Here is an article I just wrote for my zine, trying to make sense of the
standard US lab method for testing for soap in oils ("I Am Not A Chemist").
A couple of years ago a bunch of us were looking pretty hard at the
Leonardo european vocational training website (
http://koal2.cop.fi/leonardo/ ) and this titration and other good testing
info was on there- in German unfortunately. I'd love it if someone who
speaks German could tell me the the difference between the methods.
My objective was to write this up for homebrewers. If you can test
batches of biodiesel conclusively for soap, you can then eliminate one of
the two major factors when troubleshooting and doing quality
control. Yippeeee!!!!!!
I would like some feedback from chemists on whether I got the information
correct, especially the conversions back and forth from weight to volume
for unwashed biodiesel versus the number I listed (which is for commercial
washed biodiesel), and the stock solution/ normality instructions. I want
to thank Kalib Kersch for explaining the simple concept of normality to me
and for writing me the formulas for converting from variable strength
hardware store hydrochloric acid to something that the titration can use.
Please dont' reprint this until we hear if I got it correct or not...
Here it is:
Testing for Soap In Unwashed Biodiesel and Glycerol:
The standard titration for soap in oil, unwashed biodiesel, or glycerol,
uses hydrochloric acid solution as a reagent, bromophenol blue as an
indicator (.4% in water), and acetone or isopropyl alcohol as a solvent.
Bromophenol blue turns from blue to yellow at an acidic pH (4.5 if I recall
correctly). This is the point at which all the soap in a sample has been
neutralized by the hydrochloric acid.
Solvents that work for this are either acetone in water or isopropyl- which
needs to be absolutely neutral. If it's acidic, you will need to add some
of your old lye/water solution (which you'll have on hand from your free
fatty acid titration) until it is neutralized. Do this in the presence of
the bromophenol blue indicator, and add lye/water solution until it just
barely turns yellow again.
Otherwise, use acetone and distilled water: make up a solvent of 980 ml of
acetone and 20 ml of distilled water.
Procedure:
100 ml of the acetone/water solution or isopropyl
.5 ml of the bromophenol blue solution
Neutralize this by titrating to yellow using your lye/water solution that
you have in your acid value (ie free fatty acid) titration equipment
Add 100 grams of the unwashed biodiesel or 10 grams of the melted glycerol
that you are trying to determine the soap content of.
Titrate using .01N HCL solution until it reaches the yellow endpoint. This
is the amount of .01N solution that is needed to neutralize all the soap in
this sample.
Calculations:
soap in parts per million=
(ml of .o1N HCL solution)/weight of sample*30.44
residual catalyst in parts per million=
(ml of .01N HCl)/weight of sample*561
You should also do a blank titration and subtract any HCl needed for the
blank titration from the weight of the HCL needed for the actual soap test
in your calculations.
To convert from the weight measurements above to volume measurements that
we use, use the following: Biodiesel: approx .97 kg/liter (I'm not sure if
this applies to unwashed though) Glycerol: approx 1.26 kg per liter (not
sure if this applies to glycerol byproduct). You can always weigh some of
your product and do the conversion from the results you find
*********************************************************
'sidebar:'
Using Hardware store 'Muriatic Acid' as your source of hydrochloric acid:
'Muriatic acid' is HCL, sold in hardware stores for swimming pool
maintenance and for concrete etching, about $3 a gallon. Muriatic acid
varies slightly in strength. It is labeled on the bottle with the
percentage concentration, usually somewhere around 30%.
Here's how to make a .01N concentration of this acid for the titration
(assuming the label is correct and you've kept the bottle sealed):
1N- 1 mol HCL/liter (ie 1 mole of H+)
1 mol HCL is 36.5 grams
To make 1 liter of the .01N HCL solution that we need, we want to have .01
moles of HCL in 1 liter of solution
.01 moles of HCL is .365 grams (.01mol/L) (1L)
Find the percentage concentration on the bottle (it'll be close to 30%).
We want to know how much volume in milliliters to use of this
(approximately 30g per 100 ml) hardware store acid to make up .365 grams
for our .01N solution.
The formula is for finding the number of milliliters is:
X [which is milliliters to use] = .365g / ( (percentage HCL in g) (100 ml))
Substitute the actual percentage from the bottle (ie 30.7 or 29.24 or
whatever it says)
This will tell you a very tiny number, like 1.88 mililiters. You can do one
of two things:
1. Measure out the x of hydrochloric acid, and make up a solution of 1
liter of distilled water and this x ml of your hydrochloric acid. This is a
.01N solution. To make the solution, first add acid to a graduated cylinder
filled with half a liter of water, then add enough water to make up the
volume to exactly 1000 ml including the acid.
2. or, you can make a stock solution in order to get a more precise
measurement:
We sometimes make a stock solution when measuring out tiny amounts of a
liquid for titration reagent, in order to be more accurate with our
measurements. In the case of the hydrochloric, I for example might need 1.8
ml to end up in 1000 ml of water. This is a tiny amount of liquid and if I
make a mistake here it'll have a large impact on the chemistry.
We can be more accurate by making up our .01N HCL solution in two steps-
first measure ten times the smaller amount of the substance as usual, (for
instance it is easier to measure 18 ml than to measure 1.8 ml) and then add
this to 1000 ml distilled water. You will have a concentrated reagent that
is 10 times as strong as what you need for the titration. You can then make
up working dilutions of it by diluting some of this stock with further
distilled water: take 10 ml of stock solution, and add 100 ml of distilled
water. Work from this .01N solution for the titration above.
Once again, stock solution instructions:
Step one, make stock solution:
Take 10 times X ml of hydrochloric (from calculation in the box above),
with distilled water added to make up a total of 1 liter of stock solution.
This violates the usual instructions to not add water to acid, so please
put some of the water into the flask first and THEN add the acid
Label the bottle clearly as 'stock solution' with the strength marked
step two, 1 in 10 dilution:
dilute this by taking 10 ml of this stock solution, add 90 ml of distilled
water. This is now a .01N solution which you can use for the titration.
Label the bottle clearly as '.01N solution'.
Phew!
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