes with reagents. In case of an accident, flush the affected area with plenty of water.

5. Apparatus and equipment

• Coulometric titrator: consists of a detection electrode, an electrolysis electrode, a titration cell, a magnetic stirrer, and a control unit (6.1).
  • Detection electrode: this electrode indicates the titration endpoint by measuring the sudden change in the impedance of the electrolytic cell via amperometric analysis (6.1.1).
  • Electrolysis electrode: this electrode completes the cathodic reduction of iodide so that the Karl Fischer reaction continues. The electrolysis electrode is a pair of platinum gauze or platinum wire separated by a diaphragm (or not separated by a diaphragm) within a glass assembly (6.1.2).
• Titration vessel: a titration vessel of suitable capacity that can prevent moisture in the air from entering, preferably with a drain plug at the bottom for draining reagents (6.2, source-text extraction contains noise, requires confirmation against the source standard (source-text extraction)).
• Stirrer: used for stirring during titration, consisting of a magnetic stirrer and a stirring bar sealed in glass or polytetrafluoroethylene; the stirring bar is 2 cm~5 cm long, with a length suitable for the titration cell. The stirring bar should be thoroughly cleaned, rinsed with methanol, dried in an oven at 100℃ for 1 h, and stored in a desiccator before use. The stirring bar inside the electrolytic cell does not need to be cleaned and dried again in routine tests (6.3).
• Transfer syringe: use a syringe of a size matching that recommended by the instrument manufacturer; it may be a plastic or glass syringe. Glass syringes should be cleaned before use and dried at 100℃ for 1 h; plastic syringes should be discarded after one use (6.4).
• Needle: attached to the syringe, long enough to inject the sample directly below the surface of the Karl Fischer reagent, and of sufficient gauge to facilitate sample transfer (6.5).
• Septum: used to seal the sample introduction port so that the sample introduction is minimally affected by moisture in the air (6.6).
• Sealant grease: if the instrument does not use a gasket seal, use sealant grease to isolate the chamber from moisture in the air (6.7).
• Drying oven: vacuum type or forced-air circulation type (6.8).
• Desiccator: standard laboratory type, with indicating silica gel (6.9).
• Analytical balance: readability 0.1 mg (6.10).

6. Recommended operating conditions

• Instrument preparation (Chapter 10):
  • After thoroughly cleaning and drying the titration cell, assemble the titration cell according to the instrument manufacturer's recommended method. Clean according to the detailed procedure in Annex B (10.1).
  • Fill the titration cell with a suitable titration reagent according to the requirements of the instrument manual (10.2).
  • Turn on the power and allow the instrument to stabilize according to the requirements of the instrument manual (10.3, source-text extraction contains noise, requires confirmation against the source standard (source-text extraction)).
• System verification (Chapter 11): before starting the test, use an appropriate verification solution of known water content (see 7.3) to verify the titration accuracy of the instrument and reagents. It is best to use a verification solution whose water content range is close to the expected value of the sample to verify the system. Use the verification solution before starting the test; if the measured result of the verification solution exceeds the given water content value, replace the titration reagent and re-verify.
• The stirring speed should be appropriate so as not to form an oil layer on the reagent surface. If an oil layer still forms at the maximum stirring speed, stop the test and remove the oil layer (5.3).
• After assembling the titration cell and adding the solvent system, swirl the solvent system to wet the walls of the titration cell vessel (5.4).

7. Sampling and sample preparation

• Sampling of insulating liquids shall be carried out in accordance with GB/T 7597 (Chapter 9).
• If the sample is turbid or contains free water, it may be difficult to obtain a representative sample (Chapter 9).
• The sample container may significantly affect the water content in the insulating liquid sample. During storage of the sample in a glass container, the water content may increase or decrease, depending on the initial water content, the method of cleaning and drying the container, and the storage time before sample determination. In addition, the drying temperature of the sample bottle must not exceed 110℃, and the sample bottle should be rinsed with the sample to be tested before sampling (5.2).
• Use a suitable syringe and needle to draw the sample; before sampling, rinse the syringe and needle with the sample at least 3 times (12.3, source-text extraction contains noise, requires confirmation against the source standard (source-text extraction)).

