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EDTA Titration Lab Prep

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EDTA and Water Hardness Titration• Water hardness is dissolved minerals

– Ca2+ + Mg2+

• Titrate dissolved Ca2+ and Mg2+ with EDTA (Na2H2Y)

– Ca2+ + H2Y2 CaY2 + 2H+

– Use metallochromic indicator for endpointMgIn + HY3 HIn2 + MgY2

• Calculations– Reaction stoichiometry

nCa = nY = MYmLY

– Report hardness as ppm CaCO3 (mg CaCO3/L)

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Spectrophotometric Endpoint

Alternative to visual endpoint detection

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Absorption Spectra of Calmagite

HIn2

Blue form

MgIn

Red form

End point reactionMgIn + HY3 HIn2 + MgY2

Monitor absorbance at wavelength with maximum change in absorbance.

5

-0.15

-0.1

-0.05

0

40.0 40.5 41.0 41.5 42.0 42.5 43.0 43.5 44.0

Volume EDTA, mL

Ab

sorb

ance

2 3 2

Indicator present as complexed red form

Ca HY

M In

CaY

g

H

2

Adding excess EDTA

Indicator present as free blue form HIn

42.55 mL

Titration of Mg2+ (or Ca2+) with EDTAMg2+ + HY3 MgY2 + H+

223

Endpoint reaction:

MgIn HInHY MgY

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Ocean Optics Spectrometer with Fiber Optic Probe

Fro

m l

igh

t so

urc

e

To

sp

ectr

om

eter

Mirror

Lens

Analyte solution

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Summary• Water hardness titration determines dissolved Ca2+ and

Mg2+ salts.

• EDTA forms a 1:1 complex with Ca2+ or Mg2+

• Endpoint detection methods– Visual– Spectrophotometric

• Report water hardness as ppm CaCO3