# Need a pH indicator that disappears after evaporation **URL:** \ **Category:** Factual Questions\ **Created:** [May 23, 2001, 4:06pm UTC](https://boards.straightdope.com/t/need-a-ph-indicator-that-disappears-after-evaporation/66308 "2001-05-23T16:06:58Z")\ **Posts on this page:** 13\ **Page:** 1
**Author:** ![Mr.Zambezi](https://avatars.discourse-cdn.com/v4/letter/m/7cd45c/32.png) [@Mr.Zambezi](https://boards.straightdope.com/u/Mr.Zambezi)\ **Post date:** [May 23, 2001, 4:06pm UTC](https://boards.straightdope.com/t/need-a-ph-indicator-that-disappears-after-evaporation/66308/1 "2001-05-23T16:06:58Z")
I am looking for a PH indicator that is soluable in water and/or ammonia. IT should be clear at around pH 7, but should give a dark color at around 8.6 and higher. THe point is that when in a solution of ammonia and water, there should be a strong color, but it shoudl disappear when it evaporates leaving no trace (like diappearing Ink.) I have been scouring the internet but am having trouble finding anything that meets the bill. A couple look promising but I can’t find where to buy the chemical. Any links to chemical supply companies that sell in small quantities would be a great help. ---
**Author:** ![MrDeath](https://avatars.discourse-cdn.com/v4/letter/m/22d042/32.png) [@MrDeath](https://boards.straightdope.com/u/MrDeath)\ **Post date:** [May 23, 2001, 7:57pm UTC](https://boards.straightdope.com/t/need-a-ph-indicator-that-disappears-after-evaporation/66308/2 "2001-05-23T19:57:28Z")
It’s not perfect, but bromothymol blue could do the trick. It’s greenish at pH 7, pale yellow below pH 6 and royal blue above pH 8. I have no idea what happens when the solution evaporates, but the usual solution is 0.04% bromothymol blue in water, so there ought not to be _much_ of anything left. If you could set things up so that your ‘ground state’ is pH 6, the colour change would be quite dramatic. It is also non-toxic and readily-and-cheaply available from many places on the net, and certainly at your local chemistry-supply shop too. And I have no idea how soluble it is in ammonia. ---
**Author:** ![Mr.Zambezi](https://avatars.discourse-cdn.com/v4/letter/m/7cd45c/32.png) [@Mr.Zambezi](https://boards.straightdope.com/u/Mr.Zambezi)\ **Post date:** [May 23, 2001, 10:06pm UTC](https://boards.straightdope.com/t/need-a-ph-indicator-that-disappears-after-evaporation/66308/3 "2001-05-23T22:06:37Z")
I have some bromothymol on order, but I am not thrilled about the color at pH7. Phenolphthalein would be perfect but it requires such a high Ph. Litmus may also be OK. What I would love to find is a really comprehensive chart of pH indicators, but I can’t find any on the web. There are nitrophenol, Cresolphthalein, and Naphtholbenzein, but I can’t find out what they are soluable in. I am not a chemist. THis is all very confusing and I really, Really REALLY need to figure this out. ---
**Author:** ![MrDeath](https://avatars.discourse-cdn.com/v4/letter/m/22d042/32.png) [@MrDeath](https://boards.straightdope.com/u/MrDeath)\ **Post date:** [May 24, 2001, 2:12am UTC](https://boards.straightdope.com/t/need-a-ph-indicator-that-disappears-after-evaporation/66308/4 "2001-05-24T02:12:12Z")
> [@](#): > > \*Originally posted by Mr.Zambezi \* > **What I would love to find is a really comprehensive chart of pH indicators, but I can’t find any on the web.** Something like [this](http://www.101science.com/PH.html)? Or maybe [this one](http://www.dudley-chem.com/list3.htm) is more your style? From the latter, I think m-nitrophenol is your likeliest bet, assuming it is soluble in ammonia. ---
