How Using Sulfuric Acid Works When Making Biodiesel

If you’ve been making Biodiesel for a while, chances are you’ve run across oil that titrates really high. You know, that really crappy stuff that ends up making really big emulsions, creating big globs of soap, lowering your Biodiesel yield, and generally makes life a pain to make it into Biodiesel.

Most people try to avoid it at all costs, but there is a method you can use that actually can help you generate great batches from this nasty stuff…and it involves using sulfuric acid.

To explain why it works, we need to do a little chemistry explanation of the Biodiesel reaction.

To make Biodiesel, we start out with an oil molecule called a triglyceride. These are molecules with three fatty acid chains connected to a glycerol backbone.


To make Biodiesel, a catalyst is needed; either Potassium Hydroxide (KOH) or Sodium Hydroxide (NaOH). The catalyst is mixed with methanol to create methoxide. The oil is heated and the methoxide is mixed in and the reaction begins.

As the catalyst is mixed with the oil, it breaks the chemical bond between the fatty acid chain and the glycerol back bone. A methanol molecule then attaches to the fatty acid chain and a Biodiesel molecule is created (technically called a Fatty Acid Methyl Ester).


This process, technically called transesterification, repeats itself until all the fatty acid chains are stripped away from the glycerol backbone and reacted into Biodiesel.



So, this is the way it happens in theory. Break the bond between the fatty acid chain and glycerol and attach each fatty acid to a methanol resulting in Biodiesel molecules. This all happens in sequence meaning that each fatty acid chain is broken away one at a time.

To help the explanation out, it’s important to know that a glycerol molecule with 3 fatty acid chains is called a triglyceride. If it’s lost one fatty acid chain and only has two still connected it’s called a diglyceride. And if it only has one fatty acid chain left, it’s called a monoglyceride (Tri=three, Di=two, and Mono=one).

So, that’s how it happens in theory….in the real world, things are much different. What usually happens is that before the catalyst & methanol even get added, many of the oil molecules are already missing fatty acid chains.

This occurs naturally as the oil ages or as the oil is used in fryers under extreme heat. This means that waste vegetable oil tends to have a lot of diglycerides and monoglycerides in it (glycerol molecules missing some of their fatty acid chains). The older the oil is or the longer it was used under heat, the more of them there are.

When the fatty acid chains break off through aging or through heat, we refer to them as free fatty acids (or FFA’s). This is because they’re not hooked to any other molecules. There’s a lot of fancy chemistry that explains how they break away from the glycerol molecules, but for the purpose of this article, let’s just assume that the chemical bond that held the fatty acid to the glycerol has “rusted” away. This means that each “free-fatty acid” has one end that’s rusted and there’s a bit of rust still left on the glycerol backbone. We’ll designate the rust with an r.


When we try to make Biodiesel with free fatty acids in the oil, these free fatty acid chains start attacking the catalyst and form soap.


If there’s enough free fatty acids in the oil, we end up with lots of soap and not enough catalyst left over to break all the bonds of the remaining tri, di, and monoglcyrides molecules. In other words, we end up with soap, unconverted oils (tri, di, & mono’s), and some Biodiesel. I like to call this mixture partially reacted Biodiesel; because there’s still so much unreacted oil left in it.

To counter this problem, we do something called an oil titration on the oil. What we’re doing when we titrate oil is measuring how many free fatty acids there are in the oil. Think of it like literally counting all the fatty acid chains that have broken away from glycerol molecules.

Once the free fatty acids are all counted (the titration number), we use that number to tell us how much extra catalyst to add to the reaction to ensure we react all the free fatty acids into soap. Then, when we’re done making soap, we still will have enough catalyst left over to break the bonds on fatty acid chains still attached to glycerol molecules which is then reacted with methanol into Biodiesel. We call making Biodiesel this way a “base reaction”. A strong base (KOH or NaOH) reacts with the oil and methanol to make soap and Biodiesel.

The more free fatty acids that a given oil has, the less Biodiesel we will get out of it. This is because we’re reacting fatty acids into soap instead of Biodiesel–remember, it’s the fatty acids combined with methanol that make Biodiesel. If there’s a lot of free fatty acids in the oil, that means there’s a lot less fatty acids left that we can make into Biodiesel. So, the higher the titration a given oil has, the less Biodiesel you’re going to be able to get out of it.

