Thanks for your help. You are right that I dont want to mess with CO2. Did
it before with yeast and worked well but was a pain. Pressurized cylinders
too much cash and effort also. You did hit the nail with the stability of a
purely bicarbonate buffer. It raises the pH too high if I add enough but at
low concentrations its not an effective buffer. I will keep looking for a
non phos. buffer and let everyone know if I have any luck.

Thanks, John



"Dan Drake" wrote in message
news:vhIsdqY67dTD-pn2-MWCc1XoEoYty@localhost...
On Tue, 23 Dec 2003 05:57:53 UTC, "Shannon" wrote:

First I want to thank everyone for their replies. To clarify....

I have poorly buffered water from the tap (not RO, maybe my kH is low?)
and
an unstable pH which tends to drop (presumably from mulm buildup in the
gravel)... My issue with baking soda is that the pka is
somewhere in the mid 7's if I remember correctly. This would mean it is
not
much of a help as a buffer below pH 7 where my fish like it. I may have
this a little confused but the principle is there I think.

Well, so you start out knowing more than some of the people who have
replied.

However, the "effective" pKa of the bicarbonate system is a little above
6. There's a lot of room for confusion here, because the value given in a
table may or may not include a factor of 55 (a difference of log 55 in the
pKa) which represents the concentration of _water_ in the system. Also,
the calculation can be done in terms of CO2 pressure or H2CO3
concentration, related by a factor of about 30.

Here's a page that goes into detail on why carbonate works to keep blood
pH at 7.4 even though it shouldn't be very good at that _high_ a pH:
http://www.qldanaesthesia.com/AcidBaseBook/ab2_2.htm
Another take:
http://www.usbweb.com/reference2.asp?id_ref=23

Anyway, according to this and other sources, carbonate buffering should be
at its best for pH in the 6's, and I don't suppose you want to go much
below that.

Now for the bad news. In a chemical sense, bicarbonate works just fine at
pH 6, if you have enough of it. But you can't have enough of it. Your
water finds an equilibrium with the tiny amount of CO2 in the air; and the
more bicarbonate, the higher the pH. To have a low pH, you have to have
only a really low level of bicarbonate in the water -- and the low
concentration, sure enough, gives you bad stability.

You can break out of this by artificially forcing the amount of CO2 in the
water to stay waay above the level in the air. That's right, CO2
injection. (You knew that, and you don't want to do it, and that's why
you're asking, right?)

If I add sodium
bicarb my pH obviously shoots up above 7 but doesnt tend to stay there
too
long. Just curious if there are any commercial preparations that will
keep
the pH around 7 or slightly under with no phosphates in them. The
bullseye
I added doesnt state either way but I am starting to see some hair algea
so
I have my suspicions.

There are some non-phosphate buffers on the market, but I don't have them
handy. Poke around on the shelves of a good, big aquarium store, if
available, and you'll see some. I'm pretty sure SeaChem makes a pH-Down
replacement without phosphate. And they do say what's in the stuff;
generally it's a zwitterionic compound, maybe tin-something. How much
they'll help, I don't know.

--
Dan Drake

http://www.dandrake.com