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Author Topic: pH transition range confusion  (Read 4291 times)

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perwaaler

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pH transition range confusion
« on: February 13, 2013, 02:36:27 AM »

hi. Ive done this experiment that involves using a pH indicator called neutral red in order to get a rough estimate of its acidity. It's transitional range is 6,8-8,0. basically red=acidic, and yellow=basic. Im somewhat confused as to what this means in practice. Ive done a lot of googling, and this is what ive come up with:

According to wikepedia, it means that the coulour of the soulution will change somewhere within this pH interval. A solution with ph higher than 8  will allways be yellow, and a solution with pH lower than 6,8 will always be red. But within that pH interval the colour could go either way.

according to that definition, a red solution could have a pH value of 6,8 aswell as a pH value of say 7,5, cause we dont know at which point within this intervall its going to switch colours, it just means it has to be somewhere within that range.

according to another website, these two pH values - 6,8 and 8,0 - are the endpoints. a solution(with neutral red in it) will change colour at these two and only these two values(called the endpoints). a red solution(acidic) thats being titrated with a base will change over to yellow when the pH is 8,0. A yellow solution will switch to red at the pH value of 6,8. according to this definition, the point at which the solution changes colour is not some random point somewhere the transitional intreval. its fixed at either 6,8 or 8,0.

heres the link to the website http://science.jrank.org/pages/3552/Indicator-Acid-Base.html

the questions the paper is asking, is what conclusion can be drawn about the pH value of the liquid from looking at its colour(when neutral red has been added).

so yea.... im confused. can anyone help me clarify?
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Borek

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Re: pH transition range confusion
« Reply #1 on: February 13, 2013, 03:33:28 AM »

Below 6.8 it is red as the solution is dominated by the protonated molecule.

Above 8.0 it is yellow as he solution is dominated by the non-protonated molcule (conjugate base of the red form).

Between 6.8 and 8.0 you have a mixture of both yellow and red forms, and the color changes gradually.

Ratio of the concentrations of both forms is easy to calculate for a given pH if you know indicator pKa. Compare http://www.titrations.info/acid-base-titration-indicators
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perwaaler

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Re: pH transition range confusion
« Reply #2 on: February 13, 2013, 10:31:40 AM »

so does that mean that if the solution is yellow i can tell for certain that pH is above 8,0? and that if its red it is below 6,8?

I've been doing some more reading, and found to my surprise that indicators change colour gradually as they pass through their transition range. this makes allot more sense of the idea of a transition range, but does puzzle me, since it has always seemed to me that the transition from one colour to another happens very abruptly, and not gradually at all.

I'm thinking specifically about a titration experiment where we used the indicator to tell when the equivalence point was reached. think it was titration of a weak acid with a strong base - NaOH. dint remember the exact details, but i remember that we had to be extremely alert because the change of colour was very sudden, and we would have to stop the titration immediately as that happened.
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Borek

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Re: pH transition range confusion
« Reply #3 on: February 13, 2013, 10:57:01 AM »

so does that mean that if the solution is yellow i can tell for certain that pH is above 8,0? and that if its red it is below 6,8?

Yes.

Quote
I've been doing some more reading, and found to my surprise that indicators change colour gradually as they pass through their transition range. this makes allot more sense of the idea of a transition range, but does puzzle me, since it has always seemed to me that the transition from one colour to another happens very abruptly, and not gradually at all.

For a single molecule color is either yellow, or red, but the mixture is at acid-base equilibrium and contains both kinds of molecules. This is explained on the page I linked to, seems like you have not read it. Please do.

Quote
I'm thinking specifically about a titration experiment where we used the indicator to tell when the equivalence point was reached. think it was titration of a weak acid with a strong base - NaOH. dint remember the exact details, but i remember that we had to be extremely alert because the change of colour was very sudden, and we would have to stop the titration immediately as that happened.

That's because when you are close to the equivalence point single drop changes pH by several units. See http://www.titrations.info/titration-end-point-detection for more details.
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Arkcon

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Re: pH transition range confusion
« Reply #4 on: February 13, 2013, 01:15:54 PM »

Phenolphthalein is abrupt, it is either clear or pink.  But slightly orangeish-yellow turning to orange can be a little hard to recognize.
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perwaaler

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Re: pH transition range confusion
« Reply #5 on: February 13, 2013, 11:06:38 PM »

I forgot about that! now i remember that pH changes quickly because the buffert system has been worn out, sort of. It all makes sense now!

Thanks alot guys!
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Borek

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Re: pH transition range confusion
« Reply #6 on: February 14, 2013, 12:08:47 AM »

Phenolphthalein is abrupt, it is either clear or pink.

Not exactly - pink get deeper with the raising pH. Actually that's why concentration of an indicator matters when titrating against a single color indicator.
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