11. EDTA Titration of Ca2+ and Mg2+ in Natural Waters
The most common multivalent metal ions in natural waters are Ca2+ and Mg2+. In this
experiment, we will find the total concentration of metalions that can react with EDTA, and we
will assume that this equals the concentration of Ca2+ and Mg2+. In a second experiment, Ca2+
is analyzed separately after precipitating Mg(OH)2 with strongbase.
Buffer (pH 10): Add 142 mL of 28 wt% aqueous NH3 to 17.5 g of NH4Cl and dilute to 250 mL
Eriochrome black T indicator: Dissolve 0.2 g of the solid indicator in 15 mL oftriethanolamine
plus 5 mL of absolute ethanol.
50 wt% NaOH: Dissolve 100 g of NaOH in 100 g of H2O in a 250-mL plastic bottle. Store
tightly capped. When you remove solution with a pipet, try not todisturb the solid Na2CO3
Dry Na2H2EDTA.2H2O (FM 372.24) at 80û C for 1 h and cool in the desiccator. Accurately
weigh out ~0.6 g and dissolve it with heating in 400 mLof water in a 500-mL volumetric
flask. Cool to room temperature, dilute to the mark, and mix well.
Pipet a sample of unknown into a 250-mL flask. A 1.000-mL sample of seawater or a
50.00-mLsample of tap water is usually reasonable. If you use 1.000 mL of seawater, add
50 mL of distilled water. To each sample, add 3 mL of pH 10 buffer and 6 drops of
Eriochrome black T indicator.Titrate with EDTA from a 50-mL buret and note when the
color changes from wine red to blue. Practice finding the end point several times by adding
a little tap water and titrating with more EDTA. Save asolution at the end point to use as a
color comparison for other titrations.
Repeat the titration with three samples to find an accurate value of the total Ca2+ + Mg2+
concentration. Performa blank titration with 50 mL of distilled water and subtract the value
of the blank from each result.
For the determination of Ca2+, pipet four samples of unknown into clean flasks...
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