GB/T 1792-2015 Determination of mercaptan sulphur in gasoline, kerosene, jet fuel and distillate fuels — Potentiometric titration method

Summary page. This page is a compiled excerpt/summary of the standard — it is not the full standard text and does not replace it. Verify every value against the official standard and the product certificate before use.

Applies to the determination of the mercaptan sulphur content of gasoline, kerosene, jet fuel and distillate fuels.

Evidence note: a partial revision of the existing 2015 study body: the input current exponent of the meter, the sign of the initial potential, the clause references and the reporting precision have been checked against the 15-page original held. Technical review is pending; the figures and the steps not fully presented must be checked against the controlled original, and neither the current version status nor product applicability is presumed. In this round the meter input current exponent was corrected from 12 to 22, the missing minus sign at one end of the initial potential was restored, the misaligned old clause numbers were corrected, and the reporting precision was added. The 31 g in the body and the 30.6 g/L in the note for sodium sulphide nonahydrate are both retained; the acidic solvent of 20 mL water + 975 mL isopropanol + 4.6 mL glacial acetic acid is not forced to total 1000 mL; the precision limitations for out-of-range, small and diluted samples are retained. The figures and complete annexes have not been reconstructed, and technical and independent acceptance remain outstanding.

Scope: Applies to the determination of the mercaptan sulphur content of gasoline, kerosene, jet fuel and distillate fuels. Mass fraction range for mercaptan sulphur: 0.000 3% to 0.01%. Organic sulphur compounds (sulphides, disulphides and thiophenes) do not interfere with the determination. Elemental sulphur at a mass fraction below 0.000 5% does not interfere with the determination. Hydrogen sulphide interferes with the determination unless it is removed; for removal see 8.2. Samples with a mercaptan sulphur mass fraction greater than 0.01% may be analysed by reducing the sample size (the sum of the titration solvent and sample volumes must be at least more than 120 mL); the precision of the determination is not established in this standard. For sample sizes below 20 mL or for diluted samples, the precision of this standard has not been determined between laboratories.

Principle: A sample free from hydrogen sulphide is dissolved in an isopropanol titration solvent containing sodium acetate; the potential between a glass reference electrode and a silver/silver sulphide indicating electrode is used as the indicator, and titration is carried out potentiometrically with standard alcoholic silver nitrate solution. During the titration the mercaptan sulphur is precipitated as silver mercaptide, and the end point is shown by the sudden change in cell potential.

Apparatus: For the potentiometric titration apparatus see 4.2 to 4.5. Alternatively, any automatic titration system may be used provided that it uses an electrode pair meeting the requirements of 4.3, can titrate as required by clause 8, gives an end point meeting 10.1, and has a determination precision meeting or better than that of clause 13. 4.2 Meter: an electronic voltmeter operating with an input current of less than 9×10⁻¹² A, with a range of at least ±1 V and an accuracy of ±2 mV. It shall have a static-protected case, and the protective case shall be earthed. 4.3 Cell system: consists of a reference electrode and an indicating electrode. The reference electrode shall be a robust pencil-type glass electrode with an earthed shielded lead. The indicating electrode is made of silver wire of 2 mm diameter or larger fixed in an insulating support; a silver strip electrode may also be used. 4.4 Burette: capacity 10 mL, graduation 0.05 mL, with a delivery tip extending about 120 mm below the stopcock. 4.5 Titration stand: preferably made as an integral part of the meter case, fitted with an electrode holder and a magnetic stirrer, and earthed. Switching the stirrer motor on or off shall not cause an obvious lasting change in the meter reading.

