3% lye discount miscalculation, help!

PandasMom · Jan 23, 2021

So I am practicing for a soap exam. And for the life of me cant understand why the answer to this problem is 149. I keep getting 209. What am I missing. Could anyone enlighten me how to get there?

Take the SAP values of coconut, palm, and olive oils to be 256.5, 199.5, and 190 milligrams KOH/gram oil respectively. Consider a soap to be produced from 333 grams of coconut oil, 445 grams of palm oil, and 222 grams of olive oil. How many grams of NaOH should be used to saponify this oil using a 3% lye discount? (Round your answer down to the nearest whole gram.)

153
216
149
209

DeeAnna · Jan 23, 2021

You've been given KOH sap values. You want to calculate for NaOH instead. You have to convert the KOH sap value to an NaOH basis.

NaOH sap value = KOH sap value / 1.403

Or you can find the total weight of KOH (which is the 209 grams you've calculated), and convert that to the equivalent total weight of NaOH by dividing the KOH weight by 1.403.

NaOH weight = KOH weight / 1.403

SoapDaddy70 · Jan 23, 2021

it’s kind of a trick question. They are giving you the SAP values of the KOH. you need to takes those numbers and divide by 1402.50 to get the correct numbers for the NaOH.

DeeAnna said:
You've been given KOH sap values. You want to calculate for NaOH instead. You have to convert the KOH sap value to an NaOH basis.

NaOH sap value = KOH sap value / 1.403

Or you can find the total weight of KOH (which is the 209 grams you've calculated), and convert that to the equivalent total weight of NaOH by dividing the KOH weight by 1.403.

NaOH weight = KOH weight / 1.403

Darn you Deanna. I was all proud of myself for getting the answer right and you beat me to it. Lol.

DeeAnna · Jan 24, 2021

Actually it's not all that much of a trick question -- this is a point that trips up many handcrafted soap makers.

Saponification values in scientific or industrial literature are almost always reported as sap values for KOH. Basing sap values on KOH is the standard convention used outside the handcrafted soap making community. We tend to forget we're only a teeny tiny segment of the entire soap making world.

Sap values in scientific/industrial literature are often reported with no alkali mentioned (and sometimes not even units of measure.) If you see this, the default alkali is KOH.

If you're reading something specifically targeted at the handcrafted soap makers, it's hard to say what alkali was used for the basis -- many use NaOH but some use KOH. You have to hope the author clearly tells you. If it's not clear, the next best thing to do is compare the sap values given by the author with sap values from other sources that do give the basis.