ISO 13759 Petroleum products - Determination of alkyl nitrate in diesel fuels - Spectrometric method
6 Preparation of calibration curve
6.1 Not more than 24 h before each determination, prepare calibration curves in accordance with the procedure described in 6.2 to 6.11.
6.2 Carefully measure 1.0 ml, 3.0 ml, 5.0 ml, 8.0 ml and 10.0 ml of alkyl nitrate solution (4.2) from the burette (5.7) into individual 100-ml volumetric flasks (5.5), and dilute to the mark with diesel fuel (4.6).
6.3 Pipette 1.0 ml portions of each of the solutions prepared in 6.2 into five lo-ml volumetric flasks (5.5). Pipette 1.0 ml of diesel fuel into a separate 10-ml volumetric flask. Dilute the contents of all flasks to the mark with propan-2-ol (4.8).
6.4 Pipette into individual separating funnels (5.3) 1.0 ml of each of the solutions prepared in 6.3 including the blank diesel fuel.
6.5 Pipette 1.0 ml of 2,4-dimethylphenol solution (4.5) into each separating funnel and swirl to mix thoroughly.
6.6 Add 40 ml of sulfuric acid solution (4.3) from a graduated cylinder (5.8) to each separating funnel.
NOTE - If the 2,4-dimethylphenol is not added before the sulfuric acid, low or negative results will be obtained.
6.7 Secure the stoppers of the separating funnels and shake the funnels and contents for 30 min on the shaking machine (5.4).
NOTE - The amount of contact that the sample has with the sulfuric acid determines the extent of hydrolysis. Therefore, the speed and time of shaking should be identical for both test samples and calibration standards.
6.8 Add 25 ml of 2,2,4-trimethylpentane (4.7) to each separating funnel and shake for 1 min on the shaking machine. Allow the layers to separate and discard the acid (bottom) layer. Add 25 ml water to each funnel and again shake for 1 min on the shaking machine, allow to separate and discard the water (bottom) layer.
6.9 Pipette 10.0 ml of sodium hydroxide solution (4.4) into each separating funnel, shake by hand for 1 min, and allow the phases to separate for at least 10 min or until they separate cleanly (see note in 6.7). Drain a few drops of the lower (water) phase to rinse the stem, check that the solution is alkaline, and discard.
When the solution is not alkaline, add an additional 10,0 ml of sodium hydroxide solution and correct calculations (see 6.11) for the increased solution volume.
Fill an absorption cell (5.1) with solution from each separating funnel and read the absorbance on the spectrometer (5.2) with respect to water, cleaning the cell between each change of solution.
6.10 Subtract the absorbance of the blank from that of the standard solutions to obtain corrected absorbance values.
6.11 Plot the corrected absorbance values against concentration, and if the response is smooth and rectilinear, calculate the slope, C, of the calibration curve using the following equation:
C = A/B
where
A is the sum of the concentrations of alkyl nitrate standard, expressed as a percentage by volume;
B is the sum of the corrected absorbances.
