Hydrogen ion

Anion (negatively charged)

Hydrogen anions are formed when additional electrons are acquired:

• Hydride: general name referring to the negative ion of any hydrogen isotope (H⁻)
• Protide: ¹H⁻
• Deuteride: ²H⁻, D⁻
• Tritide: ³H⁻, T⁻

Uses

Hydrogen ions drive ATP synthase in photosynthesis. This happens when hydrogen ions get pushed across the membrane creating a high concentration inside the thylakoid membrane and a low concentration in the cytoplasm. However, because of osmosis, the H⁺ will force itself out of the membrane through ATP synthase. Using their kinetic energy to escape, the protons will spin the ATP synthase which in turn will create ATP. This happens in cellular respiration as well though the concentrated membrane will instead be the inner membrane of the mitochondria.

Hydrogen ions concentration, measured as pH, is also responsible for the acidic or basic nature of a compound. Water molecules split to form H⁺ and hydroxide anions. This process is referred to as the self-ionization of water.

Ocean acidification

The concentration of hydrogen ions and pH are inversely proportional; in an aqueous solution, an increased concentration of hydrogen ions yields a low pH, and subsequently, an acidic product. By definition, an acid is an ion or molecule that can donate a proton, and when introduced to a solution it will react with water molecules (H₂O) to form a hydronium ion (H₃O⁺), a conjugate acid of water. For simplistic reasoning, the hydrogen ion (H⁺) is often used to abbreviate the hydronium ion.

Ocean acidification is the direct consequence of elevated concentrations of hydrogen ions and carbonate saturation from significant absorption of carbon dioxide (CO₂) by the world's oceans. The pre-industrial state of the ocean's carbonate chemistry has been notably stable, including the balance of its pH. Following the industrial revolution, anthropogenic emissions of burning fossil fuels, cement production, and changes in land use, have increased the oceans uptake of carbon dioxide from the atmosphere by 30%. In the ocean, the absorption capacity of this greenhouse gas is 59 times higher than in the atmosphere; the ocean acts as the largest carbon sink on the planet, playing a significant role in climate regulation. In addition to carbon fluxes, the natural process of carbon sequestration from the atmosphere into the deep ocean is facilitated by two systems, the biological pump and the solubility pump. The solubility pump is a physico-chemical process that transfers CO₂ at the air-sea interface. Based on Henry's Law, the amount of dissolved CO₂ in an aqueous solution is directly proportional to the partial pressure of CO₂ in the atmosphere. To maintain equilibrium, a state of high atmospheric partial pressure of CO₂ leads to an increased oceanic exchange of this gas by molecular diffusion.

In the surface waters, dissolved atmospheric carbon dioxide (CO_2(aq)) reacts with water molecules to form carbonic acid (H₂CO₃), a weak diprotic acid. Diprotic acids consist of two ionizable hydrogen atoms in each molecule. In an aqueous solution, partial dissociation of carbonic acid releases a hydrogen proton (H⁺) and a bicarbonate ion (HCO−3), and subsequently, the bicarbonate ion dissociates into an additional hydrogen proton and a carbonate ion (CO2−3). The dissolving and dissociating of these inorganic carbon species generate an increase in the concentration of hydrogen ions and inversely lowers ambient surface ocean pH. The carbonate buffering system governs the acidity of seawater by maintaining dissolved inorganic carbon species in chemical equilibrium.

The chemical equation consists of reactants and products that may react in either direction. More reactants added to a system yield more product production (the chemical reaction shifts to the right) and if more product is added, additional reactants will form, shifting the chemical reaction to the left. Therefore, in this model, a high concentration of the beginning reactant, carbon dioxide, produces an increased amount of end-product (H⁺ and CO2−3), thus lowering pH and creating a more acidic solution. The natural buffering system of the ocean resist the change in pH by producing more bicarbonate ions generated by free acid protons reacting with carbonate ions to form an alkaline character. However, increasing atmospheric CO₂ concentrations may exceed the buffering capacity threshold, consequently resulting in higher rates of ocean acidification. Shifts in the ocean's carbonate chemistry has the potential to manipulate ocean biogeochemical cycles for many elements and compounds causing profound impacts on marine ecosystems. Furthermore, the solubility of CO₂ is temperature dependent; elevated surface water temperatures reduce CO₂ solubility. A continual rise in atmospheric partial pressure of CO₂ could potentially convert the ocean from acting as sink (the vertical transport of carbon to the depths of the ocean) to becoming a source (CO₂ degassing from the ocean), further increasing global temperatures.