Lecture 8 Ocean Carbonate Chemistry Carbonate Reactions 华盛顿大学-海洋化学课件.ppt

Gas Pi KH (0?C , S = 35) Ci (0?C, S = 35; P = 1 Atm N2 0.7808 0.80 x 10-3 62.4 x 10-3 mol kg-1 O2 0.2095 1.69 x 10-3 35.4 x 10-3 Ar 0.0093 1.83 x 10-3 0.017 x 10-3 CO2 0.00033 63 x 10-3 0.021 x 10-3 Gas Mole Fraction in Dry Air (fG) (where fG = moles gas i/total moles) N2 0.78080 O2 0.20952 Ar 9.34 x 10-3 CO2 3.3 x 10-4 Example: Gas concentrations in equilibrium with the atmosphere Atmosphere Composition Algebraic Approach – Monoprotic Acid Let acetic acid (CH3COOH) = HA. The base form (CH3COO-) = A- We need to determine the concentrations of 4 species = HA, A- , H+ and OH-. The 4 key equations are: 1. The reaction HA = H+ + A- Acid Hydrogen Anion (or base) The Equilibrium Constant = K = (H+)(A-) / (HA) 2. The reaction H2O = H+ + OH- Kw = (H+)(OH-) 3. Mass balance on A CA = [HA] + [A-] 4. Charge Balance [H+] = [A-] + [OH-] By combining equations 2 and 3 given above we can write algebraic expressions to solve for the main species of acetic acid (HA) and acetate (A-). [HA] = CA H+ / K + H+ [A-] = CA K / K + H+ The master variable is pH – How do these species vary with pH? Graphical Approach (log – log diagram) There are three regions for these graphs ; pH = pK (system point) ; pH pK; pH pK K versus K’ - pros and cons Oceanographers frequently use an equilibrium constant defined in terms of concentrations. These are called apparent or operation equilibrium constants. We use the symbol K to distinguish them from K. Formally they are equilibrium constants determined on the seawater activity scale. Apparent equilibrium constants (K) are written in the same form as K except that all species are written as concentrations. The exception is H+ which is usually written as the activity (H+). There are pros and cons for both the K and K approaches. K the pro is that we can calculate the K from DGr and one value of K