Sciencemadness Discussion Board

Making Soap - NaOH vs KOH - results of run-off - is the K available?

RogueRose - 17-10-2015 at 02:43

I've made some soaps and there can be a noticable difference between soaps made with KOH vs NaOH with the KOH seemingly being softer and more liquid in some recipes.

Also, in some recipes people add salt as a means to make the bar harder (this is usually done in relatively small quantities) and I have only ever seen NaOH mentioned in this.

I'm really not sure of the chemistry when saponifying the oils so IDK if either the Na or K molecules from the base become a useable form in the run-off from washing.

I've read that KCl is sometimes used as a fertilizer (never heard of this before..) while I don't know of any benefit to the sodium in the runoff.

I'm going to do a couple tests between using NaCl, KCl and nothing in the soaps to see if there is a difference in hardness.

So, what I really need to know is if there is any benefit (as in plant avaiable nutrient) in using one of the bases in producing the soap

deltaH - 17-10-2015 at 03:29

Adding a salt to the soap would make it much harder for the soap to dissolve and so can prolong the use of the soap bar.

This is perhaps a frugal idea since one only needs a very small amount of soap when washing anyway and I personally feel people waste way to much when using commercial bars that dissolve quickly.

The reason for this is that the soap in not very soluble in salty water. You might know this from the 'salting-out' technique that can actually precipitate pure soap when it's dissolved in water. That's a neat trick for separating the glycerine from the soap and making a very high purity soap.

KCl and NaCl are safe to add in to soap, they are more or less inert, but NaOH and KOH are not because they are very caustic and will not be neutralised by saponification (because there's an excess).

I avoid using K salts because of the higher costs. Apparently they're good for making liquid soap, but for liquid soaps, I simply prefer to isolate polyunsaturated fats whose soaps are VERY soluble in water. For example, you can make an excellent liquid soap using pure linseed oil (make sure it's free from additives) and ordinary sodium hydroxide.

When you're done making the soap by a hot method, simply dissolve the cubed soap bar in a litre of boiling water by stirring for some time. A portion of the soap won't dissolve but form a sticky gloop., this is the saturated portion, simply fish it out (and use later to make slow dissolving bar soap). The clear viscouse solution will form a gel upon cooling and you can dilute further, instant liquid soap with sodium, easy!

It also works with sunflower oil, which is cheaper, but the linseed version is apparently good for carpet shampoo, don't know why, maybe because it's low foaming.

[Edited on 17-10-2015 by deltaH]

Fulmen - 17-10-2015 at 04:04

No, the Na/K is consumed by the saponification process. Oils and fat are esters made from fatty acids and glycerine (triglycerides). The saponification process splits these glycerides into it's raw components, then neutralizes the fatty acids forming the respective salts. The lye should be added in stoichiometric proportions (or slightly less) to avoid a to caustic soap, so there shouldn't be much left in the runoff.

As for fertilizers potassium is indeed a essential nutrient for plants, sodium is not. But using the runoff from soaps sounds like a bad idea.

[Edited on 17-10-15 by Fulmen]

UC235 - 17-10-2015 at 04:10

Adding salt to soap decreases its water solubility by the common ion effect. If you make a fairly strong solution of soap in water and add a lot of salt, the soap precipitates as curds. You'll also notice that it stops foaming almost entirely. I can't really imagine that this is a good thing in a finished bar.

If you want a harder soap, use more saturated fats in the recipe. Coconut oil, palm, and soy wax make for very hard bars.

deltaH - 17-10-2015 at 04:48

Quote: Originally posted by Fulmen  
No, the Na/K is consumed by the saponification process. Oils and fat are esters made from fatty acids and glycerine (triglycerides). The saponification process splits these glycerides into it's raw components, then neutralizes the fatty acids forming the respective salts. The lye should be added in stoichiometric proportions (or slightly less) to avoid a to caustic soap, so there shouldn't be much left in the runoff.

As for fertilizers potassium is indeed a essential nutrient for plants, sodium is not. But using the runoff from soaps sounds like a bad idea.

[Edited on 17-10-15 by Fulmen]


Quote:
...but NaOH and KOH are not because they are very caustic and will not be neutralised by saponification (because there's an excess).


I meant when it's added as an excess additive. I think that's what op wanted to know, salts that could be added to harden the soap, using an excess of KOH or NaOH is a bad idea because it won't be neutralised.

Fulmen - 17-10-2015 at 04:52

UC235: I don't see any problems with adding a small amount of salt to adjust hardness, but adding too much would probably "kill" it. Choosing the right ingredients is probably the best way to go, but sometimes you're stuck with what you've got, perhaps a byproduct that's cheap. Or perhaps you've messed up a batch by using too much water, in that case a pinch of salt can perhaps save the batch without having to dry it for 6 months.

I make a batch of soap myself every few years, best soap money can ('t) buy. A mix of coconut, rape and olive works out fine for my taste, but I also have tome to let it cure for a long time which help produce a reasonably firm and long lasting.

deltaH - 17-10-2015 at 04:54

Incidentally, I had made soap once with an excess of lye in it to use as toilet bowl soap, that works very well indeed, so long as you don't get any on your hands. Most of the excess lye convert to sodium carbonate which forms crystals over the soap bar which looks very pretty, but it's far too caustic for hand soap. It cleans like a bomb though, great for the toilet.

