Wild pH Swings

To: Aquatic-Plants at actwin_com
Subject: Wild pH Swings
From: George Booth <booth at hpmtlgb1_lvld.hp.com>
Date: Fri, 29 Mar 1996 08:37:48 -0700
In-Reply-To: Your message of "Thu, 28 Mar 1996 15:39:01 EST." <199603282039.PAA23165 at looney_actwin.com>
> From: Michael Livingston <Mikel at netcraft-sw_com>
> 
> After all that, what can I do to minimize these changes.  I have read
> that kH (or is it alkalinity per recent flames :) ), pH and CO2 are
> tied together.  

pH, KH and CO2 have a relationship as shown in the table below.  The
KH vs Alkalinity confusion arises due to:

1) KH is a component of alkalinity.  If your water is relatively pure
and you don't add buffers other than carbonates, your KH value and 
alkalinity value will be very close,

2) Most hobbyist test kits actually measure alkalinity, even though 
they claim "KH",

The table below is accurate only if carbonate hardness (KH) is the 
predominate component of alkalinity as measured by your test kit. 

              The Relationship of CO2, pH and KH

-----------------------------------------------------------------------
 \ pH | 6.0     6.2     6.4     6.6     6.8    7.0    7.2    7.4    8.0
KH\   |
-----------------------------------------------------------------------
0.5   | 15      9.3     5.9     3.7     2.4    1.5    0.9    0.6    0.2
1.0   | 30      19      12      7       5      3      1.9    1.2    0.3
1.5   | 44      28      18      11      7      4      2.8    1.8    0.4
2.0   | 59      37      24      15      9      6      4      2.4    0.6
2.5   | 73      46      30      19      12     7      5      3      0.7
3.0   | 87      56      35      22      14     9      6      4      0.9
3.5   | 103     65      41      26      16     10     7      4      1.0
4.0   | 118     75      47      30      19     12     6      5      1.2
5.0   | 147     93      59      37      23     15     9      6      1.5
6.0   | 177     112     71      45      28     18     11     7      1.8
8.0   | 240     149     94      59      37     24     15     9      2.4
10    | 300     186     118     74      47     30     19     12     3
15    | 440     280     176     111     70     44     28     18     4
-----------------------------------------------------------------------
                        CO2  milligrams/liter

> My water has a kH of 4 per the Tetra tests.  Will
> raising the kH help stabalize the pH?  Can I use aquarium salt or
> baking soda to bring the kH up?  Are there other better suggestions?

If your pH is going from 7.2 to 6.2 and your KH is 4, you are going 
from an "equilibirium" value of CO2 of 6 ppm (which seems reasonable)
to an injected value of 75 ppm (which seems extremely unlikely given 
your setup, even in a 15 gallon tank).  

You need to be careful with the measurements, as any error can cause 
dramatic changes in the interpolated value of CO2.  For exmaple, if
you use a 5 ml sample with the Tetra KH kit, you have 1 dKH per drop. 
Depending on how you read the color change, you might actually have 3
dKH to 5 dKH.  You didn't mention what kind of pH test kit you have,
but typical kits have a 0.4 unit resolution, so your real pH could
be 7.0 to 7.4 or 6.0 to 6.4.  

Given this error range, your non-injected CO2 could be in the range of
4 to 15 ppm and your injected CO2 could be in the range of 35 to 147
ppm.  Going from 4 ppm to 35 ppm is much more likely. 

Getting back to your question, 4 dKH is a good value if that's what
it is.  You might verify this using a 10 ml sample where each drop is
1/2 dKH.  Also, if you are using any commercial buffers like pH-UP or
-DOWN, the table cannot be used.  If your KH seems right and the pH is
still dropping into the low 6's, you need to reduce the amount of Co2
you are injecting - slow down the flow or use a smaller plastic
container to reduce the CO2 diffusion surface area. 

George Booth