This article describes the results of one of my kitchen chemistry experiments.
Knowing chemistry in the abstract is useful, but it is also good to experience chemistry happening in real life. I decided to see what happens when I added washing soda, borax, baking soda, vinegar, and citric acid to soap. I chose these chemicals because many soap makers often add them to soap for a variety of reasons. Two common reasons include lowering the pH of soap and making laundry soap mix.
Soap with lower pH. Many soap makers wonder if adding acids will reduce the pH of their bath soap and thus make the soap milder to the skin. While it is true that acids can reduce the pH of soap, a lot of acid has to be added to get a significant change in pH. If enough acid is added to reduce the pH of the soap by as little as 1 to 1.5 units, chemical analyses show most of the soap will decompose into fatty acids. (1) At this point, the "soap is not soap any more" and is no longer an effective cleanser,
Laundry soap. Do-it-yourself recipes for laundry soap mixes abound on the internet. Some mixes are liquid, some are powders. Many are based on some variation of shredding bar soap and mixing it with borax, washing soda, and/or baking soda. A few even include acids in the soap mix. Based on what I know about the chemistry of soap when combined with these other chemicals, recipes that include baking soda or any type of acid are less effective than they should be.
Chemical properties
Chemical
pH when diluted in water
Washing soda
11
Soap
Varies, but typically 9.5-11.5
Borax
9.1
Fatty acids
pKa is 7.5-10.2 for fatty acids typically in soap (1, 2)
Baking soda
8
Citric acid
3.1
Vinegar, white
2.6
Washing soda (sodium carbonate, Na2CO3) is an alkaline salt, meaning when washing soda is dissolved in plain water, the pH of the solution is alkaline. The pH of washing soda is similar to the pH of soap.
Soap is a family of alkaline salts. When soap is dissolved in plain water, the pH will always be alkaline, but the exact pH measurement will vary from about 9.5 to about 11.5 depending on the types of fatty acids in the soap. Learn more about soap pH....
Borax (sodium tetraborate, Na2B4O7ยท10H2O) is another alkaline salt, with a pH at the lower end of the pH range for soap. Like soap, borax has the unusual ability to resist changes in pH if an acid or an alkali is added. A chemical that behaves like this is called a buffer.
The behavior of a borax buffer varies depending on the other chemicals present. If the other chemicals are less alkaline than borax (such as boric acid), the borax buffer may function in a pH range of about 8 to 9, which is below the typical pH range for soap. If there are chemicals present that are more alkaline than borax (such as NaOH), the borax buffer may function in a slightly higher pH range of about 9 to 10, which is at the low end of the pH range for soap. (4)
Most buffers have a single range of pH in which they are effective, so this behavior of a borax buffer is unique. If the borax buffer is functioning in its lower pH range, the soap can decompose because the pH is too low for soap to remain intact. If the borax buffer is functioning in its higher pH range, it will behave more like washing soda when combined with soap, and the soap should remain intact.
Fatty acids. Fatty acids are very weak acids that are slightly soluble to insoluble in water. If present in a mixture of soap diluted in water, the fatty acids will separate and form a greasy or oily layer that floats on the surface of the water-based soap mixture.
Baking soda (sodium bicarbonate, NaHCO3) is another alkaline salt, but its pH is considerably lower than the pH of soap, borax. and washing soda. When baking soda is mixed with soap, baking soda will behave as if it is a weak acid.
Just like a "real" acid, baking soda will cause soap to decompose into fatty acids when added to soap. Since baking soda behaves as such a weak acid in this situation, the amount of decomposition will vary depending on the amount of baking soda added and the types of fatty acids present in the soap.
Citric acid and vinegar (acetic acid) are acidic (pH below 7). Both are stronger acids compared with fatty acids. When these acids are added to soap, they will chemically react with soap molecules and cause the soap to decompose into fatty acids.
Experimental procedure
For this experiment, I used a liquid soap made from 25% coconut oil and 75% high oleic sunflower oil with a very small positive superfat. The fats were saponified with potassium hydroxide (KOH) using a hot process method of saponification.