Ok, with that chemistry lesson out of the way, lets get back to using sulfuric acid. Here’s how it works.

Remember I said that free fatty acids have that little bit of rust on the end? Well, it’s that rust that keeps them from bonding to methanol to form Biodiesel and instead makes them attack the catalyst to form soap.

Well, what if we could modify the rusty ends of the free fatty acid so that it could bond to methanol again? That’d mean we could react free fatty acids into Biodiesel instead of into soap, right? That would also mean we’d be able to get more Biodiesel out of high titrating oil, right? That also means less soap made too! Hey, this is great! And that’s EXACTLY what happens when we use sulfuric acid!

Here’s the analogy that I use to explain what goes on. Instead of adding catalyst to methanol, we add sulfuric acid. The sulfuric acid then reacts with the free fatty acid chains and sort of “sands down” the rusty end on several of the free fatty acids. Once the rust is removed, the methanol can then attach to the fatty acid chain and make Biodiesel. The Biodiesel produced this way is only made from the free fatty acid chains (or from the fatty acids that would have been reacted into soap). We still have all the tri, di, and mono-glyceride molecules hanging out in the oil that can still be made into Biodiesel as well!

So, after the sulfuric acid has sanded down as many free fatty acids as it can and the now sanded free fatty acid chains have bonded with methanol, we then re-titrate the oil, see how many free fatty acids are still left, figure out how much catalyst is needed, add it to the methanol and make Biodiesel out of the rest of the fatty acid chains.

Now, it’s important to note that not all of the free fatty acid chains can be reacted into Biodiesel. The way I think about it is that some of the chains have rusted so bad that not even the sulfuric acid can sand them down enough to allow them to bond to methanol. Those chains will still be made into soap during the normal Biodiesel reaction. However, there will be a heck of a lot less of them than before we started with the sulfuric acid.

So, long story short. Sulfuric acid modifies a majority of the free fatty acids in oil so that they can bond to methanol to produce Biodiesel. We call this an “acid reaction”. Then, we retitrate the oil to see how many free fatty acids are still left, add enough extra catalyst to react them into soap and then the remaining catalyst breaks the bonds of the fatty acid chains still attached to glycerol molecules, those fatty acids bond to methanol and we get even more Biodiesel; the base reaction. And THAT’s how we can use sulfuric acid to help us make Biodiesel.

Also, it’s worth noting that using sulfuric acid to make Biodiesel takes a lot longer. Most base reactions can be performed in about 2 hours, but acid reactions can take anywhere from 6 to 24 hours to perform and can sometimes take even longer, so using sulfuric acid with Biodiesel is definitely going to add to your reaction times.

So, how well does it work? It works amazingly well! When done properly, it’s possible to take oil that titrates in the high 20’s and bring the titration clear down to 1 or 2! However, there are limits to how high the titration can be.

In most cases, oil that titrates above 25 is really pushing the limit and anything titrating over 30 is pretty much a lost cause. This is because the by product of an acid reaction is water. Get too much water produced and it’ll just get in the way and hinder the reaction. It also leads to more soap being produced in the base reaction too.

Also, the benefit of using sulfuric acid on oils titrating below about a 4 is much less pronounced. Sure, there will be a benefit to using it, but the added cost of the sulfuric acid along with the extra time it will take to perform the acid reaction doesn’t really make it worth the effort.

The titration sweet spot for using sulfuric acid to make Biodiesel or the most bang for your buck & time if you will, seems to be with oils that titrate between about 7 to 15. Much higher than that and the benefit starts dropping (because too much water starts getting produced), much lower than that and the effort you put in doesn’t result in much more Biodiesel being made.

So, now you know how using sulfuric acid to make Biodiesel works and how it can be of benefit when dealing with higher titrating oils.

So the next question ought to be, “So how do I do it?” Glad you asked! We actually have a really great article that will walk you through how to make Biodiesel using sulfuric acid.

Here’s the link!
Click Here To Learn How To Make Biodiesel With Sulfuric Acid

Want to give it a try? We carry high purity sulfuric acid pre-bottled & ready to use!
Click Here to see our selection!

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2 comments on “How Using Sulfuric Acid Works When Making Biodiesel

  1. It was an excellent article. you doing very good job.

    it would also be appreciated if you upload video on you tube of making biodiesel with help of H2SO4.

    thank you alot.

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