A Halogenated Substance
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Phenolphthalein in HCl/NaOH Titration Problem...
A week ago I attempted a re-crystallization of NaOH (which had a miserable yield) from a drain cleaning product. The very large majority of the
contaminated NaOH was still in solution so I decided to titrate it.
Product MSDS: http://www.roebic.com/pdf/sds/Crystal_Drain_Opener_SDS.pdf
I set up a basic acid/base apparatus using 11.82 hydrochloric acid solution to titrate the NaOH. I added some phenolphthalein to the beaker below the
burette. However, I noticed some tiny pink globs floating around and as I stirred the solution it seemed oddly viscous, clinging to the walls of the
beaker a little bit. I dismissed this (I suspected the pink globs to be NaOH that fell out of solution during storage) and proceeded on to add the
HCl.
Upon the addition, the solution went mostly clear but the pink globs and bits of pink on the sides of the beaker remained for a while. I had only
added a little bit HCl and I swore there was significantly much more NaOH in solution. I added a couple more drops of phenolphthalein to the beaker
and saw it go pink for a while then go clear. Hmm.
So I repeated. Added some HCl, added some phenolphthalein to see watch it go from pink to clear, repeat.
Here's a video of it (Sorry for the incredibly shaky shooting): https://www.youtube.com/watch?v=GemQQbWg3so
Does anyone have an idea of what's going on? I suspect that the NaOH is definitely still present as each continuing addition of HCl creates fumes and
some heat. However, I've no clue what the phenolphthalein is doing. I also don't think the carbonate and sodium chloride contaminants are doing much.
I appreciate any feedback!
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JJay
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Phenolphthalein is actually degraded to colorless products by strong base:
http://www.chemeddl.org/alfresco/service/org/chemeddl/video....
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ficolas
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A in highly basicbasic solutions, phenolphtalein is also colorless, you need to add enough ácido for it to turn pink, and then add more until it
turns colorless again, un wich point it will have a pH of ~8.2
The pink areas are normal afaik, untill you stirr enough.
I am not the best candidate to answer you thought, im not really experienced, and I have just performed two tritrarions in my life. But I try to help
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A Halogenated Substance
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The answers above turned to be right. I kept adding the HCl slowly and the solution turned and remained a deep pink color. I continued adding until I
got a clear mixture. I ended up having 15 molar NaOH...
Thank you very much for your responses JJay and ficolas!
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DraconicAcid
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You'd be better off titrating the acid with the base, rather than the other way around, and it's also better to titrate more dilute solutions.
Please remember: "Filtrate" is not a verb.
Write up your lab reports the way your instructor wants them, not the way your ex-instructor wants them.
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Magpie
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I always titrate at near 0.1M. Try that. It works well.
Phenolphthalein is supplied in an alcohol solution. It may not be all that soluble in a strong base.
The single most important condition for a successful synthesis is good mixing - Nicodem
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byko3y
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There's tons of different indicators, phenolphthalein being shitty one but still in use these days because it's dirt cheap and because people have no
idea other indicators exist. There's https://en.wikipedia.org/wiki/Thymol_blue , which is extremely widespread, usually in a form of universal indicator. Other indicators: https://en.wikipedia.org/wiki/PH_indicator
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j_sum1
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Phenolphthalein is not completely useless, byko3y.
It does have some advantages over bromothymol blue -- namely that the colourless to pink transition is a bit clearer to spot than a yellow to blue.
But I agree in principle that having a range of indicators at your disposal and knowing how to use the is a good idea.
Also worthy of note is that Phenolphthalein has four states -- two colourless, one ping and one a lovely orange colour.
https://en.wikipedia.org/wiki/Phenolphthalein#Indicator
[edit]
Sorry, I misread. I thought you said bromothymol blue. I don't recall if I have ever actually used thymol blue.
[Edited on 27-2-2017 by j_sum1]
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byko3y
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I don't say it's uselss, I just say it's shitty. But yes, the transition of color is slightly clearer for phenolphthalein than for thymol blue, you
can see in the wiki article that thymol blue goes from clear yellow to dark blue at 8.0-9.6 range, which is not that bad. But you need to be carefull
with phenolphthalein, which is not the case for our original poster of the thread.
[Edited on 27-2-2017 by byko3y]
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