Alkalinity and carbonic acid content

　Seawater is rich in strongly electrolytic ionic components. Strongly electrolyte ionic components are those that are completely ionized in water. For example, when sodium chloride (NaCl) dissolves in water, all the dissolved Na and Cl are ionized (present apart, sharing positive and negative charges) as positive and negative ions (Na⁺ and Cl^-).

　Looking at the difference in total charge between the positive and negative ions of the strong electrolyte in seawater, the positive ion has an excess charge of about 2.3 mmol kg^-1. This "excess charge of strongly electrolyte positive ions" is called alkalinity.

　In other words, alkalinity [Alk] is expressed by the following equation

[ Alk ] =［Na⁺+K⁺+2Mg²⁺+2Ca²⁺ + ,,,］ － ［Cl^-+Br^-+2SO₄^2-+NO₃^- + ,,,］　（= 2.3 mmol kg^-1 )

　Since seawater itself is electrically neutral (i.e., it has equal amounts of positive and negative charge and is not charged*), the excess charge must be counteracted by weak electrolyte negative ions. The weak electrolyte ions used to maintain the alkalinity of seawater are bicarbonate ions, carbonate ions, boric acid ions, hydroxide ions, and hydrogen ions. The total charge of these equals the alkalinity.

　The figure above shows the weak electrolyte components that maintain alkalinity and their respective percentages. 96% of alkalinity is maintained by carbonate ions.

*Since seawater is in contact with the earth, it is not positively or negatively charged. Even if it is given an electric charge, it is absorbed by the earth as it is. On the other hand, cloud particles in the sky are charged and sometimes absorb electric charges as lightning.

Why is there an excess of strong electrolytic positive ions in the first place? Most of the light elements (hydrogen, carbon, oxygen, sulfur, chlorine,) were probably vaporized by asteroids and meteorites that impacted the Earth in the early stages of its formation. Carbon and oxygen would have combined to form carbon dioxide, hydrogen and oxygen would have formed water, sulfur and hydrogen would have formed hydrogen sulfide, and chlorine and hydrogen would have formed hydrochloric acid. Earth's early atmosphere and oceans were rich in acidic substances, which would have leached cations from rocks and formed the salt in seawater. In addition to the strong acids HCl and H₂SO₄, the weak acid carbonic acid (H₂CO₃) plays a major role in dissolving basalt. As shown in the picture below, when basalt dissolves, Ca²⁺ and HCO₃^- are produced; some of the HCO₃^- dissociates to form CO₃^2-, which sticks to Ca²⁺ to form CaCO₃. While seawater is acidic (pH below 7), the carbonate ion equilibrium is biased toward H₂CO₃ and HCO₃^-. In other words, even if Ca²⁺ dissolves, it does not easily become CaCO₃, leaving Ca²⁺ in seawater. Currently, the pH of seawater is 8, leading to the current alkalinity. Currently, Ca²⁺ is supplied to the oceans through rivers, and equilibrium is maintained in terms of quantity. (We will check the literature and update it with more reliable information on the causes of alkalinity.)

　The partial pressure of carbon dioxide in the present atmosphere is about 0.00038 atm (= 1 atm x 380 ppm), but CO₂ dissolves in raindrops to form H₂CO₃, a weak acid. The weakly acidic (pH 5.6) rainwater is still gradually dissolving rocks. (You will solve the pH of rainwater in a later exercise.)

　Currently, the alkalinity of seawater is maintained under the following conditions. The formation of calcium carbonate in the ocean is mostly due to the shell formation of organisms.