Redfield ratio details

BradB · Feb 22, 2016

I know the Redfield ratio is 106:16:1. Is that by weight or number of molecules or something else? Is it just measure Nightrogen atoms or whole Nitrate molecules?

Randy Holmes-Farley · Feb 22, 2016

It comes from this sort of reaction (cut and pasted from an article of mine). The numbers you quote relate to moles of N, P, and C atoms. The reverse reaction would be for growth of phytoplankton.

http://www.reefedition.com/nitrate-in-the-reef-aquarium/

The degradation of plankton, for example, provides nitrate. This can be shown in a simplified chemical equation describing what happens when organic “food” is digested with oxygen:

(CH2O)106(NH3)16(H3PO4) + 138 O2 → 106 CO2 + 122 H2O + 19 H+ + PO43- + 16 NO3–

which in words reads as:

plankton + oxygen → carbon dioxide + water + hydrogen ion + phosphate + nitrate

BradB · Feb 22, 2016

So 106 moles of C to 16 moles N to 1 mole P, thanks

Randy Holmes-Farley · Feb 22, 2016

Yes.

You're welcome and Happy Reefing.

BradB · Feb 22, 2016

In theory, I can lower phosphate by adding 1219 mg ethanol and 808 mg potassium nitrate at the same time?

C2H6O to KNO3

106*(12*2 + 6 + 16) / 2
to
16*(39 + 14 + 48)

I am not suggesting doing this. I'd want to try less carbon to more nitrate if I do it for real.

BradB · Feb 22, 2016

Actually a 1 to 1 ratio might be a good starting point

Eckolancer · Feb 22, 2016

Trying to understand this more. So for every ppm of phosphate I would need 16 ppm of nitrate and what is the 106 ppm of and what is a good source for it?

BradB · Feb 22, 2016

Not that simple, but more importantly, you can't measure phosphate accurately enough to use exact math.

Once in a while, my tank gets into a state where it uses nitrate faster than phosphate. Nitrate is 0, phosphate is low or undectable, corals do badly and algea grows fast. Water changes can help, but this is a poor way to deal with phosphate. This time I tried a GFO reactor, which helped a lot, but didn't fix the problem.

I want to add Nitrates until I have 5ppm, but don't want to add nitrates without carbon as I already have an algea problem. I definitely don't want to add Carbon above the Redfield ratio.

Several large water changes with a small amount of nitrates would probably fix the problem with less risk by conventional thinking, but with 400 gallons of water, I want to try adding some Carbon too.

Randy Holmes-Farley · Feb 22, 2016

BradB said:
In theory, I can lower phosphate by adding 1219 mg ethanol and 808 mg potassium nitrate at the same time?

C2H6O to KNO3

106*(12*2 + 6 + 16) / 2
to
16*(39 + 14 + 48)

I am not suggesting doing this. I'd want to try less carbon to more nitrate if I do it for real.

The Redfield ratio doesn't apply to organic carbon dosing in a reef tank because some fraction (maybe a large fraction) of the organic carbon added is used in low O2 regions for denitrification, where nitrate is used as an electron acceptor in place of oxygen. So you lose nitrate and get no organic tissue from it.

I show a typical reaction in the same article linked above:

organic + 124 NO3– + 124 H+ → 122 CO2 + 70 N2 + 208 H2O

Consequently, organic carbon dosing is typically significantly skewed toward nitrate reduction relative to phosphate, even more than the Redfield ratio typically is skewed.

Randy Holmes-Farley · Feb 22, 2016

Eckolancer said:
Trying to understand this more. So for every ppm of phosphate I would need 16 ppm of nitrate and what is the 106 ppm of and what is a good source for it?

Again, it is just not useful to think of these ratios as we have no idea what they are in reef tanks for the reasons above.

That said, the 106 is moles of carbon, which might be from organic carbon dosing (ethanol (vodka), acetic acid (vinegar), etc.0 or organics that are naturally present in the aquarium (which is how denitrification in a deep sand bed works in the absence of organic carbon dosing).

