SH/T 0246-1992 Determination of Water Content in Light Petroleum Products (Coulometric Method)

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

• Standard number: SH/T 0246-92
• Full title: Determination of Water Content in Light Petroleum Products (Coulometric Method)
• English name: not provided in the available source
• Replacement relationship: replaces SY 7411-80(88)
• Other: confirmed in 2004; approved by China Petrochemical Corporation 1992-05-20, implemented in 1992-05-20

2. Scope of application and non-applicability

• This standard specifies a method for determining the water content of samples by the coulometric method.
• This standard applies to light petroleum products, and the range for determining water content is from 1ppm to 90%(m/m).
• Not applicable: not provided in the available source (only Appendices A12 and A14 mention that hydrogen sulfide, mercaptans, aldehydes, and ketones interfere with the determination).

3. Method principle

Using chloroform, methanol, and Karl Fischer reagent as the electrolyte, 1ppm water can be quantitatively detected with a 2 to 5mL sample.

The principle of the coulometric method for determining trace water is based on the electrolytic process in an electrolyte containing a constant amount of iodine, causing iodide ions in the solution to be oxidized to iodine at the anode:

Anode: 2I⁻ − 2e⁻ → I₂

The iodine produced then reacts with water in the sample:

H₂O + I₂ + SO₂ + 3C₅H₅N → 2C₅H₅N·HI + C₅H₅N·SO₃

The pyridinium sulfate formed further reacts with methanol:

C₅H₅N·SO₃ + CH₃OH → C₅H₅N·HSO₄CH₃

The endpoint of the reaction is indicated by a pair of platinum electrodes. When the iodine concentration in the electrolyte returns to the original concentration, electrolysis stops automatically. According to Faraday's law of electrolysis, the corresponding water content in the sample can be calculated.

4. Reagents and materials (purity / preparation)

• 5.1 Anhydrous methanol: analytical reagent.
• 5.2 Pyridine: analytical reagent.
• 5.3 Iodine: analytical reagent.
• 5.4 Chloroform: analytical reagent.
• 5.5 Sulfur dioxide: may be used from a steel cylinder, or prepared by placing sodium sulfite (or sodium bisulfite) 240g into a 2L round-bottom flask and adding sulfuric acid dropwise to generate sulfur dioxide gas; before use, it must be dehydrated with sulfuric acid.

6.1 Preparation of Karl Fischer reagent

• Solution A: Dissolve 50g iodine in 80mL pyridine with constant shaking until all iodine is dissolved. Then add 260mL anhydrous methanol; at this point an orange crystalline substance appears in the solution.
• Solution B: Pass dried sulfur dioxide into 40mL pyridine to a total volume of 120mL of orange solution.
• In an ice bath, slowly add Solution B dropwise into Solution A; the crystals in Solution A gradually dissolve, yielding a dark brown Karl Fischer reagent. After the solution cools to room temperature, stopper the bottle tightly and store it in a desiccator; it may be used only after stabilizing for 24h.
• Note: When preparing, diluting, and using Karl Fischer reagent, work in a well-ventilated area. Karl Fischer reagent is strongly corrosive and highly toxic; avoid skin contact and inhalation during use.

6.2 Preparation of electrolyte

• 6.2.1 Anode electrolyte: mix uniformly in the ratio chloroform:methanol:Karl Fischer reagent = 3:3:1, and store in a stoppered brown bottle.
  • Note: If the solution is pale yellow or shows excess water, an appropriate amount of concentrated Karl Fischer reagent may be added to consume water introduced by the solvent. However, the amount of Karl Fischer reagent must not exceed 20% (the source text reads “20.I”, requires confirmation against the source standard (source-text extraction)); otherwise, it is necessary to reselect a solvent with lower water content or prepare the electrolyte with concentrated Karl Fischer reagent.
• 6.2.2 The cathode electrolyte is the same as the anode electrolyte.

