Determination of water in volcanic glasses by Karl-Fischer titration

Abstract

A method is described for the routine measurement of dissolved water in crushed and sieved volcanic glasses utilizing Karl-Fischer titration and sample pyrolysis without the addition of a flux. Sample fragments between 75 and 150 μm in size proved to be desirable for rapid measurements at low water contents because complete degassing is achieved without significant water adsorption. However, adsorbed water was found in all ground samples after long-term storage, illustrating the need for prompt water analyses after crushing. Estimates of analytical precision were based upon replicate measurements of natural rhyolitic and basaltic glasses. Results indicate a relative standard deviation for this method of < ± 5% at low water contents (0.1 wt.%) and < ± 2.5% at high (> 1.5 wt.%) water contents. The ease of sample analysis and good analytical precision make this technique quite useful for geologic interpretation of volcanic processes. Detection limits depend upon sample size but were estimated to be 60 ppm for a 1-g sample.