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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