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Author: Subject: PH indicators.
beeludwig
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[*] posted on 10-6-2019 at 16:18
PH indicators.


Hey all quick (hopefully) question. I teach high school chem. When I ordered my pH indicator weeks ago I ordered phenolphthalein but what they sent was bromothymol blue. Mix up with the bio people I think (they don’t have it. I asked. Just as a school we now have several decades worth of this one chemical now.). My question is would a typical strong acid/base titration still work? Or would the pH just blow through indicator range. Would a weak acid/base titration work better? I’ve got loads of store bought vinegar from previous projects that we could titration the hell out of.
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DraconicAcid
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[*] posted on 10-6-2019 at 16:31


Bromothymol blue will work fine for the titration of a strong acid with a strong base (or vice versa). It will not work as well for the titration of a weak acid such as vinegar. If you're stuck for an indicator for vinegar, you could use cabbage juice (which turns blue- green at the endpoint. Yellow is overshot).



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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j_sum1
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[*] posted on 10-6-2019 at 16:32


Bromothymol blue transitions at ph 7.6 compared with phenolphthalein's 8.3.
That may make a difference, but probably not in most practical situations.
However, the phenolphthalein transition is very noticeable and very abrupt. Colourless to pink is a lot easier to spot than yellow to blue. So, I think phenolphthalein is to be preferred.




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beeludwig
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[*] posted on 10-6-2019 at 16:38


Quote: Originally posted by DraconicAcid  
Bromothymol blue will work fine for the titration of a strong acid with a strong base (or vice versa). It will not work as well for the titration of a weak acid such as vinegar. If you're stuck for an indicator for vinegar, you could use cabbage juice (which turns blue- green at the endpoint. Yellow is overshot).


Ok. That’s what I thought. I read something, I can’t remember where, saying it was better for weak acids. It might have made me overthink it, and most of the lab protocols are for phenolphthalein
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PirateDocBrown
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[*] posted on 11-6-2019 at 02:32


Making phenolphthalein is not difficult, if you have phenol and phthalic anhydride.

It can also be isolated from many consumer products.

It's also pretty cheap from chemical suppliers.




Phlogiston manufacturer/supplier.

For all your phlogiston needs.
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beeludwig
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[*] posted on 11-6-2019 at 03:33


oh yeah. It's not expensive at all. The only reason i even worry about it at all is because it's for a school lab, by the time I ordered it myself and it shipped the school year would literally be over. I'll put it on my requisition list for next year. I'm just kicking myself that I didn't notice earlier. I could have sworn we had phenolphthalein somewhere.
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Ubya
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[*] posted on 11-6-2019 at 03:41


a titration of strong acid/base works well with bromothymol blue, as the usual equivalence point (ph 7) is in the indication range of the indicator




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beeludwig
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[*] posted on 11-6-2019 at 08:41


Update: Did the lab, worked great. Not sure why I fussed about it. I blame end of year stress. I do prefer the Phenolphthalein though, it really does have a nice crisp transition. But students had a big kick over the whole thing. It's not like they would know the difference. But the whole "one drop" thing just blew their minds. So much fun.
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