Answer:
The lower the hydrogen ion concentration, the higher the pH.
Step-by-step explanation:
This effect comes from the definition of pH:
pH = -log[H₃O⁺]
Consider two solutions.
Solution A has [H₃O⁺] = 10⁻¹ mol·L⁻¹; Solution B has [H₃O⁺] = 10⁻¹⁰ mol·L⁻¹.
Solution B has the lower [H₃O⁺].
Now, take the logarithm of each concentration.
Solution A: log[H₃O⁺] = log10⁻¹ = -1; Solution B: log[H₃O⁺] = log10⁻¹⁰ = -10
Solution B still has the lower value.
However, pH is the negative logarithm.
When we take negative logarithms, we get
Solution A: pH = 1; Solution B: pH = 10
Solution B has the lower [H₃O⁺], but the higher pH.
The lower the hydrogen ion concentration, the higher the pH.