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Re: HNO3 in Planted Tanks?

>Date: 20 Feb 1998 11:06:08 GMT
>From: Jonathan_Kirschner at Energetics_com (Jonathan Kirschner)
>Under what conditions would nitric acid be generated in a tank?  Is this due
>to the natural hydration of our ever-present nitrate ions? 

The nitrification cycle is equivalent to titrating the water with nitric acid.  
Two of the end products of the cycle are extra H+ ions (which is defined as 
"lower pH") and NO3- ions (nitrate).  If nitric acid is added to water, it 
completely dissociates into H+ and NO3-. Therefore the natural biological 
processes in the tank are "generating nitric acid".  

If the water is buffered, a buffer ion combines with the H+ to form another 
compound.  Thus the buffer "mops up" the excess H+ generated and, thus, the pH 
doesn't change (this is a very simplistic model). However, as the mopping up 
progresses, buffer ions are used up and the buffering capacity (how much more 
acid can be neutralized) is reduced. When all the buffer ions have been used up, 
the H+ ions then lower the pH. 

This is known as a pH crash.  pH is stable for a long time (as whatever buffer 
is in the water is used up) then, WHAMMO! it nose-dives.  If you do regular 
water changes or replenish the buffer by other means, your pH should be 
relatively stable. 

Perhaps one of the chemistry wizards could delve into how buffering *really* 
works, but I think this model is useful for conceptualizing the idea. 

George Booth in Ft. Collins, Colorado (booth at frii_com)
Need Info?  http://www.frii.com/~booth/AquaticConcepts.htm