What Does pH Stand For And Mean?

Lemons have citric acid, which has a pH of 2.2. Photo: rawpixel via Pixabay, CC0

pH stands for potential hydrogen with the “p” meaning potential and the “H” standing for hydrogen. The pH scale is a scale that is used to rank the relative basicity or acidity of substances to other substances, based around the amount of hydrogen ion activity in a substance.


The scale is logarithmic in nature, meaning that every whole pH value represents a change of 10 times the previous value. The pH scale is based around pH 7, which is neutral and represents substances which are neither an acid nor a base.

The Basis For The pH Scale

Even if you’re not intimately familiar with the concepts of acids and bases, you certainly have some exposure to them. Basic substances are things like baking soda, while acidic substances are things like orange juice and soda. Substances are classified as a base or an acid based upon the concentration of hydrogen ions that the substance has. The H in pH represents the level of hydrogen ion activity in a given solution. So it’s possible to define acidic and basic solutions this way:

An acidic solution is a solution with levels of hydrogen ions greater than the amount found in pure water (which is at the neutral seven). A basic solution, in contrast, has a hydrogen ion concentration lower than that of water.

Photo: rawpixel via Pixabay, CC0

Since pure water functions as the neutral point on the pH scale, let’s look at the properties of water and place it in the context of hydrogen ions. The ionization of water refers to how some small percentage of the water molecules in a given amount of water will spontaneously generate hydrogen ions as they dissociate, or split into smaller particles. This is sometimes referred to as autoionization. The process of autoionization produces equal numbers of hydroxide (OH-) ions and hydrogen (H+) ions. Note that the hydrogen ions will usually bond directly with a nearby molecule of water to form a hydronium (H3O+). This means that there aren’t actually a bunch of hydrogen ions floating around in the water. However, scientists still use the ionization of water and the hydrogen ions they produce as a way to create a scale that allows the comparison of hydrogen on levels in other solutions. The hydroxide ions typically just float around within the solution, unlike the hydrogen ions.

In terms of the number of hydrogen ions produced through autoionization, the amount is equal to 1 x 10^-7 M (assuming it’s pure water). The notation refers to moles per liter of water. The number of ionized water molecules are an incredibly small percentage of the total number of water molecules found in any amount of pure water.

Acid Solutions And Basic Solutions

Now that we have established a reference point for the basicity and acidity of solutions, we can discuss how acids and bases are defined. Acidic solutions are those solutions that have a greater H+ concentration than pure water, while basic solutions are those that have an H+ concentration lower than that of pure water. To put that another way:

Base = H+ concentration < 1 x 10^-7 M

Acid = H+ concentration > 1 x 10^-7 M

The concentration of hydrogen ions within a solution is usually given in pH terms, which is calculated as the inverse log of the hydrogen ion concentration for that given solution.

pH = -log10[H+]

So putting the hydrogen ion concentration of water into this formula would get you a value of 7.0, or the neutral pH on the pH scale. As you might be able to guess, substances made out of mainly water such as the cytosol found in cells or the blood in the human body, have pH values very near the neutral seven. Acids and bases can be added to a water-based solution, shifting the concentration of that solution away from the neutral pH point. Bases usually raise the pH level through the introduction of hydroxide into the environment, which collects the hydrogen ions and pulls them out of the solution. Meanwhile, acidic substances are those that increase hydrogen ion concentration by dissociating and introducing one of its hydrogen atoms into the solution.

The more powerful the acid the quicker it will degrade and release H+. An example of a strong acid is HCl, which when combined with water dissociates quickly and completely into ions of chloride and hydrogen. By contrast, weak acids like vinegar don’t dissociate completely. A strong base substance releases hydroxide ions capable of absorbing H+ when in water, completely degrading much like a strong acid substance will. An example of a strong base is sodium hydroxide.

Photo: OpenStax college, via Wikimedia Commons, CC 3.0

Strength And Examples


As previously mentioned, the pH scale helps rank various chemical compositions in terms of their basicity (also called alkalinity) and their acidity. Is logarithmic nature means that for every increase of one pH unit that occurs, there is a tenfold increase in concentrations of H+. While there are some substances which can fall outside the 0 to 14 pH range, most solutions fall within this range, and therefore the pH scale is usually represented as running from 0 to 14, with basic substances being any value above seven and acidic substances any value below neutral 7.

Substances which lie at the far ends of the pH scale, either extremely basic or extremely acidic substances can be quite harmful to organic material, doing substantial damage to cells. Both highly basic and highly acidic substances can be corrosive in nature, damaging people’s tissue. The pH of the solution, as well as the amount of solution one is exposed to and how long they are exposed to it are all factors that determine how much damage the substance will do. You may have noticed that gastric acid, your stomach acid, is a highly acidic substance. How does your body protect itself from this acid? There are certain stomach cells created specifically to act as a barrier between the gastric acid and the other cells in the body, which are constantly being created and sacrificed.

Some examples of common solutions and the pH values they possess are:

Milk – slightly acidic with a pH value of about 6.5.

Rain – slightly acidic with a pH value of around 5.5.

Coffee – mildly acidic with a pH value of about 4.8.

Tomato Juice – mildly acidic with a pH of about3.0

Battery acid – highly acidic with a pH value of around 1.0.

Blood – slightly basic with a ph of around 7.3

Seawater – slightly basic with a pH of about 8.0.

Baking soda – slightly basic with a pH of about 8.0.

Milk of magnesia – moderately basic with a pH of about 10.6

Lye – Highly basic with a pH of about 13.5

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Daniel Nelson, MS. What Does pH Stand For And Mean?, Science Trends, 2018. Available at:
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