Denatured Ethanol Analysis

By Dr. Jerry King | April 01, 2006
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The ethanol industry is expected to grow rapidly, and the need for additional testing will also increase, but this ramped-up testing must be accompanied by validated methods, acceptable quality control and lower detection levels. Currently, the majority of U.S. fuel ethanol is blended with gasoline, of course, and the chemistry of the resulting ethanol-gasoline blend is very complex, so extensive knowledge of the chemical composition is crucial.

The industry's leading laboratories are positioned to effectively analyze denatured ethanol by utilizing instruments such as Inductively Coupled Argon Plasma Emission Spectroscopy (ICP). The current version of ASTM D 4806the specification for denatured fuel ethanolhas requirements for copper and sulfur. The analytical methods require use of Atomic Absorption for copper and sulfur by UV fluorescence. Both procedures analyze for only one metal, but with newer regulations and the understanding of the level of other minerals such as sodium, phosphorus, lead and others, the ICP method would analyze for a number of metals using one instrument and one analysis. Use of ICP has been referenced and used in a variety of ASTM standards, but the instrument has yet to be validated for use in ethanol analysis, and Midwest Laboratories of Omaha, Neb., is preparing to validate the method.

ICP detection is based on the principle that each energized element emits light energy as the atom returns to its resting state. By determining the type (i.e., wavelength) and amount of light (i.e., intensity) emitted, it is possible to determine how much of a specific metal is present. The existing ICP instruments can detect a number of elements, provided the wavelengths of light are unique to a specific element. One of the most critical aspects of ICP analysis is sample preparation. The objective of the sample prep is to remove the ethanol and dissolve the metals in a liquid without losing the various metals.

Generally, use of acids or microwaves can be used to prepare the ethanol samples, but great care must be exercised since ethanol is very flammable. Also, some heating methods can also drive off more volatile minerals such as sulfur, so modifications have to be considered as the sample is prepared.

Another difficulty in determining metal concentrations in ethanol involves the ability to obtain a "true" value for an accuracy study. It is possible to use spiked samples, but the behavior of a spiked sample and a "natural" sample may be different.
The analysis of sulfate and chloride are two other analytes that are monitored in a number of ASTM standard requirements. Currently, sulfate is not part of the D 4806 standard, but chloride is. Over the past year or more, sulfate levels have been investigated as a possible source of clogging problems experienced in the automobile industry. To determine the levels of sulfate present and if the sulfate is responsible for clogging, it was important to develop a sensitive test for sulfate. The current method under review is the potentiometric lead titration method, but concerns have been raised about the inaccuracy of the method, the inability to achieve low-level detection with confidence and the presence of toxic materials in the laboratory. Midwest Laboratories has developed and validated a method using Ion Chromatography (IC) that can achieve both low-detection levels and confidence in the results. In addition, the IC method can obtain both chloride and sulfate in the same analytical run. It is very important in the laboratory area to find a way to explore new methods of analysis of denatured alcohol that can provide quicker turnaround and lower detection levels, while reducing the interference caused by denaturants in gasoline itself.

In conjunction with the Renewable Fuels Association (RFA), a nationwide survey of finished denatured alcohol was carried out to provide information about existing levels of sulfate and chloride, and also to compare two proposed sulfate analytical methods. The two methods included an evaporative IC method for chloride and sulfate, and a lead potentiometric titration method for just sulfate. The RFA provided a list of existing ethanol plants and a set of four sample bottles sent to the plant with sampling instructions. Each plant collected a finished ethanol sample and three additional samples each week for a month. When all four samples were collected, they were sent to Midwest Laboratories for analysis.

At our laboratory, each of the submitted samples were logged in, and one of the four submitted samples was selected as a duplicate. Each sample was analyzed by IC and the potentiometric titration method. The results were then compared. The design of this study allowed a comparison of the levels of sulfate between the two methods to demonstrate precision of the methods. This was also the case with the duplicate sample. At the conclusion of the study, close to 360 samples were analyzed. The data indicated no sample exceeded 10 milligrams per liter (mg/L) for sulfate, but 17 samples analyzed by potentiometric titration had levels between 4.01 mg/L and 10 mg/L, while 26 had these levels using IC. The two methods were fairly consistent in the levels detected, but the IC showed greater precision between samples than the titration method. The titration method was more sensitive to pH changes and the interpretation of the graph inflection, and the confidence at detection levels of 0.4 was quite low, while the IC had very reproducible results at the 0.4 mg/L level. In addition, the IC method was able to provide chloride results in the same run, while with the potentiometric titration method, only sulfate would be provided.

During the December ASTM meeting, the ASTM D 4806 specification was voted to include sulfate as a specified parameter at a level of 4 ppm. A round robin was set up to compare the potentiometric lead titration method and two IC methods: a direct injection method and an evaporative method. The results of the round robin will be evaluated and the result distributed to the ASTM committee for working with planned implementation in late 2006.

The denatured ethanol industry is rapidly growing, and with the government incentives for increasing E85 and biodiesel, the need for quality, affordable and accurate analyses will increase. Through participation in round robin studies, performance samples and ASTM participation, the developing analytical field will mature, and instrumentation and sample preparation methods will need to be refined to meet the new analytical needs. The quality systems are generally in place in established laboratories, and it is these laboratories that will provide the necessary method development for E85 and biodiesel analyses. In addition, a full-service ethanol laboratory will be able to provide analyses of ethanol ingredients, raw material for mycotoxins, coproducts for feed nutritional levels and amino acid profiles, and waste water for compliance purposes.

Dr. Jerry King is QA/QC contracts director for Omaha, Neb.-based Midwest Laboratories Inc. Reach him at or (334) 829-9890. Additional contacts include Heather Ramig at or (402) 829-9891, and Bill Diederich at or (402) 829-9864.