Introduction

This module covers the chemical concepts of aqueous solutions of ionic compounds in the context of a real-world problem, that of determining the potability of a community water supply. Students analyze water from a local water source (river, pond, lake, stream) to determine pH, alkalinity, total dissolved solids, coliform bacteria, dissolved oxygen, and the concentrations of the major naturally-occurring anions and cations. Connecting a local body of water to the students' chemistry laboratory experience provides relevance to their work and motivation for them to do a good job.

Often, students will be able to find evidence of leaking sewer pipes, seawater intrusion into freshwater sources, road salt contamination, agricultural runoff, leachate from landfills, and other possible sources of contamination. At the end of the module, students use the data collected by the class to write up a full report on water quality and make recommendations as to whether the water source tested could be used as drinking water. Having the class report their data to local agencies (with the appropriate caveat that the data has been acquired by beginning chemists) provides significant motivation to the students to proceed with care in the lab. It also sends a strong message that what they are learning in their chemistry laboratory is useful to society. The full module can be carried out in three to five 4 hour laboratory periods, which includes one laboratory period for students to learn how to use a computer graphing program to help with data analysis. Depending on the time available and the types of analyses used, the instructor can choose to do more or fewer analyses.

This instructor's manual is provided to help someone new to water quality analysis and discovery learning get started with this module. The following information is provided:

Learning Goals for Students

This module is designed to introduce students to the chemistry of natural waters, the analytical methods used to evaluate water quality, and the techniques of data analysis. By the end of the module, they should have acquired a variety of skills and knowledge about the subject. Specifically, the broad learning goals for students are:

Logistics

In What Courses Could the Module Be Used?

This module can be used at a variety of levels throughout the undergraduate curriculum.

 

General Chemistry

This module has been tested for eight semesters in a first semester general chemistry course at the University of California, Berkeley in special pilot laboratory sections (~23 students per section) focusing on environmental chemistry. We have also tested the module in the larger general chemistry class of ~1,300 students using a slightly different format that emphasized wet methods of analysis instead of instrumental methods. The experiments have been carried out successfully by students with a wide range of abilities. In general, the response of students to this module has been overwhelmingly positive.

Suggested Flow of the Module for General Chemistry

Week #1: Field trip for site assessment and sample collection

Week #2: Laboratory analysis of water samples and data entry

Week #3: Laboratory analysis of water samples and data entry

Week #4: Review and correction of data, computer graphing

 

Environmental Chemistry

This module could also serve as a relevant laboratory experience for Environmental Chemistry or Environmental Studies students.

Suggested Flow of the Module for Environmental Chemistry

Week #1: Field trip for site assessment and sample collection

Week #2: Laboratory analysis of water samples and data entry

Week #3: Laboratory analysis of water samples and data entry

Week #4: Review and correction of data, computer graphing

 

Instrumental Methods of Analysis

This module would also work well in an upper level instrumental methods of analysis course. With more advanced students, instructors would be able to delve deeper into the theory and practice of ion chromatography and atomic absorption spectrophotometry.

Suggested Flow of the Module for Upper-Level Analytical Chemistry

Week #1: Field trip for site assessment and sample collection

Week #2: Preparation of standards for IC and AA. Test standards for accuracy.

Week #3: Laboratory analysis of water samples and data entry

Week #4: Laboratory analysis of water samples and data entry. Review and correction of data

 

Suggested Schedule

This module is designed to occupy three to five weeks of laboratory time, assuming a 4 hour laboratory period. If students spend less than 3 hours in the laboratory, some of the shorter analyses should be chosen. The time required for each analysis is provided. The optimum scenario is:

Week 1: Field trip for sample collection. Return to lab to carry out analyses for coliform bacteria.

