Litmus as an indicator... (1 Viewer)

[Lani]

Hey all those unfortunate enough to have kept chemistry,
So, I'm trying to study for trials and I keep coming across questions about litmus as an indicator. In one it meantions that it turns red to blue from pH 5 - 8. And, in another says it is not used for volumetric anaylsis.
In both it basically asks you to explain why its not useful in volumetric anaylsis...
Does it have something to do with its fairly wide range? Or that it would need to be strong acid/strong base or weak acid/weak base to be around 7? Or is it that it only comes in paper form?
I'll laugh if its the last of my options...
Thanks!

PS I would ask my chemistry teacher, but I know she won't know the answer... I sometimes wonder whether we know more from teaching ourselves the course from the textbook than she does from teaching students for 20 years.

 

[-=«MÄLÅÇhïtÊ»=-]

your guesses are correct.
litmus does indicate acid/base change, but it's unsuitable for titration. The only reasonable explanation to that is litmus is made from fungi like substance? and it may interfere with the measurements.

 

[jims]

have u tried dipping that tiny piece of litmus into a flask and getting it out again??

its unsuitable because it doesnt change colour as the pH changes. once its red or blue or green or rainbow, it stays like that. stuff like phenothaline n methyl orange changes colour so u can tell wen the endpoint comes.

 

[underthesun]

Or is it because litmus don't have any of the endpoint at 7? I mean, for titration, isn't it better to have one of the endpoint at 7 so that when it reaches neutral state, it just stops changing colour. Or am I wrong?

thats the answer i'd give in an exam, provided that I did not study

 

[jims]

u need an indicator that has an endpoint suitable for ur titration.
if ur doing a weak acid n a strong base, ull want an indicator that changes in the slightly basic range like phenopthaline (pH 8-9).
but if ur doing a stong acid n a weak base ull want something like methyl orange (pH 3-4).
stong acid/base then it doesnt matter.

 

[inasero]

and on your point of litmus, it doesnt necessarily have to be in paper form, litmus is actually a liquid derived by boiling the litmus lichen in water....
what they then do is add a little sulfuric to make red litmus solution, and the paper form u are acquainted with is essentially normal paper dipped in this liquid....
blue litmus paper on the other hand is paper dipped into litmus solution which has a little base (typically sodium hydroxide) added to it.

 

[+:: $i[Q]u3 ::+]

isn't it supposed to be because litmus is actually a really not sensitive indicator? my teacher said that i think...

 

[mitochondria]

Originally posted by +:: $i[Q]u3 ::+
isn't it supposed to be because litmus is actually a really not sensitive indicator? my teacher said that i think...

hmm... an interesting one... because indicators are mostly weak acids which works on the principle of equilibrium displacement and the colour of the ions that it produce (which is different to its undisssociated colour). That is, if the concentration of H+ is high, it will not dissociate. Conversely, in a basic solution, the concentration of H+ is low, the indicator dissociate more relative to when it is being placed in an acid. Therefore the colour depends on the number of the other component ion of the indicator (ie. X- if the indicator has a formula of HX). So I don't see why is it not "sensitive" if it has been used as an indicator for such a long time... it would simply change colour if you give it a few shakes provided that it's time for change ... anyways

Originally posted by Lani
... it turns red to blue from pH 5 - 8. And, in another says it is not used for volumetric anaylsis....Does it have something to do with its fairly wide range?,

Yup, you are definitely correct with your first option It is not suitable to be used as an indicator because its range is too wide.. when you press a few buttons on your calculator or just simply count your fingers, that's a blanky 3 in difference.. when you look at other indicators, e.g. phenolphthalein (pH 8.3-10); methyl orange (pH 3.1-4.4); or bromothymol blue (pH 6.2-7.6)... etc, most of them have a range of less than 2... Now you look at it, and you know you are right



*figures are drawn from Conquering Chemistry*
Note: the pH range of litmus provided by Conquering Chemistry is apparently inaccurate... *pH 6-8*, that could well destroy my argument sooo I had a look on the internet and most of them show at least 2.5 in difference... However, I went on further to seek authority in the "Oxford Dictionary of Science" it has 4.5-8.3 for litmus *satisified*
Information are based on the "Oxford Dictionary of Science" and Le Chatelier's Principle

 

[Lani]

Thanks everyone... espically mitochondria for the extent you've been to try answer! I understand what you've all said, and although I'm still not really completely satisfied, I'll deal.
Why can't there just be one right answer? ahh... yea... this is chem...
Thanks heaps again!

 

[mitochondria]

what don't you understand Lani? Just tell us I mean, don't bring anything you don't understand to sleep or it'll come and bite you later - in any form and shape (heheee). Okay, maybe I should say this:

YOUR FIRST OPTION WAS DEFINITELY RIGHT - IT WAS JUST BECAUSE OF THE RANGE AND JUST BECAUSE OF THE WIDE RANGE MAKES IT UNSUITABLE FOR TITRATION/VOLUMETRIC ANALYSIS.

If you are still not satisified, just spill it all out here I'm sure many of us are more than happy to help All the best with your HSC!