If when water and carbon dioxide react they form carbonic acid, carbonate, and bicarbonate, how does seawater still have a pH of around 8? Doesn't a compound need an hydroxide ion to be a base?

  • $\begingroup$ The chemistry of the oceans make for a pretty complex situation. But for starters, think about what could be in the ocean and the geology of the materials "containing" the ocean. The adsorption of carbon dioxide of course affects the pH, but it is far from the only process at play here. What kind of minerals could dissolve to give a pH in the 8.3 range? $\endgroup$ – airhuff Feb 22 '17 at 2:51
  • $\begingroup$ It's true that I made a questionable assumption, but as I see it, none of the common salts in the sea could've affected the pH, so I would think you would need to dissolve some atmospheric gases in the ocean, or find some organic process to explain the alkalinity. $\endgroup$ – smaude Feb 22 '17 at 2:56
  • $\begingroup$ How about carbonate mineralogy and carbonate based sea shells? You are right that carbon dioxide is an acidic gas, and as the atmospheric concentration increases, more of it dissolves into the oceans, slightly increasing their acidity, but also dissolving more carbonates, acting somewhat as a buffer system. $\endgroup$ – airhuff Feb 22 '17 at 3:14
  • $\begingroup$ yeah, I guess that would be the thing. I'd just really like to figure out how it works. $\endgroup$ – smaude Feb 22 '17 at 3:26

The short answer to the title question is the ubiquitous presence of geologic and biogenic calcium carbonate $(\mathrm{p}K_\mathrm{a} = 9)$. The oceans as a whole can largely be thought of as residing on beds of calcium carbonate. The following are some excerpts from this Wikipedia page:

Eggshells, snail shells and most seashells are predominantly calcium carbonate.

Carbonate is found frequently in geologic settings and constitute an enormous carbon reservoir. Calcium carbonate occurs as aragonite, calcite and dolomite. The carbonate minerals form the rock types: limestone, chalk, marble, travertine, tufa, and others.

Calcium carbonate contributors, including plankton (such as coccoliths and planktic foraminifera), coralline algae, sponges, brachiopods, echinoderms, bryozoa and mollusks, are typically found in shallow water environments where sunlight and filterable food are more abundant. Cold-water carbonates do exist at higher latitudes but have a very slow growth rate.

Regarding your question

Doesn't a compound need an hydroxide ion to be a base?

Here you go:

$$\ce{CO3^2- (aq) + H2O (l) <=> HCO3- (aq) + OH-(aq)}$$

Although calcium carbonate is only sparingly soluble $(K_\mathrm{sp} = 3.3×10^{−9})$, as stated above the oceans are simply rife with it.

  • $\begingroup$ Also, sea water is less than $25^o C$, so the $k_w$ is less in sea water; therefore, the $H^+$ concentration is also lower, leading to a higher pH. $\endgroup$ – DrPepper Jan 22 at 12:18
  • $\begingroup$ @DrPepper You are not serious, are you? $\endgroup$ – andselisk Jan 22 at 12:20
  • $\begingroup$ @andselisk why do you think i'm joking? $\endgroup$ – DrPepper Jan 22 at 14:42
  • $\begingroup$ @DrPepper First, temperature of the sea water also exceeds 25 °C in tropical areas, which, following your logic, should result in slightly acidic water near equator, which is not true. Second, yes, $K_w$ depends on temperature, but it doesn't depend on concentrations: with more released protons there is going to be an equal amount of hydroxide-ions in the solution; formally, you get a slightly higher pH, but also the same increase in pOH and it doesn't affect alkalinity at all (as the title mentions it). $\endgroup$ – andselisk Jan 22 at 14:51
  • $\begingroup$ @DrPepper Third, there is no way to increase the formal pH value by lowering the temperature to even merely reach the pH 8 mentioned in the question. Even at 0 °C pH = pOH = 7.47! $\endgroup$ – andselisk Jan 22 at 14:52

Fellow collegaes,

The Question: how and why is seawater alkaline? Is very important.

Forget what you have ever learned and read " how to understand acid-base" by Peter Stewart.

