Testing Biodiesel For Soap Content

When Biodiesel is made, one of the by-products that gets produced along with the Biodiesel is soap. Typically, the amount of soap produced can be directly tied to how high the titration was, how wet the oil was to start with, or how much extra catalyst was used. Soap in Biodiesel is a contaminant and something that should be measured for each time you make a batch.

Why Is Soap Bad?
Soap in Biodiesel isn’t good for your diesel on many counts. The main problem we see is plugged fuel filters. This is because it tends to congeal up on the fuel filter fibers which leads to reduced throughput which leads to a plugged fuel filter…and down time for your diesel. Next in line is that when soap is burned in a diesel engine it leaves behind an ash residue. This residue can manifest itself as higher levels of soot out the tailpipe or even build up on fuel injectors and in the combustion chamber.

How Soap Is Formed
Soap is formed when free fatty acids in oils mix with water and catalyst (Sodium Hydroxide–NaOH or Potassium Hydroxide–KOH). Typically, the higher the free fatty acid level in oil leads to higher soap content in the Biodiesel. It’s why we titrate waste vegetable oil–so that we can add enough extra catalyst to react the majority of the free fatty acids into soap and still have enough left over to react the remaining oil into Biodiesel.

How To Test For It
There’s two popular tests for checking soap levels in Biodiesel. One, called The Shake Em Up Test, is a subjective test that can give you a good approximation of the soap content in Biodiesel. The other test, called The Soap Titration Test, is an extremely accurate objective test that will tell you exactly how many parts per million are in your Biodiesel sample.

Test #1: The “Shake-Em Up” Test
This test is a subjective test that will give you a good approximation if the soap content in Biodiesel is almost gone. It’s a great test for checking to see if you’re “getting close” to getting all the soap out of your Biodiesel. However, it is not an exact test and should only be used to give a “close approximation” of the level of soap in Biodiesel. Passing this test doesn’t necessarily guarantee that soap is at ASTM levels. A pass simply indicates that soap levels are significantly reduced.

How The Test Works
Distilled water is mixed into Biodiesel and then allowed to settle out. If soap is present in large amounts, it will cloud the water that settles out of the Biodiesel. If soap is present in high quantities, it will readily dissolve into the water. As the water settles out the soap will cloud the water making it hazy and somewhat opaque. The test relies on being able to see the difference between fresh, clear water and water that may have absorbed soap. It is a highly subjective test and only gives an indicator that soap may be present. It does not indicate the “amount” of soap present in the Biodiesel.

Shake Em Up Test

Passing Shake-Em Up TEst

Passing Shake-Em Up Test

Items Needed:
– Clean Glass Jar (at least 500 mL capacity)
– Distilled Water
– Biodiesel To Test

How To Perform The Test:
1) Fill a clean glass jar half-way full of Biodiesel
2) Fill the rest of the jar with distilled water
3) Cap the jar and shake it vigorously for about 10 seconds
4) Set the jar down and allow the water and Biodiesel to sit for about 30 min to 1 hour
5) Return and look at the water that has settled to the bottom.

Reading The Test:
If the water that settles out isn’t as clear as the water that went in, then soap levels are still fairly high and the Biodiesel should continue to be washed further.

If the water is as crystal clear as it was going in, there’s a good chance that soap levels are low. It’s now a good time to test with the Soap Titration Test to see what exact soap levels are.

Test #2: The Soap Titration Test
This test is an objective test that when properly performed will allow you to measure soap levels in Parts Per Million (PPM). It can be used to successfully identify exactly how much soap is present in your Biodiesel and is an excellent test to use to ensure that the soap levels in your fuel are low enough to run in a diesel engine.

How The Test Works
The test works very close to how an oil titration test works except in reverse. Biodiesel is diluted in highly pure isopropyl alcohol, a pH indicated called Bromophenol Blue is added and everything is mixed up well causing the mixture to usually take on a bluish green tint. Once mixed, small amounts of a very weak solution of Hydrochloric Acid (HCL) are added in measured amounts. If soap is present in the Biodiesel, the acid will neutralize the soap and the pH indicator will change from a greenish blue to a yellow color. The amount of HCL used to neutralize the soaps in the Biodiesel is recorded and used to calcuate the soap levels in Biodiesel in parts per million (PPM).

Items Needed:
– Bromophenol Blue pH Indicator
– 99% Pure Isopropyl Alcohol
– Hydrochloric Acid 0.01 Normal (HCL)
– Magnetic Stirrer
– Graduated Glass Beaker Capable Of Holding 150 mL Liquid
– 2 Glass Pipettes (1 mL or 10 mL works best)
– Biodiesel To Test

Pick up one of our three Biodiesel Soap Test Kits and get all the items needed to perform soap tests in a handy kit!
Basic Biodiesel Soap Test Kit | Deluxe Soap Test Kit | Ultimate Biodiesel Soap Test Kit

Test Biodiesel for soap with our soap test kit

Ultimate Soap Testing Kit

How To Perform The Test:
1) Measure 100 mL 99% Pure Isopropyl Alcohol into glass beaker
2) Place beaker on magnetic stirrer and begin stirring
2) Dissolve 12 mL (or 10 grams) of Biodiesel into the alcohol
2) Add 10-20 drops of Bromophenol Blue to tint the mixture and continue to mix
— This may tint it to a greenish blue, completely blue, or even just a light green
3) Using a glass pipette, add measured amounts of HCL until the mixture turns yellow
4) Record the amount of HCL used to turn mixture yellow
5) Calculate soap content

