Sciencemadness Discussion Board

Optimum indicator concentration?

fusso - 26-5-2018 at 00:44

I want to prepare EtOH solns of phenolphthalein and methyl orange. What's the optimum concentration? Too dilute will make colour too pale and need many drops and waste EtOH. Too concentrated will make every titration consume too much precious indicator even only 1 drop. Thx.

Disclaimer: I'm a minimalist

j_sum1 - 26-5-2018 at 01:18

https://www.google.com.au/search?q=preparation+of+phenolphth...

I think you could have done this yourself.
Practically, you are hard-pushed to purchase phenolphthalein in quantities less than 5g, and that much can last years. Minimalist or no, indicators are not a big expense.

DavidJR - 26-5-2018 at 10:22

Yeah, pre-made indicator solutions always feel like a total ripoff (cough says the person selling premixed bromocresol purple solution on eBay) but making your own is super cheap. I bought 25g of phenolphthalein for £3.60 (from betterequipped.co.uk). At 0.5% w/v this would make 5 litres of solution, which frankly is a lifetime supply for me.

fusso - 26-5-2018 at 13:11

Quote: Originally posted by j_sum1  
https://www.google.com.au/search?q=preparation+of+phenolphth...

I think you could have done this yourself.
Practically, you are hard-pushed to purchase phenolphthalein in quantities less than 5g, and that much can last years. Minimalist or no, indicators are not a big expense.

I did search about it, so I dunno whether I should use 0.5% or Nilered's 0.05% or even more dilute.
And you didn't mention methyl orange.
Also is substituting EtOH with i-PrOH ok?

aga - 26-5-2018 at 13:23

Everything is fine. It's all OK. Calm. Think happy thoughts.

The exact concentration only ever matters if you need to use it, even then, not so much.

LearnedAmateur - 26-5-2018 at 13:49

At school our phenolphthalein was always 1% w/v in ethanol, prepped on site. Not sure on the methyl orange and bromothymol blue though, but most likely between 0.5 and 2%.

DavidJR - 26-5-2018 at 15:00

I'd just go with 0.5% for phenolphthalein and be done with it. Isopropanol will work fine.