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Introduction to pH scale

Introduction: What is pH?

Before we can perform a titration, we need a way to measure the "strength" or concentration of the acid or base we are working with. We do this using the pH scale.

The Definition

The term pH stands for "potential of Hydrogen." It measures the concentration of hydrogen ions (written as H+) in a solution.

pH = -log10 [H+]

Because these concentrations are usually very tiny (like 0.0000001), the log scale turns them into simple, manageable numbers typically ranging from 0 to 14.

  • pH 2 means the decimal is 2 places over ([H+] = 0.01).

  • pH 7 means the decimal is 7 places over ([H+] = 0.0000001).

The pH Scale

0-6: Acidic
(High H+)
7: Neutral
(Pure Water)
8-14: Basic
(Low H+)

Connection to Titrations

In a titration, we watch for a color change. This change is triggered by an indicator—a chemical that stays one color at a certain pH and flips to another color when the pH crosses a specific threshold. Understanding the pH scale allows us to choose the right indicator for our reaction.

Calculating pH and Concentration

Since the pH scale is logarithmic, we can easily switch between the concentration of hydrogen ions and the pH value using these two methods:

1. Finding pH from [H+]

If the concentration of hydrogen ions is 0.0001 M (which is 10-4):

pH = -log(0.0001) = 4.0

Rule of thumb: For simple powers of 10, the pH is just the positive version of the exponent!

2. Finding [H+] from pH

If we know the pH of a solution is 9.0, we can find the concentration by using the inverse log (10-pH):

[H+] = 10-9 M

This is equal to 0.000000001 M. As you can see, the higher the pH, the smaller the amount of acid.

The Relationship: H+ and OH-

In every water-based solution, there is a balance between hydrogen ions (acidic) and hydroxide ions (basic). This relationship is constant at room temperature. If one goes up, the other must go down.

The product of these two concentrations always equals a very specific number called the Ion Product of Water (Kw):

[H+] × [OH-] = 10-14

This is why the pH scale traditionally ends at 14. If you know the concentration of the acid, you can always calculate the concentration of the base using this formula.

Why this matters for your Titration

When you add a base (OH-) to an acid (H+), they react to form water. This "neutralization" changes the concentrations, which shifts the pH. Your indicator will change color the moment the H+ and OH- concentrations reach a specific balance!