Materials Included In Kit

Additional Materials Required

Prelab Preparation

1. The Eriochrome black T•sodium chloride indicator may be prepared ahead of time. Mix 0.15 g EBT with 29 g of sodium chloride. Mix thoroughly using a mortar and pestle until homogenous.
2. For hard water unknowns, samples may include tap water if it is in an acceptable hardness range. This can be easily tested using Water Hardness Testing Paper, available from Flinn Scientific (Catalog No. AB1161).
3. Alternatively, simulated hard water samples may be prepared by dissolving calcium nitrate or calcium chloride in water to approximate the amount of calcium ions present in 120–450 ppm hard water.
4. To prepare “hard water” corresponding to 180-ppm equivalent of CaCO₃, dissolve 425 mg of calcium nitrate tetrahydrate [Ca(NO₃)₂•4H₂O] in 1 L of distilled or deionized water.

Safety Precautions

The EDTA solution and the buffer solution are strongly basic and are corrosive to eyes, skin and other body tissues. They are toxic by ingestion. Avoid all body tissue contact. Wear chemical splash goggles and chemical-resistant gloves and apron. Remind students to wash hands thoroughly with soap and water before leaving the laboratory. Please review current Safety Data Sheets for additional safety, handling and disposal information.

Disposal

Please consult your current Flinn Scientific Catalog/Reference Manual for general guidelines and specific procedures, and review all federal, state and local regulations that may apply, before proceeding. The EDTA solution, the buffer solution, water solution, and the EBT sodium chloride indicator may be disposed of according to Flinn Suggested Disposal Method #26b. The ion exchange resin may be regenerated as described below and stored for reuse.

Regeneration of the Ion Exchange Resin

1. To regenerate the resin, combine all used resin into one 500-mL beaker.
2. Add 3 M HCl to the beaker until the resin is covered.
3. Allow the acid–resin mixture to stand 5 minutes.
4. Slowly add 250 mL of DI water, stir, and let the mixture rest until the resin settles to the bottom of the beaker.
5. Carefully decant the solution from the resin. Only a few mL of solution should remain with the resin when finished.
6. Add 200 mL of distilled water to wash the resin. Decant as detailed in step 5.
7. Repeat washing and decanting as necessary. Note: To know when the resin has been properly washed, test the resin solution with blue litmus paper. If the paper turns red, more washing is necessary until the blue litmus paper stays blue when immersed into the resin solution.

Lab Hints

• Enough materials are supplied for 15 laboratory groups.
• Practice this experiment ahead of time to determine the approximate hardness level if using tap water.
• Mark all water samples with a unique water identification number.
• Ensure that all student groups have determined a working plan for Parts A and B before beginning this activity.
• In the laboratory the students may be more successful given certain parameters. This also cuts down chemical use. In Prelab Question 3 it is mentioned that certain parameters may be given to the students. The resin amounts to soften can range from 0.5 g to a maximum of 4 g per 25-mL water sample. After the water sample has been treated by the resin and is being titrated, no more than 100 drops should be used for any titration. Make note in the data table that the amount was 100+ and move onto the next trial. This conserves the EDTA titrant.
• When titrating, it may be helpful to put a white piece of paper under the flask so the color of the solution is more obvious. The indicator change is gradual, going from pink through purple to a clear blue. The endpoint occurs at the point where no more pink is present. It is helpful to keep a flask handy containing a previous titration so that the color at the endpoint can be compared. Students who suffer from color blindness will have a difficult time recognizing the pink–purple–blue indicator transitions.

Teacher Tips

• Plan the prelab information session according to the knowledge level of the students and their familiarity with guided inquiry activities.
• Vary the unknown water samples for different lab groups. This helps the students build confidence in their own data and lab group and also teaches them to depend on the experimental process for answers.
• Discuss if a hardness reading of close to zero is optimal. Water is the universal solvent. With water hardness close to zero, there is often severe corrosion of pipes.
• Based on the volume and molarity of EDTA, the concentration of the ions that cause hard water can be determined. The dropper pipets used can be calibrated for drops per mL to determine the volume of EDTA added.
• Reference point: 1 g of resin will soften 12.8 mL of water with a hardness of ≈ 270 ppm.
• Investigate a local water report to obtain further information and as a standard to compare with test results.
• After the experiment, study the geography of the area to suggest why local or regional areas have the water hardness reported at http://www.water-research.net/images/hardwatermap1.jpg Note: This website gives away that hardness is calculated as calcium carbonate (accessed July 2009).
• Research the pros and cons of having water supplier softening the water versus individual home softeners. Discuss EPA suggested guidelines for taste of water. This information can be accessed at http://www.epa.gov.safewater/ccl/sodium.html#five (accessed July 2009). The EPA recommends maintaining the sodium content between 30 and 60 ppm.

