CHM 101/102 Laboratory Manual Water Hardness: Determination with EDTA General Chemistry 101/102 Laboratory Manual University of North Carolina at Wilmington

CHM 101/102 Laboratory Manual Water Hardness: Determination with EDTA Purpose Purpose  To determine the “hardness” of a water sample using an EDTA titration.  To learn and practice quantitative techniques for determining the concentrations of solutions. Safety Considerations 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.

CHM 101/102 Laboratory Manual Water Hardness: Determination with EDTA  Water is said to be “hard” when it contains Ca 2+ and Mg 2+ ions. These ions react with soap to form an insoluble substance called “soap scum”. dihydrogen ethylenediaminetetraacetate ion H 2 EDTA 2-  EDTA has a greater affinity for Ca 2+ and Mg 2+ when it is in the form of the dihydrogen anion H 2 EDTA 2-. This is the ionic form of EDTA at pH 10.  Ca 2+ and Mg 2+ ions along with other metal ions such as Fe 3+ and Pb 2+ can be removed from hard water by the addition of EDTA (ethylenediaminetetraacetic acid).

CHM 101/102 Laboratory Manual Water Hardness: Determination with EDTA H 2 EDTA 2- Ca H + Ca 2+ ion chelated by EDTA  H 2 EDTA 2- binds to a Ca 2+ ion by forming four special covalent bonds called coordinate covalent bonds.  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.

CHM 101/102 Laboratory Manual Water Hardness: Determination with EDTA Using a Volumetric Pipet Using a Volumetric Pipet  Squeeze the pipet bulb and place the silicone end over the top of the pipet. Do not force the pipet into the bulb.  Draw liquid up into the pipet until it is just above the calibration mark. Slide the bulb off the pipet and place your index finger over the top of the pipet.

CHM 101/102 Laboratory Manual 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.

CHM 101/102 Laboratory Manual Water Hardness: Determination with EDTA Using a Buret 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. remove air bubble

CHM 101/102 Laboratory Manual Water Hardness: Determination with EDTA  Use a marker pen to create a black area on a white note card. This will be used to help locate the bottom of the meniscus when reading the buret.  Place the notecard with the black mark behind the buret and just below the meniscus. This will make the bottom of the meniscus much easier to see. Record the volume reading to the nearest 0.01 mL.

CHM 101/102 Laboratory Manual Water Hardness: Determination with EDTA  If you need to dispense less than a full drop of titrant, open the stopcock slightly to allow a small amount of titrant to accumulate on the tip of the buret.  Rinse the titrant on the tip into the flask using deionized water from your wash bottle.

CHM 101/102 Laboratory Manual Procedure (Titration of Blank) Procedure (Titration of Blank)  Use a volumetric pipet to dispense mL of deionized water (DI) into a 250 mL flask. Water Hardness: Determination with EDTA  Add 5 mL of pH 10 buffer, 2 drops of Eriochrome Black T indicator, and 15 drops of 0.03 M MgCl 2.  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. beginning color nearing endpoint endpoint color

CHM 101/102 Laboratory Manual Procedure (Titrating Blank and Sample) Procedure (Titrating Blank and Sample)  The following video clip demonstrates the proper technique for titrating both the sample and the blank. Water Hardness: Determination with EDTA

CHM 101/102 Laboratory Manual Water Hardness: Determination with EDTA Procedure (Titration of Sample) Procedure (Titration of Sample)  Use a volumetric pipet to dispense mL of your hard water sample 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 MgCl 2.  Titrate the solution with EDTA from your buret to the blue endpoint you observed when titrating the blank. endpoint color

CHM 101/102 Laboratory Manual Calculations Calculations Water Hardness: Determination with EDTA mol CaCO 3 in sample = L EDTA x M EDTA x 1 mol CaCO 3 1 mol EDTA (1) M CaCO3 = moles CaCO 3 50 x L sample (2) mg/L CaCO 3 = M CaCO3 x 100 g CaCO 3 1 mol CaCO 3 x 1000 mg 1 g (3) (4) ppm = mg/L