5. Instruments and equipment

• 4.1 Coulometric titrator: capable of supplying 50mA electrolysis current, and equipped with a delay switch, positive and negative compensation circuit, bridge current adjustment, and other devices.
• 4.2 Electrolysis cell (see Figure 1): the outer shell of the electrolysis cell is a glass cylinder with a diameter of 60mm and a height of 70mm; on the upper part, a standard ground joint of size 19 and one of size 14 are respectively fused for mounting electrodes. Below the female ground joint of size 19, a glass tube with a diameter of 18mm is fused. The lower part is closed with ground glass of size 5, and its length should preferably be controlled so that the distance from the bottom of the glass tube to the bottom of the electrolysis cell is about 10mm. Then, inside the glass tube, a platinum wire gauze is fused directly above the ground glass as the electrolysis cathode, and a platinum wire gauze of the same size is fused below the ground glass as the electrolysis anode. The anode compartment and cathode compartment are separated by a semipermeable membrane (ground glass); the semipermeable membrane can prevent diffusion of the solution, especially iodine, but allows electrons to pass. Below the male ground joint of size 14, two platinum sheets each with an area of 0.7cm² are fused in parallel, with a spacing of 0.5 to 1cm, as indicator electrodes.
• The upper part of the electrolysis cell has a sample inlet, sealed with a flanged rubber stopper; samples can be injected through the rubber stopper with a syringe. During injection, in order to keep the pressure inside the electrolysis cell always in equilibrium, a vent is left in the upper part of the electrolysis cell, connected to the atmosphere through a drying tube. A magnetic stirring bar is placed in the anode compartment to ensure uniform mixing of the solution; all standard ground joints are coated with vacuum grease.
• 4.2.1 Magnetic stirrer: with stirring bar.
• 4.2.2 Sampling bottle (Figure 2): 250mL.
• 4.2.3 Syringe: 1, 2, 5, 50mL.
• 4.2.4 Microsyringe: 0.5μL.
• Figure 1 labels: 1— indicator electrode; 2— electrolysis electrode; 3— platinum gauze; 4— platinum sheet; 5— stirring bar
• Figure 2 label: 1,2— screw clamp

6. Recommended operating conditions

• 6.3 Instrument adjustment (taking Model YS-2A as an example):
  • 6.3.1 Place a stirring bar in the pre-dried anode compartment of the electrolysis cell, then add 70mL to the anode compartment and 2mL electrolyte to the cathode compartment. Cover the titration cell cap, coat all ground joints with vacuum grease to prevent moisture absorption, and keep the electrolysis cell sealed for use.
  • 6.3.2 Wash the 250mL sampling bottle with distilled water, place it together with the silicone rubber tube in an oven at 120℃ and dry for 3 to 4h, then remove and place in a desiccator, and cool to room temperature. Before sampling, the conduit connected to the atmosphere should be protected with a molecular sieve.
  • 6.3.3 Turn on the instrument power switch and the stirrer power switch. Set the “Work—Delay” switch to “Work”, press the “Automatic” key under “Work Selection”, adjust the set knob between 7 and 8, set the gain knob to the maximum value, and press the “Measure 1” or “Measure 2” key under “Meter Switching”. At this time, if the electrolyte contains excess iodine, the microammeter pointer does not start and points at the “0” value, and the amplifier has no signal output.
  • 6.3.4 Inject an appropriate amount of water-containing methanol into the electrolysis cell until the electrolysis cell is in a slightly moist state; the microammeter pointer should slowly move toward the maximum value. The amplifier begins to operate, and at this time the digital display begins to record the electrolysis charge. If it is desired to observe the exact output current value, press the “Electrolysis” key under “Meter Switching” to read the current output.
  • In order to ensure sufficient accuracy of sample analysis results, the appropriate electrolysis current range may be selected according to the sample water content by referring to the data given in the table below, and then electrolysis is carried out to the predetermined endpoint. After reaching the endpoint, record the “set position” and observe the swing of the microammeter pointer and changes in the digital display count to judge the stable state of the electrolysis cell, and according to the stability of the electrolysis cell, appropriately adjust the compensation potentiometer so that the endpoint is stable for 1 to 2min. At this point, the instrument adjustment is complete.
• 7.2 Reference table for injection amount and electrolysis range (the original table headers are “Sample water content, ppm” and “Sampling amount, mL”, “Electrolysis range, μA”; source-text extraction the table is misaligned; the following are the data visible in the source text, requires confirmation against the source standard (source-text extraction)):

7. Sampling and sample preparation

• 6.4.1 When sampling from the unit, first open the sampling port and drain 3 to 5 times the amount of oil retained in the “dead corner”, then connect the dried sampling bottle to the sampling line, rinse the sampling bottle with at least 500mL sample, then directly take the sample into the sampling bottle; after the sample completely fills the container, immediately tighten the screw clamp.
• 6.4.2 After the sample is taken back, it should be analyzed immediately. When using a syringe to take a sample for analysis, first open screw clamp “2”. Then insert the syringe through screw clamp “1” into the middle of the sample oil layer, and slowly pull back the plunger to withdraw the sample.
  • Note: Prevent pulling too quickly, which may cause air to enter the syringe and form bubbles.
• 6.4.3 Because water in the sample may separate out upon cooling, causing low results. For this reason, before analysis, carefully observe the sample and container; if emulsification is found, or small water droplets separate out on the bottle wall, analysis must be carried out according to A9 in Appendix A using the ethylene glycol extraction method.
• 6.5 Determine the density of the sample: determine the density of the sample at the time of sampling according to GB/T 1884, GB/T 1885.
• A11 For samples with a water content less than 10ppm, it is best to use a syringe to sample directly from the unit distillation outlet for analysis. Generally, during sampling, the syringe should be rinsed repeatedly with the sample 5 to 10 times; when injecting, cotton gloves should be worn, and hands must not touch the plunger or needle to prevent contamination.

