pH and the pH scale

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pH and the pH scale : 

pH and the pH scale by Ann Riedl, Ph.D., Biology Department, Front Range Community College

What is pH? : 

What is pH? pH is a measure of the activity of hydrogen ions in a solution {H+} It approximates the molar concentration of hydrogen ions in a solution [H+]

Slide 3: 

So, what are these hydrogen ions and why do we care so much about them? H+ H+ H+ H+ H+ H+

Slide 4: 

Hydrogen atoms consist of one positively charged proton with one negatively charged electron orbiting it H

Slide 5: 

Remove the electron, and you are left with just a proton, or a positively charged hydrogen ion H+

Slide 6: 

It turns out proteins, such as enzymes, are very sensitive to the effects of hydrogen ions Too many or too few hydrogen ions in a solution can make a protein denature, or unfold, and stop working properly H+ H+ H+ H+ H+ H+ H+ H+ H+ H+

Slide 7: 

If a solution has just the right concentration of hydrogen ions to make our proteins work well, it is called neutral If a solution has too many hydrogen ions, it is called acidic If a solution has too few hydrogen ions, it is called basic acidic neutral basic

Slide 8: 

A substance that releases hydrogen ions when it is put into a solution is called an acid Cl- H+ Cl- H+ Cl- H+ Cl- H+ Cl- H+

Slide 9: 

A substance that removes hydrogen ions when it is put into a solution is called an base H+ H+ H+ H+ H+ H+ H+ OH- Na+ OH- Na+ OH- Na+ OH- Na+ H2O H2O H2O H2O

Slide 10: 

The concentration of hydrogen ions [H+] is important to normal function of our cellular proteins Trouble is, the [H+] of most biological solutions is very low For example, pure water has a [H+] of .0000001 moles/liter, or, in scientific notation, 1 x 10-7 M No one, not even scientists, like to work with numbers this small

Slide 11: 

So, the pH scale was developed to make it easier to keep track of those pesky H+ pH is the negative logarithm of [H+] Sounds complicated… but it simply means that you state the hydrogen ion concentration in scientific notation, take the exponent, and change its sign Thus water, rather than being [H+] 1 x 10-7, is simply pH 7

Slide 12: 

Let’s practice… 1. Lemon juice has a [H+] of 1 x 10-3 M What is the pH of lemon juice? 2. Household bleach has a pH of 13 What is the [H+] of household bleach? 3. Stomach acid has a [H+] of .01 M What is the pH of stomach acid? pH = 3 1 x 10-13 M = .0000000000001 M .01 M = 1 x 10-2 M pH = 2

Slide 13: 

Because scientific notation is in base ten, a change in one of the pH represents a ten-fold change in [H+] Because pH is the negative logarithm, an increase in pH represents a decrease in [H+] .000001 = 1 x 10-6 pH 6 .0000001 = 1 x 10-7 pH 7 .00000001 = 1 x 10-8 pH 8

Acids, Bases, and pH : 

Acids, Bases, and pH In summary, we use the pH scale to describe the [H+] of a solution by taking the negative logarithm of the actual [H+] pH 7 is arbitrarily described as neutral Solutions with pH below 7 have a higher [H+] and are therefore acidic Solutions with pH above 7 have a lower [H+] and are therefore basic

The pH Scale : 

The pH Scale