Procedure (Titration of Blank)

Use a volumetric pipet to dispense 25.00 mL of deionized water (DI) into a 250 mL flask.
Add 5 mL of pH 10 buffer, 2 drops of Eriochrome Black T indicator, and 15 drops of 0.03 M MgCl2.
Titrate the solution with EDTA from your buret. As you near the endpoint, the solution will turn purple. Continue to slowly add EDTA until the solution turns blue, with no trace of red.

Water Hardness: Determination with EDTA

Carefully release liquid from the pipet until the bottom of the meniscus is on the calibration line.
Release your finger and allow the liquid in the pipet to drain into a beaker or flask. Touch the tip of the pipet to the side of the beaker or flask to completely drain the pipet.
Using a Buret
Clean the buret with a buret brush, water, and a small amount of detergent. Rinse it twice with deionized water. Be sure to drain deionized water through the tip.
Rinse the buret again with two 10 mL portions of the titrant (EDTA in this experiment).
Fill the buret with titrant and drain a small amount from the buret to dispel any air bubbles that might be in the tip.

Titration of EDTA

In today’s experiment, you will determine the total concentration of calcium and magnesium ions in a hard water sample using EDTA in a solution buffered to a pH of 10.
Water Hardness: Determination with EDTA
Using a Volumetric Pipet
Squeeze the pipet bulb and do not force the pipet into the bulb.
Squeeze the pipet bulb and place the silicone end over the top of the pipet. Do not force the pipet into the bulb.

EDTA Reaction with metal ions

H2EDTA2- binds to a Ca2+ ion by forming four special covalent bonds called coordinate covalent bonds.

EDTA Structure

General Chemistry 101/102 Laboratory Manual University of North Carolina at Wilmington

Water Hardness: Determination with EDTA
• Purpose
• To determine the “hardness” of a water sample using an EDTA titration.
 To learn and practice quantitative techniques for determining the concentrations of solutionTo determine the “hardness” of a water sample using an EDTA titration.
 Safety Considerations
 Keep the pH 10 buffer in the hood. Avoid breathing ammonia vapors from the buffer.
 Eriochrome Black T will stain skin and clothes.
 All waste materials can be safely rinsed down the sink.
 Water Hardness: Determination with EDTA
 Water is said to be “hard” when it contains Ca2+ and Mg2+ ions. These ions react with soap to form an insoluble substance called “soap scum”.
Ca2+ and Mg2+ ions along with other metal ions such as Fe3+ and Pb2+ can be removed from hard water by the addition of EDTA (ethylenediaminetetraacetic acid)
 EDTA has a greater affinity for Ca2+ and Mg2+ when it is in the form of the dihydrogen anion H2EDTA2-. This is the ionic form of EDTA at pH 10.