EDTA Titration Lab

8/11/2019 EDTA Titration Lab

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Determination of Ca2+and Mg2+in Water

by EDTA Titration

What are Lewis Acid-Base Reactions?

In your study of acid-base chemistry, you learned of different models for describing the behavior of

acids and bases. In the Arrhenius model, acids are substances that increase the concentration of

hydronium ion (H3O+) in aqueous solutions, while bases increase the concentration of hydroxide ion (OH

-

) in aqueous solutions. In the Bronsted-Lowry model, acids are proton donors and bases are proton

acceptors. Finally, the Lewis model considers acids to be electron pair acceptors and bases to be

electron pair donors. In this lab, you will use a Lewis acid-base reaction to determine the concentration

of Ca2+and Mg2+in a water sample. These ions and can inhibit the action of soaps and cause

precipitates called limescale. Water with high concentrations of these ions is said to be hard.

Because, these ions have a positive charge and vacant orbitals, they are good electron pair acceptors

and therefore Lewis acids. They can accept electron pairs from a donor that has an unshared pair of

electrons (i.e., a Lewis base). When a Lewis base donates its electrons to a metal ion to form a complex

ion, it is called a ligand. An example of a Lewis acid-base reaction involving the cyanide ion as a ligand is

shown below.

The reaction of the silver ion with the cyanide ion produces a complex ion in a process called

complexation. Complex ions are similar to polyatomic ions in that they are groups of covalently bonded

atoms that carry an overall charge. Complexation reactions are reversible and are characterized by an

equilibrium constant called a formation constant, Kf. Values of Kfare generally large, which indicates

that these equilibria strongly favor products, leaving a relatively low concentration of free metal ions in

solution. In fact, complexation reactions are commonly used to remove unwanted metal ions from


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solutions. For example, EDTA is a complexing agent commonly added to food packaging in order to

complex metal ions that catalyze reactions that cause food to spoil.

What is EDTA?

EDTA is an example of a multidentate (many-toothed) ligand, which can bind metal ions through

multiple atoms. Multidentate ligands are also called chelates, which comes from a Greek word meaning

claw. Multidentate ligands, or chelating agents, wrap themselves around metal ions like a claw. The

figure below shows EDTA chelating a metal ion.

EDTA Chelating a metal ion


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A more complete structure and a shorthand line structure of EDTA are shown below.

A line structure is a shorthand way of depicting an organic molecule. The lines in these structures

represent covalent bonds. Because all organic compounds contain carbon and hydrogen, these atoms

are not shown. A carbon atom is assumed to be present at the intersection of two lines. Because

carbon typically forms four bonds, the number of hydrogen atoms bonded to each carbon atom can be

inferred.

What are the different forms of EDTA?

EDTA is an amphiprotic substance, which means that it can both donate and accept protons. The four

hydrogen atoms shown in the above line structure are acidic, and because of this, the formula of EDTA is

often abbreviated H4Y, where H4represents the four acidic hydrogen atoms and Y represents the

remaining structure. The Kavalues for the sequential loss of these protons are 1, 0.032, 0.01, and

0.0022. The lone pairs of electrons on the nitrogen atoms in EDTA are capable of accepting protons.

The Kbvalues for the protonation of these nitrogen atoms are 1.74 10-4

and 1.45 10-8

. EDTA can

therefore exist in many different forms depending on the pH of the solution. At very low pH, EDTA will

be present in its completely protonated form H6Y2+

. At very high pH, EDTA will be present in itscompletely deprotonated form, Y

4-. At intermediate pH, EDTA will be present in one of its intermediate

forms. The following structures depict the various ionized forms of EDTA, along with their abbreviations

and the pH at which each predominates.


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The negative log of an equilibrium constant is called a pK value. The pKavalue, the negative log of the Ka

value, for an acid gives the pH at which there is an equal concentration of an acid and its conjugate base.

This can be seen by solving the Henderson-Hasselbalch equation for the pH at which an equal

concentration of an acid and its conjugate base would be present in a solution. The Kaand Kbvalues canbe used to determine the predominant form of an amphiprotic substance at different values of pH. The

following table was constructed using the pKaand pKbvalues for EDTA.

