pH and pOH Notes

Characterizing Aqueous Solutions

• The ion product of water indicates that hydronium ($H^+$ or $\text{H}_3 O^+$) and hydroxide ($OH^−$) ion concentrations are inversely proportional.

  • As $[H^+]$ increases, $[OH^−]$ decreases, and vice versa.

• Neutral Solution: Equal concentrations of hydronium and hydroxide ions.

• Acidic Solution: Higher concentration of hydronium ions than hydroxide ions.

• Basic Solution: Lower concentration of hydronium ions than hydroxide ions.

pH and pOH Scales

• The p-function: $pX = -\log(X)$; used to compress large numerical spans into a more manageable range.

• pH: $pH = -\log[H_3O^+]$ (negative log of hydronium ion concentration).

• pOH: $pOH = -\log[OH^−]$ (negative log of hydroxide ion concentration).

• Hydronium and Hydroxide Ion Concentrations:

  • $[H_3O^+] = 10^{-pH}$

  • $[OH^−] = 10^{-pOH}$

Relationship Between pH and pOH

• Using the ion product of water ($K_w$) and log rules:

  • $pK_w = pH + pOH$

• At 25 degrees Celsius:

  • $K_w = 1 \times 10^{-14}$

  • $pK_w = 14$

  • Therefore, $pH + pOH = 14$

Neutral Solutions and Temperature

• Neutral solutions have equal concentrations of hydronium and hydroxide ions, but the specific pH value depends on temperature.

• At 25 degrees Celsius, a neutral solution has a pH of 7.

• At 80 degrees Celsius, the pH of neutral water is approximately 6.31.

Measuring pH

• Chemical Indicators: Substances that change color depending on pH.

• pH Paper: Paper strips impregnated with chemical indicators for quick pH estimation.

• Electronic pH Meters: Devices that measure the concentration of hydronium ions using a glass membrane and conductivity changes.

Summary of Acidity, Neutrality, and Basicity at 25 Degrees Celsius

• Acidic Solutions: Excess of hydronium ions; pH less than 7.

• Neutral Solutions: Equal concentrations of hydronium and hydroxide ions; pH equals 7.

• Basic Solutions: Excess of hydroxide ions; pH greater than 7.