Lye purity ?

[Emmanuel]

Hello evryone !
I bought KOH and the purity was 90% wich is honestly low but okay. Do you know what the 10% are ? I think they might be KCl considering the way of producing the compound but what if instead there is K2CO3 ? Because if I calculate my soap with the purity, the carbonate might makes it quite caustic to the skin. Also I have a jug of NaOH where they don't write the purity, what to do in theses cases ?
I ask you that because I'm making a 0% superfat soap and I would not like it to be caustic as just a little bit of alkaline on top is enough to shift the pH.

Have a nice day

 

[DeeAnna]

Water will be the main impurity in KOH and the secondary impurity will be potassium carbonate. Both come from exposure to the water vapor and carbon dioxide in air. There may be trace impurities such as chlorides, but I'd expect those to be very low -- well under 1%.

If you don't know the NaOH purity, you may want to ask your supplier. If your supplier doesn't know, then you're just going to have to make your best guess.

If you want to make a soap with exactly 0% superfat, meaning no excess alkali and no excess fat, there's no reliable way to do that using the methods we normally use to make soap -- hot process and cold process soap making.

These methods rely on reasonable estimates for the lye purity AND for the saponification value of the fats, but there's always some error in these estimates. You could know the lye purity exactly, but if the sap values you use for your calculations are slightly incorrect for the actual fats you're using, you will still not end up with a 0% superfat soap. (Or whatever superfat you want.)

The only surefire way to make an exactly 0% superfat soap is to use a boiled method. This is basically a trial and error saponification method that doesn't rely on numbers to get the results desired. The boiled method instead relies on testing of the saponified soap.

That said, soap is a fairly forgiving product. Calculate the recipe at zero or -1% superfat and make the soap with a CP or HP method. If there is a slight lye heaviness in the soap, that will disappear during cure as excess NaOH or KOH reacts with air to form sodium or potassium carbonate.

If you're concerned about sodium or potassium carbonate being harsh due to being alkaline, then how do you reconcile the inherent alkalinity of the soap itself? The carbonate alkalis (potassium or sodium carbonate) have a pH in the same pH range as soap. The hydroxide alkalis (potassium or sodium hydroxide) have a much higher pH at any given concentration and are much stronger alkalis.