Background:

Acid-base indicators are substances that change colour when they are placed in solutions. Depending on how their colour changes, it can indicate an approximate pH for the solution.

The Questions:

What pH values correlate with the colour changes of red cabbage juice?

Variables:

Identify the type of data you will collect to support your hypothesis and state the manipulated, responding and controlled variables in this investigation.

SAFETY REMINDER: Wear protective equipment while handling chemicals.

Materials:

• pH sensor and link to computer
• eye protection, rubber gloves
• 12 plastic cups
• grease pencil
• pipets
• 1 M Hydrochloric acid, 1 M sodium hydroxide (teacher use only)
• distilled water (if not available, substitute tap water)
• red cabbage juice
• small graduated cylinder (10 ml)

Procedure:

For this activity, a pH sensor is used to determine the pH of unknown solutions, and link pH values to red cabbage indicator colour changes.

TEACHER PREPARATION:

Using the hydrochloric acid (HCl) and sodium hydroxide base (NaOH) create 12 different unknown pH value solutions. DO NOT mix the HCl and NaOH directly! Place a pH sensor into distilled water, then, using a pipet, carefully add either HCl or NaOH. Continue to add until the desired pH level is reached. Endeavour to produce a variety of pHs, approximately from pH 3 to pH 12. Label each of the solutions 1 through 12, but it is important to label them randomly - for example, do not label them in order of increasing pH.

Create a secret key of the known pH values that correspond to the numbered solutions. The key should list the solutions from lowest to highest pH value.

Step 1:

Prepare some red cabbage juice, or use the juice provided by your teacher. Instructions for red cabbage juice preparation can be found on many Internet pages, such as this HowStuffWorks page - Make a Cabbage Juice pH Indicator.

Step 2:

Your teacher will provide you with 12 unknown pH solutions. Use the grease pencil to label 12 plastic cups by numbering them, then fill them about 1/3 of the way (about 50-60 ml) with the corresponding unknown pH solutions - for example, put unknown solution 7 into cup number 7.

Step 3:

Use the graduated cylinder to add 5 ml of the cabbage solution to each of the 12 cups. Observe the cups carefully and place them in order of colour, starting with bright pink. Place the next closest colour to the first cup, and so on. The colours should progress from pink to purple to green. Describe these colours as best as you can in an appropriate table that lists all the numbered cups, and also provides a column for pH determination with a sensor:

Step 4:

Connect the pH sensor to your computer - if you are using a USB sensor, your apparatus will look something like this:

Step 5:

Configure the computer software to collect pH data.

It would be wise to choose a digital display of the data, so you can clearly see the pH value of the unknown solution:

Step 6:

Put the pH sensor into the first cup (with the bright pink colour), and record the pH indicated by the sensor. Remove the sensor, rinse it with distilled water.

Step 7:

Repeat step 5 for all of the cups that you have to test. When you are done, clean the sensor and dispose of all solutions and cups as instructed by your teacher.

Analyzing and Interpreting:

1.	Variables come in three types - manipulated, responding and control. What variable would the colours of solution (pink, purple, etc.) be? Explain.

Forming Conclusions:

Based on the data you have collected, answer the following questions:

2.	Compare your resulting order of cups with the teacher's secret key. Did your order match? If not, explain why you think your order does not match the secret key.
3.	Is the cabbage juice successful as a pH indicator? Discuss the pros and cons of using an indicator such as cabbage juice to determine pH.

Extending:

• Discuss with your classmates how you might repeat this experiment to make the results more reliable. If you think improvements could be made, make changes to the procedure, and repeat the experiment.