Materials Included In Kit

Additional Materials Required

Prelab Preparation

The Eriochrome Black T indicator solution should be prepared fresh (within two weeks) in ethyl alcohol. Dissolve 0.15 g EBT in 30 mL ethyl alcohol. Stir to dissolve. Dispense in dropper bottles. Each team of students will need about 2 mL for the titrations. Preparing 30 mL will be more than enough for 24 students working in teams of two.

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. The EBT indicator solution contains ethyl alcohol and is flammable. Keep away from open flames and ignition sources. 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, and the EBT indicator solution may be disposed of according to Flinn Suggested Disposal Method #26b.

Lab Hints

• If pipets are not available, graduated cylinders may be used but with a loss of precision. Do not attempt to use a pipet without a pipet bulb.
• Forty-five minutes are required to carry out the titrations.
• A universal clamp or a buret clamp may be used to hold a buret. Either a 25-mL or 50-mL buret can be used. Unless the water is very hard, less than 25 mL of titrant will be required for each titration trial.

Teacher Tips

• Students will need instruction and practice in using the pipets. Even though only tap water will be pipetted, it is good technique to always use a pipet bulb. Never pipet by mouth. The pipet should be clean and should drain without leaving droplets inside the pipet. Rinse the pipet with the solution to be pipetted before measuring the liquid. Show how to draw the liquid into a pipet and let it drain until the meniscus is at the calibration mark. Place a finger over the pipet to stopper it. Then drain the liquid into a flask. The tip of the pipet should be touched to the solution in the flask to draw out some of the liquid. A small amount remains in the pipet and should be discarded.
• 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. A very small volume of liquid (less than 0.1 mL) can be dispensed from the buret by quickly twisting the stopcock one-half turn. Often, the first titration is done very quickly to estimate the volume of titrant required. Subsequent titrations can then be carried out quickly until just before the endpoint, at which time very small increments of titrant should be added.
• 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 blue–purple–pink indictor transitions.

Answers to Prelab Questions

1. What is a complex ion?

   A complex ion forms when a metal ion is surrounded by a group of anions or neutral molecules which are attached to it by coordinate covalent bonds.

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.
   {10558_Answers_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. An experiment to find water hardness required 18.2 mL of 0.0050 M EDTA solution to titrate 40.0 mL of tap water using an Eriochrome Black T indicator.

   EDTA reacts with the hard water ions in a 1:1 mole ratio. The hardness of water is often reported as ppm Ca. This means that all the hardness is assumed to be caused by calcium ions, and its concentration is reported in parts per million (ppm).

   {10558_PreLab_Equation_2}
   To find parts per million of calcium, convert the moles of metal ions present to grams of calcium. The density of tap water is 1.0 g/mL. Divide grams of calcium by grams of tap water and multiply by 1,000,000. Calculate the parts per million (ppm) calcium ions present in the tap water.
   {10558_Answers_Equation_4}

Sample Data

Molarity of EDTA ___0.005___ M
Volume of tap water ___50___ mL

Answers to Questions

1. Volume of EDTA used for titration:

   Trial 1: 19.50 mL

   Trial 2: 19.16 mL

   Trial 3: 19.24 mL

   Average of last two trials: 19.20 mL

2. Moles EDTA used:
   {10558_Answers_Equation_7}
3. Calculate ppm Ca:
   {10558_Answers_Equation_8}

Introduction

Hard water can cause major problems with scaling and damage to water supply systems. In this laboratory, the hardness of ordinary tap water will be measured using a titrimetric method.

Concepts

• Water hardness
• Titration
• Complex ions

Background

Tap water contains a number of dissolved substances such as calcium, Ca²⁺, magnesium, Mg²⁺, iron, Fe³⁺, carbonate, CO₃^2–, and chloride, Cl^–, ions. Ions that prevent soap from forming a lather are said to give the characteristic of “hardness” to water. The two ions that are primarily responsible for water hardness are Ca²⁺ and Mg²⁺. Other ions such as iron, Fe³⁺, also contribute to water hardness. The presence of these ions makes it difficult for soap to lather and causes a “scum” to form with soap.

The amount of calcium and magnesium ions in water can be determined by titration with a complexing agent called EDTA. Titration is the process of adding the exact amount of a chemical reagent that is needed to react with substances dissolved in solution. The titrating agent, also called the titrant, is added using a buret, which makes it possible to accurately measure the volume added. In this experiment, the dissolved substances will be the calcium and magnesium ions in hard water, and the titrating agent will be EDTA solution. An indicator is added to determine the point at which all of the ions in solution have reacted with EDTA. The concentration of EDTA must be known, and the volume of EDTA which is required for the reaction is measured. Based on the volume and molarity of EDTA, the concentration of the ions which cause hard water can be determined.

