TOWSON CHEM 104 - Toxicity of Copper to an Aquatic Plant

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IntroductionProcedureQuestionsChemistry 104 Fall, 2003Toxicity of Copper to an Aquatic PlantIntroductionOne of the most important functions of Environmental Toxicology is to determine whet-her a particular exposure causes harm to an organism. Humans, being animals, often aremost concerned with toxicity to animal life (including ourselves!), but toxicity to plants isalso of major concern. In the environment, plants are producer organisms—that is, theyare capable of “making their own food” from simple non-living materials (CO2, H2O,NO3-, etc.). (The fancy way to say this is that they are at the first or lowest trophic level.)They do this using energy from the sun in the process of photosynthesis. They generallydo not need to consume other organisms, like animals do. Because of this, they form thebase of virtually every food chain or food web in the environment. Any set of conditions which adversely affects plant life will therefore also adversely af-fect the animals which are the consumers of those plants (which puts animals at highertrophic levels). This is true whether the consumer organisms directly consume the plants(making them herbivores), or consume the animals that consume plants (making themcarnivores). Even if a chemical does not directly kill a plant species, decreases in thegrowth and/or reproduction of the plant can lead to a decrease in biomass, and this canhave a great impact on herbivorous and carnivorous species at higher trophic levels inthat ecosystem (since they will have less to eat, fewer places to live and hide, etc.).In this experiment, we will investigate the toxicity of copper to an aquatic plant common-ly known as duckweed (Lemna minor). Duckweed is a convenient organism to use intoxicity tests because it requires very simple growing conditions. It is a floating plant andso does not require any soil or sediment in which to root. Also, because it is aquatic, it isin intimate contact with the solution containing the chemical of interest and therefore isvery quick to respond to changes in its environment.In general, many properties can be used to judge toxicity. The death of an organism is anobvious measure of toxicity, but many other measures (such as growth, reproduction, tu-mor formation, hormone levels, etc.) can also be used as the endpoint of a toxicity test(i.e., that property which is observed during the test). In this experiment, we will use thegrowth of the plants as our endpoint. Duckweed is a very small plant (each leaf or frondis only 2-3 mm long), so several plants can conveniently be placed in a small container(like a beaker) for the experiment. You will count the original number of fronds used atthe beginning of the experiment and then count the fronds at the end of the one-week ex-posure period to determine the growth (either positive or negative) that has occurred.Almost all scientific experiments require the use of a control. The control is a treatmentin which the factor being investigated is not present. In this case, several of our experi-mental setups will not contain copper, so we can observe the growth of the duckweedunder optimum conditions (i.e., in the absence of copper). Toxicity to the plants will thenbe determined by comparing the growth of samples containing copper to the growth ofthe control plants that had no copper exposure. All of the solutions used are nutrient me-dia, which contain dissolved materials (Na+, Mg2+, SO42-, NO3-, etc.) essential to thegrowth of the duckweed.During this experiment, we will also investigate the effect of pH on the growth rate of theduckweed. As you have seen in earlier experiments, pH has a noticeable effect on theamount of “free” copper ions that are present in a solution. (Remember that it is this“free” form which generally has the most effect on living organisms.) At low pH, mostof the copper is in the “free” form, but as the pH increases, copper forms several com-plexes with OH- and the amount of “free” copper decreases dramatically.ProcedureWeek One:Each lab group (pair or three) will be assigned three specific experiments to prepare.Each experiment should be prepared in duplicate by your group (two separate beakers foreach experiment). The experiments are listed in the table below. Experiment pH Conc. of copper (ppm) 5A 5 0 5B 5 1 5C 5 10 5D 5 20 7A 7 0 7B 7 1 7C 7 10 7D 7 20 9A 9 0 9B 9 1 9C 9 10 9D 9 201. Obtain 100 mL of the appropriate copper solution for each of the experiments that youare to prepare.2. Using the instructions below, calibrate a pH probe using solutions of pH 4 and 7. 3. Adjust the pH of the copper solution to within 0.1 pH units of the specified pH foryour experiment. Do this by adding either 0.01 M NaOH (to raise the pH) or 0.01 M HCl(to lower the pH) dropwise, mixing well, and then measuring the pH of the resultingmixture. If you overshoot the pH you are trying to attain, use the other solution to bringit to the proper value. Record the exact pH to which you adjusted each solution.4. For each of your experimental solutions, place 25 mL of the pH-adjusted copper solu-tion into each of two 50-mL beakers (for a total of six beakers, two each for the three ex-periments). Label each beaker clearly with the experiment number (e.g., 5A) and someidentification of your group (initials, etc.).5. Place 15 fronds (not 15 individual plants!) of duckweed into each of the beakers, cov-er each with Parafilm® and place the beakers where instructed. These will be kept forone week in a well-lighted area to encourage growth.6. Place the remaining amount of each solution in separate 100-mL beakers, then add 1mL of 5 M NaNO3 to each and mix well. Using the copper electrodes set up in lab, ob-tain and record a reading in mV, noting which electrode you used (A or B). Record themV readings in your notebook and on the master list available in lab.Week 2:Retrieve your beakers and count the number of fronds present in each. Record thesecounts in your notebook


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