Keywords: Biodiesel, feedstock, free fatty acids, acid number, triglycerides, acid value, titration, vegetable oil, biofuel, soap, glycerin, saponification, transesterification

What is Titration Test? Titration is an indirect test for free fatty acids (FFAs) in oil. The test is done by reacting a small sample of oil containing FFAs with a measured amount of base, and using pH to tell us when the FFAs are all used up. The titration test is done to find out how much free fatty acids are present to compensate for it by adding more base catalyst so there is sufficient left over for the desired biodiesel reaction.

Why we do Titration Test? The biodiesel reaction uses alkaline base, NaOH or KOH, as a catalyst. Waste oil contains FFAs, which will react with the catalyst to make soap before the catalyst has a chance to participate in making biodiesel. This specific amount of base consumed as a side reaction is hence sacrificed to the soap making process.

What is Acid Number? The titration test gives a number called acid value or acid number. In this test, every 1 ml of titration solution used (i.e. the acid number) means to add an extra 1 gram of base for each liter of oil to compensate for the side reaction caused by the FFAs. Titration test is done on 1 ml of oil, and the results are projected to the whole batch of biodiesel. To avoid any miscalculations, it is therefore suggested to repeat the test at least three times to ensure reliable results.

Material Required:

Ø Phenolphthalein pH indicator 1%.

Ø Bottle of distilled water.

Ø Bottle to store 0.1% NaOH or KOH stock solution

Ø Isopropyl alcohol – at least 70% isopropyl rubbing alcohol or 99%

Ø Three oral syringes – graduated in milliliters.

Ø Measuring cup or beaker to measure 1 liter – or a graduated cylinder

Ø Three small jars or beakers – one for titration, one for oil sample, and one for small portion of 0.1% NaOH solution so as to avoid contamination of the whole bottle of stock solution.

Ø A scale capable to weigh 0.1g of chemicals

Procedure:

• Prepare Reference Solution 0.1% in distilled water -- This solution is stable for quite long time unless you see contamination, solids or change in color. Measure exactly 1 gram of catalyst (NaOH or KOH) and dissolved in 1 liter of distilled water. Minimize exposure to the open air since these chemicals are very hygroscopic, absorbing moisture which can invalidate the weight. To ensure accurate weights, measure 5 grams of the chemical and dissolve in 500 mL of distilled water (1% stock solution). Then dilute this solution by adding 100 mL of it to 900 mL of distilled water to make the desired 0.1% reference solution (for 500 mL of the desired 0.1% reference solution, add 50mL of the 1% stock solution to 450 mL of distilled water). Every 1 mL of the 0.1% reference solution contains 1/1000 of a gram of catalyst – an amount too small to weigh. Keep the stock solution sealed to last for many titrations. To ensure accurate titration results, use the same catalyst planned for biodiesel reaction, which may be the same batch of the catalyst acquired for making biodiesel.
• Isopropyl Alcohol + Phenolphthalein: Take 10 mL of isopropyl alcohol in a jar or beaker and add 5 drops of phenolphthalein indicator solution. Alcohol can become slightly acidic with age or exposure to air, which can affect the titration results. The alcohol is therefore neutralized with the reference solution of 0.1% before adding the oil sample. To do this, add the reference solution drop-wise until the color of isopropyl alcohol changes to pink and stays for 30 seconds. If it takes more than 12 drops, it means the isopropyl is too acidic to be used in the test. It is recommended to get fresh isopropyl alcohol. The amount of 0.1% reference solution added in this step is not required in calculations.
• Adding Oil Sample: Measure exact 1 mL of oil using a syringe, and add it to the same jar or beaker containing 10 ml of isopropyl alcohol mentioned in the previous step. Fill the syringe to 2 or 3 mL and then drop the level down by 1 mL to ensure accurate measurements. Use different syringes for isopropyl and oils to avoid cross contamination. If solid fat is used, warm it first and then take 1 mL of it. The pink color of isopropyl alcohol now changes to yellow because of the addition of the acidic oil, which brings the pH of the mixture below 6. Swirl the jar or beaker to dissolve everything.
• Adding Reference Solution 0.1%. The amount of reference solution added in this step is very crucial for titration test. Put a portion of this reference solution into another jar or beaker to avoid contamination of the whole bottle. Fill the syringe to 3 or 4 mL mark. Make sure there is no bubble in the syringe. Note the initial mark on the syringe before starting titration.
• Titration: Start adding to the jar/beaker containing isopropyl alcohol and oil a small amount (¼ milliliter at a time or drop-wise) of the reference solution and keep swirling the contents until it turns the jar/beaker contents a pink color that lasts for 30 seconds. This means that all the FFAs are neutralized by the reference solution. Note the final mark on the syringe.
• Calculate: Each 1 mL of reference solution corresponds to 1 extra gram of catalyst needed for each liter of oil in order to neutralize the free fatty acids. Average restaurant oil normally exhibit about 3 mL on titration while really bad grease can take 10, which is hard to work with.
• Projection of the test results to Larger Batches: Determine how many liters of oil are needed to be processed (one gallon has 3.784 litters of oil). The following calculation can be used to determine the amount of catalyst required.

                Total NaOH required = Number of liters of oil x (7 grams + titration results)

Example: Titration results showed 2 mL, i.e. 2 extra grams, for each liter of oil used to make biodiesel. If reacting 190 liters of oil, use the following calculation:

Total NaOH required = 190 liters of oil x (7 grams NaOH + 2) = 190 x 9 = 1710 grams

                Total KOH required = Number of liters of oil x (9 grams + titration results)

Example: Suppose titration results showed 4 ml, i.e. 4 extra grams, for each liter of oil used to make biodiesel. For KOH use 9 grams per liter instead of 7 grams per liter as base level. If reacting 190 liters of oil, use the following calculation:

Total KOH required = 190 liters of oil x (9 grams KOH + 4) = 190 x 13 = 2470 grams

•  KOH comes in a variety of purities, e.g. 93% purity. Then the above calculations are adjusted to supply sufficient amount of the catalyst. Only the base level is required to be adjusted since the titration automatically reflects the impurity level in the test. The above calculations then become like this:

               Total KOH required = No. of liters x [(9 grams KOH ÷ Percent Purity in decimal format) + titration results]

Example: Suppose titration results showed 4 ml, i.e. 4 extra grams, of 93% pure KOH for each liter of oil used to make biodiesel. If reacting 190 liters of oil, use the following calculation:

Total KOH required = 190 x [(9 grams KOH ÷ 0.93) + 4] = 2599 grams

Tricks to improve the accuracy of titration test:

§ Working with larger amounts of chemicals: Using 5 ml of oil instead of 1 mL would reduce the margin of error. The result obtained is divided by 5, in this case, to determine the amount of catalyst required per liter of oil.

§ NaOH/KOH is very hygroscopic: These chemicals readily absorb moisture while exposed to humid air, especially for low levels. Measuring more than 1 gram at a time and then diluting to lower concentrations can reduce this error.

§ Catalyst quantity: The more catalyst used, the better the quality of the biodiesel is up to certain point; however, more catalyst causes more soap production. Generally if working with high-FFA oil, over 4 ml on titration, or oil with high water contents, begin with smaller quantity of catalyst. Accurate titration measurements can make it easier to troubleshoot any problems encountered.