water hardness edta

CHM 101/102CHM 101/102

Laboratory ManualLaboratory ManualLaboratory Manual

Water Hardness: DeterminationWater Hardness: Determinationwith EDTAwith EDTA

General Chemistry 101/102General Chemistry 101/102Laboratory ManualLaboratory Manual

University of North Carolina at WilmingtonUniversity of North Carolina at Wilmington

CHM 101/102CHM 101/102


Water Hardness: Determination with EDTAWater Hardness: Determination with EDTA

• PurposePurpose To determine the “hardness” of a water sample To determine the “hardness” of a water sample

using an EDTA titration.using an EDTA titration. To learn and practice quantitative techniques for To learn and practice quantitative techniques for

determining the concentrations of solutions.determining the concentrations of solutions.

• Safety ConsiderationsSafety Considerations Keep the pH 10 buffer in the hood. Avoid Keep the pH 10 buffer in the hood. Avoid

breathing ammonia vapors from the buffer.breathing ammonia vapors from the buffer. Eriochrome Black T will stain skin and clothes.Eriochrome Black T will stain skin and clothes. All waste materials can be safely rinsed down the All waste materials can be safely rinsed down the

sink.sink.

CHM 101/102CHM 101/102


Water Hardness: Determination with EDTAWater Hardness: Determination with EDTA Water is said to be “hard” when it contains CaWater is said to be “hard” when it contains Ca2+2+ and Mg and Mg2+2+ ions. ions.

These ions react with soap to form an insoluble substance These ions react with soap to form an insoluble substance called “soap scum”.called “soap scum”.

dihydrogen ethylenediaminetetraacetate iondihydrogen ethylenediaminetetraacetate ion

HH22EDTAEDTA2-2-

EDTA has a greater affinity for CaEDTA has a greater affinity for Ca2+2+ and Mg and Mg2+2+ when it is in the when it is in the form of the dihydrogen anion Hform of the dihydrogen anion H22EDTAEDTA2-2-. This is the ionic form of . This is the ionic form of

EDTA at pH 10.EDTA at pH 10.

CaCa2+2+ and Mg and Mg2+2+ ions along with other metal ions such as Fe ions along with other metal ions such as Fe3+3+ and and PbPb2+2+ can be removed from hard water by the addition of EDTA can be removed from hard water by the addition of EDTA ((eethylenethyleneddiamineiaminettetraacetic etraacetic aacid). cid).

CHM 101/102CHM 101/102



HH22EDTAEDTA2-2-

CaCa2+2+

+ + 2 H2 H++

CaCa2+2+ ion ion chelatedchelated by EDTAby EDTA

HH22EDTAEDTA2-2- binds to a Ca binds to a Ca2+2+ ion by forming four special covalent ion by forming four special covalent

bonds called coordinate covalent bonds.bonds called coordinate covalent bonds.

In today’s experiment, you will determine the total concentration In today’s experiment, you will determine the total concentration of calcium and magnesium ions in a hard water sample using of calcium and magnesium ions in a hard water sample using EDTA in a solution buffered to a pH of 10.EDTA in a solution buffered to a pH of 10.

CHM 101/102CHM 101/102



• Using a Volumetric PipetUsing a Volumetric Pipet

Squeeze the pipet bulb and Squeeze the pipet bulb and place the silicone end over the place the silicone end over the top of the pipet. Do top of the pipet. Do notnot force force the pipet into the bulb.the pipet into the bulb.

Draw liquid up into the pipet until Draw liquid up into the pipet until it is just above the calibration it is just above the calibration mark. Slide the bulb off the mark. Slide the bulb off the pipet and place your index finger pipet and place your index finger over the top of the pipet. over the top of the pipet.

CHM 101/102CHM 101/102


Water Hardness: Determination with EDTAWater Hardness: Determination with EDTA Carefully release liquid from the Carefully release liquid from the

pipet until the bottom of the pipet until the bottom of the meniscus is on the calibration meniscus is on the calibration line. line.

Release your finger and allow Release your finger and allow the liquid in the pipet to drain the liquid in the pipet to drain into a beaker or flask. Touch into a beaker or flask. Touch the tip of the pipet to the side of the tip of the pipet to the side of the beaker or flask to completely the beaker or flask to completely drain the pipet.drain the pipet.

CHM 101/102CHM 101/102



• Using a BuretUsing a Buret Clean the buret with a buret brush, water, and a small Clean the buret with a buret brush, water, and a small

amount of detergent. Rinse it twice with deionized amount of detergent. Rinse it twice with deionized water. Be sure to drain deionized water through the water. Be sure to drain deionized water through the tip. tip.

