The patent behind C E Ferulic expired on 24 March 2025. A steady run of articles has since explained that the dupes are finally legal, listed five bottles and moved on. Almost none says what the patent actually required. That is a pity. The patent is a public document, it runs to 42 claims, and it is far stranger than the summary.
It does not require what you have been told it requires. It asks for one thing nobody checks. It caps a property no label prints. And it explains itself in a single sentence built around a number we have not seen quoted anywhere. Here is that sentence, that number, and what the pair of them do to the question of whether a cheaper serum is really the same thing.
Table of contents
- What expired, and exactly when
- What the patent actually claimed
- The number nobody quotes, and why
- Where the pH rule really comes from
- The patent holder’s own researchers now dispute the rule
- What the brands themselves disclose
- Why you still cannot tell whether a dupe matches
- What a dupe would have to match
- If you are buying one anyway
- Frequently asked questions
- References
We do not run a laboratory and we have not tested any product. We have measured no pH, and we make no claim about how any of these serums performs. Everything below comes from reading documents: US Patent 7,179,841 and the earlier US Patent 5,140,043 in full, including the granted PDF of the first; the European equivalent and one later application; a federal court docket; and each brand’s own website, all read on 14 September 2026.
Four of the scientific papers cited here sit behind paywalls, and of those we have read only the abstracts. We say so again at each one rather than let the citation imply more. Every quotation below was checked character by character against the document it came from.
What expired, and exactly when
The patent is US 7,179,841 B2, “Stabilized ascorbic acid compositions and methods therefor”, granted on 20 February 2007 to Jan E. Zielinski and Sheldon R. Pinnell. SkinCeuticals took the assignment on the day it was filed and passed it to L’Oreal USA Creative in December 2006, which is why the second company, not the first, is the one printed on the face of the granted patent.
The expiry date is worth working out for yourself, because the reason usually attached to it is wrong. A US patent of this vintage runs twenty years from its earliest non-provisional filing date, not from the day it was granted. The front page of the granted PDF gives that filing date as 11 January 2005, which puts the end of the base term at 11 January 2025. The same page adds, in its notice field, that the term is “extended or adjusted under 35 U.S.C. 154(b) by 72 days”, compensation for delay at the patent office. Count 72 days on from 11 January 2025 and you land on 24 March 2025.
So the date in circulation is right and the reasoning behind it usually is not. Twenty years from grant would have run to February 2027. Twenty years from the provisional application of 13 January 2004 is wrong too, because a provisional does not start that clock. One loose end we could not close: the maintenance-fee ledger at the patent office was unreachable from three different directions on the day we looked, so we cannot show you a record proving the fees were paid to full term rather than the patent lapsing earlier. Either way, it was expired well before today.
A patent is rarely alone, so we checked whether anything else in this family is still alive. Here it really is a single patent: no continuations, no divisionals, no continuations-in-part anywhere in the US family. The European equivalent, EP 1742710 B1, has also expired. A later, related application on stabilizing ascorbic acid, US 2008/0233076, was abandoned and never granted. The nearest live patent we found from the same owner claims an antioxidant formulation stabilized by an organic UVA sunscreen filter, which a plain vitamin C serum does not contain, so it does not reach across. Nothing in this family is in force.
What the patent actually claimed
Here is the substance of claim 1, the broadest in the patent. A composition falls inside it only if it has every one of these elements.
- “5% to 40% L-ascorbic acid” by weight. Not 15%. Fifteen percent is what the product contains; the claim covers a range nearly an order of magnitude wide.
- “0.2% to 5.0% of a cinnamic acid derivative selected from p-coumaric acid, ferulic acid, caffeic acid, sinapinic acid”, plus combinations and isomers. Ferulic acid is one permitted option of four. The patent never singled it out.
- “10% to 60% of a solvent comprising a glycol ether and an alkanediol”, and water. Nobody mentions this one, and at the top of its range it is more than half the bottle.
- “the composition having a pH of no more than about 3.5”. A ceiling, with no floor anywhere in the claim.
One condition rides along with these: where the cinnamic acid derivative exceeds 0.5%, a surfactant at 1.5% to 5.0% becomes mandatory too.
Now hold that against the name on the bottle. Of the three things in “C E Ferulic”, claim 1 strictly requires only the C. Vitamin E is absent from claim 1. It first turns up at claim 11, as an optional addition, “further comprising a form of Vitamin E and a surfactant”. It does become a required ingredient in two of the patent’s seven independent claims, 38 and 41, but both of those are methods of treating skin rather than the composition claim. The ferulic acid, meanwhile, is one of four alternatives throughout.
