Acid-Base Titrations and Liquid Soap

[arthur_]

I am doing an acid-base titration on liquid soap. I understand that buffers are used in soap. Would that affect the results of my titration? If so, how would I overcome this?

Arthur.

 

[chemisttree]

What are you trying to measure?

 

[g_midford]

sorry for the delay, chemisttree.

i'm trying to measure Sodium Lauryl Ether Sulphate

 

[chemisttree]

Buffers can titrate as acids or bases depending on the pH and the indicator you use. If a buffer is added to soap, you will be titrating buffer rather than the acid form of 'SLES'. I don't know the pKa of the monoester of lauryl ether sulfate but I'm guessing it is somewhere around -2 to -3. If you measure the pH of your soap and it is near neutral, you can be sure that all of the lauryl ether sulfate exists as the anion. You will simply be titrating the buffer

If you are actually trying to measure the SLES by titration, I would suggest that you instead measure the critical micelle concentration of a series of dilutions and back-calculate the initial concentration.

 

[g_midford]

so i cannt measure the SLES by titration?

 

[MrSid]

Not by an aqueous titration, and not without a set of conditions that can exclude the known buffering materials (those one can expect in the soap product). Most buffers can be expected as inorganic salts, and their interference might be removed by complexing agents in analytical methods found in the literature.

A nonaqueous titration uses a different solvent that presents a stronger conjugate acid than H3O(+) in the equilibrium. Water has to be rigidly excluded for these titrations to work.

The presence of other surfactants may make it difficult to determine one surfactant in the mix by CMC. These could be non-ionic, anionic, and cationic. If your soap is indeed an unknown, then other methods should be used that are more unequivocal. (NMR, other spectrometric methods).