Testing alkalinity in water systems (simulation)
This resource provides the instructions for an alkalinity investigation of water resources. The activity is conducted in the lab. Although this is no substitute for working in the natural environment, it will help students practice skills which could be applied in the real world. After participating in this activity, students should have knowledge of the chemical species that contribute alkalinity to a water system, an understanding of how a measurement is used to provide information about the health of a water resource, and experience of the skills necessary to make the measurement
Context for Use
Resource Type: Activities:Lab Activity
Grade Level: High School (9-12)
Theme: Teach the Earth:Teaching Topics:Water, Teach the Earth:Course Topics:Environmental Science
Description and Teaching Materials
Perform an alkalinity titration. At least one day prior to the experiment, prepare three beakers: one with tap water, one with a piece of granite or another igneous rock in tap water, and one with limestone, concrete, or an antacid tablet in tap water. As a class, measure and record the alkalinity of each. Add acid, such as vinegar, to each mixture, and record the amount of acid as you go. Using a test kit to measure pH, add enough acid to bring each mixture to a pH of 4.2. At this pH, all of the alkalinity is consumed. Does the amount of acid added differ among the mixtures?
Chemical background: Alkalinity is a measure of water's capacity to resist a decrease in pH. It is also referred to as the acid neutralizing capacity, and sometimes the buffering capacity. Although a true buffer is able to neutralize both acids and bases, alkalinity is a measure of the concentration of bases that neutralize acids.
Alkalinity in natural waters is primarily a function of the carbonate system. Carbonates come from limestone and other rocks containing calcium carbonate that dissolve on contact with water. They release calcium ions (Ca2+) and carbonate ions CO32-), bicarbonate ions (HCO3-), or carbonic acid (H2CO3), depending on the water's pH. The negative carbonate and bicarbonate ions then combine with the positive hydrogen ions (H+) from solution, thereby reducing the acidity and increasing the pH. Different carbonate species dominate at different pH levels as listed below.
pH > 10.33 carbonate (CO32-) is the dominant species
pH 6.4 – 10.33 bicarbonate (HCO3-) is the dominant species
pH < 6.4 carbonic acid (H2CO3) is the dominant species
Relevance: Alkalinity in natural levels is beneficial to all organisms that depend on water. Because alkalinity resists a change in pH it helps prevent acidic water (pH < 5) that is harmful to humans, wildlife, and aquatic organisms. Some acidic water also mobilizes toxic heavy metals, making them available to the environment. Alkaline compounds not only neutralize acidity, but also react with heavy metals, such as lead, arsenic, and cadmium, to remove them from the water.
Teaching Notes and Tips
Can the students answer questions about the source of alkalinity and its relevance to the natural water resource?
220.127.116.11. Personal and community health can be affected by the environment, body functions and human behavior.