MadSci Network: Chemistry
Query:

Re: how can pH go DOWN when I add sodium bicarb to water

Date: Mon Apr 10 09:58:35 2000
Posted By: ,
Area of science: Chemistry
ID: 953890767.Ch
Message:

Peter,

Sorry about the delay on the answer.

Your question concerns the nature of diprotic
acids and bases.  This means that they have more
than one dissociable location.  Sodium Hydrogen Carbonate,
(Sodium Bicarbonate) has the formula NaHCO3
The parent acid is Carbonic Acid, H2CO3.  When the
salt of a weak acid is added to water, some portion
of the salt acquires a hydrogen ion from the water.  This
leaves an excess of hydroxide ions and thus, the pH of
the solution goes up.  

The interesting thing that you have found is that when the pH
is higher than that of the salt + water alone, then the second
dissociable hydrogen of the Bicarbonate ion will act as an acid
and lower the pH.  
This is called a BUFFER system.  The solution really does not
want to change pH very much at all, and it requires fairly
large amounts of acid or base to change pH.
There is a lot of information out on the web about buffers and
Acid Base chemistry.  Try running a search using those as keywords.
You may wish to look at a first year collegiate chemistry textbook.
The discussion on polyprotic acids and bases as well as buffers will
give you plenty of information to satisfy your curiosity.

As an aside, that is about the highest pH of tap water that I have
ever heard of.  You probably live in an area with limestone or
chalk deposits underground.  

Keep down the pH

Michael M. Gallagher Ph.D.
Senior Research Chemist
J.R. Simplot Co.



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