Rinse the buret again with two 10 mL portions of the Rinse the buret again with two 10 mL portions of the titrant (EDTA in this experiment). titrant (EDTA in this experiment).

Fill the buret with titrant and drain a Fill the buret with titrant and drain a small amount from the buret to small amount from the buret to dispel any air bubbles that might be dispel any air bubbles that might be in the tip. in the tip. remove remove

air bubbleair bubble

CHM 101/102CHM 101/102



Use a marker pen to create a Use a marker pen to create a black area on a white note card. black area on a white note card. This will be used to help locate the This will be used to help locate the bottom of the meniscus when bottom of the meniscus when reading the buret. reading the buret.

Place the notecard with the black Place the notecard with the black mark behind the buret and just mark behind the buret and just below the meniscus. This will below the meniscus. This will make the bottom of the meniscus make the bottom of the meniscus much easier to see. much easier to see. Record the Record the volume reading to the nearest volume reading to the nearest 0.01 mL.0.01 mL.

CHM 101/102CHM 101/102


Water Hardness: Determination with EDTAWater Hardness: Determination with EDTA If you need to dispense less than a full If you need to dispense less than a full

drop of titrant, open the stopcock slightly drop of titrant, open the stopcock slightly to allow a small amount of titrant to to allow a small amount of titrant to accumulate on the tip of the buret. accumulate on the tip of the buret.

Rinse the titrant on the tip into the flask Rinse the titrant on the tip into the flask using deionized water from your wash using deionized water from your wash bottle.bottle.

CHM 101/102CHM 101/102


• Procedure (Titration of Blank)Procedure (Titration of Blank) Use a volumetric pipet to dispense 25.00 mL of Use a volumetric pipet to dispense 25.00 mL of

deionized water (DI) into a 250 mL flask. deionized water (DI) into a 250 mL flask.


Add 5 mL of pH 10 buffer, 2 drops of Eriochrome Add 5 mL of pH 10 buffer, 2 drops of Eriochrome Black T indicator, and 15 drops of 0.03 M MgClBlack T indicator, and 15 drops of 0.03 M MgCl22. .

Titrate the solution with EDTA from your buret. As Titrate the solution with EDTA from your buret. As you near the endpoint, the solution will turn purple. you near the endpoint, the solution will turn purple. Continue to Continue to slowlyslowly add EDTA until the solution add EDTA until the solution turns blue, with no trace of red. turns blue, with no trace of red.

beginningbeginningcolorcolor

nearingnearingendpointendpoint

endpointendpointcolorcolor

CHM 101/102CHM 101/102


• Procedure (Titrating Blank and Sample)Procedure (Titrating Blank and Sample) The following video clip demonstrates the proper The following video clip demonstrates the proper

technique for titrating both the sample and the technique for titrating both the sample and the blank.blank.


CHM 101/102CHM 101/102



• Procedure (Titration of Sample)Procedure (Titration of Sample) Use a volumetric pipet to dispense 50.00 mL of Use a volumetric pipet to dispense 50.00 mL of

your hard water sample into a 250 mL flask. your hard water sample into a 250 mL flask. Add 5 mL of pH 10 buffer, 2 drops of Eriochrome Add 5 mL of pH 10 buffer, 2 drops of Eriochrome

Black T indicator, and 15 drops of 0.03 M MgClBlack T indicator, and 15 drops of 0.03 M MgCl22. .

Titrate the solution with EDTA from your buret to Titrate the solution with EDTA from your buret to the blue endpoint you observed when titrating the the blue endpoint you observed when titrating the blank.blank.

endpointendpointcolorcolor

CHM 101/102CHM 101/102


• CalculationsCalculations


mol CaCOmol CaCO33 in sample = in sample = LLEDTAEDTA x M x MEDTAEDTA x x 1 mol CaCO1 mol CaCO33

1 mol EDTA 1 mol EDTA (1) (1)

MMCaCO3CaCO3 = = moles CaCOmoles CaCO33

50 x 1050 x 10-3-3 L sample L sample (2) (2)

mg/L CaCOmg/L CaCO33 = =

MMCaCO3CaCO3 x x

100 g CaCO100 g CaCO33

1 mol CaCO1 mol CaCO33 xx

1000 mg1000 mg

1 g 1 g (3) (3)

(4) (4)

ppm = mg/Lppm = mg/L

water hardness edta

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