Experiment Optional #4: 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 200 mL of the EDTA solution
from the carboy into a dry 250 mL beaker.
2. Rinse the buret with a few mL of the solution twice, using a 150 mL 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 25 mL DI water and 5mL of pH 10 buffer to a 250 mL 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 25 mg.
4. Add 15 drops 0.03 M 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 50 mL graduated cylinder to measure out two 50 mL 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.
Page Optional #4-1
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.
Page Optional #4-2
Report Sheet
Name:______________________
Water Hardness
A. Blank
Initial buret reading
________________ mL
Final buret reading
________________ mL
Volume of EDTA used to
titrate blank
________________ mL
B. 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
_________________ mL
_________________ mL
_________________ mL
_________________ mL
_________________ mL
_________________ mL
__________________
_________________
_________________
Volume of EDTA used
to titrate blank
Volume of EDTA
subtract the Blank
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:
ppm 
Molarity of EDTA  Volume of EDTA  100,000
Volume of water sample
Page Optional #4-3
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?
Page Optional #4-4
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.
Page Optional #4-5