Trying to break down his message: 1. First Consider pure H2O (without minerals) and it relation with temperature. PH of pure H2O changes from 7.00 at 25 degrees celcius to 7.47 at 0 degrees celcius. H2O starts dissociating less as temperature goes down. From 25 to 0 degrees both [H+] and [OH] decrease and pH goes up, although the water remains neutral ([H+]=[OH-])

  1. Second: the only independent factors (apart from temperature) capable of changing pH are: 1. Strong ion difference (alkalising) and 2. PCO2 (acidifying)

Consider all cations and anions in seawater. [Na+]+[Mg]+[Ca2+]+[K+]+[rest cations]+[H+] = [Cl]+[SO4]+[HCO3]+[rest anions]+[OH-] - All cations are "strong" (meaning fully dissociated) - Not all anions are "strong" but a minority is "weak" meaning for a small part in equilibrium with [OH-], for instance: 1. HCO3- + H2O <-> H2CO3 + OH- or 2. HPO4- + H2O <-> H2PO4 + OH-. H2CO3 and H2PO4 are unable to give of H+ due to a positive SID.

C/ the Strong Ion Difference between cations and anions (=SID) is what makes the ocean alkaline.

CO2 acidicifies but is unable to counter the SID

Consider: 1.If H2O evaporates: SID increases and pH goes up If H2O is added due to rains: SID decreases and pH goes down 2. Stewart demonstrated that a positive SID is the most potent buffer system and determines all because electroneutrality determines all. Formation of CaCO3 is only possible due to a positive SID. Any reasoning the other way around (e.q that CaCO3 could buffer the ocean) is wrong.

  • $\begingroup$ Strictly speaking, magnesium ion does show some acidity due to hydrolysis and calcium ion, in the presence of bicarbonate, can consume hydroxide ions via $\ce{Ca^{2+} + HCO3- + OH- <=> CaCO3(s) + H2O}$. But the former is much less than the alkaline bicarbonate hydrolysis and the latter requires the seawater to become alkaline first, so the basic reasoning passes muster. $\endgroup$ – Oscar Lanzi Jan 22 at 14:16

Understanding "How is seawater Alkaline" is a vital question, because life is defined by Redox and Acid/Base chemistry of Carbon in relation to Oxygen, Hydrogen, Nitrogen, Sodium and Chloride, Earth Alkali Metals and Earth Oxides.

Ocean Acid/base chemistry has traditionally been approached from the atmosphere: (CO2 and H2O<->>H+ and HCO3). This post proposes an ocean acid/base approach from the origin of earth and earth crust composition.

1.--Stellar evolution-- During stellar evolution heavier elements are formed by nucleosynthesis due to mass, temperature and pressure. Dependend upon star size stellar evolution takes either route: I) <8 solar masses: type I supernova: the element of Iron (Fe) stays central. The solar system (and the earth) is likely the result of a low mass (Type I)supernova II) 8-15 solar masses: type II supernova (more energy and elements larger than Iron)

ref: Evidence from stable isotopes and 10Be for solar system formation triggered by a low-mass supernova. Nature Communications volume 7, Article number: 13639 (2016)

These types of supernovae were originally classified based on the existence of hydrogen spectral lines: Type Ia spectra do not show hydrogen lines, while Type II spectra do.

"Classification of Supernovae". Supernovae and Gamma-Ray Bursters. Lecture Notes in Physics. 598. pp. 21–36.

Dependend upon the balance between centrifugal force and gravitational force the earth was formed as the third rock from the sun, with a solid iron innner core and liquid iron outer core. The crust or lithosphere was formed by the earth oxides:SiO2 (43%), MgO (35%), FeO (9.0%), Al2O3 (7%), CaO (4%), Na2O (0.5%), Fe2O3 (0.4%). Molecular mass respectively: SiO2: 60, MgO: 40, FeO 72, Al2O3: 102; CaO 56, Na2O 62, Fe2O3 160.

  1. -- Atmospheric evolution -- The evolution of the atmosphere can be summarized:
    • in the beginning: probably (slightly) reducing (H2 appr 0.1%)
    • now( >2.3-2.4 biljon years): oxidizing

ref: How Earth’s atmosphere evolved to an oxic state: A status report Earth and Planetary Science Letters 237 (2005) 1–20

Conclusion on 1. and 2.: With the abundance of element of Oxygen within the Earth's crust an oxidising atmosphere is logical

  1. -- H2 is very light -- During the evolution of the earth Hydrogen Gas is a key gas lost (as is Helium)

ref: Our Planet's Leaky Atmosphere. Catling & Zahnle, Scientific American may 2009.

