Evaluation for SO₄ and Cl Analyses of Ash Leachates by Turbidimetry Using a Compact Absorptiometer

Abstract

When volcanic ash is exposed to water after fell on the ground, various chemical substances will be eluted to water phase. Amounts of water soluble SO₄ and Cl and their Cl/SO₄ ratio of ash are useful for understanding eruptive activities of the volcano. For prompt evaluation of eruptive activities by water soluble components on ash, it would be useful if the analyses are made in-situ or near by the volcano instead of sending the samples to the laboratories far away. For this purpose, we utilized and established a method using a compact handheld absorptiometer for analyses of Cl and SO₄ in ash leachate. The method uses a small digital scale, a handheld absorptiometer and other equipment (PP bottles, syringes filters and etc.). The scale is for ash leachates preparation and the absorptiometer is used for the turbidimetric measurements of SO₄ and Cl. The calibration curves for SO₄ and Cl were linear and parabola for the concentration range of the standard solutions up to 81.9 mg/L and 39.6 mg/L, respectively. Eleven ash samples from Kirishima Shinmoedake and Sakurajima volcanoes were analyzed by turbidimetry method of this study and by ion chromatography method, and were compared for validation of the method. The analyzed concentrations were basically within about 10 % compared to those of ion chromatography, except for samples whose absorbance were smaller than 0.1 unit. We also checked for the interfering components for turbidimetry analyses by checking the compiled ash leachate data of Airys and Delmelle (2012) and came to conclusion that effect of interference can be usually ignored. On the other hand, some of the ash samples with very low water soluble SO₄ and Cl may be under detection limit with the proposed method. As a conclusion, ash leachate analyses for SO₄ and Cl by turbidimetry using a handheld absorptiometer used in this study is an effective method and could be used for prompt evaluation of eruptive activities especially on remotes islands where the chemical laboratories are not available.