Determination of the Hardness of Water
Introduction
Depending on the area of the country, tap water contains various amounts of ions such as calcium, iron, magnesium,
etc. The higher the concentration of these ions the “harder” the water is. Hard water is characterized by the inability of
soap to form lather and the formation of “scum” in sinks and bathtubs.
To determine the concentration of the two most common hard water ions, Ca2+ and Mg2+, a titration is performed with a
chealating agent, ethylenediaminetetraacetic acid (EDTA). Using a burette, the exact amount of EDTA needed to react
with the concentration of ions in the water is determined. The molar concentration of EDTA is 0.005. Due to steric
hindrances, EDTA will complex with calcium and magnesium in a one-to-one molar ratio.
In this experiment the indicator Eriochrome Black T (EBT) is used to signal the presence of ions in the water sample. EBT
binds with free metal ions in the water to form a pink complex. EDTA has a stronger affinity for the metal ions than EBT
so when EDTA is added it replaces the EBT and the EBT returns to its blue, uncomplexed color. The blue color is used as
the end point in the titration.
Both EDTA and EBT are weak acids and their actions are very pH dependent; thus we use a pH buffer to hold the
solutions at an appropriate pH for both the EDTA and the EBT indicator to work well. The titration is carried out at a pH
of 10 which keeps the EDTA in a half-neutralized form, where it complexes with Group IIA ions very well but does not
tend to react as readily with other cations such as Fe+ that might be present as impurities in the water.
Total water hardness is usually expressed as the milligrams of CaCO3 equivalent to the total amount of calcium and
magnesium present in one liter of water (mg/L, i.e. ppm).
The classification of the degree of water hardness according to the US Geological Survey is as follows:
Soft
Moderately Hard
Hard
Very Hard
0 – 60
61 – 120
121 – 180
> 180
ppm CaCO3 equivalents
ppm CaCO3 equivalents
ppm CaCO3 equivalents
ppm CaCO3 equivalents
Pre-Lab Questions: These are due at the beginning of tomorrow’s lab (0800). Late answers will result in a decrease of
one letter grade on your lab.
1.
2.
3.
4.
5.
Why is it important to estimate the hardness of potable water?
At what pH condition is the titration performed? Why?
What indicator is used in the analysis? What is its color change?
What is the most common way to soften hard water in households?
A 50.0 mL sample of tap water was analyzed using the EDTA procedure. An average volume of 16.34 mL of EDTA
was required for the titration. Answer the following questions:
a. Calculate the moles of EDTA used.
b. Calculate the molarity of metal ion present in the water sample (Remember: EDTA binds to metal ions
on a 1:1 molar ratio).
c. Water hardness is often expressed as ppm CaCO3 even though there may be several metal ions present.
Find the parts per million calcium ion concentrations in the sample based on your results from question
b.
Objective
A sample of tap water is treated with EBT indicator. If the indicator turns from blue to pink, metal ions such as calcium
and magnesium are present. To determine the concentration of ions present, the sample is titrated with a known molar
concentration of EDTA.
Chemicals and Equipment
EDTA 0.2% solution
Buffer solution pH 10
EBT indicator solution
50 mL graduated cylinder
250 mL Erlenmeyer flask
50 mL burette, burette clamp and ring stand
150 mL beaker (for waste)
Magnetic stirrer and accessories
Safety Equipment Required:
Rubber gloves
Aprons
Safety goggles
Safety Note: The pH buffer and EDTA solution are basic. Wash contaminated area immediately with water. The EBT
indicator is an alcohol solution which is flammable. Keep away from all sources of ignition.
Procedure
1. Rinse the burette with ≤ 5 mL EDTA solution.
2. Set up two burettes on ring stand with burette clamps. Fill one burette with 20 mL EDTA solution. Fill the other
burette with 20 mL tap water. The tap water burette will be used only if you overshoot your endpoint (it’s
easier than you think!!). Take an initial volume reading of each burette and record it on your datasheet. Set up
a magnetic stirrer underneath the EDTA burette.
3. Use a graduated cylinder and eye dropper to put 50.0 mL of tap water into an Erlenmeyer flask and add 5-10
drops of the EBT indicator and about 10 mL of buffer. The color change from blue to pink is an indication of the
presence of ions in the water sample.
4. To perform the titration, turn the stopcock on the burette so that the EDTA is added one drop at a time. Be
sure to go drop wise as you approach the endpoint. When you get close to the end point the solution will turn
purple.
5. Continue to add the EDTA one drop at a time until one drop turns the solution blue. Record the final volume of
EDTA in the burette on your datasheet.
6. If you overshoot the endpoint, add a known volume of tap water from the tap water burette and record it on
your datasheet.
7. Empty the flask, triple rinse with de-ionized water and dry. Repeat the titration two more times to get the exact
endpoint.
Chemical Disposal: Since the products of these titrations are dilute, non-hazardous solutions, it is safe to flush these
solutions down the drain with copious amounts of water.