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Can You Lower the pH of Castile Soap?

August 30, 2016

Good old, honest, traditional castile soap, loved by hippies and the  home-made set the world over for its simplicity and elegance but there is one thing that bothers me about this ingredient and that’s it’s high pH.

Why am I bothered by such a trifling matter?

A couple of reasons:

  1.  I find castile soap overly drying for my eczema prone skin and I feel the very high pH (9-11.5) doesn’t help.
  2. It stings the eyes.
  3. Many of my customers want to use castile as an ingredient in an otherwise pH balanced natural product. I’ve had requests from those wanting to use it in a combination face wash to those putting it in cleaning products.  To do this I have to be able to adjust the pH.

So is pH Adjustment possible?

Yes and no. Mainly no.

So the pH of castile soap in its natural state is between 9-11.5 which is typical for soap which naturally has a high pH whether it is in a bar form or as a liquid. This is because it relies on Lye (Sodium Hydroxide or Potassium Hydroxide) to rip the oils heads off (that’s how I imagine it) and replace them with ‘soap’ (sodium or potassium salts) while releasing the glycerin from the triglycerides.   The resulting saponified fatty acids have naturally high pH levels (usually between 8-11 but most often between 9.5-11) and any residual lye will also contribute to giving the soap bar a high pH (although soap manufacturers usually make sure there is no residual lye).

One of the reports I’ve used to establish the pH of the saponified fats is this one: The Hydrolysis of Soap Solutions. III. Values of pH and the Absence of Fatty Acid as Free Liquid or Solid JAMES W. McBAIN, P. LAURI:NT 1 and LUCILLE M. JOHN, 2 Department of Chemistry, Stanford University, California.  It is available to purchase from Springer online.

pH changes with concentration and temperature so these numbers are just to illustrate the point rather than be absolute references but you can quickly see where the high pH of soap comes from.

Potassium Stearate pH 10.05

Sodium Stearate pH of a 5% Aqueous solution is 10.7 (MSDS online)

Potassium Laurate  pH 10

Sodium Laurate pH 9.78

Potassium Myristate pH 10.28

Sodium Myristate pH 9.81

Potassium Palmitate pH 9.81

Sodium Palmitate  pH 10.1

The above ingredients and their pH’s represent what is formed when you saponify an oil. You are creating new chemicals that have some degree of water solubility and that have relatively high pH values.  So, even if ALL of the lye is used up in your soap, your soap will still have a high pH (pH 7 is neutral) because you have created chemicals that have a high pH.

Here is what stearic acid/ Sodium Stearate looks like:

Fatty acids and soaps


So what happens when you try and adjust the pH down?

There is a precipitation reaction that occurs when you add acid to the castile soap base.  This doesn’t happen straight away but it does happen relatively quickly showing us that there is a reaction going on.

Here are the results of an experiment we carried out in the New Directions Laboratory:

castile soap experiment

So we established that the pH of the New Directions Castile Soap could be reduced from 10.27 to 9.03 using a small amount of citric acid and careful stirring.

Soap and citric acid.

soap and citric acid

Next we wanted to see if the pH of Castile Soap could be changed by using another method/ acid just in case there was a particular issue with Citric Acid.

We chose to look at diluting the Castile with more Glycerin (as some customers had said that helped to make the soap more mild),  with more vegetable oil (based on the theory that any excess lye would soap up the oil and thus lower the pH) or with lactic acid – an alternative to citric.

The results are here. This batch of Castile was a bit lower than the last and measured a starting pH of 9.6.

Changing pH with other ingredients

So with regards to pH adjustment it became clear that none of the above worked but what these results did show, especially with the sunflower oil addition was that it is unlikely the resting pH of the Castile Soap was due to excess lye thus supporting what we know about the pH of the saponified fatty acids.

So what is going on with the Castile Soap when we attempt to adjust the pH downwards?

Well that’s the question.

The typical reaction we talk about it:

Acid + Base = Salt + Water.

