Citric Acid in Soap: What Does it Actually Do?

Citric acid is one of those soapmaking additives that gets credited with doing an awful lot. You’ll hear that it prevents soap scum, improves lather, softens water, and even helps prevent DOS. Some of those claims have a good bit of chemistry behind them, while others need a little more explanation.

So, what does citric acid actually do in handmade soap?

The first thing to understand is that citric acid itself isn’t really the ingredient doing the work in your finished bar. When you add citric acid to a cold process soap recipe, it reacts with some of your sodium hydroxide and creates sodium citrate.

Sodium citrate is a chelator, and that one bit of chemistry explains most of what citric acid can—and cannot—do in soap.

What happens when you add citric acid to soap?

Citric acid is an acid, and sodium hydroxide is a base. When they react with one another, the citric acid is neutralized and sodium citrate is formed.

If you’re making bar soap with sodium hydroxide (NaOH), you create sodium citrate.
If you’re making liquid soap with potassium hydroxide (KOH), you create potassium citrate.

The citrate is the part we’re interested in because it can bind with certain metal ions and make them less available to participate in other chemical reactions. That process is called chelation.

So, while we soapmakers generally say we’re “adding citric acid to soap,” what we’re actually doing is using citric acid and lye to create citrate right there in the soap pot.

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What is a chelator?

A chelator is a substance that binds with metal ions. For soapmakers, two particularly important ones are calcium and magnesium, which are largely responsible for hard water.

When traditional soap encounters calcium and magnesium, those minerals can react with the soap and form insoluble compounds. You probably know the result: soap scum. (If you’d like the complete wonderfully scummy explanation, I have an entire article about what soap scum is, why handmade soap makes it, and how to clean it.)

Citrate can bind with some of those minerals before they react with your soap, leaving fewer calcium and magnesium ions available to make soap scum. The important word there is fewer. Citric acid doesn’t magically turn hard water into soft water, and citrate won’t necessarily capture every mineral ion your soap encounters.

That distinction matters quite a bit.

Does citric acid prevent soap scum?

Citric acid can help reduce soap scum, but I would stop short of saying that it prevents it.

I originally experimented with citric acid in my own soap specifically because I wanted less soap scum. At the time, I lived in Denver, where I had fairly hard water. I added citric acid, made my soap, used it—and I still had soap scum. Now I live in Tucson, where my water is even harder, so I certainly wouldn’t expect a small amount of citrate in my bar of soap to make all those minerals disappear.

That doesn’t mean the sodium citrate wasn’t doing anything. It simply means there is a big difference between chemically binding some of the minerals in hard water and binding enough of them to completely prevent soap scum.

For me, the difference wasn’t dramatic enough to make citric acid something I routinely add solely to combat soap scum. Your experience may be completely different, especially if your water is much softer than mine. And this is exactly why I love testing this stuff instead of stopping at, “Well, the chemistry says it should work.”

Does citric acid increase lather?

This is where the wording gets especially important because citric acid is sometimes described as a lather booster.

I wouldn’t call it that.

Citric acid doesn’t change the fatty acid profile of your recipe or magically make a low-lather formulation bubbly. What sodium citrate may do, however, is help your soap retain more of its normal lather when it is being used in hard water.

When soap encounters the calcium and magnesium in hard water, some of that soap reacts with the minerals and forms insoluble soap scum. That portion of the soap is no longer available to do what you actually wanted it to do, including cleaning and making bubbles.

If citrate binds with some of those minerals first, more of your soap may remain available to lather. So, if a soapmaker tells me, “I started using citric acid and my soap lathers better in my hard water,” I wouldn’t tell them they’re imagining it. I would just describe what is happening a little differently.

Citric acid isn’t really boosting the lather. Sodium citrate may be reducing some of the hard-water interference that was suppressing the lather in the first place.

We’ll dig into that relationship much more when we talk specifically about hard water and handmade soap.

Does citric acid help prevent DOS?

This one also gets repeated quite a bit in soapmaking circles, and I think it deserves a little more nuance.

Trace metals, including iron and copper, can encourage oxidation in fats and contribute to rancidity. Because chelators bind metal ions, it makes sense that soapmakers would look at chelators as one possible tool for reducing the risk of Dreaded Orange Spots, or DOS.

That doesn’t necessarily mean that every chelator does an equally good job of preventing rancidity, though. Testing by soap chemist Kevin Dunn found sodium citrate was not particularly effective at protecting soap from rancidity under the conditions he tested.

So I wouldn’t tell you, “Add citric acid and you’ll prevent DOS.”

There are also far too many other variables involved in rancidity for any one additive to provide that kind of guarantee. Your oils, storage conditions, oxidation, contaminants, water quality, metal exposure, and formulation can all play a role.