Form of

EDTA

H6Y2+

H5H+ H4HY H3Y

- H2Y

2- HY

3- Y

4-

pH 0.0 1.5 2.0 2.66 6.16 10.24

Which form of EDTA do we start with?

EDTA is most frequently purchased as the dihdrate salt of its H2Y2- form, Na2H2Y2H2O (structure shown

below). Recall that hyrdrates are ionic compounds with loosely bound water molecules in their crystal

structure.

Which form of EDTA forms the most stable complex ions?

Very stable complex ions are formed between metal ions and EDTA in its completely deprotonated

form, Y4-. In order to get a significant portion of EDTA into this form, solutions of EDTA used in titrations

are typically buffered at high pH. In this experiment, you will use a buffer of NH3and NH4Cl to maintain

a pH of 10.


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As the equivalence point is approached, added EDTA displaces metal ions from the indicator-metal

complex ion, and the solution takes on color of the free indicator.

How can the individual concentrations of Ca2+

and Mg2+

be determined?

In this lab, you will use EDTA to determine the concentration of metal ions in a water sample. The most

common positively charged metal ions in natural waters are Ca2+

and Mg2+

. For this lab, you will assume

that these are the only metal ions present in your water. You will perform two sets of titrations. One

set of titrations will be used to determine the total concentration of Ca2+and Mg2+present in your water

sample. After selectively precipitating the Mg2+as Mg(OH)2, you will perform a second set of titrations

to determine the concentration of Ca2+in your water. The concentration of Mg2+in your water sample

can then be determined by difference.

Materials

EDTA: Na2H2EDTA2H2O (molar mass 372.24 g/mol), 0.6 g per student

pH 10 Buffer: Add 142 mL of 28 % (by mass) aqueous NH3to 17.5 g of NH4Cl and dilute to 250 mL with

deionized water.

Eriochrome black T indicator: Dissolve 0.2 g of the solid indicator in 15 mL of triethanolamine plus 5 mL

of absolute ethanol

50 % (by mass) aqueous NaOH

Water Standard: Evian bottled water or other brand if calcium and magnesium concentrations are

known.

Unknowns: Collect a water sample from a stream, lake, or ocean. Use a plastic bottle and fill it

completely to the top so that no air will be present in the sealed bottle. This will minimize the growth of

bacteria in your water.

Procedure

Solution preparation: If the EDTA has not been dried for you, start by drying about 1 g of

Na2H2EDTA2H2O (molar mass 372.24 g/mol) at 80C for 1 h and cool in a desiccator. Weigh out ~0.6 g

of the dry EDTA and record the mass to the nearest 0.0001 g. Add the EDTA to a 500 mL volumetric

flask and add about 400 mL of deionized water. Dissolve the EDTA with swirling and heating if

necessary. If you use heating, you will need to allow your EDTA to cool to room temperature before

diluting to the 500-mL mark on the neck of the volumetric flask. Use a squirt bottle of di H2O to careful

bring the volume of the solution to 500 mL.


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Water standard: In order to test your reagents and the accuracy of the titration, you may first wish to

titrate a water sample with a known concentration of calcium and magnesium. Bottled mineral water

for which these concentrations are known can serve as a standard.

Unknown water samples: If you choose to analyze unknown water samples, you will need to determine

the appropriate sample size. The ability of titrations to precisely to determine unknown concentrationsrelies on the ability to precisely deliver volumes with a buret. In order to deliver a volume to 4

significant digits using a buret, you must deliver at least 10 mL. Precision increases with the delivery of

higher volumes. Refilling the buret however will increase the number of volume readings that must be

made and therefore increase the uncertainty in determining the total volume delivered. The goal is to

deliver as much titrant as possible without having to refill the buret. Using 20-40 mL of titrant is a

reasonable goal. Because the concentrations of Mg2+and Ca2+in your unknown water sample are

unknown, it is impossible to know what sample volume will require 20-40 mL of titrant. Therefore, you

will need to perform some quick titrations (using the procedure below) with different sample volumes

to determine the appropriate sample size. Start with a 50-mL sample size and then adjust accordingly.

If the appropriate sample volume is less than 50 mL, bring the total volume up to ~50 mL by addingsufficient deionized water. Once you have selected an appropriate sample size, perform careful

titrations on at least three samples of your unknown water.

