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Soapy Science: Citric Acid in Soap Making

If you take a look at any of our recipes, you will see a common ingredient in all of them- citric acid. Citric acid is one of our favorite additives and packs a powerful punch, with a budget price. What is citric acid and what does it add to our soap? Let's take a look at the benefits of citric acid and explore how you can use it in your recipes too.

First, I want to address one of the most common misconceptions about citric acid before moving on to the good stuff. There is a lot of misinformation floating around the internet in different blogs and Youtube videos that state citric acid is used in soap making specifically to lower the pH to neutral or even an acidic pH. Many of these "resources" will add all different types of acids, including citric acid, acetic acid from vinegar, and more. I see a lot of soap makers that make claims like this one from a shampoo bar video where the host states, "I just neutralized this soap by adding citric acid to bring it down to a pH of 7 to make the most gentlest bar ever." Unfortunately, this simply is not true and not possible without causing complications in your soap. Including things like citric acid, lemon juice, and vinegar, will not lower the pH of soap and should not be added with that intention. We discuss this in great detail in our books, but as a review, soap is naturally alkaline with an average pH of 9-10, sometimes even higher. By adding an acid to your soap, especially when a higher concentration is used, it can cause the soap to lose its ability to act as an effective surfactant, and instead of getting "the most gentlest bar ever," you get a pile of mushy goop that doesn't clean, lather, or foam.

If you add citric acid to the lye solution at the beginning of your soap making process, it will not lower the pH of your soap. Instead, it will react with or "consume" some of the lye that was intended for saponification of your oils, creating a lye discount and increasing the superfat. By adding citric acid to your soap at the end of the process, be it hot process or liquid soap, the citric acid may free some of the fatty acids and again, which again, increases the superfat. (We have an awesome experiment in The Ultimate Guide to Liquid Soap coursebook that delves deeper into this topic and I highly recommend that everyone perform it!)

So, if we don't use citric acid to lower the pH of soap, then why do we use it then? After reading The Ultimate Guide to Soap books, you know that when an acid and a base are combined, a salt is created. When citric acid is combined with sodium hydroxide, it creates sodium citrate. When citric acid is combined with potassium hydroxide, it creates potassium citrate. Although you can certainly purchase sodium or potassium citrate, it must be purchased from a specialty supplier and is often more expensive. Citric acid is very cheap and can be even be purchased from your local grocery store and found in the baking aisle. When purchased online, you can find it on Amazon, eBay, and from cosmetic suppliers. Citric acid is also a common ingredient in other cosmetics, including bath bombs, shampoos, and more, so you can use it for many different purposes, not just soap.

What is so special about sodium and potassium citrate? Both of these chemicals are chelators. A chelator is a chemical that has the ability to attract, bind, and "trap" metal ions. The molecular structure of a chelator has at least two or more electrically charged ends or "claws" that can bind to and trap an electrically charged metal ion. Similar to the claw arcade game that has multiple prongs used to grab toys and carry them away, chelators also act like a claw, but instead of grabbing and trapping a stuffed toy, they bind to and trap metal ions. When we use a chelator in our soap, the chelator binds to the metal ions so that they can't bind to the soap molecules. By trapping these ions, they can't form new chemicals like soap scum or negatively affect the soap in the same manner.

Metal ions, like those found in hard water sources, certain additives, and some of our unrefined oils like shea butter and palm oil, such as calcium and magnesium, can have detrimental consequences to our soap. The two most common complications these metals and minerals cause are: rancidity and the formation of soap scum.

Rancidity is due to the oxidation of fatty acids in our soap. There are many things that accelerate oxidation, including heat, light, and water, but contact with certain contaminants, even trace amounts can result in oxidation. These risks are significantly higher in recipes that have a higher concentration of free fats and fatty acids (superfat), soaps that are made with tap water, and soaps made with a high concentration of polyunsaturated fats. Signs that your soap has experienced oxidation include DOS or Dreaded Orange Spots, other types of discoloration, soft mushy spots, and foul odors. (Want more information about oxidation? Check out this awesome blog article!)

Soap scum is the second problem that contaminants can cause. Soap scum is created by the reaction that occurs when soap is combined with metal contaminants in hard water like calcium and magnesium. For example, when sodium stearate (the soap created by the reaction between our sodium hydroxide and stearic acid) reacts with calcium, like the calcium found in hard tap water, it creates calcium stearate, a poorly soluble and hard white substance, commonly known as soap scum. The formation of soap scum drastically reduces the lathering and performance abilities of our soap. It also creates a pesky layer of soap scum residue, which can be left on the skin and hair and can leave white build up on the tub, faucets and shower walls. Soap scum left on your shower and tub can build up over time and can be quite difficult to remove, often requiring the use of special cleaning products. The more metal ions and contaminants in your water, the more soap scum, and the fewer bubbles. When soap makers use a balanced and informed recipe complain about poor lathering qualities, my first question is always "Do you have hard water?" (How do you know if you have hard water? Perform this simple and easy experiment in our blog article about distilled water)

By adding a chelator to your soap, it traps the contaminants like the calcium and magnesium ions. Instead of the soap molecules reacting with these ions, the chelator binds to them and traps the ions so that the soap can't react with them. This means that instead of forming icky soap scum or oxidizing and creating smelly orange spots, soap can be its bubbly, full-lathering, and cleansing surfactant self.

