In this lab, students investigate the use of milk of magnesia poultice to remove copper stains on masonry in copper architecture. They use chalk as the model for masonry, copper(II) chloride solution as a model for soluble copper and a freshly prepared slurry of copper phosphate as a model for a hard stain of copper on masonry. Through a series of investigations students have the opportunity to connect chemistry topics with real-world applications, such as environmental hazards, engineering practices of copper architecture, corrosion control, and structural protection.
This lesson will help prepare your students to meet the performance expectations in the following standards:
- HS-ETS1-2: Design a solution to a complex real-world problem by breaking it down into smaller, more manageable problems that can be solved through engineering.
- HS-ETS1-4: Systems and System Models:Models (e.g., physical, mathematical, computer models) can be used to simulate systems and interactions—including energy, matter, and information flows— within and between systems at different scales.
- HS-PS1-2: Construct and revise an explanation for the outcome of a simple chemical reaction based on the outermost electron states of atoms, trends in the periodic table, and knowledge of the patterns of chemical properties.
- HS-ETS1-3: Evaluate a solution to a complex real-world problem based on prioritized criteria and trade-offs that account for a range of constraints, including cost, safety, reliability, and aesthetics as well as possible social, cultural, and environmental impacts.
- HS-PS2-6: Communicate scientific and technical information about why the molecular-level structure is important in the functioning of designed materials.
- Scientific and Engineering Practices:
- Constructing Explanations and Designing Solutions
- Engaging in Argument from Evidence
- Obtaining, Evaluating, and Communicating Information
By the end of this lab, students should be able to
- Analyze and delineate the factors of building design, building age, type of copper architecture, and quantity of copper that contribute to the development of green staining of copper on masonry.
- Design the approach methodology to develop a stain remover for copper stains on masonry.
- Describe the chemical and physical properties of milk of magnesia which make it a suitable candidate for copper stain removal from masonry.
- Justify the selection of dustless black board chalk as a model to simulate masonry in lab investigations.
- Predict the outcomes of the chemical reaction between copper(II) and common cleaning agents based on the general periodic trends and pH of the medium and then revise an explanation after direct experimental observations of the reaction between copper(II) chloride and the common cleaning agents.
- Demonstrate the poultice method to remove copper stain from chalk to about 98% efficiency from very small to big masonry structures and from plain surfaces to complex designs.
- Determine the cost of the milk of magnesia poultice for a 1 m2 area.
- Communicate the eco-friendliness of the developed poultice and the poultice technique.
This lab supports students’ understanding of
- Solubility Rules
- Chemical Change
- Redox reactions
- Corrosion Protection Engineering
- Structural Cleaning and Maintenance
Teacher Preparation: 2 hours initially; 30-40 minutes when used again
- Engage: 20 minutes
- Explore: 30 minutes
- Explain: 40 minutes
- Elaborate: 30 minutes
- Evaluate: 40 - 60 minutes
Materials (per group)
- 1 roll of white masking tape
- 24, 10 ml glass bottles or test tubes
- 24, 2 ml droppers
- 6 cotton swabs
- 2 large (16 oz) cups
- 2 small cups without lid
- 1 small cup with lid
- 24 pieces of extruded dustless white chalk from Crayola
- 0.05M and 1M Copper (II) chloride solution
- 1 12 oz bottle of Milk of Magnesia
- 2 pieces of facial tissue
- 1 spray bottle with water
- 2 small hand towels or white cotton rags
- Paper towel
- 1 magnifying glass
- 5 white Styrofoam plates
- 1 pavestone concrete block
- 1 floor and wall porcelain tiles (10 cm x 10 cm)
- 1 pavestone edge stone red (30 cm x 30 cm)
- 25ml of each cleaning supply:
- 10% Ammonium sulfate solution
- 4% Ammonium oxalate solution in water
- 25 ml
of the following solutions need to be prepared:
- 20% Sodium metabisulfite solution prepared by dissolving 20 g of sodium metabisulfite in water
- Saturated sodium carbonate solution 100 ml made by dissolving 3.4 grams of sodium carbonate in 100 ml water
- Saturated solution of baking soda
- 2% solution of iodate free sodium chloride (Morton salt) in 20% sodium metabisulfite solution
- Safety goggles, gloves, and apron needed for doing this lab.
- All chemical compounds including copper metal are not to be tasted.
- Students should wash their hands thoroughly before leaving the lab.
- Brick, edge stone, etc. are quite heavy; they should be arranged on a stable and even platform or table. Sufficient working space and movement space need to be allowed.
- When students complete the lab, instruct them how to tidy up their place of work
- Remind students to dump the copper solutions in the respective bottles.
- Recycling copper solution: Keep two labeled bottles in a place in the lab for students to dump the copper solutions; one bottle is for copper phosphate and the other is for all other copper solutions. You can acidify the non-phosphate solutions with concentrated hydrochloric acid and save it for use in single replacement labs for your curriculum. The phosphate solutions can be used to prepare stained specimens for this lab.
