Materials Included In Kit

Additional Materials Required

Prelab Preparation

1. Dilute the 0.04 M EDTA to 0.01 M. In the 250-mL graduated cylinder measure 250 mL of 0.04 M EDTA and transfer to a clean, dry 1-L volumetric flask. Add distilled or deionized water so that the flask is ⅔ full. Mix solution and fill to designated mark with DI water. Repeat as needed.
2. Prepare a fresh Eriochrome Black T indicator solution. The Eriochrome Black T indicator solution should be prepared fresh (within two weeks) in ethyl alcohol. Dissolve 0.15 g EBT in 30 mL of ethyl alcohol. Stir to dissolve. Dispense in dropper bottles. Each team of students will need less than 2 mL total for the titrations.
3. Dilute the 1 M HCl to 0.1 M. Measure 100 mL of 1 M HCl using a graduated cylinder and dilute to 1 L in a beaker or flask. Analytical precision is not required for the HCl concentration. Repeat as needed.

Safety Precautions

Ammonium oxalate is moderately toxic by ingestion and inhalation and corrosive to body tissue. The EDTA solution and the buffer solution are strongly basic and are corrosive to eyes, skin and other body tissue and toxic by ingestion. The EBT indicator solution contains ethyl alcohol and is flammable. Keep away from open flames and ignition sources. Avoid contact of all chemicals with eyes and skin. Wear chemical splash goggles, chemical-resistant gloves and a chemical-resistant apron. Remind students to wash their 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, ammonium oxalate solution, and the EBT indicator solution may be disposed of according to Flinn Suggested Disposal Method #26b. The precipitated calcium oxalate may be disposed of according to Flinn Suggested Disposal Method #26a.

Lab Hints

• Enough materials are supplied for 15 laboratory groups.
• A couple steps of the laboratory procedure need process time. Optimize use of time by preparing setups and doing other analysis while waiting. Have students dissolve the CMA solid, separate the needed aliquots and add the ammonium oxalate solution to one CMA aliquot before all other setups.
• If the precipitate (calcium oxalate) will be used in calculations, realize the precipitate formed is calcium oxalate monohydrate. The conversion to CaC₂O₄•H₂O is necessary as shown.
  {12378_Hints_Equation_5}
• A bonus question may be, “Theoretically, what is the mass of the filtered precipitate?”
• The EDTA titrant may be standardized using freshly prepared magnesium chloride solution, if desired.

Teacher Tips

• Show how to use a buret properly. Be sure it is clean and drains freely, and then rinse it with the EDTA solution before filling.Be sure the tip is filled. The liquid volume in a buret is read at the bottom of the meniscus and the volume can be estimated to ±0.02 mL. It should be read with the eye at the level of the liquid in the buret. The meniscus can be seen better if a paper with a black mark on it is held behind the buret so that the black mark is a few millimeters below the meniscus. The black mark will shadow the meniscus and show it more clearly against the white background.
• When titrating, it is a good idea to put a white piece of paper under the flask so the color of the solution is more obvious.Initially large volumes of titrant can be added, but as the endpoint approaches, very small volumes should be used.
• The indicator change is gradual, going from pink through purple to blue. The endpoint occurs at the point where no more pink is present. The endpoint color in the actual titration will not be sky blue but more of a purplish blue. Students who suffer from color blindness will have a difficult time recognizing the pink–purple–blue indictor transitions. For best results, we suggest titrating to the purple endpoint; it is much more distinct and easier to recognize than various shades of blue.
• Demonstrate how to use proper filtering technique.
• Water softening and water hardness determinations commonly use a similar EDTA titration. Kits on these subjects are available from Flinn Scientific—Determination of the Hardness of Water, Catalog No. AP9091, How Hard Is Your Water?, Catalog No. FB0411, Water Softening Guided-Inquiry Kit, Catalog No. AP7352.

Answers to Prelab Questions

A 3.50-gram sample of CMA was dissolved in HCl and diluted to 500.0 mL. A 10.0-mL aliquot of the sample was titrated with 31.12 mL of 0.011 M EDTA. The calcium was removed from a second 10.0-mL aliquot by precipitation with ammonium oxalate, and the remaining filtrate was titrated with 21.64 mL of 0.011 M EDTA.

