You are here:
HYDROLYSIS OF MTBE TO TBA IN GROUND WATER SAMPLES WITH HYDROCHLORIC ACID
Citation:
McLoughlin, P., R. J. Pirkle, D. Fine, AND J T. Wilson*. HYDROLYSIS OF MTBE TO TBA IN GROUND WATER SAMPLES WITH HYDROCHLORIC ACID. 2002 Petroleum Hydrocarbons and Organic Chemiclas in Ground Water Prevention, Assessment, and Remediation, Atlanta, GA, 11/6-8/2002.
Description:
Conventional sampling and analytical protocols have poor sensitivity for fuel oxygenates that are alcohols, such as tert-butyl alcohol (TBA). Because alcohols are miscible or highly soluble in water, alcohols are not efficiently transferred to the gas chromatograph for analysis. A common technique to improve the efficiency of transfer is to heat the water sample to 80 C. 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 sample is heated, the acid used for preservation causes 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. The samples were analyzed 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. The extent of MTBE hydrolysis to TBA during sample analysis varied from 19% to 87%; the average extent of hydrolysis was 59%. The confirm and document the importance of acid hydrolysis of MTBE at higher temperatures during sample preparation, the rate of hydrolysis of MTBE was measured at 80 C. The rate of hydrolysis at pH-1 was 1.22 per hours; while the rate at pH = 2 was 0.15 per hour. Previously published data and the Arrhenius relation predict that the rate of hydrolysis of MTBE is trivial at temperatures below 10 C. To confirm this prediction, samples were preserved with hydrochloric acid to a pH<2, then held in storage at 5 C. The average loss over 237 days of storage was 5.3% of the initial concentration, corresponding to a rate of loss of 0.00001 per hour.