If you are an advocate of laboratory instruction as an integral and essential part of chemistry education (as I am), you have to face several obstacles.
The first obstacle is that you have to have the materials to do laboratory work. This wasn’t a problem some forty or fifty years ago as there were countless funds available to outfit and supply the laboratory.
Second, you have to overcome the mentality that work in the lab serves to prove what was said in the lecture is correct. If you are using the lab as part of the learning process, you have to create experiments that don’t prove the lecture correct but show how to use the lecture or perhaps even develop what is covered in the lecture.
Then, there is the problem with disposal. If we are to have a “green” chemistry lab, we have to be sensitive to the materials that we use. It would be nice if the raw materials that we use and produce were environmentally friendly but that is not always the case.
And finally, how do you relate societal issues such as “green” chemistry and recycling to the issues of a chemistry class. Some textbooks simply add a few lines of commentary to a textbook problem but never really address the issue.
In what I like to consider “a favorite problem”, I have created a scenario that addresses the environmental question and works within the context of the chemistry course.
My favorite problem
An article (written in the early 1980s) in the Journal of Chemical Education discussed the problem of toxic waste disposal in freshman chemistry laboratories.
The essence of the problem was “what to do with some Co2+ solution that was left after an analytical problem. Should the solution be diluted to a safe level and disposed of by pouring down the drain or shipped off as liquid waste; should it be precipitated and shipped off to a landfill as solid waste; or should it be recycled and used again during the next semester. The calculations for this problem are typical calculations for an introductory chemistry course and one can set up the calculations to be dependent on the size of the class. The only information that an instructor would be need would be the cost of the original raw materials as well the cost of shipping liquid and solid wastes.
In order for the students to do the calculations, they need to know how many students are in the course in question.
It probably bothers some people that I give the exact same question each time but the beauty of it is that since this number will vary from semester to semester, the answers to some of the questions will vary as well.
But, from an environmental standpoint, it always appears that that recycling is the best solution.
Assume that one of the experiments for this class requires that each student use 100.-mL of a 0.050 M Cr(NO3)3 solution. At the completion of the experiment, each student will have 200.-mL of a 0.025 M solution that must be dealt with in some manner.
- What is the accepted IUPAC name for the compound used to make the solution?
The EPA-designated toxic threshold concentration for total chromium is 5.0 ppm. Note that while only Cr(VI) is considered toxic and we are using Cr(III), the threshold limit is for total chromium. Solutions containing more than 5.0 mg/L of chromium in any form are considered toxic and cannot legally go down the drain. It is also illegal to dilute a solution to a concentration below the threshold limit and pour it down the drain.
- What concentration, in ppm of chromium, will each student end up with?
- Can students dispose of their solution by pouring it down the drain?
- What is the total volume of solution that this class would have to dispose of? Note assume that for the purposes of this test there are ten students enrolled in the course at the time of the experiment.
- To what volume must the combined solutions be diluted in order to reduce the concentration to acceptable limits?
- Would diluting the combined solutions be a reasonable solution?
- Can the combined solutions be disposed of by pouring them down the drain?
An acceptable and legal alternative to dilution would be to send the solution to an EPA-approved landfill.
- Assuming the cost of packing, transporting, and disposing of these solutions is $6.08/L, what would it cost to send this class’ chromium waste to such a landfill?
A third alternative would be to precipitate the Cr(III) as the hydroxide.
- What amount of solid NaOH is needed to precipitate the Cr3+ as the hydroxide?
- How much chromium (III) hydroxide is made?
- Assuming that the total amount of solid waste can fit into a 500-mL beaker, what will it cost to ship this waste? (Use the same shipping costs as before.)
Care must be taken to assure that the chromium (III) hydroxide does not go back into solution as the tetrahydroxochormate(III) complex as indicated in the following reaction, Cr(OH)3(s) + OH–(aq)w Cr(OH)4–(aq) Kf = .4.
- If the complex cannot be more that 5 mg/L (the legal limit for chromium in a solution), what pH must the solution be in order to insure that the precipitate does not dissolve and from the complex
- At this pH, what is the molar concentration of Cr3+ in the solution (Ksp for Cr(OH)3 = 6.7 x 10-31)?
- Using current catalog information found on the worldwide web, what does 500 g of Cr(NO3)3 cost? (Don’t forget to include the reference you obtained.)
- What would it cost to prepare the necessary amount of Cr(NO3)3 solution for next year’s class, assuming the size of the class remains constant.
- What does 500 mL of hydrochloric acid (stock solution, 13.6 M) cost?
- What would it cost to prepare the necessary amount of chromium (III) solution by dissolving the leftover Cr(OH)3 from this year’s experiment in hydrochloric acid?
- Of the various alternatives presented in this problem (dilution and disposal into the public water system, transportation of the liquid waste to an authorized landfill, transportation of the solid waste to an authorized landfill, recycling of waste), which is the most logical? Why?