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Testing For Soap In Unwashed Biodiesel And Glycerol
by Maria "Mark" Alovert of Local B100
Questions? Comments? Feedback? Click Here

Source: Biodiesel Homebrew Guide: Everything you need to know to make quality alternative diesel fuel from waste restaurant fryer oil

The standard titration for soap in oil, unwashed biodiesel, or glycerol, uses hydrochloric acid solution as a reagent, bromophenol blue as an indicator (0.4% 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.


Click on the images below for a larger version. 

Some of the Ingredients water burette and acid 100 ml of the acetone/water solution or isopropyl


0.5 ml of the bromophenol blue solution Add 100 grams of the unwashed biodiesel or 10 grams of the melted glycerol that you are trying to determine the soap content of.
Starting with this color Titrate using 0.01N HCL solution


Neutralize this by titration to yellow using your lye/water solution that you have in your acid value (i.e. free fatty acid) titration equipment Starting to turn pH4.5 Swirling.

Almost There until it reaches the yellow endpoint.  This is the amount of 0.01N solution that is needed to neutralize all the soap in this sample.

soap in parts per million=
(ml of 0.01N HCL solution)/weight of sample*30.44 --this not correct.dkenny..5.5.2009 edit

B * A * S / 1000 * W = grams of soap/gram of sample
B - mls of HCL solution
A - N of HCL solution - usually 0.1 or 0.01
S - molecular weight of the soap in question. 304.4 for sodium, 320.56 for potassium
W - weight of sample in grams
for PPM multiply by 1 million

residual catalyst in parts per million=
(ml of 0.01N HCl)/weight of sample*561 -> this is also not correct..dkenny 5.5.2009 edit

B * A * S / 1000 * W = grams of KOH catalyst/gram of sample

B - mls of HCL solution
A - N of HCL solution - usually 0.1 or 0.01
S - grams per mole of catalysts. 40 for NaOH, 56.1 for KOH
w - grams of sample

formula are from Biodiesl Anyatical Methods August 2002 through Jan 2004.

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 .87 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


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 0.01N 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 0.01N HCL solution that we need, we want to have 0.01 moles of HCL in 1 liter of solution
0.01 moles of HCL is .365 grams (0.01mol/L) (1L)

Find the percentage concentration on the bottle (it'll be close to 30%, probably 31.45%).

We want to know how much volume in milliliters to use of this (approximately 30g per 100 ml) hardware store acid to make up 0.365 grams for our 0.01N solution.

The formula is for finding the number of milliliters is:
[edit: these are the corrected numbers I think: x= (0.365 x 100) / percentage from the bottle . See several posts down for more information]

X [which is milliliters to use] = 0.365g / ( (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 milliliters. 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 982 ml distilled water [edit: it's probably going to actually be 11.6 ml of acid, going into about 988 ml of water, if your acid is of 31.45% strength which is common]. 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 [edit: should be 90 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 0.01N solution which you can use for the titration. Label the bottle clearly as '0.01N solution'.

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