Editing PH (section)

=== pH in the ocean ===
{{Anchor|Seawater}}{{See also|Seawater#pH value|Ocean#pH and alkalinity|Ocean acidification}}

The pH of [[seawater]] plays an important role in the ocean's [[Carbon cycle#Ocean|carbon cycle]]. There is evidence of ongoing [[ocean acidification]] (meaning a drop in pH value): Between 1950 and 2020, the average pH of the ocean surface fell from approximately 8.15 to 8.05.<ref>{{Cite journal |last1=Terhaar |first1=Jens |last2=Frölicher |first2=Thomas L. |last3=Joos |first3=Fortunat |date=2023 |title=Ocean acidification in emission-driven temperature stabilization scenarios: the role of TCRE and non-{{CO2}} greenhouse gases |journal=Environmental Research Letters |language=en |volume=18 |issue=2 |pages=024033 |bibcode=2023ERL....18b4033T |doi=10.1088/1748-9326/acaf91 |issn=1748-9326 |s2cid=255431338|doi-access=free }}</ref> [[Carbon dioxide emissions]] from human activities are the primary cause of ocean acidification, with [[Carbon dioxide in Earth's atmosphere|atmospheric carbon dioxide levels]] at 430 ppm {{CO2}} at [[Mauna Loa]] observatory in 2025.<ref name="NOAA_CO2">{{cite web | title=Trends in CO<sub>2</sub> | website=NOAA Global Monitoring Laboratory | date=2025-04-22 | url=https://gml.noaa.gov/ccgg/trends/monthly.html | access-date=2025-04-22}}</ref> In 2024, the annual atmospheric {{CO2}} increase measured by the [[National Oceanic and Atmospheric Administration|NOAA]]’s Global Monitoring Laboratory was 3.75 ppm {{CO2}}/year.<ref name="Berwyn2025">{{cite web | last1=Berwyn | first1=Bob | title=A grim signal: Atmospheric CO<sub>2</sub> soared in 2024 | website=Ars Technica | date=2025-04-25 | url=https://arstechnica.com/science/2025/04/a-grim-signal-atmospheric-co2-soared-in-2024/ | access-date=2025-04-27}}</ref> CO<sub>2</sub> from the [[atmosphere]] is absorbed by the oceans. This produces [[carbonic acid]] (H<sub>2</sub>CO<sub>3</sub>) which dissociates into a [[bicarbonate ion]] ({{Chem|HCO|3|-|}}) and a [[Hydron|hydrogen cation]] (H<sup>+</sup>). The presence of free hydrogen cations (H<sup>+</sup>) lowers the pH of the ocean. 

==== Three pH scales in oceanography ====
The measurement of pH in seawater is complicated by the [[Chemical property|chemical properties]] of seawater, and three distinct pH scales exist in [[chemical oceanography]].<ref name="zeebe2">Zeebe, R. E. and Wolf-Gladrow, D. (2001) ''CO<sub>2</sub> in seawater: equilibrium, kinetics, isotopes'', Elsevier Science B.V., Amsterdam, Netherlands {{ISBN|0-444-50946-1}}</ref> In practical terms, the three seawater pH scales differ in their pH values up to 0.10, differences that are much larger than the accuracy of pH measurements typically required, in particular, in relation to the ocean's [[Total inorganic carbon|carbonate system]].<ref name="zeebe2" /> Since it omits consideration of sulfate and fluoride ions, the ''free scale'' is significantly different from both the total and seawater scales. Because of the relative unimportance of the fluoride ion, the total and seawater scales differ only very slightly.

