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Author Topic: Soap titration - phenolphtalein, water, ethanol  (Read 321 times)

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Imperius123

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Soap titration - phenolphtalein, water, ethanol
« on: August 06, 2018, 01:59:40 AM »

Hello,

When titrating soap for determining the possibility of excess alkali present in the "finished" product, is the water content important? And more specifically, does a "low as possible" water content in the sample tested mean a higher accuracy of results?

I ask because in my experiments (brief example below), a high water content seems to invalidate the procedure entirely - curiously enough however, some well-respected authors claim that a soap sample dissolved entirely in water can still be tested for excess alkali... I'm confused. It seems that if one followed that advice, the soap would break completely back into free fatty acids/water and potassium citrate, no?

Example (numbers are representational as I don't have my notebook at the moment, but the principle is there - the numbers rise with water content):
-5g of (concentrated) KOH soap (of which 40% water) dissolved in 50g ethanol required 1g of citric acid to be neutralized (sample no longer pink)
-5g of (diluted) KOH soap (of which water 75%) dissolved in 50g ethanol required 1.2g of citric acid to be neutralized (sample no longer pink)
-5g of concentrated soap (40% water) dissolved in 50g water - it required upwards of 15g of citric acid to remove the pink.

What is the deal here? Thanks :)
« Last Edit: August 06, 2018, 02:10:27 AM by Imperius123 »
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Arkcon

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Re: Soap titration - phenolphtalein, water, ethanol
« Reply #1 on: August 06, 2018, 04:13:57 AM »

Briefly, you misunderstand pH and neutralization.  Water is better for neutralization reactions -- neutralization doesn't happen without water.

Try this for yourself:  neutralize your concentrated soap, your ethanol soap, and diluted water soap to phenolphthalein endpoint.  Then, dilute you concentrated sample, in water, until they are as dilute as you soap solution.  Are the "neutalized" concentrated soap samples now pink again?  Also, review this: https://en.wikipedia.org/wiki/Phenolphthalein#Indicator
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Imperius123

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Re: Soap titration - phenolphtalein, water, ethanol
« Reply #2 on: August 08, 2018, 01:36:27 AM »

Briefly, you misunderstand pH and neutralization.  Water is better for neutralization reactions -- neutralization doesn't happen without water.

Try this for yourself:  neutralize your concentrated soap, your ethanol soap, and diluted water soap to phenolphthalein endpoint.  Then, dilute you concentrated sample, in water, until they are as dilute as you soap solution.  Are the "neutalized" concentrated soap samples now pink again?  Also, review this: https://en.wikipedia.org/wiki/Phenolphthalein#Indicator

Well no, the neutralized ethanol-dissolved, low-water sample does NOT become pink again with the addition of water.

Is it then possible to detect unreacted alkali in a purely water-soap solution (no ethanol?) and why does it require so much more acid (since the same amount of alkali is theoretically present)?

Doesn't a soap solution in water measure pH rather than unreacted alkali?
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