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Re: Alkalinity and Phosphates

Scott H. wrote:

> My experience with the Hach CO2 test is that the changes in
> color (it's a titration test for a color change) are very
> subtle, so much so that readings need to be "generalized"
> as a range of about plus/minus 5 or so parts ppm.

A standard electrometric titration for acidity has a pH
endpoint of 8.3.  Judging from the color you describe it
sounds like the Hach kit might be using an indicator called
Metacresol Purple, which changes from yellow to purple
between pH 7.4 and 9.0.  If the colors are too pale to judge
when the change occurs, then maybe Hach hasn't included
enough of the indicator in the test.

You might check their website or call the friendly folks 
at Hach to see if the behavior is normal and to get detailed
instructions.  I expect that the test reaches the titration
endpoint the first time you see any permanent color change.
> Anyway, if you have organic compounds, don't you have acids
> and those goof up the results using the pH/KH/CO2 table --
> but then wouldn't those same acids cause a (false) high
> reading on the CO2 titration test?

Some of the same acids might interfere with both tests.
Generally the different tests should be effected by different
acids.  If an acid effects both tests then it will have a
different effect on one test than on the other; large on one
and small on the other.

The alkalinity (KH) titration is interfered with by acids that 
become associated (grab a hydrogen ion) at a pH between the 
initial pH of the sample and the endpoint pH of about 4.5.  
The CO2 (acidity) titration is interfered with by acids that 
become associated at a pH between the initial pH of the sample 
and the endpoint pH of about 8.3.  An acid that is partly 
associated at the initial pH of the sample will interfere to 
some degree with both tests.  I think the acid will have an 
equal effect on both test only if the sample pH happens to be 
equal to the pKa of the acid.

I don't claim that the acidity test is subject to less
interference than the alkalinity test.  They both have problems.

> Just to be clear, the organics cause a false high reading
> on the KH tiration test?

If they have any effect then it is to cause a false high KH
reading and false high KH gives rise to a false high CO2
result.  It isn't established that organics in aquariums have 
a general effect.  That could vary from case to case.

Roger Miller

P.S.  I'm responding to letters in the archives because 
delivery of the digest has been very unpredictable.