Soapless detergent -- acidic ?!

2007-04-23 7:39 am

Some soapless detergents are slightly acidic, depend on the ionic heads them.
E.g. the one containing SO3- as the ionic head is acidic in its solution.
However, since H2SO3 is a weak acid in nature, when that detergent is dissolved in water, the ionic head should combine with H+ ion to form H2SO3 so as to maintain the equilibrium. Hence, excess OH- should be left and the solution should be alkaline.
BUT why, it is ACIDIC instead ?

THZ

回答 (1)

2007-04-23 10:15 pm

✔ 最佳答案

the SO3- containing detergent you mentioned is a sulphonate
it is not sulphite
they differ from the structure shown below:

sulphite: R - O - S (=O) - OH
sulphonate: R - S (=O) (=O) - OH

in other words, sulphonate comes from a sulphuric acid molecule, having one of the hydroxyl group replaced by an alkyl group;

whereas in alkyl sulphite, only the hydrogen atom on one of the hydroxyl group on the sulphurous acid is replaced by an alkyl group

i am not sure whether alkyl sulphite exist
but sulphonic acids are stronger acids than sulphurous acid since more oxygen atom is directly linked to the central sulphur atom
pKa of sulphurous acid is 1.9, whereas for sulphonic acid detergent are <1

since sulphonic acids are quite strong, it dissociate readily in water
they do not need to be in a alkaline medium to make it become an anion

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