Simultaneous determination of arsenic and arsenious acids in geothermal water

A flow system using ion-exclusion chromatography and continuous hydride/atomic absorption spectrometry was developed for the simultaneous determination of arsenious acid (As(III)) and arsenic acid (As(V)) in geothermal water at the ppm level. The precision (C.V.) was ±2.4 and ±1.1% for As(III) and As(V) (1 ppm As each), respectively. The sampling rate was 5.5 samples hr^-1. The arsenic species could be measured in the concentration range 0.01-10 ppm As by changing the injection volume of geothermal water. Total concentration of As in geothermal waters collected from geothermal fields in Kyushu, Japan, and Wairakei, New Zealand, was in the range of 0.5-4.6 ppm As. The arsenic was mostly present as As(III) in the geothermal waters directly discharged from geothermal wells through the water-vapor separator, but the As(III) was rapidly oxidized to As(V). This contrasts with slow oxidation of As(III) in standard solutions, which suggests that a catalyzer (probably Fe or Mn compounds) accelerating the oxidation reaction of As(III) to As(V) is present in the geothermal waters. For the speciation of As(III) and As(V), it was found that the geothermal water sample should be acidified with hydrochloric acid to pH 2 immediately after sampling to preserve the state of As.