Materials Included In Kit

Additional Materials Required

Safety Precautions

Hydrochloric acid is toxic by ingestion or inhalation and is severly corrosive to skin and eyes. Sodium hydroxide is a corrosive liquid that causes skin burns and is very corrosive to the eyes. The buffer solutions in a low pH range are strongly acidic and those in a high pH range are strongly alkaline. Review the Safety Data Sheets before beginning this activity. Wear chemical splash goggles, chemical-resistant gloves and a chemical-resistant apron. Please review current Material Safety Data Sheets for additional safety, handling and disposal information.

Disposal

Please consult your current Flinn Scientific Catalog/Reference Manual for general guidelines and specific procedures, and review all federal, state and local regulations that may apply, before proceeding. Universal indicator solution should be disposed of according to Flinn Suggested Disposal Method #18b. Hydrochloric acid should be disposed of according to Flinn Suggested Disposal Method #24b. Sodium hydroxide should be disposed of according to Flinn Suggested Disposal Method #10. The buffer solutions should be disposed of according to Flinn Suggested Disposal Method #26b.

Teacher Tips

• Use cotton swabs for the cleaning of the micro strips. Insist on using clean micro strips.
• White paper can be used as a background to improve the reading of the colors in the micro-strip wells.
• Stick a piece of paper to the transparent tape at each end of the 1 x 12 micro-strip. This will make it easier to remove the tape at the end of the experiment. Also, the paper can be used to identify both ends of the pH scale by marking one end “pH–1” and the other with “pH–12.”
• Have 12 beakers or flasks of pH-adjusted liquids ready beforehand. Use the pH 1-adjusted solution as is and prepare the pH 2–12 solutions by disolving each capsule in 100 mL of distilled or deionized water. To reduce the number of culture bottles and pipets, have a chemical station with the 12 pH solutions and 8 sample solutions. As for the Universal indicator, hydrochloric acid, and sodium hydroxide solutions, groups can be given a pipet for
  each of these 3 liquids only.
• Be sure to filter the sea/lake/pond water before using it.
• pHs of some liquids

  Grapefruit Juice                  3.1
  Tap Water                           6–7
  Rainwater                            6.2
  White Vinegar                    2.8
  NaOH, 0.1 M                     13.0
  Hydrochloric Acid, 0.1 M  1.1
  Soda, 7-Up^®                       3.0
  Seawater                              8.5
  Lake/Pond Water               Varies
  Ammonia Water                 13.0

• Use only 1 drop of indicator, more than 1 drop may cause significant error. Be sure the thin-stem pipets used to dispense the indicator have stems “drawn out” so that the diameter of the stems are reduced in size. (See the Teacher PDF.) This will help avoid dispensing too much indicator. The “drawn out” stems will deliver about 50 drops per mL, as compared to the standard stems which will deliver about 25 drops per mL.

Further Extensions

• An extension of this experiment would be to have the students substitute red cabbage juice for the Universal indicator.

Sample Data

Observations

How many drops of sodium hydroxide are required to match the color in the upper-middle cup containing a neutral solution (water)?

10 drops

Answers to Questions

Answers to Post-Lab Questions

1. What effect does the addition of sodium hydroxide have upon the Universal indicator?

   Since the addition of sodium hydroxide changes the pH of the liquid, the Universal indicator changes in color, too.

2. What effect does the addition of sodium hydroxide have upon the solution itself with respect to the concentration of the hydronium ions, [H⁺]?

   The addition of sodium hydroxide will reduce the number of hydronium ions in solution. This in turn will change the pH of the solution.

Teacher Handouts

12577_Teacher1.pdf

Introduction

In this experiment, Universal Indicator will be used to observe the effects of an acid and a base on an indicator. A “pH meter” will be constructed to determine the effects of dilution on pH, and to find the pH and [H⁺] of various liquid substances.

Concepts

• Indicators
• pH
• Acids and bases

Background

In 1909, the pH scale was invented by a Danish chemist by the name of Soren Sorensen while he was working at the Carlsberg Brewery in Copenhagen on problems with the brewing of beer! The normal pH scale has a range between 0 and 14. The use of these simple values avoids the use of awkward, scientific numbers representing the concentration of hydronium ions [H⁺].

