Question

Acid-Base Behavior

In addition to following the general safety rules, chemicals need to be handled properly. In particular, two very important classes of compounds called acids and bases require special attention. These compounds are commonly used reagents in the laboratory; therefore, understanding their proper disposal is beneficial.

Physical differences between acids and bases can be detected by the some of the five senses, including taste and touch. Acids have a sour or tart taste and can produce a stinging sensation to broken skin. For example, lemon juice tastes sour. Alternatively, bases have a bitter taste and a slippery texture. Soap and many cleaning products are bases.

Table 1: pH Range of Common Foods
Food	pH Range
Lime	1.8 - 2.0
Soft Drinks	2.0 - 4.0
Apple	3.3 - 3.9
Tomato	4.3 - 4.9
Cheese	4.8 - 6.4
Potato	5.6 - 6.0
Drinking Water	6.5 - 8.0
Tea	7.2
Eggs	7.6 - 8.0

Acids and bases are measured on a scale called pH. pH, or potential hydrogen, is calculated using a mathematical equation that accounts for some of the chemical differences in acidic and basic compounds. This scale helps us quickly determine if a solution is very acidic, a little acidic, neutral (neither acidic nor basic), a little basic, or very basic. pH values range from less than 1 to 14. A pH of 1 is highly acidic, a pH of 14 is highly basic (or alkaline), and a pH of 7 is neutral. Table 1 lists the pH of several foods.

pH indicators, which change color under a certain pH level, can be used to determine whether a solution is acidic or basic. For example, litmus paper is made by coating a piece of paper with litmus, which changes color at around a pH of 7. Either red or blue litmus paper can be purchased depending on the experimental needs. Blue litmus paper remains blue when dipped in a base, but turns red when dipped in an acid, while red litmus paper stays red when dipped in an acid, but turns blue when in contact with a base.

Acids and bases can react with each other. In this case, the two opposites cancel each other out resulting in a product that is neither an acid nor a base. This type of reaction is called a neutralization reaction. Neutralization of an acid or base is a technique frequently used for the proper disposal of the compound. The neutralized product can usually be disposed of by flushing it down a sink.

Keep in mind that in a typical laboratory strong acids and bases, such as hydrochloric acid (HCl) and sodium hydroxide (NaOH), are commonly utilized. As mentioned previously, the experiments in this lab manual are designed with a green approach. That means that the acids and bases, in addition to the other chemicals classes, used in this manual are safe for direct disposal.

In this experiment, you will learn how to properly neutralize and dispose of acidic and basic solutions.

	Materials 5 mL 4.5% Acetic Acid (vinegar), C2H4O2 (1) 250 mL Beaker (1) 10 mL Graduated Cylinder (1) 100 mL Graduated Cylinder 8 Litmus Test Strips (Neutral) Permanent Marker 2 Pipettes	0.5 g Sodium Bicarbonate (baking soda), NaHCO3 2 Weigh Boats *Water *You Must Provide

Procedure:

1. Use the permanent marker to label two of the weigh boats as A and C.

2. Use the permanent marker to label a 250 mL beaker as B.

3. Set the containers in the order A, B, and C.

4.Measure and pour approximately 5 mL of water into weigh boat "A".

5.Add 0.5 g sodium bicarbonate to weigh boat "B".

6.Use the 100 mL graduated cylinder to measure and pour 100 mL of water into beaker "B". Gently pipette the solution up and down until the sodium bicarbonate is fully dissolved in the water. Leave this pipette in beaker B to mix the solution in future steps.

7.Use the 10 mL graduate cylinder to measure and pour 5 mL acetic acid solution to weigh boat "C".

8.Develop a hypothesis as to what the pH of the contents of each container is. Record your hypothesis in Post-Lab Question 1.

9.Use the litmus test strips to determine if the substances in containers A - C are acidic, basic, or neutral. This is accomplished by briefly dipping an unused strip of the litmus paper in each of the weigh boats. Record your color results in Table 2. Record results immediately after dipping. The litmus paper may become darker as it dries.

10. Pipette 1 mL of acetic acid from weigh boat "C" into beaker "B". Ensure the transfer pipette does not touch the solution in beaker "B".

11. Gently pipette the solution in beaker B with its pipette to mix.

12. Test the pH of beaker "B" using new litmus paper and immediately record your result in Table 3.

13. Repeat Steps 10 - 12 four more times until all the acetic acid has been added to beaker "B".

Questions

1. State your hypothesis (developed in Step 8) here. Be sure to include what you think the pH will be, and why.

2. What is a neutralization reaction?

3. When might neutralization reactions be used in a laboratory setting?

4. At what point was the solution in beaker “B” neutralized?

5. What do you think would have been the results if a stronger solution of sodium bicarbonate was used? Would it take more or less to neutralize? What about a weaker concentration of sodium bicarbonate?

Answer 1