Alkalinity rising from dosing nitrate?

[Bmasculine]

Testing my water last night after work, I noticed my dKH had gone from 8 to 9.1 without a rise in anything else in 3 days. I had tested Sat. at an 8 which where I try to keep it. I started dosing nitrate and have been checking that daily as everything else usually runs pretty stable. My wife said it’s from dosing nitrates and after looking at her silly and saying they have nothing to do with each other I’m wondering today if it could possibly be the nitrate? Lol

 

[Randy Holmes-Farley Verified Community Expert]

Always listen to your wife. She’s right, at least she may be right.

Consumption of nitrate in a reef tank boosts alk. If the nitrate came from foods (from organics to ammonia to nitrate) that process depleted alk and the consumption and reuse adds back the exact amount lost. So the net effect is no alk change.

But if you add supplemental nitrate, you missed that first depletion step and got only the second part where alk was added.

 

[Bmasculine]

So how do I keep my nitrate up without my alk rising?

 

[Randy Holmes-Farley Verified Community Expert]

You are dosing no alkalinity?

 

[Randy Holmes-Farley Verified Community Expert]

The effect is not large. 50 ppm of nitrate consumed gives 2.3 dKH of alk. Often that gets lost in the noise of other alk dosing needs.

 

[Bmasculine]

I dose limewater with my top off and it keeps up for the most part. If I get behind I’ll dose to catch up. I did a water change Sat. Mainly to get po4 down from the over feeding. Alk in my tank at that time was 7.9, new mixed water was 8.1. Nitrate was at zero when I did the water change

 

[Bmasculine]

Over a three day period I raised nitrate to 2 but alk came up full point. Really scratching my head because I set this tank up to be a simple but effective system that has me chasing my tail right now.

 

[dson78]

I just started dosing KN03 and i noticed the same thing, my ALK went up. Randy, is there a way to raise Nitrate without the ALK raising? I really dont want to feed more because that is adding PO4 to the system and im having to clean the glass basically every other day i think.

 

[Bmasculine]

I just started dosing KN03 and i noticed the same thing, my ALK went up. Randy, is there a way to raise Nitrate without the ALK raising? I really dont want to feed more because that is adding PO4 to the system and im having to clean the glass basically every other day i think.

I never did figure out how to get it all to run together. I quit dosing kno3 so I could keep alk stable, stopped feeding so much so my po4 would come back down and now my tank runs 0 kno3 or undetectable on Red Sea test kit .03 po4 ulr Hanna checker 8 dkh 460 ca 1500mg. Some stuff looks good some doesn’t. Seems like growth is slow and sps that should be red/dark pink look pink/pale. But parameters stay stable so I don’t mess with it anymore.

 

[Constrictor6090]

The effect is not large. 50 ppm of nitrate consumed gives 2.3 dKH of alk. Often that gets lost in the noise of other alk dosing needs.

I'm very curious as to how this happens. Would you be willing to explain how this works?

 

[Lasse]

@Randy Holmes-Farley Do you mean that the enzymatic conversion of NO3 into NH4 before uptake will rise alk or is it only the denitrification process that will rise it in this case?

Sincerely Lasse

 

[Randy Holmes-Farley Verified Community Expert]

@Randy Holmes-Farley Do you mean that the enzymatic conversion of NO3 into NH4 before uptake will rise alk or is it only the denitrification process that will rise it in this case?

Sincerely Lasse

Any conversion of nitrate to make organics (at least any naturally occurring organics such as proteins), N2, or ammonia, will all release the same amount of alkalinity, 2.3 dKH per 50 ppm of nitrate consumed (1 mole of alk for 1 mole of nitrate).

Examples:

denitrification to N2:
4NO3- + 5/6 C6H12O6 (glucose) + 4H2O + 2 N2 + 7H2O + 4HCO3- + CO2

uptake by macroalgae:
122 CO2 + 122 H2O + 16 NO3- -->C106H260O106N16 + 138 O2 + 16 HCO3-

conversion to ammonia:
NO3- + H+ + H2O --> NH3 + 2O2

 

[BlueDevil]

How would we juggle having to dose nitrates (if feeding doesn’t keep it at a desirable level) without raising alk in the process? Reduce the daily amount of alk (and corresponding ca) dosed?

 

[Lasse]

uptake by macroalgae:
122 CO2 + 122 H2O + 16 NO3- -->C106H260O106N16 + 138 O2 + 16 HCO3-

You use redfield ratio C/ N (106/16) here. But in macroalgae - is it the same ratio as in microalgae? The ratio N/P in macros are more like 50-70/1 as I know

Sincerely Lasse

 

[Randy Holmes-Farley Verified Community Expert]

You use redfield ratio C/ N (106/16) here. But in macroalgae - is it the same ratio as in microalgae? The ratio N/P in macros are more like 50-70/1 as I know

Sincerely Lasse

As noted, i used roughly the N/C for marine phytoplankton, but the C/N ratio doesn't impact the alkalinity released per unit of nitrate consumed. It is the conversion of N to a different chemical state that matters. Whether more or less carbon is attached elsewhere in the molecule is unimportant.

Take an extreme case. For the most simple amino acid formation:

2 NO3- + 6 CO2 + 6 H2O --> 2 NH2-CH2-CO2H (glycine) + 2HCO3- + 7 O2

Molar C/N in the product (glycine) is 2:1, but the amount of alk produced per nitrate consumes is stil the same, 1:1.

I should add to the above statement that a sulfur denitrator is a totally different type of chemistry. So the consumption of nitrate there depletes alkalinity, not adds it. The nitrate part still adds alkalinity, but the conversion of S to SO4-- consumes a large amount of alkalinity.

 

[Coronus]

Old thread but a good one. So........how to keep alk stable while dosing no3?? This was the main question I believe and was never answered.
If dosing alk and cal, I guess you would lower alk dosing but keep cal the same?
@Randy Holmes-Farley

 

[Lasse]

I guess you would lower alk dosing but keep cal the same?

It is what I would do if it shown to be a problem.


Sincerely Lasse