- L-ascorbic acid, 5 to 40 percent
- A cinnamic acid derivative, any of four
- A glycol ether plus an alkanediol, 10 to 60 percent
- pH no more than about 3.5
The narrower figure that circulates as part of this patent, a pH range of 2.0 to 3.5, is not in the patent. We searched the full text for it. What the document contains is the ceiling above, repeated in every one of the seven independent claims, plus a tighter band in two dependent claims: “wherein the pH is 2.5 to 3.0”. No floor of 2.0 appears anywhere. A serum at pH 1.8 would have satisfied the pH element of claim 1 perfectly well.
The number nobody quotes, and why
The patent explains why it cares about pH exactly once, in one sentence of the detailed description. Here it is:
“The required pH of the composition ensures that greater than 82% of the ascorbic acid remains in a protonated, uncharged form”
82%
of the ascorbic acid is protonated at pH 3.5, given a pKa of 4.17. The patent’s number is its own pH ceiling, restated in different units
That is the mechanism, in the patent’s own words, and it is a statement about the ionization state of the molecule. Ascorbic acid is a weak acid: the handbook value for its dissociation constants is “pK1 = 4.17; pK2 = 11.57”, so below roughly pH 3.5 the great majority of it sits in the uncharged form rather than as the ascorbate anion. Hold the formula under that line and most of the vitamin C is the neutral species.
Work the number out, though, and it stops looking like a finding. Ascorbic acid loses its first proton at a pKa of 4.17, so the protonated fraction at pH 3.5 is 82.4 percent. At pH 3.0 it is 93.7 percent, at 4.17 it is exactly half, at pH 6 it is under two. The phrase “greater than 82%” is not a discovery sitting behind the ceiling. It is what the ceiling means once you know the pKa, the same rule written in different units. Which is also the honest answer to why nobody quotes it: it carries no information the ceiling had not already given you.
And the sentence does not really explain itself. It hands the job to another document. The word penetration never appears in this patent at all, and percutaneous only twice, both in front-page bibliography entries. The reasoning is not missing. It is in the patent this one points at, and that is worth reading, because it is more revealing than either the patent or the folklore built on it.
Where the pH rule really comes from
That sentence does not stand alone. It ends by pointing at an earlier patent, US 5,140,043, “Stable ascorbic acid compositions”, granted in 1992 to David Darr and the same Sheldon Pinnell, and folding it in by reference. That is where the trail actually leads, and the older document is more candid than anything written about it since.
Its own claim 1 asks for at least 1% ascorbic acid in water with a carrier, and a pH of no more than about 3.5. Then, in the description, it says why:
“Although not wishing to be bound by theory, we believe that the protonated form of ascorbic acid used in our invention is important dermatologically for several reasons”
“because the protonated form of ascorbic acid is uncharged, entry into the skin (which itself has a pH of about 3-5) should be facilitated”
Two things are going on there, and it is worth separating them, because the popular version collapses them. Being protonated is the mechanism. The patent then gives two purposes for it, and it numbers them: “First”, the uncharged form “removes the ionic repulsion of the two oxygen groups, thus stabilizing the molecule”, and “Second”, being uncharged, “entry into the skin” should be easier. Stability and penetration are not competing explanations here. They are two consequences the inventors hoped would follow from the same chemistry.
What is striking is how carefully both are hedged. “Although not wishing to be bound by theory, we believe” is standard patent drafting, but it means what it says. “should be facilitated” is a prediction, not a measurement, and the authority offered for it is a 1982 textbook chapter on skin permeability. The summary section hedges again in different words, saying the composition “appears to allow ready penetration into the skin”. Three statements of the idea, three hedges.
None of which makes the idea eccentric. It is the pH partition hypothesis, a piece of ordinary membrane pharmacology much older than either patent: uncharged molecules cross lipid barriers more readily than their ions. That is precisely what a 1982 chapter called “pH Dependence” in a book called Skin Permeability would set out, and both patents cite it. So the rule did not begin as marketing. It began as textbook chemistry applied to a new molecule, and stated as a hope rather than a result.
The measurement came later, and it is real. Pinnell and colleagues published percutaneous absorption studies in 2001, applying L-ascorbic acid to pig skin and measuring how much reached the tissue. One sentence in the results is the source of the number everyone repeats: “L-ascorbic acid must be formulated at pH levels less than 3.5 to enter the skin”. The same abstract adds that “Maximal concentration for optimal percutaneous absorption was 20%”, and closes with “Delivery of topical L-ascorbic acid into the skin is critically dependent on formulation characteristics”.
