Laboratory Notes for BIO 1003

© 30 August 1999, John H. Wahlert & Mary Jean Holland


pH

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The pH scale is used as a measure of acidity in the environment. In this lab you will learn to measure pH with a pH meter, determine the pH of some solutions, and investigate the properties of buffers.

The symbol pH is derived from the French for "hydrogen power." The numbers in the pH scale stand for negative logarithms (10-n) of the concentration of hydronium ions (H3O+), here verbally simplified to hydrogen ions (H+) expressed in moles per liter of water. To obtain a mole of any substance, you simply measure out its molecular weight in grams. One gram of hydrogen atoms is a mole; 44 grams of CO2 is a mole; the atomic weight of C = 12, that of O = 16, and 12 + 2(16) = 44. A mole of any substance contains the same number of particles—Avogadro’s number, 6.02 x 1023. The liter is a particular volume of water.

Thus a pH of 0 means a hydrogen ion concentration of 100 molar or moles per liter, which is one mole.

pHConcentration of H+ions in moles per liter
0100=1.0molar
110-1=0.1molar
510-5=0.00001molar
710-7=0.0000001molar
1010-10=0.0000000001molar

Since normal dissociation of water molecules into H+ and OH- yields 10-7 moles of H+, pure water has a pH of 7, the neutral point in the scale. Since the pH scale is logarithmic, a difference in 1 unit is a 10-fold increase or decrease in concentration of hydrogen ions.

I. Measuring pH

Your instructor will demonstrate the use of the pH meter and of litmus paper. pH meters have been calibrated—DO NOT CHANGE ANY OF THE CONTROL SETTINGS.

Pour about 15 ml (enough to cover the pH meter electrode) of each solution into a small beaker.

  • Lift electrode arm out fo the standard pH 7 buffer (yellow solution).
  • Rinse electrode thoroughly using blue capped distilled water wash bottle.
  • Dry electrode completely from top to bottom and in groove using Kimwipes.
  • Lower electrode into test solution.
  • Read pH from meter.
  • Raise electrode and repeat rinsing procedure.
  • Go on to next test solution.

Put a drop of each solution on pink and on blue litmus paper in a watch glass and record the color change. What is the effect of an acid and of a base on each color?

Solution

pH

litmus test

Is the solution acidic or basic?

    pink paper blue paper  
tap water        
lemon juice        
cola soft drink        
detergent/soap*        
household ammonia        
         
         

* clean the tip of the electrode thoroughly; detergent and soapy water both leave a film.

What would be the resulting pH if you mix equal amounts of an acid and a base that are the same numerical distance from neutrality (e.g., pH 5 + pH 9)?

When finished with the lab exercise, clean up:

  • Rinse electrode of pH meter thoroughly, dry cmpletely and place back into standard pH 7 buffer (yellow solution).
  • Dispose of all test solutions in the sink (they are household products). Do not discard the yellow standard buffer solution!
  • Remove labels from all glassware.
  • Rinse glassware thoroughly and return it to its original place.
  • Wipe up any spills in your area.
  • Leave the lab as it was when you began.


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Last updated 18 August 2019 (JHW)