Why alkalinity consumption is always higher than calcium

[Gogo007]

Why we always notice higher consumption of kh than calcium in our reef tank normally twice. if the corals got the kh as caco3 or ca(Hco3)2 is totally different in chemical balance .. my thought is at our ph levels most of the alkalinity in water in bicarbonate so corals will take one calcium ion and two bicrabonate molecules .. could anyone confirm that right or wrong ?

 

[plankton]

Why we always notice higher consumption of kh than calcium in our reef tank normally twice. if the corals got the kh as caco3 or ca(Hco3)2 is totally different in chemical balance .. my thought is at our ph levels most of the alkalinity in water in bicarbonate so corals will take one calcium ion and two bicrabonate molecules .. could anyone confirm that right or wrong ?

great question and randy will have the answer.

in the meantime I’m experiencing the same thing and attribute the ‘extra’ alk consumption due to biological activity in the tank beyond just laying SPS or lps skeleton down.

 

[Randy Holmes-Farley]

I'm not certain of the reason behind the question, which may bear on what sort of answer you are looking for.

The chemistry of pure calcium carbonate is 2.8 dKH (1 meq/L) of alkalinity for each 20 ppm of calcium. That corresponds to one calcium to one carbonate or two bicarbonate ions.

Seawater has a lot more calcium than alkalinity, so the ratio above corresponds to a far higher proportion of the alkalinity than the calcium, and that is why alkalinity drops "faster".

There are many other related issues that might actually be what you are asking about, such as the volume of liquid supplements used, why the ratio above can be deviated from etc. More clarification of the question is needed.

 

[Hans-Werner]

It is not totally different if the corals get the alkalinity as carbonate or bicarbonate. Carbonate is a bivalent anion CO3(2-) and bicarbonate is a monovalent anion HCO3-. Nevertheless both can be converted into each other with CO2 and H2O:

CO3(2-) + CO2 + H2O <--> 2 HCO3-
Both sides have 2 H + 2 C + 6 O + 2 e-

The sodium salts are Na2CO3 and NaHCO3. For each Mol of Na2CO3 you need two Mols of NaHCO3 to get the same alkalinity because HCO3- and CO3(2-) both are destroyed by acid during titration and the resulting CO2 should have the chance to volatilize by shaking the sample.