Re: Alkalinity

---------- Forwarded message ----------
From: psears at emr1_NRCan.gc.ca (Paul Sears)
Subject: Re:  Alkalinity
To: aquatic-plants at actwin_com
Date: Thu, 16 Jan 1997 09:12:59 -0500 (EST)

> From: "James Purchase" <jpp at inforamp_net>
> Subject: Re: Water Chemistry Reference
> What is phenolphthalein alkalinity and how does it differ from total
> alkalinity (I measured my tank's water with both test kits and while the
> LaMotte and HACH test kits give identical readings for Total Alkalinity,
> the HACH kit indicates that my water has zero phenolphthalein alkalinity. 
	Phenolphthalein alkalinity is given by the amount of acid that
is required to titrate the sample _down_ to pH 8.3, the endpoint for
phenolphthalein.  In effect, you are measuring the total of OH- and
CO3-- ions in your solution.  If your pH _starts_ below 8.3, both
these are very low and you get a zero result.
	The total alkalinity is given by a titration down to pH 4.3;
the methyl orange endpoint.  This effectively measures HCO3-, CO3--
and OH-.  If the phenolphthalein result was zero (as it would be for
most aquaria with plants) it just means that your "alkalinity" was
just about all bicarbonate.

> Can anyone recommend a comprehensive book on Water Chemistry as it applies
> to freshwater aquaculture? George Booth has made several references to one
> published by HACH but they advised me that it is no longer available. Are
> there any others which go from A-Z, in detail?

	In general, aquarium books are at best weak on water chemistry.
Some screw things up really badly.  All I can suggest is that you get
some general chemistry textbooks and read up first on equilibria in 
general, and then look at the carbonate/bicarbonate system.  The
only equilibria you really have to consider are:

	H2O  <->  H+  +  OH-         ([H+][OH-] = 10^-14)
	H2CO3  <->  H+  +  HCO3-     ([H+][HCO3-]/[H2CO3] = 4.16 x 10^-7)
	HCO3- <->  H+  +  CO3--      ([H+][CO3--]/[HCO3-] = 4.84 x 10^-11)

	[z] means the concentration of [z] in moles per litre.
	All the numbers are temperature-dependent to some extent.

	For almost all aquarium conditions, the only one you really have
to worry about is the second, but for a complete understanding, look
at all three, remembering that _all_ must be satisfied at the same time,
and that there must be a charge balance as well (all the ions in solution).

	For a start, at any given pH, [OH-] is fixed, and the
ratios of [H2CO3] to [HCO3-] and [HCO3-] to [CO3--] are also fixed.

Paul Sears        Ottawa, Canada

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