The pH protocol is the fifth in a series of eight hydrology protocols:

-

Junior High and Up Estimated Time: 5 Minutes for actual measurement, 10 – 15 minutes in class and 5 minutes in the field for calibration.

-

The pH or acidity of water is a key factor in determining what type of plant or animal life can live there. When we measure the pH of a solution, we are measuring its acidity in terms of its concentration of hydrogen ions. The pH scale ranges from 1 to 14, with each successive number representing a logarithmic increase. This means that a pH of 2 has ten times more acidity than a pH of 1. There are several ways to measure the pH of a water sample; through the use of pH indictor paper, a pH pen or a pH meter. GLOBE suggests that beginning level students use pH indicator paper, intermediate level students use a pH pen, and advanced level students use a pH meter. Method 1: pH indicator paper Students must fill a beaker about halfway with sample water and then proceed to dip one strip of indicator paper into the water for at least a minute. Students must then remove the paper from the water and compare the resultant four color segments with the chart on the back of the pH indicator paper box. Students should try to find a sequence where all four segments on the sample paper match all four segments on the box. Please note: pH paper readings may not be accurate if your water sample has if your water sample has a very low level charge. Method 2: pH pen or pH Meter In order to measure the pH of your water sample using the pH pen or the pH meter students must perform the following four steps: 1) Prepare buffer solutions Measure 50 mL of a pre-mixed buffer solution into a graduated cylinder. Pour the buffer solution into a labeled bottle and cap tightly. 2) Calibrate the pH pen or pH Meter In order to calibrate the pH pen or the pH meter and make sure that it will accurately measure the pH of water in the field students must immerse the pen in a buffer solution whose pH is at 7. By doing this, students are instructing the instruments as to how a pH 7 solution reads. Students must first rinse the electrode (glass probe) of the pen or the meter and the area around it with distilled water using a squeeze bottle and then blot it dry with a soft tissue, but be careful not to touch it with fingers. Next, students working with a pen must turn the pen on with the switch on top and students working with a meter must press the ON/OFF button. Students must then immerse the pen or meter in the pH 7 buffer solution, gently stirring the buffer while waiting for the reading to stabilize. Students working with a pen can use a jewelry screwdriver to turn the small screw in the hole in the back of the pen until the reading is exactly 7.0. Students working with a meter must press the HOLD/CON button of the meter and repeat this action with a pH 4 buffer and a pH 10 buffer. After calibrating the pen or meter, students can remove them from the buffer solutions and rinse the electrodes with distilled water. 3) Recheck pen or meter in the Field When traveling to the field site, students must take the pH buffer solutions along and test them along with the water sample so as to have a comparison by a known value. Students must record all values on their data sheets. 4) Measure the pH of the water sample Students must first rinse the electrode with distilled water and blot the area dry before immersing either the pen or meter in the water sample. Once the display value stabilizes students can record the data. Students should find the pH value for two water samples whose values should remain within 0.2. After taking the samples, students can rinse the glass probes with distilled water and blot them dry with a tissue.

-

For Method 1: pH indicator paper 50 or 100 mL beakers For Method 2: pH pen One jewelry screwdriver Three 50 or 100 mL beakers 50 mL polyethylene bottle pH buffer solution for pH 7 Or pH meter Five 50 or 100 mL beakers Three 50 mL polyethylene bottles Three pH buffer solutions for pH 4, 7, and 10 For both: 100 mL graduated cylinder Paper towels Soft tissues Distilled water in a squeeze bottle Stirring rod or spoon Masking tape Permanent marker Latex gloves and safety goggles

-

Students should wear the appropriate safety equipment listed and follow carefully all safety instructions specified by your test kit, including any accompanying Material Safety Data Sheets.

-

1) A student tests the pH of the water sample using pH indicator paper. 2) A student compares his pH indicator paper with the color chart. 3) The student calibrates the pH pen using the buffer solution. 4) Before calibration and testing, the student cleans the glass probe with distilled water. 5)Students work together to calibrate their pens and meters.

About | Field Sites | Protocols | Field Sites | Data | Standards
Forums | Resources | Calendar | FAQs | Home