We have read the abstract of that paper and not the full text, which is paywalled, and we could not establish from the abstract whether the pig skin was living or excised. Three things follow, and a reader should weigh each. It is a single study. Searching PubMed and Europe PMC on 14 September 2026 we did not find an independent replication of that pH titration by any other group, which is not the same as there being none. And its senior author is a named inventor on both patents above and the founding scientist of the company that sold the product, which is a fact about the provenance of the evidence rather than an accusation about its quality.
The same relationship runs through the two later studies of the combination. Lin and colleagues in 2005 reported that adding ferulic acid to vitamins C and E “improved chemical stability of the vitamins (C+E) and doubled photoprotection to solar-simulated irradiation of skin from 4-fold to approximately 8-fold”. Murray and colleagues in 2008 put the combination on human skin and noted among their own limitations that “The number of patients evaluated was relatively small”. Both are paywalled, both were read here as abstracts only, and Pinnell is an author on both.
One more wrinkle, and it cuts both ways. The older patent’s own Example III adjusted five percent ascorbic acid solutions to a range of pH values, stored them “in the dark” at four degrees, and reported that “the L-ascorbic acid at a 5% concentration is quite stable” even at relatively high pH. Left there, that undercuts the stability case for a low pH, and it is the half usually quoted at you.
Example IV is the other half, and it is the half nobody quotes. It reports that “reactions are occurring at pH 4.2 which lead to spectral broadening of the major peak into the near UV”, visible as yellowing. So the patent’s own position is narrower and more interesting than either camp uses it for: at higher pH the assay still finds the acid, and the solution is going off anyway. Refrigerated and dark is also not a bathroom shelf.
The patent holder’s own researchers now dispute the rule
In 2025, seven researchers from L’Oreal Research and Innovation published a study of a stabilized 12% vitamin C serum formulated at pH 6, and wrote this into their conclusion:
“is not a prerequisite to an efficient VC cosmetic formula, as has long been accepted in the cosmetics industry”
The full sentence names a very acidic medium of around pH 3 as the thing that is not a prerequisite, and the authors add that “Such a low pH can fragilize the skin equilibrium over time”. So the company that owns this patent has put its name to research arguing against the low-pH rule.
Be precise about the target, though, because we were not at first. The paper is arguing with an industry belief, not with the patent. Its serum is stabilized by some other means and sits at pH 6, which puts it outside the expired claims entirely rather than inside them at a higher pH. Nothing in it says the patent’s own formulation system would have worked above 3.5. What it disputes is the general rule that a vitamin C product must be acidic to be worth anything, which is the thing the internet actually believes.
Hold the enthusiasm. The two studies are not measuring the same thing, and the newer one is weaker in a way that matters. The 2025 paper is described by its own authors as an open, uncontrolled study of eighty women with no placebo arm, and what it graded was the appearance of aging signs: glow, tone, firmness, fine lines. The 2001 study measured how much ascorbic acid was actually in the skin. A formula can produce a visible change without much vitamin C getting in, and this design cannot tell the two apart, which the authors partly concede when they note the possible contribution of glycerol and of the hydrating cream used alongside. We read the abstract only.
What survives is uncertainty, not a reversal. The pH ceiling rests on one animal-skin study by an interested party, unreplicated as far as we could find, and the challenge to it rests on one uncontrolled industry trial measuring something else. That is the honest state of the question, and it is a long way from the confidence with which the number gets repeated in either direction.
What the brands themselves disclose
Which brings us to the part we did not expect. If a pH ceiling is the one checkable mark of a real dupe, you would expect the original to publish its pH and the copies to keep quiet about theirs. It is the other way round.
We read ten vitamin C serums on their manufacturers’ own websites. Six state no pH. Four state one. The original is in the first group.
| Serum | pH the brand publishes |
|---|---|
| SkinCeuticals C E Ferulic | Not stated |
| Timeless 20% Vitamin C + E Ferulic | Not stated |
| La Roche-Posay Vitamin C12 | Not stated |
| The Ordinary Vitamin C Suspension 23% | Not stated |
| Mad Hippie Vitamin C Serum | Not stated |
| CeraVe Skin Renewing Vitamin C | Not stated |
| Drunk Elephant C-Firma Fresh | 2.5 |
| Maelove The Glow Maker | 3.1 to 3.4 |
| Paula’s Choice C15 | 3.10 to 3.50 |
| Naturium Vitamin C Complex | 5.50 to 6.50 |
pH 3.5
What the patent required
An upper bound, recited in every one of the seven independent claims. Of everything in the patent, it is the one formulation property a brand could simply choose to tell you.
