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Re: Tap water mystery

> Date: Thu, 16 Oct 1997 23:57:15 -0500
> From: Ed Hengel <hengel at computer_net>
> While checking the water parameters on my discus grow out tank i
> decided to test for co2 on a hunch that it might be higher than
> normal.  What I did not expect was the greater than 50 ppm co2.
> Thinking this was odd I checked the tap water and sure enough it was
> >50 ppm co2.  So I filled a glass with tap water, let it sit all day
> and checked it again this evening along with the pH and Kh. The co2 is
> still >50 ppm, the pH is 5.96 and the Kh is 3.36.

CO2 test kits are sensitive to "strong mineral acids" (I'll let the
chemistry gurus explain what those are).  The LaMotte
kit, for example, titrates the water sample to a phenophthalein end
point with Sodium Hydroxide Standard Solution. Anything in the water
that mimics the acidic properties of dissolved CO2 will give a false
high reading. 

During a testing binge recently, I was attempting to determine if the
Tetra KH measurement does anything other than measure total alkalinity
(it doesn't). Specifically, I was hoping that the test kit did
something special to sequester phosphate so that the phosphate
buffering compounds we sometimes use would not give false KH readings
and mess up the use of the pH/KH/CO2 tables (it doesn't and phosphates
WILL mess up your use of the tables). In the process of this testing,
I was running Tetra KH and GH tests, Lamotte alkalinity, pH, CO2, Ca
hardness, Mg hardness and TDS tests.  

To add phosphates to the water, I used some SeaChem Neutral Buffer
(1/2 tsp in 10 liters). Not only did this read as KH and alkalinity,
but it also changed the CO2 reading from 3 mg/l to 25 mg/l. 

So, I suspect that your tap water has "strong mineral acids" and you
don't really have 50 ppm CO2.  I don't suspect phosphates, since the
pH and KH are lower than I would suspect with phosphates. 

> And if it did, what could be causing the LaMott test kit to give a
> false reading that coincides with the pH, Kh, co2 chart?

pH 5.96, 3.36 dKH and 50 ppm CO2 don't coincide on my chart. 

That pH and KH shows 110 ppm CO2, 
That KH and CO2 shows 6.22 pH,
That pH and CO2 shows 1.5 dKH.

(Even thought the pH is close, that area of the chart has very extreme pH