Determination by ion chromatography of selected organic and inorganic acids in rainwater at Maracaibo, Venezuela

Abstract

Chemically suppressed ion chromatography with a Dionex Model 2000i/sp, IonPac AS11, IonPac AG11, ASRS-I system was used for the analysis of major organic and inorganic acids in precipitation samples collected in the city of Maracaibo during a one-year period. Two different isocratic methods were used. First, inorganic (sulfuric, nitric and hydrochloric) acids were adequately determined by using 21 m M NaOH as eluent. Second, organic (formic and acetic) and hydrofluoric acids were determined by using 0.5 m M NaOH as eluent; however, occasional poor resolution of acetic and hydrofluoric acid peaks were obtained. The methods showed good reproducibility (R.S.D. ca. 5%) for primary inorganic anions plus formate and relatively poor reproducibility (R.S.D. ca. 15%) for fluoride plus acetate. In the Maracaibo rainwater system, the concentrations of the typical urban pollutants (SO 4 *, NO 3 −, Cl *, H +) are significantly larger than those found in Caracas precipitation, suggesting that Maracaibo is affected to a greater extent by air pollution. NO 3 − and Cl − dominated the anionic composition. About 92% (as SO 4 *) and 50% (as Cl *) of the total each ion concentration was represented by non-marine-derived species. Organic and inorganic anion balances showed that potential acid rain (volume–weighted average pH=5.35) in Maracaibo is mainly caused (ca. 90%) by sulfuric, nitric and hydrochloric acids. Only about 4% of the original acidity of the rainwater was found to be free acidity, indicating a high degree of neutralization in the Maracaibo precipitation.