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Titrating for saponification
I have a quantity of used fryer oil that I would like to use to make soap. I do not know the oil composition, so I would like to titrate the
saponification value. Here is what I have so far. Thanks for any feedback!
Goal:
To find out how many grams of NaOH are needed to convert 1000 grams of oil into soap.
Materials:
10 grams oil
2 grams NaOH
1M HCl solution
10 ml Water
12 ml syringe
Phenolphtalein
Background:
Most vegetable oils need 13% to 14% NaOH by weight for saponification.
Some saturated oils (eg, coconut) may need up to 19%.
(Reference: Scientific Soapmaking, Kevin Dunn)
Many oils (used fryer oil for example) are mixtures of unsaturated,
saturated, and partially saturated oils of unknown proportions.
By using an excess of NaOH (20%), there will be NaOH left in the soap.
I will titrate this excess NaOH with 1M HCl solution and phenolphtalein.
Excess water is used to speed up the reaction and titration.
Misc:
NaOH = 40.0g/mol
2 g NaOH = 0.050 mol
HCl 32% = 10M
Expected titration: aprox. 0.015 mol (or 15ml of titrant).
Using speed stir for saponification and titration.
Procedure:
. Mix 2.00g NaOH with 10g water.
. Add 10.0g oil into NaOH solution.
. Stir vigorously until completely reacted (viscocity?).
. Add (5?) drops of phenolphtalein. The liquid should turn bright pink, indicating excess NaOH.
. Fill 12ml syringe with titrant and add to solution.
. Fill 12ml syringe again and slowly add, stopping when color changes.
. Each ml of titrant used represents 0.001 mol of excess NaOH.
. NaOH required per 10g oil = 2.0g - titrant(ml) * 0.040
. NaOH per 1000g oil = 200g - titrant(ml) * 4.0g
Open questions:
. Is the NaOH heat of solution sufficient, or is additional heat needed?
. Is a cosolvent (an alcohol) needed for a reasonably prompt reaction?
Bibliography:
Titration with KOH, alcohol, and ether.
http://vlab.amrita.edu/?sub=3&brch=63&sim=688&cn...
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aga
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AFAIK you need to evaluate the sap number of a small sample of your oil using KOH before you can saponify the bulk with certainty.
Beyond that, i have no idea.
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blogfast25
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The saponification of the fat (prior to titration) is slow in heterogeneous phase. Better to dissolve the sample in ethanol, for that reason. To be
certain you can also heat the mixture of oil, EtOH and NaOH slightly for few minutes. Then cool under a tap prior to titration.
The required amounts of NaOH for full saponification can be found in many 'soap making tables', for most types of oil/fat, though...
[Edited on 6-6-2015 by blogfast25]
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unfrozen
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Quote: Originally posted by blogfast25  |
The saponification of the fat (prior to titration) is slow in heterogeneous phase. Better to dissolve the sample in ethanol, for that reason.
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Okay. I have to use alcohol (IPA) for titrating the amount of fatty acids for biodiesel, so this should be similar.
| Quote: |
The required amounts of NaOH for full saponification can be found in many 'soap making tables', for most types of oil/fat, though...
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Yes, the book "Scientific Soapmaking" has plenty of tables for pretty much every oil. But I don't know how much is Canola, corn, or soybean, and how
much of it is hydrogenated, and how much of it has been converted to free fatty acids in the fryer, I think the best way is titration.
Thanks!
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blogfast25
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Hydrogenation will have very little effect on any readings, since as hydrogen is so light.
Titration is definitely best. You might want to include a 'standard' like sunflower oil which has a pretty fixed FA profile and a well known SAP
number.
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unionised
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This sort of procedure
http://lib.njutcm.edu.cn/yaodian/ep/EP5.0/02_methods_of_anal...
will get you more accuracy than you need, but it does require the use of alcohol as the solvent.
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unfrozen
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UPDATE:
It looks like I will need a lot more water for this. The saponification took only a few minutes, then everything turned into a semi-solid gel. Argh.
At least the cleanup was pretty easy.
I was hoping for a thin, soapy solution. Oh, well. This recipe was more like a prank for beginning chemistry students.
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zed
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KOH usually produces liquid soaps. Like Dr. Bronners. NaOH more solid soaps. As always, regular water won't quite do. Calcium and Magnesium
present may produce insoluble fatty acid salts.
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