But we have an acid and a salt so does that mean we make a salt and water?

Or do we make nothing?

We must be doing something or there wouldn’t be a precipitation happening…..

When using Citric Acid we could rip the sodium from the Sodium Myristate/ Stearate etc and form Sodium Citrate salt but would that really happen?  If it did happen would there be a precipitate?  The answer is probably not and there would be no precipitate as Sodium Citrate is readily water soluble.

So is there some kind of complex happening?  There must be something going on..

There are a few things that might be going on, Let’s look at each one in turn.

Firstly there is a reaction called ‘Protonation’. 

So if we remember what our soap is made of (and refer to the hand-drawn picture above) we see the bit of the molecule that makes the soap water soluble has a Na+ (or a K+) part and a COO- bit when it is in water.

When an acid comes along it can donate a proton to the COO- group making it COOH. You can read more about this reaction here. 

Converting a soap to an acid is something that can happen but it is unlikely to happen in every situation unless the resulting compounds are less energy intensive to make (thermodynamically attractive).  Citric (or lactic) acids are weak acids so they are not exactly powerful enough to do this reaction on their own as the salt is still the favoured format given the (still) high pH and the relative weakness of the acid. However, if this reaction could occur  it is true that the salt form of the molecule is more soluble than the acid and in the case of these huge carboxylic acids this head-swap leads to the precipitation of the insoluble fatty acid.

So is it likely that the precipitate is formed by a reaction of glycerin and the citric acid?

Glycerin is a bi-product of saponification and is retained in castile soap to help keep it liquid.  It is well known that biopolymers can be manufactured by combining citric acid with glycerin so it is reasonable to question if this is happening here, especially given that the resulting polymer would form a precipitate.   However, in order to form a precipitate the  polymerisation reaction has to take place at temperatures in excess of 100C and over a prolonged period of time rather than instantaneously at room temperature.  So I guess it remains a possibility that something like this is happening but it is unlikely to be what is going on here. Here is some more information on this reaction.

Does the citric acid just reverse (or un-do) the lye reaction?

So we have triglycerides, add lye and get soap.

So why not take soap, add an acid and get triglycerides?

Or even monoglycerides

Or what else….. Fatty acids?

Is there a chance that the citric acid can replace the -O-Na+ with a -OH?

That’s what we discussed in the protonation bit above. It’s not that likely.

OK so what on earth is going on?

Well, my theory centres on something more physical than chemical which is all too often the case with cosmetic chemistry.  I think that the addition of acid interferes with the micelle formation and that the cloudiness is caused by the water-hating tail groups being temporarily (or permanently) flipped open or disrupted.  Acidity in the water disrupts the status quo of the micelles and if too much acid is added the micelle structure changes completely, the stearates are unable to orientate into micelles which leaves their water insoluble part with no option but to run for the hills – or the top of the beaker in most cases.  I think this is happening as the pH is not changing enough to provide the right environment for such a vigorous chemical reaction but I have been wrong before and I am sure I could be wrong again.

The bottom line.

Castile soap is lovely and natural.  It can have a pH of between 9-11.5 in its natural state and you have limited scope to change that without changing the micelle structure of the product (based on my logic).  Adding excess oil into the formula is something that many people choose to do to re-fat the skin as it is washed so as to avoid the skin being left feeling dry. This is acceptable but won’t change the pH.

So that’s that.

Amanda x

PS: Thanks to New Directions and especially to Alyce for helping with the experimental part of this piece.






17 Comments leave one →
  1. Julie E permalink
    April 28, 2018 4:52 am

    Thank you for this thorough explanation! I would love to make a lower ph shampoo bar so that it’s not so “harsh” on the hair cuticles, and it’s more environmentally friendly than a bottle of shampoo. Sounds like there’s really not a way of doing that yet.

