Ascorbic acid reduction of residual active chlorine in potable water priorto halocarboxylate determination

In studies on the formation of disinfection byproducts (DBPs), it is necess ary to scavenge residual active (oxidizing) chlorine in order to rx the chl orination byproducts (such as haloethanoates) at a point in time. Such rese arch projects often have distinct needs from requirements for regulatory co mpliance monitoring. Thus, methods designed for compliance monitoring are n ot always directly applicable, but must be adapted. This research describes an adaptation of EPA Method 552 in which ascorbic acid treatment is shown to be a satisfactory means for reducing residual oxidizing chlorine, i.e., HOCl, ClO-, and Cl-2, prior to determining concentrations of halocarboxylat es. Ascorbic acid rapidly reduces oxidizing chlorine compounds, and it has the advantage of producing inorganic halides and dehydroascorbic acid as op posed to halogenated organic molecules as byproducts. In deionized water an d a sample of chlorinated tap water, systematic biases relative to strict a dherence to Method 552 were precise and could be corrected for using simila rly treated standards and analyte-fortified (spiked) samples. This was demo nstrated for the quantitation of chloroethanoate, bromoethanoate, 2,2-dichl oropropanoate (dalapon), trichloroethanoate, bromochloroethanoate, and brom odichloroethanoate when extracted, as the acids, into tert-butyl methyl eth er (MTBE) and esterified with diazomethane prior to gas chromatography with electron capture detection (GC-ECD). Recoveries for chloroethanoate, bromo ethanoate, dalapon, dichloroethanoate, trichloroethanoate, bromochloroethan oate, bromodichloroethanoate, dibromoethanoate, and 2-bromopropanoate at co ncentrations near the lower limit of detection were acceptable. Ascorbic ac id reduction appears to be the best option presently available when there i s a need to quench residual oxidants fast in a DBP formation study without generating other halospecies but must be implemented cautiously to ensure n o untoward interactions in the matrix.