ASTM D7319 Standard Test Method for Determination of Existent and Potential Sulfate and Inorganic Chloride in Fuel Ethanol by Direct Injection Suppressed Ion Chromatography
4. Summary of Test Method
4.1 For existent inorganic sulfate and total chloride, a small volume of an ethanol sample is directly injected into a suitably configured ion chromatograph in accordance with manufacturer's recommendations for this test method. For potential sulfate, 0.5 mL of 30 % hydrogen peroxide solution is added to 9.5 mL of the ethanol sample, and then injected into the ion chromatograph. Ions are separated based on their affinity for exchange sites ofthe resin with respect to the resin's affinity for the eluent. The suppressor increases the sensitivity of the test method by both increasing the conductivity of the analytes and decreasing the conductivity of the eluent. It also converts the eluent and analytes to the corresponding hydrogen forms of anions. Anions are quantified by integration of their responses compared with an external calibration curve, and are measured as milligrams/litre, then converted to milligrams/kilograms. The calibration standards are prepared in an aqueous matrix.
4.1.1 The use of acid suppression technology is more significant for the precision of sulfate measurement than for chloride. Sulfate precision will be worse if acid suppression is not used.
4.2 Similar methods for chloride and sulfate determinations can be found in EN 15492 for total chloride, in Test Method D7328 for total chloride and existent and potential inorganic sulfate by ion chromatography with aqueous sample injection, and in Test Method D7318 for existent inorganic sulfate by potentiometric lead titration and Test Method D5827 for chloride and other anions in engine coolants by ion chromatography.
5. Significance and Use
5.1 Sulfates and chlorides can 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 can be used to minimize or resolve most interference problems.
6.2 A water dip (system void, negative peak as shown in Fig. 1) can 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 away from the water dip.
6.3 Given the trace amounts of chloride and sulfate determined in this test method, interferences can be caused by contamination of glassware, eluent, reagents, and the like. For example, sodium leaching from glassware can precipitate with sulfate giving low sulfate results. Take care to ensure that contamination is kept at the lowest possible levels. The use of powder-free gloves is highly recommended to prevent sample contamination.
