1.1: Purpose and Background

Purpose of the Experiment

This is an integrated experiment, which combines techniques from Organic, Biological, Physical, and Analytical Chemistry, its purpose is to introduce students to:

• The colorimetric method as an analytical technique
• Spectrophotometric analysis
• Water sample analysis
• Quantitative techniques in volumetric analysis

This experiment will also improve students’ lab and computational techniques in the following areas:

• Learning the correct handling and operation of a UV-VIS spectrometer and performing calculations using Beer’s Law
• Precise handling and measurement techniques for sampling water from the Charles River
• Making up solutions using serial dilution
• Using Microsoft Excel for providing graphical and numerical output analysis to prepare a calibration curve and perform an error analysis on the results

Background

In this experiment students will work with water samples, which they will obtain directly from the Charles River. The Charles River extends itself in a serpentine path some 80 miles through more than 20 cities and towns from its starting point in Hopkinton, Massachusetts.³ The river ends at Boston Harbor, which opens to the Atlantic. Dozens of lakes and ponds are part of this river. The oxygen that dissolves in the river plays an important role in supporting all types of aquatic life and microbes. The dissolved oxygen depends on many factors including the surrounding air pressure and temperature. It is also influenced by its surroundings, which could deposit substances into the water. The three major sources of pollutants & organic matter that find their way into the river include municipalities (sewage & runoff), agriculture (fertilizers) and industrial (various). The organic matter, fertilizers and detergent waste provide material for explosive growth in vegetation and algae. The vegetation eventually dies and falls to the bottom of the river and combines with other organic matter to decompose through the action of various microorganisms and bacteria. Enormous amounts of oxygen are used up during the decay process. Various bacteria, as well as other organisms such as algae and fish use up the oxygen that makes its way into the water through the process of photosynthesis and diffusion from the surrounding atmosphere. There are many different levels of oxygen that have been discussed in terms of a satisfactory baseline number needed to support life and, from all accounts, that appears to be somewhere around 5.0 mg/L of dissolved oxygen. DO (Dissolved Oxygen) levels from 3.0 to 4.0 mg/L can create a stressful environment for fish.⁴ This might lead to erratic movement or lack of interest in feeding. Levels of DO which are needed can vary from organism to organism with some fish species such as trout and small mouth bass requiring minimum levels of DO equal to 6.5 mg/L Other fish such as mud dwellers like catfish and carp can survive on as little as 2.0 to 3.0 mg/L of DO.⁵ Once DO gets down to levels less than around 2 mg/L then the dead fish floating in the Charles River “eutrophication” becomes a problem. On an ongoing basis, this experiment will measure the gaseous oxygen dissolved in the aqueous environment of the Charles River at various locations and under various seasonal conditions (temperature and pressure). The experiment will also study algae blooms fueled by phosphate via a colorimetric study to determine the phosphate concentrations present in the Charles River. Phosphates present in detergents are a nutrient for algae growth.