Expt 024 -- pH Rainbow Tube

Start with a tube filled with green solution, add a few drops of acid to one end and a few drops of base to the other, and a whole spectrum of colors appears.

Chemical Concepts

1. Universal indicators can be used to illustrate a whole range of pH conditions (usually because they are made up of a mixture of different indicators, all of which change at different pHs).
2. As an acid is diluted, its pH increases -- but never beyond 7.0. Likewise, as a base is diluted, its pH decreases -- but, again, never less than 7.0.
3. Buffers are solutions that are resistant to pH changes.

Safety

• Wear goggles and apron. Wash any spilled acid or base with plenty of water.
• Use ordinary laboratory safety procedures.

Procedure

1. With one end securely stoppered, fill the tube to within 3-4 cm of the top with a solution of universal indicator in water. Then add a few drops of HCl to the tube, stopper securely while keeping a hand on the bottom stopper to hold it in place! Invert and observe the colors produced.

   !!!Click here to See Movie.

2. Remove the top stopper (which used to be on bottom), add an equal number of drops of NaOH to the tube, re-stopper securely while keeping a hand on the bottom stopper to hold it in place! Invert and observe. The full indicator spectrum is now spread throughout the tube.

   !!!Click here to See Movie.

   !!!Click here to See Movie.

   Note the differences between the tube before and after the base was added.

   !!!Click here to See Picture.

3. Ask students to predict how many times the tube would need to be inverted back and forth before the entire solution was evenly mixed to restore the homogeneous green color throughout. Try it.

   !!!Click here to See Picture. After 5 times.

4. Set up a second tube as a wall exhibit. Leave the tube undisturbed -- clamped in a vertical position for a few hours (days?). Make periodic observations. The series of changes that the system undergoes is quite interesting -- though not always reproducible!

   !!!Click here to See Picture.

5. Once the solution is brought back to its original uniform color, it is ready to be used again. In fact, depending on the longevity of the universal indicator being used, the same solution can be recycled many times.

Optional variation

You can also use a weak acid (such as acetic) and a weak base (such as ammonia) and the demonstration works just as well...the first few times. After that, the spectrum of colors loses its extremes, and the solution eventually becomes resistant to any color changes at all. This variation provides a good demonstration for the effect of buffers!

Questions

1. Predict how many times the tube must to be inverted back and forth before the entire solution is evenly mixed to restore the homogeneous green color throughout.
2. Write the equation of the ionization of water.
3. What is the highest pH observed when only acid was added to distilled water.
4. If 10^-10 M HCl is present in a section of the tube, what is the pH in that section? What color would you expect to see?

Handout Makeup

Name ___________________________ Class ________

Teacher __________________________

BeckerDemos 024 pH RAINBOW TUBE

Watch the movies and record your observations. The original solution is neutral as indicated by the green color.

Acid added:

Base added:

Use the pictures to answer the questions.

Curriculum-

Use this experiment during discussion of pH of solutions and the use of indicators to measure pH. You may wish to follow the initial demonstration several days later with the weak acid and/or weak base variation when you are discussing buffers.

Activity-

Demonstration - Student or Teacher

After the initial demonstration, a wall exhibit is suggested where students can observe the diffusion over a long period of time is suggested. Coming back to the demonstration periodically reinforces the ideas.

Safety-

• Wear goggles and apron. Wash any spilled acid or base with plenty of water.
• Use ordinary laboratory safety procedures.

Time-

Teacher Preparation: 5 minutes

Class Time: 5 -10 minutes

Materials-

• 0.5 mL of 0.1 M NaOH
• 0.5 mL of 0.1 M HCl
• deionized water
• 100-200mL of a 5% universal indicator solution (or red cabbage extract) The concentration must be visually adjusted; see Lab Hints below.
• 2 clear glass or plastic tubes, ID: 1-3 cm, length: 40-80 cm
• 2 solid rubber stoppers to fit tube
• 1 beaker or cup (volume must exceed that of the tube)
• 2 eyedroppers or disposable pipets

Disposal-

Dispose at the sink.

Lab Hints-

1. Caution: When pushing in the stopper on top, keep in mind that pressure is easily transferred through a liquid. In other words, always keep a hand on the bottom stopper to hold it in place!
2. The precise proportions of the indicator/water solution depend on the type of indicator used as well as the dimensions of the tube. Start with a 1:20 mixture, then adjust it from there to produce rich colors in the tube.
3. The number of drops of acid and base are not critical. 3-4 drops work well for a small, 100 mL tube; a larger tube might require a proportionately larger number of drops.

Observations-

• The half color spectrum that you see after adding just the HCl and inverting the tube is comparable to what one would see after performing a series dilution with the HCl. The solution ranges from the neutral color of the indicator which is green in this case to the color of the indicator in strong acid. Although the HCl concentration might range from 1 x 10^-1 M at the bottom of the tube to something like 1 x 10^-13 M at the top of the tube, the corresponding pH's are not 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13. Rather, the corresponding pH's are approximately 1, 2, 3, 4, 5, 6, 7, 7, 7, 7... It is the 1 x 10^-7 M H₃O⁺ already present in the water that is ultimately responsible for the pH reaching this plateau of 7 and for the color not surpassing the neutral (green) point. This is important to point out to students who might think that they can use the log of the HCl concentration, no matter how dilute, to derive the pH of the solution. Certainly you can never add HCl to neutral water and expect to get a pH greater than 7! Likewise, you cannot expect dilute base solutions to have pH values of less than 7.
• To derive the entire pH scale by series dilutions, you must therefore use both an acid and a base, starting, of course, at opposite ends. This is exactly what is accomplished in the tube.

Answers-

Q1. Predict how many times the tube must to be inverted back and forth before the entire solution is evenly mixed to restore the homogeneous green color throughout.

A1. Most students underestimate the number. It turns out that the bubble is surprisingly inefficient at mixing the system.

Q2. Write the equation of the ionization of water.

A2. H₂O --> H⁺ + OH^-

Q3. What is the highest pH observed when only acid was added to distilled water?

A3. 7 indicated by the green color.

Q4. If 10^-10 M HCl is present in a section of the tube, what is the pH in that section? What color would you expect to see?

A4. pH 7, green

References-

Acknowledgments: I believe that this idea is an original one -- although the technique is so simple, I cannot imagine something like it has never been tried before! I first published it in Chem 13 News, Waterloo Ontario, in December of 1990.

Key Words 1-

pH, acid, base, indicator, universal indicator, buffers