KOH/NaOH 75:25 question

[Soapman Ryan]

When calculating a shave soap I wanted to make one that has both KOH and NaOH. If I wanted 75:KOH and 25:NaOH would I just calculate the recipe using both KOH, then NaOH and multiple the LYE total by the percentage? I've uploaded a pic of both KOH and NaOH to verify if my calculations would be correct? This is not a shave soap recipe, but just numbers for verification.

The totals came to
KOH=85.75g 75% would be 64.31g
NaOH=61.14g 25% would be 15.59g

Is this how 75:25 would be calculated?
Thanks.

 

[Susie]

Why don't you try this calculator:

http://summerbeemeadow.com/content/advanced-calculator-solid-cream-or-liquid-soaps

It allows you to use any proportion of KOH to NaOH, and is geared for cream soaps.

 

[Soapman Ryan]

That site is very inconvenient. It works in ounces, not grams and I can't type in percents. Once I can figure out the calculations from the numbers soapcalc gives, I can make myself a spreadsheet.

 

[DeeAnna]

Yes, it looks like you're doing the calculation correctly, Ryan. The following is a cut-and-paste of how I explain it to others. I'm providing this just so you can confirm you and I are on the same page with our thinking -----

Using soapcalc with a recipe with 2 lyes, you can calculate the recipe this way:

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. Use the same fats, the same superfat %, etc. for both versions -- just the lye is different. TIP: For soapcalc, be sure to put a check mark in the box for 90% KOH purity.

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.

Next figure the actual amount of KOH and NaOH needed.

If you want a mixture of 40% NaOH and 60% KOH for the recipe, multiply the NaOH weight times 0.40. And multiply the KOH weight times 0.60. This will give you the correct weights of each lye to mix together to make the recipe with 40% NaOH and 60% KOH.

 

[Soapman Ryan]

Awesome, this is the exact explanation. This let's me know I will be doing it right. Why should I put the check mark in the box for 90% KOH purity?

 

[boyago]

I run the recipe twice. In this instance I would run 75% of the weight as KOH, then again with 25% weight as NaOH and add the two together. I do it at 0 SF then add the SF per whatever I am trying. So if you are aiming for 100g I would do 75 as KOH and then 25g as NaOH.
If you like doing math you can also do the recipe as KOH then take 25% of the weight and multiply by the rate of weight difference of NaOH but I usually go back to the calc to check my work anyway.

Why should I put the check mark in the box for 90% KOH purity?

This is because most commercial grade KOH is 90% pure. Tech grades might be higher but they cost LOTS more.

 

[Soapman Ryan]

This is because most commercial grade KOH is 90% pure. Tech grades might be higher but they cost LOTS more.

Thanks, Good to know. I will now check the 90% pure box. I checked the specs on Essential Depot and this is what I found.

"Potassium Hydroxide KOH for liquid soap making and demanding biodiesel manufacture. High quality, food grade. 90% Pure Flakes. Synonyms: Caustic potash; potassium hydrate."

 

[Seawolfe]

Awesome, this is the exact explanation. This let's me know I will be doing it right. Why should I put the check mark in the box for 90% KOH purity?

Unless you are using expensive technical grade KOH, 90% is the most common purity. You should be able to find the info on the label or the manufacturers website (where you bought it).

FYI here is a calculator that will do both NaOH and KOH: http://www.soapguild.org/Certified-Lye/lye-calculator.php
so its handy for double checking to see if you are in the ball park. I think its missing some oils so I just use the method DeeAnna mentioned, its easier in the long run.

 

[DeeAnna]

"...Why should I put the check mark in the box for 90% KOH purity? ..."

Because many people use KOH with 90% purity (Essential Depot KOH being a prime example). The procedure was written for people who are really unsure about what they're doing, so I didn't get into the whys and wherefores of adjusting for purity; I just gave a black and white instruction for using SoapCalc that will work for most of the people most of the time.

If you're going to take the trouble to make a spreadsheet, you should include an adjustment for KOH purity, especially if you may use it for making liquid soap. This will help you avoid problems with separation due to excess superfat or lye heaviness -- both of which are more obvious and troubling problems with LS than with bar soaps. If you're only going to use this for bar or shave soap, you can just assume KOH is at 100% purity and accept that your soap will have some amount of "hidden" superfat due to the difference between the purity of the KOH that you're using vs. 100% purity.

 

[topofmurrayhill]

I thought it might be of interest to mention that the bulk of the impurities in the KOH we use is water. That and some potassium carbonate from reacting with CO2 in the air is almost all of it, with traces of other things.