Week 37 – pH Indicators

To determine whether a solution is an acid, neutral, or a base, we need a tool.  In the lab, we can use a pH probe to obtain a quantitative pH value (an actual number).  Back in the day, we used pH strips to estimate the pH of different solutions.  Some strips are more sensitive than others, but the common theme is the strips rely on the user matching the color of the strip to a color chart which then estimates the pH.

At home (or in high school classrooms with limited funding) we can create a colorimetric indicator using cabbage! Colorimetric pH indicators provide us with a semi-quantitative measurement of pH.  By comparing the color of the indicator to a scale showing the color at a known pH, we can estimate the pH visually.  Color alone would be a qualitative data point (a description) while matching the color to a number provides us with a quantitative data point (an actual number).

To be fair, scientists use laboratory-grade colorimetric indicators in the lab all the time, and then use machines called spectrophotometers to quantitatively determine the optical density of the light passing through…remember the ELISA post from last week?

If you watched the Week 36 Intro video, you will soon realize that it also introduced you to this part of our lesson.  Prepare to be dazzled by the wizardry that is red cabbage juice indicator:

What is the actual chemistry behind red cabbage juice indicator?  Click the picture below and find out:


You now have what you need to complete the pH Analysis Gizmo.  The Gizmo was sent as a PDF attachment on Monday morning at around 8:00 am to the Week 37 – Chemistry Lesson email.

Anticipated answers to the question, “How do I turn in the Gizmo?”

  • If you have access to a printer, print the Gizmo and then:
    • Scan and email your completed work to Mr. Swart
    • Send pictures of your completed work Mr. Swart
    • Insert pictures of your completed work into a Google Doc and share with Mr. Swart.
  • If you do not have access to a printer:
    • Write answers on a piece of paper and then see above.
    • Write answers in a Google Doc and then see above.
    • Add comments to the PDF and share with Mr. Swart
    • This is 2020 – get creative!

Extend your learning!  For more on acid-base indicators, read Lesson 117 in the online textbook.  Note: this is not an assignment and you are not required to turn in any work related to lesson 117.

Return to Week 37 – Acids and Bases and continue working.


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