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Topic: Colour of Phenolphthalein at Certain pH  (Read 348 times)

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Offline MJCormier

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Colour of Phenolphthalein at Certain pH
« on: February 16, 2021, 01:14:00 PM »
I am posting this in the undergrad section because I believe the level of the course I am taking and the question I have is closer to year 1 university chemistry than to highschool chemistry.

I am a Year 2 IB student and am doing a paper on an Acid-Base titration of excess 1M HCL with a differing # of tablets of a certain Calcium Carbonate (tums) antacid tablet, and 2M NaOH added until the Phenolphthalein indicator turns pink (at Basic). I ran my experiments until the indicator turned a dark/solid pink and got consistent results that seemed to follow my hypothesis, but I did not realize when doing the experiment that the solution does not turn pink at neutral, but rather at a pH that is basic. Can anybody help me find out what pH the solution turns solid pink? I've seen results online saying anything from 8.0pH to 10pH and that is a very wide range with the concentrations and amounts used.

I am not going to redo the 25 observations.

Offline chenbeier

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Re: Colour of Phenolphthalein at Certain pH
« Reply #1 on: February 16, 2021, 02:39:58 PM »
Every indicator has a turning point of colour at a certain pH. For phenolphthalein its pH 8.2 . Now it depends which titration you do , which acid and alcaline compound is involved. If you titrate NaOH with HCl then there is a sharp change in the curve. Its easy to see. In your case it should be ok. If you take acetic acid or ammonia then you have to consider the turning point.  Go in a table book and choose the right indicator. Or better use a pH probe and read the pH Numbers.

Offline Borek

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Re: Colour of Phenolphthalein at Certain pH
« Reply #2 on: February 16, 2021, 05:14:50 PM »
The color doesn't change "at pH", it changes over a pH range.

See discussion here: http://www.titrations.info/acid-base-titration-indicators
ChemBuddy chemical calculators - stoichiometry, pH, concentration, buffer preparation, titrations.info, pH-meter.info

Offline DrCMS

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Re: Colour of Phenolphthalein at Certain pH
« Reply #3 on: February 17, 2021, 04:48:02 AM »
I ran my experiments until the indicator turned a dark/solid pink

This is not the ideal way to do titrations with colour changing indicators.  Your results might be consistent but are likely to be wrong.  Aim to titrate until you see the faintest colour that does not disappear when mixed.

Offline MJCormier

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Re: Colour of Phenolphthalein at Certain pH
« Reply #4 on: February 17, 2021, 08:29:11 AM »

Your results might be consistent but are likely to be wrong.  Aim to titrate until you see the faintest colour that does not disappear when mixed.
The reason why I did not stop at the faintest colour was 1: because I did not really know to do so (teacher told me otherwise) and 2: the solution of calcium carbonate and HCl was cloudy and hard to see the faint change.

I know they are wrong either way because I didn't look far enough into how the indicator worked and my teacher had told me just to use the pheno. that being said since I took the measurements at the same point (same pH) for each observation, I can still calculate what the point of neutralization is. I just don't know exactly what pH I was taking my measurements at, which is my fault. Maybe I will redo 3 or 4 observations that cover the range of my data and when I reach the indication that I had before, I can check the pH with a digital pH meter. Would that work to some extent?

I could then use the range of that pH found in order to find the uncertainty of my observations.

Offline MJCormier

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Re: Colour of Phenolphthalein at Certain pH
« Reply #5 on: February 17, 2021, 08:36:09 AM »
it changes over a pH range.

Yes, but if I stop my observations at the same point, should the pH not be roughly the same. I would not expect the range of pH to be over 2 if I am stopping the observations at the same point, and if that was the case my results would be much wider.

I know I am not really giving enough information for anyone to calculate the uncertainty in pH and I don't expect anyone to do that, I am just wondering if that point can be roughly calculated.

Offline Meter

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Re: Colour of Phenolphthalein at Certain pH
« Reply #6 on: February 18, 2021, 05:03:00 AM »
The point is that at very slightly pink, you know you are at the beginning of the pH range where the indicator starts changing, whereas at "definitely pink" and "a little more pink than that", it is hard to tell where in the range you are. Plus, it's easier to see when something goes from not-pink to slightly pink, but harder to accurately gauge transitions in shades of pink.

I don't know what to say, you botched your experiment. If we did something like that in our titration labs, we were told to just scrap it and start over.

Offline MJCormier

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Re: Colour of Phenolphthalein at Certain pH
« Reply #7 on: February 18, 2021, 12:55:14 PM »
you botched your experiment.

I definitely see what you're saying. And if the deadline for the paper was not so close to when it was assigned, I would definitely re-do or change the method. That being said, I think I am just going to re-do a few observations with the exact same conditions as before, however, when I reach the point of the indicator being that dark pink, I will use a pH meter to see what it says. that way I shouldn't have to redo the experiments as I don't have time at this point. I wouldn't consider it totally botched, just not planned well enough.

Thanks.

Offline Meter

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Re: Colour of Phenolphthalein at Certain pH
« Reply #8 on: February 18, 2021, 01:36:51 PM »
you botched your experiment.

I definitely see what you're saying. And if the deadline for the paper was not so close to when it was assigned, I would definitely re-do or change the method. That being said, I think I am just going to re-do a few observations with the exact same conditions as before, however, when I reach the point of the indicator being that dark pink, I will use a pH meter to see what it says. that way I shouldn't have to redo the experiments as I don't have time at this point. I wouldn't consider it totally botched, just not planned well enough.

Thanks.
Hey, sometimes explaining very clearly exactly how and where you messed up shows understanding. In HS, we could get full points on certain assignments if we could explain the theory with sufficient accuracy, even if our data was corrupted for whatever reason. Hope it goes well, cheers.

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