# Boiling off KH



## Edward Shave (20 Sep 2018)

Just wondering if anyone calculates KH by first boiling it off to get permanent hardness?

So KH should equal GH - Permanent hardness.

Would this method be viable/accurate?

I'm assuming the water lost through the boiling would be made up using di water before testing.


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## dw1305 (20 Sep 2018)

Hi all,





Edward Shave said:


> Just wondering if anyone calculates KH by first boiling it off to get permanent hardness? So KH should equal GH - Permanent hardness.


If you assume that all the dGH/dKH come from calcium carbonate (CaCO3), then dGH will be the same as the dKH. The reason for this is that they are defined in terms of <"calcium oxide (CaO)">. 





Edward Shave said:


> I'm assuming the water lost through the boiling would be made up using di water before testing


Yes, you can remove the temporary (carbonate) hardness by boiling. You need  to pour the near boiling waterthrough a fine sieve, and then make it back up to the original volume with DI water, when the boiled water has cooled.  

You have to pour the boiling water off the limescale, because as the water cools down the carbonate reserve will go back into solution. Carbonate solubility is a bit strange because carbonates become less soluble at higher temperatures (this is because of the solubility of gases and the carbonate ~ CO2 equilibrium).





Edward Shave said:


> Would this method be viable/accurate?


Theoretically it would, but because you've already made the assumption that all your dGH/dKH is from CaCO3 you could just measure the alkalinity of the original water (dKH tests are all really alkalinity test kits, that use a semi-quantitative acid base titration).

If the original water is your tap water? your water supplier should be able to give you accurate calcium and hardness values.

cheers Darrel


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## Edward Shave (22 Sep 2018)

Hi Darrel. 

Many thanks for your answer. Just one thing...

I didn't as you say make the assumption that all the dGH/dKH is from CaCO3.
I guess you got this idea from the part where I stated.. "So KH should equal GH - Permanent hardness."
By which I meant KH should equal GH minus permanent hardness. Where Permanent hardness = GH after boiling.

Hope that makes better sense.

The background to this...
After having been thoroughly confused by references to KH test being for alkalinity rather than hardness and the KH,PH,CO2 Table being wrong if the KH (alkalinity) test didn't reflect actual KH,  I was looking for a way to get an accurate measure.

Thanks again,
Ed


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## dw1305 (22 Sep 2018)

Hi all, 





Edward Shave said:


> Hope that makes better sense.


It does. 





Edward Shave said:


> By which I meant KH should equal GH minus permanent hardness. Where Permanent hardness = GH after boiling


The terminology doesn't help, but the "temporary hardness" is the carbonate hardness (dKH) that can be removed by boiling, and the "permanent hardness" is the amount of multivalent ions in solution, which can't be removed by boiling. If the source of both types of hardness is calcium carbonate (CaCO3) then 1dGH = 1dKH.

You can  have a situation where the dKH exceeds the dGH,  this occurs when a source of carbonates is a salt like potassium bicarbonate (KHCO3), in this case you've added dKH (HCO3-), but not any dGH (K+ is a monovalent cation). 

The same can apply for dGH when you add a salt like calcium chloride (CaCl), you've added a divalent cation (Ca++), but you haven't added any dKH. 





Edward Shave said:


> After having been thoroughly confused by references to KH test being for alkalinity rather than hardness and the KH,PH,CO2 Table being wrong if the KH (alkalinity) test didn't reflect actual KH, I was looking for a way to get an accurate measure.


It isn't straight forward to measure either dGH, or dKH, directly, so the dKH test (an acid base titration), or the CO2/dKH/pH tables, are proxies that give you a ball-park figure.  

There is a way to get the CO2/dKH/pH table to work, which is to use a drop checker with a solution made up to 4dKH  and a narrow range pH indicator (bromothymol blue). In this case the only source of acids is the CO2 which has diffused across the air gap and the only source of bases the KHCO3 etc. you've used to make up your solution (starting with DI water). 

