L3.3: pH Scale

• pH Scale

  ● pH stands for "power of hydrogen" and it measures how acidic or basic (alkaline) a solution is, based on the concentration of hydrogen ions.
• Reading | pH

  • Acidic solution | < 7.00
  • Neutral solution | = 7.00
  • Basic solution | > 7.00
• ● The higher the pH, the lower the H3O+ and the less acidic the solution.

  ↑ pH =↓ H3O+ =↓ acidity

  ● The lower the pH, the higher the H3O+ and the more acidic the solution.

  ↓ pH = ↑ H3O+ = ↑ acidity
• Relationship between Ka & pKa

  ● A lower value of pKa = higher value of Ka

  ★ Recall: Ka is the measure of an acid’s strength; a high value = strong acid.
• Relationship of pH, pOH & pKw

  Since Kw is a constant, the values of pH, pOH, [H3O+], [OH-] are interrelated:

  ● If [H3O+] increases, [OH-] decreases (and vice versa).

  ● If pH increases, pOH decreases (and vice versa).
• pH Paper

  ○ A strip of paper soaked in pH-sensitive dye that changes color based on acidity or basicity of the solution.

  ○ Cheap, quick, and easy-to-use, however is not precise since sometimes color can be hard to interpret in colored or dark solutions.
• pH Meter

  ○ A digital device with an electrode (probe) that measures voltage related to [H+] in solution and then converts the voltage into an exact pH value.