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Takeshita, Y, Martz TR, Coletti LJ, Dickson AG, Jannasch HW, Johnson KS.  2017.  The effects of pressure on pH of Tris buffer in synthetic seawater. Marine Chemistry. 188:1-5.   10.1016/j.marchem.2016.11.002   AbstractWebsite

Equimolar Tris (2-amino-2-hydroxymethyl-propane-1,3-diol) buffer prepared in artificial seawater media is a widely accepted pH standard for oceanographic pH measurements, though its change in pH over pressure is largely unknown. The change in volume (Delta V) of dissociation reactions can be used to estimate the effects of pressure on the dissociation constant of weak acid and bases. The Delta V of Tris in seawater media of salinity 35 (Delta V-Tris*) was determined between 10 and 30 degrees C using potentiometry. The potentiometric cell consisted of a modified high pressure tolerant Ion Sensitive Field Effect Transistor pH sensor and a Chloride-Ion Selective Electrode directly exposed to solution. The effects of pressure on the potentiometric cell were quantified in aqueous HCl solution prior to measurements in Tris buffer. The experimentally determined Delta V-Tris* were fitted to the equation Delta V-Tris*= 4528 +0.04912t where t is temperature in Celsius; the resultant fit agreed to experimental data within uncertainty of the measurements, which was estimated to be 0.9 cm(-3) mol(-1). Using the results presented here, change in pH of Tris buffer due to pressure can be constrained to better than 0.003 at 200 bar, and can be expressed as: DpH(Tris) = -(4.528 + 0.04912t)p/ln(10)RT. where T is temperature in Kelvin, R is the universal gas constant (83.145 cm(3) bar K-1 mol(-1)), and Pis gauge pressure in bar. On average, pH of Tris buffer changes by approximately -0.02 at 200 bar. (C) 2016 Elsevier B.V. All rights reserved.

McLaughlin, K, Dickson A, Weisberg SB, Coale K, Elrod V, Hunter C, Johnson KS, Kram S, Kudela R, Martz T, Negrey K, Passow U, Shaughnessy F, Smith JE, Tadesse D, Washburn L, Weis KR.  2017.  An evaluation of ISFET sensors for coastal pH monitoring applications. Regional Studies in Marine Science. 12:11-18.   10.1016/j.rsma.2017.02.008   AbstractWebsite

The accuracy and precision of ion sensitive field effect transistor (ISFET) pH sensors have been well documented, but primarily by ocean chemistry specialists employing the technology at single locations. Here we examine their performance in a network context through comparison to discrete measurements of pH, using different configurations of the Honeywell DuraFET pH sensor deployed in six coastal settings by operators with a range of experience. Experience of the operator had the largest effect on performance. The average difference between discrete and ISFET pH was 0.005 pH units, but ranged from -0.030 to 0.083 among operators, with more experienced operators within +/- 0.02 pH units of the discrete measurement. In addition, experienced operators achieved a narrower range of variance in difference between discrete bottle measurements and ISFET sensor readings compared to novice operators and novice operators had a higher proportion of data failing quality control screening. There were no statistically significant differences in data uncertainty associated with sensor manufacturer or deployment environment (pier-mounted, flowthrough system, and buoy-mounted). The variation we observed among operators highlights the necessity of best practices and training when instruments are to be used in a network where comparison across data streams is desired. However, while opportunities remain for improving the performance of the ISFET sensors when deployed by less experienced operators, the uncertainty associated with their deployment and validation was several-fold less than the observed natural temporal variability in pH, demonstrating the utility of these sensors in tracking local changes in acidification. (C) 2017 Elsevier B.V. All rights reserved.