Sodium Bicarb in Aquaculture

[ozfish]

I am using sodium bicarbonate in an closed recirculating aquaculture system indoors. The biofilters are submerged in each tank. The nitrification process is producing acids and reducing pH. We buffer twice daily with sodium bicarbonate.

Is it possible that the bicarb is also breaking down to produce carbonic acid or CO2 and hence dropping the pH? - hence countering its effect.

Andrew

 

[PeterAB]

Representing acids by a simple proton H+ in the following chemical equation, sodium bicarbonate will buffer acids as follows:

NaHCO3 + H+ = CO2 + H2O + Na+

In other words, yes, sodium bicarbonate can break down to CO2 (which gases off) in the presence of other acids, and by so doing it neutralizes that acid.

Sodium bicarbonate is useful for protecting an aqueous system from pH swings, either up or down. If you consistently need to RAISE the pH, then consider using sodium carbonate (Na2CO3) which can neutralize two protons instead of only one.
In swimming pools, sodium bicarbonate is used as pH buffer (raising alkalinity), while sodium carbonate is used for raising the pH. I don't know what the practice is for fish tanks, so I must add the disclaimer that you must determine for yourself the suitability of using sodium carbonate there.

Peter.

 

[jmj5152]

You may consider running the water through a weak base anion ion exchange resin column, in free base form. It will "absorb" the acidity, eliminating the addition of Na salts into your water system.

 

[orenda1168]

Ozfish:

Take a look at

http://www.infoecp.com,

particularly about the application of this technology to RAS systems.

Orenda

 

[bimr]

No, it is not possible that the bicarb is also breaking down to produce carbonic acid or CO2 and hence dropping the pH.

C02 is abosorbed from the air and will tend to decrease the pH. If you add bicarb, then the bicarb will neutralize the CO2 and raise the pH.

If you add too much bicarb, it will precipitate out as carbonate.

See:

http://jove.geol.niu.edu/faculty/loubere/Oc1046W04.htm

 

[ozfish]

I have revisited this thread because I am still encounterign some problems.

I can make a few more points.

a. The biofilters and the fish create the CO2 which drops the pH - hence the reason for adding the bicarb in the first place (raise pH and buffer against constant CO2 production)

b. PeterAB notes that CO2 can be formed by the breakdown of the sodium bicarbonate. - Is this pH dependant - ie I have been advised that the CO2 will be bound by the carbonate and released only if the pH is allowed to drop to lowe levels day >6

Thanks for contribution

 

[bimr]

Don't forget about the organic waste breakdown:

http://saltaquarium.about.com/cs/phalkalinity/a/aa061301.htm

http://www.aquahobby.com/articles/e_adjusting_pH.php