I used liquid soap (soap made with potassium hydroxide) in this experiment because I had it on hand and it is much easier to dilute than bar soap. I could have used bar soap (soap made with sodium hydroxide) instead. My knowledge of soap chemistry tells me either type of soap will give essentially the same results, however, so I opted for convenience.
Into each of six clear glass jars, I measured 25 grams of this soap paste. The amounts of soap and other chemicals used were arbitrary.
To five of the jars, I added 150 grams of distilled water heated to a simmer (180-200 F/80-95 C). I chose this amount of water to ensure the baking soda was fully dissolved, since baking soda was the least soluble chemical used in this experiment.
The soap and warm water mixtures were stirred until the soap was fully dissolved. The first jar was a control sample, so nothing more was done to it.
I added 5 grams washing soda to the second jar, 5 grams borax to the third, and 5 grams baking soda to the fourth. I added 2 grams of citric acid powder to the fifth jar.
To the soap in the sixth jar, I added 110 grams of warm distilled water and stirred to mix. I then added 40 grams of commercial white vinegar at 5% acidity. This amount of vinegar is equivalent to about 2 grams of pure acetic acid. The water in the vinegar plus the added distilled water totaled about 150 grams.
Jar
Additive
1
Control, no additive
2
Washing soda, 5 g
3
Borax, 5 g
4
Baking soda, 5 g
5
Citric acid, 2 g
6
Vinegar, 40 g (2 g acetic acid)
The mixtures were stirred until well mixed and allowed to sit quietly for about 1 hour. Each sample was evaluated and photographed at that time. The samples were then allowed to sit quietly overnight and then they were again evaluated and photographed.
Experimental results
Control. The control sample was a clear, pale amber mixture. After standing one hour, a small amount of fluffy white sediment had settled on the bottom of the jar. There was nothing floating on the surface of the soap solution. There was no change in appearance after standing overnight or after standing for one week.
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Control sample, one hour
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| Control sample, next day | ||
Washing soda. The washing soda sample was pale amber. Compared with the control, the mixture was very slightly more cloudy and there was slightly more fluffy sediment at the bottom. Nothing was floating on the surface. There was no change in the appearance after standing overnight or after standing for one week.
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Washing soda, one hour
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| Washing soda, next day | ||
Borax. The borax sample looked essentially the same as the washing soda sample. Compared with the control, the mixture was very slightly more cloudy, there was slightly more fluffy sediment on the bottom, and there was nothing floating on the surface. The sample did not change its appearance after standing overnight or after standing for one week.
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Borax, one hour
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| Borax, next day | ||
Baking soda. The baking soda sample was initially very cloudy but still translucent -- light could pass through the liquid, but I could not see through it. After standing one hour, there was no obvious separation in this sample. When I wetted a fingertip with the liquid at the surface of the sample and rubbed the liquid with my fingertips, my skin felt faintly oily. After rinsing my fingers in room temperature water, a faint oily residue remained on the skin.
After standing overnight, a layer of opaque white material had separated out of the mixture and was floating on top. The liquid below that layer was more cloudy than at the one hour mark with threads of white opaque material floating throughout. When I rubbed a bit of material from the top layer on my skin, it felt oily and did not rinse off in room temperature water.
After one week of standing, the white opaque layer was still floating on the soap and appeared to be stable.
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Baking soda, one hour
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| Baking soda, next day | ||
Citric acid. In the citric acid sample, a sticky white curd formed seconds after the acid was added to the soap solution. After standing for one hour, some of the curd coated the sides of the jar and spatula and patches were also floating on the surface of the soap solution. The curd felt greasy and slightly sticky on my skin. It left an obvious, greasy coating on my fingertips even after I rinsed them with room temperature water.
Ignoring the curd, the main part of the soap solution at one hour was much cloudier than the baking soda sample. The solution passed some light, but I could not see anything through it.
After standing overnight, a lot of the curd had dissipated and a thin layer of clear yellow liquid had formed and was floating on the surface. When tested on the fingers, this liquid felt oily and did not rinse off. The main part of the soap solution was cloudy but less opaque compared with its appearance at the one hour mark.
