UNPUR-PUR DATA COMPILATION

Consistent monitoring of seawater spectrophotometric pH on the total hydrogen ion scale (pHT) has been challenged by an evolving method, with changes in formulation and the purity of the meta-cresol purple (mCP). Critically, the method still lacks metrological traceability to the International System of Units (SI). Using real seawater samples, we demonstrate that spectrophotometric pHT measurements obtained with unpurified (UNPUR) and purified (PUR) mCP can be harmonized to within 0.003 pH units, the climate-goal threshold. This level of agreement is only achieved when mCP impurities are quantified for both the UNPUR and PUR mCP, and impurity-corrected absorbance data is used in the same formulation to calculate pHT. We applied this approach to a ship-based pHT time-series that transitioned from UNPUR to PUR mCP measurements, achieving agreement better than 0.003 pHT units. Our results show that previous claims suggesting that UNPUR mCP underestimates pHT in the upper pH range are misleading, as they were based on the inappropriate use of absorbances obtained with UNPUR mCP with a formulation developed for PUR mCP. In fact, our data reveal better agreement between UNPUR and PUR pHT in the upper pH range, while UNPUR mCP tends to overestimate pHT in the lower pH range. These findings highlight the urgent need for the global chemical oceanography community to establish a spectrophotometric pHT method with full SI traceability, along with certified reference materials and characterized mCP. This work supports the need for harmonization efforts to ensure the reliability of pHT data in global synthesis products.