Seawaterp H calculator

The marine carbonate system has four measurable variables - pH, total alkalinity, dissolved inorganic carbon and the partial pressure of CO2 - and any two of them fix the other two. This calculator takes the alkalinity and pCO2 pair and solves for hydrogen ion concentration, then reports pH as the negative base-10 logarithm of that activity. The first and second dissociation constants of carbonic acid, expressed here as pK1 and pK2, are not fixed numbers: they are empirical functions of absolute temperature and salinity, which is why changing the temperature field moves the answer even when the carbon inputs are unchanged. Total alkalinity, entered in micromoles per kilogram and typically near 2300 in open surface water, is the excess of proton acceptors over donors and quantifies how much acid the water can absorb before pH shifts appreciably. The saturation states omega for aragonite and calcite are ratios of the in-situ calcium and carbonate ion product to each mineral solubility product. Aragonite is the more soluble polymorph, so its omega falls below 1 first, and it is the form corals and pteropods build with. Surface tropical water commonly sits near 3 to 4; polar surface water is much closer to 1. The depth adjustment reflects a real pressure dependence, since pressure raises carbonate solubility and helps define the carbonate compensation depth near 3500 to 5000 m below which carbonate sediment does not accumulate. Two limitations matter. Results depend on which pH scale is intended, because the total, free, seawater and NBS scales differ by several hundredths of a unit. And this is a simplified equilibrium solution that handles borate, phosphate and silicate alkalinity only approximately, so research reporting should use a full implementation such as CO2SYS.

The pH notation itself comes from Soren Sorensen at the Carlsberg Laboratory in Copenhagen, who introduced it in 1909 for brewing chemistry. Applying it to seawater proved far harder than to dilute solutions, because the high ionic strength of seawater makes activity coefficients awkward and forced oceanographers to define their own concentration-based scales rather than use the freshwater NBS convention. Through the 1960s the field consolidated around the carbonate equilibrium framework, but progress was limited by disagreement over the values of the dissociation constants under marine conditions. The key measurements came in 1973, when Alan Mehrbach and coworkers determined the apparent constants K1 and K2 across oceanographic ranges of temperature and salinity; Andrew Dickson and Frank Millero refit that dataset in 1987 to the total hydrogen ion scale, and Timothy Lueker with coauthors revised the fit again in 2000, producing the parameterizations used in most modern software. Robert Byrne and Tonya Clayton demonstrated a spectrophotometric method using the indicator dye meta-cresol purple in 1993, pushing routine precision toward 0.001 units. Ernie Lewis and Doug Wallace released the CO2SYS program through the Oak Ridge National Laboratory carbon dioxide information center in 1998, which standardized how the system is solved. Comparability across laboratories rests on certified reference materials for alkalinity and dissolved inorganic carbon distributed from Andrew Dickson laboratory at Scripps Institution of Oceanography since the early 1990s.