Do Nitrates decrease pH

[Fred Dulley]

This cropped up (on another forum ) and people seem certain that nitrates decrease the pH. They say that's why cycles can stall sometimes, so they add bi-carb to buffer the system.
Surely this can't be right? I've never heard of nitrates decreasing pH.

My thoughts on the matter... (this is for a fishless cycle btw)

The tank doesn't get a water change because you're trying to cycle the tank. Inevitablly the nitrates are going to build up as will metabollic wastes and organic wastes from the bacteria. When the pH crashes, people test their water and oh look nitrate is high, it must be causing the crash....false associations. It's the same thing people accusing nitrates of killing thier fish. Some heavily stocked tanks woudnt get enough water changes and fish would start to die off. Someone tests the water and finds that the nitrates are high therefore assume it's the nitrates that are causing the deaths. When infact what else have they tested for? Just assumed its the nitrates and made it "fact".

Any inputs are appreciated (I'm looking at you Clive )

 

[George Farmer]

http://www.skepticalaquarist.com/docs/w ... acid.shtml

 

[Ed Seeley]

As George's link says it's not the nitrates that lower the pH but an active biological filter that produces an acidification. Regular water changes will more than deal with this though, no need to add bicarb.

 

[ceg4048]

Yeah, as discussed in that link George shows, there are a few different acids produced in the tank including nitric acid. I'm not sure you can attribute the majority of the nitric acid production to the inorganic KNO3 that we add though. That's happening due to the organic nitrate formation from the nitrogen cycle which starts out as ammonia and other nitrogenous waste. The nitric and other organic acid production in the water column is enough to affect the tank pH measurement which is the reason that tank water in a dropchecker is corrupted and gives a false high acidic reading. That's why we have to use distilled water in a dropchecker.

The nitric acid buildup takes a really long time. I don't think it will happen in the few weeks in which people are doing fishless cycling. Adding ammonia to a tank and then complaining about nitrates because your fish die is simply unreal.

Cheers,

 

[Fred Dulley]

Can always count on UKAPS. Cheers guys.

 

[Robert1979b]

As the web page posted by George shows the Ammonia exists in solution as an equilibrium between ammonia and ammonium. As pH changes the equilibrium shifts, the higher the pH the greater the ammonia.

Taking water at 25ºC, and 100 mg/l of ammonia and ammonium the concentration of ammonia at different pH would be
pH 10 - 85 mg/l
pH 9 - 36 mg/l
pH 8 - 5 mg/l
pH 7 - 0.6 mg/l
pH 6.5 - 0.2 mg/l
pH 6 - 0.06 mg/l

The bacteria that we culture in filters when cycling tanks can’t use the ammonium as efficiently (or at all in the case of some of the species) and the optimum pH range for Nitrosomona (sp) is between 7.8 and 8.0, the optimum pH range for Nitrobacter is between 7.3 and 7.5, the addition of the bi-carb keeps the pH up and the ammonia/ammonium equilibrium towards ammonia and keeps the bacterial happy.

As ammonia is more toxic to fish (than ammonium) you would expect more fish deaths to occur when the pH rises rather than falls.

Rob

 

[dw1305]

Hi all,

That's happening due to the organic nitrate formation from the nitrogen cycle which starts out as ammonia and other nitrogenous waste."

The process in which bacteria transform ammonia/ammonium into nitrite ("nitritation"), produces unstable nitrous acid (HNO2)........Nitrous acid has a pH-dependent equilibrium with nitrite, together, nitrous acid and nitrite act as an acid/base buffer, not unlike the more familiar acid/base buffer of carbonic acid and carbonate..........The nitrification cycle, in which ammonia is eventually metabolized to nitrate, has an additional side effect that generates more acid: the bacterially-oxidized molecule of ammonium finally produces--— in addition to nitrate--— a molecule of water and two protons (H+). Those dissociated H+ ions released into the water additionally lower the pH. Since the nitrifying communities also contribute CO2, they are an essential part of bio-acidification."

As suggested that is the the answer. I like Brett, (the Skeptical Aquarist) he produced the most readable account of the scientific processes in the aquarium I've ever found: Here is his "Nitrogen Cycle" page http://www.skepticalaquarist.com/docs/nutrient/nitcyc.shtml.

