Activity 24 Diluting the Problem

Learning Objective You will explore dilution as a disposal method for copper-containing solutions produced during circuit-board manufacturing. You will construct a serial dilution of 100,000 ppm used copper chloride solution and add ammonia to demonstrate that chemical tests can reveal concentrations lower than those visible to the human eye. You then use the serial dilution for comparison in determining the concentration of the copper chloride waste from Activity 23, “Producing Circuit Boards.” This slide is optional. Use for online make-up work.

SAFETY Wear protective eyewear during the activity. Solutions used in this activity, such as ammonia and the copper chloride waste solution, are toxic and corrosive. Avoid contact with skin and eyes. Wash with water for 2–3 minutes any area that comes in contact with the solution. If contact with an eye occurs, rinse eye for 15–30 minutes, and consult a doctor.

Disposal It is important that you dispose of the liquid waste generated in this activity and others in the unit in accordance with your local regulations. It contains copper, a regulated heavy metal, in concentrations of 1–100,000 parts per million (ppm).

Introducing the copper chloride waste problem. Recall the copper chloride waste solution produced in Activity 23. The main components of the solution are water and copper-containing compounds formed when the copper-etching solution chemically reacted with the copper metal. Once the circuit board is etched, the copper chloride solution is left over, and manufacturers must decide how to handle the resulting waste.

TS: What should we do with our used copper chloride solution? Pouring the solution down the drain and delivering the solution to a waste-collection site is an option, however it is not always safe. The harm to animals and humans, when too much copper is released into the water systems, is a health hazard. Like other toxic substances, the disposal of copper is regulated by laws that set limits on what wastes can be released into the environment. While living things need copper in small amounts, in larger amounts copper can be toxic and is a problem too big to ignore.

Important In most places, the concentration of copper in wastewater cannot exceed 1–10 parts per million (ppm). Parts per million (ppm)is a way to measure the concentration of one component compared to the amount of the whole mixture.

in this activity You will determine the concentration of copper chloride in the used copper chloride solution in parts per million (ppm). This will help you to determine how much water it would take to dispose of the waste according to legal regulations. Many units can communicate concentration, but parts per-million is often used when talking about substances for which a very small amount in a large volume is significant because the solution can still be toxic.

Discuss what is meant when concentration is expressed in parts per million. You may be familiar with concentrations expressed in percentages, such as the 5% alcohol solution used in this activity. Parts per million is a measurement that describes the amount, or concentration, of one substance in a mixture, as in one part copper in a solution of a million parts. In this activity you will determine the concentration of copper in a rinse-water solution. Show a container of hydrogen peroxide or rubbing alcohol that shows the percent composition.

Let’s look at the prepared solution. Now, let’s pour half the solution into a second cup. What is the concentration of the solution in the second cup—more, less, or the same as the first cup? The answer is … that it is the same. Next add water to the second cup, and repeat the question, What is the concentration of the solution in the second cup—more, less, or the same as the first cup? This time, the answer is… less because when more water was added it lowered the amount of food coloring in water. This is a more dilute solution.

Adding more water, and making the amount of red food coloring in the cup dilutes the solution. The solution in the first cup is more concentrated, while the solution in the second cup is more dilute. Generally, if a solution is colored—as with food coloring or tea—the color becomes less visible as it becomes more dilute.

How could you make the solution in the second cup (or the first cup) more dilute? You may suggest adding more water, or removing some of the food coloring. Both of these methods decrease the amount of food coloring per volume of water, or concentration. Understanding the concept of concentration will help you as you construct a serial dilution and determine how many parts per million of copper are in the waste solution from Activity 23,“Producing Circuit Boards.”

demonstrating parts per million A video of CO2 in our atmosphere over time in a similar demonstration, in reverse. To demonstrate parts per million show students a plastic cup containing red food coloring and water.

