Vitamin C in a Korean Formula: 13, 23 and the Derivative Route

If you want the short answer: buy the 13, not the 23. COSRX The Vitamin C 13 Serum and Advanced The Vitamin C 23 Serum both declare pure L-ascorbic acid straight in the ingredient list, at 13% and 23%. The higher number is not a stronger version of the lower one. It is a different product with a different base, a different feel and a much shorter fuse, and almost everybody who buys it because 23 is bigger than 13 ends up using it twice a week and letting it oxidise on the shelf. If your skin has never met a straight ascorbic acid serum, start at 13. If you have used one happily for months and want more, go to 23. If you have reactive skin, skip both and take the derivative route with Dr. Althea Gentle Vitamin C Serum.

That is the whole recommendation. The rest of this piece is the reasoning, and every number in it is read off the ingredient list on the box we ship, not off a marketing page.

What does the number on a Korean vitamin C serum actually mean?

Korean cosmetic labelling puts declared figures inside the ingredient list itself, in parentheses, immediately after the ingredient. That convention is why our catalogue can be this specific. Across the 715 products in our range whose full ingredient list we hold, 104 carry some form of vitamin C, and the split matters more than the count:

A percentage only means something when you also know which of those six things it is a percentage of. Twenty-three percent ascorbic acid and twenty-three percent ascorbyl glucoside are not comparable claims, and a brand that prints a big number without printing the form is telling you nothing. Every product below prints both.

How do the COSRX 13 and 23 serums actually differ?

Read the two lists side by side and the difference stops being a number.

The 13 opens: Water, Ascorbic Acid (13%), Butylene Glycol, Dipropylene Glycol, Tromethamine, 3-O-Ethyl Ascorbic Acid, Panthenol, Acetyl Glucosamine, Caffeine, Sodium Hyaluronate, Sodium Sulfite, Disodium EDTA, Glutathione, Adenosine. It is a water serum. Tromethamine is there to lift the pH off the floor so 13% acid is wearable. Sodium sulfite and disodium EDTA are an antioxidant and a chelator, both of which exist to slow the browning that ascorbic acid does in water. Glutathione, niacinamide, tocotrienols and tocopherol sit further down as a supporting antioxidant bench. Licorice root extract closes it out. Twenty-five ingredients, 20 ml.

The 23 opens: Aqua/Water, Ascorbic Acid (23%), Butylene Glycol, Dimethicone, Panthenol, 3-O-Ethyl Ascorbic Acid, Squalane, Sodium Hydroxide, Caffeine, Sodium Hyaluronate, Sodium Metaphosphate, Adenosine. Dimethicone is in fourth position and squalane in seventh, with methyl trimethicone, sunflower, safflower and camellia seed oils and palm oil below. The base has been rebuilt with silicone and lipids because 23% ascorbic acid in a bare water base is uncomfortable, and the oils buffer the feel of it. Sodium hydroxide replaces tromethamine as the neutraliser; sodium metaphosphate does the chelating.

So the honest framing is: the 13 is a light aqueous serum that most people can wear every morning, and the 23 is a cushioned, slightly slippery, considerably more assertive serum that most people should build up to. Both are 20 units of product. Both carry the same antioxidant bench. The 23 is not the 13 with more of the good stuff. It is a different vehicle carrying a load the first vehicle could not.

One practical consequence: neither is a product you buy in bulk. Ascorbic acid in water browns. Twenty millilitres is the right size for a serum you should finish inside a season, and the fact that both come in 20 units rather than 50 is a formulation decision, not a stinginess one. Buy one, finish it, buy another. Both live on the serums and ampoules shelf alongside the rest of the Korean-market COSRX range.

Is there a 23% serum that is easier to live with?

Yes, and it is the most interesting bottle in this category: Isntree Hyper Vitamin C 23 Serum. Seventeen ingredients, total: Water, Ascorbic Acid (23%), Propanediol, Betaine, 1,2-Hexanediol, Polyglycerin-3, Tromethamine, Ethyl Ascorbyl Ether, Hydroxyethyl Urea, Betula Platyphylla Japonica Juice, Ethylhexylglycerin, Adenosine, Disodium EDTA, Panthenol, Gardenia Florida Fruit Extract, Dextrin, Tocopherol.

No silicone. No oils. No niacinamide. What it does instead is stack humectants (propanediol, betaine, polyglycerin-3, hydroxyethyl urea, birch juice) around the acid, so the discomfort is managed by hydration rather than by slip. And it runs a second vitamin C route alongside the first: ethyl ascorbyl ether in eighth position, which is stable where the L-ascorbic acid is not. That is a deliberate belt-and-braces design, and it is the kind of thing you only see when you read the list.

If you like a short ingredient list and you dislike the feel of silicone, this is your 23. If you want the antioxidant bench and the cushioned finish, the COSRX 23 is. The rest of the Isntree range is built on the same humectant-first philosophy, which is why their Hyaluronic Acid Toner lists sodium hyaluronate before water.

