Sciencemadness Discussion Board

Indicaor problems

Kagutsuchi - 26-6-2015 at 08:40

I used fenolftaleine to see when my KOH solution becomes neutralised by HCl, but when I added it to the very-very concentrated KOH and the fenolftaleine gave no color at all! What could have possibly caused it? Also, if someone has a paper on a broad number of indcators, I'd be happy if he shared it with me.

blogfast25 - 26-6-2015 at 08:54

Quote: Originally posted by Kagutsuchi  
I used fenolftaleine to see when my KOH solution becomes neutralised by HCl, but when I added it to the very-very concentrated KOH and the fenolftaleine gave no color at all! What could have possibly caused it? Also, if someone has a paper on a broad number of indcators, I'd be happy if he shared it with me.


It's normal: above pH 13 that indicator is colourless:

https://en.wikipedia.org/?title=Phenolphthalein#Indicator

Start neutralising and the fuchsia will appear. Then continue adding HCl until it disappears.

But neutralising very concentrated KOH solutions can be dangerous: very considerable (neutralisation) heat is evolved, even more so when the solubility limit of KCl is exceeded and solid KCl forms: the dissolution of KCl is endothermic, so the precipitation is exothermic. Splattering of a highly caustic solution is a real concern, trust me: first hand experience! :( Slow addition of HCl with intermittent cooling is strongly advisable!

[Edited on 26-6-2015 by blogfast25]