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This is a single, focused recipe page: how to make a 20% L-ascorbic acid vitamin C serum from powder, with exact grams, the pH target, how 20% compares with 10% and 15%, and how to tell when your batch has oxidized. For a broader introduction to ascorbic acid powder and a beginner walkthrough, see our Ascorbic Acid Powder for Skin DIY Guide.
Why This Recipe Uses 20%
The most useful study on how much L-ascorbic acid reaches the skin is Pinnell and colleagues (Dermatologic Surgery, 2001). Working with pig skin, a common laboratory model for human skin, they found:
- L-ascorbic acid entered the skin only when the formula pH was below 3.5.
- Skin levels rose as concentration increased from 5% to 20%, with the maximum at 20% (tested at pH 3.2).
- Concentrations above 20% gave lower skin levels, not higher.
- Tissue levels saturated after three daily applications, and the vitamin C in the skin had a half-life of about four days.
That makes 20% the highest evidence-based strength for a plain L-ascorbic acid serum, and it is the strength I formulate this recipe at. Going above 20% adds powder that the study measured as delivering less, and it raises the chance of stinging.
10% vs 15% vs 20%: What Changes
| Factor | 10% | 15% | 20% |
|---|---|---|---|
| Skin levels in Pinnell 2001 (pig skin) | Lower than 20% | Lower than 20% | Highest measured |
| Published combination research | Limited | Lin 2005: 15% L-ascorbic acid with 1% vitamin E and ferulic acid | Pinnell 2001: maximum skin levels |
| Powder to dissolve | Least | Moderate | Most, still fully soluble in this recipe |
| Sensation on skin | Mildest | Moderate | Most likely to tingle, especially on dry or freshly exfoliated skin |
If 20% stings, the fix in this recipe is frequency, not strength: apply it every other morning for the first two weeks, then daily.
What You Need
- L-ascorbic acid powder, cosmetic or food grade, fine powder
- Distilled water (not tap water, which carries minerals that can speed oxidation)
- Vegetable glycerin
- A digital scale that reads to 0.01 g
- A small glass beaker or shot glass and a glass or stainless stirring rod
- A dark amber or cobalt glass dropper bottle, 10 to 30 ml
- pH strips with a 0 to 6 range, or a calibrated pH meter
- 70% isopropyl alcohol for sanitizing
The 20% Vitamin C Serum Recipe
| Ingredient | % | 10 g batch (about 1 week) | 30 g batch (about 1 fl oz) |
|---|---|---|---|
| Distilled water | 75.0 | 7.50 g | 22.50 g |
| L-ascorbic acid powder | 20.0 | 2.00 g | 6.00 g |
| Vegetable glycerin | 5.0 | 0.50 g | 1.50 g |
| Total | 100.0 | 10.00 g | 30.00 g |
I recommend the 10 g batch. A daily dose of 4 to 5 drops uses only a fraction of a gram, so a 30 g batch outlives its freshness long before you finish it.
Method
- Sanitize the beaker, stirring rod and bottle with 70% isopropyl alcohol and let them air dry completely.
- Weigh the water into the beaker at room temperature. Do not heat it. Heat speeds oxidation.
- Add the L-ascorbic acid powder and stir gently until the liquid is completely clear. At 20% this can take 5 to 10 minutes. Undissolved grains feel gritty and sting.
- Add the glycerin and stir for another 30 seconds.
- Check the pH. An unbuffered 20% solution reads roughly pH 2 to 2.5. Confirm it is below 3.5. Do not add anything to raise it.
- Bottle immediately, cap tightly, label with the date, and refrigerate.
How to use it
- Apply 4 to 5 drops to clean, dry skin in the morning. Dry skin matters because water on the surface dilutes the serum.
- Wait 2 minutes, then moisturizer and a broad-spectrum sunscreen.
- Keep it away from the eye area and freshly shaved or exfoliated skin.
- Patch test on the side of the neck for three days before the first full-face use.
- Use retinol at night, not in the same routine. Our retinol pairing guide explains why.