1. Multiply the concentration (molarity) of EDTA by the volume (in liters) of EDTA added in the first titration to determine the combined amount (moles) of calcium and magnesium ions in a 10.0-mL CMA aliquot.
   {12378_PreLab_Equation_2}
   (0.011 moles/L) x (0.03112 L) = 0.000342 moles of Ca²⁺ and Mg²⁺
2. Calculate the amount (moles) of magnesium in a 10.0-mL CMA aliquot by multiplying the concentration of EDTA by the volume (in liters) of EDTA added in the second titration (after removal of the calcium ions by precipitation and filtration).
   {12378_PreLab_Equation_3}
   (0.011 moles/L) x (0.02164 L) = 0.000238 moles of Mg²⁺
3. Determine the number of moles of calcium in a 10.0-mL CMA aliquot.

   (0.000342 moles of Ca²⁺ and Mg²⁺) − (0.000238 moles of Mg²⁺) = 0.000104 moles of Ca²⁺

4. Multiply the number of moles of each ion by the appropriate molar mass to determine the mass of magnesium acetate and the mass of calcium acetate in a 10.0-mL CMA aliquot. Molar mass: Calcium acetate, 158.2 g/mole; magnesium acetate,142.3 g/mole.

   0.000238 moles of Mg²⁺ x 142.3 g/mole = 0.0339 g magnesium acetate
   0.000104 moles of Ca²⁺ x 158.2 g/mole = 0.0165 g calcium acetate

5. Divide the original mass of the CMA sample by the initial solution volume (before removing aliquots), and multiply by the aliquot volume to determine the mass of CMA in a 10.0-mL aliquot.

   (3.50 g CMA/500.0 mL) x (10.0 mL) = 0.070 g CMA

6. Calculate the percent composition of calcium acetate and the percent composition of magnesium acetate in CMA.
   {12378_PreLab_Equation_4}
   Percent magnesium acetate = (0.0339 g/0.070 g) x 100% = 48.4%
   Percent calcium acetate = (0.0165 g/0.070 g) x 100% = 23.6%

Sample Data

Answers to Questions

1. What evidence was there that a reaction occurred when ammonium oxalate was added to the CMA solution?

   A white precipitate was observed.

2. Calculate the number of moles of Ca²⁺ and the number of moles of Mg²⁺ in the 10-mL aliquot. Show all calculations. Hint: See Prelab Questions 1–3.
   {12378_Answers_Equation_6}
   *Actual concentration of EDTA used to obtain sample data was 0.0099 M.
3. How many grams of magnesium acetate are in the 10-mL aliquot?

   Mass of magnesium acetate = (1.92 x 10^–4 moles) (142.3 g/mole)

   = 0.027 g

4. How many grams of calcium acetate are in the 10-mL aliquot?

   Mass of calcium acetate = (1.1 x 10^–4 moles) (158.2 g/mole)

   = 0.017 g

5. Divide the initial mass of CMA used by 250.0 mL (the initial solution volume) and multiply by 10.0 mL (aliquot volume) to determine the mass of CMA in the 10.0 mL aliquot.
   {12378_Answers_Equation_7}
6. Calculate the percent composition by mass of calcium acetate and the percent composition of magnesium acetate in CMA.

   Percent calcium acetate = (0.017 g/.040 g) x 100% = 42.5%
   Percent magnesium acetate = (0.027 g/.040 g) x 100% = 67.5%

7. Identify potential sources of error in the materials or experiment that may account for the calculated percent composition of CMA being less than or greater than 100%.

   The calculated percent composition of calcium and magnesium acetate was 110%.

   {12378_Answers_Equation_8}
   Several sources of error may account for this value being greater than 100%. Not all of the calcium may have precipitated out in solution B, so that some of it was double-counted in the titrations. The indistinct color changes at the endpoint may have led to over-titration as well.

References

Lantz, J. M., Feindt J. E., Lewellyn, E. P. B., Walczak, M. M. The Bridge of Mandolin County: Case Teaching Note; Drew University: Madison, NJ, St. Olaf College: Northfield, MN Supplement to: J. of Chem. Ed.

Introduction

Calcium magnesium acetate, CMA, is a deicer which is also used to melt snow in the winter. Find out the make-up of this commercial product by determining the percent composition of calcium and magnesium.