**Author:** ![tomndebb](https://avatars.discourse-cdn.com/v4/letter/t/b9e5f3/32.png) [@tomndebb](https://boards.straightdope.com/u/tomndebb)\ **Post date:** [May 24, 2001, 3:07am UTC](https://boards.straightdope.com/t/need-a-ph-indicator-that-disappears-after-evaporation/66308/5 "2001-05-24T03:07:11Z")
I dropped a link to Carolina Biological Supply in your [Where cna I buy chemicals on-line?](http://boards.straightdope.com/sdmb/showthread.php?threadid=71844) thread. They are a science supply outfit for schools and students. ---
**Author:** ![False\_God](https://avatars.discourse-cdn.com/v4/letter/f/ee7513/32.png) [@False\_God](https://boards.straightdope.com/u/False_God)\ **Post date:** [May 24, 2001, 12:29pm UTC](https://boards.straightdope.com/t/need-a-ph-indicator-that-disappears-after-evaporation/66308/6 "2001-05-24T12:29:14Z")
Uh, this thread, taken into consideration with the “where can I buy chemicals” thread also here in GQ makes me think that something may be in a state of advanced decomposition in a small Scandinavian state. Remember, we’re fighting ignorance here but there’s some stuff you shouldn’t ask in an open-ended manner without atribution for your motives. People might get the wrong idea and set Echelon upon you. ---
**Author:** ![Mr.Zambezi](https://avatars.discourse-cdn.com/v4/letter/m/7cd45c/32.png) [@Mr.Zambezi](https://boards.straightdope.com/u/Mr.Zambezi)\ **Post date:** [May 24, 2001, 1:42pm UTC](https://boards.straightdope.com/t/need-a-ph-indicator-that-disappears-after-evaporation/66308/7 "2001-05-24T13:42:53Z")
Sure, false, we all know that pH indicators, especially colorless ones, when mixed with ammonia are used to make…what, meth? Nuclear weapons? Neurotoxin? Actually I have to be a little cagey because I am an inventor and I can’t tell you what I am making. But should the gov’t wish to investigate my motives, they certainly may. They will find only a crank inventor trying to prevent workplace injuries for the food service industry. Thanks for the links guys. If anyone else has info, please elet me know and I promise I will refuse to give even the NSA your name for fear not of reprisal, but because I don’t want your name on my patent. ---
**Author:** ![Spritle](https://avatars.discourse-cdn.com/v4/letter/s/5daacb/32.png) [@Spritle](https://boards.straightdope.com/u/Spritle)\ **Post date:** [May 24, 2001, 7:42pm UTC](https://boards.straightdope.com/t/need-a-ph-indicator-that-disappears-after-evaporation/66308/8 "2001-05-24T19:42:44Z")
::Chemist enters the room:: Did you say food service industry?!? Don’t even consider phenolphthalein; it’s a laxative!! Here’s the problem with what you are looking for. You want an indicator that evaporates away. Considering that indicators are ions that change colors when in an environment that is either electron rich (bases) or electron poor (acids) evaporation is not possible. The solvent that the indicator is in may evaporate, but the indicator ions themselves won’t. Looking at your ammonia example… Phenolphthalein is soluble in alcohols. Thus the liquid Phen is actually a tincture. In water it’s colorless and in bases it’s fucsia. The color change occurs at a relatively high pH, but no so high that it’s “insurmountable”; recall that the pH scale is a log scale. Anyway, back to the solution. Suppose we have water with a few drops of Phen added. It’s colorless. We add ammonia until it turns light pink. Picking up the base ions. As the ammonia evaporates from the solution (slowly) it becomes less basic and the color fades and disappears. However, the Phen is still there. This is just an example to show the problems with trying to get the indicator to “evaporate” away. **MrDeath** , having the indicator soluble in ammonia is not necessary. If the indicator is soluble in water, it can be used. So, too, if it’s soluble in alcohol. The only problem would be if it’s only soluble in lipids. (Then, we have to get into a discussion of crown ethers.) ---
