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Re: Aquatic Plants Digest V4 #454

Chuck Gadd writes:

> > > PO4 - 0 to 0.5 ppm
>  > 
>  > Phosphates in your water will be seen by your "KH" test kit as 
additional 
> KH.
>  > It's not really KH, but additional alkalinity. Remember, KH test kits 
are 
> really
>  > measuring total alkalinity with the assumption that carbonates are the 
> only
>  > contributor. If you have phosphates in the tank, this assumption is 
false. 
> Be
>  > careful here.
>  
>  Are there any KH kits that actually measure carbonates?
>  
>  George, Do you have any impression of how much phosphate it takes to
>  throw off the KH measurements?   
>  
>  > The CO2/KH/pH tables assume that the only alkalinity is from KH. If you 
> have
>  > phosphates, the tables will NOT give you correct results.
>  
>  > > With the following tank parameters, what can I expect regarding pH and
>  > > hardness changes. (I'll be turning the system off at lights out)
>  > >
>  > > pH - 7.0
>  > > dkH - 4.0
>  
>  > No, with a KH of 4 and 15 ppm of CO2, you will get ph = 6.9.
>  
>  You say that 15ppm of CO2 will be a pH of 6.9, but his pH
>  is already at 7.0 before CO2 injection.  So, the chart would suggest
>  that with a KH of 4, and a pH of 7, he's already got 12ppm before
>  adding any CO2!  But we know that's not true.    
>  
>  It's probably the phosphates throwing off the numbers.    Which
>  makes the values on the chart incorrect.
>  
>  BUT! With the phosphates influencing the buffering, does the 
>  relationship itself hold?  Will increasing the CO2 10 times still
>  drop the pH by one full point?  Or is that relationship broken by
>  the phosphates as well?
>  
>  If that relationship still works, then:
>  
>  If we assume that his non-injected water has ABOUT 2-3ppm C02, then with
>  a pH of 7, his "real" KH would be about 1 degree, so if he targeted a
>  pH of 6.3-6.4, that would put the CO2 in the right ballpark, about
>  12-15ppm.
>  
>  Is this completely off?    If so, it might be a way to still let people
>  use the chart when they have high phosphates

It seems to me, and I'm not a chemist, that phophate alkalinity should be 
counted in the mix here as KH.  The charts allow you to figure in for 
alkalinity.  You need to do this, because alkalinity and acidity are directly 
related within a given body of water.  The KH tells you what the lowest pH 
can be under "normal", non-CO2-injected circumstances.  This is related to 
total alkalinity, including phosphates.  You calculate CO2 concentration by 
the pH swing from there, because the CO2 increases the level of H2CO3, which 
in turn lowers pH, and H2CO3 is unaffected by buffers, whether they be CO3 or 
PO4, because carbonic acid is hydrogen carbonate.