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How to measure the ethanol content of gasoline

Testing ethanol content of
gas"Ok, so an (Amazon Associate paid link to a) graduate cylinder, and what I assume is fuel. But no details on what test you're doing, how you're doing it, or how to interpret the results. Let me rush right out to click your link, buy the cylinder, and pour some gas into it. Not that I'd know what the hell anything meant, but hey, you'd make a dime, right?"

While this comment is a remarkably trollish response to a blog post meant to show you tidbits of our personal life, I had been meaning to give our readers more details on how simple it is to perform a test for ethanol content of gasoline.  All you need is something that easily measures volume --- a 100 mL or larger graduated cylinder takes nearly all the math out of your hands, but you could just as easily use a ruler in a straight-sided glass cup or jar.

The idea is that when you mix water with gas, any ethanol in the gas comes out of solution and joins the water instead.  So all you have to do is know how much water you initially added to the gas, subtract that out of the clear layer at the bottom of your graduated cylinder at the end of the experiment, and the rest of the clear substance is ethanol.

When starting your ethanol test, the first step is to take your gas sample carefully.  As a far-more-constructive commenter mentioned, you should run at least a gallon of gas into your car before taking a sample from any pump that uses a shared hose.  Then pump a sample into a container and bring the gas home to experiment.

Marking 110 mL on a graduated cylinder

In the meantime, you should take a minute to prep your graduated cylinder (assuming you're like us and only bought a 100 mL one instead of a cylinder with a larger capacity).  Later in the experiment, you'll need to know where the 110 mL line is, which is easy to guestimate by measuring the distance between the 90 and 100 mL lines, then measuring that same distance above the 100 mL line.  To keep things simple, use a piece of tape to wrap around the graduated cylinder at the 110 mL line, marking its location.

Now you're ready to add the gas.  I found it much easier to pour some gas into a small container rather than trying to fill the graduated cylinder from the gas can.  You want to add gas up to the 100 mL line, and don't forget your chemistry lessons --- read from the bottom of the meniscus!

Mixing gasoline and water

Top the gas off with 10 mL of water, meaning that the total liquid level should match the taped 110 mL line.  Then mix the contents.

Graduated cylinder mixing

(Mixing was the hardest part for me because our graduated cylinder didn't come with a stopper, and I ended up slopping a bit of liquid out while plugging the top with my palm.  I think a better solution would have been to use a sandwich baggie pulled over the top of the graduated cylinder, or to invest in some parafilm.  Either way, thorough mixing is imperative.)

Ethanol test results

After mixing, the clear ethanol and water will settle to the bottom, while the colored gas will sit above it.  You can easily measure how much water and ethanol is present, then subtract 10 mL from that to find the percent ethanol in the water.  Probably because of my sloppy mixing, our clear layer came to 17 mL, producing a reading of 7% ethanol instead of the 10% listed on the pump at the gas station.  (If you didn't use the exact amounts of gas and water I listed above, you'll have to do a bit more math: ((Clear layer - Water)/(Gas))x100%.)

As Mark has mentioned previously, ethanol in gas can wreak havoc on many of the small engines found on our farm, so it's useful to know for sure that the ethanol-free gas we've hunted down really doesn't have any ethanol in it.  We'll be testing gas stations soon and hoping to find one near us that is really ethanol-free.

Our automatic chicken waterer makes it easy to leave home without worrying about your flock.

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Hi Anna and Mark,

Just got back from Lex, VA. I wonder whether I was near you?

Also, It would seem that if only small amounts of ethanol fuel are needed you could add water to your fuel can, mix and then decant "clean" fuel?

And confirm things are OK by remeasuring the ethanol in the output product?

Since Anna is a chemist, what do you think?


Comment by john Tue Aug 6 10:16:17 2013

I don’t know why you even bothered to give “Anonymous” space on your site. Even though you didn’t provide allot of details in the post it didn’t take much thought to understand what you were attempting and how to go about it.

I completely enjoy your site. In fact it is only one of two that I check daily. The other belongs to a family member. You are constantly doing something new or updating a previous project and are generous enough with your time to share it with us. Sometimes the posts leave me scratching my head but they always give me something to think about.

Keep up the good work and thank you for sharing. It is appreciated.


Comment by Ned Newby Tue Aug 6 12:47:33 2013
Very interesting process. Thanks for taking us through it step by step. I was surprised that the gas was clear. Didn't it used to be golden? Way back when?
Comment by Robin E. Tue Aug 6 13:50:11 2013

You don't have to tape off 110 ml. Just fill it up with 90 ml of gasoline, and add 10 ml of water. Approximately* everything over 10 ml is ethanol. ( * If you mix 50 ml of ethanol with 50 ml of water you don't end up with 100 ml, but slightly less because you're dealing with partial molar volumes)

Say you do it like this and get a level of 19 ml doing this. That means ≅ 19 - 10 = 9 ml of water soluable stuff (presumably ethanol) in the gasoline. The ethanol content of the gasoline is then ≅ 9/90 = 10% by volume. This is of course different from the content by weight

Comment by Roland_Smith Tue Aug 6 14:49:49 2013
In Australia the Ethanol amount indicated is the maximum they can put in, if your rules are the same then that could explain 7%.
Comment by Trevor Tue Aug 6 20:52:35 2013
Thanks for doing it step by step. Appreciated!
Comment by Lois Fulo Sat Aug 13 01:46:43 2016

One very unique homestead, $1,500 per acre, the opportunity of a lifetime