Let’s Read, page b-60

perform a serial dilution of a copper chloride solution that has a known concentration. A serial dilution is a way to approximate concentration of a substance. You will take a substance of known concentration (100,000 ppm) and dilute it. Then you will compare this to the concentration of the used copper chloride from Activity 23, “Producing Circuit Boards,” to determine its concentration.

proper use of the dropper bottles When SEPUP dropper bottles are held upside down and vertically, the bottles produce drops of consistent size, allowing for reproducible measurements. This is especially important when constructing a serial dilution, a tool that will assist you in determining concentration. Approximately 14-20 drops from the SEPUP dropper bottles equals 1 mL. as shown on Science Skills Transparency 2,

CAution Cup 8 will contain water only, this is to be used as a control for comparison purposes. Make sure you understand the importance of cleaning the dropper in between cups so as not to unnecessarily add additional copper to any of the cups in the serial dilution.

As the students work on the investigation, circulate around the classroom. Monitor and assist where needed, but encourage each pair of students to work as independently as possible. This activity requires students to read a scientific procedure. Encourage students to be independent as they read and complete the procedure. If you o bserve they need support, model the strategies on Literacy Transparency 3, “Reading Scientific Procedures”. Learning and practicing these skills will help students become more independent in laboratory activities and become stronger readers.

PART B: estimate the concentration of copper in the used copper chloride solution. In Part B of the Procedure you will use the serial dilution you created in Part A with the solution of known concentration to estimate the unknown concentration of copper in the used copper chloride solution, from Activity 23, “Producing Circuit Boards.” You will add ammonia to detect the presence of copper ions at a concentration greater than 100 ppm.

When ammonia is mixed with the copper containing solutions, the ammonia initially forms a precipitate of milky-green copper hydroxide. As more ammonia is added, the precipitate dissolves and the solution turns blue, indicating the presence of the copper-ammonia ion. Seeing if this complex chemical reaction occurs is a more reliable test for detecting unsafe levels of copper than judging solution color alone. If in Procedure Step 10 adding 5 drops of ammonia to Cup 1 produces only a milky-green precipitate, increase the number of drops one at a time until a blue color is obtained. If it takes more than 10 drops to produce the blue color, you should replace the ammonia with fresh ammonia.

Constructing the serial dilution while using the ammonia indictor allows students to determine the concentration of the copper chloride waste. Typically students’ results after the serial dilution lead them to estimate the concentration of copper chloride in the waste solution to be approximately 50,000 ppm through 100,000 ppm.

Clean Up Instructions After you have completed the lab, you should pour the waste solutions into the waste container provided and use your droppers to transfer any remaining liquid into the waste container as well. To ensure that no copper-containing wastes are washed down the drain, use paper towels to wipe off remaining copper-containing solutions from trays and equipment before washing all equipment.

Share the observations you’ve recorded in your tables. calculate the copper concentration in the used copper chloride solutions in parts per million and as a fraction. Share the observations you’ve recorded in your tables. The concentration of the solution in Cup 1 is 100,000 ppm as shown on the bottle. Next 100,000 ppm is equivalent to the fraction 100,000/1,000,000. Write this value in the column titled “As a fraction.” Show This fraction can be reduced, or simplified, to 1/10 Write this value in the column titled “As a simplified fraction” on Student Sheet 24.1, “Determining Concentration.” 1/10 is the same as one part in 10. Display Transparency 24.1, “Diluting and Testing Used Copper Chloride,” and point out

The concentration in Cup 2 is 1/100, or one part in 100. Each cup was diluted with nine drops, or parts, water to one drop, or part, solution. This means that each cup was 1/10 as concentrated as the cup before it. To determine the concentration in Cup 2 the denominator is increased by a factor of 10; do this by multiplying the denominator 10 (the concentration in Cup 1) by 10. The concentration in Cup 2 is 1/100, or one part in 100. You may wish to show Transparency 24.1, “Diluting and Testing Copper Chloride,” to review the Procedure. Use optional Student Sheet 24.3, “Diluting Used Copper Chloride Solution to 1 PPM Copper,” to demonstrate this concentration.

The class discusses dilution as a treatment for the used copper chloride. In this activity you considered one way to treat the used copper chloride waste from Activity 23, “Producing Circuit Boards” —dilution. In this, and in the next series of activities you will explore three more methods of dealing with the waste — incineration, precipitation with solids, and precipitation with liquids. Project Transparency 24.2, “Treating Waste Concept Map.”

