Preparation of 2‐amino‐2‐hydroxymethyl‐1,3‐propanediol ( TRIS ) pH T buffers in synthetic seawater

Buffers of known quality for the calibration of seawater pH T measurements are not widely or commercially available. Although there exist published compositions for the 0.04 mol kg‐H 2 O −1 equimolar buffer 2‐amino‐2‐hydroxymethyl‐1,3‐propanediol (TRIS)‐TRIS · H + in synthetic seawater, there are no explicit procedures that describe preparing this buffer to achieve a particular pH T with a known uncertainty. Such a procedure is described here which makes use of easily acquired laboratory equipment and techniques to produce a buffer with a pH T within 0.006 of the published pH T value originally assigned by DelValls and Dickson (1998), 8.094 at 25°C. Such a buffer will be suitable for the calibration of pH measurements expected to fulfil the “weather” uncertainty goal of the Global Ocean Acidification Observation Network of 0.02 in pH T , an uncertainty goal appropriate to “identify relative spatial patterns and short‐term variation.”