Answers to Prelab Questions

1. Appliances usually have a suggested water hardness range for optimal operation. Explain why manufacturers include these guidelines in their appliance manuals.

   Manufacturers make guidelines so that the unit will operate without failure of heating elements, etc. Some manufacturers also give detergent and rinse agent recommendations based on water hardness.

2. Draw the Lewis dot formula of the EDTA^4– ion that exists in basic solution and indicate the nonbonding electron pairs that are used to bond with metal ions.
   {12836_PreLabAnswers_Figure_3}

   The nonbonding electron pairs on the two nitrogen atoms and one pair from each of the negative oxygen atoms bond with the metal ions.

3. EDTA titrations are usually carried out with a control to ensure the correct endpoint has been reached. What type of water sample should be used as an effective control in this experiment?

   The perfect control would be a sample the same as the unknown sample with only the hardness ions removed. Good substitutes for a control would be distilled or deionized water.

4. The purpose of this guided inquiry experiment is to determine the minimum (optimum) amount of resin needed to soften water. As observations and results are obtained, it will be necessary to make decisions regarding the next logical trial. Draw a flow chart (diagram) to show how decisions will be made after each resin treatment/titration result is obtained. Use quantitative terms for the first trial only, followed by qualitative terms, since results will not be known until the experiment is carried out. The teacher may discuss parameters of the experiment.
   {12836_PreLabAnswers_Figure_4}
5. Using the metal chloride salts provided in the Materials section, describe a qualitative test that could be used to identify which cations are removed by the resin during the water softening process. Write a procedure for this test.

   All correct answers should explain a logical process. One of the easiest is to use the already titrated samples and add a smaller amount of chemical to an individual sample to see if the blue color converts back to pink. (For example: Add calcium chloride to one flask, sodium chloride to another, and so forth.) The calcium chloride and the magnesium chloride will turn the blue titrated samples back to pink with a very small amount. The potassium chloride and sodium chloride will have no effect. Another possibility that students may use is to add a small amount of chemical to their water, soften it with the resin and then titrate to determine if the titration amounts change. This uses the water sample as a control and tests the water sample after the addition of a particular chemical to determine if there is an effect.

Sample Data

Water identification ___Sample 1___

Sample Flow Chart for Resin Amount Determination
As observations and experimental results occur, decisions regarding the next trial are necessary. Draw the flowchart of the decision making process before each decision is made. Note: This should have been completed with general terms during the Prelab Questions. Observations

Answers to Questions

1. How much resin would be needed to effectively soften one liter of the unknown hard water sample? Explain how this number was determined and show your calculations.

   Student results will vary based on the water hardness of their samples. Calculation should show the resin amount needed for their aliquot—from data and multiplied by the factor to determine an amount of 250 mL. For example: If 25 mL samples were used, the result would be multiplied by 10.

2. Discuss the strategy of finding how much resin was needed to effectively soften the water sample.

   The amount of resin needed to soften a water sample is determined by an EDTA titration. If the titration amount is very high, more resin is needed. If the titration amount is less than 5 drops, less resin is needed. Eventually the correct amount (range) of resin can be determined.

3. Did the strategy change depending on the previous trial result?

   Absolutely, the strategy would depend on the titration results—if the water is softened or if the number of drops of EDTA for the titration is high. Eventually there becomes a point where the water is just softened and resin is not wasted.

4. What ion(s) is the resin most likely removing from hard water? Explain based on the evidence from Part B.

   Calcium and magnesium are likely candidates because those samples reverted back to the color before the sample was titrated. Other options may include a procedure to add a little bit of a chemical to a water sample versus an identical sample without the additive, use the resin to soften it and then determine if the sample takes more titrant (EDTA), to reach the endpoint, concluding the hardness is the result of the added chemical.

References

“How Hard is Your Water?” Flinn ChemTopic™ Labs, Volume 22; Cesa, I., Editor; Flinn Scientific, Inc.: Batavia, IL (2006).

Introduction

Water softening is a type of water treatment used to remove certain cations from water and thus prevent hard-water deposits in water lines and in water-based household appliances (e.g., dishwashers, washing machines). Many households are affected by hard water—let’s investigate which ions cause hard water and explore one common method of removing these ions.