  2. June 12, 2019 8:47 am

    thanks for the accurate information, it was what I was looking for

  3. Anna permalink
    April 4, 2020 8:21 pm

    Hi Amanda, are you still researching soaps / shampoo bars?
    Kind regards Anna Hughes

    • RealizeBeautyEd permalink*
      April 5, 2020 9:50 pm

      No, that finished a while ago thanks

  4. Terra permalink
    June 2, 2020 6:55 am

    When it comes to Castile soap it’s like wine… Times plays a a good role in making it mild and, in my opinion, the nicest soap (along with Aleppo soap). Castille soaps have to be cured for 6 months to a year. I leave mine a year before selling it. Yes, when it’s just made the PH is around 9. In time gets whiter, starts to look like wax (not opaque anymore and) and the PH changes to around 7.

  5. Terra permalink
    June 2, 2020 6:58 am

    When it comes to Castile soap it’s like wine… Time plays a a good role in making it mild and, in my opinion, the nicest soap (along with Aleppo soap). Castille soaps have to be cured for 6 months to a year. I leave mine a year before selling it. Yes, when it’s just made the PH is around 9. In time gets whiter, starts to look like wax (not opaque anymore) and the PH changes to around 7.

    • RealizeBeautyEd permalink*
      June 2, 2020 8:36 am

      If you are talking about bar soap, yes of course it does need the curing time. If, like me here, you are taking about liquid castile then no, not quite the same and the pH can’t be lowered much or you will end up with insoluble flakes in the liquid.

  6. laurensnobody permalink
    June 7, 2020 12:36 am

    looks like I’ve found a new favourite blog!!!! I love reading easy-to-understand, yet in-depth, chemistry 🙂

    • RealizeBeautyEd permalink*
      June 8, 2020 3:26 pm

      Good stuff, thanks and enjoy! Lots of articles built up over the last 13 years of blogging. Some that I’d no doubt cringe over now but all done with my chemists intention to help people understand

  7. September 7, 2020 11:39 am

    I have lowered the ph of Castile soap with pure lemon juice and brought it down to an 7-8 the color or form did not change. It can be done😊

    • RealizeBeautyEd permalink*
      September 7, 2020 6:37 pm

      Not if you want to end up with soap it can’t. You end up producing an insoluble salt as per the article.

  8. October 3, 2020 6:32 am

    While true soap does not offer a wide range of pH adjustment, the overly drying effect, noted in article, can also be contributed by the type(s) of oil used to make the castile soap. All else being equal, a soap made from a high percentage of saturated oils (ie coconut, palm kernal) will be more drying than a soap made from high levels of unsaturated oils (ie olive, sunflower).

    Castile soaps can have a wide-range of characteristics that, more often than not, go unnoticed.

    • RealizeBeautyEd permalink*
      October 5, 2020 4:50 pm

      That’s a good point indeed. Some oils also contain a high unsaponifiable fraction that persists as oil after soap making and can help boost barrier functioning.

  9. Ralph Vincent Burias permalink
    October 19, 2020 3:33 am

    Hi, Im just wondering if the castille soap were talking here is pure olive oil? and what is its % solid content? thanks

    • RealizeBeautyEd permalink*
      October 25, 2020 4:47 pm

      That isn’t relevant as whatever you make the soap out of, the resulting soap will be pH responsive turning to the insoluble salt once the pH is reduced.

  10. Jim permalink
    August 9, 2021 6:11 pm

    If only focusing on the pH and its study, highly understood that the higher the pH, the more drying effect to the skin. However, with castille soap made of extra virgin olive oil, we find that the soap is hydrating and creating a layer to protect skin’s moisture.

    • RealizeBeautyEd permalink*
      September 2, 2021 1:01 pm

      Well yes, there are what I could refer to as protective factors in some formulations that make them less irritating and drying to the skin. The choice of oil, the soap concentration (pH is a log scale so you can have quite a range of active caustic % in a formula and record the same pH, presence of humectants, perfumes and colours etc. That’s why thinking of a product as a whole is so important. It’s hard to get that nuance across when breaking things down in pieces but sometimes doing that is useful too.

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