April 21, 2019, 04:31:06 AM
Forum Rules: Read This Before Posting


Topic: Determination of free alkali in potassium soap  (Read 399 times)

0 Members and 1 Guest are viewing this topic.

Offline Imperius123

  • New Member
  • **
  • Posts: 4
  • Mole Snacks: +0/-0
Determination of free alkali in potassium soap
« on: February 03, 2019, 09:14:02 AM »
Hello,

If one were to titrate X grams of soap in an aquaeous solution; the endpoint would be elusive, since the soap would get continually hydrolysed by the acid - subsequently more alkali freed up, and more acid being required (and also breaking the soap as a whole). Not a valid method.

But if instead one were to titrate X grams of soap in an ethanolic solution, the process simply works and the endpoint indicates that all the free alkali is neutralized, and no soap is hydrolyzed. Purpose achieved - we know how much acid is required to produce a soap with no free alkali, as well as keeping the soap itself quite intact.

Why is that? How does ethanol 'prevent the hydrolysis', making it a valid method for this determination? And is the method valid at all, or am I assuming too much (the neutralization of all present alkalis  (caustic and non-caustic (such as carbonates) is not a problem - I see no reason to filtrate for their separation).

Thanks

Offline Borek

  • Mr. pH
  • Administrator
  • Deity Member
  • *
  • Posts: 24943
  • Mole Snacks: +1636/-395
  • Gender: Male
  • I am known to be occasionally wrong.
    • Chembuddy
Re: Determination of free alkali in potassium soap
« Reply #1 on: February 03, 2019, 06:03:36 PM »
What it is that gets hydrolyzed in soap? Isn't soap made by hydrolyzing fats, so technically there should be nothing left for hydrolyzing?

Where does the information about different results of titration in water and ethanol come from? I am not saying it is wrong, but good source should explain why it is so. It can be a matter of kinetics.
Chembuddy chemical calculators - stoichiometry, pH, concentration, buffer preparation, titrations.info, pH-meter.info, PZWT_s1

Offline Imperius123

  • New Member
  • **
  • Posts: 4
  • Mole Snacks: +0/-0
Re: Determination of free alkali in potassium soap
« Reply #2 on: February 04, 2019, 06:55:53 AM »
What it is that gets hydrolyzed in soap? Isn't soap made by hydrolyzing fats, so technically there should be nothing left for hydrolyzing?

Where does the information about different results of titration in water and ethanol come from? I am not saying it is wrong, but good source should explain why it is so. It can be a matter of kinetics.


Hello Borek, thanks for the reply.

I am not a chemist, so take my "technical terms" with a spoon of salt. By hydrolysis of soap via addition of acid in a water solution (as part of the titration process) I meant:

-as I add (citric) acid for the neutralization of any free potassium hydroxide (which is caustic and therefore unwanted in the finished product) in soap until the indicator endpoint in an AQUAEOUS solution, the endpoint never really comes because the  (citric) acid also "breaks" the soap (in this case the Potassium Cocoate bond) and keeps "moving" the endpoint due to the potassium hydroxide being continually made available by separation from the lauric acid.
Essentially in a water solution I keep "breaking" the soap until the presence of enough fatty acids lowers the pH enough that the phenolphtalein ceases to indicate pink/alkalinity. Meanwhile the productitself becomes a useless oily mess.


On the other hand, when doing it (adding citric acid) in an ethanol solution, (example 2g of soap dissolved in 50g of ethanol), no free fatty acids are liberated from their potassium bonds when I reach the endpoint, but rather only the free unbonded alkali is reacted into potassium citrate.


Learned from: ASTM procedures; D 460-91, as well as a "dumbed down" version from Scientific Soapmaking (Kevin Dunn)


I would like to understand why this is; predominantly to understand if and how I am making a safe product with no free caustic alkali in it.


Sponsored Links