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HYDROLYSIS OF MTBE IN GROUND WATER SAMPLES PRESERVED WITIH HYDROCHLORIC ACID
Citation:
McLoughlin, P., J T. Wilson*, D. Fine, AND R. J. Pirkle. HYDROLYSIS OF MTBE IN GROUND WATER SAMPLES PRESERVED WITIH HYDROCHLORIC ACID. Presented at 2002 Petroleum Hydrocarbons Conference, Atlanta, GA, November 05 - 08, 2002.
Impact/Purpose:
To inform the public.
Description:
Conventional sampling and analytical protocols have poor sensitivity for fuel oxygenates that are alcohols, such as TBA. Because alcohols tend to stay with the water samples, they are not efficiently transferred to the gas chromatograph for separation and analysis. A common technique to improve the efficiency of transfer is to heat the water sample to 80 degrees Celsius. Traditionally, ground water samples that are collected for analysis of gasoline constituents are preserved by adjusting the pH of the sample to less than 2 with hydrochloric acid. When the water is heated, the acid used to preserve the sample can cause chemical hydrolysis of the ether oxygenates including MTBE, TAME, and ETBE.
This effect is well illustrated in data collected by U.S. EPA/ORD on a plume of MTBE in California. To attain a method detection limit for TBA that was less than 3 g/liter, the samples were prepared using a static headspace sampler heated to 80°C. The ground water samples were preserved in the field with hydrochloric acid to a pH less than two. In fifteen ground water samples it was possible to estimate the extent of MTBE hydrolysis during sample preparation; the extent of hydrolysis of MTBE varied from 19% to 87%; the average extent of hydrolysis was 59%.
To confirm and document the importance of acid hydrolysis of MTBE at higher temperatures during sample preparation, the rate of hydrolysis of MTBE at was determined at 80°C C in pyrophosphate buffer at pH=1 and pH=2. Acid hydrolysis of MTBE was very temperature dependant. The published rate of MTBE hydrolysis at pH=1 and 37°C is 0.00323 per hour; this study extracted a rate of hydrolysis of 1.22 per hour at pH=1 at 80 °C. The published rate of hydrolysis at pH=2 and 37°C is 0.000383 per hour; this study extracted a rate of hydrolysis of 0.15 per hour at pH=2 at 80°C.
U.S. EPA/ORD has developed an alternative procedure for preserving water samples that uses a base (tri-sodium phosphate) instead of acid. Because ethers are not subject to base catalyzed hydrolysis, there is no possibility of hydrolysis of the ether fuel oxygenates. This study shows that tri-sodium phosphate is at least as effective as hydrochloric acid as a preservative for ground water samples that are intended for analysis of MTBE and BTEX compounds.