Titrating for Soaps in Biodiesel
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Titrating for Soaps in Biodiesel

Topics Discussed:

Why Titrate for Soaps?

Titrating for soaps in your biodiesel can help you understand the success of your biodiesel process before and after washing. When used after washing, quite simply, it tells you how well you’ve washed your biodiesel. When used before washing, it gives you an idea of how close your process is to perfection and even more importantly, how much washing will need to be done, whether you use water washing or dry-washing.

Supplies Needed for the Titration

  • Isopropyl Alcohol
  • 0.1N or 0.01 N Solution of HCL (Instructions on how to make it provided)
  • Bromophenol Blue Indicator in water
  • 1% Phenolphthalein Indictator in alcohol
  • Pocket Scale or Accurate weighing device
  • Syringes
  • 250 ml disposable beaker

The 0.01N Hydrochloric Acid Solution

We make a 0.01N Hydrochlrocic Acid solution for soap titrations. We recommend using our already made solution since we make it in large batches, which leaves less room for error. If you purchase this, you can skip right to the soap titration.

However, you may also prepare your own 0.01N HCL solution by using concentrated hydrochloric acid and distilled water. To prepare a 0.01N Hydrochloric Acid solution from concentrated hydrochloric acid, start by downloading our excel sheet on converting concentrated hydrochloric acid to a specific molarity.

Next, you need to know the percent by weight of the concentrated hydrochloric acid you will be using. This usually does not change very much by the manufacturer, but it’s best to be sure what percent it is when you buy it. Check the current certificate of analysis for the hydrochloric acid’s concentration percent when you buy the hydrochloric acid, and make note of that concentration. To try and make your calculations even more accurate, we try to print links to the exact certificate of analysis for that particular HCL on the bottle. It is best to use that COA if available.

Open the excel sheet and enter the percent of the hydrchloric acid in the "% Concentration" field. It is usually around 32%. Next, enter the "# of Mols in Solution" that you desire. In this case, you should prefer to do a 0.01N solution for this titration, but some will prefer to make 0.1N. Finally, enter the total volume of the solution (in ml) you will make into the "Desired Volume of N Solution" field. Usually, most users will make a 1 liter solution. In this case set the volume to 1000 ml.

The Results:

The excel sheet will tell you the volume or weight of the HCL needed to make the 0.01N solution. It is best to use the weight, since that’s easily measured with a pocket scale. If using our 1 liter bottles, place it on the pocket scale and tare/zero it. Use a syringe or pipette to carefully dispense the hydrochloric acid into the bottle up to the amount needed in grams. Since it’s usually around 1 gram, you may want to do only 1 drop at a time and try to get as close as you can. Try not to go over too much as you’ll need to rinse out the bottle, let it dry and start all over again if you get too much in there. Don’t worry if it’s off by a few hundreths of a gram. Remember that these pocket scales are very sensitive. After each drop, give it a moment to settle. It will initially read the force of the drop hitting the scale, and then the actual weight when it balances out. Dropping too fast often results in too much HCL.

One you’ve put the right amount of HCL into the bottle, fill the rest of it up with distilled water to make exactly 1 liter. If using our bottles, 1 liter is to the bottom of the neck of the bottle. If you’re looking to be extremely accurate on the water, use a bigger scale sensitive to the gram, tare/zero it out and place the bottle onto it. Fill up to the amount of grams needed to make the 1 liter solution. 1 gram = 1 ml of water at 20°C, so you will need 1000 grams or 1 Kg of water to make a liter. The ml/g of water needed is also displayed in the results of the excel sheet for your convenience.

If you’d like to be more accurate in making your solution, simply increase the volume of the total solution to be made. For example, 3000 ml solution instead of 1000 ml. The more HCL you need to dispense, the more accurate your solution will be.

When using 0.1N Hydrochloric Acid

In order to obtain 0.01N HCL from 0.1N HCL, you simply need to dillute it with water down to 10%. Add 10 ml of 0.1N HCL to 90 ml of distilled water and it will give you 100 ml of 0.01N HCL. This is what you will need for the soap titration.

