11.1 pH, Acids, and Alkalinity
Key Takeaways
- The pH scale runs from 0 to 14; 7 is neutral (distilled water). Values below 7 are acidic; values above 7 are alkaline, and each whole-number step is a tenfold change in hydrogen-ion activity.
- Healthy skin is slightly acidic. The acid mantle sits near pH 4.5–5.5 and is the barrier film of sebum, sweat, lipids, and flora you are trying not to strip.
- Alkaline products swell and soften keratin; acids contract and chemically exfoliate surface cells. Traditional true soaps are often near pH 9–10; syndet cleansers can be buffered closer to skin pH.
- Illinois scope is a depth rule, not a published peel-pH cutoff: superficial AHA, BHA, and enzyme peels that stay on the stratum corneum can be esthetics; medium-depth TCA and deep phenol peels intended for living layers are not (IDFPR Statement on Prohibited Practices, 04/06/2026).
- Neutralize or rinse only as the manufacturer directs; over-timing a superficial acid is how a stratum-corneum service becomes a living-layer injury.
pH (potential of hydrogen) is how Illinois estheticians predict whether a product will respect the barrier or fight it. The National Interstate Council of State Boards of Cosmetology (NIC) tests this under Scientific Concepts I.H (ingredients, labeling, function, pH). Illinois tests the same science as product chemistry and chemical service procedures under 68 Ill. Adm. Code 1175.710, and writes chemistry — understanding chemicals and their use into the 150-hour Scientific Concepts block of 1175.835. The legal overlay is not a secret pH number. A product or technique intended to affect living layers of the skin is not an esthetics service, even if you can recite its pH from the bottle.
The pH scale: 0 to 14, 7 is neutral
pH describes how acidic or alkaline a water-based solution is.
- 0–6.9 is acidic. Acids donate hydrogen ions (H+).
- 7 is neutral. Pure distilled water is the classic reference point.
- 7.1–14 is alkaline (also called basic). Alkalis accept hydrogen ions or contribute hydroxide (OH−).
The scale is logarithmic. A product at pH 5 has ten times the hydrogen-ion activity of a product at pH 6, and one hundred times that of pH 7. “Just one number lower” on a peel label is not a small change. It is an order-of-magnitude change in how aggressively that acid can work.
Anhydrous oils do not have a meaningful pH until they mix with water or sit on wet skin. That is why the manufacturer’s indication for use — not a guess from how oily the product feels — controls application (1175.120). pH literacy starts with “is this an aqueous system?” and then with “where does it sit on 0–14?”
Acids contract and exfoliate; alkalis swell and soften
NIC chemistry is tissue prediction, not trivia.
Acids cause contraction. On the stratum corneum they loosen the bonds between dead corneocytes, so superficial acids exfoliate. The surface can feel tighter. Superficial alpha hydroxy acids (AHAs), beta hydroxy acids (BHAs), and enzyme products work this way when they stay on dead surface cells.
Alkaline products cause swelling and softening. High-pH soap, leftover alkali from saponification, and many chemical depilatories (strongly alkaline thioglycolate systems) swell keratin so it loosens. On facial skin, leftover alkalinity from true soap can leave tightness, redness, and a disrupted barrier. If a stem asks what an alkaline cleanser does to the skin, the exam answer is swell and soften, not “hydrate.” If it asks what a superficial acid does, the answer is contract and exfoliate surface cells.
The acid mantle: skin is slightly acidic
Healthy skin is slightly acidic. The acid mantle is the thin film of sebum, sweat, lipids, and resident flora on the surface. The teaching range you must memorize is about 4.5–5.5. That acidity:
- Supports barrier lipids and corneocyte cohesion
- Discourages many pathogenic organisms that prefer a more alkaline environment
- Is the “home pH” a ten-minute cleanse should not destroy
A product far more alkaline than the mantle, used daily, can raise surface pH long enough to increase transepidermal water loss (TEWL) and leave the client “squeaky.” Squeaky is stripped, not clean. After an alkaline wash, the mantle recovers, but not instantly. That recovery window is why older routines used an acidic toner after soap, and why modern syndet (synthetic detergent) cleansers are buffered closer to skin pH.
Skin is not “neutral 7.” Distilled water is. If an item offers 7 as the pH of healthy facial skin, it is trapping candidates who mixed up the midpoint of the scale with the biology of the face.
Soaps versus syndets
True soap is made by saponification: fats or oils plus a strong alkali such as sodium hydroxide or potassium hydroxide. Residual alkali commonly leaves bar soap around pH 9–10. That alkalinity is an effective cleanser and an acid-mantle disruptor. It swells and softens, which is useful on some soils and harsh on a compromised face.
