EDTA Titration Lab Prep

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 endpoint MgIn + HY3  HIn2 + MgY2 Calculations Reaction stoichiometry nCa = nY = MYmLY Report hardness as ppm CaCO3 (mg CaCO3/L)

Spectrophotometric Endpoint Alternative to visual endpoint detection

Absorption Spectra of Calmagite End point reaction MgIn + HY3  HIn2 + MgY2 MgIn Red form HIn2 Blue form Monitor absorbance at wavelength with maximum change in absorbance.

Titration of Mg2+ (or Ca2+) with EDTA Mg2+ + HY3  MgY2 + H+ 42.55 mL

Ocean Optics Spectrometer with Fiber Optic Probe From light source To spectrometer Lens Analyte solution Mirror

Data File. Absorbance vs Volume EDTA mL

End Point Determination Equivalence point occurs at intersection of the two straight lines 1. Graphical measurement 2. Algebraic approach. Set rhs of two equations equal, solve for x (where x = volume at equivalence point).

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