Having said that this is much less relevant to the planted tank, where levels of fixed nitrogen (NH3, NO2 & NO3) will naturally decline over time rather than increase. Unless you are massively over-stocked you can ignore both the pH of the water and the ammonia/ammonium equilibrium as well.

cheers Darrel

 

[sWozzAres]

<snip>

As ammonia is more toxic to fish (than ammonium) you would expect more fish deaths to occur when the pH rises rather than falls.

Rob

Yes, the balance of ammonia NH3 and ammonium NH4 changes depending on pH. So for instance 1mg/l of NH4 = 0.006 mg/l ammonia at pH 7, but at 7.5 it changes to 0.02 mg/l ammonia, which is toxic to fish long term.

Something to consider if you turn your CO2 off at night

 

[dw1305]

Hi all,

Yes, the balance of ammonia NH3 and ammonium NH4 changes depending on pH. So for instance 1mg/l of NH4 = 0.006 mg/l ammonia at pH 7, but at 7.5 it changes to 0.02 mg/l ammonia, which is toxic to fish long term. Something to consider if you turn your CO2 off at night".

This is true, but it shouldn't bother us. As a general rule, plants will preferentially take up ammonia as their nitrogen source and in a planted tank (where the biological filtration is working) ammonia will be at vanishingly low levels. You are much more likely to suffocate your fish at night with excess CO2, rather than kill them by ammonia poisoning (as the CO2 levels decline to equilibrium with atmospheric levels) as the pH rises.

cheers Darrel

 

[George Farmer]

Out of interest do plants produce NH3/4 when they respire i.e. at night? We know they produce CO2 and uptake O2 at night, so I was wondering what other processes are involved.

 

[dw1305]

Hi all,

Out of interest do plants produce NH3/4 when they respire i.e. at night?

No, not really, the ammonia is coming from the break down of proteins, (proteases break the peptide bonds (of proteins) releasing the amino acids and then deaminases break the amino group off the amino acids, releasing ammonia). This is happening all the time with ammonia released via via fishes gills etc. Terrestrial organisms can't just let the ammonia diffuse (and this toxic ammonia is normally converted to urea and is excreted in urine), but aquatic ones just release ammonia which is diluted by the huge volume of water (it is when you don't have a huge volume of water that problems occur).

There is also the ammonia production and oxygen demand from decomposition. The invisible biochemical processes of aerobic decomposition of organic material are usually described as the Biological or Biochemical Oxygen Demand (BOD), and the total biological demand for oxygen as the "bio-load." BOD represents the combined demand of all the aerobic metabolisms at work in the water, not merely that of the fishes and microscopic plankton, but also of the aerobic bacterial community. The metabolism involved in decomposition requires oxygen, often even more oxygen than respiration does. This is why water high in decaying organic material is characteristically also low in oxygen.
The decomposition reaction can be summarized as:

Oxidizable material + bacteria + nutrient + O2 → CO2 + H2O + oxidized inorganic such as nitrate (NO3) or (Sulf(ph)ate) SO4.

Plants that aren't getting enough light to actively photosynthesize make their own contribution to BOD. It is easy to ignore the constant respiration of algae and plants, because photosynthesis produces such vast quantities of oxygen that it swamps the effects of cellular respiration during the day, but algae and plants are constantly respiring at the cellular level, night and day.

At night we don't have any photosynthesis, but respiration is still producing CO2, via:
C6H12O6 + 6O2→ 6CO2 + 6H2O + 2880 kJ/mol or
glucose + oxygen → carbon dioxide + water + energy.

During the day respiration and de-composition continue, but you have oxygen produced during photosynthesis. The simplified overall formula for photosynthesis is:

6CO2 + 6H2O + photons → C6H12O6 + 6O2 (carbon dioxide + water + light → glucose + oxygen),

Last but definitely not least, there is "Nitrification", One of the most significant oxygen utilising processes, the biological oxidation of ammonia (NH3) to nitrite (NO2-) and then nitrate (NO3-).

NH3 + CO2 + 1.5 O2 → NO2- + CO2 + 0.5 O2 → NO3-

cheers Darrel