[Prev][Next][Index]

KH/pH/CO2 and non-carbonate buffers

> From: Cathy Engel <vandi at well_com>
> Date: Mon, 20 Nov 1995 09:01:03 -0800
> Subject: KH/pH/CO2 and Non-Carbonate Buffer Systems
> 
> I'm wondering if somone can enlighten me on the concept of KH/pH/CO2 and
> the use of non-carbonate buffers.  I understand that KH can be
> influenced by non-carbonate buffers (such as phosphate buffers).  The
> reason I am asking this is because of the natural pH of my water as well
> as the KH builder that I'm using.  My tap water has a pH of 7.8.  Now, I
> use RO/DI filtered water so I need to regenerate it with something.
> I've tried both a mix of RO with tap water, and RO with Kent products.
> Kent's KH builder (called pH Stable) is a carbonate based buffer and has
> a natural pH of 7.8 same as my tap water.
>
	Your tap water doesn't have a "natural" pH.  Since it has a high
KH, the concentration of bicarbonate ions in it is high, and the pH will
depend on the CO2 concentration in it.

	The buffering reaction is:

	H2CO3    <->   HCO3-   +    H+ 

	The equilibrium constant for this reaction is 4.16 x 10^-7
(It depends a bit on temperature), which means that 

	[H+][HCO3-]/[H2CO3]   =  4.16 x 10^-7  

where [something]  is the concentration of the something in moles/litre.

	If we use KH as a measure of [HCO3-], and ppm CO2 in solution as
a measure of [H2CO3], we can write:

	pH  =  7.58  +  log(KH)  -  log(ppm CO2)

	The entire pH/KH/CO2 table can be derived from this. 

	Your KH is about 13.4 (see below), so:

	7.8  =  7.58  +  log(13.4)  -  log(ppm CO2)

	from which your CO2 concentration would be about 8 ppm.
If you leave the water lying around, or blow air through it, the
CO2 concentration will drop to about 0.5 ppm (equilibrium with
atmospheric CO2), and the pH will rise to about 9.0.
	If Kent's buffer is indeed a "carbonate" buffer, then it will
involve the same reaction, and its effect will be the same.  It will
just be a lot more expensive than either your own water or some baking
soda.

> I keep Discus in my plant tank and I like to keep my pH at around 6.6.
> I don't like having to drop my pH so far and would like to lower my pH.
> However, adding an acid to the water just depletes the KH rather than
> drop the pH.  Kent's pH Minus is simply sulfuric acid.  After I add it,
> the pH does drop a bit but over the course of an hour will rise back to
> 7.8.  I've tested this is a 10g tank filled with KH=5 water.  I added
> enough pH Minus to drop the pH to 4.0.  By next morning, the pH was back
> to 7.8.  The KH had dropped to 2.  After adding more, I managed to get
> the pH to drop but at the expense of having no KH.
> 
	The pH rises again because adding the acid destroyed bicarbonate,
adding CO2 to the solution.  The created CO2 leaves over hours or days,
and up goes the pH.  Eventually, if you keep on adding acid, you get the
KH low enough that the pH stays where you want it even with low CO2
concentration in the water.  If you want pH 6.6 at a CO2 concentration
of 0.5 ppm, your KH will have to be about 0.8!  If you double the CO2
concentration, double the KH and you get the same pH.
	If you want pH 6.6 and any significant KH, you will have to add
CO2 to the system, or use some other buffering system.
 
> SeaChem makes an Acid buffer that is a Zwitterionic buffer.  They say it
> is not so much a pH Minus product but a product to buffer your water at
> a certain pH.  All I know is that it is a non-phosphate buffer.
>
	Using other buffers is possible.  A buffering product will
consist of a mixture of an acid with a salt of its anion:

	H(xxx)   and   M(xxx)

The idea is that the realtive concentrations of the two are such that
the H+ concentration is effectively fixed by the equilibrium:

	H(xxx)   <->   H+   +   (xxx)-

	This depends on swamping any other acid/base equilibria that are
going on in the solution, by sheer quantity of both H(xxx) and (xxx)-.
If SeaChem's pruduct is a Zwitterion, this means that it contains
ions with a both a positive and negative charge.  An example would
be an amino acid, which can exist as:

	H2NxxxxxCOOH   or   +H3NxxxxxCOO-  

	_Both_ ends of this get involved in equilibria with hydrogen ions.

> I'm wanting to drop the tanks natural pH to 7.0 so that with CO2
> injection I only have to drop it a few tenths of a point (to 6.6) rather
> than 1.2 points.  This became a problem the other day when I changed CO2
> cylinders.  I must have screwed up the needle valve setting when I moved
> the regulator to the other tank.  The bubble rate was too slow to keep
> up with CO2 loss and the solenoid heated up, causing the entire assembly
> to become warm.  This caused the needle valve to expand and completely
> shut off CO2 flow.  Overnight the pH went from 6.6 to 7.8.  I was
> suprised not to find a bunch of sick fish.  Amazingly the Discus and
> their tankmates looked perfectly happy.  I'm now in the process of
> dropping the pH back down but I'm holding at 6.9 until I get some
> answers.
>
> I guess I need to separate the solenoid from the needle-valve so that
> the temperature doesn't vary so much.  I could also switch to a 12V
> solenoid and use a transformer.
>  
> Would the addition of SeaChem's product screw up anything? 

	What is in it?  A bit more information is needed.

> I suspect it
> would make my KH read artificially high. 

	I expect so, it would involve weak acids.

> Would I really care if all of
> my buffering capacity was made up of SeaChem's buffer as opposed to a
> carbonate buffer? 

	That depends on what it is.

> Would my Lamotte CO2 test kit still be accurate?

	Almost certainly not.

> Would I still target the 5 degree KH mark as I always have?

	What for?  You are now using another buffer.

> From what I
> understand, alkalinity/KH test kts don't test specificly for carbonate
> buffers, is this correct?  My Lamotte alkalinity test says it is testing
> ppm CaCO3 and that does confuse me.
> 
	The "alkalinity" tests actually measure the anions of weak acids.
They thus measure HCO3- in carbonate buffered systems, by titrating
with a strong acid.

	HCO3-  +  H+  ->  H2CO3  (taken essentially to completion)

	They will measure anions of _any_ weak acids, this includes your
buffer from SeaChem.  The measurement as ppm CaCO3 is indeed confusing.
At any pH suitable for fish, there is very little CO3-- in solution,
it is all HCO3-.  What the test does is measure the HCO3-, work out
how much Ca++ would go with it, and then tell you how much CaCO3 would
contain that much calcium.  

> Sorry for the babbling.  Maybe I'm being too obsessive about this, but I
> do like knowing exactly what is in my water and being able to control
> it and I'm restricted from using pure tap water.  The water is clean,
> but is very hard (300ppm) and has a very high alkalinity (240ppm).
>  
>
	I don't think you are being at all obsessive in wanting to know
what is going on!


Paul Sears,    Ottawa,  Canada.