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: Identifying the acidic or basic nature of liquid substances
a) Measuring the solution: Use a graduated cylinder to measure 20 mL of the test solution (solution #1).
b) Transferring to a beaker: Pour the measured 20 mL into a 50mL beaker.
c) Test with Litmus paper: Immerse separately a red litmus paper and a blue litmus paper in the solution.
d) Test with pH indicator paper: Also dip a pH indicator paper.
e) Analyzing the results: Compare the obtained colors with the pH chart to determine the acidic or basic character.
f) Cleaning and repeating: Rinse the graduated cylinder and repeat the steps for solutions #2 and #3.
Part 2: Using universal indicator for pH
a) Sampling the solution: With a dropper, take 1 mL of the solution (solution #1).
b) Placing in the well plate: Place the solution in a well of a spot plate.
c) Adding indicator: Add a drop of universal pH indicator.
d) Mixing: Gently stir with a glass rod.
e) Interpretation: Compare the resulting color with the pH chart.
f) Repetition: Repeat the steps for solutions #2 and #3.
Part 3: Accurate measurement of pH with a pH meter
a) Immersing the electrode: Insert the pH meter electrode into the solution (solution #1).
b) Reading pH: Record the value displayed on the digital dial.
c) Cleaning the electrode: Rinse and dry the electrode before moving on to solutions #2 and #3.
Part 4: Determining the pH of a solid substance
a) Weighing the substance: Weigh approximately 1.8 g of ammonium sulfate.
b) Preparing the solution: Dissolve the powder in 100 mL of water in a 250 mL beaker.
c) Stirring: Mix with a glass rod.
d) Tests with Litmus and pH indicator papers: Perform pH tests as described in Part 1.
e) Measuring with a pH Meter: As in Part 3, measure the pH with the pH meter for increased accuracy.
Anticipated Outcomes
- Red litmus paper turns blue if the solution is alkaline.
- Blue litmus paper turns red if the solution is acidic.
- The tested solution 1 is 5% v/v acetic acid. The pH is about 2.4.
- The tested solution 2 is 0.1M NaOH. The pH is about 13.
- The tested solution 3 is distilled water. The pH is about 7.
- The solubility of ammonium sulfate is 700g/L, so it will dissolve completely.
The pH of the ammonium sulfate solution is about 4.7.
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
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
Ammonium sulfate (powder)
pH indicator (liquid)
Unknown liquids (#1, #2 & #3)