Concepts

• Stoichiometry
• Titration
• Percent composition
• Complex ions

Background

The removal of snow and ice for safety is a billion-dollar undertaking in the United States. Snow is moved by plows and shovels but there is another category of deicers—chemical deicers. Chemical deicers are used to make driving safer on roadways or runways by lowering the temperature at which water changes from a solid to a liquid. Chemical deicers have a finite temperature range where they are useful. If the temperature is too low, for example below –30 °C, chemicals alone do not easily melt ice. Many states use sodium chloride (table salt) on roadways. The main disadvantages of sodium chloride are the corrosion and damage it can cause to roadways, bridges, vehicles, and vegetation, and the contamination of drinking water through the soil and surface water of salted areas. Calcium magnesium acetate is sold commercially as a less corrosive “green” chemical deicer. The disadvantage of CMA is its cost—it is twenty times more expensive than sodium chloride.

Calcium magnesium acetate is formed from dolomitic lime and acetic acid. Lime is limestone that has been ground into a powder. Calcitic lime is mainly calcium carbonate, while dolomitic limestone contains both calcium and magnesium carbonates. Because CMA is prepared from natural sources, the relative amounts of calcium and magnesium acetate may vary. In this lab, the percent composition of magnesium and calcium in CMA will be determined using titration, precipitation, and secondary titration.

The following four-step procedure will be used:

1. First, a sample of CMA will be dissolved in dilute acid to form a solution.
2. One aliquot of the solution will be titrated with EDTA, a complexing agent, to determine the combined amount of calcium and magnesium in the original sample. EDTA binds to many metal ions, including both calcium and magnesium.
3. The calcium ions will be removed from a second aliquot of the CMA solution by precipitation with ammonium oxalate.
4. The remaining filtrate will also be titrated with EDTA to determine the amount of magnesium in the original sample.

EDTA is an abbreviation for ethylenediaminetetraacetic acid, an organic compound having the formula C₁₀H₁₆N₂O₈. EDTA is called a complexing agent because it binds to metal ions in solution to form complex ions. The large EDTA molecule “wraps” around the metal ions, trapping them in solution. EDTA is often used to complex ions to prevent them from causing other reactions. For example, adding EDTA to hard water allows soap to lather without forming scum. EDTA is also used clinically as an antidote in cases of poisoning by lead ions. EDTA complexes 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. In basic solution, the four acidic hydrogens in EDTA are pulled away from the molecule, leaving a tetraanion EDTA^4–. The 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 a metal ion. A basic buffer (pH = 10) is used to ensure that the EDTA is in the correct anionic form to bond with metal ions. An indicator is also added to show when the titration is complete. Eriochrome Black T, called EBT, is blue in basic solution. However, when EBT forms complex ions with calcium and magnesium, the indicator is pink. From the titration of a CMA solution with EDTA, the total percent of magnesium and calcium combined can be calculated. To determine the percent of magnesium or calcium, one of the ions must be removed from the sample. Calcium ions are removed from the sample solution by adding ammonium oxalate, which forms a precipitate with the calcium ions (Equation 1). The precipitate is filtered and removed from the sample and the resulting filtrate containing only the magnesium ions is titrated. With the moles of magnesium as well as the combined magnesium and calcium known, the amount of calcium can be derived. Sample calculations for the percent composition are previewed in the Prelab Questions.

Experiment Overview

The purpose of this experiment is to determine the percent composition of a commercial product, calcium magnesium acetate. The overall calcium and magnesium portion will be determined through an EDTA titration. Then the calcium will be removed by precipitation and the remaining magnesium ions will be titrated.

Materials

Prelab Questions

A 3.50 gram sample of CMA was dissolved in HCl and diluted to 500.0 mL. A 10.0-mL aliquot of the sample was titrated with 31.12 mL of 0.011 M EDTA. The calcium was removed from a second 10.0-mL aliquot by precipitation with ammonium oxalate, and the remaining filtrate was titrated with 21.64 mL of 0.011 M EDTA.

