PDA

View Full Version : So lemme get this straight....


speakerguy
03/02/2006, 03:46 PM
....any time you have a balanced calcium and alkalinity supplementation scheme, you have to add 1 mole of calcium and 2 moles of alkalinity. This means that we can, for instance, add

CaCl2 + Na2CO3 -----> CaCO3 + 2NaCl

to get calcium carbonate. But if instead we use baking soda

CaCl2 + 2NaHCO3 -----> CaCO3 + 2NaCL + H20 + CO2

Or if we instead use kalk reactors,

Ca(OH)2 ------> (Ca++) + 2(OH-)

2(OH-) + CO2 ------> (CO3--) + H2O

(Ca++) + (CO3--) ------> CaCO3

And yet if we use calcium reactors,

CO2 + H20 ------> H2CO3 ------> (H+) + (HCO3-)

CaCO3 + (H+) ------> (Ca++) + (HCO3-)

(Ca++) + 2(HCO3-) ------> CaCO3 + H20 + CO2

OK, so I can see why in case 2 and 4 we can have a lowering of pH because we have to blow off excess CO2, and in case 3 we have the problem of high pH because we need to suck up excess CO2 from the atmosphere to make the reaction work.

But with case 1, no equilibriation with the atmosphere is necessary, and it should therefore have no net effect on pH once driven to completion.

Question: Why does supplementation scheme #1 (two-part system with baked baking soda) tend to have a pH raising effect? Does it when driven to completion? Or does the added CO3-- ions tend to suck up H+ (thus increasing pH) to form bicarbonate until the rxn is driven to completion by calcifying organisms?

Just trying to wrap my head around all this stuff :)

Randy Holmes-Farley
03/03/2006, 05:14 AM
Good question, and this answer you gave is correct: :thumbsup:

"Or does the added CO3-- ions tend to suck up H+ (thus increasing pH) to form bicarbonate until the rxn is driven to completion by calcifying organisms?"



Adding carbonate to balance calcification has no permanent effect on pH AFTER the alkalinity has returned back to where it started, but in the short term, you boosted carbonate and most of it became bicarbonate in seawater at pH values below 8.6:

CO3-- + H2O ---> HCO3- + OH-

That serves to raise pH. :)