As a way to keep track of what you are learning about each method, and as a tool to help you with your final decision about how to treat the waste, you will record information about each method on Student Sheet 24.2, “Treating Waste.” Let us now, fill out the “Dilution” column to summarize the treatment method.

Analysis and Conclusions Not over yet… Analysis and Conclusions

1. Which contains a more dilute solution, Cup 1 or Cup 2? Explain. The solution in Cup 2 is less concentrated because it is the 100,000 ppm copper chloride solution from Cup 1 with water added to dilute it.

2. Was there copper in Cups 1-6 2. Was there copper in Cups 1-6? How did you know it was or was not there? Explain, using evidence from this activity. Understand that toxic waste is not always visible in a solution when in low concentrations. This creates the need for a test like the ammonia test. When first observing the color of the solutions, it appeared that cups 1 and 2 were the only ones that had copper. Then, when the ammonia indicator was added, it showed that copper was present in cups 1 ,2, 3, and 4. make sure that students have come to the understanding that toxic waste is not always visible in a solution when in low concentrations. This creates the need for a test like the ammonia test.

3.What is the concentration of copper in the class’s used copper chloride solution from Activity 23, “Producing Circuit Boards”? Use the evidence from the activity to support your answer. The concentration of the copper is about 100,000 ppm. I know this because the color of Cup 7 was closest to the color of Cup 1 when I added the ammonia indictor, and Cup 1 has aconcentration of 100,000 ppm.

4. How many liters of water would you need to dilute 1 liter of used copper chloride solution from 100,000 ppm of copper to less than 1 ppm? The quantities of water described in Analysis Questions 4 and 5 are in terms of the water needs for a family of four. About 1,000 liters (250 gallons) of water per day Or about 30,000 liters (7,500 gallons) of water per month. That is equivalent to 15,000 2-liter bottles! To dilute one liter—just one liter—of 100,000 ppm toxic waste to 1 ppm It would take 99,999 liters of water to dilute 1 liter of the copper solution to less than 1 ppm. It may help students understand the quantities of water described in Analysis Questions 4 and 5 if you express them in terms of the water needs for a family of four.

5. Based on your answers to Analysis Questions 3 and 4, how many liters of water would it take to dilute your waste from Activity 23, “Producing Circuit Boards,” to 1 ppm, an acceptable concentration to pour down the drain? According to the calculations in Analysis Questions 3 and 4, it would take 99,999 liters of water to dilute 1 liter of the copper waste solution to1ppm.

The quantities of toxic copper chloride waste produced by various industrial processes are much greater than one liter— we’re talking about thousands and millions of liters. The amount of water needed to dilute only 100 liters of 100,000 ppm used copper chloride solution to a safe level of less than 1 ppm is the same as a family’s water needs for over 27 years! That’s a lot of water that a manufacturer would need in a short amount of time.

6. Is dilution an acceptable way to dispose of the used copper chloride solution? It would take a lot of water to dilute a very little bit of waste. So it is not really feasible for a company that is producing circuit boards, unless they are willing to incorporate large volumes of water, which does not seem good for the environment either. Also, even though the amount of copper is smaller, relative to the amount of water when diluted, the copper will end up down line in the water system and may accumulate in another part, harming an ecosystem.

In Reality… One important point for you to keep in mind is that, regardless of the concentration, the total amount of toxic waste released into an aquatic environment (stream, river, lake, or ocean) remains the same for heavy metal substances such as copper, mercury, and lead. Thus, even if the concentrations of the metals are minute when they are disposed of by dilution, they accumulate in ecosystems, eventually threatening humans and animals. Because the major goal of pollution prevention is to reduce the amount of toxic materials released into the environment, dilution is not usually an effective option for wastes that are particularly hazardous.

Today, it is illegal to dispose of copper solutions above a certain concentration and volume by pouring them into drains, sewers, or bodies of water. In most areas, local water districts establish and enforce these standards, which limit the concentration of copper that can be deposited into the sewers from one source in a single day to 1–10 ppm. Many municipal wastewater treatment plants are not equipped to reduce the copper concentration low enough to be considered safe.