Not stated
What the original publishes
SkinCeuticals gives 15% vitamin C, 1% vitamin E, 0.5% ferulic acid and 0.3% carnosine on its own page. It gives no pH at all. Four of the cheaper serums here do.
That absence is a real check, not a failure to look properly. On the SkinCeuticals page the strings “15% pure vitamin C”, “1% Alpha Tocopherol” and “0.5% ferulic acid” are all present in the rendered page, and “pH” occurs zero times in it. An earlier pass of ours returned zero for all four at once, which meant the page had not finished loading, not that anything was missing, so we threw that result away and looked again.
We ran the same discipline on the other five, because an absence you have not tested for is just a page you skimmed. Each one had to show us ingredient strings we knew should be there before its lack of a pH counted: Timeless, The Ordinary and Mad Hippie from the page source, La Roche-Posay and CeraVe from the rendered page, since those two load their ingredient panels late. Two near misses are worth reporting. Timeless appears to state a pH until you look at the match, which is the letters p and H inside a stray piece of page code. The Ordinary appears to state six, all of which turn out to be sitting inside an encoded image. Neither brand states a pH. And in The Ordinary’s case it would be a strange thing to state, because that product contains no water, and pH is a property of a water solution.
The brands that do publish a figure repay a close read. Maelove states that “Glow Maker is formulated at a precise pH of 3.1 to 3.4”, then explains why in the same answer, naming names: “According to landmark research by Dr. Pinnell at Duke University (published in 2001), Vitamin C (L-ascorbic acid) can only penetrate your skin” if the formula is acidic enough. A copy is citing the originator’s own inventor, for a number the originator does not print.
Paula’s Choice lists “Has an optimal pH range of 3.10-3.50” among its at-a-glance bullets. Drunk Elephant says its serum is “Formulated at a skin-friendly pH of 2.5”, the lowest figure here and comfortably under the expired ceiling. Naturium publishes “Formula pH Level: 5.50-6.50”, far above the patent’s ceiling and squarely in the territory the 2025 study was defending. Whatever else that is, it is a brand prepared to state a number it knows will be argued with.
Why you still cannot tell whether a dupe matches
Now the disappointing part, and the reason this piece is a method rather than a shopping list. Three of claim 1’s four elements are percentages, and a percentage is exactly what an ingredient list cannot give you.
We went through this at length in our piece on what an ingredient list actually tells you about concentration: order is real information, one stated figure anchors everything printed below it, and beyond that the numbers are not disclosed to consumers in either the US or the EU. So when claim 1 asks for 10 to 60 percent of a glycol ether and an alkanediol, you can check whether such ingredients are present. You cannot check how much. For most of this patent, the question does this dupe match? cannot be answered from public information.
What you can do is read for the species rather than the quantity. The original’s own list runs “Aqua / Water / Eau, Ethoxydiglycol, Ascorbic Acid, Glycerin, Propylene Glycol, Laureth-23, Tocopherol” and onwards. Ethoxydiglycol is the glycol ether the patent names in a dependent claim; propylene glycol is the alkanediol it names in the next one. Both sit near the top of the list. Timeless uses the same two in nearly the same positions. Drunk Elephant uses neither, reaching for dimethyl isosorbide instead. Those are facts about a label, they are free to check, and they still do not tell you whether any of the three would have infringed anything, because the quantities are missing and because infringement is a question for a court, not for a blog.
Which, as it happens, is not hypothetical. The patent record links this patent to a federal case, and the docket confirms that L’Oreal USA Creative sued Drunk Elephant in the Western District of Texas on 14 November 2018, cause of action “28:1338 Patent Infringement”, terminated on 27 October 2020. We read the docket header and not the complaint, so we cannot tell you from the court file which patents were asserted or on what terms it ended, and we are not going to guess at either. One more reason not to join the dots: the ingredient list we just read is the 2026 one, and the case was filed in 2018, against a product sold under an earlier name. Whatever was in the bottle then is not something we have looked at.