- Engage: Use the attached PowerPoint Presentation to introduce copper architecture and the current heightened growth in copper architecture.
- Highlight the merits of copper, ease of maintenance, beauty, and sustainability (Slide 2).
- Make sure to explain that there is one problem with copper which results in the green stains caused by copper on masonry structures (Slide 3).
- Bring to their attention the reasons why copper stains on masonry are increasingly noticed now (Slide 4).
- Use the stain removal triad (Figure 1, Slide 5) to bring to focus the three entities simultaneously involved in stain removal. The entities are: Substrate, Stain, and Cleaning agent. Emphasize that the three entities operate in an integrated manner like a system of parameters. See Note 1.
- Indicate some proverbs on stains and their relevance to the context of copper stains (Slide 6) and show to them how copper stains the masonry (Slide 7).
- Now ask them to consolidate in a chart what they consider as the requisites for a good cleaning agent (Slide 8). See Note 2.
- Discuss that the poultice technique is the popular technique for stain removal on masonry, especially for hard stains, and it is highly suited for vertical structures. Let them know that poultice technique is not anything new; it has been known historically and traditionally used in medicine as well as beauty treatments (Slide 9). Use the videos in the slides to demonstrate the poultice.
- A poultice generally consists of an adsorbent material to which a suitable cleaning/curing agent is added. However, bring to their attention that people generally decide the components of the poultice based on the nature of the stain and they take care to see that the poultice does not cause damage to the substrate.
- Indicate that as of now, there are a few clays based poultice formulations available commercially for copper stains (Slide 10)
- Their cleaning ingredients (ammonium compounds) can cause damage to the masonry in the long run and have some occupational hazards associated with them. They are also costly. See Note 2.
- Inform students that there is a handicap in having other cleaning agents substituted for ammonium compounds in the poultice because it is said that common cleaning agents do not remove copper stains; instead, they aggravate copper stains and make it more prominent and resistant. Share with them that seeing what happens when copper ions meet conventional cleaning agents would be able to confirm veracity of this statement (see for instance, a discussion here) plus it will provide insights for solving the problem in copper stain removal.
- Share about how chemistry can solve real world problems (Slide 10). You may want to do some brainstorming on how very big consumer products in the market are based on simple chemistry and there is always a scope for easy, more effective, and less expensive alternatives for most of the products we have in stores.
- Discuss with students the need for models to investigate complex systems and interactively discuss the models chosen for the substrate (extruded calcium carbonate chalk for masonry) and the soluble stain (copper (II) chloride), and insoluble stain (freshly prepared copper phosphate slurry), (Slide 11).
- Note 1: If time permits as per the need of the class, you may want to engage in students’ encounters with stains. Depending on the needs of the class, you may also want to refer to this video on absorption and adsorption.
- Note 2: While talking about cost, you may want to inform that any cleaner or poultice that would arrive into the market for removing copper stains is likely to be very costly. This would be so because owners of copper architecture (who have made massive investment in copper architecture) may not mind the cost at all as they need an immediate solution.
- Part 1: Understanding the Problem: Why conventional cleaning agents fail to clean copper stains and what message do they convey by their behavior? (Slide 12)
- Remind students that conventional cleaning agents are not able to remove copper stains from masonry. Share with them that seeing what happens when copper ions meet conventional cleaning agents would be able to provide wealth of information on the chemistry of copper.
- Review with students that in acidic media and in the presence of complexing substances, copper will remain in solution. Let them recall that metal cations tend to precipitate if they are not in acidic environment.
- Students now analyze the given data (Slide 13) and propose hypothesis (Slide 14) as to whether the cleaning agent each will form a precipitate or not. Students should complete this on the provided student sheet. The data they are given in the table is: the names of the conventional cleaning agents, their pHs, the solubility product of possible compounds formed in such pHs. They will identify the anion and will predict if there will be formation of a precipitate or not. They also will predict if the masonry will be affected by the reagent and how. (AP level students will be able to identify the anions but regular students might need to be provided with the anions).
- Next, they will conduct a quick lab (Quick Lab1) and validate their hypothesis (Slide 15), refer to the Student Handout. The Quick Lab results are in Slides 16 and 17 for your reference.
- Use Slide 18 for discussing noteworthy observations. let students have a close look at how cuprammonium compounds get absorbed on chalk and enable them to figure out that ammonia and ammonium compounds are not well suited for copper stain removal as they generate a differently colored more percolating copper. Their toxicity is also major concern.
- Enable all students to answer the Post-Lab Inquiry I Questions (Slide 20). Whereas most of their predictions and hypotheses, which students made will prove correct, they may encounter anomalous behavior with alkaline tartrate and sodium thiosulfate contrary to expectations – which students might not take a serious note of - that is okay because they are not expected to know these reactions and such reactions are not aligned to their curriculum as well. However, some students with curious minds would query about these anomalous behaviors or you may want to engage some advanced learners. Draw their attention (preferably if they are in the AP class) to the following resources understand the anomalous behavior.