Syndets are synthetic detergent systems. They can be buffered near pH 5–7, closer to the mantle. Many facial cleansers labeled pH-balanced are syndets, not lye soap. Choosing a syndet is chemistry in the service of the barrier, not a brand preference.
| Product class | Typical teaching pH | What it does on skin | Illinois exam hook |
|---|---|---|---|
| Distilled water | 7 | Neutral reference | Midpoint of the 0–14 scale |
| Acid mantle / intact skin | ~4.5–5.5 | Slightly acidic barrier | Skin is not “neutral 7” |
| Traditional true soap | often ~9–10 | Alkaline; swells and softens | High-pH cleanse can disrupt the mantle |
| Syndet facial cleanser | often closer to 5–7 | Cleanses with less pH shock | Match cleanser to barrier, not to squeaky-clean |
| Superficial AHA, BHA, or enzyme | acidic; stratum corneum target | Contract or digest surface cells | In-scope only as superficial |
| Medium-depth TCA or deep phenol | intended for living layers | Medical-depth injury | Out of Illinois esthetics (IDFPR 04/06/2026) |
Soap, water, and mantle numbers are chemistry teaching values. They are not an Illinois statute. Part 1175 does not publish a numeric peel-pH cutoff that turns a bottle legal or illegal. Scope is a depth question: stratum corneum versus living layers.
Superficial versus living-layer peels — the Illinois line
225 ILCS 410/3A-1 limits esthetics to cosmetic work on the surface of the skin. IDFPR’s Statement on Prohibited Practices (dated 04/06/2026) lists chemical peels except superficial peels among procedures that are the practice of medicine unless they are physician-delegated and not held out as esthetics.
Map the national peel vocabulary onto that line:
- Superficial peels — AHA (glycolic, lactic), BHA (salicylic), and enzymes (papain, bromelain) used to exfoliate the stratum corneum. These stay in Illinois esthetics only as superficial / stratum corneum services, used exactly as the manufacturer indicates (1175.120).
- Medium-depth peels — typically trichloroacetic acid (TCA) used at strengths and techniques intended to reach living epidermis (and often papillary dermis). That is not esthetics in Illinois.
- Deep peels — phenol (including classic Baker-Gordon-type formulas) intended for deeper living tissue, historically the reticular dermis. That is not esthetics in Illinois.
Do not answer an Illinois item with an invented rule such as “peels below pH 3 are illegal” or “anything above a stated glycolic percentage is automatically medical.” Those cutoffs are not in Part 1175. If the product or technique is intended to affect living layers, it is out. If it is a labeled superficial exfoliant for the stratum corneum, and you follow the manufacturer, it can be in-scope.
Physician delegation does not let you advertise a medium peel as an “esthetician TCA facial.” The 04/06/2026 statement is explicit: even delegated medical procedures are not held out as esthetics.
Neutralization and why timing is chemistry
Neutralization means bringing leftover acid toward a safer, more skin-compatible pH so the reaction stops. Some systems use a dedicated alkaline buffer. Some AHA protocols are timed, then water-rinsed. Many BHA (salicylic) and enzyme protocols are rinsed, not chemically neutralized. Follow the SDS and the manufacturer. Inventing a baking-soda splash because a classmate did it is both a burn risk and unprofessional conduct under 1175.120 (use contrary to manufacturer indication).
Two chemistry rules travel with every acid:
- Time is dose. A superficial peel left on past the labeled window is no longer the product you consented. Over-timing is how a stratum-corneum service becomes a living-layer injury.
- Mixing order is acid into water, never water into concentrated acid. Water into acid can boil and splash because dilution of strong acid is exothermic. That procedure is Section 11.3; the chemistry reason belongs here.
After any in-scope chemical exfoliation, the barrier is thinner. Broad-spectrum SPF and sun avoidance are not optional aftercare; they are how you keep a superficial peel from becoming a pigment injury. A legally and chemically correct sequence is: syndet or labeled cleanse, confirm the peel is superficial, protect eyes and mucosa, time the product, neutralize or rinse as labeled, restore with hydrators and SPF, and refer the client who wants a living-layer TCA. pH literacy protects the acid mantle. Depth literacy protects the Illinois license. The exam will test both in the same stem.
On the pH scale used in NIC product chemistry, which statement is correct?
What is the teaching pH range of the skin’s acid mantle, and is intact skin acidic or alkaline?
Compared with a syndet cleanser buffered near skin pH, what do traditional true soaps typically do to keratin on the skin?
An Illinois candidate is asked whether a “pH 2.8 TCA” is automatically legal if the number is written on the bottle. What is the correct Illinois rule?