Testing the Percentage of Ethanol in E10 Pump Gas
At present the vast majority of gas stations in the U.S. sell E10 gasoline.  Several areas in the U.S. have been providing E10 gasoline for over 10 years.  As a result of the Clean Air Act of 1990 some areas, including Northern Virginia near Washington, DC, were using E10 by January of 2003.  The State of Virginia converted entirely from MTBE to Ethanol in 2006.  Further, in the U.S. the Environmental Protection Agency (EPA) approved E15 gas for sale for use in 2001, or newer, passenger vehicles and light trucks, during 2011.  While not in wide use, E15 was first offered in the U.S.at a gas station in Kansas during July 2012 and is now sold in some gas stations in Kansas, Iowa, Illinois, Nebraska and South Dakota.   At this time, E15 gas is not approved in the U.S. for use in motorcycles, and a variety of other vehicles, and is not offered for sales widely in the U.S.
E10 gas sold in the U.S. may have up to 10% ethanol.  It is however, not required to have 10% ethanol. The percentage of ethanol actually in the gas at the pump varies based on tax incentives, economics, seasonal environmental air pollution conditions, and other factors.  The actual ethanol level contained in E10 gas delivered to a station is required to be noted on the invoice for the gasoline, although most attendants in gas stations will not have a clue about this or have access to this information.  Since it was not possible for me to obtain the actual ethanol content of many of the gasoline test samples which I have collected, I can not state how accurate the following measurements are.  However, the results of this testing are consistent with other measurements that I have made on other gas samples in Northern Virginia, and consistent with the information which I have been able to obtain from drivers of fuel delivery vehicles as they have refilled tanks in local fuel stations.
The following is a look at how to determine whether ethanol exists in a gas sample, and how one can determine approximately how much ethanol is contained in the gasoline.
Measuring Ethanol Content in E10 Gasoline
Ethanol content in gasoline can be measured through phase separation.  In order to phase separate ethanol from E10 gasoline, an amount of water greater than that which can dissolve in the ethanol contained in the gasoline, is added to the E10 gasoline.  The excess water saturates the E10 gasoline, and causes the blended ethanol, and any water that it might be carrying, to phase separate out of the mixture. Due to the differing specific gravities of the fluids, the gasoline content of the E10 will float on top of the water and ethanol.
To measure the amount of ethanol in the E10 gasoline in gas samples, I measured specific volumes of E10 gasoline and added a proportionally large volume of water.  I used a specially calibrated test tube to make reading the percentages of the fluids easier.  (This tube was originally developed for the aviation community to help pilots of small planes check the purity of fuel.).

Using this procedure, first water is added to the "fill line" in the tube.  This provides the additional water volume which will cause phase separation in the fuel sample.

Water measured in

Next the gasoline sample to be tested is added to reach the fuel "fill line" at the top of the tube. The small white vial in the below picture contains an ethanol marking dye that will be added in the next step.

Gas Measured in

To aid in identifying the ethanol which will separate from the E10 fuel sample, a drop of blue marker dye is added to the tube.  When the water/fuel sample is shaken, this dye will color the water/alcohol in the mixture, but not the gasoline.


Dye Added

The gas/water/dye mixture is then shaken to mix the water through the E10 gasoline and overload the ability of the E10 gasoline to suspend the dissolved ethanol.  The tube is then allowed to rest, and the ethanol content which had been dissolved in the gasoline will settle to the bottom of the tube.  The phase separated ethanol settling to the bottom of the tube will cause the total volume of water/ethanol to increase in the bottom of the tube.  The blue dye will color the water/ethanol in the bottom of the tube to aid in an accurate reading of the new water/alcohol level.  And, because of the way the volume measurements have been calibrated in this test tube, the percentage of ethanol previously held in the gas sample can now be read directly from the scale line on the side of the tube.
This particular gas sample was collected on June 18, 2010 and the ethanol content tested that same day.  This measurement shows 7.5% ethanol in this particular sample.

Ethanol Level
The above was a sample of 93 octane Sunoco gasoline from the northern Virginia area near Washington, DC taken on June 18, 2010.  On that day I drew four separate fuel samples, one from each of the octane levels offered for sale by the station.  Of note is the fact that each of the four different octane levels sold at this station (87, 89, 91 and 93) all tested to 7.5% ethanol on this date.  Also, other brand fuels from nearby gas stations had approximately the same amount of ethanol - between 5% and 8 % - on that date.  This testing was done on the date that I collected the fuel, which was only a few days after delivery to the station.

NOTE:  I held additional samples of each octane, and re-ran the test on the stored samples of this gas after 14 months of storage, to determine how ethanol content in E10 gas might vary after prolonged storage.
Alternately, on November 1, 2011, samples of Sunoco 93 octane E10 gas that I tested (which was marked as having 9 - 10% ethanol by the distributor), all showed the ethanol content to be 7%. While this method of measuring ethanol content through phase separation is said to be fairly accurate, I have still not had a chance to personally verify the accuracy of this testing method by testing a known sample of E10 gas.  However, at least at the gas stations that I have been testing, the E10 gas in my area is not overblended.
The test kit which I used for these measurements was initially developed for the aviation industry to show the existence and relative percentage of ethanol which may be present in gasoline.  This method of ethanol measurement is not intended to be as precise as that conducted in a laboratory.  However, the measurement is generally accurate within the tolerance of the markings on the tube and is good for showing relative comparisons of ethanol in E10 gas from different sources.  Measurement of many samples of E10 fuel that I have collected throughout northern Virginia indicates that there is not a problem with E10 fuel being overblended to contain excessive ethanol, at least in my area.  It is more likely that seasonal blending of gasoline will result in slightly lower levels of ethanol content.

It is important to note though that in order to cause the ethanol to separate from the E10 gasolin, a significant amount of additional water must be added to the E10 gasoline and the mixture shaken, While E10 gasoline may lose its oxygenating ability and some of its alcohol content if stored - which equates to loss of octane - E10 gasoline will not phase separate simply because it has been stored for a period of time. 

a look at the effects of storing E10 gas for a period of time on phase separation, click here.

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