I assumed my lye was at least a couple percent impure, despite my buying it from a chemical shop and knowing that it didn't sit in a warehouse for a year. (Real estate is too expensive for warehousing chemicals around here.)
I did all my soap calculations assuming there would be an implicit 2.5% superfat. Today I titrated the lye with citric acid that had been carefully baked. I messed up the procedure a few times since I don't do a lot of chemistry, but I was extremely careful. I mixed powders with water immediately to limit water absorption from the air. I baked the CA in a thin layer on foil. I weighed everything subtractively so there was no chance for scale normalization or substance residue to reduce accuracy. I included the mixers and cups in all weighings. I was dismayed to find the lye was 99.6% pure.
It's been a long time since I did this type of math, but it seems there should be ±0.5% error due to the scale's limited precision when I measured the citric acid (about a gram). The other measurements, which were all ≥3 grams, don't contribute significantly to the error. I also assumed the citric acid was 99.5% pure. I wonder if that was a bad assumption, due to difficulty of removing all the water? The dehydration procedure was harder than I was led to believe, with the citric acid melting at the drop of a hat.
Moral of the experience: don't assume your lye is pure, but also don't assume it's not.
I did all my soap calculations assuming there would be an implicit 2.5% superfat. Today I titrated the lye with citric acid that had been carefully baked. I messed up the procedure a few times since I don't do a lot of chemistry, but I was extremely careful. I mixed powders with water immediately to limit water absorption from the air. I baked the CA in a thin layer on foil. I weighed everything subtractively so there was no chance for scale normalization or substance residue to reduce accuracy. I included the mixers and cups in all weighings. I was dismayed to find the lye was 99.6% pure.
It's been a long time since I did this type of math, but it seems there should be ±0.5% error due to the scale's limited precision when I measured the citric acid (about a gram). The other measurements, which were all ≥3 grams, don't contribute significantly to the error. I also assumed the citric acid was 99.5% pure. I wonder if that was a bad assumption, due to difficulty of removing all the water? The dehydration procedure was harder than I was led to believe, with the citric acid melting at the drop of a hat.
Moral of the experience: don't assume your lye is pure, but also don't assume it's not.