After standing one week, the remaining white curds had dissipated, but the layer of oily yellow liquid still remained on the top of the soap solution.
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Citric acid, one hour
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| Citric acid, next day | ||
Vinegar (acetic acid). The vinegar sample quickly turned white and opaque -- almost no light was able to pass through it. After an hour of sitting quietly, a thin layer of clear yellow liquid had separated from the main solution and was floating on top.
The floating yellow liquid felt slick and oily on my fingers. An obvious, oily residue remained on my fingertips after rinsing with room temperature water.
After standing overnight, the soap solution was still cloudy, but translucent. The floating layer of oily liquid appeared to be very slightly thicker than the day before.
After standing one week, the sample had not changed in appearance.
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Vinegar, one hour
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| Vinegar, next day | ||
Discussion
The sticky floating curds that formed initially in the citric acid sample looked familiar to me based on experience troubleshooting problems with liquid soap making. Many times people add acid to liquid soap to neutralize excess lye, but they often add too much. That results in cries for help and many pictures of soap covered with a thick layer of white, sticky curds.
What was not familiar to me was the floating oily liquid that formed in the vinegar sample and in the overnight citric acid sample. After studying this further, I found the reason why it formed.
I assumed most fatty acids would be waxy solids at room temperature, much like the stearic acid I buy for making lotions and shave soap. My assumption was wrong.
The particular soap I used in this experiment was high in oleic acid (64% oleic), so oleic acid would have to be the main fatty acid when this soap was decomposed. I learned oleic acid is a liquid at room temperature with a melting point of 56 F / 13 C. (3)
It makes sense that a fatty acid layer on this soap would be an oily liquid of mostly oleic acid. If I had used soap that was high in stearic acid for this experiment, I would probably have seen a greasy semi-solid paste of fatty acids instead.
The thick white layer floating on the baking soda sample after resting overnight also confirmed baking soda causes decomposition. The baking soda reacted with the soap more slowly, however, compared with citric acid and vinegar.
This slow decomposition makes sense, because baking soda functions only as very weak acid when combined with the more-alkaline soap. Vinegar and citric acid are much stronger acids in comparison and should react with soap more quickly and completely. That was the outcome observed in this experiment.
The only chemicals that did not visually change the soap solution were washing soda and borax. Washing soda seems to be an additive that is fairly safe to use with soap; there are many recipes for household cleaning soaps that call for washing soda.
Borax also performed well in this experiment, but some soap makers have found borax causes their liquid soap to decompose, similar to the acids and baking soda tested in this study. As I mentioned earlier, borax has the ability to form a complex chemical system called a buffer, but the borax buffer is unusual in that its characteristic range of pH can vary somewhat depending on the other chemicals present in the buffer solution.
Why did borax not "break" the soap in this experiment, but it does cause soap to decompose in other situations? I speculate that borax might might perform well in soap if it forms the higher pH buffer. If it forms the lower pH version, the borax buffer would cause soap to decompose much like baking soda did in this experiment.
Conclusion
The results of this experiment fits with the chemistry facts -- stronger alkalis such as washing soda and borax are not likely to decompose soap into fatty acids.
Acids such as vinegar and citric acid definitely cause soap to decompose, changing its appearance within a very short time.
Even baking soda affects soap, albeit more slowly than citric acid or vinegar. Adding baking soda to soap does more harm than good.
The overall conclusions I have drawn from this experiment is that alkalis that have a pH similar to soap are the least likely to chemically alter the soap. On the other hand, acids and some weak alkalis react easily with soap and cause it to decompose.
References
(1) Dunn, Kevin. Chapter 13: Soaps and detergents. Scientific Soapmaking. Clavicula Press. 2010. Table 13-1, page 229.
(2) The pH of a mixture of 50% soap and 50% fatty acids (or any 50:50 mixture of undissociated salt and dissociated ions) has a specific name -- the pKa value.
(3) Wikipedia. Oleic acid. Version last edited 16 December 2020. https://en.wikipedia.org/wiki/Oleic_acid
(4) Thorsten, C. Chemistry of the Borate-Boric Acid Buffer System. CR Scientific. March 2013. http://www.crscientific.com/experiment4.html
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