Concepts

• Water hardness
• Ion exchange
• Complex ions
• Titration

Background

Water from natural sources, including rivers, lakes, streams, ponds, reservoirs, springs and groundwater wells, may contain a number of dissolved substances. The amount and nature of these dissolved substances varies depending on the geography of the area and the journey the water has taken. As water travels through the ground or over the surface of the land, it can dissolve naturally occurring minerals and/or substances resulting from animal and human activity. As minerals dissolve in the water, the neutral compounds separate into their respective ions, both positively and negatively charged. The positively charged ions are called cations and the negatively charged ions are called anions. Specific minerals are said to give the characteristic of “hardness” to water. Actual water is not physically harder or softer but the expression “hard” water came about when people had a hard time using the water for cleaning. The presence of some mineral ions makes it difficult for soaps to lather and causes a “scum” to form. Compounds containing these ions are less soluble in warm water and tend to precipitate out of solution onto heating elements and in hot water lines. The precipitates, also called scale, are very hard to remove and reduce efficiency as water flow is impeded in pipes. Scale also acts as an insulator, so that heating elements with heavy scale buildup use more energy to heat the same amount of water. If the scale layer precipitating onto a heating element becomes thick enough, the element can actually overheat and fail. Studies of homes with hard water show that over time plumbing and water-based appliances need to be replaced much earlier than in households with softer water. The taste of even moderately hard water is often considered unpalatable.

There are many different ways to “soften” water. One of the most common ways to remove ions is by ion exchange. The ion exchange process typically uses a resin to replace some of the ions that cause hardness or scale with ions that do not. The resin used for water softening comes in the form of beads and consists of negatively charged groups covalently bonded to a water-insoluble organic polymer backbone. The negatively charged groups provide sites for interaction with the mineral ions in hard water.

Hardness is typically measured in terms of grains per gallon or milligrams per liter (also known as parts per million), and is classified by the U.S. Department of Interior and the Water Quality Association as follows:

When water has been softened, energy use, clogged pipes, and soap scum become less of a problem. Households using softened water also use less soap because the cleaning processes are not inhibited by dissolved ions. Not all tap water is hard, but in the United States about 85% of tap water can be described as having some degree of hardness. In some parts of the country, water is extremely hard.

The ions in water that cause hardness can be analyzed by titration with a complexing agent called EDTA. EDTA is an abbreviation for ethylenediaminetetraacetic acid, an organic compound having the formula C₁₀H₁₆N₂O₈. EDTA is called a complexing agent because it has the ability to bond to metal ions in solution, forming “complex ions.” The large EDTA molecule literally wraps itself around the metal ions in solution. EDTA is often used to complex ions that may be present in solutions to prevent them from causing other reactions. For example, adding EDTA to hard water will allow soap to lather without forming scum. The ability of EDTA to wrap itself around metal ions has other uses as well. EDTA is used clinically as an antidote in cases of poisoning by lead ions. The EDTA molecules tightly wrap around the lead ions in the stomach of the patient, and the lead passes harmlessly out of the body. The structural formula of EDTA is shown in Figure 1. Titration is the process of adding the exact amount of a chemical reagent that will react with substances dissolved in solution. By measuring and comparing the amount of the titrating agent, also called the titrant, needed to analyze different samples, it is possible to compare the amount of the substance in the samples. In this experiment, the dissolved substances will be the specific ions in hard water, and the titrating agent will be EDTA solution. An indicator will be added to determine the point at which all of the ions in solution have reacted with EDTA. The concentration of EDTA is constant, and the volume of EDTA required for the reaction will be measured dropwise.

In basic solution, the acidic hydrogens in EDTA are pulled away from the molecule. That leaves each of these four oxygen atoms with three nonbonding pairs of electrons and a negative charge. The two nitrogen atoms each have a nonbonding pair of electrons as well. The resulting EDTA^4– tetraanion can bond to metal ions in six places—the four negatively charged oxygen atoms and the two nitrogen atoms can each donate a pair of electrons to the metal ion. A basic buffer (pH = 10) is used to ensure that the EDTA is in the correct form to bond with metal ions. An indicator is used in complex-ion titrations to show when the titration is complete. Eriochrome Black T, called EBT, is blue in basic solution. However, when EBT forms complex ions with the metal ions primarily responsible for water hardness, the indicator is pink.

During the titration used in this experiment, the following processes will occur:

1. A water sample containing hardness ions is placed in a flask. Buffer solution, pH = 10, is added to maintain a basic pH and ensure that the EDTA will bond to the metal ions.
2. A small amount of EBT/NaCl indicator is added. The EBT immediately bonds with some of the free metal ions to give a pink complex.
3. EDTA is then added dropwise. Initially, the EDTA will complex with the free hardness ions present in solution. After all of the metal ions are complexed with either the EDTA or the EBT indicator, a “competition” occurs between the EDTA and the EBT for the metal ions. The EDTA forms stronger complexes with the metal ions than EBT does. EDTA will begin to pull the hardness ions away from EBT. As ions are removed from EBT, the indicator changes from the pink color of the EBT complex to the sky blue of EBT with no metal ion attached. This signals the endpoint of the titration.