What a dupe would have to match
Put the pieces together and the checklist is short, awkward, and for the most part not something you can answer. One distinction first, because the article has been sliding between two questions. This is the list for matching the patent. The list for matching the product is the familiar three, vitamin C and vitamin E and ferulic acid, plus the solvent pair, because that is what is in the bottle. Almost nobody shopping for a dupe cares about the first list. It is worth knowing anyway, because it is the one that tells you what the chemistry was actually for.
- L-ascorbic acid between 5 and 40 percent. Usually printed on the front of the bottle, so this one you can check. Seven of the ten serums above state a figure; Naturium and Mad Hippie do not, and Mad Hippie is not using L-ascorbic acid in the first place.
- One of four cinnamic acid derivatives. Ferulic acid is the famous one, but p-coumaric, caffeic and sinapinic acid sat just as squarely within the claim. Present or absent shows on a label; the amount does not.
- A glycol ether and an alkanediol together making 10 to 60 percent. Visible as names, invisible as quantities. This is the element the roundups never mention and the hardest to hit by accident.
- A pH of no more than about 3.5. Not on any label. Four of the ten brands here volunteer it anyway, which is more than the original does.
- Vitamin E, if you want the version in the name. Optional in the broadest claim, required in two of the seven independent ones.
And then the thing that makes the whole exercise less useful than it first sounds: none of it matters legally anymore. The claims expired on 24 March 2025. Matching them is not a license, and missing them is not an infringement. What the patent is good for now is as a specification written by people who had actually measured the formula, which beats a marketing page as a guide, and as a reminder of how narrow the famous trio really was.
There is also a fair argument that the original has already moved on. SkinCeuticals now describes C E Ferulic on its own site as an “enhanced patent-pending formula”, adding “0.3% Carnosine and 1.0% Endophytol” to the familiar three. Patent pending is not a patent. It confers nothing enforceable unless and until something is granted, and an application is not normally published for eighteen months, so we cannot show you what is claimed in it. But the formula a dupe would be copying today is not the formula the expired patent describes.
Where this leaves you
The expiry changed less than the headlines suggest, and the ingredient list changed nothing at all
The patent everyone is celebrating the end of required a solvent system nobody discusses, treated ferulic acid as one option of four, made vitamin E optional in its broadest claim, and capped pH without setting any floor. Three of those four things cannot be checked from a label. The fourth, pH, is not on a label either, yet four of the ten brands we read publish it of their own accord and the brand that held the patent does not. That is the most useful thing we found, and it has nothing to do with the patent expiring.
If you are buying one anyway
We have not tested any of these and we are not going to rank them, because we have not measured a single one. What we can say is which ones state enough about themselves to be checked, which is a narrower and more defensible question. On that measure, the two serums that publish both a full concentration set and a pH are Maelove The Glow Maker (paid link) and Paula’s Choice C15 (paid link). Timeless 20% Vitamin C + E Ferulic Acid (paid link) states its concentrations and its full ingredient list but no pH. Drunk Elephant C-Firma Fresh (paid link) publishes the lowest pH figure of the ten. The SkinCeuticals original (paid link) remains the reference formula and the least forthcoming about the number this article is about.
Two caveats on those links, and both say something about the state of this market. The pH figure we quoted for Paula’s Choice comes from its reformulated product, and the only standalone listing we could find is the earlier formula, which the listing itself labels as such. And the SkinCeuticals listing was showing as unavailable, with no restock date, when we checked it on 15 September 2026. Worth knowing before you click.
Those are links we earn a commission on, which is why the disclosure sits at the top of this page rather than down here. It changes nothing written above: the pH column was filled in from the brands’ own words before any link went into the piece, and two of the products we have linked come out of it looking better than the original.
Frequently asked questions
US Patent 7,179,841 expired on 24 March 2025. The term ran twenty years from its non-provisional filing date of 11 January 2005, plus a 72 day patent term adjustment printed on the front page of the granted patent. It did not run from the grant date in 2007, and the provisional application filed in January 2004 does not start the clock.
A pH of no more than about 3.5. That upper bound appears in all seven independent claims. Two dependent claims narrow it to 2.5 to 3.0. There is no lower limit of 2.0 anywhere in the patent, despite a range of 2.0 to 3.5 being widely attributed to it.
Not in its broadest claim. Claim 1 requires L-ascorbic acid, a cinnamic acid derivative chosen from four options of which ferulic acid is one, a glycol ether and alkanediol solvent system, and the pH ceiling. Vitamin E appears first at claim 11 as an optional addition, and is a required ingredient only in independent claims 38 and 41, which are methods of treating skin.
