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Biodiesel Quality Testing Procedures

In my three part series called Introduction to Biodiesel, I explained what biodiesel is, and how it is made, stored, and used. You’ll find links to it in the resources section below.

Previously I explained how to perform the titration of the oil. You can read it here: How to Perform Biodiesel Titrations. In today’s post, I will describe some other tests you’ll need to perform as part of the process. It is important that we confirm the finished product is good enough to burn in our vehicles, and these tests will help us do that.

The 27/3 Methanol Test
The purpose of this test is to determine if all of the vegetable oil has been converted to biodiesel. While unconverted oil can be burned in a diesel engine, it requires modifications to the fueling system where biodiesel does not require such a conversion. So we want to be sure 100% of the oil is converted.

The premise of the test is that biodiesel will completely dissolve in methanol but oil will not. The test was originally proposed by a guy named Jan Warnqvist as a 25/225 test. He took a 25ml sample of the biodiesel and mixed with 225ml of methanol. If the conversion is 100%, the mixture should become clear. If not, any unconverted oil will settle out to the bottom. Every 1 ml of settled oil represents 4% unconverted oil in the whole batch.

The test has since been modified to use 3ml of oil in 27ml of methanol, probably because of the cost of methanol for small batch home brewers. I do my test in a clean test tube. Place 27ml of methanol into a test tube, add exactly 3ml of biodiesel and shake vigorously. Wait a few minutes to see if any oil settles out. If not, all of the oil was converted. If 100% of the oil was converted, the methanol will be crystal clear with a slight yellowish color. In the picture below, you can see a result showing a trace of unconverted oil. The tiny oil droplets have formed on the inside of the test tube:

27/3 Methanol Test with Trace of Oil

27/3 Methanol Test with Trace of Oil


For my process, we estimate the amount of oil in grains of rice. We’ll use this number in one of the steps of the process which will be discussed in detail in a later post. For now, that’s how the test works.

The Soap Test
The purpose of this test is to determine if there is any soap in the biodiesel. Soap is a result or unconverted oil, catalyst, and water. It is not good for a diesel engine and fuel system.

The test is performed by mixing 50/50 biodiesel and water. After a few minutes, they will separate with the biodiesel on top of the water. Any residual soap will settle to a layer between the two.

The test needs to be performed using a graduated cylinder which gives a more accurate result. I’ll explain what this means. All of the soap forms a thin layer between the biodiesel and the water. So in a given size sample, there is a certain amount of soap – hopefully none but we’re focused on the case when you have some soap. If you perform this test with a wide container such as a Ball jar, the soap will be spread out over a larger area and might not be as visible. When you perform the test in a graduated cylinder with the same size sample, the same amount of soap is concentrated in a much smaller area and would be more visible. Conversely if the test still shows no soap, you can be more confident in your positive result.

I use a 100ml glass graduated cylinder. Place a 50ml sample of biodiesel in it. Add 50ml of distilled water, place your hand over the open end, and shake vigorously for a few seconds. Now wait 10-15 minutes for it to completely separate. The time will vary – just be patient. Below are some pictures of the test and both passing and failing results.

In this first picture, the biodiesel and water are mixed and will take a few minutes to settle:

Soap Test - Shaken Up and Beginning to Separate

Soap Test – Shaken Up and Beginning to Separate

In this picture, you can see what a negative result looks like. There is a noticeable white layer between the water and the biodiesel. Also the water on the bottom is cloudy. That is full of contaminants that we don’t want in our finished product:

Soap Test - Negative Result

Soap Test – Negative Result

This final picture is of a positive result. I took it from an angle looking up. The bottom of the biodiesel layer is visible through the water. You can see there is no white soap layer and the water is very clear. This biodiesel is clean and free of contaminants:

Soap Test - Positive Result

Soap Test – Positive Result

pH Test
The pH test is less common than the previous 2 tests. The goal is still to determine if there is any catalyst left in the biodiesel. Catalyst is a base and will affect the pH. We could try to test the pH of the biodiesel directly, but reasonably priced pH meters or even test strips will not work in biodiesel. They work much better in water.

What we can do is test the pH of the wash water before and after the wash. When the pH of the water doesn’t change during the washing process, we have all the catalyst out. I personally don’t use this test anymore in favor of the soap test. I’ve found that it isn’t necessary because of the way we wash the biodiesel, as I will discuss in future posts.

Water Test

We don’t want water in the finished biodiesel. There are elaborate test kits you can buy, but these are typically used by large scale producers. I produce smaller batches of 11-12 gallons and don’t use a test kit.

The rule of thumb is that the biodiesel will be cloudy at room temperature if water is present, but it will be clear if it is sufficiently dry. I heat the biodiesel to 130F and let it cool down to room temperature. If it is crystal clear and looks like iced tea, it is dry. If not, I heat it again. It rarely takes more than 1 heating cycle.

This picture is of a 1-gallon beaker of biodiesel that has been heated and allowed to cool down. You can see that it is very clear and “dry”:

Crystal Clear After Heating and Cooling Down

Crystal Clear After Heating and Cooling Down

What’s Next?
These test procedures will help you verify the quality of your biodiesel. Now you need to build yourself a processor and start making it. In my next post about biodiesel, I’ll talk about the processors we built and how they work. Then later I’ll describe the 2-stage process we use. Please join me next time to learn more.

Resources for this post:
Introduction to Biodiesel – Part 1
Introduction to Biodiesel – Part 2.
Introduction to Biodiesel – Part 3.
How to Do Biodiesel Titrations.

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