6.2 - pH Value

pH Scale and Measurement

• The pH scale, from 0 to 14, indicates the acidity or alkalinity of an aqueous solution.
  • pH < 7: acidic; a lower value means greater acidity.
  • pH = 7: neutral.
  • pH > 7: alkaline; a higher value means greater alkalinity.
• pH may be determined with a universal indicator, pH paper or a pH meter.
• pH is a logarithmic measure of hydrogen-ion concentration:

$\mathrm{pH}=-\log [{\mathrm{H}}^{+}]$

where $[{\mathrm{H}}^{+}]$ is measured in $\mathrm{mol}\mathrm{d}{\mathrm{m}}^{-3}$.

Calculating pH and pOH

$\mathrm{pOH}=-\log [\mathrm{O}{\mathrm{H}}^{-}]$

$\mathrm{pH}+\mathrm{pOH}=14$

• For an acid, calculate pH directly from $[{\mathrm{H}}^{+}]$.
• For an alkali, calculate pOH from $[\mathrm{O}{\mathrm{H}}^{-}]$, then use $\mathrm{pH}=14-\mathrm{pOH}$.
• Reverse calculations use $[{\mathrm{H}}^{+}]=10^{-\mathrm{pH}}$ or $[\mathrm{O}{\mathrm{H}}^{-}]=10^{-\mathrm{pOH}}$.
• Example: $[{\mathrm{H}}^{+}]=0.01\mathrm{mol}\mathrm{d}{\mathrm{m}}^{-3}$ gives pH 2; $[\mathrm{O}{\mathrm{H}}^{-}]=0.01\mathrm{mol}\mathrm{d}{\mathrm{m}}^{-3}$ gives pOH 2 and pH 12.
• The number of decimal places in pH follows the number of significant figures in the stated ion concentration.

Ion Concentration and pH

Experiment 6.3: Measure the pH of $0.1$, $0.01$ and $0.001\mathrm{mol}\mathrm{d}{\mathrm{m}}^{-3}$ hydrochloric acid with a pH meter.

• Aim: study the relationship between hydrogen-ion concentration and acid pH.
• Apparatus: three $100\mathrm{c}{\mathrm{m}}^3$ beakers and a pH meter.
• Procedure:
  1. Pour $20\mathrm{c}{\mathrm{m}}^3$ of each hydrochloric-acid concentration into a separate beaker.
  2. Measure each solution’s pH with the pH meter.
  3. Compare acid concentration, hydrogen-ion concentration and pH.
• Hypothesis: The higher $[{\mathrm{H}}^{+}]$, the lower the pH.
• Variables: concentration of ${\mathrm{H}}^{+}$ (manipulated), pH (responding), type of acid (fixed).
• Dilution decreases $[{\mathrm{H}}^{+}]$, raises pH and lowers acidity.

Experiment 6.4: Repeat with $0.1$, $0.01$ and $0.001\mathrm{mol}\mathrm{d}{\mathrm{m}}^{-3}$ sodium hydroxide.

• Aim and apparatus: study hydroxide-ion concentration and alkali pH using $100\mathrm{c}{\mathrm{m}}^3$ beakers and a pH meter.
• A higher $[\mathrm{O}{\mathrm{H}}^{-}]$ gives a higher pH and greater alkalinity.
• Dilution decreases $[\mathrm{O}{\mathrm{H}}^{-}]$, lowers pH and lowers alkalinity.

pH in Daily Life

• Everyday aqueous substances may be compared using universal indicator or a natural indicator such as purple-cabbage extract.
• Examples investigated include soap water, milk tea, carbonated drink, lime juice, coffee and tap water.