Project Description

In this design project, you will be looking at the interaction of arsenopyrite with acidic solutions and some of the variables that affect the dissolution process. You will perform the experiments at low pH (typically between 1 to 4) because this will avoid precipitation of Fe(OH)₃ and because low pH mimics the environment produced by the mineral reactions. You are expected to design an experiment to test the importance of one experimental variable in controlling the overall mineral release rate. Variables that might be of interest include the presence or absence of oxygen, presence or absence of Fe³⁺, pH, and temperature.

The reaction rates appear to be limited by the amount of sample area that is exposed to the solution. If this is true, then you would expect that the reaction rate will be proportional to the surface area of your sample, not just its weight. Consequently, it is important to know the mass of your sample and its surface area. The reactions we are investigating are fairly slow, and you will not see any change in the mass of your samples. To detect the arsenic/iron release on a short (about a week) time scale, you will be using samples that have been crushed to provide a higher surface area. These samples have also been placed through a sieve so that the individual particles all have roughly the same diameter. From the mass and the average particle size, you can estimate the total surface area of your sample. It is important to remember that the reactions you are looking at are occurring on the surface of the mineral samples; however, it is nearly impossible to detect what is actually on the surface. So, you are measuring what is released into solution and how this changes as a function of various control variables. You will use your results to make conclusions about what is happening on the surface.

Project Design

To begin your project design, you will meet with your group and 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. Healthy group discussions should have advocates, skeptics, cheerleaders, strategists, dreamers, etc. Sometimes individuals play one of the roles but the best groups have each person playing all the roles (at different times).

Design an experiment to measure the rate at which iron is released into solution. Each group will receive approximately 1.5 g of arsenopyrite to analyze. You should use the lab experiment “Spectrophotometric Determination of Fe”¹⁶ as a model. Instead of using bipyridyl, consider using phenanthroline as a complexing agent.¹⁷ Your experimental design must give consideration towards the following:

1. From the suggestions below, pick a variable to investigate:
   • See how pH values (ranging from 1 – 4 pH units) affect the dissolution rate. If Fe(OH)₃ precipitation does not occur, does [H⁺] affect the dissolution rate?
   • Since Fe³⁺ is proposed to act as the primary oxidant, you could investigate how adding ferric iron to solution either helps or hampers the dissolution of arsenopyrite. What is the relationship between [Fe³⁺] and the oxidation rate? Is it linear? Quadratic? Logarithmic? Exponential?
   • Does the presence of ferrous ion (Fe²⁺) affect the rate? If so, does the rate depend on the ratio of Fe³⁺ to Fe²⁺?
   • Oxygen is also proposed to act as an oxidant. You could investigate the dissolution of pyrite in oxygen rich vs. oxygen deficient environments.
2. Determine how you can estimate the exposed surface area of your sample from knowledge of the weight and the average particle size. Will this estimate result in a random or systematic error in your results? Will it affect your ability to detect trends when comparing a sample and a control? How will you make sure your samples are sufficiently homogeneous that you can get reproducible data?
3. Will you analyze the ferric or ferrous form of the cation? How will you oxidize or reduce the iron to obtain the correct form?
4. What range of iron concentrations will you use for your standard solutions? Your answer will depend on the iron concentration you expect to obtain from your dissolution reactions and on the iron concentrations you can measure with the spectrophotometer.
5. What pH will you use to perform your experiment? Phenanthroline is a weak acid. The form it takes in solution depends on the pH. You should figure out which form of the complexing agent is needed to complex with the iron in solution.
6. How will you control the pH of your solutions?
7. What wavelength will you pick for your analysis?
8. How will the experimental results allow you to make conclusions about how quickly the minerals will release iron and sulfuric acid into the environment?

Your group should have sound answers to all of the points above and should be able to discuss them intelligently with your instructors. It’s a good idea to use library resources to find current journal articles that are related to the topic. Some articles listed in the reference section of this document may be useful. Your group will submit a written design proposal at the end of the first lab period.

Conducting your experiments

Begin collecting data. There are many interesting aspects to this experiment that could be covered if your group has the desire and time to do it. At the very least, you should take aliquots of the dissolving iron over several days time, and observe how the iron concentration changes. Does it increase linearly with time? Can you relate the buildup in iron concentration to a specific reaction rate normalized to the available surface area? The pH is another important variable. Can you predict anything about the expected influence of pH on the reaction rate, based on the overall reaction stoichiometry? Remember that research is a process that sheds light on the truth. Your group may make several attempts at getting the experiment 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 Report

Part way through the project your group will submit a written report where you summarize your progress. Suitable materials include: brief outlines of experimental procedures, plots of your experimental results, and brief summary of results and future plans. Exactly what you turn in is up to you, but keep in mind that you are trying to demonstrate your group's progress in a concise, scientific fashion.

Final Written Report

After completing your experiments you will write and submit a final report. The format should be similar to a scientific paper. An overview of the format is given in the next section. More information on writing a scientific paper can be found in The ACS Style Guide.¹⁸

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 is then followed by ~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.