Experiment Overview

Water samples with unknown hardness will be treated with ion exchange resin and analyzed by EDTA titration. The purpose of this guided-inquiry experiment is to systematically vary the amount of resin and determine the (minimum) optimum amount of resin needed to soften the water. In the second part of the experiment, a qualitative procedure will be developed and tested in order to identify the specific metal ion(s) that were removed by treatment with the resin.

Materials

Prelab Questions

1. Appliances usually have a suggested water hardness range for optimal operation. Explain why manufacturers include theseguidelines in their appliance manuals.
2. Draw the Lewis dot formula of the EDTA^4– ion that exists in basic solution and indicate the nonbonding electron pairs that are used to bond with metal ions.
3. EDTA titrations are usually carried out with a control to ensure the correct endpoint has been reached. What type of water sample should be used as an effective control in this experiment?
4. The purpose of this guided inquiry experiment is to determine the minimum (optimum) amount of resin needed to soften water. As observations and results are obtained, it will be necessary to make decisions regarding the next logical trial. Draw a flow chart (diagram) to show how decisions will be made after each resin treatment/titration result is obtained. Use quantitative terms for the first trial only, followed by qualitative terms, since results will not be known until the experiment is carried out. The teacher may discuss parameters of the experiment.
5. Using the metal chloride salts provided in the Materials section, describe a qualitative test that could be used to identify which cations are removed by the resin during the water softening process. Write a procedure for this test.

Safety Precautions

The EDTA solution and the buffer solution are strongly basic and are corrosive to eyes, skin and other body tissues. They are slightly toxic by ingestion. Avoid all body tissue contact. Wear chemical splash goggles and chemical-resistant gloves and apron. Thoroughly wash hands with soap and water before leaving the laboratory.

Procedure

Imquiry and Design

Part A. Determine the Amount of Resin Needed to Soften Water

1. Obtain 250 mL of the hard water to be analyzed in this experiment. Each sample trial (resin treatment and titration) will be carried out on a 25-mL water sample.
2. Using a graduated cylinder, measure and pour 25 mL of the hard water sample into a clean 100-mL beaker.

Resin Treatment

3. Add a measured amount of resin to the water sample for the first trial. Hint: The amount of resin used in the first trial should be between 0.5 g and 4 g.
4. Mix with a stirring rod or a magnetic stir bar and stir plate for a minimum of 5 minutes
5. Let the resin/water mixture rest until the resin settles to the bottom of the beaker.
6. Decant the treated water into a 50-mL Erlenmeyer titration flask.

Titrate a Control Sample

7. Add 25 mL of distilled or deionized water, the same sample size as step 3, to a separate Erlenmeyer flask.
8. Add 5 mL of pH 10 buffer solution to the water aliquot.
9. Add a very small amount—the tip of a microspatula—of EBT•NaCl solid indicator to the water sample.
10. Using a Beral-type pipet, add the EDTA solution dropwise to the flask while swirling. Count the number of drops being added.
11. When nearing the endpoint of the titration, the pink color of the indicator will begin to turn purple.
12. Continue adding EDTA drop-by-drop to the water sample until the EDTA turns the solution sky blue.
13. Record the number of drops of EDTA added on the Water Softening Worksheet. Retain the water sample to use as a color reference (standard).

Titrate the Treated Water Samples

14. Add 5 mL of pH 10 buffer solution to the decanted, treated water sample.
15. Add a very small amount (about the tip of a microspatula-full) of EBT•NaCl solid indicator to the water sample.
16. Using a graduated Beral pipet, add the EDTA solution dropwise to the flask while swirling. Count and record the exact number of drops required to give the indicator color change.
17. Use a clean Erlenmeyer flask for each trial—rinse the flask with distilled water before each new titration. Repeat steps 3–6 and 14–16 at least four more times to determine the optimum amount of resin needed to soften the hard water. Note: An acceptable definition of “softened” for the purposes of this experiment is that a treated water sample requires ≤5 drops of EDTA to reach the expected indicator endpoint (color change).

Part B. Determine Ions Removed During Ion Exchange

18. Using the metal chlorides described in the Materials list, determine which ions were removed from the “hard water” by ion exchange. Follow the procedure developed in Preab Question 5.
19. Please dispose of all chemicals according to teacher instructions. Resin will be collected for regeneration and reuse.

Student Worksheet PDF

12836_Student1.pdf