This laboratory session introduces students to the colorimetry technique for determining the pH of a lake water sample, utilizing known pH standards and a pH indicator. The objectives are designed to give students practical experience in environmental chemistry, focusing on assessing the acidity or basicity of aquatic solutions.
Educational Goals
- Colorimetry scale preparation: Teach students to create a colorimetry scale for pH using a chemical indicator, enabling visual comparison of pH levels in various solutions.
- Developing practical skills: Enhance students’ ability to handle standards for constructing a visual pH reference, emphasizing the manipulation and preparation of solutions.
- Lake water pH determination: Apply the colorimetry scale to determine the lake water sample’s pH by visually comparing the color change induced by the pH indicator.
- Results validation: Use more precise equipment, such as a pH meter, to validate the colorimetry findings and ensure the accuracy of visual assessments.
This session offers an in-depth exploration of colorimetry as a method for estimating aquatic solutions’ pH, crucial in environmental and analytical chemistry. It highlights the importance of corroborating visual methods with precise measurement tools, providing reliable and accurate results. Through this laboratory, students gain essential competencies in environmental chemistry, underlining the practical application of chemistry principles in real-world scenarios.
Protocol
Part 1: Prepare a colorimetry scale using the pH indicator available to you
- Using the graduated cylinder, measure 25 mL of pH3 solution and then pour into test tube 1.
- Repeat step a) with the pH 4,5,6 and 7 solutions and test tubes 2,3,4 and 5.
- Using the dropper, add 5 drops of pH indicator to each of test tubes 1 to 5.
- Mix the contents of the test tubes with a glass rod, or by putting on a stopper and gently mixing from right to left.
- Using the graduated cylinder, measure 25 mL of the lake water sample and then pour into test tube 6.
- Using the dropper, add 5 drops of pH indicator to test tube 6.
- Mix the contents of the test tube with a glass rod, or by putting on a stopper and gently mixing from right to left.
Part 2: Determine the pH of river water using the colorimetric scale
- Determine the pH of river water by comparing the color of the sample in test tube 6 with the colorimetric scale on the poster that is on the counter.
-> If the pH seems to be above 7, complete the colorimetry scale using the pH 8 and 9 solutions and test tubes 7 and 8. If the pH is below 7, move on to Part 3 to validate the pH of each solution with the pH meter.
- Using the graduated cylinder, measure 25 mL of pH 8 solution and then pour into test tube 7.
- Using the dropper, add 5 drops of pH indicator into test tube 7.
- Using the graduated cylinder, measure 25 mL of pH 9 solution and then pour into test tube 8.
- Using the dropper, add 5 drops of pH indicator into test tube 8.
- Mix the contents of the test tubes with a glass rod, or by putting on a stopper and gently mixing from right to left.
- Compare again the pH of the river water using this new scale (pH 5 to 9).
Part 3: Validate the pH of each solution with the pH meter
- Insert the pH meter electrode into the test tube containing the river water (test tube 6) and into the other test tubes to validate the pH of each solution.
- Record the value displayed on the digital dial of the pH meter.
Anticipated Outcomes
- The lake water should have a pH between 3 and 9, different each time the experience is restarted.
- The results will follow the universal pH indicator chart.
Summary of Assignment by Grade Range
Grades 3-5 (Ages 8-10)
- Focus: Basic introduction to pH and simple observations.
- Activities: Observing pH changes using color indicators, simple demonstrations of acidic and basic solutions, basic safety instructions.
Grades 6-8 (Ages 11-13)
- Focus: Intermediate understanding of pH and colorimetry techniques.
- Activities: Creating a colorimetry scale using pH standards and indicators, comparing the pH of lake water samples visually, following detailed safety protocols.
Grades 9-12 (Ages 14-18)
- Focus: Advanced understanding of pH determination and colorimetry.
- Activities: Preparing a detailed colorimetry scale, accurately determining the pH of lake water samples, validating results using a pH meter, detailed recording and analysis, adhering to advanced safety protocols.
Laboratory essentials
Instruments
- Beakers (50ml, 250ml & 1000ml)
- Bucket plate
- Droppers
- Electronic Scale
- Erlenmeyer (250ml)
- Glass Rod
- Graduated Cylinders (25ml & 100ml)
- Hot plate
- Magnetic stirrer
- Paper towel
- pH meter
- Pipette
- Spatulas
- Test Tubes
- Thermometers
- Timer
Products
- pH solutions (#3 to #9)
- Lake water sample