What is the derivative route, and who should take it?

Straight ascorbic acid works at a low pH. Low pH is exactly what stings a compromised barrier, a rosacea-prone cheek or skin that is three weeks into a retinoid. The derivative route trades some directness for a formula that sits near skin-neutral pH and does not sting.

Dr. Althea Gentle Vitamin C Serum is the clean version of that trade. Twenty ingredients. Niacinamide is fourth. Camellia sinensis leaf water (green tea) is the seventh, sea buckthorn fruit extract the eleventh, and 3-O-ethyl ascorbic acid the fifteenth. There is no L-ascorbic acid in it at all. Nothing here will bite, and nothing here will do very much very quickly. That is the point of it, and it is the correct first vitamin C for anyone whose skin has a history of reacting.

Dr. Althea Vitamin C Boosting Serum is the same brand taking the opposite view: rather than one gentle vitamin C, put four pigment-facing ingredients in one bottle and let them cover different mechanisms. Sea buckthorn water is the first ingredient, centella leaf water second, niacinamide third. Tranexamic acid appears eleventh, 3-O-ethyl ascorbic acid further down, and then alpha-arbutin, ascorbic acid, ceramide NP, ubiquinone, bisabolol and bifida ferment lysate. This is a tone serum with a vitamin C component rather than a vitamin C serum, and if uneven tone rather than dullness is your actual complaint, it is the more sensible purchase. Across our whole range, tranexamic acid appears in 32 products and arbutin in 29, so the pairing is a recognised Korean approach rather than a one-off. Both sit in the Dr. Althea range.

Why does Purito's vitamin C serum look so different?

Because it is not built like a Western vitamin C serum at all. As published, the list on Purito Pure Vitamin C Serum opens with Sodium Hyaluronate, then Ascorbic Acid, 1,2-Hexanediol, Niacinamide, Arginine, Human-Oligopeptide-1, Hydrolyzed Collagen, and then a run of botanicals: lotus flower, magnolia bud, tiger lily, peony root, white truffle, apple, red ginseng. Arginine is doing the pH work. There is a growth-factor peptide and hydrolysed collagen in the top eight.

No declared percentage, so we will not print one. At 60 ml, it is sized as a daily conditioning serum with a vitamin C component rather than a short-course active. Different job, different bottle size. It sits with the rest of the Purito Seoul range.

Which Korean vitamin C serum should I buy?

How do I actually use it without wrecking my skin?

Morning, on clean dry skin, before anything with slip in it. Ascorbic acid wants to be the first thing on the face, and it wants a couple of minutes to itself before you go over the top.

• Start at three mornings a week. Not daily. Not every other day. Three, for two weeks, then four, then daily if nothing complains.
• Do not layer it with an exfoliating acid in the same session. If you use a BHA toner, put it in your evening routine. The Anua BHA 2% Gentle Exfoliating Toner is a PM product on a vitamin C schedule.
• Niacinamide is fine alongside it. The old internet rule against combining them is a lab artefact at high heat, not a bathroom reality, and COSRX put niacinamide inside both of their ascorbic acid serums. It is in 396 of the 715 formulas we hold full lists for.
• Seal it. A vitamin C serum on bare skin evaporates and stings. Follow it with something from the moisturizers shelf.
• Wear sun care over it. A morning antioxidant and a morning sunscreen are two halves of the same routine, and the antioxidant is the half that does less on its own. Beauty of Joseon Relief Sun: Rice + Probiotics and COSRX Aloe 54.2 Aqua Tone-Up both layer cleanly over an aqueous serum.
• Watch the colour. Straw yellow is normal. Amber is late. Brown means the ascorbic acid has oxidised and the serum is finished, whatever is left in the bottle.

Why is this the Korean-market version and not the US one?

Because that is the only version we buy. The formulas above are the ones sold in Korea, sourced for the Korean market, with the Korean ingredient list on the carton, which is precisely why the declared percentages are in the list at all. A US-shelf reformulation of the same product name can carry a different base, a different filter system or a different figure, and we wrote about exactly where those splits happen in Korean formulas and US formulas, what actually differs. If you want to check the box that arrives against the batch it was made in, how to read the batch code on your Korean skincare walks the codes brand by brand, and our position on original Korean formulas sets out what we will and will not put on a shelf.

Where do I start?

Buy the 13. Use it three mornings a week for a fortnight. If your skin is bored rather than irritated at the end of that, move up to the Isntree 23 or the COSRX 23 depending on whether you like slip. If your skin complains in week one, stop and take the derivative route instead. It is not the consolation prize, it is the right formula for a barrier that is not ready.

Everything named here is in stock and ships from within the United States with duties prepaid. Browse the full serum and ampoule shelf, or start from the whole Korean-market range.