Analysis of Phosphorus Concentrations

You will compare two spectrophotometric methods used to determine the concentration of total phosphorus in natural water systems. In the first part of the project you will determine and compare the linear range and the method detection limits of the two methods. In the second part of the project, you will use the best method to determine the concentration of total phosphorus in Lake Mendota. And in the third part of the project, you will use the best method of phosphorus analysis to answer a question you ask about the concentration of phosphorus in a freshwater system. Throughout the project, you will design, test and refine your experimental procedures so that you can obtain meaningful results from which you can learn something about the analysis of phosphorus in environmental samples and the chemistry of phosphorus in freshwater systems.

Experimental Objectives

Part 1: Compare two spectrophotometric methods of analysis for total phosphorus

• Compare the ascorbic acid method as described in Standard Methods for the Analysis of Water and Wastewater (Eaton et al., 2005) to the malachite green method proposed by Van Veldhoven and Mannaerts (1986).
• Make an absorption spectrum for each method and verify that the wavelengths given for each analysis is appropriate.
• Calculate the absorptivity constant for each method.
• Determine the linear range for both methods.
• Calculate the method detection limit for both methods using the U.S. EPA Method Detection Limit procedure outlined in Title 40 Code of Federal Regulations Part 136 (40 CFR, appendix B, revision 1.11). Use the persulfate method for sample digestion. The “EPA Method Detection Limit Procedure” is posted on our webpage on Learn@UW.
• Use appropriate quality assurance measures in your analyses.
• Compare the two methods based on reproducibility, linear range and method detection limit. Given the typical concentrations of total phosphorus found in freshwater lakes, is one method better than the other for determining concentrations in lake water?

Part 2: Measure total phosphorus in a Lake Mendota water sample

• Collect a grab water sample from Lake Mendota.
• Choose the best method from Part 1 to determine the concentration of total phosphorus in the Lake Mendota water sample. Use the persulfate method to digest your sample.
• Use appropriate quality assurance measures in your analyses.
• Experimentally test for matrix effects in the method chosen for analysis.

Part 3: Design and carry out an experiment in which you use one of the methods of analysis to answer a question you ask about concentrations of phosphorus in a freshwater system

• Possible variables to consider include: pH, oxygen, calcium, iron, aluminum, form of phosphorus, and algal growth (chlorophyll).
• Calcium, iron, and aluminum can easily be measured using Inductively Coupled Plasma Atomic Emission Spectroscopy (ICP-AES).
• Chlorophyll a is a characteristic algal pigment constituting 1-2% dry weight of algal biomass that can be extracted using 90% acetone/10% water solution and measured spectrophotometrically.
• You will not be able to measure the concentration of phosphorus in lake sediment because the digestion method for sediments requires special equipment and safety precautions; however, you can (and probably should) use sediment in your experiments. Sediment samples can be collected from Lake Mendota off one of the piers using an Ekman dredge.
• If using sediment samples in your experiments, you will need to know the mass of dry sediment. Before weighing, first let the sample air dry for several days and then dry it in the oven at 105°C.

Project Description

Introductory Assignment and Project Design

You will be receiving a "Getting Started" handout with further instructions for the Introductory Assignment. Before coming to lab, you will need to read the "Getting Started" handout in addition to the two papers that describe each of the analytical methods you will be using. These documents will all be posted on our course website. As you work through the project, you will likely need to consult other resources or articles that are related to the project. Some articles listed in the reference section may be helpful.

To begin your project design, you will meet with your group to address a set of questions related to the project and to discuss your ideas. Try to get everyone in the group to participate in the discussions. It is important to build good team skills, not only for this course but also for your future. Good team skills are one of the most important qualities that employers look for and one of the keys to success. In a healthy group discussion, everyone should feel comfortable expressing their ideas. You won't always agree with each other and that is okay; you can disagree respectfully, exchange ideas and come up with a better plan based on input from everyone.

Begin Collecting Data

Research is an investigative and iterative process. Your group may make several attempts at getting the experiments to work before obtaining results that make sense. This may seem frustrating at first, but you will be learning analytical chemistry and the scientific method as you work through this project, and the rewards will be there in the end. Some of what you do will not have a “right” or a “wrong” answer that we are expecting – your goal is to be a scientist in the truest sense. Be patient and persistent! Keep in mind that your group needs to plan and organize. Every member needs to participate and be productive. Although everyone should be knowledgeable about all aspects of the project, tasks should be delegated to increase efficiency. Having one person preparing solutions and three people watching is not a good use of your resources!

Progress Reports

During the course of the project, you will submit two written progress reports in which you summarize your progress in a concise, scientific fashion. Suitable materials include: brief outlines of experimental procedures, plots of your experimental results, and a brief summary of results and future plans.

Final Written Report

After completing your experiments you will write and submit a final report. The format should be similar to a scientific paper; more specific information will be provided later. The ACS Style Guide (Coghill and Garson, 2006) is a good resource for scientific writers.

Oral Presentation

Your group will also discuss your results with the instructors in an oral presentation at the end of the semester. Your group should plan on a brief presentation (~5 minutes) where you highlight your experimental method and anything of particular interest about the data. The presentation will be followed by approximately 15 minutes of questions. The questions will, in general, be posed to specific individuals. You may “handoff” the question to another group member, but remember that all group members should be aware of what was done with all aspects of the project. The primary goals of the questions are to determine your knowledge of the project itself and your analysis of your data.

Grading

The project counts for a total of 90 points toward your final grade. These 90 points are broken down into the following categories:

Introductory Assignment	10 pts
First Progress report	10 pts
Second Progress report	10 pts
Final report	35 pts
TA work distribution/efficiency grade	10 pts
Oral presentation	15 pts

Note that grades for the first four categories are assigned to the group as a whole. Grades for the last two categories are assigned individually.

(Example) Due Dates

Introductory Assignment:	Monday, March 12 (at beginning of lab)
First Progress Report:	Wednesday, March 21 (at beginning of lecture)
Second Progress Report:	Friday, March 30 (at discussion)
Final Written Report:	Monday, April 30 (at beginning of lab)
Oral Presentations:	Monday, May 7

Acknowledgements

This Design Project was developed by Dr. Pamela Doolittle*, Dr. Linda Zelewski, Dr. Cheri Barta and Patrick Robichaux.

*Please direct questions about this project to Dr. Pamela Doolittle, U.W. Madison Chemistry, 1101 University Avenue, Madison WI, 53706. [e-mail]