TCA - Is Percentage % or pH More Important?

First, what's the difference between % and pH

Percentage simply tells us how much of the entire product is actually active vs. inert (inactive). A product that is 50% TCA has been watered down (diluted) with about half of the contents being inert and half being the active acid.

pH, on the other hand, is a method of measuring the acidity or alkalinity of substances. When the skin's proteins react with an acid of low pH, the acidic environment destroys the existing tissues, and the body's repair system works to replace the destroyed tissue with new structures. The lower the pH, the stronger the visible peeling action. 

Example:
When I dilute my 50% TCA chemical peel with distilled water, I am lowering the percentage, not the pH since water is neutral. If I mix a separate container of my 50% TCA with ammonium hydroxide (ammonia), lye, calcium carbonate, or another highly alkaline base, it will increase the pH and drastically reduce its acidity or ability to cause visible peeling, although the percentage of TCA may be the same as the one I mixed with water. These two solutions will cause completely different reactions in my skin!

Should I buy a high or low Percentage?
I personally like to buy higher percentage products because I feel like it's a better bargain because I can dilute or buffer the product to meet different goals without having to buy several different products.

So, both percentage and pH are important, but it depends on my goals:

When buying a chemical peel, pH is very important, but so is percentage (%), but really, it depends on my goals. One rule I use above all: if a product does not state the pH level, and no one can tell me what it is, I wouldn’t buy it since I won’t know if it’s going to meet my needs.

Which pH do I buy?

If I want a medium or deep peel, which causes obvious peeling of the skin and induces collagen production more quickly, I shop for chemical peel agents with a pH level between .05 - 1.0.

If I want a lighter peel to deal with hyperpigmentation, acne, spots, or moisture retention, which will not necessarily cause detectible peeling, I shop for a higher pH level product, something I can use more often, with a pH level above 1.5 or 2.

Higher pH level chemical peels are effective at inducing collagen synthesis--it just takes longer. If my skin is sensitive, I may also want a higher pH product, which is effective, but it may not irritate my skin as much or cause apparent tissue damage.

Here's a quote from a clinical study that compared the efficacy of acids with different pH levels: "When using a 70% glycolic acid solution, the lower pH products (below pH 2) create more necrosis than the partially neutralized products with a pH above 2. At this time there is no evidence that creating necrosis leads to a more favorable result of the peel. Therefore, the use of partially neutralized glycolic acid solutions seems prudent, since they have a better safety profile than low pH solutions, which contain only free glycolic acid."

Remember, peeling the outer layers of skin is not the real purpose of a medium or deep peel, although it does kick-start the body's healing process faster.

How can I measure my Chemical Peel if pH is not Listed?

The pH scale measures the acid or alkaline level of chemical peels (and any other substance that is measurable), and it ranges from 0 to 14. As a reference, here are the pH levels of some common substances:

Testing pH Levels

To test my solutions, I use pH strips that measure 0.0 - 14.0 because they are more accurate (show pH level in increments of .02, which is good for testing products) than the pH strips that measure the body's pH level, which usually show pH levels between 4.0 - 8.0.

Buffering

Anytime the natural pH level of an ingredient or product is altered, it’s called buffering. Buffering can either increase or decrease a pH level. Buffered and non-buffered products can carry the same percentage of an active ingredient, but the pH level is not in its original range.

When I dilute my 50% TCA with distilled water, I am lowering the percentage, not the pH since water is neutral. If I mix my 50% TCA with ammonium hydroxide (ammonia), lye, calcium carbonate, or another highly alkaline base, it will increase the pH and drastically reduce its acidity or ability to cause visible peeling, although the percentage of TCA may be the same as the one I mixed with water.

Let's take a different approach at understanding pH; for example, this 30% glycolic acid has a pH level of 2.1; it has been chemically altered to give it a higher pH level so that the inexperienced at-home user has a safer peel; interestingly, their 50% glycolic acid also has a pH of 2.1, which means that the percentage of acid to other ingredients is higher than in the 30% product—but not the pH. What do you think the outcome of using the first product would be compared to the second?

A leave-on product, such as this 15% Glycolic and Hyaluronic acid serum has a pH level of 3.4, which  allows me to keep this on my skin without feeling that constant stinging, while still benefiting from the acid, although it will take more applications over time to achieve the same results, but there's no down time!