8. Analytical procedure

• After the instrument is normal, allow the instrument to re-stabilize before use (12.1).
• Determine the sample amount according to the instrument manufacturer's instructions and the expected range of water content (12.2).
• Use a suitable syringe and needle to draw the sample; before sampling, rinse the syringe and needle with the sample at least 3 times (12.3).
• Obtain the sample mass by weighing the difference in mass of the syringe before and after injecting the sample; the sample mass shall be retained to three significant figures. Alternatively, after determining the volume based on the density of the sample at the test temperature, inject a certain volume of sample with the syringe (12.4).
• The titration reagent can be used until the measured result of the verification solution is no longer accurate; see the instructions for use of the verification solution in Chapter 11 for details (12.5, source-text extraction contains noise, requires confirmation against the source standard (source-text extraction)).
• In routine analysis, if residual oil from the sample is not completely removed from the solvent system, the measured result may be low. When the amount of residual oil is excessive, mixing with the solvent system may be insufficient, resulting in incomplete determination of water in the sample. If the measurement result is inaccurate, discard the electrolytic cell solution (5.3, source-text extraction contains noise, requires confirmation against the source standard (source-text extraction)).

9. Qualitative and quantitative determination and calculation formula

• Most commercially available Karl Fischer coulometric instruments automatically calculate the water content expressed in mg/kg. If the instrument cannot calculate automatically, the water content in the sample is calculated according to formula (3) (Chapter 13):
  • X = A / B
  • Where:
    • X—— water content, mg/kg;
    • A—— mass of water, μg (instrument reading);
    • B—— sample mass, g.
• Report: water content of the sample, rounded to an integer, expressed in mg/kg (14.1.1); sample number (14.1.2); sample temperature (if possible) (14.1.3).

10. Precision

• The precision of this method is based on test results from 9 samples in 37 laboratories. The precision of test results for new and used insulating liquids (95% confidence level) (15.1.1) shall be judged according to the following rules.
• Repeatability r: For the same operator using the same instrument, in the same laboratory and under the same operating conditions, within the water content range of 0 mg/kg~50 mg/kg, the difference between two consecutive test results on the same sample shall not exceed 7 mg/kg (15.1.2).
• Reproducibility R: For different operators in different laboratories, using different instruments, within the water content range of 0 mg/kg~50 mg/kg, the difference between two single, independent test results on the same sample shall not exceed 14 mg/kg (15.1.3).
• Bias: Because there is no suitable reference material with known accurate water content, the bias of this standard cannot yet be obtained through interlaboratory collaborative study (15.2).
• The typical detection limit (MDL) of this standard for water content in mineral oil type samples is 8 mg/kg (15.3).
  • Note 1: In this standard, MDL is defined as the lowest concentration that can be determined for a 7 mL sample (at 95% confidence level) (15.3 Note 1).
  • Note 2: For a single laboratory, it can be calculated from n replicate determination results of the same sample according to formula (4): MDL0.95 = t(n-1, 0.95) × S (15.3 Note 2, source-text extraction has noise, requires confirmation against the source standard (source-text extraction)).
    • Where: t(n-1, 0.95)—— degrees of freedom n-1, the t value at 95% confidence level; S—— standard deviation of replicate determinations.
  • The detection limit calculated by a single laboratory may differ from the detection limit calculated in an interlaboratory collaborative study (15.3 Note 2).
• Appendix A.7: Because there are many types of insulating liquids and solvent systems, this standard cannot provide precision and bias under such improved conditions.

11. Report

• Report the following information (14.1):
  • Water content of the sample, rounded to an integer, expressed in mg/kg (14.1.1).
  • Sample number (14.1.2).
  • Sample temperature (if possible) (14.1.3).

12. Normative References

• GB/T 6682 Water specification and test methods for analytical laboratory use (GB/T 6682—2008, ISO 3696:1987 MOD) (Chapter 2)
• GB/T 7597 Sampling method for power oils (transformer oil, turbine oil) (Chapter 2)
• ASTM D1533-00 (2005) (Foreword, modified adoption)

13. Source and verification

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14. Quick Q&A Reference