Kh Ph

[isaac newton]

If you have a higher Kh is it always true u will always have high ph??? Im a bit confused about this.. (I.E. I have a kh of 10 but a ph of 7.. Is it possable that someone else have a kh of 10 but a ph of 8?)

[TomFromStLouis]

I am far from a chemist, but water hardness and acidity/alkalinity are two different things. So no, it is not true that you will always have a high pH with a high KH. I add CO2 which lowers pH but KH stays at 4.

Let's let someone who really knows come in and tell how the minerals which make KH higher also buffer something or other making lowering pH harder to do or something. I think I share your confusion a little - they ARE related somehow. For example I think calcium carbonate raises hardness and pH, right? And peat lowers both right?

Is there a chemist in the house?

[superjohnny]

Originally posted by TomFromStLouis
...For example I think calcium carbonate raises hardness and pH, right? And peat lowers both right?
...

Yes that's right. I use calcium carbonate to raise my hardness. Out of the tap my water is about ph 7, kh ~1. After CC it gets up to around 3, ph stays around 7 with DIY CO2. I'm not sure about peat.

This is a good question, one I would also like to know the answer to. Some people have a really high tap PH or KH and it makes having an aquarium quite difficult. Other people have great water... it depends on where you are I guess.

[RTR]

KH, pH, & CO2 are inseparable. They are the "Bermuda triangle" of water chemistry. Any change to one will affect one or both of the others.

KH, alkalinity, is also refered to as "carbonate hardness" to distinguish it from "general hardness" which is in fact calcium and magnesium hardness. So to say that "hardness" and alkalinity are two distict things, you must specify which hardness you are talking about.

In non-CO2 supplemented tanks, increasing the KH will always increase the pH. Tom's CO2 supplemented tank shows lowered pH only so long as CO2 is being added. That is because a tiny fraction of disolved CO2 always forms carbonic acid in water, and the acid reduces the pH. Turn off the CO2 and within hours the tank pH will return to the level set by the KH and atmospheric equilibrium with CO2. So he is counter-balancing his "natural pH" with added acid to give a lower pH.

For more than you want to know on the topic (fairly heavy going):

http://www.tomgriffin.com/aquasource/hardwater.shtml

HTH

[Bergebo]

Originally posted by RTR
KH, pH, & CO2 are inseparable. They are the "Bermuda triangle" of water chemistry. Any change to one will affect one or both of the others.


I'm gaining a slow understanding of this, but have a question if you don't mind.

If the water has a high, say 200ppm carbonate hardness, will the carbonate need to be "used" up before the pH will start to drop?

I realize there are not simplistic answers to my simplistic question, but this is how I currently understand the term "buffer."

On target?

Thanks.

[RTR]

No, if you are talking about CO2 addition. Supplementing to planted tank levels will still decrease the pH, but not as easily read and calulated as at lower KH values.

But in general you are correct, that large amounts of buffering takes large amounts of acid to overcome. In water at 200ppm CaCo3 equivalent (or ~11.2 degrees), you are unlikely to ever see significant KH drop from normal nitrification, where in a tank at 20-40ppm you would be on the edge of crash much of the time. For CO2 effect, have you studied the charts on the Krib?

http://www.thekrib.com/Plants/CO2/kh-ph-co2-chart.html

Unfortunately, the changes in the slope of those curves as the KH increases is telling you that KH 11+ is going to require a bit of CO2 to read. I believe someone gives the equation from which the data is derived, so you could generate your own curve for hard water, if you have the math and the patience.

[Bergebo]

Originally posted by RTR
No, if you are talking about CO2 addition. Supplementing to planted tank levels will still decrease the pH, but not as easily read and calulated as at lower KH values.

I think I get it. That's because of the higher slope at lower KH levels, as shown by the graph.

So, CO2 will lower the pH. Readily at low KH levels, but not so easy at high KH. Yet adding lots of CO2 to high KH water will ultimately lower that KH as long as the additional CO2 is present.

If your goal was to lower high pH/ high KH water, using CO2 would be difficult. Easier to filter the water through peat and use that to add to the tank. That way the KH would "naturally" be lower.

[RTR]

No, not quite, the CO2 will lower the pH, leaving the KH where it is. Turn the CO2 off and the pH will over hours rise back to the pre-CO2 point.

The pH/KH/CO2 charts do not work for peat extracted water - the tannic acids distort the results.

[wetmanNY]

My understanding of peat-- while RTR is at my shoulder to correct me-- is that its humates have many negatively charged sites, which trap the positively charged calcium and magnesium ions. Dissolved humic substances (in peatwater for instance) also bind the Ca and Mg. The positive ion, H+, that is released in this ion-exchange, is that same acidifying H+ as in pH.

In practice, it takes a bucketful of peat with a little hole in the bottom and water trickling through, to effect much softening in very alkaline (=high KH) water. A couple of tablespooons in the filter won't do it.