Tutorials

# What Is pH?

It is a measure that indicates whether a liquid solution is acidic or alkaline. In our everyday routine, we encounter and drink liquids with different pH, such as water, milk or soda. The acidity or alkalinity of these substances is an important property, since it affects how they interact with one another and with our bodies. Read on to learn how acidity is measured and why it is important!

## What Does pH Stand For?

The symbol is made up of two letters: a lowercase “p” which stands for “power” and an uppercase “H” for the element hydrogen. Together, it is an abbreviation for “power of hydrogen.” This terminology is used because the scale is logarithmic – it has to do with powers of ten.

Strong Acids and Bases

Properties of Acids and Bases

Acid Base Chemistry

Buffers

Acid Base Theories: Arrhenius & Brønsted-Lowry Acids

## Formula

pH = −log [H+]

The formula for pH is shown above. It is a logarithmic measure of the concentration of hydrogen ions in a solution, which means that increasing the pH by 1 corresponds to multiplying the concentration of H+ ions by 10! So even though the difference between 6 and 7 might sound small, it’s actually quite sizeable. For these calculations, the concentration of hydrogen ions is always a molar concentration, that is, moles of H+ per liter.

## Equation

In some situations, we know the pH and need to convert to the molar concentration of H+ ions. For this, we need to invert the logarithm from the first equation, by raising 10 to the power of the negative pH.

[H+] = 10-pH

Using this equation, we find that if the pH of a solution is 7, then [H+] = 10-7 M.

• It is the most abundant element in the universe.
• It is the lightest element in the periodic table.

## What Is the pH Scale?

As the name implies, this is a scale, and it starts from the number 0 and ends at the number 14. These numbers allow the classification of substances; the most acidic substances will be close to 0, while the most basic or alkaline substances will be close to 14. The lower the pH, the more H+ ions and the stronger the acid would be, and the most basic or alkaline substances will have a classification between 7 and 14 and in this case the more OH- ions the stronger the base.

## Water and its pH

Pure water has a pH of 7 on the pH scale, meaning that it is neutral. However, dissolved acids, bases, or salts can make it either acidic or basic.

## Kw in Chemistry

Kw is the dissociation constant or ionization constant of water.

When water ionizes, it splits into a hydrogen ion (H+) and a hydroxide ion (OH). H+ is unstable in water on its own and prefers to form a hydronium ion (H3O+), but for convenience we usually still refer to it as H+.

The ionization constant of water represents the degree to which it exists as ions versus together as a molecules and equals the concentration of H+ times the concentration of OH:

Kw = [H+][OH]

pKw = -log Kw

At room temperature, Kw = 10-14 and pKw = 14. We can use this constant to convert between the pH and pOH via the equation below:

pH + pOH = pKw

## Why is pH important?

Some chemical reactions only take place under certain pH conditions. Sometimes this is because H+ or OH acts as a reactant in the reaction. In other cases, acid or base can catalyze a reaction, meaning that they affect the rate of the reaction.

Living organisms rely on a wide variety of biochemical reactions and processes, most of which require specific pH ranges. As a result, ecosystems like lakes and rivers thrive under the pH conditions that are favorable to the biochemistry of the local flora and fauna.

Like an ecosystem, the human body has a certain pH that allows the proper functioning of the different tasks that our body performs. We require one value in our blood, and a totally different one (much more acidic) in our digestive fluids. Otherwise, normal biochemistry could break down, causing serious health issues. Luckily, humans and many other creatures have blood that is buffered, so that the pH cannot change easily. This is why if you drink a bottle of alkaline water or acidic soda, your blood will stay nearly the same, keeping you safe from the effects of imbalance!

Another consequence of this safety net built into your blood is that the common health benefits associated with alkaline water are mostly made-up. Even if your blood were somehow too acidic, drinking some water with pH 8 would barely change it!

## pOH

The term pOH is similar to pH, but refers to alkalinity or basicity, that is, the concentration of hydroxide ion (OH-) in a solution. The two scales function identically, except that the scale is reversed. A neutral substance has both pH and pOH of 7. The equation for pOH is the same as that shown earlier except using the concentration of hydroxide:

pOH = −log [OH]