pH - means "potential hydrogen"

pH is a measure of the [ H₃O⁺ ] in solution.

• The brackets, [   ], represent concentration in moles per cubic decimeter (dm³), or Molarity of the ion in solution.
   
• Water ionizes according to the equation:
  2H₂O_(l) H₃O⁺_(aq) + OH⁻_(aq)  
• The ion product constant of water:
  K_w = [H₃O⁺] [OH⁻] = 1 X 10⁻¹⁴  
• This provides the basis for the pH scale.

Calculating pH:

• The equation is: pH = − log [H₃O⁺]
   
• [H₃O⁺] is expressed in powers of 10 from 10⁻¹⁴ to 10 ⁰
   
• If [H₃O⁺] = 1 X 10⁻⁷, the negative log of [H₃O⁺] = 7.
   
• A pH of 7 indicates a neutral solution.
   
• pH calculations always give numbers between 0 and 14.

Sample pH Calculations

1. What is the pH of a solution with a [H₃O⁺] of 1.00 X 10⁻⁴ M?

Problem Solution:

• pH = −log [H₃O⁺]
• pH = −log (1 X 10⁻⁴)
• Do the following on your calculator:
  1. Enter 1 X 10⁻⁴
  2. Press " LOG "
     (Some calculators reverse steps 1 and 2)
  3. Press " ⁺/- " or "(−)"
• pH = 4

Tip: when the interger of the scientific notation is 1, the pH is the exponent of 10 as a positive number.

2. Find the pH of a solution whose [H₃O⁺] is 9.5 X 10⁻⁸

Problem Solution:

• pH = −log [H₃O⁺]
• pH = −log (9.5 X 10⁻⁸)
• pH = − (log 9.5 + log 10⁻⁸)
• pH = − (.98 + (−8))
• pH = − (−7.02)
• pH = 7.02

3. Calculate the [H₃O⁺] of a solution with a pH of 3.70.

Problem Solution:

• pH = −log [H₃O⁺]
• −pH = log [H₃O⁺]
• −3.70 = log [H₃O⁺]
• antilog −3.70 = [H₃O⁺]
• Do the following on your calculator:
  • Enter −3.70
  • antilog is usually "INV", "shift", or "2^nd" + "log" on a calculator. Now is the time to find out which of these it is on your calculator.
• [H₃O⁺] = 2 X 10⁻⁴ M

pH Calculations

 
pOH, "potential hydroxide", = −log [OH⁻]

• Hydronium ion concentration is expressed in powers of 10 from 10⁻¹⁴ to 10⁻⁰
• [H₃O⁺] [OH⁻] = 1 X 10⁻¹⁴
• pH + pOH = 14

1. What is the pH of a solution with a pOH of 1.36?

Problem Solution:

• pH + pOH = 14
• pH = 14 − pOH
• pH = 14 &minus 1.36
• pH = 12.64

2. What is the pOH of a solution with a [OH⁻] of 2.97 X 10⁻¹⁰

Problem Solution:

• pOH = −log [OH⁻]
• pOH = −log (2.97 X 10⁻¹⁰)
• pOH = − (log 2.97 + log 10⁻¹⁰)
• pOH = − (.47 + (−10))
• pOH = − (−9.53)
• pOH = 9.53

• pH meter:
   
  • An electronic device that measures pH directly.
     
  • pH meters are used in most professional lab settings today.
   
• Indicators:
   
  • Weak organic acids and bases whose colors differ from the colors of their conjugate acids or bases.
     
  • The color is best viewed from above against a white background.
     
  • Chart of Indicator Color Change

Buffer Systems:

• Buffers provide ions that react with H₃O⁺ or OH⁻, if they are introduced into the solution. As either ion is neutralized, the pH of the system remains nearly constant.
   
• Buffer solutions are prepared using a weak acid or base with one of its salts. The weak acid or base reacts with one of the added ions, the salt reacts with the other ion. The combination keeps the pH constant - up to a point.
   
• By choosing the correct weak acid or base, a buffer solution can be prepared that will maintain almost any pH.

Measuring pH

Serial Dilution