Experiment 15: Water Hardness

One of the factors in establishing the quality of the water supply is its degree of hardness. The hardness of water is defined in terms of its content of calcium and magnesium ions. In the days when soap was more commonly used for washing clothes, and when people bathed in tubs rather than showers, water hardness was more often directly observed than it is now. Both calcium and magnesium ions form insoluble salts with soaps producing a scum that sticks to clothes or to the bath tubs.

Water hardness can be readily determined by titration with EDTA, a chemical which reacts with both ions. In this experiment we will use the EDTA solution to determine the hardness of a water sample. We will use an indicator called Erichrome Black T to detect the end point of the titration.

Procedure

A: Determine the Blank (DI Water)

  1. Clean your buret thoroughly as directed by your instructor. Obtain about 200mL of the EDTA solution from the carboy into a dry 250mL beaker.
  2. Rinse the buret with a few mL of the solution twice,using a 150mL beaker to collect the waste. Drain the solution through the stopcock and then fill the buret with the EDTA solution until the level of the EDTA is above the zero mark. Drain through the stopcock to remove any air bubbles and until the level of EDTA is between 0 and 2 mL. Record the volume reading from the buret to the second decimal place.
  3. Add 25mL DI water and 5mL of pH 10 buffer to a 250mL Erlenmeyer flask. To this solution, add a small amount of solid Eriochrome Black T indicator; you just need enough to cover the tip of a spatula, about 25mg.
  4. Add 15 drops 0.03M magnesium chloride (MgCl2) solution to the Erlenmeyer flask. The solution should turn to wine red.
  5. Place the Erlenmeyer flask under the buret so that its tip is just below the rim of the flask. Add EDTA slowly with constant swirling until the last tinge of purple just disappears. Only a few mL will be needed for this titration. Record the volume to the second decimal place.
  6. Subtract the initial volume reading from the final volume reading to find the total volume of EDTA required to titrate the blank. This value must be subtracted from the total EDTA volume used in each of the following titrations.

B. Titration of the Water Sample

  1. Refill the buret with EDTA solution. Read and record the volume. Obtain a water unknown sample from your instructor.
  2. Use a 50mL graduated cylinder to measure out two 50mL portion of the water sample into two beakers (250mL and 400mL).
  3. To each beaker add 5mL of the pH 10 buffer solution, a small amount of indicator and 15 drops of the 0.03M magnesium chloride solution.
  4. Place the beaker under the buret and titrate one of the solution slowly until its color matches the titrated blank from Part A of this experiment(that is until the last tinge of purple just disappears). Record the volume of EDTA.
  5. Refill the buret, read and record the volume. Titrate the second solution as before.
  6. If time permits, you should titrate a different sample of household water that you brought to the class, this time use 100mL of the water sample instead of 50mL.
  7. The chemical waste from this lab may be poured down the sink. Empty the buret, rinse it with 20mL of DI water and place the buret in the proper container after cleaning.

Report SheetName:______

Water Hardness

  1. Blank

Initial buret reading ______mL

Final buret reading______mL

Volume of EDTA used to

titrate blank______mL

  1. Water Hardness

Unknown Number ______

Sample I / Sample II / Household Sample
Volume of water used / ______mL / ______mL / ______mL
Initial buret reading / ______mL / ______mL / ______mL
Final buret reading / ______mL / ______mL / ______mL
Volume EDTA / ______mL / ______mL / ______mL
Volume of EDTA used
to titrate blank / ______mL / ______mL / ______mL
Volume of EDTA subtract the Blank / ______mL / ______mL / ______mL
Hardness of water / ______/ ______/ ______

Molarity of EDTA (should be on bottle):______

Water Hardness is expressed as ppm (parts per million), using the following equation to calculate the ppm:

What problems may you observe around the house if you have hard water?

A new car wash apparatus (the Mr. Clean Carwash) deionizes ordinary household water which is supplied to it by a garden hose. This deionizer is good for several uses, but it must eventually be replaced. After washing a car with a special soap, the user is instructed to rinse the car with regular water, then rinse it with the deionized water produced by the apparatus. The car can then be allowed to air dry. What is the advantage of using this apparatus? How is it effective?

Materials for this experiment:

The materials required are very similar to those used in Chem. 131L:

  1. 0.015 M EDTA solution (2sig fig) 250mL/student ( 2 bottles)
  2. pH 10 buffer 15mL/student ( 2 bottles)
  3. Erichrome Black indicator 0.2 g/student (2 bottles)
  4. 3.0 M sulfuric acid
  5. 0.030M magnesium chloride (2 bottles)
  6. 120 mL unknown water sample/student (2 sig fig is all we need), use same concentration for a small class. (800ppm will be just fine) They are using 50mL per titration.

7. Perform a “test run” of this experiment well in advance of the lab period, and notify me how much EDTA solution you needed to titrate the blank with.

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