soap titration/total alkali test - scientific soapmaking

[Sapo]

Does anyone titrate their soap using the total alkali test from Scientific Soapmaking to learn exactly how much acid to add in order to neutralize any potential free alkali?

I've noticed that, when using citric acid, things aren't so simple. If using HCl, the test is straightforward and linearly applicable. With citric acid however, the base logic doesn't quite follow (a 2g sample of soap might require 1g of 0.5% citric acid, but the entire stock of product, say 2000g, will not require 1000g of acid, but rather 1500g (subsequent tests still yield alkaline (pink sample) soap, instead of being neutral)). As a consequence you need to test and neutralize your batch like 3 times before the sample finally stops being pink.

With HCl, the test and bigger batch neutralization was completely linear. 2g-1g meant 2000g-1000g. The very next test after the first yields a colorless solution.

Any ideas as to why that is? Not a chemist, but I suspect it is due to using a weak acid to neutralize a strong base.

Are buffering agents present or formed? Can this be the reason?

 

[shunt2011]

Nope, as when I've made LS I make the recipe in the files on the forum and calculate a 3% SF therefore no excess to neutralize.

 

[SaltedFig]

Does anyone titrate their soap using the total alkali test from Scientific Soapmaking to learn exactly how much acid to add in order to neutralize any potential free alkali?

I've noticed that, when using citric acid, things aren't so simple. If using HCl, the test is straightforward and linearly applicable. With citric acid however, the base logic doesn't quite follow (a 2g sample of soap might require 1g of 0.5% citric acid, but the entire stock of product, say 2000g, will not require 1000g of acid, but rather 1500g (subsequent tests still yield alkaline (pink sample) soap, instead of being neutral)). As a consequence you need to test and neutralize your batch like 3 times before the sample finally stops being pink.

With HCl, the test and bigger batch neutralization was completely linear. 2g-1g meant 2000g-1000g. The very next test after the first yields a colorless solution.

Any ideas as to why that is? Not a chemist, but I suspect it is due to using a weak acid to neutralize a strong base.

Are buffering agents present or formed? Can this be the reason?

Citric acid is a weak acid, Hydrochloric acid is a strong acid and Sodium hydroxide is a strong base.

Yes, there is a buffering effect with the weak acid and strong base reaction.

This might interest you: https://groups.chem.ubc.ca/courseware/pH/section14/index.html
(it explains the buffering effect about a third of the way down the page )

 

[Sapo]

Citric acid is a weak acid, Hydrochloric acid is a strong acid and Sodium hydroxide is a strong base.

Yes, there is a buffering effect with the weak acid and strong base reaction.

This might interest you: https://groups.chem.ubc.ca/courseware/pH/section14/index.html
(it explains the buffering effect about a third of the way down the page )

Does the buffering effect mean that more citric acid is required to neutralize the free KOH in ACTUALLITY (rephrased question: does the buffer material/solution "eat" some of the acid? Thus requiring more overall?) or does that simply mean that the test and indicator itself are less reliable due to it's presence (rephrased: all the KOH is already in potassium citrate form, but the test "lies" due to the buffer)?

The equivalence point at the linked example seems to be the same in regards to the quantity of titrant added (x axis). Just the pH is different... Basically trying to understand how this effect translates to the test in question specifically (titrating soap with slight excess of KOH with citric acid in an aquaeous solution).

 

[SaltedFig]

I haven't run large and small volume tests of Citric acid in reaction with NaOH, but my guess would be that the difference you are seeing in the two volumes is influenced by the degree of accuracy between 1g and 1000grams.

A weak acid with a strong base will tend towards basic, so you will need more (by molecular weight) of the weak acid (over a strong acid) to bring the solution to neutral.

*Edited to add:
This has a bit more detail:
https://www.ck12.org/book/CK-12-Chemistry-Intermediate/section/21.4/