Sciencemadness Discussion Board

Phenolphthalein Decomposition

agorot - 8-8-2010 at 18:52

Recently I made 200 mL of Nitric Acid and have it stored in an amber glass bottle with a teflon-sealed cap. I wanted to be sure the bottle wasn't leaking though, so I took a small test tube and filled it with NaHCO3 solution, and then added a few drops of phenolphthalein 1% solution (it turned pink just like you would expect it to), then taped this test tube to the bottle in an upright position, and then put the whole thing in a large ziploc bag to ensure that vapors would be largely contained.

I thought that if the bottle was leaking, the vapors would mostly float around in the bag and would dissolve in the NaHCO3 solution and make it neutral or acidic, and so the liquid in the test tube would turn clear and I would know that the bottle was leaking.

I did all of this two weeks ago.

I checked the bottle again tonight. To my horror, I found a very transparent liquid in the test tube. I ran to my desk and threw on my gas mask and nitrile gloves, and then I went as calmly as I possibly could to my back yard with the bottle. Then I took some universal indicator pH paper and wetted it with distilled water, and opened up the bag just a little and inserted the paper.

Nothing happened.

Next, I took the pH paper and dipped it in the liquid of the test tube, and it turned a deep blue color indicating base. I determined that the bottle's seals were good and my lungs were safe.

So now I'm wondering why the liquid is clear? Is it possible that after a long time, basic conditions can decompose phenolphthalein?

EDIT: It just occurred to me that all of the ethanol in which the PHPH was dissolved could have evaporated, and since PHPH is insoluble in water, there was no color being exhibited. I just added some 95% ethanol to the test tube and it did turn a very faint pink, but I stress the faintness of this pink. The depth of the color is not what it was originally.

Perhaps some of the PHPH did decompose then? Any ideas would be much appreciated. I'm soon going back to school and will be leaving this stuff at home, and I want my parents to check on this stuff while I'm gone to make sure the bottles are fine, etc, and I wanted an easy way for them to tell that the bottles weren't leaking.

[Edited on 9-8-2010 by agorot]

densest - 8-8-2010 at 21:34

It's a matter of proportions & math... in your bag, the tiny amount of nitrogen oxides escaping had two weeks to deal with (oxidize, change color, whatever) the phenophthalein. One minute is thousands less time to affect the pH paper.

HNO3 outgasses. Depending on purity and temperature, it is a lot or a little. So store your HNO3 in as cold a place as you can, in as clean a bottle as you can, away from light.

My analytical grade HNO3 in its original bottles dissolved the plastic sealing band around the cap in about 2 years. I keep a plate covered with sodium bicarbonate in the cabinet with the bottles now.

woelen - 8-8-2010 at 23:10

Try adding some NaOH to your test tube with almost colorless phenolphtalein. Does it turn deep pink? If so, then it did not decompose and then some acid went into the test tube. If you used NaHCO3 to start with (which itself is very weakly basic), then a VERY tiny amount of acid is enough to decolorize the phenolphtalein.

entropy51 - 9-8-2010 at 16:32

Quote: Originally posted by agorot  
To my horror, I found a very transparent liquid in the test tube. I ran to my desk and threw on my gas mask and nitrile gloves, and then I went as calmly as I possibly could to my back yard with the bottle.
Glad you didn't over react.:D Remain calm!

Magpie - 9-8-2010 at 17:40

I keep my nitric acid in a ground glass stoppered reagent bottle in a plastic tub and don't give it a second thought.

My HCl is the one that has given me trouble. I keep that in a glass bottle with a PTFE lined lid. It probably still leaks gas, but slowly.

These are my "day use" bottles kept in my lab in my garage. My 2.5L acid bottles I keep in a garden shed.

agorot - 9-8-2010 at 20:08

Quote: Originally posted by woelen  
Try adding some NaOH to your test tube with almost colorless phenolphtalein. Does it turn deep pink? If so, then it did not decompose and then some acid went into the test tube. If you used NaHCO3 to start with (which itself is very weakly basic), then a VERY tiny amount of acid is enough to decolorize the phenolphtalein.


Just did. Almost no color change.

I also did another thing. I took a watch glass and put 3 drops of 1% Phenolphtalein on it and then a flake (~.5g) KOH on it. It immediately turned a deep pink color. I then left it for an hour, and upon returning, everything had evaporated. I then put some 95% ethanol on it and it did not change color to pink again. PHPH is a solid powder, so it should not have evaporated away, so the only thing I can think of is that the PHPH was destroyed...

Try it yourself, exactly as I have described above.

unionised - 10-8-2010 at 12:43

Under strongly alkaline conditions (pH >12 or so) phenolphthalein is doubly deprotonated and goes clear again.

http://en.wikipedia.org/wiki/Phenolphthalein

Also, phenoxide ions like the sodium salt of this stuff are very easily oxidised by air.

agorot - 10-8-2010 at 12:56

Quote: Originally posted by unionised  
Under strongly alkaline conditions (pH >12 or so) phenolphthalein is doubly deprotonated and goes clear again.

http://en.wikipedia.org/wiki/Phenolphthalein

Also, phenoxide ions like the sodium salt of this stuff are very easily oxidised by air.


I don't think that the pH is above 12 because all I've added is maybe 2g of NaHCO3 and .5g NaOH...

But the ions were definately in contact with air. That could have been the issue.

Well, does anyone have any other ideas on how to make an indicator system to easily check if bottles are leaking? I can't keep this stuff at 0 degrees because I don't have a separate refrigerator, and I'd rather not put it next to my strawberries.