4.-- The element Oxygen is most prevalent by mass in both in the earth and (even more) in the human body.

enter image description here

5.-- The ratio Oxygen/Hydrogen ratio on earth is 328:1 (Oxygen: 460.000 ppm Hydrogen: 1.400 ppm). NH4(+) may still be competing for hydrogen with HCO3 (+) and other oxides HSO4(1-) and H2PO4 (2-): both within the earth crust and within the ocean.

Conclusion based on 1-5: it is very well possible that elements are competing for hydrogen to fulfil energy transfer and realise both Redox and Acid-Base interaction.

Additional arguments regarding the alkalinity of the ocean:

6.-- Due to residence time the current concentration of the prevalent ions in the ocean are (in mM): Cations: Na(1+) 480, Mg (2+) 54.14, Ca (2+) 10.53, K(1+) 10.46; Sr (2+) 0.09: Total cations (in mEq): 620.55 mEq. Anions (in mM): Chloride (1-): 559.40, SO4(2-) 28.93, HCO3(1-) 2.11, Br(1-) 0.87, F(1-) 0.07; Total Anions (in mEq/l) 620.31 mEq/l. In all solutions: concentration of [Cations]=[Anions] (electrochemical neutrality)

  1. A mayor determinant of seawater pH is the Strong Ion Difference (SID): (Na(1+)+ Mg(2+) +Ca(2+) + K(1+)+ other cations) minus (-) (Cl(1-) + Br (1-) + F (1-)+ other anions); leaving HCO3- to dissociate into (mainly) CO2 and OH-.

  2. Most early earth compounds: MgO (35%), FeO (9%), CaO (4%) and Na2O (0.5%) favour alkalinity versus acidic earth compound Al2O3 (7%) (SiO2 (43%) does not dissove =like sand).

  3. The presence of CO3(2-), Br- and F- indicates the alkaline ocean pH is probably caused by another equilibrium. Even a low concentration of NaHCO3 (2 mmol/l) within a solution of 660mM NaCl produces an alkaline pH of 7.69 (25 degrees). Weather NaHCO3(1-) is the most important salt in determining the Alkaline pH of the ocean is not established.

enter image description here

  1. SID (and NaHCO3) may not be the only factor determinining pH of the ocean. Another factor of interest regarding the alkalinity of the ocean may be NH3 because of of neutral nature in metabolism (NH3) and basic nature in aqueous solution (NH4+).

Based on 1-5 a) the hypothesis is that the earth is currently competing for hydrogen ion to equalize intrinsic molecular energy in relation to solar energy. b) an oxidizing environment (both lithospere and atmosphere) follows mainly from the earth's formation during solar system formation.

Based on 6-9 a) the ocean is alkaline because of Strong cations> Strong Anions. b) it may be more appropriate to approach acid/base within the ocean considering salts and earthoxides (instead of strong acids or bases) c) despite a long residence time HCO3 (1-) has a displaced other strong anions within the ocean. This can be a sign of the power of the attraction of Hydrogen to 3 Oxygen atoms (with Carbon) compared to F (1-) or Br (1-). d) NH3 is a point of interest in ocean alkalinity

  • $\begingroup$ This is wrong on so many levels... $\endgroup$ – andselisk Jan 22 at 12:16
  • $\begingroup$ While this link may answer the question, it is better to include the essential parts of the answer here and provide the link for reference. Link-only answers can become invalid if the linked page changes. - From Review $\endgroup$ – A.K. Jan 22 at 13:41
  • $\begingroup$ If you start with pure water, you can make it alkaline by adding a compound that contains hydrogen but does not contain oxygen (e.g. the base $\ce{NH3}$. This counter example demonstrates that alkalinity is not strictly related to the proportion of oxygen and hydrogen in a sample. To give you another counter example: Dissolving elemental oxygen in pure water does not make it alkaline, even though I now more than the 1:2 ratio of total oxygen atoms to total hydrogen atoms in the sample. $\endgroup$ – Karsten Theis Jan 22 at 16:35
  • $\begingroup$ I have undeleted this because a large improvement was made, but I have no way to validate its content, so I'll leave that up to the community to decide. $\endgroup$ – jonsca Jan 28 at 2:51

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