Sodium hydroxide versus potassium hydroxide in soap production

Soaps are sodium or potassium salts of fatty acids, prepared in a chemical reaction called saponification. Originally soap was made by boiling lard or other animal fat together with lye (sodium hydroxide) or potash (potassium hydroxide), as a result of which crude soap and glycerol was formed. Nowadays animal fat is more often replaced with all kinds of plant oils and butters such as olive oil, coconut oil, castor oil, palm oil, cocoa butter, shea butter etc. Yet, still sodium and potassium hydroxide are used in the process of soap making. 

I am no expert on soap making so I will leave writing great blog posts about soap making to those that have years of experience with this craft. I am rather new to this topic and have recently made my very first soap. This post is not about the properties of soap making oils, or instructions on how to make soap. Actually you can find all of this information on great websites such as:

http://www.lovinsoap.com/oils-chart/

http://www.soap-making-resource.com/

http://www.soapdelicatessen.com/make-soap.html

http://www.soapqueen.com

What I do want to write about in this post is something I was wondering about while making my very first soap - does it make a difference whether I use sodium hydroxide or potassium hydroxide to make my soap?

Anyone who took some basic chemistry classes in high school probably remember that both sodium hydroxide and potassium hydroxide are caustic bases formed from an alkali metal ionically bound to a hydroxide group. Potassium is heavier then sodium, therefore potassium hydroxide has a higher atomic weight then sodium hydroxide. Both sodium and potassium hydroxide react with water generating heat (exothermic reaction). The difference between these two hydroxides is that potassium hydroxide reacts with water slightly less exothermic. Another difference is that potassium hydroxide is more soluble in water then sodium hydroxide (121 g of potassium hydroxide will dissolve in 100 ml of water compared to only 100 g of sodium hydroxide in the same amount of water). Also potassium hydroxide is cheaper then sodium hydroxide.

Getting back to my question, does it matter if I use sodium or potassium hydroxide to make soap? 

Will it give me the same results? 

After a bit of research I learned that sodium hydroxide is used to make bar or solid soap, whereas potassium hydroxide is used to make liquid soap. Potassium hydroxide doesn't make a hard bar of soap unless you use animal fat (lard, tallow etc). According to some people liquid soap can also be made using sodium hydroxide (I've seen the recipes and the outcome), but supposedly it doesn't make as nice of a soap as potassium hydroxide. You can also mix sodium hydroxide with potassium hydroxide and make what is called a hybrid soap. This gives a nice creamy soap. This difference between soap made from potassium and sodium hydroxide can be somewhat explained by the fact that sodium hydroxide soaps form crystals that clump together giving harder soaps, whereas potassium soaps form strands leaving them more liquid

Since we are already talking about soaps and sodium and potassium hydroxide I wanted to make it clear that real soap always has to be made from one of them! I often stumble across blog posts where people ask can you substitute the lye (sodium hydroxide) for something else in the soap. The answer is no. Yes, you can you sodium lauryl sulphate as for example the brand Lush does in some of its "soaps" (don't get me wrong I love most of their products). Its just that lets make it clear that something made form a detergent (like sodium lauryl sulphate) is not real soap anymore. Its just a detergent in a solid form. 

And for those which are a bit afraid of a chemical like sodium hydroxide or potassium hydroxide let me assure you that if the soap is produced in a proper way all of the hydroxide is neutralised and in its place a salt is formed (to make sure the soap is completely neutralised pH papers are used). 

Here a photo of glycerine melt and pour soap, which I have recently made