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Re: Carbonate hardness



On Wed, 20 Sep 2000, George S. wrote:

> It seems that we are talking here a bit past each other, probably
> because the term "Carbonate Hardness" is often misapplied to mean
> "Alkalinity".
>
> If one wants to measure ALKALINITY, then titration is both an easy and
> accurate method. Of course it does not tell one where the alkalinity is
> coming from, but that is another story.

I think that bicarbonate concentration is the quantity we actually want to
get at -- it's just that we normally use it in units of hardness, rather
than as the actual bicarbonate concentration.  Temporary hardness (your
carbonate hardness) isn't something we want to analyse.

Incidentally, the USGS and EPA standard methods both analyse bicarbonate
with the alkalinity titration, with the results reported as ppm HCO3-
rather than as ppm CaCO3.  Other acids can effect the alkalinity
titration, and these are regarded as interference in the analysis.  Our
problem is to determine when we have interference and to try to work
around the problem.

> If, as you say, the EPA talks about "non-carbonate hardness", then, by
> implication, there IS a "carbonate hardness" (perhaps of no interest to
> them), and, I am certain, that they do not mean "Alkalinity" by it!

That's a pretty solid logic, but they use the term "temporary hardness"
instead.  Perhaps they would just like to avoid the confusion inherent in
associating the word "carbonate" with something that actually measures
calcium and magnesium, not carbonate.

> The example you cite with your water is right: Your water has 1 "degree"
> of Carbonate Hardness and 7 "degrees" of Alkalinity. Nothing more,
> nothing less. I place "degree" in quotes, because *a priori* one would
> not know WHICH "degree" you have in mind -- American, English, French,
> German, or ... As you know, the EPA uses milligrams-per-liter (or ppm)
> as units for Water Hardness. Wouldn't it be a good idea for aquarists
> also to use it, exclusively?

As long as I'm using consistent units, it makes no difference who's degree
I use, or whether I use ppm -- the conclusion is the same.  My kits
measure in degrees, and I like using small round numbers, so I see no
value in converting things.

> Unless I misunderstood your other example, the boiling method CAN be
> used on waters without any Permanent Hardness: The second value
> (Permanent Hardness) just will be "zero". Subtracted from Total Hardness
> it will give you the correct answer -- all water hardness is due to
> Temporary or Carbonate Hardness.

True, but I think in the original context George B. was talking about
bicarbonate concentration (however you want to express it), not hardness.

> What I had in mind, mentioning "other alkalies" in the water in the SW
> US, was borates and silicates. Perhaps my geographical designation was
> not accurate enough.

Ah.  Silicates are present in significant quantities in most groundwater
supplies, but borates are pretty rare.  In either case, they won't effect
alkalinity unless the water your titrating starts out with a pH > 9.
That's fairly unusual in aquarium water.  The lowest pKa for boric acid is
9.14, and the lowest pKa for silicic acid is 9.66.


Roger Miller