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–.