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
4. Summary of Test Method
4.1 For existent inorganic sulfate and chloride, a small volume of a sample is evaporated to dryness and reconstituted to the initial sample volume with deionized water, and injected into an ion chromatograph consisting of appropriate ion exchange columns, suppressor and a conductivity detector. For potential sulfate, a small volume of a sample is evaporated to dryness and reconstituted to the initial sample volume with 0.90 % hydrogen peroxide solution in water, and injected into an ion chromatograph. Ions are separated based on their affinity for exchange sites of the resin with respect to the resin's affinity for the eluent. The suppressor increases the sensitivity of the method by both increasing the conductivity of the analytes and decreasing the conductivity of the eluent. The suppressor converts the eluent and the analytes to the corresponding hydrogen form acids. Anions in the aqueous sample are quantified by integration of their responses compared with an external calibration curve, calculated as mg/L for each ion. The calibration standards are prepared from suitable salts dissolved in water solutions. Existent or potential inorganic sulfate and chloride concentrations may be calculated as mg/kg by measuring the density of the original sample.
4.2 Similar methods for chloride and sulfate determinations can be found in Test Method D5827 for engine coolant, and for ethanol in ISO/CEN15492, Test Method D7319 by ion chromatography with direct sample injection, and for sulfate only in Test Method D7318 by potentiometric lead titration.
5. Significance and Use
5.1 Sulfates and chlorides may be found in filter plugging deposits and fuel injector deposits. The acceptability for use of the fuel components and the finished fuels depends on the sulfate and chloride content.
5.2 Existent and potential inorganic sulfate and total chloride content, as measured by this test method, can be used as one measure of the acceptability of gasoline components for automotive spark-ignition engine fuel use.
6. Interferences
6.1 Interferences can be caused by substances with similar ion chromatographic retention times, especially if they are in high concentration compared to the analyte of interest. Sample dilution or standard addition can be used to minimize or resolve most interference problems.
6.2 A water dip (system void, negative peak as shown in Fig. 1) may cause interference with some integrators. Usually, for chloride and sulfate determinations, the water dip should not be a problem since the chloride and sulfate peaks are far enough away from the water dip.
6.3 Given the trace amounts of chloride and sulfate determined by this method, interferences can be caused by contamination of glassware, eluents, reagents, etc. Great care must be taken to ensure that contamination is kept at the lowest possible levels. The use of powder-free gloves is highly recommended to prevent sample contamination.