Reagents and reference materials: Reagent purity: all tests use analytical-grade reagents, which shall comply with the relevant national or industry standards unless otherwise stated; reagents of other grades may also be used provided their purity is shown to be sufficient and the accuracy of the determination is not reduced. 5.1.1 Commercial solutions whose concentration has been shown to meet the requirements may replace laboratory-prepared solutions. 5.1.2 Larger quantities of solutions and solvents may be prepared and kept at the required concentration provided the concentration remains unchanged. 5.2 Water: not lower than grade 3 water according to GB/T 6682-2008. 5.3 Cadmium sulphate: prepare an acidic cadmium sulphate solution. Dissolve 150 g of cadmium sulphate hydrate (3CdSO₄·8H₂O) in water, add 10 mL of dilute sulphuric acid and dilute to 1 L with water. WARNING: toxic; swallowing or inhalation may be fatal; known carcinogen (positive in animals). 5.4 Potassium iodide: prepare an approximately 0.1 mol/L potassium iodide standard solution. Dissolve 17 g of potassium iodide (KI) (weighed to 0.01 g) in 100 mL of water in a 1 L volumetric flask, dilute to 1 L with water and calculate the exact molar concentration. 5.5 Isopropanol: analytical grade. WARNING: flammable; stored in a container exposed to air it forms peroxides and may explode when evaporated to dryness; suspected peroxide may be removed by percolation through an activated alumina column. 5.6 Silver nitrate: analytical grade. 5.6.1 0.1 mol/L alcoholic silver nitrate standard solution: dissolve 17 g of silver nitrate (AgNO₃) in 100 mL of water in a 1 L volumetric flask and dilute to 1 L with isopropanol; store in a dark bottle and standardize periodically, sufficiently often to detect a change of 0.000 5 mol/L or more. NOTE: it is essential that the isopropanol is freed from peroxides through an activated alumina column; failure to remove them leads to low results; where testing shows no peroxides, removal is unnecessary. Standardization: measure 100 mL of water into a 200 mL, 250 mL or 300 mL beaker, add 6 drops of concentrated nitric acid (relative density 1.42) and boil for 5 min to remove nitrogen oxides; after cooling to room temperature, pipette 5 mL of the 0.1 mol/L potassium iodide standard solution into the same beaker and titrate with the alcoholic silver nitrate standard solution; take the inflection point of the titration curve as the end point and calculate the exact molar concentration. WARNING: nitric acid is toxic, causes severe burns, and is harmful or fatal if swallowed or inhaled. 5.6.2 0.010 mol/L alcoholic silver nitrate standard solution: prepare on the day of the test by diluting the 0.1 mol/L alcoholic silver nitrate standard solution. Pipette 100 mL of the 0.1 mol/L alcoholic silver nitrate standard solution into a 1 L volumetric flask, dilute to the mark with isopropanol and calculate the exact molar concentration. 5.7 Sodium sulphide: prepare a 10 g/L sodium sulphide solution. Dissolve 10 g of sodium sulphide (Na₂S) or 31 g of sodium sulphide hydrate (Na₂S·9H₂O) in water and dilute to 1 L with water, preparing fresh when necessary. NOTE: anhydrous sodium sulphide is difficult to purchase and expensive; an aqueous solution of 30.6 g/L sodium sulphide hydrate (Na₂S·9H₂O) may be used. 5.8 Sulphuric acid: prepare a 1:5 (by volume) dilute sulphuric acid solution. Add 1 volume of sulphuric acid (relative density 1.84) slowly to 5 volumes of water with constant stirring. Only 10 mL of dilute sulphuric acid is needed per litre of acidic cadmium sulphate solution. WARNING: adding water produces heat, so mix thoroughly; if the water begins to boil, cool first before adding more acid. 5.9 Titration solvent: low relative molecular mass mercaptans (usually present in gasoline) are readily lost from acidic titration solutions and shall be determined with an alkaline titration solvent; higher relative molecular mass mercaptans (usually present in kerosene, jet fuel and distillate fuels) shall be determined with an acidic titration solvent, which helps to reach equilibrium more quickly. 5.9.1 Alkaline titration solvent: dissolve 2.7 g of sodium acetate hydrate (NaC₂H₃O₂·3H₂O) or 1.6 g of anhydrous sodium acetate (NaC₂H₃O₂) in 25 mL of oxygen-free water and pour into 975 mL of isopropanol. If necessary, remove dissolved oxygen before daily use by passing a rapid stream of nitrogen for 10 min; exclude air by a nitrogen blanket; blanket with nitrogen before sealing to reduce dissolved oxygen during storage. 5.9.2 Acidic titration solvent: dissolve 2.7 g of sodium acetate hydrate (NaC₂H₃O₂·3H₂O) or 1.6 g of anhydrous sodium acetate (NaC₂H₃O₂) in 20 mL of oxygen-free water, pour into 975 mL of isopropanol and add 4.6 mL of glacial acetic acid. If necessary, remove dissolved oxygen before daily use by passing a rapid stream of nitrogen for 10 min; exclude air by a nitrogen blanket; blanket with nitrogen before sealing to reduce dissolved oxygen during storage. 5.10 Abrasive paper or cloth: average abrasive particle size 6 µm to 20 µm.

Key conditions: Sample size: weigh or measure 20 mL to 50 mL of the original or treated sample into a 200 mL, 250 mL or 300 mL beaker containing 100 mL of the appropriate titration solvent. Electrode immersion: adjust the position of the titration stand so that the electrodes are about half immersed. Burette tip: extend about 25 mm below the surface of the liquid in the beaker. Stirring: adjust the stirring speed so that stirring is vigorous without splashing. Titrant: 0.01 mol/L alcoholic silver nitrate standard solution. Initial potential: in the presence of mercaptans the potential reading is usually between −250 mV and −350 mV (NOTE: on some instruments the algebraic sign of the potential may be reversed). Criterion for constant potential: a potential change of less than 6 mV/min is regarded as constant; near the end point, a potential change of less than 6 mV per 0.1 mL added is regarded as constant. End-point potential: when titrated to a constant potential, the cell potential reading is usually close to +300 mV. Low-temperature condition for light gasoline: light gasoline fractions with a final boiling point not above 85 °C may contain a high mercaptan sulphur content (> 100 mg/kg) and are very temperature-sensitive during titration; the sample should be kept below 4 °C; where necessary, light gasoline samples containing methanethiol or higher mercaptans should be titrated with the apparatus cooled and maintained below 4 °C. Sample storage: untreated light gasoline known or believed to contain methanethiol (boiling point 6.2 °C) should be stored below 4 °C.

