Sodium hydroxide is in a ton of beauty products. But as one of my favourite unreliable sources says (the “favourite” is sarcastic by the way, just to clarify):
Sodium Hydroxide is, however, a known irritant…
The National Institute for Occupation Safety and Health … recommends that consumers prevent skin and eye contact
The CDC reports that “Skin contact with sodium hydroxide can cause severe burns with deep ulcerations. Pain and irritation are evident within 3 minutes, but contact with dilute solutions may not cause symptoms for several hours.
Solutions as weak as 0.12% have shown to destruct healthy skin cells within one hour.
– Truth in Aging
It sounds pretty nasty, and it’s nasty even at a super low concentration of 0.12%!
But then why is it in everything? Let’s talk basic (haha) chemistry…
What Does Sodium Hydroxide Do?
Sodium hydroxide, with the formula NaOH, is usually used in products as a pH adjuster. It’s also used in soap-making to turn fats and oils into soap. You can find sodium hydroxide (also called caustic soda or lye) in its pure form in the cleaning aisle, since it’s also really handy for clearing up clogged drains.
But here’s something interesting…
Sodium Hydroxide Isn’t Really in Most Products
To really understand why we don’t need to freak out about sodium hydroxide, we need to understand what happens when we put sodium hydroxide into a product.
Sodium hydroxide is a soluble inorganic compound and a strong base, and this makes it really unusual because, unlike most other ingredients, the amount you put into a product isn’t the amount you end up with in the product at the end.
Sodium Hydroxide Chemistry
When sodium hydroxide dissolves in water, it breaks up (dissociates) into sodium ions (Na+) and hydroxide ions (OH–):
The sodium ions are pretty benign. Table salt is sodium chloride (NaCl), and when that dissolves in water (or in your food), it splits up into sodium ions (Na+) and chloride ions (Cl–). The sodium ions from sodium hydroxide and the sodium ions from salt are indistinguishable after the substances have broken up:
There’s tons of sodium ions everywhere: there’s almost 1 gram per litre of sweat, and seawater is 1.08% sodium ions by mass. We have lots of sodium ions in our bodies, and it’s essential for the way our body functions.
If you don’t have a high enough concentration of sodium ions in your blood, you end up with hyponatremia, which can kill you (the name comes from natrium, the Latin name for sodium). This is the reason that water in large doses is toxic.
It’s really the hydroxide ions that are the cause of the trouble.
We don’t talk much about hydroxide ions in skincare, and we really should!
We talk about hydrogen ions (H+) a lot, since hydrogen ions are the reason for acidity. pH is a measure of the concentration of hydrogen ions in a water-based substance. If you don’t know much about pH yet, check out this guide to pH that I wrote. (Don’t worry, this post will still be here when you get back!)
The key things about pH you’ll need to know to understand what comes next:
- pH 7 means neutral. If it’s lower than 7 it’s acidic, and if it’s higher than 7 it’s basic.
- Lower pH means a higher concentration of H+ is present.
- Acidic substances donate H+ ions in water.
- Alkaline substances absorb (accept) H+ ions in water.
- The formula for calculating pH is pH = -log10[H+].
(Side note: Hydrogen ions are more correctly referred to as hydronium ions, but for the sake of simplicity I’m sticking with hydrogen ions.)
Here’s the thing about hydroxide ions – they react with hydrogen ions to form water:
H+ + OH– → H2O(l)
This is how they work as pH adjusters – they neutralise the hydrogen ions and turn them into water, raising the pH of the product. Most of the added hydroxide ions will turn into water, and sodium ions will float off.
Water can also break apart to form hydrogen ions and hydroxide ions:
H2O(l) → H+ + OH–
This is why pH 7 is neutral, even though you can get less hydrogen ions if you have a higher pH. At pH 7, you have the same amount of hydrogen ions and hydroxide ions (at 25 °C). At lower pH you have an excess of H+, and at higher pH you have an excess of OH–.
Because hydrogen ions and hydroxide ions are linked to water in this way, you can actually work out the concentration of hydroxide ions in a product if you know the pH. The formula is a little bit complicated:
Concentration of hydroxide ions ([OH-]) = 10-(14-pH) mol L-1
Basically it’s just the opposite of pH. pH 14 has the same concentration of hydroxide ions as pH 0 has of hydrogen ions, and so on.
What does this mean for sodium hydroxide in skincare products?
Here’s the nitty gritty.
The only reason sodium hydroxide can be harmful in products is because of the hydroxide ions, and you know exactly how dangerous the hydroxide ions are if you know the pH of the product.
If the product is at pH 7, it has the same concentration of hydroxide ions as water (i.e. not very scary). You don’t need to worry about sodium hydroxide.
If the product is at a pH below 7, it has less hydroxide ions than plain water (still not very scary… but if you go too low then you need to start worrying about hydrogen ions). You don’t need to worry about sodium hydroxide at all.
If the product is at a pH above 7, then it will have more hydroxide ions than water. In this case, look at the pH – the higher the pH, the more dangerous the hydroxide will be. In general, anything below pH 10 is fine if it’s in contact with your skin for a short amount of time, although you’ll probably want to rinse your skin afterwards (soap is pH 9-10).
Speaking of soap…
Sodium hydroxide in soap
The issue of sodium hydroxide in soap is a bit like sodium hydroxide as a pH adjuster – in the final product, there isn’t much sodium hydroxide there. In soap, the sodium hydroxide reacts with a fat or oil, turning it into soap.
More information on soap chemistry and saponification: Make Your Own Soap! Part 1: The Chemistry Behind Soap Making
That means that once you have a soap, you shouldn’t have any sodium hydroxide left. Usually soap-makers actually play it safe and add extra fat or oil (“superfatting”) to make sure there’s no excess sodium hydroxide, so you don’t burn your skin off accidentally.
However, the ingredient lists in soaps are usually listed as the raw ingredients that were mixed together to make the soap, so sodium hydroxide will be at a disturbingly high position. But don’t worry – if the soap’s been made properly, the sodium hydroxide will be used up.