Differences in ammonia testing

[jonnythan]

I ran into some trouble regarding ammonia levels. I'm still a little confused, but I think I have it figured out.


When talking about stressful or toxic ammonia levels, most sites seem to be talking about total ammonia - NH3 + NH4. And this is what most test kits seem to talk about.

The relative concentrations of NH3 and NH4 vary with the water pH.

However, NH4 is relatively non-toxic and can constitute the bulk of the total ammonia reading.

I ran into some confusion when trying to fishless cycle because I have an in-tank meter that apparently reads only "toxic" NH3. At my water's pH, about 7.8, getting the NH3 level to 0.5 - which was my goal - apparently creates a total ammonia level of 25+ ppm according to the little chart that came with the in-tank meter.

I'm told that very high ammonia levels can stall the cycling, so I'm going to have to do a significant water change to get ammonia levels back down to where they should be. But I'll leave it be until my Aquarium Pharma liquid test kit comes in.

I think I have this correct. Please correct me if I'm wrong.

 

[jsoong]

You've got the basic concepts right.

The 25+ total level seems too high (according to MY chart.)

According to this calculator from Purdue University:
Free Ammonia Calculator (Javascript)

Your total ammonia should be about 12 with NH3 of 0.5.

You should aim for free ammonia of 0.2 if you want a total ammonia of 4.

 

[jonnythan]

Oh, you're right. I was looking at the chart for a pH of 7.5.

Thanks for confirming my thoughts.

 

[CHurst5841]

In case it helps, in acidic conditions (pH < 7.0) ammonia (NH3) is converted to the ammonium ion (NH4+). In alkaline (basic) solution (pH > 7.0) the ammonium ion in converted back to ammonia. This is according to the formula:

NH3 + H2O <-> NH4(+) + OH(-)

Additionally, the overall NH4:NH3+ ratio is affected by temperature. Low temperature facilitate the formation of NH4+ and high NH3.

In summary...

(1) Low temperature acidic conditions push the reaction to the right
(2) High temperature alkaline conditions push the reaction to the left

If you are interested, there is a very good article written by John Sawyer from the Department of Agronomy at Iowa State University University Extension entitled "Surface Waters: Ammonium is Not Ammonia" that discusses this very subject. It is available at http://www.extension.iastate.edu/CropNews/2008/0421JohnSawyer.htm.

 

[jonnythan]

Good article, makes perfect sense. I have just enough chemistry to follow that.

 

[OldMan]

Your basic statement was right Jonnythan. You have too much ammonia in your tank for the bacteria that you want to culture. It is time to lower it to the 4 to 5 ppm range to get the right bacteria going.