- Help students understand that chalk can have several catalytic sites that might trigger novel and unconventional reaction pathways.
- Answers for the Post-Lab Inquiry I for Quick Lab 1 are presented in Slide 21 for your reference.
- Part 2: Exploring if Milk of Magnesia can be a poultice for copper stain removal
- To begin, have students recall how copper stains are produced. Then divert their attention to the different types of acnes that grow on the face and compare it with the copper stains that have developed on masonry and chalk (Slide 22).
- Indicate to them that these are all reactive eruptions on the skin surface involving the subsurface. Copper stain seems like it is like acnes in being reactive eruptions on masonry that have different shapes, structures, colors, coarseness, and strength.
- Tell students that one of the interesting discoveries in the poultice method for acne removal is milk of magnesia, which also has multiple other uses (Slides 22 and 23).
- Given these everyday applications of milk of magnesia, ask the students if they see the possibility of applying milk of magnesia as a poultice for copper stain removal (Slide 20). There will be generally, a unified approbation and enthuse their investigating this aspect of milk of magnesia. As per the need of the class, you may want to use this video or this video to brainstorm how milk of magnesia demonstrates the acid-base chemistry, chemical equilibrium, and chemical kinetics.
- With this enthusiasm, facilitate Quick Lab investigations 2 to 4. In Quick Lab 2, students will study the reaction between lime water and milk of magnesia with copper(II) chloride solution (0.05 M). In Quick Lab 3, they will observe the effect of lime water on milk of magnesia. In Quick Lab 4, they will study the absorption of copper(II) chloride on chalk without and with milk of magnesia coat. Information, answers and expected results for these labs are included in the PowerPoint. Also, they are each outlined in the Student Handout.
- Explain: Students answer the Post-Lab Inquiry Questions for Part I (Slide 19) and Part II (Slide 26, 28, and 30) of the exploration activities. You can appropriately assist them in this activity. Slides 30 to 35 provide questions and answers in a sequence.
- Extend: (Optional) In this section, students extend the application of milk of magnesia poultice to the removal of hardened copper stain on chalk as well as other real-world cases of copper stained masonry. Herein, they employ the milk of magnesia poultice to remove copper stains from small to big structures.
- Small size structures: Chalk, Small unglazed porcelain tiles
- Medium size structures: Ceramic Tile
- Medium size uneven surfaces: Backside of ceramic tiles
- Big Structures: Bricks
- Note that a student handout is not provided for this optional section. Refer to Slides 36 to 42 provide step by step procedures for carrying out the optional Extend activity. At the end of the investigation, students will answer data collection questions (Slide 43). Answers to the data collection questions are presented in Slide 44.
- Preparing Copper-Stained Substrates:
- Place copper phosphate slurry (a mixture copper chloride and sodium phosphate solutions) and using the dropper place the slurry on the top of the surface of the substrate. Allow to be absorbed and you can repeat the process when until you see sufficiently intense adherent stain. Prepare these specimens at least two days ahead.
- Evaluate: In this section, students will determine the cost per m2 of the poultice technique for the removal of light, moderate, and hard stains of copper from masonry (Slide 45). A sample answer to this question is provided in Slide 46. They will also prepare a flyer on the technique communicating the cost-effectiveness and eco-friendliness of the developed poultice and the poultice technique, which includes the method of preparing the recipe for public use (Slide 47). The electronic platform for the school or the teacher can be used for this publication activity. A sample flyer has been provided.
- National Mall and Memorial Parks in Washington DC has copper staining in many of the historical structures. Contact the Public Affairs Division at National Parks Services at the National Mall Washington DC and share with them the findings of your research with milk of magnesia in removing copper stains. Especially they undertake a lot of restoration projects at the memorial parks and your learning might be found useful by them. You may also be interested in applying for summer jobs and volunteering opportunities to work at the National Mall and Memorial Parks. To contact the Public Affairs, follow this link. (This question is presented in Slide 48).
- L. Shao, Y. Zhou, J.F. Chen, W. Wu, and S.C. Lu (2005). Buffer behavior of brucite in removing copper from acidic solution, Minerals Engineering, 18 (6)639–641; http://www.sciencedirect.com/science/article/pii/S0892687504002456
- Henley’s Twentieth Century Formulas, Recipes, and Processes; https://archive.org/stream/henleystwentieth00hiscrich/henleystwentieth00hiscrich_djvu.txt
- Christine C. Gaylarde, Peter M. Gaylarde, and Iwona B. Beech (2008). Deterioration of limestone structures associated with copper staining, Journal of Archeological Science, 62(2)179–185
- Kenneth S. Warren (1957). Differential Toxicity of Ammonium Salts.
- Debarati Nandy, Soumen Banerjee, and S.K. Bhattacharjee (2016). Effect of Hydration in Magnesia Based Refractories for Lime Kiln Industry; http://www.meconlimited.co.in/Writereaddata/pub/MECON%20Technical%20Paper.%20IREFCON.%2016.pdf