Testing alkalinity in water systems (simulation)
Initial Publication Date: December 23, 2009
Summary
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
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Learning Goals
Students will use titration to measure the alkalinity of a water sample. Students will compare the effects of substrate material on alkalinity. Students will read and discuss the relevance of alkaline chemistry as it relates to natural water resources.
Context for Use
This investigation could be completed in one class session. It requires a laboratory setting (sinks and pH measuring materials). Students should have some understanding of ionization, pH, and neutralization reactions. This investigation is related to a study of the characteristics of a natural water system. It could easily be used in physical science, chemistry, or environmental studies classes.
Description and Teaching Materials
This activity appears in: "Healthy Water, Healthy People Field Monitoring Guide" (2003) by the Project WET International Foundation www.projectwet.org
1. Alkalinity
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.
1. Alkalinity
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
This activity refers to pH and toxic heavy metals. The teacher should have background knowledge in these areas and be prepared with references for students to continue their inquiry online. Although acids and bases are common topics for both chemistry and biology, the introduction of toxic heavy metals is unique in this context. It gives an important practical reference for the titration. Heavy metals are often dangerous in trace amounts and accumulate in living organisms. Unless one is prepared to catch fish and puree fish livers and kidneys to check for the metals, measurement of the metals in the water resource is difficult. Alkalinity provides an accessible measuring tool which can be related to the risk of active toxic metals in the water.
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Assessment
Do the titration results reflect the precision possible with the lab equipment available?
Can the students answer questions about the source of alkalinity and its relevance to the natural water resource?
Can the students answer questions about the source of alkalinity and its relevance to the natural water resource?
Standards
(2009 Minnesota Academic Science Standards) 9.4.4.1. Human activity has consequences on living organisms and ecosystems.
9.4.4.2. Personal and community health can be affected by the environment, body functions and human behavior.
9.4.4.2. Personal and community health can be affected by the environment, body functions and human behavior.