As part of its [[operational definition]] of the pH scale, the [[IUPAC]] defines a series of [[Buffer solution]]s across a range of pH values (often denoted with [[National Bureau of Standards]] (NBS) or [[National Institute of Standards and Technology]] (NIST) designation). These solutions have a relatively low [[ionic strength]] (≈&nbsp;0.1) compared to that of seawater (≈&nbsp;0.7), and, as a consequence, are not recommended for use in characterizing the pH of seawater, since the ionic strength differences cause changes in [[Standard electrode potential|electrode potential]]. To resolve this problem, an alternative series of buffers based on [[artificial seawater]] was developed.<ref>{{cite journal |author=Hansson, I. |year=1973 |title=A new set of pH-scales and standard buffers for seawater |journal=Deep-Sea Research |volume=20 |issue=5 |pages=479–491 |bibcode=1973DSRA...20..479H |doi=10.1016/0011-7471(73)90101-0}}</ref> This new series resolves the problem of ionic strength differences between samples and the buffers, and the new pH scale is referred to as the ''total scale'', often denoted as pH<sub>T</sub>. The total scale was defined using a medium containing [[sulfate]] ions. These ions experience [[protonation]], {{chem2|H+}} + {{chem|SO|4|2-|↔ HSO|4|-}}, such that the total scale includes the effect of both [[Proton|protons]] (free hydrogen cations) and hydrogen sulfate ions:
: [{{chem2|H+}}]<sub>T</sub> = [{{chem2|H+}}]<sub>F</sub> + [{{chem|HSO|4|-}}]

An alternative scale, the ''free scale'', often denoted pH<sub>F</sub>, omits this consideration and focuses solely on [{{chem2|H+}}]<sub>F</sub>, in principle making it a simpler representation of hydrogen ion concentration. Only [{{chem2|H+}}]<sub>T</sub> can be determined,<ref>{{cite journal |author=Dickson, A. G. |year=1984 |title=pH scales and proton-transfer reactions in saline media such as sea water |journal=Geochim. Cosmochim. Acta |volume=48 |issue=11 |pages=2299–2308 |bibcode=1984GeCoA..48.2299D |doi=10.1016/0016-7037(84)90225-4}}</ref> therefore [{{chem2|H+}}]<sub>F</sub> must be estimated using the [{{chem|SO|4|2-}}] and the stability constant of {{chem|HSO|4|-}}, {{nowrap|''K''{{su|b=S|p=*}}}}:
: [{{chem2|H+}}]<sub>F</sub> = [{{chem2|H+}}]<sub>T</sub> − [{{chem|HSO|4|-}}] = [{{chem2|H+}}]<sub>T</sub> ( 1 + [{{chem|SO|4|2-}}] / ''K''{{su|b=S|p=*}} )<sup>−1</sup>

However, it is difficult to estimate ''K''{{su|b=S|p=*}} in seawater, limiting the utility of the otherwise more straightforward free scale.

Another scale, known as the ''seawater scale'', often denoted pH<sub>SWS</sub>, takes account of a further protonation relationship between hydrogen cations and [[fluoride]] ions, {{chem2|H+}} + {{chem2|F-}} ⇌ HF. Resulting in the following expression for [{{chem2|H+}}]<sub>SWS</sub>:
: [{{chem2|H+}}]<sub>SWS</sub> = [{{chem2|H+}}]<sub>F</sub> + [{{chem|HSO|4|-}}] + [HF]

However, the advantage of considering this additional complexity is dependent upon the abundance of fluoride in the medium. In seawater, for instance, sulfate ions occur at much greater concentrations (>&nbsp;400 times) than those of fluoride. As a consequence, for most practical purposes, the difference between the total and seawater scales is very small.

The following three equations summarize the three scales of pH:
: pH<sub>F</sub> = −log<sub>10</sub>[{{chem2|H+}}]<sub>F</sub>
: pH<sub>T</sub> = −log<sub>10</sub>([{{chem2|H+}}]<sub>F</sub> + [{{chem|HSO|4|-}}]) = −log<sub>10</sub>[{{chem2|H+}}]<sub>T</sub>
: pH<sub>SWS</sub> = −log<sub>10</sub>({{chem2|H+}}]<sub>F</sub> + [{{chem|HSO|4|-}}] + [HF]) = −log<sub>10</sub>[v]<sub>SWS</sub>