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Ion-Product of Water

Explains the self-ionization of water and how it relates to pH.

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More Than Meets the Eye

More Than Meets the Eye

Credit: Redshirt
Source: https://commons.wikimedia.org/wiki/File:Poipu_Beach.jpg
License: CC BY-NC 3.0

The water looks cool and clear. The surface is marred only by a few small waves from the incoming tide. A nice, uncluttered scene. But underneath is a different matter. Sand swirls from the moving water. Colorful fish skitter here and there, searching for food or just enjoying life. Plant life is there, not so colorful as the fish, but always present. Is there coral? A living material, painful to touch and harmful to the plant. A lot going on under that placid surface.

Why It Matters

  • Water is an intriguing material. Without it, life cannot function. This molecule has its atoms attached by covalent bonds, yet dissociates to form H+ and OH- ions. The dissociation is temperature-dependent and has become the basis for establishing whether a given material is an acid or a base. The hydrogen ion content of water at specific conditions determines how we talk about hydrogen and hydroxide ion concentrations.
  • Our focus is usually on the hydrogen ion concentration. This is the value we use in determining acidity. The proton is also used as the foundation for definitions of acid in two of the three classifications of acids and bases. The pH is easy to manipulate and control, just by adding a certain amount of acid (often HCl since there are few side issues to deal with when working with this acid.)
  • Credit: Kristine Paulus
    Source: http://www.flickr.com/photos/kpaulus/7451969320/
    License: CC BY-NC 3.0

    pH can easily be tested using simple kits that will show you the pH of a sample based on the resulting color [Figure2]


  • The other side of the acid-base coin is not often considered separately. We have a pOH parameter defined and we can calculate values based on pOH. But where are the pOH meters? Where are the devices that monitor and adjust pOH in the same way we do pH? From a practical standpoint, it is just easier and more convenient to deal with pH. Could we build a pOH meter? There don’t seem to be any advertised on-line.
  • Watch a video about pH meters at the link below:


Show What You Know

Use the links below to learn more about acids and bases. Then answer the following questions.

  1. What is another term for H3O+?
  2. Which definitions of acids and bases rely on an understanding of proton chemistry?
  3. How can a Lewis acid involve protons?
  4. Can pOH be used in a non-aqueous situation?

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Image Attributions

  1. [1]^ Credit: Redshirt; Source: https://commons.wikimedia.org/wiki/File:Poipu_Beach.jpg; License: CC BY-NC 3.0
  2. [2]^ Credit: Kristine Paulus; Source: http://www.flickr.com/photos/kpaulus/7451969320/; License: CC BY-NC 3.0

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