MAR 510 Chemical Oceanography

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Richard Phelps
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1 Carbonate Equilibrium -Key Concepts- Major buffer system influencing ph (master variable) Linked to geological, biological and climatological cycles Complex chemistry involving gaseous, dissolved, and solid phases Cycle undergoing significant anthropogenic perturbation mostly from fossil fuel burning Oceanic Carbonate System controls atmospheric CO 2 on >1000 yr timescales (60 times more CO 2 in ocean) Carbonate System 1 #1

2 Species of Importance CO 2 H 2 CO 3 (+ CO 2(aq) = H 2 CO 3 *) HCO - 3 CO 3 2- H + OH - Carbonate System 1 #2

3 Carbonate Equilibrium Equations CO 2 (g) CO 2 (aq) CO 2(aq) + H 2 O H 2 CO 3 H 2 CO 3 H + + HCO - 3 HCO - 3 H + + CO 2-3 Ca 2+ + CO 2-3 CaCO 3(s) (calcite or aragonite) Carbonate System 1 #3

4 CONCEPT OF ph H 2 O H + + OH - Kw = (ah + aoh - )/ah 2 O = [H + ][OH - ] γh + γoh - Kw* = [H + ][OH - ]=10-14 pkw(pure water) = [H + ]=[OH - ]=10-7 ph = - log [H+] = 7.0 in pure water poh = 14 - ph pkw(seawater) = [H + ] Total = [H + ] Free +[HF]+[HSO 4- ] 85% of H+ is Free 34% of OH- is Free Carbonate System 1 #4

5 Equilibrium Constants K 0 (or K H ) = [H 2 CO 3 *]/pco 2 K 1 = [H + ] [HCO 3- ]/[H 2 CO 3* ] K 2 = [H + ] [CO 2-3 ]/[HCO 3- ] K sp = [Ca 2+ ] [CO 2-3 ] C T =ΤCO 2 =ΣCO 2 = [H 2 CO 3 ] + [HCO 3- ]+[CO 2-3 ] (for practical calculations K s are K* s and a function of T,S, & P) Carbonate System 1 #5

6 Carbonate System 1 #6

7 H 2 CO 3 * = CO 2 (aq) + H 2 CO 3 (CO 2 * in Millero) [CO 2 ] (aq) + [H 2 O] [H 2 CO 3 ] (slow) K = [H 2 CO 3 ]/[CO 2 ] (aq) [H 2 O] = [H 2 CO 3 ] [H + ] + [HCO 3- ] (fast) K (H2CO3) = [H + ] [HCO 3- ]/[H 2 CO 3 ] = Carbonate System 1 #7

8 H 2 CO 3 * (cont.) [CO 2 ] (aq) + [H 2 O] [H 2 CO 3 ] [H + ] + [HCO 3- ] H 2 CO 3 + CO 2(aq) = H 2 CO 3 * K 1 = K K H2CO3 = K 1 = [H + ] [HCO 3- ]/[H 2 CO 3* ] K K H2CO3 Carbonate System 1 #8

9 Ionization Fractions (e.g. [H 2 CO 3* ] = C T α H2CO3* ) α H2CO3* = (1 + K 1 /[H + ] + K 1 K 2 /[H + ] 2 ) -1 α HCO3 = (1 + [H + ]/K 1 + K 2 /[H + ]) -1 α CO3 = (1 + [H + ]/K 2 + [H + ] 2 /K 1 K 2 ) -1 Carbonate System 1 #9

10 Carbonate System 1 #10

11 Parameters of the CO 2 System in Seawater ph Total Alkalinity (TA) TCO 2 (C T, ΣCO 2 ) pco 2 Any two of above parameters can define carbonate speciation Carbonate System 1 #11

12 METHODS OF ANALYSIS ph Potentiometry ± Spectrophotometry ± TA Potentiometry ± 2 µm Spectrophotometry ± 2 µm TCO2 Potentiometry ± 3 µm Coulometry ± 1 µm pco2 Gas Chromatography ± 0.5 µatm Infrared Spect. ± 0.5 µatm Batch Surface ± 2 µatm Batch Deep ± 2 µatm Carbonate System 1 #12

