Characteristics of pH Swings

[DSenn]

Large pH swings due to a low KH/pH crash and KH in general are some of the most useful concepts I have learned since joining this forum 150 posts ago However, I have never actually experienced one of these swings first hand. So, I guess my newest round of questions are these:

If/When one has depleted their KH and the pH begins to swing, how much of a swing can be expected? From a 7 to a 6? Even greater, like from a 7 to a 5?

Also, how often would a pH swing? For example, would it be in a continuous state of fluctuation, like from a 7 to a 6 to an 8? Would it make a major swing once every few minutes? Every hour?

And finally, is it a 'guarantee' that your pH will begin to swing with no KH? Can you pH swing with 1-2 dKH? I have around 2 degrees, and I haven't noticed any swings yet.

Thanks!

 

[gzeiger]

KH acts as a buffer for pH variations. A buffer is simply a chemical that can react with, and neutralize, either an acid or a base. Adding acid or base to a buffered solution causes only a small change in the pH (some buffer gets used up) until you've added enough that all the buffer has reacted. At that point, the water is effectively not buffered in one direction. That is, if you added acid until the buffer is exactly used up, then the chemical resulting from that reaction will still buffer a base but will no longer buffer acids.

In an unbuffered solution, pH changes based on the addition of acid or base with no dampening effect. The pH will not "swing" randomly, and certainly not minute by minute, without some cause. Typically there are a few acid sources in your environment though. Rotting wood and dissolving carbon dioxide from the air add acid to the water and will cause a downward trend in pH, but it will reach an equilibrium in either case. The equilibrium value for atmospheric CO2 will be about 5.7 with no buffer present.

Remember that pH is a logarithmic scale. Each .3 you drop on the scale represents approximately a doubling of the concentration of hydronium (acid).

 

[jsoong]

If you have no KH, the pH will move in proportion to addition of base or acid.

In a typical tank, there isn't a source of base, but there are lots of acid sources: rotting plants, fish waste, a dead fish ... etc. Typically, what you see with depletion of KH is a slow downward move in the pH for some time, then something happens in the tank - a bit of overfeeding, a dead fish/snail - and the pH suddenly bottoms out. I have read posts where all the fish died & the pH is found to be 1 or 2.

Another bit of food for thought .. the nitrifying bacteria are inhibited at low pH (5), so you also get to deal with rising ammonia with a dropping pH ...