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Saponification. The soapy facts

March 26, 2010

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!

9 Comments leave one →
  1. Ian Antonio permalink
    August 29, 2014 4:58 pm do i know the percentage concentration of lye to be used in a certain formulation for soap?
    2. what is the ratio of water to sodium hydroxide?

    • RealizeBeautyEd permalink*
      August 30, 2014 6:48 am

      There are some Lye calculators that you can look up on soap professional websites such as the Soap Queen. I’d go and see what they have as I’m no expert.

  2. September 25, 2017 2:16 am

    Ugh… No wonder! So why DO people use saponified oils to make shampoo? Or what is the safest type of shampoo (or soap) to use? I’ve tried some shampoo that leaves my hair a little stringy but overall pretty good, one brand that is so good I don’t need conditioner, and one brand that I tried recently that leaves it horrible – and it uses saponified oils. It leaves it dry and greasy at the same time – it’s weird. Wish I had read this first and I would have avoided it.

    • RealizeBeautyEd permalink*
      September 25, 2017 9:47 am

      Hi Heather, I think there are many reasons why people still do it including the fact that some people do like it – I remember my dad washing his hair with soap all of my young childhood (after that age I stopped noticing what he did). Soap based shampoo probably does work best for people with oilier hair and/or for people who don’t wash their hair that often and/or have thicker hair. That’s my theory anyway, it may be wrong. In any case I think the key is to wash it out well and if you find the after-effects too drying then try an apple cider vinegar rinse afterwards.

      With regards to non-saponified shampoo/ hair products I’m not sure of your definition of ‘safe’ so I can’t tell you what is the safest but if you are sensitive to hair products I’d aim towards baby products first as they tend to have the lowest concentration of actives that might irritate you. The second thing to look out for is the preservative. I’m personally reactive to MIT (methylisothiazolinone) and this is a very widely used preservative. I can pretty much use any shampoo or any surfactant combination as long as the product isn’t preserved with this. If it is, my scalp burns and flakes. You may not have this issue but I thought it worth bringing up. It is sometimes hard to find cost effective shampoos and conditioners without this ingredient which again, might push people towards saponified products including liquid castile – the manufacturers of these types of shampoos generally won’t use this preservative system as most are either hand crafters, natural brands or brands seeking to be ‘different’ from the mainstream – avoiding MIT is one way to do this.
      There are other ingredients and formulating strategies that can cause irritation in some situations but I’d work backwards from the preservative and see how you go.
      Generally if you are looking for a very good shampoo I’d go for one with anionic surfactants and something conditioning present too (Sodium Cocoyl Glutamate and Panthenol might be one such combo) formulated to be mild so low surfactant actives. Some people can find Cocamidopropyl Betaine irritating and if they do Coco Betaine may well be more so. However, the irritation potential can sometimes come from impurities in the cocamidopropyl betaine rather than the chemistry so again, that can make people jump to false conclusions – larger brands should understand the risks of using inferior material whereas smaller brands buying from small repackers may not have the resources or experience to know a good quality from a less so – again this is only a ‘maybe’.
      Quite a lot to think about but hopefully some of that helps you.


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