I think the more accurate takeaway is that sodium citrate is useful as a hard-water chelator, but I would not rely on it as my primary strategy for preventing DOS.

That’s a little less exciting than promising that one ingredient will solve everything, but chemistry does not exist to make our marketing copy more exciting. 😂

How much citric acid should you use in soap?

You’ll see a fairly wide range of suggested usage rates among soapmakers. A common range is around 1–2% of your total oil weight, although some soapmakers use as much as 3%, particularly when working with harder water.

For a batch containing 1,000 grams of oils, that would look like:

1% citric acid: 10 grams
2% citric acid: 20 grams
3% citric acid: 30 grams

I would start conservatively rather than assuming more must be better. Higher levels of citrate can sometimes produce white crystalline deposits on the finished soap as it ages. Those crystals may be harmless, but they probably aren’t the decorative effect you had planned.

As with so many soapmaking additives, start with a reasonable amount and test it in your recipe with your water. If you live somewhere with relatively soft water, your experience could be very different from mine here in Tucson.

You need to adjust the sodium hydroxide

This is the part you absolutely do not want to skip.

Remember that citric acid reacts with sodium hydroxide. That means some of the NaOH you calculated for your soap will be used to neutralize the citric acid instead of saponifying your oils.

If you simply add citric acid to an existing recipe without accounting for that reaction, you will effectively increase the superfat of your finished soap.

For bar soap made with sodium hydroxide:
1 gram of citric acid consumes approximately 0.624 grams of sodium hydroxide.

So, if you want to use 20 grams of citric acid:
20 × 0.624 = 12.48 grams of additional NaOH

Let’s say your original formulation requires 135 grams of sodium hydroxide to saponify your oils at your chosen superfat. Once you add 20 grams of citric acid, your total NaOH would become:
135 g + 12.48 g = 147.48 g NaOH

The original 135 grams is still doing the job of saponifying your oils. The additional 12.48 grams is there to react with your citric acid.

You aren’t decreasing your intended superfat by adding that extra NaOH. You’re compensating for the lye that the citric acid will consume so your original superfat remains where you intended it to be.

A simple citric acid calculation

Let’s put the whole calculation together using a soap recipe with 1,000 grams of oils and a citric acid usage rate of 2%.

First, calculate your citric acid:
1,000 × 0.02 = 20 grams citric acid

Then calculate how much additional NaOH the citric acid will consume:
20 × 0.624 = 12.48 grams additional NaOH

You would add that 12.48 grams of NaOH to whatever amount of sodium hydroxide your normal soap calculation requires for your oils and chosen superfat.

This is why I don’t recommend simply adding citric acid to a recipe as an afterthought. The citric acid and additional sodium hydroxide are part of the same calculation. If you change one, you need to recalculate the other.

How do you add citric acid to cold process soap?

You have a couple of options for physically incorporating citric acid into your recipe. One simple method is to dissolve the measured citric acid in a portion of the water already included in your formulation before incorporating it into the batch.

You can also dissolve the citric acid in the water that will be used for your lye solution. If you use this method, be aware that adding sodium hydroxide to an acidic solution can produce additional heat and bubbling. Add the NaOH slowly while stirring and follow your normal lye-safety procedures.

Whatever method you use, make sure your formulation contains enough water to comfortably dissolve both the sodium hydroxide and the citric acid.

And, of course, our standard soapmaking safety rule still applies: always add sodium hydroxide to your liquid, never liquid onto dry sodium hydroxide.

If working with concentrated lye solutions is still fairly new to you, my 1:1 Master Batch Lye Solution tutorial goes into much more detail about safely measuring, mixing, storing, and working with sodium hydroxide solutions.

Can I just add sodium citrate instead?

Yep!

You can purchase sodium citrate directly and add it to your soap rather than creating sodium citrate by reacting citric acid with sodium hydroxide.

There is one especially nice advantage to doing it this way: sodium citrate is already neutralized, so you don’t need to add extra sodium hydroxide to compensate for it.

Citric acid requires additional lye because it is an acid and will consume some of your NaOH. Sodium citrate has already undergone that neutralization.

The usage rates aren’t directly interchangeable gram for gram, however. One gram of citric acid creates more than one gram of sodium citrate after it reacts with sodium hydroxide, so sodium citrate is generally used at a somewhat higher percentage than citric acid.

If your entire reason for using citric acid is simply to get sodium citrate into the finished soap, purchasing sodium citrate directly can make the formulation easier. Citric acid, however, is inexpensive, readily available, and something many soapmakers already have around the house or soap room, which is probably one reason it remains so popular.

What about lemon juice and other citrus juices?

Ah yes, the wonderfully irresistible soapmaker thought: if citrus fruit contains citric acid, can I just make soap with lemon juice?

You absolutely can use citrus juice as part or all of the liquid in a soap formulation, but it isn’t nearly as predictable as weighing pure citric acid.

