re: Alkalinity-CO2-Test Kits

---------- Forwarded message ----------
From: psears at emr1_NRCan.gc.ca (Paul Sears)
Subject: Re: Alkalinity-CO2-Test Kits
To: Aquatic-Plants at ActWin_com
Date: Mon, 13 Jan 1997 15:15:47 -0500 (EST)

> From: Daniel Marsh <dmarsh at premier1_net>
> Subject: Alkalinity-CO2-Test Kits
> We have set up several planted tanks w\CO2 and TWP DI water
> (reconstituted with their Electro-Right 

	Do you have any idea what is in this?  Adding unknown
things to the aquarium water can be expected to cause problems 
with getting meaningful measurements. 

> The CO2 test kit has a very indestinct end point as it
> is such a light pink. 

	Phenolphthalein endpoint.  It should _start_ to be visible at
somewhere a bit above pH 8.3, but as you say, it is _very_ pale in
that range.

> I called tech support and they said go for the PH
> 8.3 endpoint on the meter which is not pink at all!

	It is about 2 pH points higher than the pKa of "carbonic acid",
so at that point you will have netralized about 99% of the CO2.  The
phenolphthalein should give a similar _result_ even if the pH is a bit 
higher.  It should change rapidly in that range.

> I've calibrated the
> meter very carefully and it will read my buffer solutions 4.0, 7.0, 10.0
> precisely but the Hach PH kit reads off the scale low when my meter
> reads 6.85. The CO2 kit reads from 12.5 ppm (meter 8.3 endpoint) to
> 21.25 ppm visual extremely light pink.

	A difference of that size indicates to me that either the pH meter
is misbehaving, or the stuff you are adding to the water is screwing 
things up.  When you use indicators, how much are you adding?  Don't
forget they are acids/bases themselves, and will _themselves_ alter
the pH of a weakly buffered solution if you add too much.

Paul Sears        Ottawa, Canada

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