Reef Chemistry Question of the Day #211 ORP and Additives

[Randy Holmes-Farley]

Reef Chemistry Question of the Day #211

Which of the following additives would be expected to chemically reduce the ORP of a reef aquarium? Assume each one is only what is says it is, with no impurities, and is added in amounts reefers might typically use. List all you think will reduce the ORP. By "chemically reduce" I am looking for a pure chemical effect, not one that, say, relies on organisms to use the chemical and reduce ORP through other processes they carry out.

A. Calcium chloride (common calcium additive)
B. Ferrous gluconate (e.g., DIY iron using Fergon tablets)
C. Lugol's Solution (iodine supplement)
D. Sodium carbonate (alkalinity additive)
E. Sodium bicarbonate (alkalinity additive)

 

[coral-boss]

c & d maybe

 

[greg 45]

B

 

[sghera64]

Iron and iodine can both be reduced. I'm Just not sure that the amounts we typically add would be enough to move the needle enough to see a shift outside of normal variation.

So: B and C

 

[CNDReef]

C

 

[Richard baker]

C

 

[Keiffer the reefer]

B & c

 

[dragon99]

B and D

 

[Randy Holmes-Farley]

And the answer is...

B. Ferrous gluconate (e.g., DIY iron using Fergon tablets)
D. Sodium carbonate (alkalinity additive)

Ferrous (Fe++) is a reduced form of iron in seawater. At any ORP, there is a mix of Fe++ and Fe+++ (Fe+++ dominates in normal aerated seawater). Add more Fe++ and the ORP will be lowered, add Fe+++, and the ORP will be raised. The same would be true of many transition metals (manganese, etc.).

Sodium carbonate will raise pH. Raising pH in reef aquarium water tends to reduce ORP.

Calcium carbonate would not be expected to impact ORP, either directly or via pH.
Lugols solution contains some amount of I2, an oxidizer which will tend to raise ORP.
Sodium bicarbonate reduces pH, which will tend to raise ORP.

This has more on ORP effects:

ORP and the Reef Aquarium - Reefkeeping.com
http://reefkeeping.com/issues/2003-12/rhf/feature/index.htm