BradB · Feb 22, 2016

I agree denitrification uses Carbon without creating organic tissue, however Nitrate is still used, although with a different ratio. Of course, different bacteria may also use different ratios. As I said, I don't think exact math is useful, I just included it as sometimes I am off by a factor of 10 or 100.

I still feel dosing both Nitrate and Carbon at the same time will be useful to my tank. The actual ratio will be found experimentally, but I need to start somewhere. Redfield is a good starting point, factoring in that too much Carbon will be far worse than too much Nitrate.

Randy Holmes-Farley · Feb 22, 2016

BradB said:
I agree denitrification uses Carbon without creating organic tissue, however Nitrate is still used, although with a different ratio. Of course, different bacteria may also use different ratios. As I said, I don't think exact math is useful, I just included it as sometimes I am off by a factor of 10 or 100.

I still feel dosing both Nitrate and Carbon at the same time will be useful to my tank. The actual ratio will be found experimentally, but I need to start somewhere. Redfield is a good starting point, factoring in that too much Carbon will be far worse than too much Nitrate.

Yes, it may be useful to dose both. Many people do, with the goal to reduce phosphate. I just don't think the ratio will be helpful int he endeavor, but I agree that you have to start somewhere, and if you prefer that way as opposed to the dosing schedules people sometimes recommend, that's certainly fine.

Kungpaoshizi · Feb 22, 2016

https://en.wikipedia.org/wiki/Redfield_ratio

It does not apply fully to bacterial interactions of carbon dosing. It was originally for phyto research, though there is a fair comparison to bacterial interaction.

BradB · Feb 22, 2016

I do want to dose both with the goal of reducing phosphates. I haven't found anything on the forums about this, I thought I'd be the first. My concern is Carbon dosing a Nitrate starved environment. Bacteria will probably still form and further use Nitrate, to the detriment of everything else that needs it.

I want to add enough Nitrate it goes up to around 5ppm, and at the same time use ethanol to prevent the current algea outbreak from getting worse.

Randy Holmes-Farley · Feb 22, 2016

BradB said:
I do want to dose both with the goal of reducing phosphates. I haven't found anything on the forums about this, I thought I'd be the first. My concern is Carbon dosing a Nitrate starved environment. Bacteria will probably still form and further use Nitrate, to the detriment of everything else that needs it.

I want to add enough Nitrate it goes up to around 5ppm, and at the same time use ethanol to prevent the current algea outbreak from getting worse.

Check this current thread starting at post 26 for someone doing exactly this by dosing nitrate and organic carbon.

https://www.reef2reef.com/threads/starting-to-dose-nitrate.235096/

BradB · Feb 23, 2016

Great thread and almost exactly my situation. Unfortunately, it doesn't help much figuring out how much to start dosing.

Do you think dosing equal parts ethanol and potassium nitrate is a good starting point?

Randy Holmes-Farley · Feb 23, 2016

BradB said:
Great thread and almost exactly my situation. Unfortunately, it doesn't help much figuring out how much to start dosing.

Do you think dosing equal parts ethanol and potassium nitrate is a good starting point?

That may be a bit high on nitrate, but it may depend on how much ethanol you are dosing. You can always back off on the nitrate if it rises (or increase it if you never detect any).

If a typical ethanol dose might be 0.02 grams (20 mg) per gallon per day, that's 5 ppm per day. Most people dosing nitrate are dosing less than that. Maybe 1-2 ppm per day.

BradB · Feb 23, 2016

I don't know the effect of dosing a net Carbon gain to a Nitrate starved aquarium. I don't want to find out.

If I dose 2 ppm Nitrate and 4 ppm Carbon, will nitrate eventually start accumulating? If not, what about 2ppm Nitrate and 3 ppm Carbon?

2ppm Nitrate with 2ppm Carbon seems better than 2 ppm Nitrate alone.