Affects of KH on pH

[flanque]

Hello,

Firstly lets define a few things in my view. A "proper" pH is 8.2 and a "proper" KH is between 180-200ppm.

Now...

I had a debate today with a work colleage about the affects of KH on pH. It got me thinking because in my early days of marine aquariums I was told "raise your KH to the correct value and your pH will go up to the correct value then stop".

Curious I read more articles and discovered that that may not be true. It was my understanding that having a "proper" KH will maintain a stable pH. In other words, it is quite possible to have a pH of 8.2 for instance, but have a KH lower than what's "recommended" (note this varies as well in my experience). In this instance the pH is more capable of shifting depending on other water chemistry.

The reverse also, it is possible to have a "proper" KH but the pH may not be correct.

I tried to explain this to her, but she resisted.

So now I am reading and it seems that I may be correct, but may not be depending on what I read. So I ask you all, what is your view on the KH-pH relationship?

It's still my understanding that a proper KH will make your water's ability to resist changes in pH possible (i.e. not shift +/-). It's my understanding that also doesn't necessarily mean that by raising the KH without buffering the pH to the appropriate level, that it will just naturally shift towards 8.2.

I am of the belief from what I have read, that there is a positive relationship between raising KH and maintaining a stable pH, and that there is a "break even" point where you can actually have a "proper" KH without necessarily having a "proper" pH.

Honestly I don't mind being wrong - it's a great learning tool - but from what I have read the level of KH is about the water's ability to buffer pH changes rather than actually "setting" a proper pH.

Thoughts?

 

[localhost]

I am of the belief from what I have read, that there is a positive relationship between raising KH and maintaining a stable pH, and that there is a "break even" point where you can actually have a "proper" KH without necessarily having a "proper" pH.

I think this is correct. You have to set the ph properly first, then the kh will prevent it from changing. The things that causes pH to change first affects the KH before it can affect the pH. Like a bodyguard protects somebody. I think of it as a protection instead of a buffer.

A buffer IMO is things like arganite. From what I've read, it will release 'something' that will help raise the ph back to the pH when the pH gets too low.

I could be totally wrong, this is just my views from what I've learned here and there.

 

[batfire]

Totaly agree, I never test Ph only Kh.

 

[flanque]

Yeah I was reading a little more last night and basically what I can see is that the carbonate hardness is the buffering capacity of the water. In other words the pH will be at a given level and if the carbonate hardness is high then the addition of acids will be absorbed (probably not actually absorbing) by the carbonate/bicarbonate bases.

Perhaps it "naturally" goes up as the carbonate hardness is higher because it is affected less by the aquarium environment (such as sand, rock, phosphates, etc), or possibly because it just seems to be low at lower carbonate hardness levels due to the larger fluctuations? I'm not entirely sure.

It seems to be a chemical equasion matter. I'm no chemist but it seems to be related to the positively and negatively charged state of ions.

So far from what I've read, it's all about the stabilisation of the pH and not about changing the pH.

 

[steve-s]

Actually, pH is a factor of CO2. The more CO2 in the system the lower the pH becomes. By adding buffers to increase alkalinity, you drive out the CO2 which allows the pH to rise up. Unfortunately this is also the worst means to control pH. If alkalinity is in the normal to higher range, adding more buffer to increase problematic pH will just lead to more problems. In fact, the tank becomes so CO2 inhibited the pH can actually crash and become acidic. You can directly measure this affect in your tank via algae respiration. At night algaes will consume O2 and respire CO2, the reverse is true in the day time. Algaes consume CO2 and respire O2. It is not the increase in O2 that affects the pH but rather the decrease in CO2. O2 levels unto itself has no impact on pH. Where alkalinity comes in is it as stated above... a proper KH will make your water's ability to resist changes in pH possible (i.e. not shift +/-). In otherwords helps prevent drops in pH from the introduction of acids.

pH can still be low where alkalinity is high or conversely pH can be high where alkalinity is low. The more important factor being CO2 but alkalinity defiantely plays a large role.

http://www.advancedaquarist.com/issues/may2002/chem.htm

Cheers
Steve

 

[flanque]

Hi Steve,

Thanks for that. I'm sorry I didn't mention the parts I read about CO2. I read what you explained but what I read said that the reverse was also possible in that the more O2 in the water can affect pH as well. This was only on one website though.

Aside from the CO2 element, you pretty much confirmed what I thought, though I am curious that if by addition of carbonates/bicarbonates will drive out CO2 then wouldn't that mean the pH should rise? That being said I guess that's where the "breakpoint" is reached and adding more can have a negative affect as you explained.

I'm not entirely sure of the CO2 pH relationship here. I guess the point is without getting to chemically engaged, your pH is related to your CO2 levels and your carbonates/bicarbonates at the appropriate level will "consume" acid bases to stop changes in pH.

I'll read the article you provided but if you could clarify the CO2 being "driven out" issue that would be great.

 

[steve-s]

Thanks for that. I'm sorry I didn't mention the parts I read about CO2. I read what you explained but what I read said that the reverse was also possible in that the more O2 in the water can affect pH as well. This was only on one website though.

I can assure you, O2 levels alone have zero effect on pH.

I'm not entirely sure of the CO2 pH relationship here. I guess the point is without getting to chemically engaged, your pH is related to your CO2 levels and your carbonates/bicarbonates at the appropriate level will "consume" acid bases to stop changes in pH.

Loosely speaking pretty close, yes.

I'll read the article you provided but if you could clarify the CO2 being "driven out" issue that would be great.

Driven out means to reduce the level of. This can be done through many means. More commonly the problem tends to be trapped ambient CO2. Not enough to affect us but definately enough to affect the pH in a closed system. This can be accomplished simpley by opening a window. Other factors are tank tops without ample open sump space, gas appliances improperly installed/leaking, low/improper tank water flow and so on.

Cheers
Steve

 

[localhost]

Until somebody mentioned CO2, I had forgotten about all that planted tank research I did. Freshwater planted people use kh and ph #'s to calculate the amount of CO2 in the water.

http://www.csd.net/~cgadd/aqua/art_plant_co2chart.htm

 

[flanque]

I supose you could do the same for your saltwater as well?

 

[steve-s]

Don't see why not although it does indicate that phosphates will skew the results. Marine tanks usually have some form of P even if registering zero on a test kit. The other thing to consider is it does not really help evaluate the source of the elevated CO2, only that it may be present.

Using that calculator, I got a CO2 level of 1.075 ppm. My pH is usually around 8.3-8.5 with an alk of 3.15-3.25 mEq/l

Cheers
Steve