In pure water, scientific measurements have shown that the [H⁺] is 1.0 x 10^–7 moles per liter. As a solution is made more acidic, the [H⁺] increases from 10^–7, to 10^–6, to 10^–5 and so on. Likewise, as the solution is made more basic, the [H⁺] decreases from 10^–7, to 10^–8, to 10^–9 and so on.

Sorensen realized that the log of the reciprocal of the concentration of the hydronium ion of pure water produces the value of 7. As the solution becomes more and more acidic, the pH values decrease from 7 to 6 to 5 to 4 and so on. As the solution becomes more and more basic, the pH values increase from 7 to 8 to 9 to 10 and so on. Using this mathematical concept, Sorensen developed the pH scale as we know it today.

The pH of a solution may be defined as

Materials

Safety Precautions

Hydrochloric acid is toxic by ingestion or inhalation and is severly corrosive to skin and eyes. Sodium hydroxide is a corrosive liquid that causes skin burns and is very corrosive to the eyes. The buffer solutions in a low pH range are strongly acidic and those in a high pH range are strongly alkaline. Wear chemical splash goggles, chemical-resistant gloves and a chemical-resistant apron. Wash hands thoroughly with soap and water before leaving the laboratory.

Procedure

Observing the Effects of an Acid/Base on an Indicator

1. Fill 4 small cups ½ full of water and arrange them as in Figure 1.
   {12577_Procedure_Figure_1}
2. Place 5 drops of Universal Indicator into each cup. Note the color change.
3. Into the upper-left cup place 10 drops of 0.1 M hydrochloric acid. Note the color change.
4. Into the upper-right cup place 10 drops of 0.1 M sodium hydroxide, a base. Note the color change.
5. Place 10 drops of 0.1 M hydrochloric acid in the lower middle cup. Note the color change. Begin to add 20 drops of 0.1 M sodium hydroxide, drop by drop. Don’t forget to stir between each addition and note the color change. Record the number of drops of hydroxide required to match the color of that found in the upper-middle cup which represents a neutral solution (pure water). Continue the process until all 20 drops of sodium hydroxide have been added. Note the color of the liquid in the lower-middle cup at the end of the process.

Construction of a “pH Meter”

6. Using a 1 x 12 well micro strip, rotate the micro strip so that the numbers assigned each well (1–12) read from left to right (see Figure 2). Place 8 drops of a pH–1 solution into well 1, place 8 drops of a pH–2 solution into well 2, place 8 drops of a pH–3 solution into well 3. Continue the process until all 12 wells are filled with the corresponding solution.
   {12577_Procedure_Figure_2}
7. Place one drop of Universal Indicator into each of the 12 wells. Note the color change.
8. Place a strip of transparent tape across the top of all 12 wells to seal them (see Figure 3). Each well will represent the color code for the Universal Indicator representing pH 1 through 12 (e.g., well 1 for pH–1, well 2 for pH–2, well 3 for pH–3).
   {12577_Procedure_Figure_3}

The Effects of Dilution on pH

9. Using a clean, dry, 1 x 8-well micro strip, place 9 drops of distilled or deionized water into wells 2 through 6. Place 10 drops of hydrochloric acid (0.1 M) into well 1. Begin a series dilution process, with the remaining cups that contain water, that is to say, place one drop of solution from well 1 into well 2 then mix, place one drop of solution from well 2 into well 3 then mix, etc. This should produce 6 concentrations of hydrochloric acid (0.1, 0.01, 0.001, 0.0001, 0.00001, 0.000001 molar). Place 1 drop of Universal Indicator into each of the 6 wells, mix each well thoroughly, and check the pH of each concentration against your “pH meter.” Record the observations in Data Table 1.
10. Wash and dry the 1 x 8-well micro strip and repeat the investigation except this time use sodium hydroxide (0.1 M).

Determining the pH and [H⁺] of Various Liquid Substances

11. Place 8 drops of tap water into one of the wells of the clean, 1 x 8-well micro strip. Add 1 drop of Universal Indicator to the well and mix thoroughly with a toothpick. Note the color change. Determine the pH of the tap water by matching its color change to that of one of the wells of your “pH meter” (see Figure 4). Record the observations in Data Table 2.
    {12577_Procedure_Figure_4}
12. Repeat this process with all of the given liquids to be tested.
13. Calculate the hydronium ion concentration for each of the liquids tested and record the results in Data Table 2.
14. See instructor for appropriate disposal procedures.

Student Worksheet PDF

12577_Student 1.pdf