Total alkalinity determined through experiments comparing usage of high-density polyethylene (HDPE) and borosilicate glass bottles for collection and storage of water samples collected on the GEOTRACES GP17-OCE cruise (R/V Roger Revelle RR2214)

Total alkalinity plays an important role in buffering seawater and determining how much anthropogenic carbon dioxide the oceans can absorb and mitigate the rise in atmospheric concentrations. Total alkalinity varies with location, depth, and time making it an important variable needed to quantify and monitor ocean acidification, and potentially for ocean alkalinity enhancement interventions. Currently, best practices are to use expensive high quality borosilicate glass bottles for collecting and storing these samples. However, unlike other carbon system variables, total alkalinity is not affected by gas exchange meaning plastic bottles may be suitable for total alkalinity sample storage. Plastic bottles are lighter, cheaper, and less prone to breakage making them easier to handle and ship. Here, we test the suitability of high-density polyethylene (HDPE) for collection and long-term storage of total alkalinity samples. Duplicate samples were collected in standard borosilicate glass bottles and HDPE bottles on GEOTRACES GP17-OCE. The samples were analyzed in the laboratory over a year later and the storage methods were compared. It was determined that HDPE is not suitable for long-term storage of total alkalinity samples as there were large changes in total alkalinity over time and precision of duplicate samples was very poor. We hypothesize that HDPE plastic is slightly porous leading to leaching of alkalinity either into or out of the bottle over time impacting the value of the sample. Use of HDPE bottles for total alkalinity samples is not recommended for long term sample storage.