**Author:** ![Mr.Zambezi](https://avatars.discourse-cdn.com/v4/letter/m/7cd45c/32.png) [@Mr.Zambezi](https://boards.straightdope.com/u/Mr.Zambezi)\ **Post date:** [May 24, 2001, 9:52pm UTC](https://boards.straightdope.com/t/need-a-ph-indicator-that-disappears-after-evaporation/66308/9 "2001-05-24T21:52:09Z")
Sprittle, I was not clear enough. I don’t care if the indicator evaporates. I just want the _color_ to disappear when the solution evaporates, exactly like disappearing ink. SO the ammonia and water evaporate and the liquid goes from some color to nothing. Since the concentrations are less than 1%, I am not concerned about the leftover indicator. Also, the indicator will be in an ammonia solution and will not be used on any areas that are touched by food. I think that thymol is less dangerous to humans than ammonia and phosphoric acid. So do you have any suggestions for a good indicator and where I can buy it? Now since you mentioned it, a lipid soluble indicator would be verrry helpfull. Care to point me to a few? ---
**Author:** ![Mr.Zambezi](https://avatars.discourse-cdn.com/v4/letter/m/7cd45c/32.png) [@Mr.Zambezi](https://boards.straightdope.com/u/Mr.Zambezi)\ **Post date:** [May 24, 2001, 9:59pm UTC](https://boards.straightdope.com/t/need-a-ph-indicator-that-disappears-after-evaporation/66308/10 "2001-05-24T21:59:38Z")
**Chronos** , they _were_ separate threads: 1) What is the right indicator and 2) where can I buy chemicals on line. That is, until everyone started talking about building bombs and the government getting involved and questioning my motives. Then I had to explain why. Hell, I haven’t posted for 3 months and now I get yelled at for asking **TWO** questions! Sheesh! ---
**Author:** ![tomndebb](https://avatars.discourse-cdn.com/v4/letter/t/b9e5f3/32.png) [@tomndebb](https://boards.straightdope.com/u/tomndebb)\ **Post date:** [May 25, 2001, 1:14am UTC](https://boards.straightdope.com/t/need-a-ph-indicator-that-disappears-after-evaporation/66308/11 "2001-05-25T01:14:27Z")
Sorry I sandbagged your thread, **Mr. Z**. I hadn’t realized that the sense (or recognition) of humor in smart alec remarks had fallen so low, over here. ---
**Author:** ![Mr.Zambezi](https://avatars.discourse-cdn.com/v4/letter/m/7cd45c/32.png) [@Mr.Zambezi](https://boards.straightdope.com/u/Mr.Zambezi)\ **Post date:** [May 25, 2001, 3:11pm UTC](https://boards.straightdope.com/t/need-a-ph-indicator-that-disappears-after-evaporation/66308/12 "2001-05-25T15:11:02Z")
Everybody justs needs to lighten up. It is not your fault tomndeb. I will make sure that my next post contains a disclaimer waiving all liabilities to any poster and a discussion of my reasons for asking te question. ---
**Author:** ![Adrigole](https://avatars.discourse-cdn.com/v4/letter/a/919ad9/32.png) [@Adrigole](https://boards.straightdope.com/u/Adrigole)\ **Post date:** [January 20, 2012, 11:07pm UTC](https://boards.straightdope.com/t/need-a-ph-indicator-that-disappears-after-evaporation/66308/13 "2012-01-20T23:07:36Z")
> [@MrDeath](#): > > It’s not perfect, but bromothymol blue could do the trick. It’s greenish at pH 7, pale yellow below pH 6 and royal blue above pH 8. I have no idea what happens when the solution evaporates, but the usual solution is 0.04% bromothymol blue in water, so there ought not to be _much_ of anything left. If you could set things up so that your ‘ground state’ is pH 6, the colour change would be quite dramatic. It is also non-toxic and readily-and-cheaply available from many places on the net, and certainly at your local chemistry-supply shop too. And I have no idea how soluble it is in ammonia. Thank you Mr death and others in the thread : would you have an opinion on what might be suitable as a non toxic ( ie to microrganisms) at a low ph ( eg circs 3-4) : Ideally we are looking for a dramatic colour change - maybe his could be achieved by buffering the solution - any suggestiosn on buffers? Thank You