Meeting the Analytical Requirements for Sulfate in Ethanol

By Derrick W. Rowe | October 02, 2006
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Contamination of fuel ethanol with soluble salt ions, such as chloride, nitrate and sulfate, can affect the performance ofand cause corrosion withinan engine system. An ASTM standard for a maximum sulfate contamination level has been proposed at the four parts per million (ppm) level. A chloride contamination limit is proposed at 40 ppm with some discussions to reduce this to as low as one ppm. With forthcoming ASTM methods for the analysis, many companies are working to provide a viable solution to the industry requirement. Ion chromatography (IC) is at the leading edge of this effort. The application of this technique for the satisfaction of the operational and analytical requirements will be discussed.

There are several analytical techniques for the determination of chloride and sulfate by independent analyses. Of these methods, IC is ideal for analysis factors, such as selectivity, sensitivity, automation, ruggedness and flexibility. IC uses a high-pressure analytical pump to deliver a liquid solution to separate components on a packed column. Specifically, a simple sodium carbonate solution is used to separate ions like chloride and sulfate, from an injected sample on an anion exchange resin-packed column. The separated ions pass through a highly accurate conductivity detector and can then be quantified automatically using data processing software.

A key component for both the sensitivity and ruggedness of the method is called a suppressor. In anion exchange IC, the suppressor acts as a cation exchanger by replacing cations and metals with a hydronium ion. Through this process it reduces the conductive background and increases the sensitivity of the ions to the detector. Combined, this process yields quantitative limits for chloride and sulfate in the parts per billion range. Many suppressors will experience an incompatibility or significantly shortened life if contacted by solvents like ethanol and its denaturants. Great care must be taken to select a compatible suppressor for rugged performance of the method. Since ion chromatographs are software-controlled, full automation for the analysis of multiple samples can be programmed for unattended operation. A fully enclosed and portable system design ensures rugged operation in variable environments.

Since IC systems use the same basic components as high-performance liquid chromatography (HPLC), the system can be easily converted for HPLC analyses on fermentable sugars and organic acids, which may be required for the production process.
The Metrohm 861 Compact IC system has been used in the development of the IC method that is, at press time, in the final round robin stages. Still, many ethanol plants have already employed the system for quality control ahead of the expected standard release.

As with any analytical method, the method effectiveness is measured by a series of tests to determine how accurately and precisely quantification can be controlled in real-world samples. Titration methods are operating at the bottom limit of the technique's range, which increases the risk for error and leaves no room for detection improvement. A proposed IC method requires evaporation of a measured volume of ethanol with reconstitution in deionized water prior to analysis. The potential loss of recovery, however, can adversely affect the precision of the analysis. Additionally, the evaporation process requires preparation time and operator effort prior to analysis. Therefore, a simple direct-injection method will give repeatable and timely results with superior sensitivity. The Metrohm IC direct injection method has been used to demonstrate the direct-injection analysis of chloride and sulfate in denatured ethanol samples.

In the production process, sulfite and other forms of sulfur may be formed that will oxidize into sulfate over time. Depending on the age of the sample at analysis, a discrepancy can exist between results. To determine the potential sulfate that could be formed during transportation of ethanol to a blending terminal, for example, a known amount of hydrogen peroxide is added to the sample prior to analysis. This ensures agreement between production and point of use sulfate values.

A key consideration for the analysis is the method ruggedness for long-term use. There are several components that can be negatively affected by repeated injection of compounds, like some of the denaturants, hydrogen peroxide and even the ethanol itself. Ongoing deterioration of the column or suppressor will lead to lower responses (increasing error), frequent calibration and a higher cost to maintain the operational integrity of the system. The Metrohm IC uses a suppression technology that is not only the most rugged for the analysis; it is also backed by an industry leading 10-year warranty.

Table 2 shows the initial concentration of a denatured ethanol sample at the installation of the application, and the injection of the same sample after 1500 injections. The table shows stable retention times (column performance), repeatable area counts (suppressor performance) and consistent concentration values (calibration stability). During the test, none of the system components were replaced or changed. So, not only is the system shown to be rugged, it is dependable and offers a low operating cost.

Since many ethanol plants operate around the clock, the instrument must be user-friendly for numerous operators at differing familiarity levels. The Metrohm system uses control software that has a built-in method for the application and a simplified interface for novice users. Operational instructions are also provided in the technical application note AW-US6-0110-072005.
The Metrohm IC is well-suited for the analysis of chloride and sulfate in ethanol samples for monitoring compliance with ASTM D4806 specifications. For application information on determining chloride and sulfate in ethanol, fermentable sugars, or other ion chromatography applications, please contact Metrohm-Peak LLC.

Derrick W. Rowe is vice president of technical operations for Metrohm-Peak LLC. Reach him at (800) 410-7118.