Total Mg2+

and Ca2+

determination: Pipet a 50-mL sample of water into a 250-mL Erlenmyer flask. To

the sample add 3 mL of the pH 10 buffer and 6 drops of Eriochrome black T indicator. Rinse and fill a

clean 50-mL buret with your EDTA solution and record the initial volume. Titrate your water sample

until the color changes from wine red to blue. Perform a quick titration to determine the approximate

volume needed to reach the endpoint. In order to accurately identify the endpoint, where the last trace

of red has been removed from the solution, you may want to add small volumes of mineral water to

your titrated sample and practice reaching the endpoint. Once you have confidently identified theendpoint, save this sample as a reference for subsequent titrations. Carefully titrate 3 more samples of

the water.

Blank titration: Titrate a 50-mL sample of the lab water that was used to prepare your solutions to

determine whether it contains any calcium or magnesium ions. If the lab water requires the addition of

EDTA to reach the endpoint, you will need to subtract this volume from each of your titration volumes.

Ca2+

determination: If you wish to determine the individual concentration of calcium and magnesium in

your water sample, use the following procedures for precipitating the Mg2+and titrating the Ca2+that

remains in solution. Pipet four water samples into separate Erlenmyer flasks. To precipitate the Mg2+,

add 30 drops of 50% NaOH to each sample and swirl for 2 min. The precipitate may not be visible.

Because Eriochrome black T does not work well at this elevated pH, you will use hydroxynaphthol blue

as the indicator in these titrations. Add ~0.1 g of hydroxynaphthol blue to each sample. Perform one

quick titration to determine the approximate endpoint, and practice finding the endpoint if necessary.

To accurately determine the endpoint of each of the remaining samples, titrate to the blue endpoint and

then allow the sample to sit for 5 min with occasional swirling. This will allow any Ca(OH)2that may

have precipitated to redissolve. If the solution has turned back to red, add additional titrant to reach


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the blue endpoint. Following the same procedure, perform a blank titration on a 50-mL sample of lab

water and make any volume corrections necessary.

Calculations: From the precise mass of EDTA that you weighed out, calculate the concentration of your

EDTA. Use the volume of EDTA needed to reach the endpoint in each of your titrations to determine

either the total concentration of Ca2+

and Mg2+

or the concentration of Ca2+

alone. Recall that EDTAreacts with either of these ions in a 1:1 mole ratio. If you performed the Ca

2+titration, determine the

Mg2+

concentration by difference from the total of Ca2+

and Mg2+

. Calculate the relative standard

deviation of replicate titrations. If bottled water was titrated, calculated the relative difference of your

Ca2+

and Mg2+

concentrations to those listed by the manufacturer.


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Pre-lab Questions Determination of Ca2+and Mg2+

in Water by EDTA Titration

Name:_______________________

Instructor:____________________

1. If a student prepared an EDTA solution for a complexiometric titration by dissolving 0.5946 g of

Na2H2EDTA2H2O (molar mass 372.24 g/mol) in enough water to bring the total solution volume to 500.0

mL, what would the molar concentration of EDTA in the solution be? Clearly show any required

calculations with proper units and significant digits.

2. A student titrates a 50.00-mL sample of water with a 0.003125 M EDTA solution. If the titration

requires 34.64 mL of EDTA to reach the endpoint, what is the total concentration of Mg2+

and Ca2+

in the

water sample? Clearly show any required calculations with proper units and significant digits.

3. The same student then adds NaOH to another 50.00 mL sample of the same water and then titrates

again with EDTA. This time the titration requires 23.67 mL of EDTA solution. Determine the individual

concentrations of Ca2+

and Mg2+

in the water sample. Clearly show any required calculations with

proper units and significant digits.


