Abstract
AbstractMeasuring pH in glacial meltwaters is challenging, because they are cold, remote, subject to freeze‐thaw cycles and have low ionic strength. Traditional methods often perform poorly there; glass electrodes have high drift and long response times, and spectrophotometric techniques are unpractical in cold, remote environments. Ion selective field effect transistor (ISFET) sensors are a promising alternative, proven in marine and industrial applications. We assess the suitability of two models of ISFET, the Honeywell Durafet and Campbell Scientific Sentron, for use in glacial melt through a series of lab and field experiments. The sensors have excellent tolerance of freeze‐thaw and minimal long‐term drift, with the Durafet experiencing less drift than the Sentron model. They have predictable response to temperature, although the Durafet housing causes some lag during rapid cycling, and the impact of stirring is an order of magnitude less than that of glass electrodes. At low ionic strength (< 1 mmol L−1), there is measurable error, but this is quantifiable, and less than glass electrodes. Field tests demonstrated low battery consumption, excellent longevity and resistance to extreme conditions, and revealed biogeochemical processes that were unlikely to be recorded by standard methods. Meltwater pH in two glacial catchments in Greenland remained > 7 with consistent diurnal cycles from the very first meltwater flows. We recommend that ISFET sensors are used to assess the pH of glacial meltwater, since their tolerance is significantly better than alternative methods: the Durafet is accurate to ± 0.2 pH when waters are > 1 mmol L−1 ionic strength, and ± 0.3 pH at < 1 mmol L−1.
Highlights
ion selective field effect transistor (ISFET) sensors have been the subject of extensive investigation over the past two decades (Bergveld 2003), and ISFETs for pH measurement are commercially available from a number of suppliers (Honeywell, Campbell Scientific, Microsens, Mettler Toledo, Hach Lange)
This study aims to assess the performance of two commercially available examples of ISFET sensors for determining pH in glacial meltwaters, and use them to reveal high resolution cycling in glacial meltwater river pH hitherto unseen in the early season melt from the Greenland ice sheet
Sampling regimes to date have relied on regular spot sampling, conducted during the height of the melt season. These data pH in glacial meltwaters have been invaluable in discovering how meltwaters behave at peak flow, but are insufficient for understanding how biogeochemical processes operate from melt onset, and for coverage of the full diurnal flow cycle
Summary
Potentiometric electrodes offer the convenience and instant analysis desired for field applications, but are not always suited to long-term monitoring of meltwater systems They are compromised by low temperatures, frequent freezethaw, high concentrations of suspended sediment, and low ionic strength. ISFET sensors have been the subject of extensive investigation over the past two decades (Bergveld 2003), and ISFETs for pH measurement are commercially available from a number of suppliers (Honeywell, Campbell Scientific, Microsens, Mettler Toledo, Hach Lange) These sensors perform over a wide range of the pH scale, and are typically robust and easy to use (Martz et al 2010). This study aims to assess the performance of two commercially available examples of ISFET sensors for determining pH in glacial meltwaters, and use them to reveal high resolution cycling in glacial meltwater river pH hitherto unseen in the early season melt from the Greenland ice sheet
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