This is an explanation in <"CO2 relationship to KH.."> and linked threads.

cheers Darrel


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## Edward Shave (22 Sep 2018)

Your right Darrel, about the terminology which is what threw me in the first place. 
I should have said that since my initial confusion I did delve into the chemistry somewhat but its not an easy subject I find.
Some of what you said doesn't quite gel with what I thought I new, so it's back to the text books for me lol.

I know that in theory you can boil off the temporary hardness and thus again in theory be left with just the permanent hardness.
I was interested to know if this works in practice. 

I have in fact just carried out a test run to see what sort of results I might get...


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## dw1305 (22 Sep 2018)

Hi all, 





Edward Shave said:


> I did delve into the chemistry somewhat but its not an easy subject I find.


I know, it isn't easy. I'm not a proper chemist (I'm a botanist) and I've really struggled with the application of some of the concepts.

I'm lucky that I've worked with people who can explain "buffering" etc to me in terms that I can understand. I've also used "@gerald", "@Mike wise" and <"@regani"> on Apistogramma forums and they have been very useful, because they are really experienced fish keepers and scientists.

Your results are probably about what you'd expect, you've removed some of the ions and the conductivity has fallen (the TDS measurement is really an electrical conductivity measurement). We know that the dGH should still be the same (we haven't removed any of the permanent hardness) so that is probably to do with the way the test kit works. I'll add in @alto, he is a lot more up on these things than I am.

You could measure the dGH accurately (by combining the values for magnesium and calcium ions) with an <"ICP or AAS analyser">, the actual procedure is pretty straightforward, but you need access to the equipment.

The change in pH is a bit strange, you would expect the pH to be much the same (you still have some carbonate) and about pH8. The pH is to do with the CO2 ~ carbonate equilibrium, basically if you have bicarbonate in solution you will always end with a pH about pH7.8 - pH8. The amount of acids you have to add to lower the pH depends upon the reserve of bases ("buffering"), if you have soft water (this is "dKH" we are interested in) a small addition, of weak acids, will reduce pH. If you have hard water then you need to add a lot more acid before the pH falls.

In practical terms it is really difficult to remove solutes from water, without an RO unit, and your water is quite hard, so even the reduction in temporary hardness still leaves you with fairly hard water.

I'm a rain-water user, I know it isn't for every-one, but I've never had any problems with it.

cheers Darrel


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## Edward Shave (22 Sep 2018)

dw1305 said:


> We know that the dGH should still be the same (we haven't removed any of the permanent hardness)


Now I'm getting confused again. lol.

I was working on the assumption that GH (Total Hardness) is the sum of temporary + permanent hardness?
Thus after boiling off the temporary hardness I expected to see a reduction in GH?


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## dw1305 (22 Sep 2018)

Hi all, 





Edward Shave said:


> was working on the assumption that GH (Total Hardness) is the sum of temporary + permanent hardness?
> Thus after boiling off the temporary hardness I expected to see a reduction in GH?


No, the temporary hardness (dKH) is the carbonate content that can be removed by boiling, but boiling doesn't remove the Ca++ (or Mg++) ions that we measure as dGH, they are the "permanent hardness", that can't be removed by boiling. 





dw1305 said:


> ...........but the "temporary hardness" is the carbonate hardness (dKH) that can be removed by boiling, and the "permanent hardness" is the amount of multivalent ions in solution, which can't be removed by boiling. If the source of both types of hardness is calcium carbonate (CaCO3) then 1dGH = 1dKH.
> 
> You can have a situation where the dKH exceeds the dGH, this occurs when a source of carbonates is a salt like potassium bicarbonate (KHCO3), in this case you've added dKH (HCO3-), but not any dGH (K+ is a monovalent cation).
> 
> The same can apply for dGH when you add a salt like calcium chloride (CaCl), you've added a divalent cation (Ca++), but you haven't added any dKH.


cheers Darrel


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## Edward Shave (22 Sep 2018)

Well you live and learn. 

Thanks Darrel.


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