EDTA is an abbreviation for ethylenediaminetetraacetic acid, a common 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. 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.

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. The resulting EDTA^4– tetra-anion that is formed in basic solution can bond to calcium or magnesium 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. The resulting Ca–EDTA and Mg–EDTA complexes have a negative two charge.

An indicator must be used to tell when the titration is complete. Eriochrome Black T, called EBT, is blue in basic solution. However, when EBT forms complex ions with Ca²⁺ or Mg²⁺, the solution is pink in color.

During the titration the following processes occur:

1. Tap water containing Ca²⁺ and Mg²⁺ ions is placed in a flask. Buffer solution, pH = 10, is added to maintain a basic pH. This ensures that the singly-bonded oxygen atoms in EDTA are not bonded to hydrogen atoms and can bond to the metal ions.
2. A small amount of EBT indicator is added. The EBT immediately bonds with some of the free metal ions forming a pink complex.
3. EDTA is then added from a buret. Initially, the EDTA will complex with the free calcium and magnesium 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 formation constants for the EDTA and EBT complexes of Ca²⁺ and Mg²⁺ are given:
   {10558_Background_Equation_1}
   The formation constants show that EDTA forms stronger complexes with the metal ions than EBT does. EDTA will begin to pull first calcium and then magnesium ions away from EBT. As magnesium ions are removed from EBT, the indicator changes from the pink color of the Mg–EBT complex to the sky blue of EBT with no metal ion attached. This signals the endpoint of the titration.

Materials

Prelab Questions

1. What is a complex ion?
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. An experiment to find water hardness required 18.2 mL of 0.0050 M EDTA solution to titrate 40.0 mL of tap water using an Eriochrome Black T indicator.

   EDTA reacts with the hard water ions in a 1:1 mole ratio. The hardness of water is often reported as ppm Ca. This means that all the hardness is assumed to be caused by calcium ions, and its concentration is reported in parts per million (ppm).

   {10558_PreLab_Equation_2}
   To find parts per million of calcium, convert the moles of metal ions present to grams of calcium. The density of tap water is 1.0 g/mL. Divide grams of calcium by grams of tap water and multiply by 1,000,000. Calculate the parts per million (ppm) calcium ions present in the tap 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. The EBT indicator solution contains ethyl alcohol and is flammable. Keep away from open flames and ignition sources. Wear chemical splash goggles and chemical-resistant gloves and apron. Thoroughly wash hands with soap and water before leaving the laboratory. Please review current Safety Data Sheets for additional safety, handling and disposal information. 

Procedure

Part I. Set up a buret.

1. Obtain a buret and buret clamp. Clean the buret with detergent so that it drains without streaking. Rinse it with tap water and then distilled water.
2. Clean and dry a 100-mL beaker. Pour about 75 mL of the EDTA solution into the beaker to use as a supply to fill your buret.
3. Pour about 10 mL of EDTA solution into the buret, and rinse the buret well with EDTA solution. Don’t forget to rinse the tip as well as the barrel.
4. Drain the buret, and repeat the rinse with another 10 mL of EDTA.
5. Fill the buret above the 0-mL mark, and drain some EDTA so that the buret tip is filled and no air bubbles are present. The meniscus should be on the graduated part of the buret.

Part II. Prepare the water sample.

1. Clean a 150-mL Erlenmeyer flask and rinse it with distilled water.
2. Pipet 50.0 mL of tap water into the flask.
3. Add about 10 mL of pH 10 buffer solution and 5 drops of EBT indicator solution to the water sample.

Part III. Carry out the titration.

1. Record the initial level of the EDTA solution in the buret before beginning the titration. (This is called the initial volume.)
2. Add the EDTA solution to the flask with swirling (see Figure 2).
   {10558_Procedure_Figure_2}
3. When nearing the endpoint of the titration, the pink color of the indicator will begin to turn purple.
4. Continue adding EDTA drop by drop to the water sample until one drop of EDTA turns the solution sky blue.
5. Record the final level of the EDTA solution in the buret. (This is called the final solution.)
6. Since the endpoint is not known exactly, this first trial may be done quickly to get an approximate value.
7. Rinse the Erlenmeyer flask with distilled water and repeat the titration two more times to measure the exact endpoint.
8. Dispose of all solutions as directed by the instructor.

Student Worksheet PDF

10558_Student1.pdf