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Water Chemistry III: pH

The pH of a liquid is a measure of the number of H+ ions present. These ions are present in water, as a fraction of the molecules naturally break up into H+ and OH- ions. Pure water has a pH of about 6-7 (slightly acidic to neutral).

Measuring pH

The pH of a solution is defined as -log[H+], or the negative log of the H+ concentration in moles per liter. (For example, a solution with a pH of 2 would have a H+ concentration of 0.01 molar.) The lower the pH, the more H+ ions are present. The pH scale ranges from 0 (strongly acidic) to 14 (strongly basic).

pH Chart

After dipping pH paper in various liquids, we matched the colors to this chart to determine pH.

This photo is labeled with the solutions tested and their approximate pH values.

Buffers

Buffers are chemicals that minimize changes to the pH of a solution when acids or bases are added. (Think of them as pH insulators -- they slow any change in pH). Buffers can take up extra hydrogen ions, or release them when the number of H+ ions in the solution drops.

We investigated the buffering properties of four solutions in lab: water, sodium chloride, milk, and sodium phosphate. The solutions with the least pH change were the best buffers: milk and sodium phosphate.

Solution Initial pH pH after adding acid
Milk
7
6
Sodium chloride
7
3
Sodium phosphate
7
7
Water
7
4
How does it work?
Milk proteins have charged areas which bind H+. Phosphate ions have a -3 charge and can bind up to three protons.
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