pH, pH Scale, and pOH

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1. pH

Definition: pH is a measure of the acidity or alkalinity of a solution. It quantifies the concentration of hydrogen ions (H⁺) in a solution, which reflects how acidic or basic the solution is.

Formula: $\text{pH} = -\log[\text{H}^+]$ Where $[\text{H}^+]$ is the concentration of hydrogen ions in moles per liter (M).

Interpretation:

• Acidic Solutions: pH < 7
  • Higher concentration of hydrogen ions.
  • Example: Lemon juice (pH ~ 2) and vinegar (pH ~ 3).
• Neutral Solutions: pH = 7
  • Equal concentration of hydrogen ions and hydroxide ions.
  • Example: Pure water.
• Basic (Alkaline) Solutions: pH > 7
  • Lower concentration of hydrogen ions.
  • Example: Soap (pH ~ 10) and bleach (pH ~ 12).

2. pH Scale

Overview: The pH scale is a logarithmic scale ranging from 0 to 14, used to represent the acidity or alkalinity of a solution. Each unit change on the pH scale represents a tenfold change in hydrogen ion concentration.

Details:

• 0 to 6.9: Acidic range.
• 7: Neutral point.
• 7.1 to 14: Basic (alkaline) range.

Applications:

• Environmental Monitoring: Measuring soil pH for agricultural purposes.
• Industrial Processes: Adjusting pH in manufacturing and water treatment.
• Healthcare: Monitoring blood and urine pH for diagnostic purposes.

3. pOH

Definition: pOH is a measure of the concentration of hydroxide ions (OH⁻) in a solution. It complements the pH scale and is used to quantify how basic or alkaline a solution is.

Formula: $\text{pOH} = -\log[\text{OH}^-]$ 

Where $[\text{OH}^-]$ is the concentration of hydroxide ions in moles per liter (M).

Relationship with pH:

• The sum of pH and pOH is always 14 at 25°C. $\text{pH}+\text{pOH}=\mathrm{14 \; }$

Interpretation:

• Basic Solutions: pOH < 7
  • Higher concentration of hydroxide ions.
  • Example: Ammonia solution (pOH ~ 4).
• Neutral Solutions: pOH = 7
  • Equal concentrations of hydrogen and hydroxide ions.
  • Example: Pure water.
• Acidic Solutions: pOH > 7
  • Lower concentration of hydroxide ions.
  • Example: Hydrochloric acid (pOH ~ 1).

Applications:

• Water Chemistry: Assessing the alkalinity of natural waters.
• Industrial Processes: Managing pH and pOH in chemical manufacturing.

Summary

• pH measures the concentration of hydrogen ions, indicating how acidic or basic a solution is.
• pH Scale ranges from 0 to 14 and is logarithmic, representing the concentration of hydrogen ions.
• pOH measures the concentration of hydroxide ions and complements the pH scale.

External References

For more detailed information, you can explore the following resources:

• The pH Scale - Chemistry LibreTexts
• PubChem - pH and pOH