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Re: CO2 and hardness help
> From: "Buckley, Kevin" <kevin.buckley at nectech_co.uk>
> Subject: CO2 & hardness help (long)
> Since I started doing this back in August 99 I've only ever put tap water in
> my tank &, lately, Hagen Aquaplus at each water change.
> Every time I ever tested my tap & tank water it has been pH=8.2.
> My tap water also tests ~40ppm Nitrates so since December I do all water
> changes via an inline, salt-water rechargeable, Nitrate & Phosphate filter
> (available in the UK, called a 'Nitragon').
Do you really want to put chloride in your water? That is what
this thing will do if it is an ion exchange resin, and it would appear
> I also have a sachet of a Nitrate removing resin called 'Cleanwater' & 200g
> of GAC in my filter which I change every few months.
What is "GAC"? Granulated Active Carbon?
> The hardness test kit I bought ('Interpet') tests for 'Total' hardness (blue
> solution added 1 drop at a time to 5ml of test sample - turns pink initially
> then turns blue on completion.
This will be measuring total Ca++ and Mg++.
> Number of drops = degrees of hardness) &
Sounds like (one of) the usual tests
> 'Temporary' hardness (orange solution, turns blue initially then turns
> orange/yellow on completion. Again, number of drops = degrees of hardness).
This sounds a bit odd. I would expect a straight titration with
HCl, using methyl orange as the indicator: Orange to orange/yellow -
> Firstly - is 'Temporary Hardness' the same thing as KH (or Alkalinity?
> What's the difference?)? I've read all about this on the Krib but it still
> hasn't 'clicked' for me.
"Hardness" is total Ca++ and Mg++ - the anion(s) don't matter.
Temporary hardness is the part that can be removed by boiling the water.
The reduction is the result of:
Ca++ + 2HCO3- -> CaCO3(precipitates) + CO2(leaves) + H20
For this to happen, you must have both Ca++ (or Mg++) and HCO3- in the
solution. Permanent hardness is the part that is left.
"Alkalinity" is the total of the weak acid anions in the water.
For most of us, that is HCO3-, which is KH.
> My tap water tested 9 degrees 'Temporary' & 16 degrees 'Total'. However,
> for the first time ever, the pH has dropped to 7.3! That would seem to
> indicate a CO2 concentration between 14ppm & 17ppm (depending on which
> formula you use) unless the water company is putting some other acid in the
> water. Is that likely?
It's probably CO2. Another acid would destroy the KH.
> My tank water (with current level of DIY CO2) tested 3.5 degrees 'Temporary'
> & 15 degrees 'Total'. The pH measured 7.6. So CO2 ought to be 3ppm
> (probably reasonable - I don't have a very efficient CO2 reactor & my main
> filter generates a lot of surface agitation & I have an airstone).
> So where did the missing 5.5 degrees of 'Temporary' hardness go & why is the
> 'Total' hardness unchanged?
The resin that removes nitrate and phosphate is probably removing
HCO3- as well. I would expect it to. It won't touch the Ca++, because
it is an anion exchange resin.
> I bubbled air into the tank sample for about 15 minutes & re-tested - result
> was 3.5 degrees 'Temporary' & 14 degrees 'Total'. pH was 8.2 which I
> expected & which indicates CO2 around 0.6 - 0.8 which also sounds reasonable
> I think?
> At this point I re-tested my tap water - pH=7.3. So I guess the test kit's
> OK & the tap water really is more acidic than usual.
I would think so.
> Finally, I tried blowing into a sample of tank water (as recommended by
> someone, somewhere) to raise the CO2 to 60ppm (is that correct?). The pH
> dropped to 6.4 which indicates CO2 between around 42ppm & 53ppm (using
> 'Temporary' hardness = 3.5). So is 60ppm not correct? (I think that info.
> came from the Krib).
That "60" number will be very rough. How heavily had you been
exercising? That would make a difference.
> The same experiment done with tap water gave a pH of 6.7, so CO2 is between
> 54ppm & 68ppm (using 'Temporary' hardness = 9).
> The one thing I didn't do (but wish I had) was to aerate a sample of tap
> water & then re-test the pH.
You did a pretty full set of experiments, which always helps to
sort things out.
> So, to summarise the questions I have:
> 1) Are 'Temporary' hardness', KH & alkalinity the same thing?
Stricly, no, but in practice, yes. The odd result will come
when you have _more_ HCO3- than the Ca++ can take. That means there is
another cation in there. You can't have _more_ temporary than total
hardness, but you can have more KH.
> 2) Is it reasonable to expect (in the absence of CO2 injection) that my tank
> water has dropped 5.5 degrees of 'Temporary' hardness in 4 weeks? Or, has
> my recent DIY CO2 injection (about 2 weeks worth) somehow affected the
> 'Temporary' hardness?
The CO2 injection didn't affect it. Your resin would.
> 3) Why didn't total hardness (which presumably is 'Temporary' hardness plus
> some other hardness) also drop by 5.5?
I think I covered that.
> 4) Is it likely that the 'Nitragon' filter, as well as removing Nitrates &
> Phosphates is also swapping Calcium for Sodium - like a regular water
> softener does (which might explain question #3 above)? If so, is that good,
> bad or indifferent for the plants & fish?
No. You know that, because the hardness (total) didn't drop.
Paul Sears Ottawa, Canada