How to determine the alcohol content of petrol

To determine how much alcohol has been blended into the petrol by a petroleum company and to guage how much water is present in the fuel, the following test can be done. This is a simple yet valuable test, a procedure that almost anyone can do.

CAUTION - Always take extreme care when handling any flammable liquids or gas such as petrol and always be sure to have adequate ventilation in the storage or test area. Avoid sparks, naked flame, excessive heat and/or any other source of ignition for flammable liquids such as petroleum, alcohol, gas or similar liquids. Fuel vaporization occurs from any fuel and/or container left open or partially open which can be extremely HAZARDOUS and DANGEROUS !

You will require a graduated glass cylinder with increments marked in milliliters (ml).

1. Pour 250ml of petroleum to be tested into the graduated glass cylinder.

2. Add 50ml of water to the 250ml of sample petroleum in the glass cylinder.

3. Agitate the sample in order to mix the petrol and water thoroughly.
4. Place down firmly and let the liquids come to rest for a minute.


If there is no alcohol in the sample, there will still be only 50ml of water that has separated into a layer at the bottom of the glass cylinder.

If any amount of alcohol is present in the sample, the amount of water/alcohol will accumulate at the bottom of the glass cylinder and will have increased by the amount of alcohol that is present in the 250ml petrol sample.

Now you must subtract the original 50ml of water that was added at the start of the test from the amount present and then you can determine the percentage of alky that has been added to the petroleum by the oil company.

N.B - If there was any water present in the petrol sample, the water will be included in the amount seen at the bottom of the glass cylinder. Therefore, any amount present in the test sample that exceeds the listed amount of alcohol (either 85% for E85 or 10% for E10), must be water.

It is estimated that there can be anywhere from 65% to 85% for E85 and not more above 10% alcohol for E10, but the figures can be lower. If it is lower, there could be water present in the fuel.

A larger amount of water will cause two layers in the glass test tube to separate out at the bottom of the vessel when testing alcohol-containing fuel if the amount of water present is sufficient to saturate the alcohol. Therefore, the layer seen at the very bottom of the glass cylinder will be water, followed upwards by a blend of water/alcohol and petrol above these layers.

  • Always fill up at high volume fueling stations.
  • Do not store petrol for any longer than what is necessary.
  • Petrol older than 3 months can cause severe damage to your engine.

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