titration indicators (1 Viewer)

[abc123doremi]

weak acid-strong base --- phenol
strong acid-strong base --- bromo blue
strong acid-weak base -- meth o

so when we did prac titrating HCl and NaOH why did we use phenolphthalein instead of bromothymol blue?

coz it's just easier to see colour transition from clear to salmon pink rather than from yellow to green? or is NaOH a wayyyyy stronger base than HCl is an acid? i'm sure HCl is really strong as well, so is it just like coz we're used to it?

and does it even really matter?


and bonus question (ahem)

why is the equivalence point not always neutral point (pH 7)
i mean, if you're doing a neutralisation reaction, then why do you get an equivalence point that's like basic or acidic. doesn't that mean it's not neutral (well duh), so then how can you say they've neutralised

i suppose that strong base + weak acid -> basic salt, is that something to do with it?

 

[dp624]

no, it doesnt matter what you use, really, for strong-strong titrations
cos within the addition of one drop the pH moves from like 5 to 9 in one jump.
so any of the 3 indicators is fine.

and its easier to see too, lol



and for q2:
say you have NH3 and HCl
At equivalence, you have equal amounts of NH3 and HCl. It's kinda like having a solution of NH4Cl
NH4CL is acidic.

etc

 

[abc123doremi]

alright thanks

 

[Pwnage101]

yeh, my techer said even though bromothymol blue changes in mid range, for strong acid-strong base u use either methyl orange or phenolphthalein (the latter ebing better because of the fact that colourless and pink are very distinct from each other)

if you look at the titration curve of a strong acid - strong base titration like this: http://img.sparknotes.com/figures/3/3a5994498f24d59f5d5d762b40844a2a/sasb.gif , you'll see that yeh the line is so steep that when it reaches a pH of 9, and the pheno changes colour, that is approx. the equivalence point and the volume added is just about equal to that for a pH of 7