Making Biodiesel: Titration of Oil with Curry Indicator
Channel: Orion Lawlor
Date: 2015-08-06

I'm measuring the free fatty acids (FFA) in used fry oil using a curry pH indicator.  

Inside the syringe is a "reference base solution", made by mixing 1 gram of KOH (potassium hydroxide) with 1 liter of water.  This makes 1 milligram of KOH per milliliter of water.  This solution needs to be kept covered, because it will slowly pull CO2 out of the air and neutralize the base.

In the white cup I've previously prepared an indicator solution, by soaking curry powder (turmeric spice) in isopropyl alcohol, and pouring off the deeply colored liquid.  This indicator liquid is yellow in an acidic or neutral solution, and turns a deep red in a base solution (pH above about 8), due to the magic of curry.  

We measure the free fatty acid in oil by dissolving 1 mL of oil in 10mL of the curry/alcohol indicator.  We then use the syringe to squirt in a few milligrams of base at a time, until the curry turns red.  This is the number of milligrams of base needed to neutralize the acid in 1 mL of oil.

Be sure to do a "blank titration" on the same batch of indicator before you add any oil, since some sources of curry or isopropyl alcohol are already somewhat acidic--my curry+isopropyl took 8mL of reference solution (8mg of base) on its own.

I did this so I know how much base to use to neutralize the free fatty acids when making biodiesel from used oils, but this titration process is frankly quite beautiful on its own!