If it is convenient and possible to take the students to a nearby water source (pond, stream, lake, etc.), a field trip is an excellent beginning to the module. Students will learn the "lay of the land" and will then be able to correlate a site on their sampling map with a possible source of contamination. Check with your local geology department---they run field trips frequently---and ask them about the details of arranging transportation. Often it is cheaper than carrying out a typical experiment and disposing of the waste! If there is a water source within walking distance, all the better. The coliform tests are best done within 24 hours of collection of the sample, and this short (20-30 minutes) procedure fits nicely at the end of a field trip day. If a field trip is not an option (large classes, no nearby water source, no funding), samples should be collected for the students and stored at 4°C before analysis. Coliform tests are not particularly trustworthy after the samples have been stored (even at 4°C) for more than 3 days. Samples that will be tested for nitrate or phosphate should be filtered through 0.45 mm filters within 3 days of sample collection, since these components are "eaten" by bacteria present in the sample. Filtration removes most of the bacteria and preserves the sample.

 

Weeks 2-3 (4): Analysis of selected water quality parameters.

Choose several of the possible water quality parameters from the methods provided, depending on the equipment available at your institution. Analyses requiring expensive equipment can often be carried out in groups. Rotating the groups through a set of analyses is a workable way to obtain a variety of water quality parameters on the samples and expose the students to several different types of analyses. Alternatively, if time is a factor, you may wish to spend only one week on water analyses and have different groups within the class perform different analyses.

 

Week 4: Tutorial on computer analysis of data.

At the freshman level, provision of supervised instruction in computer analysis of data is necessary if the students are going to be expected to produce graphs or carry out extensive statistical analysis on the data. A worksheet that steps the students through exercises that demonstrate the power of the computer is contained in Appendix C in the module text. If your institution lacks the computer hardware, this part of the module can be eliminated; however, students could still compute averages for sample sites and produce graphs by hand.

 

Materials and Instrumentation

All materials and equipment not normally found in a chemistry stockroom are listed in the Stockroom Preparations sections with a source, catalog number, and approximate price. It is assumed that students have access to balances and the usual collection of flasks, beakers, clamps, buret, etc. in their lab locker.

Wet methods of analysis are provided for each analysis; however, analysis of water samples for trace levels of ionic components can also be carried out using an ion chromatograph for anions and an atomic absorption spectrophotometer (AAS) for cations. Background reading and generic procedures for use of these two instruments are provided in the laboratory manual. Because an expensive instrument usually has limited availability, it is often necessary to rotate students through the different analyses. An autosampler can also relieve congestion.

Logistically, it is highly desirable to have all instruments in the same lab room or an adjacent room. If you rotate students through a number of different analyses, all of which are running simultaneously and require supervision, you will quickly find that there is not enough of you to go around!

 

If an ion chromatograph or AA spectrometer is not available at your institution and you would like to look into the purchase of one, it might be worth considering the submission of an Instrumentation and Laboratory Improvement (ILI) proposal to the National Science Foundation (http://stis.nsf.gov). Ion chromatographs range in cost from approximately $15,000 to $50,000. Dionex, Hewlett-Packard, and Waters all make excellent instruments. Atomic absorption spectrophotometers range in cost from $30,000 to $100,000.

Chemical Concepts and Experimental Methods

In this module, a number of concepts and laboratory skills are introduced. The concepts can be taught at a variety of levels, depending on the background of the students and the equipment available for carrying out the analyses. An average level of detail is presented in the text of the module. For students with weaker backgrounds in chemistry, the instructor can refer students to materials in the textbook to fill the gaps.

A choice of experimental methods, from very simple chemical tests to sophisticated instrumentation, is provided for most of the analyses. For example, if an AA is available and the class size is small enough, you may wish to determine the water hardness ions calcium and magnesium by AA. This provides an opportunity to discuss atomic spectroscopy, Beer's law, and electromagnetic radiation. Alternatively, calcium and magnesium can be determined by a simple EDTA titration requiring only a few chemicals and some glassware.

At the simplest level (introductory chemistry, no instruments available), a selection of the following concepts and experimental techniques could be introduced:

At a more sophisticated level (an analytical chemistry class, or if a wider selection of instruments are available for a freshman class), the concept and techniques list to select from might look more like the following:

Not all concepts and techniques need be discussed. Each instructor may select concepts and techniques appropriate for his or her particular class. The laboratory manual is structured such that the introductory material on a particular topic is followed immediately by the procedures for that analysis. This arrangement allows the instructor to assign a contiguous block of reading in the laboratory manual for a particular analysis.


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