1. Multiply the concentration (molarity) of EDTA by the volume (in liters) of EDTA added in the first titration to determine the combined amount (moles) of calcium and magnesium ions in a 10.0-mL CMA aliquot.
   {12378_PreLab_Equation_2}
2. Calculate the amount (moles) of magnesium in a 10.0-mL CMA aliquot by multiplying the concentration of EDTA by the volume (in liters) of EDTA added in the second titration (after removal of the calcium ions by precipitation and filtration).
   {12378_PreLab_Equation_3}
3. Determine the number of moles of calcium in a 10.0-mL CMA aliquot.
4. Multiply the number of moles of each ion by the appropriate molar mass to determine the mass of magnesium acetate and the mass of calcium acetate in a 10.0-mL CMA aliquot. Molar mass: Calcium acetate, 158.2 g/mole; magnesium acetate, 142.3 g/mole.
5. Divide the original mass of the CMA sample by the initial solution volume (before removing aliquots), and multiply by the aliquot volume to determine the mass of CMA in a 10.0-mL aliquot.
6. Calculate the percent composition of calcium acetate and the percent composition of magnesium acetate in CMA.
   {12378_PreLab_Equation_4}

Safety Precautions

Ammonium oxalate is moderately toxic by ingestion and inhalation. Hydrochloric acid is corrosive to body tissue. The EDTA solution and the buffer solution are basic and irritating to eyes and skin. The EBT indicator solution contains ethyl alcohol and is flammable. Keep away from open flames and ignition sources. Avoid contact of all chemicals with eyes and skin. Wear chemical splash goggles, chemical-resistant gloves and a chemical-resistant apron. Wash hands thoroughly with soap and water before leaving the laboratory.

Procedure

Part A. Preparing the Solutions

1. Mass 1.0 g of calcium magnesium acetate, CMA, into a tared weighing dish. Record the precise mass in the data table.
2. Transfer the CMA solid to a 400-mL beaker.
3. Using a 250-mL graduated cylinder, add 250.0 mL of 0.1 M HCl solution and mix well with a stirring rod.
4. While waiting for the CMA to dissolve, set up the buret for the titration (see Figure 2).
   {12378_Procedure_Figure_2}
5. Obtain a buret and buret clamp. Rinse the buret with tap water and then distilled water and check that it drains without streaking.
6. Obtain 100 mL of 0.01 M EDTA solution in a clean 150-mL beaker.
7. Pour about 5 mL of EDTA solution into the buret, and rinse the buret well with EDTA solution. Do not forget to rinse the tip as well as the barrel.
8. Drain the buret.
9. Fill the buret with the EDTA solution above the 0-mL mark, and drain some liquid so that the buret tip is filled and no air bubbles are present. Note: The meniscus should be on the graduated part of the buret. 
10. Check the dissolved CMA solution prepared in step 3. When the solid is completely dissolved, transfer a 10.0-mL sample of the CMA solution into a 125-mL Erlenmeyer flask using a 10-mL pipet or graduated cylinder. Label this solution A.
11. Transfer another 10.0-mL sample of the CMA solution into a 100-mL beaker. Label this beaker B.
12. Using a 10-mL graduated cylinder, add 2 mL of ammonium oxalate to solution B.
13. Record observations of the reaction of the CMA solution and ammonium oxalate in the data table, including the time the ammonium oxalate was added.
14. Set aside the mixture in beaker B until step 24.

Part B. Titrate the CMA Sample

15. Using a clean 10-mL graduated cylinder, add 5 mL of pH 10 buffer solution to solution A (step 10).
16. Using a Beral-type pipet, add 8 drops of Eriochrome Black T (EBT) indicator solution to the flask. Swirl to mix. Record the color.
17. Record the initial level of the EDTA solution in the buret before beginning the titration. (This is called the initial volume.)
18. Open the buret stopcock and add EDTA to the CMA solution in small portions. Observe the color changes and gently swirl the flask to mix the solution (see Figure 2).
19. When nearing the endpoint of the titration, the pink color of the indicator will fade. Record observations in the data table.
20. Continue adding the EDTA drop by drop to the sample until one drop of EDTA turns the solution purple and no traces of pink remain.
21. Record the final level of the EDTA solution in the buret. (This is called the final volume.)
22. Rinse the Erlenmeyer flask with DI water and repeat the titration if desired.

Part C. Filter

23. Set up a gravity filtration station (see Figure 3a–e).
24. Allow the mixture in beaker B to sit for at least 10 minutes to allow the precipitate to fully form.
25. Filter the contents of the 100-mL beaker into a 250-mL Erlenmeyer flask.
26. Rinse the precipitate with deionized water. Collect the rinse water with the filtrate in the same 250-mL flask. Label this solution C.

Part D. Titrate the Filtered Sample

27. Titrate solution C with EDTA following the same procedure as the first titration (steps 15–23).
    {12378_Procedure_Figure_3}

Student Worksheet PDF

12378_Student1.pdf