ASTM D7328 Standard Test Method for Determination of Existent and Potential Inorganic Sulfate and Total Inorganic Chloride in Fuel Ethanol by Ion Chromatography Using Aqueous Sample Injection
11. Procedure
11.1 Obtain samples in accordance with Practice D4057 or Practice D4177. Samples should be well-mixed to ensure homogeneity. A representative portion shall be taken from each sample for analysis. Samples should be collected in (preferably glass or polyethylene) containers with closures that seal well to prevent evaporation. Sample containers shall not contain any residual or extractable sulfate or chloride. If containers have been cleaned and rinsed with water, they shall be thoroughly rinsed with Type II reagent water and dried prior to use.
11.1.1 Thoroughly mix the samples in their containers immediately prior to withdrawal of the test specimen.

11.1.2 Samples may be analyzed for either or both existent and potential inorganic sulfate, as well as total chloride.

11.2 Set up the ion chromatograph in accordance with the manufacturer's instructions.
11.2.1 Equilibrate the system by pumping eluent for 15 min to 30 min, until a stable baseline is obtained.

11.3 Start the chromatographic run in accordance with the manufacturer's instructions.

11.4 Existent Inorganic Sulfate and Total Chloride:
11.4.1 Carefully add 2.00 mL of the ethanol test specimen into a clean, dry, tared 15 mL glass vial without its screw cap closure. See Note 5.

NOTE 5 - Larger amounts of test specimen may be prepared (as needed for autosampler use) as long as the preparation maintains the prescribed ratio of components. Preparation of larger test specimen might increase drying time. Precision was determined with 2.00 mL aliquots.

11.4.2 Place the vial with sample in a hot block at 65 °C and blow a steady stream of nitrogen gas over the sample at 2 mL/min to 3 mL/min flow. Allow the sample to dry completely, this may take 5 min to 10 min. When all of the liquid is gone, remove the vial from the hot block and allow it to cool to room temperature (60 °F to 80 °F).

NOTE 6 - It is possible that a slight oily residue from the ethanol denaturant could remain. Do not worry about this residue if it is a thin film, as any sulfide or chloride in it will be extracted into the water phase.

NOTE 7 - Various commercial apparatus are used to evaporate the sample. Depending on the apparatus used, the flow can be adjusted to optimize the evaporation.

NOTE 8 - The purpose of the nitrogen is to sweep the ethanol vapors away from the sample tube so they do not condense back into the sample. There should be enough flow to sweep the ethanol vapors without creating turbulence in the sample and possible loss of analyte. When the flow gets too high, the cooling effect from the nitrogen gas can work against the intent. If the temperature in the sample block does not reach 65 °C, this can also extend the time to evaporate. Typical nitrogen flows are two to three milliliters per minute. The flow may be varied to accommodate available laboratory apparatus.

11.4.3 Carefully add 2.00 mL of Type II water to the dried sample. Seal the vial with a screw cap, and shake the vial vigorously to dissolve all of the solid salts.

11.4.4 Inject the resulting solution into the ion chromatograph as in 11.6.

11.5 Potential Sulfate:
11.5.1 Carefully add 2.00 mL of the ethanol test specimen into a clean, dry, tared 15 mL glass vial without its screw cap closure. See Note 5.

11.5.2 Place the vial with sample in a hot block at 65 °C and blow a steady stream of nitrogen over the sample. Allow the sample to dry completely, this may take 5 min to 10 min. When all of the liquid is gone, remove the vial from the hot block and allow it to cool to room temperature (60 °F to 80 °F). See Note 6.

11.5.3 Carefully add 2.00 mL of 0.90 % hydrogen peroxide solution (9.3) to the dried sample. Seal the vial with a screw cap, and shake the vial vigorously to dissolve all of the solid salts.

11.5.4 Inject the resulting solution into the ion chromatograph as in 11.6.

11.6 Inject 25 µL of sample solution from 11.4 or 11.5 into the ion chromatograph, and measure the areas of the peaks corresponding to chloride and sulfate ions.

11.7 If the concentration of the anion of interest exceeds that of the highest calibration solution, dilute the sample solution with Type I water containing no measurable sulfate or chloride as appropriate, and repeat the sample measurement. Take into account the dilution factor in the calculation of sulfate content in the ethanol sample, and calculate the dilution factor as shown in Eq 4.
F = Vf/Vi
where:
F = dilution factor,
Vi = volume of the initial sample diluted, mL, and
Vf = volume of the final solution, mL.

12. Calculations
12.1 The concentrations of sulfate or chloride in the ethanol sample as mg/L (mg/mL) are calculated as shown in Eq 5.
C = A x Rf x F
where:
C = concentration of anion in the original ethanol sample, mg/L,
A = anion peak area, from the ion chromatogram in 11.6, counts,
Rf = calibration plot response factor from 10.5, mg/L/counts, and
F = dilution factor from Eq 4 (= 1.0 if no dilution).

12.2 The units of mg/L (volume) can be converted to mg/kg (weight) using the density of the same denatured ethanol as is present in the sample, as shown in Eq 6. Denatured ethanol density (d) at room temperature may be measured using Test Method D4052.
sulfate (mg/kg) = sulfate (mg/L)/d
where:
d = density of denatured ethanol, g/mL.