Traceable Reference Standards for Seawater pH

Ocean acidification is primarily driven by increased anthropogenic atmospheric CO₂. Our present understanding of the effects of an increasingly acidic ocean is largely founded on metrologically unlinked collections of diverse environmental observations and laboratory studies with vast temporal and geographic extents. To understand these complex implications better, it is critical to establish rigorous metrological traceability in seawater carbon-system measurements and to develop a comprehensive evaluation of the measurement uncertainty. This poster communicates the development of a NIST reference material to extend metrological traceability to measurements of seawater pH. Traceability of seawater pH to the International System of Units (SI) is achieved through a multi-standard suite of synthetic seawater buffers certified using primary pH metrology (Harned cell measurements). The primary measurements of these buffer materials extend to lower salinities and pH values than those from previous investigations. The lifetime of these liquid buffers (months) is too short for use as a reference material (years). To leverage the measurement to a stable calibrator, a purified batch of colorimetric pH indicator is characterized spectrophotometrically using these same primary buffers. Use of a multi-standard suite of primary buffers traceably links the solid reference material to the primary pH measurements in a manner compliant with current practice for calibration of secondary pH measurements. An uncertainty analysis compliant with presently accepted metrological guidelines for pH determined with the reference material is presented.