dkh v. ph, please help me understand

[Reef Bass]

It bothers me that I don't understand the relationship. I've done some searching and read lots of interesting stuff, but haven't come across the answer that somehow causes understanding to crystalize in my thick head.

I humbly ask that someone clue me in, or point me to some links. Flame me if you must, but I wish to get this.

Here's my question. My buddy's tank (really, not mine!) has been generally running on the lower side of the alk scale, like 6-7, as measured in dkh. His corals look fabulous, btw. However, his ph was creeping up to 8.5.

So his alkalinity is on the low side as measured in dkh, yet his tank is too alkaline as measured by ph. Can someone please shed some light on what seems to me to be contradictory? Is this just an unfortunate, coincidental and not appropriate use of the term alkaline for both measures?

Thank you.

 

[mm949]

does he top off with kalk? NSW has alk range of 7-9, so its within range.
my tanks run at 8 dkh with ph 0f 8-8.3 with a kalk doser
what are you using to test pH? maybe have someone else test the ph with a dif test kit to see if the numbers are similiar

 

[Reef Bass]

He tops off with ro/di, not kalk. He was manually dosing diy two part, then converted to automated dosing and he felt the rise in ph was about that time.

Good advice about testing pH with a second test kit just to confirm the allegedly high readings.

It's not the mismatch in numbers (6-7 verus 8.5), it's not understanding how the apparently similar term is used for two different measures, one of which shows low and the other high, that makes me wonder.

 

[KSzegi]

I sympathize with you ..... at first I had a hard time wrapping my brain around the idea that ph and alk weren't really the same thing! I don't know if I can help - but I'll give it a shot. I think I understand it now but that doesn't mean I can explain it!! :lolspin:

PH is a measurement of the degree to which the water is acid or alkaline.... pretty much measuring the hydrogen ions as compared to pure (neutral) water. An acidic solution has more hydrogen ions, a base/alkaline solution has less hydrogen ions.

ALK (KH/dKH) is measuring the buffering capacity, or ability of the solution to maintain its PH when when acid is added..... or how resistant the tank will be to a ph change. Rather than measuring hydrogen ions ..... it is measuring available carbonates (carbonate and bicarbonate for dKH and also borate and hydroxides for a true 'total alkalinity) which combine with excess hydrogen ions and "absorbs" acids .... maintaining PH. As KH levels drop, PH becomes unstable and carbonate becomes unavailable as a buffer or a nutrient..... when carbonates are used up, extra hydrogen ions are found .....

 

[disc1]

Here's an experiment that will help you understand.

I make a solution that is 2.0mM in sodium bicarbonate. That's 168mg in 1L. It has an alkalinity of 4.0 meq/L. That's double the concentration since carbonate counts twice for alkalinity. And it will have a pH between 10 and 11.

If I adjust the pH by adding acid, some of the carbonate will react with the acid to become bicarbonate. A solution that has both a base (carbonate) and its conjugate acid (bicarbonate) is a buffer. The pH is set by the ratio of the two.

If I adjust the pH to 8.3, then 99% of the carbonate has become bicarbonate. Look here to see how I did that. http://en.wikipedia.org/wiki/Henderson–Hasselbalch_equation

Now the kH has gone down to just above 2.0meq/L because it's almost all bicarbonate which counts only once towards alk. And the pH has gone down too. So adding acid drops them both.


Now let's take our solution, it still has 2.0mM carbonate in it (99% of which is in the bicarbonate form) and a pH of 8.3. Let's dilute 10mL of this solution to 1L. Now we have cut the carbonate concentration by a factor of 100. Now we have 0.02mM carbonate 99% of which is in the bicarbonate form. Now the alkalinity is 0.02meq/L.

But guess what the pH is? It's still 8.3! We didn't change the ratio of carbonate to bicarbonate, we just diluted it. Since the pH is set by the ratio and not the total concentration of the buffer, the pH remains the same. So in this case, alkalinity went down by a ton and pH didn't change at all.

 

[bertoni]

This article might help:

http://reefkeeping.com/issues/2006-06/rhf/index.php

The term "alkalinity" was poorly chosen, IMO. "Acid neutralizing capacity" would be better. I think that's the British term.

If the high pH is accurate, more aeration might help by bringing carbon dioxide into the water, although pH at 8.5 is fine, IMO. I'd worry some if it got over 8.6.

 

[Reef Bass]

Thank you very much gentlemen. This is very helpful.

 

[bertoni]

You're welcome!

 

[reefinghabit]

yes the alkalinity or dkh or kh is as stated the buffering ability to maintain a stable ph.without first having a stable kh of 3.5 meq/l or a scale of 10-12 kh u will have a difficult time maintaining ph,but if the ph is to high,it could be due to over adding a ph buffer product or dosing to much kalkwasser drip. ideal ranges are ph of 7.8-8.5 and kh of 10-12 or 3.5 meq/l

 

[HighlandReefer]

Alkalinity effects pH and the amount of CO2 in your water effects it as well.

Randy shows the effect of alkalinity as well as CO2 on the pH of your tank water in this diagram:

Low pH: Causes and Cures
http://reefkeeping.com/issues/2004-09/rhf/index.htm

Figure 1. The relationship between alkalinity and pH for seawater equilibrated with air containing normal and elevated carbon dioxide levels. The green dot shows natural seawater equilibrated with normal air, and the curves reflect the result that would be obtained if the alkalinity were artificially raised or lowered.

 

[HighlandReefer]

If you don't have enough circulation of water in your tank and your CO2 level drops below normal this will cause your tank pH to rise. Randy depicts this scenario in this graph:

High pH: Causes and Cures
http://reefkeeping.com/issues/2005-03/rhf/index.htm

Figure 2. The relationship between alkalinity and pH for seawater with normal carbon dioxide
levels (black), excess carbon dioxide (purple) or deficient carbon dioxide (blue). The green
area represents normal seawater.

 

[Reef Bass]

Wow. I knew excess C02 could contribute to lower pH, but didn't realize excess (or deficient) could affect alk as well, if I'm reading that graph correctly.

My bud's got good flow but I appreciate your pointing out how the situation of low alk and higher pH could be.

 

[Randy Holmes-Farley]

No, the graph is not showing that CO2 impacts alkalinity. It does not.

What is shows is that if the water is equilibrated with the air, that at a given alk, the pH is fixed by the amount of CO2 in the air.