Lye calculation incorrect on dual lye recipe

[darciecousins]

I've entered a dual lye recipe with a 40/60 split NaOH/KOH and a 6% superfat.
The recipe calculator calculates the NaOH at 40% but KOH at 66%. It implies the KOH calculation doesn't account for the superfat.

 

[Yooper]

What do you have your impurity of the KOH set for? That could be it. KOH and NaOH don’t have the same SAP values so they aren’t interchangeable. However, I will dig into this too.

 

[Yooper]

I just doubled checked, and it is working correctly.

The reason is that there is a different SAP number for KOH and even with the same pyrenees level (which isn’t the case most of the time), you’d need more KOH than NaOH to saponify the same amount and type of oils. Therefore, a 50/50 mix isn’t using the same amount of both types of lye; it is using the amount needed to saponify your oils 50% each.

I found this link to explain it better (with the math shown):

How to Calculate a Dual-Lye Recipe

"How do I calculate a dual-lye recipe?" The most common and obvious answer is "Use a calculator." Well duh, why didn't I think of that? One of the most popular soap calculator websites, and one of the select few that offers a dual-lye option, is no longer functioning and has left soap makers...

www.ultimateguidetosoap.com

 

[darciecousins]

What I did was I entered all my oils into Soapcalc and ran the recipe using only NaOH. Then I ran it again using 90% purity KOH. I took each of the lye amounts and multiplied them by the percentage I wanted in the recipe. So I ran the NaOH amount x 40% and the KOH amount x 60%. The NaOH calculation I made matched what the Soapmaking Friend calculator came out with. But the KOH amount that Soapmaking friend calculated was 66% instead of 60%. I chose 6% superfat in Soapcalc each time I ran the calculation, so the implication was that when SoapMaking Friend ran the dual lye calculation it didn't account for the superfat but I didn't test it any further to see if it would be off by 5% or 10% if I changed the superfat, so I acknowledge it could be a red herring.

I'll add some images but it will be easier to do from my laptop in a few minutes.

 

[Yooper]

I understand what you’re saying, I think. But because the SAP values are different, the amount won’t be exactly 50%. I did it with 5% superfat and it seemed to calculate appropriately although the total weight of the KOH was quite a bit more than the NaOH.

If you have screenshots we can show the developer, he can dig into the software and ensure it’s correctly calculating, and if not, he can correct it.

 

[darciecousins]

 

[darciecousins]

What I'm seeing above is that for the oils specified in a 500g batch, Soapcalc gives
NaOH = 66.74g
KOH = 93.60g

For a 40% NaOH / 60% KOH recipe:
NaOH 66.74 x 0.4 = 26.696 - Soapmaking Friend calculation matches
KOH 93.60g x 0.6 = 56.16 - Soapmaking Friend calculation shows 62.4g

56.16 / 93.60 = 0.6
62.4 / 93.6 = 0.66667

I couldn't figure out why it was giving me 40% / 66% until I remembered my superfat is 6% so as I've said, that's my first thought. I'll need to try some other superfats to confirm.

 

[Yooper]

Thanks! I’ll dig into this today. One thing that jumps out at me right away is the stearic acid- I didn’t include that in my testing.

 

[darciecousins]

I've tried with a 10% superfat and it still calculated the KOH at 66% so the superfat is a red herring. But the calculation still didn't match what I expected.

10% superfat

Soapcalc calcuations:
NaOH = 63.9 x 0.4 = 25.56 (Soapmaking friend matches when specifying dual lye)
KOH = 89.62 x 0.6 = 53.772 (Soapmaking friend when specifying dual lye: 59.74)

53.772 / 89.62 = 0.6
59.74 / 89.62 = 0.6666

When I run the recipe through Soapmaking friend as KOH only, the lye amount it calculates is 99.58g instead of 89.62 that Soapcalc calculates. The NaOH amounts match between the two calculators.

 

[darciecousins]

How are you calculating the lye amounts needed for KOH?

I have compared the SAP values that SoapCalc lists for the oils in my recipe and they match the SAP values in SMF. Then I calculated a 500g batch of soap using 100% of each oil, and looked at the lye amount for NaOH, and ran it again using only KOH. In every single calculation, the two calculators matched on NaOH, but on KOH the SMF calculator was at least 10g higher than SoapCalc.

 

[darciecousins]

I must apologise - you can close this ticket. I've been examining everything and comparing SoapCalc values to SMF values, trying out single oil recipes, etc., trying to work out where the discrepancy lies and it was me. The '90%' box was cleared in the course of re-running the SoapCalc going back and forth between the calculations. Facepalm.