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Re: Total Alkalinity and Carbonate Hardness

> Date: Mon, 23 Jun 1997 20:06:15 GMT
> From: mike at microspan_com (Mike Roberts)
> Subject: Re: Amano's CO2/KH/PH
> I think this answers one of my test kit questions, but I would like to double
> check before buying them.
> I was planning on getting the Hach "Total Alkalinity" and the Hach "Carbonate
> Hardness" test kits.
> The first will tell be how "hard" the water is for the purposes of making the
> fish and their eggs osmotically "happy."  The second test kit will tell me how
> much C02 is dissolved in the water by using 'the chart' as a lookup table
> against carbonate hardness and pH.
> Am I getting warm?
> *Mike Roberts - mike at microspan_com

I don't think so.  "Hardness" is one of the most abused terms in the 
whole water lexicon, and the confusion tends to entangle "alkalinity" as 
well.  I'll give you my best understanding of these terms as used in 
technical and regulatory literature, and hope that Hach's use of the 
terms is similar.

I understand the good people at Hach are forthcoming with answers, should 
you need details about their kits.

"Total Alkalinity" should simply be alkalinity - in most natural waters
this is synomymous with the bicarbonate concentration, just expressed in
different terms.  In fact, I know of no other standard method for
determining bicarbonate.  I don't think this is what you want to measure
for breeding purposes.  It is what you need to determine your CO2 

Hang with me a moment while I do details.  Alkalinity is measured by
titrating a sample with acid from its natural pH to an endpoint pH of
about 4.5, determining the number of "milliequivalents" of acid used, and
then expressing that quantity as milligrams/liter (mg/l) or parts per
million (ppm) of CaCO3.  There are 50 mg/l of CaCO3 per milliequivalent. 
A number of other units (including degrees) can be used.  Any acid in the
sample that becomes associated during the titration will add to the
alkalinity.  In heavily polluted water there may be a number of organic
acids that might contribute to alkalinity.  I think that in an aquarium
and in a few natural waters, the chelating agent EDTA (a polyprotic acid)
will also add to alkalinity. 

The word "hardness" should probably never be used without qualifiers - it 
is simply too commonly abused.  General hardness (or total hardness or 
Ca-Mg hardness) measures the total concentration of the alkaline earth 
elements calcium, magnesium, strontium and barium.  In most instances 
this is almost entirely calcium and magnesium.  I think this is the 
hardness that you need to know about for breeding purposes.

Total hardness can be measured by titration, but I'm not sure how that
titration works.  Most commonly, it is calculated from the separately
determined concentrations of calcium and magnesium (and Sr and Ba if
they're available).  Either way, the concentrations are converted to
milliequivalents, then the milliequivalents are (just like for alkalinity)
expressed as mg/l of CaCO3.  This can also be converted to degrees
(several types) or grains per gallon and probably any number of other

"Carbonate hardness" is the part of the total hardness that is balanced by
alkalinity.  I don't know how a test kit will determine that value - I
think its normally done by calculation.  If the total hardness is greater
than the alkalinity, then there is also some "noncarbonate hardness".  If
the total hardness is less than the alkalinity then there is no
noncarbonate hardness.  It is fairly common for the total hardness to be
very nearly equal to the alkalinity.  Then total hardness, carbonate
hardness and alkalinity are all about the same.  Too bad.  That just adds 
to the confusion.

I have seen the term "carbonate hardness" used when alkalinity is meant.  
That is improper.  I've also seen the term "hardness" used when the 
writer (or speaker) is describing the water's total solute content.  
Again, that is improper.  The total salt content is best described as 
total dissolved solids (TDS) or with specific conductance (an indirect 

Hmmm.  Sorry about the really long note.  At least I'm not accompanying 
it with a mime-encoded binary, or repeating the entire digest!

Roger Miller