RogueRose - 17-10-2015 at 14:48

Thanks for all the informative responses!

I've made more batches than I care to mention and it is fun learning how the different oils and bases interact as well as additives - that is partially what got me "re-interested" in chemistry.

As for my OP, I wasn't considering adding an excess of K/NaOH unless it was a specialty soap for something like cleaning very greasy things. Basically I wanted to know if the K or Na (from the base) was made available after the saponification process.

So, as for adding salt to harden the bar, this is indeed correct. I've only ever used NaCl and wanted to know if KCl would be preferrable as I would guess the K would be available in the water run-off.

deltaH - 17-10-2015 at 20:11

Ah okay, I'm with you now. The potassium in potassium soap is available in the runoff even if it was just from KOH used in saponification. This is because in solution the ions dissociate and mix with other ions, it's just a soup of ions, the potassium isn't glued to the heads of the soap molecules in the micelles formed. It can be exchanged for other ions. Indeed, what usually happens, even so soon as in your bath, is that the soap precipitates by forming an insoluble calcium 'soap', i.e. the soap scum sticking the bath wall.

Bottom line, all the potassium ends up in the runoff... yet another reason not to use KOH in soap making!

Fulmen - 18-10-2015 at 02:04

Quote: Originally posted by deltaH  
Bottom line, all the potassium ends up in the runoff... yet another reason not to use KOH in soap making!

I don't follow, potassium isn't harmful. Quite the contrary, it's a plant nutrient.

otherwise I agree, all of the Na/K will be available in the runoff. Soaps should be perfectly biodegradable, so as long as it's not too concentrated I don't see any major problems. But I don't think you'll ever use enough soap to get any real gain either, so use whatever makes the best soap.

deltaH - 18-10-2015 at 02:27

Quote: Originally posted by Fulmen  
Quote: Originally posted by deltaH  
Bottom line, all the potassium ends up in the runoff... yet another reason not to use KOH in soap making!

I don't follow, potassium isn't harmful. Quite the contrary, it's a plant nutrient.

otherwise I agree, all of the Na/K will be available in the runoff. Soaps should be perfectly biodegradable, so as long as it's not too concentrated I don't see any major problems. But I don't think you'll ever use enough soap to get any real gain either, so use whatever makes the best soap.


I'm no biologist, but I've heard that plant macronutrients like phosphates used in detergents are bad for the environment because they end up in river water. I assumed that K, being another plant macronutrient, might be detrimental for similar reasons.

It's probably only a problem if the soap were produced on a large scale, niche uses probably have negligible effect.

Fulmen - 18-10-2015 at 03:19

You're right, plant nutrients can be bad if too much gets into the waters as it can cause unwanted algae growth. But that doesn't mean that plant nutrients are bad, just that over-fertilizing is. I doubt that he'll use enough soap to cause such problems.

j_sum1 - 18-10-2015 at 03:40

Quote: Originally posted by Fulmen  
You're right, plant nutrients can be bad if too much gets into the waters as it can cause unwanted algae growth. But that doesn't mean that plant nutrients are bad, just that over-fertilizing is. I doubt that he'll use enough soap to cause such problems.

To add, I have not heard of the same problems with potassium as with phosphates. Phosphates tend to cause algal bloom in waterways. I don't think that K does the same. K promotes strong root growth for most plants -- which might be the main difference. (Unqualified stab in the dark there. I expect to be corrected.)

Fulmen - 18-10-2015 at 06:28

Seawater (where algae blooming seems to be worst) contains 400ppm of K, so, that should not be a limiting factor for growth. It could be a problem in fresh water, but I really don't know.

deltaH - 18-10-2015 at 07:01

Quote: Originally posted by j_sum1  
Quote: Originally posted by Fulmen  
You're right, plant nutrients can be bad if too much gets into the waters as it can cause unwanted algae growth. But that doesn't mean that plant nutrients are bad, just that over-fertilizing is. I doubt that he'll use enough soap to cause such problems.

To add, I have not heard of the same problems with potassium as with phosphates. Phosphates tend to cause algal bloom in waterways. I don't think that K does the same. K promotes strong root growth for most plants -- which might be the main difference. (Unqualified stab in the dark there. I expect to be corrected.)


I thought it was P that promoted root growth, K for flower and fruits and N for foliage. Where's aga when you need him...

Fulmen - 18-10-2015 at 22:14

We're talking about algae here which have neither :-)

j_sum1 - 18-10-2015 at 23:19

I was under the simplified impression that for most plants
* Nitrogen makes it green
* Phosphate helps it reproduce -- flowers and fruits
* Potassium promotes strong root growth
(With the qualifier that I might have the P and K the wrong way around)

On reading, this is a gross simplification. I have not found a simple summary. Nitrogen definitely promotes greening. But all three macronutrients are needed for a range of plant systems. In any case, I would not expect algae to conform to such gross generalisations.

I do know however that phosphates cause havoc in fresh waterways by promoting growth of aquatic species; with the effect that the system is later starved of available oxygen.

Back to the OP. I am sure that K from soap run-off will be bioavailable if it gets into the environment. I suspect that you would need to dispose of a lot before it caused any environmental problems. (I think that phosphates would in general be more problematic, but that is off topic.) Na is unlikey to have any positive or negative effect until it reached the concentration where it interfered with osmotic processes in plants.