Troubleshooting the Mix
| Problem | Likely cause | Fix |
|---|---|---|
| Powder will not fully dissolve | Coarse crystals, cold water, or not enough stirring time | Use a fine powder, start with room-temperature water, and keep stirring for up to 10 minutes. Do not heat. |
| Serum feels gritty on skin | Undissolved grains | Let the bottle sit for 10 minutes and shake again before use. If grains remain, remake the batch. |
| Strong stinging every time | Applying to damp or freshly exfoliated skin | Apply to fully dry skin. Skip mornings after an acid or retinol night. |
| Sticky finish | Glycerin weighed heavy, or too many drops | Recheck the scale, and use 4 to 5 drops for the whole face. |
| Turns yellow within 2 to 3 days | Light or air exposure, or warm storage | Use dark glass, keep the cap tight, store in the refrigerator, and make the 10 g batch. |
| pH strip reads above 3.5 | Something other than water, powder and glycerin was added, or the strip is expired | Remake with only the three listed ingredients and fresh strips or a calibrated meter. |
Your Morning Routine With the Serum
| Step | Product | Wait |
|---|---|---|
| 1 | Gentle cleanser | Pat completely dry |
| 2 | 20% vitamin C serum, 4 to 5 drops | 2 minutes |
| 3 | Peptide or hyaluronic serum, optional | 1 minute |
| 4 | Moisturizer | 2 to 3 minutes |
| 5 | Broad-spectrum sunscreen | Let it set before makeup |
Vitamin C goes first because it needs direct contact with dry skin at its low pH. Everything else layers on top once it has absorbed.
Signs Your Vitamin C Serum Has Oxidized
L-ascorbic acid in water reacts with oxygen and light. As it oxidizes, the serum changes color in a predictable way, which makes it easy to judge freshness by eye.
| What you see | What it means | What to do |
|---|---|---|
| Clear and colorless | Fresh | Use as normal |
| Very pale straw tint | Early oxidation has started | Use within a few days and mix a new batch |
| Clear yellow | Significant oxidation | Discard |
| Deep yellow, orange or brown | Heavily oxidized | Discard. Do not apply. |
| Crystals forming in the bottle | Water has evaporated or the batch got too cold | Discard and make a smaller batch |
| Cloudiness, film or odd smell | Possible contamination | Discard immediately |
A simple habit that helps: compare your bottle against a white sheet of paper in daylight each morning. Color shifts are easy to miss under bathroom lighting. For a full guide to slowing oxidation, see our page on keeping DIY vitamin C from oxidizing.
Why This Recipe Has No Vitamin E or Ferulic Acid
The best-known stabilized vitamin C combination comes from Lin and colleagues (Journal of Investigative Dermatology, 2005). They found that adding ferulic acid to a solution of 15% L-ascorbic acid and 1% alpha-tocopherol improved the chemical stability of the vitamins and roughly doubled photoprotection, from about 4-fold to about 8-fold, measured by redness and sunburn cell formation. That is a professionally formulated system. Vitamin E is oil-soluble and will not dissolve in a simple water and glycerin serum, so adding it to this recipe would leave it floating on top. If you want the vitamin C, vitamin E and ferulic acid combination without mixing, our ready-made Vitamin C Serum with Hyaluronic Acid contains vitamins C and E, ferulic acid and hyaluronic acid.
Storage Rules
- Refrigerate between uses.
- Keep the cap tight and the bottle upright.
- Use dark glass only. Clear bottles let light speed oxidation.
- Mix a fresh 10 g batch every week, even if the color still looks clear.
- Store the dry powder tightly closed, cool and dark. Dry powder lasts far longer than the mixed serum.
Frequently Asked Questions
Why 20% and not higher?
In Pinnell 2001, skin levels peaked at 20% and were lower at higher concentrations. Above 20%, you add stinging potential without measured benefit.
My serum tingles. Is that normal?
A brief tingle at pH 2 to 2.5 is common. Apply to fully dry skin and use it every other morning for two weeks. If you feel burning or see lasting redness, stop and rinse.
Can I add this powder to my moisturizer instead?
Not as a 20% serum. Most moisturizers sit at pH 5 to 6, well above the 3.5 ceiling that Pinnell 2001 found necessary for L-ascorbic acid to enter the skin, and the powder can destabilize the cream.
How long does a batch last?
Plan on one week in the refrigerator. Discard it sooner if it turns clearly yellow.
Can I use it at night?
Yes, on nights you do not use retinol or acids. Morning use under sunscreen is my first choice, because that is when environmental exposure is highest.
Sources
- Pinnell SR, Yang H, Omar M, et al. Topical L-ascorbic acid: percutaneous absorption studies. Dermatologic Surgery. 2001;27(2):137-142. Pig skin model; pH below 3.5 required; maximum skin levels at 20%. Publication summary
- 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):826-832. Duke Scholars record
These statements have not been evaluated by the Food and Drug Administration. These products are not intended to diagnose, treat, cure, or prevent any disease.
About the Author
Written by Kari Thomas, cosmetic formulator and founder of Skin Perfection.