Slight Unintended Glycerin Rivers : An Error in Thinking, and how I solved it.

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Sep 9, 2023
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Southern California
Hi All,

I wanted to post this in case somebody who is using a master batch of NaOH and converting it to a desired concentration doesn't make the same mistake that I did.

I had made a 50% master batch solution of NaOH. No problem there. I would convert to a 33% NaOH solution and make soap. Some soap seemed a bit... well... like they had hints of glycerin rivers. Since I'm relatively new to soap making it was only when I was reading in this forum about Glycerin Rivers and seeing pictures that I had that "Hey, some of my soap kinda has that look!" thought. I read about different causes of glycerin rivers and how low NaOH concentration can help induce glycerin rivers. This in turn caused me to wonder about by conversion from 50% solution to 33% solution (did I not calculate correctly?)... I re-thought my formula: Initial Concentration (C1) X Initial Volume (V1) = Final Concentration (C2) X Final Volume (V2) that is: (C1)(V1) = (C2)(V2) ... Hm, what was wrong? What was wrong was staring me in the face! Instead of using VOLUME (ml) I was WEIGHING (grams) it! The problem with this, is density... Pure water (H2O) has a density of 1 gram/ml. Thus, with water, 1 ml of water also equals 1 gram of water. So, for water, yes, you can weigh it out like I was, but for NaOH, the density is higher. NaOH has a density of 1.53 grams/ml in solution at room temperature. What this means ultimately is that I was under-calculating the amount of NaOH @ 50% solution that I needed to use, and over-calculating the amount of water I needed! Thus, while I wanted a 33% solution of NaOH, I was actually ending up with with less. My NaOH solution was more along the lines of 29.98%. Not super low, but not 33% either. Add in the other causes for glycerin rivers and I think this concentration was just low enough to cause some of the hints of glycerin rivers that I've been seeing in a few batches of soap.

I've spared you all the how I went back and recalculated, but if anyone is interested I will post how I came to the correct usage of my master batch and water to get a true 33% solution of NaOH. Happy Soaping everyone!!!
That's interesting, Michael, since (a) I have always used weight over volume unless told otherwise since it has been beaten over my head early on as a baker, and subsequently in soapmaking and (b) that is a pretty big difference (30% v. 33%) in some instances, especially if you are not aware of it (I certainly wasn't). Although I guess if we all *mean* 30% when we say 33% - ie; most/all of us use weight over volume, which I think might be the case? - then it sort of works out in the end. Hm. I for one will forgo the actual math since it might make me cry :).
@not_ally, In this case, it's my fault for using a formula (C1V1=C2V2) that is specifically meant for concentrations and volumes when trying to convert my 50% master batch solution to a 33% solution and not using volume but weight! 🤦🏻‍♂️

@dmcgee5034, Ah! I didn't know that feature was available on that calculator! Seems I will be using a new soap calculator! Thank you for that!!🙏
Yep, basically C1V1=C2V2 is for dilute aqueous solutions only. A 50% NaOH isn't a "dilute" solution anymore. Good save!

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