Soap Titration for Unwashed Biodiesel

By Rick Da Tech


Testing Biodiesel for SoapThis biodiesel titration test is used to determine the amount of soap in unwashed biodiesel. It is not part of ASTM D6751 biodiesel testing. It is used for determining the amount of Magnesol or synthetic magnesium silicate to use when washing without water. Other uses include process improvement and biodiesel analysis.

The test procedure is a variation the one found in the Biodiesel Analytical Methods by the NREL pg 41, and has been tuned specifically for unwashed biodiesel analyasis.

The test uses isopropyl alcohol as a solvent, 0.01N aqueous solution of hydrochloric acid as reagent, 0.04% bromophenol blue and 1% phenolphthalein as indicators. In this test we use a triple beam and determine the ratio of soap to biodiesel by weight.


Materials need for test:

  • 100 ml Isopropyl Alcohol
  • 0.01N Aqueous solution of HCL
  • 0.04% Bromophenol Blue in water
  • Triple Beam Balance

Requirements for Materials

You will need isopropyl alcohol for the test. It must be pH neutral. You can tell by testing it with pH strips. If it is not at 7.0 then add enough of your lye water solution that you use in your normal WVO titration to bring it to a pH of 7.0

You can buy 0.01N aqueous solution of HCL at most laboratory supply houses for about $16 per liter. Shipping will be another $20 to $30 after paying the hazmat fees. 

Titrating for Soap

Now we will continue to add the 0.01N aqueous HCL to break apart all the soap.

  1. Add 20 drops of Bromophenol blue to the solution; it should turn blue.
  2. Weigh the sample and record the weight. (the starting weight)
  3. Remove the beaker from the scale.
  4. While stirring add 0.01 aqueous HCL until the Bromophenol blue changes to yellow.
  5. Weigh again and record the weight (the ending weight)

The Math the hard way

["B" ml of additional 0.01N HCl added] x [1 liter/1000ml] x [0.1 moles of HCl/liter] x [1 mole of soap/mole of HCl] x [320.56 g/mole of soap] / ["W" grams of sample] = grams of soap (as potassium oleate) per gram of sample.


soap test forumla

B = The ml of 0.01 aqueous HCL
W = grams of biodiesel in sample (should be 10g)

Use 304.4 instead of 320.56 if using NaOH or catalyst.

Multiply by one million to get grams of soap per million grams of sample

The Math the easy way

Subtract the starting weight from the ending weight for the net weight.

Multiply the net weight by 320 for the ppm of soap if using KOH

Multiply the net weight by 304 for the ppm of soap if using NaOH.


How Much Magnesol?

Use 1% by weight per 1000ppm of soap, according to the Dallas Group. If we take the specific gravity of biodiesel to be .860 then we add 8.6 grams of Magnesol per liter per 1000ppm of soap to give the formula:

["A" ppm of soap] x ["B" Liters of Biodiesel] x [8.6] / [1000] = Grams of Magnesol


Related Links

Soap and Catalyst Measurement - LSU Ag Center Biodiesel Program

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