What Does pH Mean in Chemistry?

pH is a scale of hydrogen concentration.
••• glass image by Mikhail Olykainen from Fotolia.com

The pH scale is a method for representing how acidic or basic a substance is. The scale seems counterintuitive at first glance, yet it is widely used in biology, chemistry, geology and other physical sciences. Communication in these fields requires an understanding of the concept of pH. Once understood, the pH scale is a useful index for communicating a critical physical property of substances.


pH, which is always written with a lowercase "p," is a scale of how much ionic hydrogen is in a substance. The scale runs from 0 to 14. Completely pure water has a pH of 7, which is neutral. The lower the number on the scale, the more acidic the substance is. The higher the number on the scale, the more basic, or alkaline, the substance is. Substances that are extremely acidic or basic tend to be corrosive, or burn-causing. The scale is logarithmic, meaning that it is based on tens. Thus, a substance with a pH of 4 is 10 times more acidic than a substance with a pH of 5.

Calculating pH

Mathematically, pH means the negative log of the concentration of hydrogen ions in a substance. This takes the concentration of hydrogen ions in a substance and converts it to a value on the pH scale, where it can be better understood. The concentration of hydrogen is found in the form of molarity, or moles per liter. Then, the negative log of the concentration is taken. So pH in chemistry is a streamlined way of representing how many hydrogen ions are in a given substance.

An Example

For example, if a sample is 1 L and has 0.02 g of hydrogen in it, you can calculate its pH. You must first determine its molarity. Because a mole of hydrogen is roughly 1 g, 0.02 grams per liter is the same as 0.02 moles per liter, giving a molarity of 0.02. In scientific notation, this would be 2 x 10^-2. So, the negative log of this number would be the exponent with a negative sign in front of it (--2 = 2). Thus, the pH of this sample would be 2.

Finding pH through pOH

You can also use an indirect way to measure pH. If you know the pOH, you can calculate the pH. The pOH is the negative log of the hydroxide, or OH- group, ion. Because of the way hydrogen and hydroxide interact, the pOH is essentially the reverse of the pH. So, if you know the hydroxide concentration, you can calculate the pH with the equation 14 - pOH = pH. For example, if a substance has a pOH of 12, it would have a pH of 2. You can apply the same principal in reverse to find pOH from pH, using the formula 14 - pH = pOH.

Related Articles

What Is Alkaline pH?
How to Calculate the pH of Ammonia Water Using KB
How to Calculate ph And pOH
How to Calculate Hydrogen Ion Concentration
How to Calculate the Percentage of Ionization Given...
The Properties of Acidic Substances
How Can You Tell If a Substance Is Acidic?
What Is the Meaning of a Ph Scale?
How to Calculate the pH of a Strong Acid
General Characteristics of Acids & Bases
What Type of Molecule Prevents Extensive Changes in...
What Is the pH of Distilled Water?
How to Find pH for a Given Molarity
The Effects of Temperature on the pH of Water
What Is Alkaline pH?
How Does PH Paper Work?
How to Find Hydroxide Ion Concentration
Three Variables That Can Affect Activities of an Enzyme
What Are Buffer Solutions Used In?

Dont Go!

We Have More Great Sciencing Articles!