The citric acid concentration of citrus juice varies according to the type of fruit, variety, ripeness, growing conditions, and processing. Lemon and lime juice generally contain considerably more citric acid than orange juice, but even two lemons aren’t necessarily chemically identical.

That makes calculating exactly how much sodium hydroxide the juice will consume much less precise.

So citrus juice can absolutely be something you intentionally experiment with. I just wouldn’t treat “the juice from one lemon” as a precise substitute for a carefully weighed amount of pure citric acid.

Soapmaking chemistry likes grams considerably more than it likes lemons. 🍋

So, is citric acid worth adding to soap?

Here comes my wonderfully annoying soapmaking answer: it depends on why you’re adding it.

If you have hard water and want to experiment with reducing soap scum, citric acid is absolutely worth testing. If you’re hoping it will turn a low-lather recipe into a bubble monster, probably not. If you’re adding it simply because someone told you that every good soap recipe needs citric acid, I’d rather you understand what it’s doing first and decide whether it actually serves a purpose in your formulation.

And if your primary goal is preventing DOS, I wouldn’t depend on sodium citrate alone for that job.

For me personally, citric acid hasn’t made enough difference in soap scum to earn a permanent place in every batch I make. But that conclusion comes from my soap and my tap water, and your results could be entirely different. Your tap water (the water you wash with) is part of your soapmaking experience whether or not you write it down on your recipe sheet.

Instead of asking, “Should soapmakers use citric acid?” I think there is a much more useful question:
What problem am I trying to solve, and does citrate actually make a noticeable difference in my soap?

If you really want to know, make two otherwise identical small test batches. Add citric acid to one and leave it out of the other. Cure them under the same conditions, and then compare them using the same water.

Pay attention to:

How easily each soap begins to lather
The amount and quality of the lather
How the soap feels during rinsing
How much soap scum develops on your sink or shower
Whether you notice any difference as the bars age

That simple side-by-side experiment will teach you much more about whether citric acid belongs in your soap than adding it automatically because someone else’s recipe told you to.

Frequently Asked Questions About Citric Acid in Soap

Does citric acid make soap more acidic?

Not when it is properly formulated. Citric acid reacts with sodium hydroxide and forms sodium citrate. If you properly compensate for the NaOH consumed by that reaction, you aren’t simply leaving a bunch of free citric acid in your finished soap to lower its pH.

Is citric acid a preservative in soap?

I wouldn’t describe it that way. Properly formulated bar soap generally doesn’t require a conventional preservative simply because water was used to make it. The role we’re discussing here is primarily chelation, not preservation.

Is citric acid a lather booster?

Not in the usual sense. Sodium citrate may help soap perform better in hard water by binding with some of the calcium and magnesium that would otherwise react with the soap. That can leave more soap available to lather, but it doesn’t inherently make your recipe more bubbly.

Does citric acid stop soap scum?

It can help reduce soap scum by creating citrate that binds with some hard-water minerals, but it doesn’t guarantee that soap scum will disappear, especially if you have very hard water.

Do I need extra lye when using citric acid?

Yes. For NaOH bar soap, every gram of anhydrous citric acid consumes approximately 0.624 grams of sodium hydroxide. You need to account for that additional NaOH if you want to maintain the intended superfat of your formulation.

Do I need extra lye when using sodium citrate?

No. Sodium citrate has already been neutralized, so you don’t compensate with additional sodium hydroxide the way you do when adding citric acid.

Can citric acid prevent DOS?

I wouldn’t rely on it for that purpose. Although chelation can be useful when dealing with metals involved in oxidation, sodium citrate has not performed particularly well as a rancidity preventative in published soap testing. If DOS is your concern, it’s better to look at the broader causes of rancidity rather than depending on citric acid alone.

Continue Learning with Soapy Friends

Soapmaking gets much more interesting when you start asking why an ingredient is in your recipe instead of simply following an ingredient list. Citric acid is a perfect example because one seemingly simple additive leads us straight into chelation, hard water, lather, superfat calculations, soap scum, and even questions about rancidity.

If you’d like to continue learning and soaping along with me, here are a few great next steps:

Soap Scum 101: What Is Soap Scum and How to Clean It — See exactly what happens when handmade soap meets hard-water minerals and why even a chelator may not make soap scum disappear.
DOS Soap: Dealing with Dreaded Orange Spots in Handmade Soap — If rancidity prevention is why you’re considering citric acid, this is the place to understand the bigger picture behind DOS.
Get the Free Soapy Friends Starter Kit — Build your soapmaking knowledge with free classes, charts, worksheets, labels, and other resources from the Soapy Friends learning library.
Explore Soapy Friends Memberships — If you enjoy understanding what is actually happening in your soap pot, membership gives you more opportunities to experiment, learn, and soap along with me.

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