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Report Sheet Determination of Ca2+and Mg2+


Name:_______________________

Instructor:____________________

Mass of Na2H2EDTA2H2O used to prepare EDTA solution (g) _______________

Concentration of EDTA solution (M) ______________________

Water standard

Total Mg2+

and Ca2+

determination:

Volume of EDTA titrant

Initial Volume

(mL)

Final

Volume (mL)

Volume

Delivered (mL)

Volume for

blank (mL)

Corrected

Volume (mL)

Rough Trial

Trial 1

Trial 2

Trial 3

Blank

Total concentration of Ca2+and Mg2+(M)

Trial 1

Trial 2

Trial 3

Mean

Standard Deviation

Relative Standard Deviation (%)

Ca2+

determination:


Initial Volume

(mL)

Final

Volume (mL)

Volume

Delivered (mL)

Volume for

blank (mL)

Corrected

Volume (mL)

Rough Trial

Trial 1

Trial 2

Trial 3

Blank


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Concentration of Ca2+

(M)

Trial 1

Trial 2

Trial 3

Mean

Standard Deviation


Total concentration of Ca2+and

Mg2+(M)

Mean Concentration of Ca2+(M) Concentration of Mg2+by

difference(M)

Comparison of manufacturer-reported and experimentally-determined Mg2+and Ca2+concentrations

Ca2+concentration (M) Mg2+concentration (M)

Reported by Manufacturer

Experimental

Relative Difference (%)

Sample calculations (use proper units and significant digits):

EDTA concentration:

Total concentration of Ca2+

and Mg2+

:


:

Concentration of Mg2+

:


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Report Sheet Determination of Ca2+and Mg2+


Name:_______________________

Instructor:____________________

Unknown water samples

Volume of water sample used for titration (mL)__________________________

Total Mg2+

and Ca2+

determination:


Initial Volume

(mL)

Final

Volume (mL)

Volume

Delivered (mL)

Volume for

blank (mL)

Corrected

Volume (mL)

Rough Trial

Trial 1

Trial 2Trial 3

Blank


and Mg2+

(M)

Trial 1

Trial 2

Trial 3

MeanStandard Deviation


Ca2+

determination:


Initial Volume

(mL)

Final

Volume (mL)

Volume

Delivered (mL)

Volume for

blank (mL)

Corrected

Volume (mL)

Rough Trial

Trial 1

Trial 2

Trial 3

Blank


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(M)

Trial 1

Trial 2

Trial 3

Mean

Standard Deviation


Total concentration of Ca2+and

Mg2+(M)

Mean Concentration of Ca2+(M) Concentration of Mg2+by

difference(M)

Sample calculations (use proper units and significant digits):

EDTA concentration:


and Mg2+

:


:

Concentration of Mg2+

:


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Post-lab questions Determination of Ca2+and Mg2+


Name:_______________________

Instructor:____________________

1. Write out the net ionic equation for the reaction that was used to remove Mg2+

from your water

sample so that Ca2+

alone could be determined.

Why is it that Mg2+

is removed from solution but Ca2+

is not? The Kspvalues for the hydroxides of Mg2+

and Ca2+

are 1.8 10-11

and 6.5 10-6

respectively.

2. Calcium Disodium Versenate (see structure below) is a form of EDTA that can be used to treat lead

poisoning in humans. In this drug, EDTA is present as a complex ion with Ca2+

. How is it possible for this

drug to remove Pb2+

from a persons blood if it is already complexed to Ca2+

? The formation constants,

Kfvalues, for the EDTA- Ca2+

and EDTA- Pb2+

complex ions are 4.9 1010

and 1.1 1018

respectively.

3. Two indicators, Funky Green (FG) and Infernal Orange (IO), are being considered for an EDTA titration

for the determination of Mg2+. The formation constants, Kfvalues, for the indicator- Mg2+complex ionare 2.34 10

5for FG and 1.76 10

9for IO. The formation constant for the EDTA-Mg

2+complex ion is

6.17 108. Which indicator would be the appropriate choice and why?


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4. Challenge Question. Calcium Disodium Versenate (see structure below) is a form of EDTA that can

be used to treat lead poisoning in humans who have blood lead levels in the range of 20-70 micrograms

per deciliter. The recommended dosage of this drug for an adult is about 1000 mg. The average blood

volume of an adult is about 5 L. If the formation constant, Kf, for the EDTA-Pb2+

complex ion is 1.1

1018, what would be the concentration of free Pb2+in an average adults blood if she had an initial blood

Pb2+concentration of 50 micrograms per deciliter and received a 1000 mg dose of calcium disodium

versenate . Assume that there are no other competing reactions.

Documents

EDTA Titration Lab