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carb - Carbonate chemistry Paradigm carbonate system...

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1 Carbonate chemistry Paradigm: carbonate system dominates pH control in most natural aquatic systems Carbonate system: CO 2(g) = CO 2(aq) + H 2 O = H 2 CO 3 = H + + HCO 3 - HCO 3 - = H + + CO 3 2- [H 2 CO 3 *] = [CO 2(aq) ] + [H 2 CO 3 ] 99.85% 0.15% Total Inorganic Carbon (C T ) = [H 2 CO 3 *] + [HCO 3 - ] + [CO 3 2- ] carbonic acid and bicarbonate are weak acids: H 2 CO 3 * = H + + HCO 3 - K a1 = 10 -6.3 HCO 3 - = H + + CO 3 2- K a2 = 10 -10.3 strong acids: HCl K 10 3 H 2 SO 4 K a1 10 3 HNO 3 K = 1 Open system (logC-pH diagram): 1. What is [H 2 CO 3 *]? [H 2 CO 3 *] = K H · pCO 2 (K H = 10 -1.47 ) 2. What is [HCO 3 - ]? [HCO 3 - ] = K H K a1 pCO 2 /[H + ] 3. What is [CO 3 2- ]? [CO 3 2- ] = Example 1: Add base (e.g. Na OH) to open system to raise the pH to 6.3, 7.3, 8.3, 10.3.
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2 Approach: Determine the concentrations of H 2 CO 3 *, HCO 3 - , and CO 3 2- in water of pH 6.3 that is in equilibrium with air containing 10 -3.5 atm of CO 2 . [H 2 CO 3 *] = pCO 2(g) · K H [H 2 CO 3 *] = (365 x 10 -6 atm) (3.4 x 10 -2 M atm -1 ) [H 2 CO 3 *] = 1.24 x 10 -5 M For [HCO 3 - ]: [HCO 3 - ][H + ] K a1 = –––––––––––––––– [H 2 CO 3 *] [HCO 3 - ] = [H 2 CO 3 *] K a1 / [H + ] [HCO 3 - ] = (1.24 x 10 -5 M)( 10 -6.3 )/10 -6.3 [HCO 3 - ] = 1.24 x 10 -5 M For [CO 3 2- ]:
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