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Name: Gene S.
Status: educator
Age: 40s
Location: N/A
Country: N/A
Date: 2001 - 2002


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.


Replies:
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


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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