Let’s discuss titrations. In nearly any biodiesel tutorial, one of the first things that people learn is “how to titrate oil”. Typically, these tutorials would have you:
- Carefully make a 1 gram per liter solution of distilled water and either potassium or sodium hydroxide. (for my examples here we will use sodium hydroxide) This will henceforth be called the caustic solution.
- Dissolve a 1 ml sample of oil into about 10 ml of isopropyl alcohol.
- Add some ph indicator.
- Slowly add the caustic solution until the ph indicator shows a basic solution.
- The number ml of the caustic solution used shows the number of grams of NaOH needed to neutralize the free fatty acids present in the oil. This quantity is then added to base amount of NaOH that is always use in processing to give you
a total quantity of base needed per liter of oil to be processed.
This is all well and good, except that all of the following steps depend on the accuracy of step 1. If anything throws off the accuracy of your carefully prepared 1 gram per liter solution of NaOH, then the whole thing is bunk. Not only that, but there is no way to even tell if you did step one correctly. And even if you did do it right, the fact that a caustic solution will degrade rather quickly with time means that the next time you use this carefully prepared solution, it will be of a slightly different strength than the time before. (Incidentally, the two primary sources of sodium hydroxide degradation involves its reacting with glass to form sodium silicate, and its reacting with carbon dioxide to form sodium carbonate. Therefore an airtight dark plastic container is typically ideal for NaOH solution storage.)
The following is a simple way to test the strength of your caustic solution prepared in step one. This will allow you to have confidence in the accuracy of your titration every time. This is true even if your caustic solution is mis-measured or degraded.
- Purchase a stock solution of .0100N HCl (hydrochloric acid) or KHP (potassium hydrogen phthalate). These stock solutions are very carefully measured to be exactly what is claimed on the bottle. The .0100N represents .01 moles of solute (HCl or KHP) per liter of solution. These types of stock solutions can be found from most chemical suppliers.
- Mix 10 ml of this .0100 N solution with a drop of ph indicator (phenol red or phenolphthalein) and 10 ml of distilled water.
- Titrate this solution with your caustic solution carefully counting the exact number of ml needed to get a color change.
- Divide 4 by this number to find the real strength of your caustic solution in grams of NaOH per liter of water.
- Don’t try to change the concentration of your caustic solution to try to make it come out at exactly 1. Just titrate your oil as you normally would with this solution. Take the result of your titration and multiply it by the real strength number that you obtained in step 4.
Let’s look at an example:
- I grab an old caustic solution off the shelf and I want to use it to titrate some oil. So first of all, I get a clean beaker and add 10 ml of distilled water and 10 ml of my .0100 N KHP solution and a drop of phenolphthalein.
- Then I slowly add my caustic solution while stirring until the mixture turns pink. I find that this takes 5.3 ml
- I divide 4 by 5.3 to get 0.755. This means that my basic solution really only has an equivalent of 0.755 grams of NaOH per liter.
- Now, if it takes 6.3 ml of this solution to neutralize the FFA in 1 ml of oil, I know that the real neutralization number is only 6.3X0.755= 4.76
Not too bad.