pH and the tank cycle

[JulesH]

Hi, I would like to understand a part of the water chemistry that occurs when a tank cycles.

I am working away from home. Prior to leaving home I set up a pH probe on my new tank. I added RO water and salt a week before I started the cycle. I waited for the salinity and temperature to settle before 'officially' starting the cycle. During that time I logged my pH over a period of time and it was steady at 8.0.

Just before I left home I seeded the tank with bio balls from my other tank and added NH3 in the form of Brightwell quikcycl. Watching my pH probe remotely has seen a drastic decrease in pH by nearly 2 orders of magnitude to a value of 6.0 over two days but now I see an upward trend.

I am assuming the drop in the pH is due to breaking of the single bonds between the N and the H of the NH3 due to bacteria converting the molecule to Nitrite, with the abundance of free H decreasing the pH. If that is true why do I see an increase in pH now?

Thanks for taking the time to read.

Julian

 

[Randy Holmes-Farley]

A pH of 6 is test error. THere is a small lowering of pH during cycling, but it definitely won't go below the low 7's and typically stays in the upper 7's.

During cycling, there is a small lowering of alk and pH due to the conversion of ammonia to nitrate/. It si the release of H+ in the conversion to nitrite that causes both of these effects:

NH4+ + 3/2 O2 --> NO2- + 2H+ + H2O

NO2- + ½ O2 --> NO3-

pH will rise after an initial lowering when adding acid due to the blowing off of the excess CO2 produced:

H+ + HCO3- ---> H2CO3 --> H2O + CO2 (which slowly gets aerated away if it is present in excess)

 

[JulesH]

Thank you very much Randy for your reply.

So much for reefing pH probes, I did calibrate it prior to re-installing it.

As I understand the bond between N and H in NH3 is covalent, sharing an electron so when the bond is broken the N strips the H of the electron hence the H+?

 

[Randy Holmes-Farley]

Thank you very much Randy for your reply.

So much for reefing pH probes, I did calibrate it prior to re-installing it.

As I understand the bond between N and H in NH3 is covalent, sharing an electron so when the bond is broken the N strips the H of the electron hence the H+?

Not exactly. That’s a little bit if an artificial distinction, but the H loses both of the shared electrons it shared with nitrogen, but they do not go to the nitrogen. They go to the oxygen atoms that comprise nitrite.

 

[JulesH]

Thank you Randy.

NH4+ is Ammonium, I am not familiar with 3/2 O2?

 

[Randy Holmes-Farley]

Thank you Randy.

NH4+ is Ammonium, I am not familiar with 3/2 O2?

O2 is oxygen. The 3/2 just means it takes one and a half O2 molecules per ammonium.

 

[taricha]

in a sealed container, you can use a pH indicator to see the pH drop from nitrification happening. Below is instant ocean with a little phenol red.

tubes from left to right....
ammonia in instant ocean
ammonia in IO + biospira (fast ammonia processing)
ammonia in IO + one and only (this bottle of O&O was slow or a dud. might've been frozen in winter shipping)
IO + ammonia + sand
IO + ammonia + sand + biospira
IO + ammonia + sand + one and only.

The aragonite sand buffers the pH drop and prevents the pH indicator from showing much of an effect.

 

[Volcom6981]

I am cycling a tank right now, my PH is very low, 7.59-7.8 if it even gets that high. Now I’m sure some is because of high co2 levels, but I’m also sure some is because of the cycle. I’m doing a fishless cycle since I’m heading out for vacation, so the end of next week I’ll be doing the first water change, turning on the skimmer for the first time, adding filter stuff. So will see for sure how low my PH really is after that. I’m planning on using kalk, and trying a co2 scrubber to help out. I just hope my PH is not really going to be this low.

 

[Randy Holmes-Farley]

Conversion of ammonia to nitrate adds acid, lowering alk and pH.