Mostly no. Three of the four elements of claim 1 are percentages, and percentages are not disclosed on an ingredient list. You can see whether a glycol ether and an alkanediol are present, and roughly where they sit in the order, but not whether they total between 10 and 60 percent. pH is not on a label at all.
Of the ten brands we read on 14 September 2026, four published a figure on their own site: Drunk Elephant C-Firma Fresh at 2.5, Maelove The Glow Maker at 3.1 to 3.4, Paula’s Choice C15 at 3.10 to 3.50, and Naturium Vitamin C Complex at 5.50 to 6.50. SkinCeuticals does not publish one for C E Ferulic.
The evidence is thinner than the confidence around it, in both directions. The underlying idea is ordinary membrane pharmacology and long predates the patents: uncharged molecules cross lipid barriers more readily than their ions. The specific measurement for ascorbic acid traces to a single pig-skin study from 2001 whose senior author was a named inventor on the patent, and a PubMed and Europe PMC search on 14 September 2026 did not turn up an independent replication of it. In 2025 researchers at the patent holder published a study of a pH 6 serum arguing that a very acidic medium is not a prerequisite, but that study was open and uncontrolled and measured the appearance of aging signs rather than how much vitamin C entered the skin. We do not have a laboratory and we have measured nothing ourselves.
References
- US Patent 7,179,841 B2, Zielinski and Pinnell, “Stabilized ascorbic acid compositions and methods therefor”, filed 11 January 2005, granted 20 February 2007. patents.google.com. Full text and all 42 claims read on 14 September 2026; the front-page notice and filing date were read from the granted PDF.
- US Patent 5,140,043, Darr and Pinnell, “Stable ascorbic acid compositions”, granted 18 August 1992, incorporated by reference into the patent above. patents.google.com. Full text read.
- European Patent EP 1742710 B1, the European equivalent, expired. patents.google.com.
- US Patent Application 2008/0233076, “Process for stabilizing ascorbic acid compositions”, abandoned. patents.google.com.
- L’Oreal USA Creative, Inc. v. Drunk Elephant, LLC, 1:18-cv-00982 (W.D. Tex.), filed 14 November 2018, terminated 27 October 2020. courtlistener.com. Docket header only; the complaint was not retrieved.
- Pinnell SR, Yang H, Omar M, et al. Topical L-ascorbic acid: percutaneous absorption studies. Dermatologic Surgery 2001;27(2):137-42. PubMed 11207686. Abstract only; full text paywalled and not read.
- Lin FH, Lin JY, Gupta RD, et al. Ferulic acid stabilizes a solution of vitamins C and E and doubles its photoprotection of skin. Journal of Investigative Dermatology 2005;125(4):826-32. PubMed 16185284. Abstract only; full text paywalled and not read. Two of its authors are the named inventors on the patent above.
- Murray JC, Burch JA, Streilein RD, et al. A topical antioxidant solution containing vitamins C and E stabilized by ferulic acid provides protection for human skin against damage caused by ultraviolet irradiation. Journal of the American Academy of Dermatology 2008;59(3):418-25. PubMed 18603326. Abstract only; full text paywalled and not read.
- Isoir-Ingrez M, Veriato A, Figueiredo S, et al. In vivo efficacy of a stabilized Vitamin C-based serum at pH 6 on some ageing facial signs of women of different ages and phototypes. International Journal of Cosmetic Science 2025;47(3):424-33. PubMed 39788601. Abstract only; full text paywalled and not read. All seven authors are employed by the patent holder.
- Dissociation constants of ascorbic acid, from the Merck Index as cataloged by PubChem. pubchem.ncbi.nlm.nih.gov.
- SkinCeuticals, C E Ferulic with 15% L-Ascorbic Acid, product page and ingredient list as published by the manufacturer. skinceuticals.com. Read 14 September 2026.
- Maelove, The Glow Maker, product page as published by the manufacturer. maelove.com. Read 14 September 2026.
- Paula’s Choice, C15 Brightening Super Booster, product page as published by the manufacturer. paulaschoice.com. Read 14 September 2026.
- Drunk Elephant, C-Firma Fresh Day Serum, product page as published by the manufacturer. drunkelephant.com. Read 14 September 2026.
- Naturium, Vitamin C Complex Serum, product page as published by the manufacturer. naturium.com. Read 14 September 2026.
- Timeless Skin Care, 20% Vitamin C + E Ferulic Acid Serum, product page as published by the manufacturer. timelessha.com. Read 14 September 2026.