Magnesium Ascorbyl Phosphate: A Stable Vitamin C Derivative

Ascorbic acid, plain vitamin C, is a genuinely effective antioxidant and brightening ingredient, and it's also a formulator's headache. It oxidizes quickly, turns a serum brown within weeks if the packaging and pH aren't perfect, and needs a low pH that can irritate sensitive skin. Magnesium ascorbyl phosphate exists largely to solve that problem, and it's worth understanding as its own distinct ingredient rather than just "the stable version of vitamin C" without knowing why that stability exists.

What makes it different from ascorbic acid

Magnesium ascorbyl phosphate is a derivative where a phosphate group is attached to the ascorbic acid molecule, with magnesium as the counter-ion. That phosphate group is the whole point. It protects the reactive part of the vitamin C molecule from oxidizing on contact with air and light, which is exactly what makes plain ascorbic acid so hard to keep stable in a finished product.

Once it's on skin, the theory is that naturally occurring enzymes (phosphatases) in skin cleave that phosphate group off, releasing active ascorbic acid at the site. Whether that conversion happens efficiently and at what rate is a more nuanced question than marketing copy usually lets on, and it's an area where formulators reasonably differ in how they message the ingredient's benefits. What's not in dispute is the stability advantage during storage and shelf life, which is the practical reason most makers reach for it.

It's also water soluble and works at a more skin-friendly, closer-to-neutral pH than ascorbic acid needs, which is another reason it shows up in gentler, more sensitive-skin-oriented formulas.

INCI and CAS identification

Property	Detail
INCI name	Magnesium Ascorbyl Phosphate
CAS number	113170-55-1
Typical function	Antioxidant, skin conditioning
Typical use level	Often in the range of 1% to 10% depending on the formula and desired effect, with many finished products landing toward the lower to middle part of that range
Solubility	Water soluble
pH	Performs at a more moderate pH than ascorbic acid, generally friendlier for sensitive-skin formulations

Compare this to plain ascorbic acid, INCI "Ascorbic Acid," CAS number 50-81-7, which needs a low pH environment and much more careful packaging (airless pumps, opaque containers, sometimes nitrogen-flushed) to hold up over a product's shelf life. If your ingredient deck says "Vitamin C" in supplier shorthand, always confirm which specific derivative or form you're actually using before you list it, since ascorbic acid, magnesium ascorbyl phosphate, sodium ascorbyl phosphate, and ascorbyl glucoside are all distinct INCI entries with distinct CAS numbers, not interchangeable synonyms for a filing.

Why this distinction matters for your notification

Health Canada's Cosmetic Notification Form wants the specific INCI name of what you actually used, with a concentration or concentration range. Listing "Vitamin C" as a catch-all isn't accurate enough, and it risks a mismatch if a reviewer or later audit checks your formula against your supplier documentation. If your certificate of analysis says Magnesium Ascorbyl Phosphate, CAS 113170-55-1, that's what belongs on your filing, not a generic vitamin C placeholder.

This also matters for marketing accuracy. If your product copy says "stabilized vitamin C" or "gentle vitamin C derivative," that's consistent with magnesium ascorbyl phosphate. If your copy implies you're using pure ascorbic acid at a meaningful percentage for maximum potency, and you're actually using the phosphate derivative, that's a mismatch between your claims and your formula worth catching before launch, not after a customer asks a pointed question.

A quick gut check before filing

Pull your supplier's certificate of analysis or technical data sheet and confirm the exact INCI name and CAS number match what you're listing. If you're buying a "vitamin C complex" or blend product, get the actual breakdown so you know whether magnesium ascorbyl phosphate is the primary vitamin C source or one of several forms combined, since that changes both your labeling accuracy and your percentage calculation for the notification.

This is precisely the kind of mapping problem Cosmetic Comply's ingredient matching step is built to catch, taking a supplier's trade name or shorthand and confirming the correct INCI and CAS pairing before a real reviewer signs off and the notification gets filed.