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Author Topic: Phenolphthalein Questions  (Read 1740 times)

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schmidling

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Phenolphthalein Questions
« on: February 05, 2013, 06:06:00 PM »

Phenolphthalein is not soluble in water so it is dissolved in alcohol for use in pH testing as everyone here know but I only learned recently in the process of learning soap chemistry.

It turns pink at pH 8.2 so it is taught that putting a drop on a bar of soap will indicate a safe soap if the drop does not turn pink.

To measure the Total Alkaline (TA) in soap, a sample is dissolved in alcohol to which phph is added and if it turns pink, citric acid is added until the pink goes away and if we measured everything carefully, we know how much TA is in the soap.,

Most of the time I have made this test, the sample does not turn pink so I have wasted a lot of time.  Seems like I could have learned this on the bar itself without  all the dissolving and measuring but it does not seem easy to test the bar and it has something to do with the insolubility of the phph.

Putting a drop of phph on soap is  kind of nutty, sometimes turning pink, sometimes not and sometimes adding a drop of water or alcohol changes everthing.

Most recently, I have a liquid soap that measures pH 10.5 on a meter, a test strip turns green which is supposed to mean 8 and dropping phph into a sample does nothing.  When I add a drop of water it instantly turns pink and totally confuses me.

So the questions are:  why does the phph have to be dissolved in alcohol?

Why does it not turn pink until water is added as in the above?

Thanks,

Jack Schmidling 
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Arkcon

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Re: Phenolphthalein Questions
« Reply #1 on: February 06, 2013, 01:52:14 AM »

Briefly, phenolphthalein is a large organic molecule, and it doesn't dissolve in water.  A tiny amount in a solution of alcohol makes a decent solution.  However, phenolphthalein is a large, organic acid, and its structure changes, depending on whether its neutralized, or not -- and its color changes, depending on its structure.  But acid-base neutralization only happens in water.  So you have the system, use a small amount of alcoholic indicator solution in your water solution of your soap, for a qualitative test -- pink color if there's excess base, or a quantitative test -- how much acid needed to turn pink soap solution clear equals how much excess base you have.

I don't really see what the problem is:  your soap may have excess base, but we can't tell until its dissolved in water to test it.  Then again, dry, on  a shelf, it can't hurt anyone, so we get existential as to if there is base there, or not.  ;D
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schmidling

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Re: Phenolphthalein Questions
« Reply #2 on: February 06, 2013, 04:11:06 AM »


I don't really see what the problem is:  your soap may have excess base, but we can't tell until its dissolved in water to test it.  Then again, dry, on  a shelf, it can't hurt anyone, so we get existential as to if there is base there, or not.  ;D

I  have two problems:

1.  The TA test has one dissolving the soap sample in alcohol with a drop of phph in it.

2.  The test cited to was on a liquid soap which is about 75% water and the phph showed no color until a drop of water was added.

After the paste has saponafied, an approximately equal amount of water was added to get the viscosity I wanted so why does water have to be added after the phph?

js
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Arkcon

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Re: Phenolphthalein Questions
« Reply #3 on: February 06, 2013, 05:26:45 AM »

Because you need some water for the excess base to dissolve.  You need some water to disperse the soap paste.  You need some water so the phenolphthalein, in alcohol solution, can find the base, the soap (which is likewise a weak base) and any free acids (unsaponified fatty acids from your fats, or the citric you've added for a quantifying test) and then the phenolphthalein can react, to form the pink color, or not.
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Tittywahah

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Re: Phenolphthalein Questions
« Reply #4 on: March 13, 2013, 11:48:52 PM »

Wonder if you could tell me Why you need to add water?  For example I read on a scientific lab report where they used 0.1g of Phph to to just 95% ethanol 5% water, although I must say the usual rule seems to be 0.1g to 50:50mLs water ethanol.  But there seems to be absolutely no agreement about ratios of water to ethanol, just the usual 0.1  0.05 amounts in 100mLs.  Besides, water and alchohol make milky phph at least when I do it.
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schmidling

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Re: Phenolphthalein Questions
« Reply #5 on: March 14, 2013, 03:36:49 AM »

The problem I am pondering is possible misinformation on all of the soap making lists.

Most of the people just trust to the lye tables or at most, tongue the finished soap for the so called zap test.

The more advanced makers put a drop of phph on their bars and if it does not turn pink it is deemed safe to use or sell.

My experience is that it almost never turns pink even if the pH is over 10 on a meter unless stirred into the bar with a bit of water and then it almost always does turn pink.
 
Fresh soap is rarely under 10 and frequently takes days or weeks to get there if ever.

Not being a chemist, I am reluctant to point out to the nice ladies that their tests are invalid the way they are doing it.

As it turns out pH 10 is perfectly safe but they all claim their soap is nominally pH8 because it passes the phph test.

js
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Tittywahah

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Re: Phenolphthalein Questions
« Reply #6 on: March 14, 2013, 03:56:53 AM »

Hi, well, I had a thought reading your thread and it may or may not be an idea for you, but in this situation I would get hold of a PH Litmus paper with a fixed range, say for example, 6.5 - 8.5. This would be the cheaper suggestion. They go in 0.5 increments, I have just ordered some as a back up with my phph so that I can narrow down the accuracy in titrations and neutralisations.
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schmidling

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Re: Phenolphthalein Questions
« Reply #7 on: March 14, 2013, 06:28:58 AM »

Hi, well, I had a thought reading your thread and it may or may not be an idea for you, but in this situation I would get hold of a PH Litmus paper with a fixed range, say for example, 6.5 - 8.5. This would be the cheaper suggestion. They go in 0.5 increments, I have just ordered some as a back up with my phph so that I can narrow down the accuracy in titrations and neutralisations.

I have a good pH meter but I was thinking about the people who rely on phph for soap without adding a bit of water.

js
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Arkcon

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Re: Phenolphthalein Questions
« Reply #8 on: March 14, 2013, 09:52:26 AM »

I only tried making soap at home once, and it didn't go well, so I've stopped ...

The problem I am pondering is possible misinformation on all of the soap making lists.

Most of the people just trust to the lye tables

Might work adequately well, for very specific, limited recipes.  Probably best for fats like olive oil, which have a very defined composition, and less good for animal fats, especially mixed sources.

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or at most, tongue the finished soap for the so called zap test.

A hah ha ha.  Oh wait, you're serious.  Yikes.

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The more advanced makers put a drop of phph on their bars and if it does not turn pink it is deemed safe to use or sell.

Hmmm ... no, that's not what I've suggested in this thread.

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My experience is that it almost never turns pink even if the pH is over 10 on a meter unless stirred into the bar with a bit of water and then it almost always does turn pink.
 
Fresh soap is rarely under 10 and frequently takes days or weeks to get there if ever.

Now this I like hearing, I'm glad you've put effort into learning something, and applying it to your hobby.

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Not being a chemist, I am reluctant to point out to the nice ladies that their tests are invalid the way they are doing it.

Suggestion: Wait until they complain they are being burnt.  Then suggest they should dissolve their soap in water and check its pH with a meter.  Then you will have helped them, without challenging their preconceptions.

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As it turns out pH 10 is perfectly safe but they all claim their soap is nominally pH8 because it passes the phph test.

js

Erm.  No.  That's not correct.
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