Calculation and results: The mass fraction X (%) of mercaptan sulphur in the sample is calculated by equation (1), (2), (3) or (4): X = [D·M·(A₁ - A₀) × 3.206] / m (1) X = [D·M·(A₁ - A₀) × 3.206] / (d·V) (2) D = (m + I) / m (3) D = (V + J) / V (4) where: A₁ is the volume of alcoholic silver nitrate standard solution required to reach the end point at about +300 mV (see Figure 1) when titrating the sample, mL; A₀ is the volume of alcoholic silver nitrate standard solution required to reach the end point at about +300 mV (see Figure 1) when titrating the blank, mL (see NOTE); d is the density of the sample at the sampling temperature, g/mL; D is the dilution factor (if necessary, see NOTE 2 of 8.3.2); I is the mass of diluent used, g; J is the volume of diluent used, mL; M is the molar concentration of the alcoholic silver nitrate standard solution, mol/L; m is the mass of sample used, g; 3.206 is 100 × the mass in grams of sulphur per millimole of mercaptan; V is the volume of sample used, mL. NOTE: A₀ is unchanged in the equations. 10.1 Data treatment: plot the cumulative volume of 0.01 mol/L alcoholic silver nitrate standard solution added against the corresponding cell potential. Take as the end point the inflection point of the steepest part of each "break" in the titration curve shown in Figure 1. The shape of the titration curve may vary with the instrument, but the interpretation of the end point does not change. 10.2 Mercaptans only: the titration gives the first type of curve in Figure 1, with an initial plateau at a potential equal to or more negative than −250 mV; the end point is reached when the potential change is less than 6 mV/min and the change in titrant potential (mV/min) decreases with each increment added. 10.3 Mercaptans and elemental sulphur together: an interactive chemical reaction occurs and silver sulphide (Ag₂S) is precipitated during the titration. 10.3.1 Mercaptan in excess: the silver sulphide precipitation end point appears at about −550 mV to −350 mV, followed by precipitation of silver mercaptide, whose end point is reached in the region close to +300 mV (middle curve of Figure 1). Since all the silver sulphide is produced from an equivalent amount of mercaptan, the total titre reaching the mercaptide end point shall be used to calculate the mercaptan sulphur content. 10.3.2 Elemental sulphur in excess: the silver sulphide precipitation end point occurs in the same region as in the mercaptide case (+300 mV), and the silver sulphide precipitation end point shall be used as the mercaptan sulphur end point to calculate the mercaptan sulphur content. 10.4 Light gasoline samples containing methanethiol or higher mercaptans may give anomalous results; it is necessary first to cool the test apparatus and keep it below 4 °C, then titrate as in 8.2 to 8.4, which gives more reproducible results.

Reporting precision and removal boundaries: Original clause 12: the result of the determination is reported to a mass fraction of 0.000 1% or 1 mg/kg; the number of reported digits must not be omitted. Hydrogen sulphide interferes with the determination; for arbitration the designated removal procedure of 8.2.1 shall be followed. Alternative removal procedures for non-arbitration use, or modified procedures for automatic instruments, are considered only under the conditions given in the original; data obtained with such modifications are not covered by the precision of this standard, and samples containing hydrogen sulphide must not be applied directly to the precision. Figure 1 is a schematic titration curve, not an actual sample curve; Figure 2 is the precision curve. Annexes A and B are not set out in full on this page, and this study summary is not to be used as an operating procedure.

Quality control: The precision was established from interlaboratory results by statistical analysis (95% confidence level). 13.1.1 Repeatability r: the difference between two consecutive results obtained by the same operator using the same apparatus on the same sample shall not exceed the value calculated by equation (5): r = 0.000 07 + 0.027X (5) where X is the arithmetic mean of the mercaptan sulphur mass fractions of the two consecutive determinations, %. 13.1.2 Reproducibility R: the difference between two single and independent results obtained by different operators in different laboratories on the same sample shall not exceed the value calculated by equation (6): R = 0.000 31 + 0.042X (6) where X is the arithmetic mean of the mercaptan sulphur mass fractions of the two single and independent determinations, %. 13.1 NOTE: the precision is shown in Figure 2 (precision curve for mercaptan sulphur in gasoline, kerosene, jet fuel and distillate fuels). 13.2 Bias: the bias of this standard has not been established.

Normative references: GB/T 1884 Crude petroleum and liquid petroleum products — Laboratory determination of density (Hydrometer method) GB/T 1885 Petroleum measurement tables GB/T 4756 Petroleum liquids — Manual sampling GB/T 6682-2008 Water for analytical laboratory use — Specification and test methods GB/T 27867 Petroleum liquids — Automatic pipeline sampling SH/T 0604 Crude petroleum and petroleum products — Determination of density (Oscillating U-tube method) Annex B refers to: NB/SH/T 0843, ASTM MNL7 (general quality control).

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