Here's something I wrote awhile back about calculating the lye weights for a mixed lye soap:
Three molecules of KOH are needed to saponify one molecule of fat. Three molecules of NaOH are needed to saponify one molecule of fat. Sounds easy peasy, right? Just count out the right numbers of molecules and make your soap!
Oh, there's a catch -- we humans can't count molecules, so we have to measure by weight. And it turns out that one molecule of KOH weighs more than one molecule of NaOH. So if you want to make a soap using, for example, 57% NaOH molecules and 43% KOH molecules, what is the correct way to figure the
weights for the KOH and NaOH lyes?
There are several ways to do this correctly. Some people use the summerbeemeadow.com
soap calculator. It will allow you to specify the percentages of the two lyes and will calculate the weights of the two lyes for you.
Another way is to do the calculations by hand or create a spreadsheet to calculate the lye amounts.
A third way to figure the correct lye weights is to use your favorite
soap recipe calculator, such as soapcalc.net. Here's how --
You are going to calculate the exact same soap recipe twice. The
only difference between the two versions is your choice of lye -- use NaOH as the lye for the first version and KOH as the lye for the second version. TIP: If using soapcalc, be sure to put a check mark in the box for 90% KOH purity if needed for the KOH you are using.
For the first version, the recipe calculator will tell you how much NaOH that you would need to use if you wanted NaOH as ALL of the lye. In the second version, it will tell you how much KOH you would need to use if KOH was ALL of the lye.
The last step is to calculate the weights you really need for your specific recipe. Since you want a mixture of 57% NaOH and 43% KOH for the recipe, multiply the NaOH weight times 0.57. And multiply the KOH weight times 0.43. This will give you the correct weights of each lye to mix together to make the recipe with 57% NaOH and 43% KOH.
