046 – pH

This laboratory session is dedicated to teaching and practicing the identification of acid-base properties and the measurement of pH in various substances, encompassing both liquids and solids. The main objective is to acquaint students with the necessary laboratory techniques for determining pH levels and to enhance their understanding of the acid-base behavior of substances through a variety of tools and methodologies.

Educational Goals

  • Understanding pH concepts: Participants will delve into the concept of pH and its role in reflecting the acid-base character of a substance, aiming to deepen their understanding of chemical properties.
  • Utilization of pH indicators: Students will be introduced to using different pH indicators, such as litmus papers (red and blue), pH indicator paper, and universal indicators, for qualitatively determining the acid-base nature of solutions.
  • Precision with pH meters: The session will teach students the accurate use of digital pH meters for precise pH measurements, highlighting the importance of exactitude in chemical analysis.
  • Solution preparation skills: Participants will develop skills in manipulating and preparing solutions for pH testing, enhancing their practical chemistry capabilities.
  • Observation and measurement techniques: The laboratory will foster students’ practical understanding of how to observe and measure chemical properties in a controlled setting.

This laboratory session provides a comprehensive exploration of pH measurement techniques, essential for grasping the chemical properties of substances. By combining theoretical insights with hands-on activities, students will not only familiarize themselves with various methods of determining pH but also refine their laboratory skills. This experience highlights the significance of precise pH measurement in understanding the acid-base behavior of substances, offering valuable insights into the practical application of chemistry principles.

Protocol

PART 1: Determine the acid base character and the pH of a liquid substance

  1. Measure 20 mL of solution 1 using the graduated cylinder.
  2. Pour 20 mL of solution 1 into a 50 mL beaker.
  3. Dip a red litmus paper into it.
  4. Then dip a blue litmus paper.
  5. Finally dip a pH indicator paper.
  6. Compare the colors obtained with those available on the pH chart.
  7. Rinse the graduated cylinder with distilled water.
  8. Repeat steps 1 to 7 with substances 2 and 3.

PART 2: Determine the pH of a liquid substance using a universal indicator

  1. Using the dropper; take solution 1 from the 50 mL beaker.
  2. Then empty the dropper into well no. 1 of the well plate.
  3. Add one drop of universal pH indicator into the same well.
  4. Stir using the glass rod.
  5. Compare the color obtained with those available on the pH chart.
  6. Repeat steps 1 to 5 with substances 2 and 3.

PART 3: Determine precisely the pH of a liquid substance

  1. Immerse the pH meter electrode in the beaker containing solution 1.
  2. Read the measurements on the digital display.
  3. Rinse the electrode with distilled water.
  4. Dry the electrode with absorbent paper.
  5. Repeat steps 1 to 4 with substances 2 and 3.

PART 4: Determine the pH of a solid substance

  1. Weigh about 1.8 g of ammonium sulfate powder using the electronic balance.
  2. Measure 100 mL of distilled water using a graduated cylinder.
  3. Pour the weighed ammonium sulfate powder into a 250 mL beaker.
  4. Pour the contents of the graduated cylinder into the same beaker.
  5. Stir using the glass rod for 5 seconds.
  6. Dip a red litmus paper into the 250 mL beaker that contains the ammonium sulfate solution
  7. Dip a blue litmus paper into the 250 mL beaker that contains the ammonium sulfate solution
  8. Dip a pH indicator paper into the 250 mL beaker that contains the ammonium sulfate solution.
  9. Compare the colors obtained with those available on the pH chart.
  10. Immerse the pH meter electrode in the ammonium sulfate solution.
  11. Read the measurement on the digital display.
  12. Rinse the electrode with distilled water.
  13. Dry the electrode with absorbent paper.

Anticipated Outcomes

Steps 1, 2, and 3

  • Tested solution 1 is 5% v/v ethanoic acid. The pH is approximately 2.4.
  • Tested solution 2 is 0.1M NaOH. The pH is approximately 13.
  • Tested solution 3 is distilled water. The pH is approximately 7.
  • Red litmus paper turns blue if the solution is alkaline.
  • Blue litmus paper turns red if the solution is acidic.

Step 4

  • The solubility of ammonium sulfate is 700 g/L, therefore, it will dissolve completely.
  • The pH of the ammonium sulfate solution is approximately 2.6.

Summary of Assignment by Grade Range

Grades 3-5 (Ages 8-10)

  • Focus: Basic introduction to pH concepts and simple measurements.
  • Activities: Observing pH changes using litmus paper, simple demonstrations of acidic and basic solutions, basic safety instructions.

Grades 6-8 (Ages 11-13)

  • Focus: Intermediate understanding of pH and acid-base properties.
  • Activities: Using pH indicator paper and universal indicators to test various substances, measuring pH with digital pH meters, preparing solutions for pH testing, following detailed safety protocols.

Grades 9-12 (Ages 14-18)

  • Focus: Advanced understanding of pH concepts, precise measurement techniques, and chemical analysis.
  • Activities: Using a variety of pH indicators, precise measurement of pH with digital meters, preparing and manipulating solutions for testing, detailed observation and recording of results, adhering to advanced safety protocols.

Laboratory essentials

Instruments

  • Beaker (250ml)
  • Bucket plate
  • Droppers
  • Electronic Scale
  • Glass Rod
  • Graduated Cylinders (25ml & 100ml)
  • Paper towel
  • pH meter
  • Spatula (1mL)
  • pH chart

Products

  • Ammonium sulfate (powder)
  • pH indicator (liquid)
  • Unknown liquids (#1, #2 & #3)
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