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Chemistry Forums for Students => Undergraduate General Chemistry Forum => Topic started by: zodiacfire on October 29, 2007, 11:46:59 PM

Title: Strange unknown reaction...any suggestions?
Post by: zodiacfire on October 29, 2007, 11:46:59 PM

I am a teaching assistant at university running a gen chem lab for chemistry majors.  Last week we did the usual experiment to test the effectiveness of Tums antacid vs. generic brand by first neutralizing each tablet with an excess of 3M HCl and then back titrating with 0.3M NaOH solution.  Before titrating, the students added 5.00 mL of the acid and then 50.00 mL of DI water.  They then heated the solution on a hot plate until they saw all the tablet was completely dissolved.  Instead of the usual cloudy white watery liquid, one group's solution turned very frothy and bright white and would not settle.  After almost 15 minutes on a hot plate at medium heat, the solution took on a light yellow color then proceeded to turn entirely bright yellow, looking almost like drawn butter with a layer of thick white foam on top.  Having no clue what this product was, I treated it as unknown waste and discarded it appropriately.  Any ideas what this strange product could be?  I suspect they may have added 0.3M NaOH instead of the HCl, then heated it, but I am at a loss as to what they may have created!!  Thanks for any and all suggestions.

Title: Re: Strange unknown reaction...any suggestions?
Post by: Borek on October 30, 2007, 04:19:31 AM

No idea what composition Tums antacid is, so it is hard to even speculate. But you may try to do the experiment just as you suppose they did (ie adding NaOH instead of HCl) to see if you can repeat the problem.

Title: Re: Strange unknown reaction...any suggestions?
Post by: AWK on October 30, 2007, 08:30:54 AM

Tum is simply CaCO3.
Note - NaOH can contain some CO2, also deionized water can contain some CO2.
Antacids also contain some starch, sugar, and flavor.

Title: Re: Strange unknown reaction...any suggestions?
Post by: Borek on October 30, 2007, 09:17:33 AM

Quote from: AWK on October 30, 2007, 08:30:54 AM

Antacids also contain some starch, sugar, and flavor.

When writing about unknown composition I was referring to these additives - most likely these are responsible for the observed color changes.

Title: Re: Strange unknown reaction...any suggestions?
Post by: zodiacfire on October 30, 2007, 08:55:32 PM

I spoke with my advisor today and he mentioned that oftentimes the inert binders in antacid tablets contain a stearate (in some form or another) so the students may have initiated a polymerization reaction in their flask! 
Thanks for your suggestions.

Title: Re: Strange unknown reaction...any suggestions?
Post by: AWK on October 31, 2007, 01:59:36 AM

Almost all organic compounds warmed  with NaOH give yellow solutions.
I do not believe in polymerization of stearate