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
Prepare a colorimetry scale using the pH indicator at your disposal.
a) Locate the 50mL beaker filled with lake water, collected earlier this morning.
b) Take 25mL of pH3 solution and place it in test tube 1.
c) Repeat step b) with the pH 4,5,6 and 7 solutions and test tubes 2,3,4 and 5.
d) Add 5 drops of indicator to each of the test tubes using the dropper.
e) Mix the contents of the test tubes with a glass rod, or by putting a stopper and gently mixing from right to left.
f) Collect 25 mL of the lake water sample, place it in test tube #6, and add 5 drops of the same indicator used for the colorimetric scale.
g) Mix the contents of the test tube with a glass rod, or by putting a cap on and gently shaking it from right to left.
2. Compare the color of the colorimetric scale thus created with the color chart on the counter.
a) Determine the pH of the lake water using this scale. If the pH appears to be above 7, empty and clean the contents of test tubes 1 and 2, and continue the scale with pH 8 and 9, using these test tubes.
b) Compare again the pH of the lake water using this new scale (pH 5 to 9).
3. Validate the pH of each solution with the pH meter.
Insert the electrode of the pH meter into the lake water solutions.
Note the value displayed on the digital dial of the pH meter.
Anticipated Outcomes
The lake water should have a pH between 3 an 9, different each time the experience is restarted.
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.