Science Junction

Water What-ifs
pH Mini-workshop

Introduction The determination of the pH of a substance is the measurement of the H+ ions found in that particular substance. pH is determined and recorded as a number between 0 and 14. Pure deionized water has a pH of 7 which is neutral. This means that the level of H+ and OH- ions in pure water are equal.

If the level of H+ ions increases, the substance is considered an acid and the pH number is below 7. If the level of OH- ions increases, the substance is considered to be alkaline or basic and the pH number is above 7. 

An acid has a range of 0 to any numerical value below 7. For example, 6.9 would be a weak acid. A base has a range of any numerical value above 7 to 14 with 7 being a neutral value. A one-unit change in the pH, from 4 to 3, is a ten-fold change in how acidic the substance has become. This means that a pH of 3 is ten times more acidic than a pH of 4. 

In the United States the pH of most natural water systems range from 6.5-8.5, but wide variations can occur due to increases in the atmosphere of nitrogen oxides and sulfur dioxides from automobile and coal-fired power plant emissions. These oxides are converted to nitric and sulfuric acids in the atmosphere and fall to earth as acid rain or snow. This acidic precipitation can adversely affect the pH of aquatic ecosystems. 

Most organisms are adapted to a specific pH level. When pH increases or decreases, the diversity of an ecosystem can be changed appreciably. 

Goals and Objectives

pH Objectives

After completing the tutorial you should be able to:
  • explain what a pH number is a measure of and how it is determined and recorded.
  • interpret pH levels and determine their meaning with the use of the LaMotte pH test kit.

Learn the skills

pH Test Kit (LaMotte)

pH should be measured immediately after collection because a change in temperature can affect the pH levels.
  1. Rinse the test tube with sample water. 
  2. Fill the test tube (0230) to the 5 ml mark with sample water. 
  3. Holding the dropper bottle vertically, add ten drops of indicator solution to the test tube with the sample water. 
  4. Cap and invert three times. 
  5. Place test tube into Octet Comparator. Match sample color to a color standard. Record pH number off of the Octet Comparator.
NOTE: If more than one pH reading is taken, report the most common value, the mode, rather than the average. 

Normal values for most freshwater systems are 6.5-8.5.

CBL pH Probe (TI-83/Vernier)

  1. Press "PRGM." Arrow down until "CHEMBIO" is highlighted. (Check to see if "CHEMBIO" is loaded on your calculator. If not, use the TI-GraphLink to load the program from another calculator or a computer). Press "ENTER" and "ENTER" again to go to the title screen. Press "ENTER" to go to the main menu.
  2. On the main menu screen select "1:SET UP PROBES" and enter the number of probes being used.
  3. After you enter the number of probes being used, the "SELECT PROBE" menu will appear.
  4. Select "PH" under the "SELECT PROBE" screen and press "ENTER."
  5. After selecting "PH", a calibration menu will appear.
  6. Select "USE STORED" if the buffers needed for calibration are unavailable. (Note: It is always better to calibrate the pH probes before using.) If buffers are available, (buffers of pH 4 and 10 should be used) select "PERFORM NEW CALIBRATION." Follow the directions on the screen (push the trigger button on the CBL unit and enter the known value of the pH buffer). Repeat this step for the second buffer.
  7. The "MAIN MENU" will appear. Select "2:COLLECT DATA" to set up your experiment.
  8. Monitor input by placing the probe in the sample to be tested. The pH data will be displayed, but not recorded. The values must be recorded manually in a data table or chart.
  9. Rinse the pH probe thoroughly with distilled or deionized water before testing each sample and before storage.

For review, click here

Check yourself

Is a pH of 3 considered acid, base, or neutral?

Is a pH of 7 considered acid, base, or neutral?

In the United States, the pH readings for natural water systems usually fall in what range?


Workshop Wrap-up

Once you have completed the mini-workshops, test your knowledge with the Water Quality Post-test. Once your completed post-test is submitted, you will receive a password which will give you and your students complete access to all areas of the Water What-ifs web site. Thanks for taking part in this research project on water quality.
| Water What-ifs Home | Teacher Tutorial | temperature mini-workshop |

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Last Modified: 8/15/01

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