Calculating Soap Content:

Soap Calculation Multiplier:
NaOH Reacted Biodiesel – 304
KOH Reacted Biodiesel – 320
Times the amount of HCL required to turn yellow added to the multiplier
Example: 0.05 mL x 304 =  15.2 PPM

Acceptable ASTM Limits For Soap
– NaOH Reacted Biodiesel – No more than 41 PPM (0.0041%)
– KOH Reacted Biodiesel – No more than 66 PPM (0.0066%)

Reading The Test:
If your fuel tests within the ASTM limits, congratulations!
If your fuel is 100-200 PPM or less, it shouldn’t pose any real risks to plugging fuel filters or to diesel engines.
If your fuel is 200-300 PPM, it’s really right on the edge of what you should be using
If your fuel is 300-400 PPM, filter clogging may occur and soap content is getting high
If your fuel is above 400 PPM, it really should be washed again to lower the soap content
If your fuel is above 500, there’s a good chance you’ll see cloudy water in The Shake-Em Up Test

Video Example:
We have a great video example for this test available online. Click below to see the video

Video Example Of How Ion Exchange Resins Deal With Soap:
Soap testing of the Biodiesel is typically performed when using dry wash ion exchange media. This is because it’s so difficult to visually identify if the ion exchange resin really did it’s job of removing the soap. Below is a great video that explains what the ion exchange resins are actually doing to the soap.

Additional Resource
Explanation on soap testing and how it’s calculated – Infopop Biodiesel Forum

About Utah Biodiesel Supply

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20 comments on “Testing Biodiesel For Soap Content

  1. Many thanks for your efforts to have decided to put these things together on this site. Mary and i also very much valued your knowledge through your articles with certain things. I recognize that you have many demands on program therefore the fact that you took all the time just like you did to help people just like us by this article is also highly loved.

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  3. Greetings,

    Early in the video, the speaker says that he tests the alcohol with indicator to “make sure its not severely basic or acidic”. What is the indication that the alcohol is appropriately neutral for this test? What color is he looking for? If the alcohol does not pass the test, what is the indication and what is the fix to make it “neutral”?


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    • The method is based off of the AOCS (American Oil Chemists Society) test for soap content. It assumes that the “basic” portion of the biodiesel is soap when testing with bromophenol blue & isopropyl alcohol.

      Not sure what you mean by number though.
      The test ties indirectly to EN 14538:2006 in the ASTM Standard, which is a test to identify sodium & potassium in a liquid. The Biodiesel ASTM Standard allows for no more than 5 ppm of Sodium or Potassium.

      The soap test we calculate approximates that 41 ppm of Sodium based soap or 66 ppm of Potassium based soap is equivalent to 5 ppm of Sodium or Potassium content.

      • Hi,

        What I mean is the complete test method basis for this test (ex. ASTM D6751). If this is based on AOCS, can you please inform the complete test method number(AOCS- ???). Lastly, in the computation there were multiplier factor used, 320 for KOH and 304 for NaOH. Can you confirm if it can be used for a sodium methoxide or sodium methylate catalyst and can you also explained on how to arrive at 304 and 320 multiplier?
        Thanks to response

        • I’ll have to dig around & see if I can find the exact AOCS method number. Not sure what it is. I learned about it from our Dry Wash Column supplier several years ago.

          The 320 and 304 were numbers that some chemists arrived at several years ago that tie back to the 5 ppm of Sodium or Potassium in Biodiesel. I’m not sure how they came up with them though. You can use it with Sodium Methoxide, should be able to with Sodium Methylate (there’s plants that use the same soap test method using sodium methylate).

          Hope that helps.

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  9. I need some urgent advise please. ! In RSA we get HCL acid at 0,1N can I use this like it is or do I have to dilute it? I did my first soap test and using diluted HCL ( I added 10ml HCL 0.1N to 90ml distilled H2o.) after adding 2ml of HCL there still was no colour change. I then tried the test with HCL 0.1N and I got a reading of 0.2ml = 64 ppm. Is this correct ? please inform.
    2) I then tested another batch of biodiesel and this time the colour did not change to blue with the Bromophenol? I went as far as 25 drops and it changer to a yellow,green colour. when I added the HCL 0.1N it changed to a bright yellow with 0.25ml HCL 0.1N. I would really appreciate you feedback on this . Kind regards. Braam Landsberg

    • Braam,
      I just emailed you a more in depth article that talks about how to test with either normality of HCL. As far as the not changing to blue, that’s somewhat normal. It just means that batch has very little soap (soap is basic, if it’s not there, you may only get a slight tinge of green).

      Another trick is to add the bromo to the isopropyl BEFORE adding any Biodiesel to get a nice blue color. Then add the bio. If it goes right to greenish blue, you know there’s a good chance that there’s not much soap in your batch. Do the full test and then see what it comes in at. It sounds like you have low soap levels though on batch #2.

      Check the article I emailed you too.
      – Utah Biodiesel

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