4.1 Introduction to pH

1. Introduction to pH

The concept of pH was first introduced in 1909 by the Danish biochemist S.P.L. Sorenson. He published a paper in Biochem Z discussing the effect of $H^{+}$ ions on enzyme activity.

Etymology of pH:

• The letter 'p' is derived from the German word "potenz", meaning power or exponent of, specifically referring to base 10.
• Sorenson originally defined pH as $-\log [H^{+}]$. A proposed, though not universally accepted, Latin meaning for pH is "pondus hydrogenii".

Evolution of the Definition:

• Initially, pH was defined in terms of the concentration of $H^{+}$ ions.
• In 1924, Sorenson refined his definition, recognizing that pH is a function of the "activity" ($a\{H^{+}\}$) of the $H^{+}$ ion, not just its concentration. Activity represents the effective concentration of an ion.

Activity vs. Concentration:

• Concentration: Abbreviated using square brackets, e.g., $[{\mathrm{H}}_3{\mathrm{O}}^{+}]$ for the concentration of hydronium ion.
• Activity: Abbreviated using 'a' with curly brackets, e.g., $a\{H^{+}\}$ for the activity of $H^{+}$ ions. The activity of an ion depends on various factors, including its concentration.

2. The pH Scale and Formula

The pH scale provides a convenient way to express the concentration of hydrogen ions (or hydronium ions, ${\mathrm{H}}_3{\mathrm{O}}^{+}$) in aqueous solutions. It is defined as the logarithm to base 10 of the reciprocal of the numerical value of the hydrogen ion concentration or, more precisely, its activity.

Mathematical Definition of pH: $\mathrm{pH}=-\log a\{H^{+}\}$

For practical purposes in many introductory contexts, especially for dilute solutions, activity is often approximated by concentration: $\mathrm{pH}=-\log [H^{+}]$

Or, equivalently, using the hydronium ion concentration: $\mathrm{pH}=-\log [{\mathrm{H}}_3{\mathrm{O}}^{+}]$

This can also be expressed as: $\mathrm{pH}=\log \frac{1}{[{\mathrm{H}}_3{\mathrm{O}}^{+}]}$

3. The pOH Scale

Similar to pH, the concentration of hydroxide ions (${\mathrm{OH}}^{-}$) in a solution can be expressed using the pOH scale.

Mathematical Definition of pOH: $\mathrm{pOH}=-\log [{\mathrm{OH}}^{-}]$

4. Relationship Between pH and pOH

In any aqueous solution, there is an equilibrium between hydrogen ions (or hydronium ions) and hydroxide ions due to the autoionization of water. This equilibrium is governed by the ion-product constant for water, $K_w$.

The relationship between pH and pOH is given by: ${\mathrm{pK}}_w=\mathrm{pH}+\mathrm{pOH}$

Where ${\mathrm{pK}}_w=-\log K_w$.

At $25^{\circ }\mathrm{C}$: The value of $K_w$ is approximately $1.0\times 10^{-14}$. Therefore, ${\mathrm{pK}}_w=-\log (1.0\times 10^{-14})=14$. $\mathrm{pH}+\mathrm{pOH}=14$

This equation is crucial for converting between pH and pOH values at standard temperature.

Worked Examples

No specific numerical examples were provided in the source text. However, a typical example would involve calculating pH from a given $H^{+}$ concentration or calculating pOH from $O{H}^{-}$ concentration and then finding the pH.

Possible Questions/Answers

Q: Who introduced the pH scale, and what does the 'p' in pH signify?

A: The pH scale was introduced by S.P.L. Sorenson in 1909. The 'p' signifies "potenz" (German for power or exponent of 10).

Q: What is the primary difference between the activity of an ion and its concentration in the context of pH?

A: Concentration refers to the number of moles of solute per unit volume. Activity is the effective concentration of an ion, which accounts for non-ideal behavior in solutions (due to interionic attractions) and is the more accurate measure for defining pH. Activity is denoted by $a\{H^{+}\}$, while concentration by $[H^{+}]$.

Q: If the pOH of a solution at $25^{\circ }\mathrm{C}$ is 3.5, what is its pH?

A: At $25^{\circ }\mathrm{C}$, $\mathrm{pH}+\mathrm{pOH}=14$. Therefore, $\mathrm{pH}=14-\mathrm{pOH}=14-3.5=10.5$.

Q: Write the mathematical formula for pH in terms of hydronium ion concentration.

A: $\mathrm{pH}=-\log [{\mathrm{H}}_3{\mathrm{O}}^{+}]$