Ask A Scientist Chemistry Archive

name         Gene S.
status       educator
age          40s

Question -   I am a chemistry teacher who is trying to help a student
prepare for the Science Olympiad Qualitative Analysis competition.  The
problem is that to differentiate sodium acetate from sodium chloride and
sucrose, the directions stipulate that phenolphthalein will turn pink
when added to the sodium acetate, but not to the other two.  Here's the
problem-- it does not work.  Apparently the hydrolysis of acetate is not
sufficient to carry the indicator over.  I tried adding the indicator to
the solid, then to a solution of the salt, all to no avail.  We tried
three different sources of the acetate salt, one being ACS reagent
grade.  Finally, I tried it on some sodium acetate that had seeped out of
a chemical hand warmer-- the darn stuff turned pink!  Have any
ideas?  I'm stumped.
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Yes, an aqueous sodium acetate solution should be more alkaline than either
sodium chloride or sucrose solutions.  But phenolphthalein's pink endpoint
is actually at a pretty high pH.  It is pushing things to expect sodium
acetate to reliably deprotonate phenolphthalein.

My guess is that the stuff in the chemical hand warmer was slowly
hydrolyzing and evaporating over time:

   NaOAc + H2O <==> HOAc + NaOH
   HOAc(l) --> HOAc(g)

The equilibrium of the first reaction is to the left, but the second
reaction removes the acetic acid product.  The end result is a slow
accumulation of sodium hydroxide (which will actually give sodium carbonate
by absorption of atmospheric carbon dioxide, but let us not confuse things
too much), which makes the material more alkaline.

Perhaps the folks who wrote the directions should have tried them out
themselves on several different samples of sodium acetate, hmm?

Richard E. Barrans Jr., Ph.D.
Assistant Director
PG Research Foundation, Darien, Illinois
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I think the attached web site answers your question. Dissolved CO2 lowers the
pH sufficiently that the phenolphthalein is still colorless. The solution
that leaked from the hand warmer probably is mixed with other ingredients
that raise the pH so that the indicator turns pink.
http://jchemed.chem.wisc.edu/Journal/Issues/2002/Jan/abs29_1.html

Vince Calder
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