Testing for extra NaOH or KOH in unwashed biodiesel

Unwashed biodiesel can contain left over catalyst which throws off the results of the soap titration. Before titrating for soaps, it is best to test for this.

  1. Pour 100 ml of isopropyl alcohol into your 250ml beaker, then place it onto your scale and tare/zero it.
  2. Add 10 grams of your biodiesel.
  3. Next add about 5 drops of 1% Phenolphthalein solution. If the liquids in the beaker stay clear, then there is no catalyst in your biodiesel and you may proceed to the soap titration.
  4. If the liquids turn magenta, then you will need to neutralize the extra catalyst. Do this by adding the 0.01N HCL solution very slowly while stirring the liquid in the beaker. Stop when the liquid turns from magenta to clear.

Note: Using a magnetic stirrer is much easier when performing titrations than stirring manually.

Titrating for Soaps

Now the 0.01N HCL solution will be added to test for the soap content of the Biodiesel.

  1. While stirring the liquid in the beaker, add about 20 or so drops of bromophenol blue indicator solution to it. It should turn a dark blue.
  2. Weigh the beaker with the solution and record the weight.
  3. While stirring the solution, slowly add the 0.01 N HCL solution until the stirring solution turns from blue to yellow.
  4. Weigh the beaker with the solution again and record the final weight.

The Math to Figure Out Soap in PPM

Take the difference of the two weights recorded and that will be how many g of 0.01N HCL was added. Since the solution is basically water, 1 gram = 1 ml.

The Equation:

soap titration equation math


  • B = ml (or grams) of 0.01N HCL added.
  • C = Catalyst Factor (320.56 for KOH, 304.4 for NaOH)
  • W = grams of biodiesel in solution (should be 10g)

Multiply the final result of the equation by one million to get grams of soap per million grams of the sample. (ppm)

The Easy Way

The easier way to obtain ppm is to just multiply the amount of 0.01N HCL used in the titration (difference in weight in grams or total ml used) by the catalyst factor and you get soap content in PPM.

Using the Results for Dry-Washing


When using Magnesol, 1% by weight per 1000 ppm of soap should be used. The specific gravity of Biodiesel on average is about 0.860 so 8.6 grams of magnesol should be used for every 1000 ppm of soap per liter of biodiesel to be dry-washed. See below for the easy formula:

(PPM of Soap) x (Liters of Biodiesel) x (8.6) / 1000 = Grams of Magnesol Required

To get the most accurate results, it’s best to check the density of your biodiesel. Weigh out 100 ml of biodiesel on your pocket scale.

Divide the weight by 100 to get the density. For example, if your 100 ml of biodiesel weighs 86 grams, take 86g/100ml = 8.60g/ml (common density of biodiesel)

Ion-Exchange Resins

When using an Ion-Exchange resin, knowing the soap content of your unwashed biodiesel is important for maximizing the use of your resin. See our Ion-Exchange Resin Guide for more information.

ASTM Standards & Understanding PPM of Soap After Washing

The soap titration can be performed again after washing to see if the biodiesel is of enough quality for use in your diesel engine.

ASTM Standards call for no more than 41 PPM of soap when NaOH is used as your catalyst and 66 PPM when KOH is used. If the fuel tests at or below these standards, you have quality fuel. ASTM standards do not usually need to be met for clean operation, but are required for commercial use of the fuel. See the chart below for generally accepted values for home-brewers.

Analyzing the quality of Biodiesel based on soap content chart

Soap Content Fuel Quality
at or below 41 PPM (NaOH) or 66 PPM (KOH) Within ASTM standards
Above ASTM Standards but Below 200 PPM Should not pose any threat to a fuel filter or engine
200-300 PPM maximum soap content which should be allowed in fuel
300-400 PPM May clog fuel filters, not recommended, wash more
400-500 PPM High soap content, not recommended, wash more
Above 500 PPM Can possibly leave ash in your engine and cause longterm damage, not recommended, wash more

Fuel which is around or above 300 PPM of soap should be washed more before use. It is important to be sure your biodiesel is low enough in soap content to preserve your fuel filters and prevent longterm engine damage.

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