pH of Skin
What is pH?
When an acid is put into water, it dissociates and reacts with water to form hydronium. Hydronium, or H3O+, is the ion that gives acids their properties – sour taste and corrosiveness, for instance.
A higher concentration of hydronium makes a substance more acidic while a lower concentration of hydronium makes a substance more alkaline (basic). As such, the properties of a substance can be predicted given the hydronium concentration. Concentration is usually measured in moles per liter, also known as molarity and abbreviated M.
However, these concentrations are usually in the form of unwieldy numbers such as .000002 or 1×10-3.21. Hardly practical for everyday use. Instead, the negative base-ten logarithm of the concentration is used. With a hydronium concentration of Z, the pH is equal to -log10(Z). This operation results in a more practical number, usually between zero and fourteen. The inverse, hydronium concentration given pH is simple as well. With a pH of Z, the concentration is equal to 1.0×10-Z.
The concentration of hydronium in pure water at standard temperature – 25º C or 77º F – is equal to 1.0×10-7 M. Pure water has a neutral pH of 7. pH values less than seven are acidic while pH values greater than seven are alkaline. When water is heated, it becomes slightly more acidic. When it is cooled. It becomes slightly more basic.
Always keep in mind that the pH scale is logarithmic, not linear. This means that an acid with a pH of 3 is ten times stronger than one with a pH of 4 and one hundred times stronger than an acid with a pH of 5.
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What is the natural pH of skin?
Oddly enough, healthy skin is not neutral but in fact has a pH of 5.5, which makes it slightly acidic. This is due to a thin layer on the surface of the skin made of sebum, sweat, broken down fatty acids, and dead skin cells known as the acid mantle.
The acidity of the mantle allows it to fight off dangerous microbes and free radicals that could cause harm if untreated. Like all other organs in the body, the skin does its job best within a very narrow range of conditions. The pH range for healthy skin is 4.4 to 5.6. Anything lower or higher can cause damage.
Extremely acidic skin results in acne, reddening, inflammation, and dryness. Having overly alkaline skin results in tightness, dryness, inflammation, scaling, signs of aging due to dropping collagen levels, and increased permeability. This in turn can result in athlete’s foot and yeast infections. pH maintenance of skin is a delicate balancing act. Various factors can affect skin pH besides cosmetics.
These include air pollution, humidity, excessive face washing, detergents, sweat, sun exposure, and tap water. Bar soaps are another common culprit. While ok for use on hands, they are usually made with lye, a strong base, and have a pH of 9, which is far too high for healthy facial skin.
What are the pH of skin care products ?
Cosmetics range between 2.5 and 7.5 on the pH scale. Don’t worry about disrupting the effects of one by layering it with another. It’s far easier to disrupt the balance of your skin than it is to disrupt other cosmetics. Sunscreens are the most alkaline products with a pH range of 5.5 to 7.5.
Next come moisturizers and toners with a range of 5.5 to 6.5.
After that are the cleansers with a range of 4.5 to 7.0.
Serums have pH values between 4.0 and 6.0.
Retinol-based products have a range of 3.7 to 5.0.
Exfoliants made with AHA or BHA range from 3.2 to 3.9, though most dermatologists agree that any reading between 3.0 and 4.0 is effective.
Most acidic by far are products made with citric acid (otherwise known as vitamin C). They have a pH of 2.6 to 3.2. Use them only in moderation.
How is pH of skin care producs measured?
There are several ways to determine pH. The most scientific and accurate involves calculating the exact concentration of hydronium ions based on the presence of various acids and bases.
First, one determines the concentration of each acid and then calculates how much of it is converted into hydronium based on its relative strength.
Next, these values are added up to get the total concentration. Finally, the negative logarithm is taken to give a cleaner value.
However, this requires an extensive knowledge of chemistry and isn’t practical for skincare products due to the number of ingredients in most formulations. The next most accurate is a digital pH meter.
They can approximate pH values out to the thousands by measuring the conductivity of a solution. These work by immersing three nodes in a solution. One is a voltmeter, one is a pH responsive electrode, and one is a control electrode.
Together, they act as a battery and measure how many ions are in the solution based on its conductivity. The cheapest and simplest method is to use litmus strips. These are small strips of paper coated with a chemical indicator. There are many different indicators for different pH ranges.
The most common is a universal indicator that is accurate from 0 to 14. It is green when neutral, blue or violet when alkaline, and red, orange, or yellow when acidic. Two other common indicators are red and blue litmus.
Red litmus is red when neutral or acidic and blue when basic. Blue litmus is blue when neutral or basic and red when acidic. While cheap, litmus strips rely on one’s ability to differentiate shades of color and are only approximations.
Obviously, the first two aren’t practical for measuring skin pH, but can still be used to measure the pH of cosmetic products. Some products even advertise their pH on their labels! Most beauty stores sell paper strips for at-home testing while some dermatologists offer liquid pH testing.
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