You are at The Krib ->Chemistry [E-mail]



  1. (F) lower water pH without adding phosphate
    by (Pat White) (21 Jan 92)
  2. Phosphate Test Kit
    by Greg Morin <greg/> (Tue, 8 Jan 2002)
  3. Seachem phosphate test kit
    by Greg Morin <greg/> (Wed, 9 Jan 2002)

(F) lower water pH without adding phosphate

by (Pat White)
Date: 21 Jan 92
Newsgroup: rec.aquaria

In article <> (Xuyong Liu) writes:
>I was told that all the pH down contains some sort of phosphate.  And
>to alleviate the algae problem I have now, I'd rather stop using
>phosphate to lower pH.  I don't know whether biophosphate presents the
>same algae problem.

	Yup, all phosphate will present the same algae problem.  As I
understand it, it all gets mineralized into the same phosphate-containg
	BTW, they use phosphate because it is convenient.  These pH changers do
so via a buffer reaction.  A buffer reaction is a reversible reaction where the
concentration of H+ ions (ie. the pH) determins how much of each chemical there
will be when it reaches equilibrium.  These reactions each have their own
point where very small pH changes shift the equilibrium point between the two
compounds a great amount.  That's the pH the buffer reaction will keep.
	So, the pH UP stuff is one chemical of the pair, the pH DOWN is the
other.  If you add only one of these chemicals it forms the other by absorbing
H+ or releasing H+ till the reaction is at an equilibrium.  If you add them
both in the proper proportions, you strengthen the buffer so that it can absorb
or release more H+ ions before it runs out of one compound or the other (the pH
changes more as one gets closer to running out of a compound).

	I believe the phosphoric buffer reaction they use is:

	H2PO4 <---> HPO4- + H+

	Which, as I recall, conveniently trys to keep the pH around 7.2.
There was a posting a while back, and reposted again recently I think, and
probably in the FAQ by now, that explains exactly the chemicals and
pH they tend to keep.  Check it for the exact reactions and pH numbers.
	Trying to use this buffer reaction to get away from a pH of 7.2 will
only result in failure -- the pH will tend to drift back to 7.2 over time, or,
if your fish add pH changing stuff to teh water, the pH will continue to drift
away from where you adjusted it to.  The reason is that to get the pH away
from 7.2, you almost depleted the reaction of one chemical or the other.. so it
can't act as much of a buffer anymore.

	There are many other buffer reactions -- the one the marine guys are
always interested in comes to mind:

	H2CO3  <---> HCO3- + H+

	It has a different equilibrium pH (around 7.4 or 7.8 I think).

	If one can find some buffer compounds that don't kill the fish or
plants, and are not fertilizers, then one could produce a buffer that will get
the pH where you want it without the side effects.  Obviously such a buffer is
hard to find, and the people who sell pH changers have no interest in selling
it (they all sell algae killers too.. which one will need if one uses the
phorphous-based pH changers :-)
>Peat filter seems to be the way to go.  But it's
>not very convenient when you are doing a water change and have to
>lower the pH of tap water quickly.

	I take it filling some buckets with water, adding peat and something to
circulate the water, then using that as your water source for water changes is
out of the question?
	That's really best as it allows any CO2 to come to equilibrium, any
hardness to go away (peat is supposed to soften the water), and any chlorine
and other nasties to dissipate.

>Are there any other alternatives
>to lower pH?

	Any acid will do it.  Some are safer for life than others.  ALL have
the problem that it is easy to overshoot the desired point.  And they all also
have the problem that, not being a buffer, the pH won't tend to stay to where
you adjusted it.
	Personally, I wouldn't recommend it.. even if you do know what you are
doing.. it will only result in subjecting the fish to pH swings.  Swings are
probably worse for them than a nice stable pH they can adjust to.

>slightly acidic (say around 6.6 - 6.8).  The pH of tap water here is
>typically 7.4 (with KH of 4 and GH of 7).  My tank water conditions

	Trying to get he pH to 6.8 with a phorphous-based buffer is probably
doomed to failure -- it will tend to drift back to the 7.2 pH the buffer can
keep best (unless your fish add acidic stuff to teh water.. in which case the
pH will constantly drift down, never stabelizing at any pH).

good luck,
Pat White (
--  ...!{tektronix!nosun,uunet}techbook!patbob
PDaXcess BBS at (503) 644-8135 (1200/2400) Voice: +1 503 646-8257
Paid public access user --- Not affiliated with TECHbooks

Phosphate Test Kit

by Greg Morin <greg/>
Date: Tue, 8 Jan 2002

>I've been using a Seachem Phosphate Test Kit for about a year now. Before I
>started dosing KH2PO4, the phosphate level was so low (~ 0.1 mg/l) that I
>didn't think much about it.  But now, on Tom Barr's advice, I'm trying to
>keep the level at 0.5 mg/l.  And, lately, I've begun to doubt the results of
>the test.  This morning, I tested the reference solution that comes with the
>kit and, instead of getting the 1.0 mg/l I should have, I got about half
>So, my questions are: Do the reagents "expire" in as little as a year, or is
>this test kit inherently inaccurate?  If the latter, does anyone have an
>opinion on which of the more expensive Hach or Lamotte kits would be better?

With the phosphate kit it is very, very , very important that exactly 
one full stem of sample be used. A little bit more or less will throw 
off the color. Also, take the reading between 10-30 seconds as the 
color does shift off scale the longer it sits. If the cap on 
phosphate reagent#1 has not been on or has been kept loose then the 
reagent can evaporate slightly and this will also alter the test 
result (normally not a problem but I mention it because very old 
phosphate kits won't work because of this i.e. 5+ years).
- -- 

Gregory Morin, Ph.D.  ~~~~~~~Research Director~~~~~~~~~~~~~~
Seachem Laboratories, Inc.     888-SEACHEM

Seachem phosphate test kit

by Greg Morin <greg/>
Date: Wed, 9 Jan 2002

>You kind of answered a question I was going to pose to
>you before I asked it with your last post.  I should
>go with whatever the kit says at 30 seconds regardless
>of any change that might take place after that?  The
>color will often keep giving a higher and higher
>reading until it gets to about .5 ppm, when it then
>changes to a pale turquoise color that doesn't match
>anything on the slide.  For a while, I was thinking
>that whatever reading I got before it turned to
>turquoise was the right one.  Not so?

Correct, because before it turns turquoise even then the green color 
that it is at is not the same hue of green on the color scale (more 
blue green and less yellow green). As long as the hue of green 
matches the hue on the scale you're ok to take a reading, but as soon 
as that hue changes it's not valid and would be read as a false high 
reading. You can play around with the reference to see this more 
clearly under controlled conditions. If you test the reference the 
color develops rapidly and completely in about 10 seconds... you then 
have from 10-20 seconds to read it accurately (i.e. the green hue 
won't change) after that it will keep getting darker and darker which 
would otherwise be read as a false high, however you should notice 
that this "higher" green is not the same hue as the green on the 
scale, it's more blue and less yellow. So, if it doesn't match the 
hue of the scale then it's not a valid reading.
- -- 

Gregory Morin, Ph.D.  ~~~~~~~Research Director~~~~~~~~~~~~~~
Seachem Laboratories, Inc.     888-SEACHEM

Up to Chemistry <- The Krib
This page was last updated 17 February 2002