# 3d. Computation of the Alkalinity in Seawater

The alkalinity of seawater has been computed using the observed values of temperature, salinity, pCO_{2} and the concentrations of total CO_{2}, silicate, and phosphate. For our computation, the total alkalinity (TALK) in seawater is defined by:

TALK = Ac + Ab + Asi + Ap + Aw

where

Ac | = | Carbonate alkalinity = [HCO_{3}^{-}] + 2[CO_{3}^{=}], |

Ab | = | Borate alkalinity = [H_{2}BO_{3}^{-}], |

Asi | = | Silicate alkalinity = [H_{3}SiO_{4}^{-}], |

Ap | = | Phosphate alkalinity = [H_{2}PO_{4}^{-}] + 2[HPO_{4}^{=}] + 3[PO_{4}^{-3}], |

Aw | = | Water alkalinity = [OH^{-}] - [H^{+}]. |

The total concentration of borate (TB) has been assumed to be proportional to salinity: TB (µmol/kg) = 410.6*(Sal/35). The borate alkalinity ranges between about 40 µeq/kg for deep waters and 100 µeq/kg for surface waters. Since the silicate concentration may be ashigh as 150 µmol/kg in deep waters, the silicate alkalinity is as high as 6 µeq/kg for deep water but it is negligibly small for surface waters. The phosphate alkalinity ranges from 0.5 µeq/kg for surface waters to about 5 µeq/kg in deep waters. The following apparent dissociation constants of acid in seawater were used; Merhbach et al. (1973) for carbonic acid; Lyman (1956) for boric acid; Kester and Pytkowicz (1967) for phosphoric acid; Ingri (1959) for silicic acid; and Millero (1979) and Culberson and Pytkowicz (1973) for water. The expressions used to compute these constants as a function of temperature and salinity and the computational scheme are described in Peng et al. (1987).