Re: Alkalinity and phosphates

[Roger Miller <roger at spinn_net>]

Scott wrote:

> With organic acids present you
> could have no CO2 (or carbonic acid) and the table would
> show that you did because of the organic acids effect on pH

The error would not be caused by the effects of the organic 
on pH.  It would be caused by the effect of the organic on the 
alkalinity (KH) reading.  

> -- I guess my question is, is that the same effect that
> disturbs the KH reading or another phenomenon confounding
> the use of the table?  (albeit a small confounding ;-)  )

The tables and formulas determine CO2 using the pH and 
*bicarbonate* content of the water.  We use KH tests to 
determine the bicarbonate concentration.  The KH reading is 
disturbed because some acids (organic and inorganic) can act 
like bicarbonate in the test, producing a false reading.

The test measures the tendency of the water to resist a drop in 
pH.  When you perform the test you titrate the sample with a 
mixture of an indicator solution and a strong acid.  The strong 
acid adds hydrogen ions to the sample and the indicator 
solution changes color when the hydrogen ion concentration
is high enough that pH of the sample reaches about 4.5.
 
pH measures the concentration of hydrogen ion; as the
hydrogen ion concentration increases the pH decrease. It's a 
logarithmic thing.  Water resists a decline in pH (increase in
hydrogen ion concentration) when any compound in the 
water associates with hydrogen ions.  That takes the free 
hydrogen ions out of the water and cancels out the change in 
pH.  The test measures the total concentration of compounds
that can take hydrogen out of the water.

Bicarbonate ion is usually the only compound present that 
will combine with hydrogen ions in the pH range of the test.
That pH range is from the starting pH of the sample to the
ending pH of the test -- about 4.5.  Because bicarbonate ion
is usually the only ion that effects the test, the test can be
regarded as a test for bicarbonate.

There are compounds other than bicarbonate that will grab 
hydrogen ions out of the water in the pH range of most tests.  
That includes hydrogen and dihydrogen phosphate and some 
organics.  The test does not distinguish between the bicarbonate 
and any other compound that acts the same way during the test.  
If those compounds are present then they are read as part of the 
KH and our assumption that KH is equal to the bicarbonate 
concentration is wrong.

> If the organics can present acid that lowers the pH does it
> then also reduce KH?  If so, does that counteract, somewhat
> the effect of inflating the KH reading?

If the organics lower the pH they will cause a usually small 
decrease in the bicarbonate concentration.  At the same time,
the presence of the acid -- or at least some acids -- adds to 
the KH reading, thus effecting the test.  It doesn't hurt the test if 
something other than bicarb and CO2 causes a change in pH.  
The tables and formulas don't assume that the pH is determined 
just by bicarbonate and CO2.

It's theoretically possible that a compound could exist in just
the right concentratrion that it increases both pH and alkalinity 
(KH) to just such a degree that the tables and formulae came 
out right.  That would be quite a chemistry exercise, or a very
slim random chance.  But then, if a room full of monkeys with
typewriters were given enough time they could eventually 
reproduce a Shakespeare play.  Who's to say that the effects 
wouldn't all work out right?


Roger Miller