8. Analysis procedure

• 7.1 Before injection, set an appropriate time on the “Time Selection” key, then press the “Start” button. After the delay indicator lights up, use a microsyringe to inject an appropriate amount of water-containing methanol into the electrolysis cell so that the electrolysis cell contains a trace amount of water; the instrument pointer slowly moves toward 50μA, adjust the electrolysis range to the required position, and when the delay time is reached, the instrument automatically performs electrolysis and automatically stops electrolysis after the endpoint; the result is displayed on the instrument panel in millicoulombs. If the endpoint can remain stable for 1 to 2min, it is ready for sample injection; if the displayed endpoint is unstable, the electrolysis cell needs to be rebalanced and electrolyzed to the endpoint.
• 7.2 When sampling with a syringe, before injection first rinse the syringe with the sample to be analyzed 5 to 7 times, then refer to the table above and quickly inject through the rubber stopper of the electrolysis cell inlet into the electrolysis cell.
• 7.3 Press the "Automatic" knob, time selection is available 100s, then press the "Start" knob; the indicator light being on indicates that the electrolytic cell is reacting; after the predetermined time is reached, the instrument begins electrolysis; after the endpoint, the microammeter pointer returns to the predetermined position, and the digital display stops jumping. Thereafter, if the instrument can stabilize 1min, it can be considered that the analysis endpoint has been reached. Record the millicoulomb value displayed on the digital display.
• 7.4 Repeatedly press 7.1 to 7.3 to perform blank value determination, and record the blank delay time and millicoulomb value.

9. Qualitative and Quantitative and Calculation Formulas

The water content of the sample X(ppm) is calculated according to the following formula:

X = [Q₁ − (Q₂/t₂ × t₁)] × 18 × 10³ / (2 × 96500 × V × ρ)

Or written as:

X = [Q₁ − (Q₂/t₂ × t₁)] × 10³ / (10722 × V × ρ)

Where:

• Q₁ — electricity consumed by the sample, mC;
• Q₂ — electricity consumed by the blank test, mC;
• V — sample volume, mL;
• ρ — density of the sample at sampling, g/mL;
• t₁ — time consumed by sample analysis, s;
• t₂ — time consumed by the blank test, s;
• 18 — molecular weight of water;
• 2 — equivalent number of water;
• 96500 — the amount of electricity consumed by 1 gram-equivalent of water, mC.

(Note: the original formula source-text extraction is misaligned; the above is organized according to the visible symbols in the original text, requires confirmation against the source standard (source-text extraction).)

Appendix A12 Interference Correction Formula

When the analysis sample contains hydrogen sulfide and mercaptans, the determination will be interfered with. In order to obtain accurate and reliable analysis results, an appropriate method may be used to determine their content, and then the true water content in the sample X₂(ppm) is calculated according to formula (A1):

X₂ = [Q₁ − (Q₂/t₂ × t₁)] × 10³ / (10722 × V × ρ) − 9S − 9R (original formula A1, source-text extraction is misaligned, requires confirmation against the source standard (source-text extraction))

Where:

• S — concentration of mercaptans in the sample calculated as sulfur, ppm;
• R — concentration of hydrogen sulfide in the sample calculated as sulfur, ppm;
• Q, V, ρ, t are all the same as the calculation formulas in Chapter 8.

10. Precision

Repeatability: the difference between two repeated determination results and the arithmetic mean shall not be greater than the following values.

(Note: the original precision table source-text extraction is misaligned; 1～10 corresponding to "10% of the arithmetic mean" and >10～50 corresponding to "5% of the arithmetic mean" are organized according to the visible content of the original text, requires confirmation against the source standard (source-text extraction).)

11. Report

Take the arithmetic mean of two repeated determination results as the determination result.

12. Reference Documents

• GB/T 1884 Standard Test Method for Density of Crude Petroleum and Liquid Petroleum Products by Laboratory Determination (Hydrometer Method)
• GB/T 1885 Petroleum Measurement Tables

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 Q&A