Solutions, Acids and Bases

Students are expected to read pages 1 to 3 before coming to the lab to complete the experiments.

Print this entire lab packet and bring it to the laboratory. You must submit the completed lab worksheet for credit. Please provide a FULL lab report for this experiment.

Objectives:

After completing this laboratory assignment, students will be able to:

· Measure the pH of various liquids.

· Demonstrate that buffers stabilize the pH of a liquid.

· Measure the ability of commercial antacids to buffer the pH of a liquid.

Acids:

Acids are ionic compounds (compounds with a positive or negative charge) that break apart in water to form hydrogen ions (H+). The strength of an acid is based on the concentration of H+ in the solution.

The greater the concentration of H+, the stronger the acid.

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Characteristics of Acids:

· Acids taste sour.

· Acids react strongly with metals (Zn + HCl).

· Strong acids are dangerous and can burn your skin.

Examples of Acids:

· Vinegar

· Stomach Acid (HCl)

· Citrus Fruits

Bases:

Figure 1: Hydrochloric acid (HCl) in water

Bases are ionic compounds that break apart to form a negatively charged hydroxide ion (OH-) in water. The strength of a base is determined by the concentration of hydroxide ions (OH-). The greater the concentration of OH-, the stronger the base. Solutions containing bases are often called alkaline .

Characteristics of Bases:

· Bases taste bitter.

· Bases feel slippery.

· Strong bases are very dangerous and can burn your skin.

Examples of Bases:

· Sodium hydroxide (lye)

· Ammonia

Figure 2: Sodium

Hydroxide (NaOH) in water

pH Scale and Indicators
The strength of an acid or a base in a solution is measured on a pH scale. The pH scale is a measure of the hydrogen ion (H+) concentration. It spans from 0 to 14 with the middle point (pH 7) being neutral (neither acidic nor basic). Any pH number greater than 7 is considered a base and any pH number less than 7 is considered an acid. 0 is the strongest acid and 14 is the strongest base. An indicator is a special type of compound that changes color as the pH of a solution changes, thus telling us the pH of the solution.

Figure 3: A pH scale indicating the pH of common substances.

Measuring pH
A convenient way of measuring the pH of a solution is with pH paper. pH paper is treated with a chemical indicator that changes color depending on the concentration of H+ in the solution that it has contacted. The color chart on the container of the pH paper is used to compare the color of the pH paper to determine the pH of the solution (see Table 1).

Table 1: Examples of pH indicators.

Indicator

Range

Color Change

Methyl violet

0.2 – 3.0

Yellow to blue-violet

Bromphenol blue

3.0 – 4.6

Yellow to blue

Methyl red

4.4 – 6.2

Red to yellow

Litmus

4.5 – 8.3

Red to blue

Bromcresol purple

5.2 – 6.8

Yellow to purple

Phenol red

6.8 – 8.0

Yellow to red

Thymol blue

8.0 – 9.6

Yellow to blue

Phenolphthalein

8.3 – 10.0

Colorless to red

Buffers
Buffers are mixtures of two chemicals that stabilize the pH of a solution by resisting changes in the pH.

· If the pH is too low, one chemical will bind some of the hydrogen ions and raise the pH.

· If the pH is too high, the other chemical will donate some hydrogen ions to lower the pH.

pH in the body
The blood pH must be maintained very close to 7.4 which is slightly alkaline. A change of 0.2 pH units in either direction is considered dangerous and potentially fatal. Hydrogen ions are extremely reactive and affects many molecules which regulate physiological processes.

The body regulates pH in several ways including eliminating CO2 by the lungs and eliminating other acids and bases by the kidneys. The most important way to minimize pH changes in the body is using buffers. All body fluids, inside or outside of the cells, have buffers which defend the body against pH changes. The most important buffer in extracellular fluids, including blood, is a mixture of carbon dioxide (CO2) and bicarbonate anion (HCO3). CO2 acts as an acid because it forms carbonic acid when it is dissolved in water and donates hydrogen ions when they are needed. HCO3 is a base, soaking up excess hydrogen ions. There are also other buffers in the blood, such as proteins and phosphates, but they are less important in maintaining the overall pH of the blood. Blood pH is established by a balance between bicarbonate and CO2.