Chelators are excellent additions to soap, especially for those who have hard water. The addition of a chelator can drastically improve a soap's lathering performance and significantly improves the lather volume, life-span, and rate of lather formation when used in hard water.

Citrate vs EDTA

Citric acid is derived from plants and is a natural constituent of many living organisms. In fact, citric acid is essential in humans and other animals for energy metabolism in a process known as the citric acid cycle, or the Krebs cycle. Citric acid and its sodium and potassium salts are readily biodegradable, weakly toxic to aquatic organisms, and pose a very low hazard to human health. These qualities are in sharp contrast to many of the more popular chelators, like EDTA, which is poorly biodegradable and remains as a persistent substance in the environment. Because of its poor biodegradability, EDTA's contribution to heavy metal bioavailability and remobilization processes in the environment, in addition to its roles in plant and animal toxicity, is of major concern. For this reason, we don't recommend its use, but rather a safer alternative.

*It should be noted that sodium citrate has been found to be much more effective against oxidation when used in conjunction with another antioxidant and/or chelator, such as ROE, BHT, sodium gluconate, and/or EDTA. Sodium citrate has less "claws" than other chelation options. This makes it weaker than say EDTA which has six, but unlike EDTA, sodium and potassium citrate are biodegradable and are much less hazardous to the environment.

How to Use Citric Acid Soap Making

We use citric acid to create a citrate in just about every recipe that we make. Remember, citric acid isn't the chelator, but rather the citrate, meaning the dissociated salt of sodium/potassium hydroxide and citric acid. The suggested usage rate in bar soap is approximately 1-2% of the total oil weight used. In liquid soap, you can also calculate the percentage based on total oil weight, or for a more accurate and potent formulation, you would calculate it based on 0.5-1% of the final solution for even more chelating action and would need to calculate your total soap solution weight upfront or post dilution (We spend an entire chapter in UG2LS covering soap solution concentrations!).

To use citric acid to create sodium or potassium citrate, simply calculate, add, and dissolve the citric acid in your lye water before adding your alkali. Because a portion of the lye will react with the citric acid and "consumes" it, you may need to add additional lye to accommodate for this if you do not wish to have any additional superfat. This may be of special importance in liquid soap making where we need to be very considerate of free fats to prevent separation and clouding.

To calculate the total amount of additional lye needed, you will determine the amount of sodium/potassium citrate you want to include in your recipe, calculate the additional lye necessary, and then add that to the total lye needed for your recipe.

10g citric acid neutralizes 6g of NaOH

10g citric acid neutralizes 8g of KOH

Total lye required= Additional lye needed to accommodate for sodium/potassium citrate + lye needed to saponify recipe

(Example: If your bar soap recipe is made from 1000g of oil and you want to add 2% citric acid based on the total oil weight to create sodium citrate without adding any additional superfat, you would add 20g of citric acid (0.02×1000=20) and would include an extra 12g of NaOH in the lye solution (20/10=2×6=12) (We at UG2Soap personally don't calculate the extra NaOH in bar soap recipes due to the minor amount and our personal love of superfatting, although it is a personal decision and should be part of your recipe formulating process)

I purchased a 10lb bag of citric acid for $20 and it will last me forever. Other options for citric acid include using citrus juices, like lemon, orange and grapefruit juice. (Lemon juice has roughly 3-8% citric acid, orange juice has 1-2%, and grapefruit juice has 2-3%.)

Another side benefit of citric acid and it's lye salts are their antioxidant and preservative properties. Citric acid, sodium citrate, and potassium citrate are all preservatives. These ingredients can be found in a large percentage of food products like canned foods, ice cream, baked goods, and more. Bar soap made with glycerin has a final water availability that is not suitable for microbial growth, but liquid soap does.

Citric acid is extremely affordable, is plant-based and biodegradable, can easily be added to the lye solution to create sodium/potassium citrate (which allows my soap to do its job as a cleansing agent in hard water), protects my lather from metal ions so that it can produce a full, bubbly, and long-lasting lather, and it provides a layer of protection against rancidity. It is super easy to use, just sprinkle some in the lye water, and bonus, it has a super long shelf life so you don't need to worry about it expiring or using it all right away. It's the perfect ingredient!

Do you use citric acid or another chelator in your soap? Share your story below!


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