LAUSR

Changes in seawater pH resulting from anthropogenic influence, termed ocean acidification, have significant implications. Monitoring and evaluating ocean acidification requires highly precise measurements comparable among many laboratories over decades. Short‐term repeatability can be achieved with spectrophotometric methods, but comparability among studies is much less certain. The indicator used plays an important role in measurement quality. Meta‐cresol purple (mCp), which is used for most seawater pH measurements, is known to break down under exposure to ultraviolet light, which can impact the long‐term stability of the indicator, and thus the quality of the measurements. Here, certified reference material and 2‐hydroxymethyl‐1,3‐propanediol (TRIS) buffers were used to assess the long‐term stability of mCp solutions. Purified indicator solutions were found to be stable (within 0.0025) at room temperature in a dark container for at least 5.3 years. Uncertainties in unpurified indicators made the stability assessment inconclusive. Such long‐term stability minimizes one potential source of uncertainty when comparing measurements and can reduce costs and waste by not prematurely disposing of indicator solutions that remain useable. Changes in the pH of the indicator solution indicate the indicator perturbation correction should be regularly determined. The A434imp correction method for determining impurities could be used as a quality assurance measure by making measurements over the life of the indicator solution to monitor for changes in the solution. However, the choice of molar absorptivities required for the calculation impacts the magnitude of the correction and thus warrants further study to improve best practices for making corrections to impure indicators.