Saponification. The soapy facts

Simple Soap Making Kit

I am in the middle of my first term of chemistry teaching for a local home school group and thought that soap making would be a great activity for our class before Easter. Many people have asked me about the saponification reaction and so I thought I’d better  explain the science without putting you to sleep!

Saponification is the reaction that makes oil turn into soap.  Get yourself some vegetable oil out of the cupboard and have a little play with it. It’s greasy, runny, oily looking and slippery. While it can moisturise the skin it finds it difficult to clean the skin.

In order to turn that oil into a soap we need to add a strong alkali to it. This alkali comes in the form of Sodium Hydroxide or Potassium Hydroxide.  Way back before the chemical revolution people would take the left over fat from an animal caucass and react it with caustic potash (potassium hydroxide) that had been created from burnt wood ash.  When wood is burned the ash contains a mix of chemicals that formed as part of the burning process.  Potassium Carbonate is one of the big ones!

Once you have potassium carbonate you are only  a stones throw from potassium hydroxide – your lye or base that is required for soap making.   Popping the ash into water and boiling it up until the excess water evaporates leaves behind some powdered lye.  Bucket chemistry at it’s best!

These days we can buy our ‘lye’ (which is another word for a base or an alkali) can be purchased from any hardware store making the whole process of soap making much easier and quicker than those heady caveman days!

The amount of lye required for soap making varies depending on the oil used but whatever amount you need, you will first need to take your Potassium or Sodium Hydroxide and turn it into a solution. This requires a great deal of care and attention as the lye is extremely toxic if inhaled or spilled on the skin. Further, tipping the caustic pellets into water generates an awful lot of heat so you need to be prepared for that. Finally the reaction should be carried out in a plastic or pyrex glass container and not in metal.

I have some Caustic Soda here from a company called Pascoes and their website advises adding 3 cups of water (approx 720 ml) to 300g of caustic soda. The caustic is 98% pure at the beginning meaning that your solution is about 40% strength once finished which is still strong!

So what do we do with the Lye?

Recipes vary but the one on the Pascoes site advises adding the  lye to the vegetable oil  rather than the other way around. The oil and water should be at the same temperature – cold is fine but it will take longer to set. You will see your oil turning cloudy and whitish. It should thicken up a bit until it looks like a thick custard. That needs leaving overnight to set.

And what have we formed?

Unless you are a chemist or one of my students the chemical reaction is probably of little interest but I will just mention it quickly!

The vegetable oil consists of fats known as triglycerides which live on a hydrocarbon backbone. These triglycerides get split off by a process known as hydrolysis when the lye is added.  These split-up triglycerides are also known as carboxylic acids due to their structure and as they have been ripped off the original chemical they are in the mood for making friends with something else – REMEMBER THAT!

Next the lye – SODIUM or POTASSIUM hydroxide joins with the newly freed triglycerides to form a soap and the remaining stuff in the jar combines to form glycerine.

Soap is what you get when you add a sodium or a potassium to a carboxylic acid. The carboxylic acid came from the triglyceride that’s part of the vegetable oil.  It’s easy really!

And in a nutshell, that’s it!

Soap produced via saponification is ALWAYS going to have a basic pH – between pH 8 – pH 11 and it is the high pH that makes soap a less-than-perfect thing to use when washing sensitive skin or hair.  Our skin usually sits at pH 4.5-5.5 which is acidic and while it can cope with a bit of soap and water, those suffering with eczema or dry skin may find traditional soap a bit on the drying side.

As far as hair goes we have an even bigger problem. Hair sits at a pH of 5 ish also BUT hair is made of a protein called keratin and keratin can be dissolved by alkali solutions. Saponified soap can at worse damage the hair and at best, leave it dull and tricky to comb.

Soap for kids.

This reaction is far too caustic to make at school but it is worth demonstrating as part of an acid/base reaction class.  However, if you do want to make soap with your kids it is totally do-able as long as you remain vigilant. Here is a great little recipe that works every time – you can also make it more fun by allowing the kids to add a toy to the soap mix. Extra motivation for washing their hands after the loo!