PH (Meaning and Explanation)

We explain what pH is and what instruments are used to measure it. The pH scale and examples of acidic, neutral and base compounds.

The pH is nothing more than the indicator of the hydrogen potential.

What is pH?

pH is a measurement that It is used to establish the level of acidity or alkalinity of a solution. The “p” is for “potential”, that is why the pH is called: hydrogen potential.

It is expressed as the negative base 10 logarithm of the hydrogen ion concentration. The following equation represents this definition:

Equation 1: Equations to calculate pH and POH.

Besides, pOH is a measure of the concentration of hydroxyl ions in a solution. It is expressed as the negative base 10 logarithm of the concentration of hydroxyl ions and, unlike pH, is used to measure the level of alkalinity of a solution.

An additional fact is that in aqueous solution at 25 ºC, the sum of pH and pOH is equal to 14.

What relationship exists between acidity level and pH?

Acidic solutions have a high amount of hydrogen ions. This means that they have low pH values (see equation 1) and, therefore, their acidity level is high. Thus, a solution will be more acidic or less acidic depending on the amount of hydrogen ions it has.

Besides, basic (alkaline) solutions have low amounts of hydrogen ions. This means that they have high pH values (see equation 1) and, therefore, their acidity level is low.

The pH measurement scale

The pH scale It is used to measure the degree of acidity of a solution and, since pH is related to pOH (see equation 1), then knowing the degree of acidity of a solution, we can also know its degree of basicity.

Thus, the pH scale goes from the value 0 to 14. For example, substances with a pH value=0 are the most acidic (least basic), those with pH=7 are neutral, and those with pH= 14, are the least acidic (most basic).

Examples of acidic, basic and neutral compounds

Examples of acidic compounds

Battery acids. They have pH values between 0 and 1. Their acid level is so strong that it is harmful to the species.
Acid rain. It is a phenomenon that occurs due to the accumulation of acids from fossils and fuels. These rains can take pH values between 2 and 5 on the pH scale. When the pH approaches 2 it can cause the death of fish, plants and other species. When the pH approaches 5 it produces less damage, but it still affects aquatic and terrestrial life.
Lemon juice It has pH values between 2 and 3.
Coffee It has a pH value of 5, or very close values.

Examples of neutral compounds

Blood
Milk

Examples of basic compounds

Milk of magnesia. In the pH table it is between values 10 and 11. This product is for consumption
Bleach or chlorine. It has pH values around 13. It is used for cleaning the home, bathrooms, kitchen and has the power to discolor clothing.

How is pH measured?

Litmus paper — Tornasol paper allows us to measure pH.

The way to distinguish between an acidic compound and a basic one is by measuring its pH value. Currently there are numerous methods to measure the pH of a substance.

Using acid-base indicators Indicators are compounds that change color when the pH of the solution in which they are found changes. For example, phenolphthalein is a liquid that turns pink if added to a base and becomes colorless if added to an acid. Another example is litmus paper: if a fragment is immersed in an acidic solution it turns red-orange, and if it is immersed in a basic solution it darkens, taking on a blue color. There are also some types of litmus paper with more specific color scales that indicate more accurate pH values.
Using a potentiometer or pH-meter It is an electronic device that directly gives us the pH value of a solution. Measuring pH using this equipment is more accurate than using litmus paper.

What are Buffer solutions or Buffers?

Buffer solutions or Buffers are solutions that have the property of keeping the pH of a solution constant even when certain amounts of acid or base are added.