Re: ammonia (NH3) vs. ammonium (NH4+)

Karen says:
>Well, if I understand correctly, it's not a complete change, it's 
>a sliding scale.  Below 7.0, all ammonia is in the form of 
>ammoniUM.  Above 7.0, there is more ammoniA and less ammoniUM, 
>until, at some point, it is all in the form of ammoniA. 

Karen, , you are correct that it is a sliding scale but there is not more
ammonia (NH3) above pH 7.0. From my paper originally published in FAMA,
reprinted in TAG and available on the Krib, Table 1 shows the percentages.
You comments may follow from the general notion that the total ammonia is
much less toxic when the pH is below 7.0.

Table 1. Un-ionized NH3 as a percent of total ammonia (by temperature and pH).
		Percent NH3 of total ammonia
Temp				 pH
		6.5	7.0	7.5	8.0	8.5
68		.13	.40	1.24	8.82	11.2
77		.18	.57	1.77	5.38	15.3
82		.22	.70	2.17	6.56	18.2
86		.26	.80	2.48	7.46	20.3

NH3 is much more dependent on pH than temperature.  Within the pH range
shown, an increase of one pH unit will increase the NH3 concentration about

What is also VERY important is the relative toxicity of NH3 and NH4+ at
different pH.

The next table shows that small concentrations of both ammonIA (NH3) and
ammonIUM (NH4), or the total of both, can be fatal to the animals in a tank.
The concentrations below 7.0 ARE important. Although I do not have empirical
evidence, I don't believe ammonia stays around very long in a heavily
planted tank. I discoved that small amounts below pH 7.0, however, can be
fatal. I found this out the hard way when I experimented with adding ammonia
salts to feed my plants (in form or Ammonium nitrate and ammonium chloride).
The article immediately followed <g>.
Table 2.Lethal ammonia concentrations at 86 degrees F.
       (by pH, and duration of exposure)

pH	duration	Lethal* Ammonia Concentration (mg/l)

			total	NH3

6.5	1-hr		14.3	0.036
	4-day		 0.73	0.002

7.0	1-hr		11.6	0.093
	4-day		 0.74	0.006

7.5	1-hr		 7.3	0.181
	4-day		 0.74	0.019

8.0	1-hr		 3.5	0.26
	4-day		 0.47	0.035

8.5	1-hr		 1.3	0.26
	4-day		 0.17	0.035

*Lethal concetrations are derived from levels at which half of the exposed
individuals die.

Neil Frank                 Aquatic Gardeners Association,  Raleigh NC