13 MEASURABLE PARAMETERS Parameter Precision Accuracy ph ± ± TA ± 1 µm ± 2 µm TCO 2 ± 1 µm ± 2 µm pco 2 ± 0.5 µatm ± 1 µatm Carbonate System 1 #13

14 What s Alkalinity? Carbonate alkalinity (CA, A c ) CA = [HCO 3- ] + 2[CO 3 2- ] Total alkalinity TA = [HCO 3- ]+ 2[CO 2-3 ] + [B(OH) 4- ]+[OH - ] -[H + ] + [SiO(OH) 3- ] + [HPO 2-4 ]+ 2[PO 3-4 ] + [Organics] Carbonate System 1 #14

15 Total Alkalinity Definition 1: The sum of the anions of weak acids as determined by titration by HCl to the carbonic acid endpoint Definition 2: the excess positive charge from the cations of strong bases Carbonate System 1 #15

16 COMPONENTS OF TOTAL ALKALINITY SPECIES % TA HCO µm CO B(OH) SiO(OH) OH HPO µm ANOXIC WATERS HS - and NH 3 are Important Carbonate System 1 #16

17 Carbonate Species at Fixed CA & ph [HCO 3- ] = CA [H + ]/([H + ] + 2K 2 ) [CO 2-3 ] = CA K 2 /([H + ] + 2K 2 ) [H 2 CO 3 *] = CA[H + ] 2 /([H + ]K 1 + 2K 1 K 2 ) pco 2 = [H 2 CO 3 *]/K 0 C T = ΤCO 2 = [H 2 CO 3 ] + [HCO 3- ]+[CO 2-3 ] Carbonate System 1 #17

18 Carbonate Species at Fixed ph and C T H 2 CO 3* = C T [H + ] 2 /[H + ] 2 + K 1 [H + ] + K 1 K 2 HCO 3 - = C T K 1 [H + ] /[H + ] 2 + K 1 [H + ] + K 1 K 2 CO 3 2- = C T K 1 K 2 /[H + ] 2 + K 1 [H + ] + K 1 K 2 pco 2 = [H 2 CO 3 *]/K 0 C T = ΤCO 2 = [H 2 CO 3 ] + [HCO 3- ]+[CO 3 2- ] Carbonate System 1 #18

19 Carbonate Species at Fixed pco 2 & ph (K H = K 0 in Millero) [H 2 CO 3* ] = K 0 pco 2 [HCO 3- ] = K 1 K 0 pco 2 /[H + ] [CO 3 2- ] = K 1 K 2 K 0 pco 2 /[H + ] 2 Carbonate System 1 #19

20 Carbonate System 1 #20

21 CHANGES IN CO 2 IN OCEAN WATERS REMOVAL 1.) Photosynthesis CO 2 CHO 2 2.) Formation of CaCO 3 CO Ca 2+ CaCO 3 3.) Solar Heating CO 2 (aq) CO 2 (g) ADDITION 1.) Oxidation of Organic C 2.) Dissolution of CaCO 3 3.) Dissolution of CO 2 from Atm from Fossil Fuel Burning Carbonate System 1 #21

22 1160 Carbonate System 1 #22

23 Carbonate System 1 #23

24 Carbonate System 1 #24

25 CA =[HCO 3 - ]+ 2[CO 3 = ] OM remin. + CaCO3 diss. CaCO3 dissolution only! Carbonate System 1 #25 OM remin. wins!

26 Carbonate System 1 #26

Chapter 12: Acids and Bases: Ocean Carbonate System James Murray 4/30/01 Univ. Washington

Chapter 12: Acids and Bases: Ocean Carbonate System James Murray 4/30/01 Univ. Washington Last lecture was concerned with gas exchange and one example we looked at was the solubility of CO 2. Next we have