051 Determining the Hardness of Water

Expt 051 -- Determining the Hardness of Water

The concentration of certain dissolved ions such as calcium and magnesium ions that make water hard is determined by a titration procedure using EDTA.

Background

Water containing certain ions reacts with soap forming a precipitate called soap scum. For example,

2 NaC₁₅H₃₁COO + Ca²⁺ --> Ca(C₁₅H₃₁COO)₂ + 2 Na⁺
Contemporary society deals with this problem in two ways. One way is to use detergents whose calcium salts are soluble. Another is to treat the water by softening -- usually replacing calcium and magnesium with sodium.
Hardness is reported as mg CaCO₃ because dissolved limestone is the major cause of hardness in most regions. (Dissolved Mg²⁺ and sometimes Fe²⁺ also contribute.)
Calmagite has a red color when complexed with Mg²⁺ and a blue color when Mg²⁺ is chelated by the EDTA. Ca²⁺ is more tightly bound to EDTA then Mg²⁺. Consequently, the color change for Mg²⁺ occurs after all Ca²⁺ is chelated.
Eriochrome Black T indicator has a red color when complexed with Ca²⁺ or Mg²⁺. It is blue when these ions are removed. A change from red to blue is the basis of the endpoint.
The EDTA is a weak acid. A pH 10 buffer is used to keep the acid groups deprotonated. (The EDTA can be envisioned to wrap itself around the divalent cation.)

Safety

The buffer is toxic and gives off toxic vapors. Wear goggles and apron. Use is a well ventilated laboratory area. Use the buffer from a 1.0-mL microtip pipet or a pulled plastic transfer pipet. Wash hands after the experiment.

Procedure

Use either Calmagite or Eriochrome Black T as the indicator. Movies and pictures labeled "ET" use Eriochrome Black T; all other movies and pictures display Calmagite.
Obtain a 24-well plate.
Add 5 or six drops of distilled water to one well. Add 2 drops of buffer solution and 1 drop of indicator solution to the well. The solution will be red. Add EDTA until the solution is completely blue. One drop of EDTA will be enough if the distilled water is pure. Add a drop beyond the endpoint. Use this blank for a color comparison during your titrations.

!!!Click here to See Picture.
!!!Click here to See Picture.
Set up solutions for three separate identical trials in adjoining clean wells of the plate.
To each well, add 20 drops of the calcium standard to be tested to a clean well of the plate. Add 2 drops of buffer. Add 1 drop of indicator solution. Stir each well. Compare the color to the blank.

!!!Click here to See Movie.
!!!Click here to See Movie.
Add EDTA with rapid stirring until the solution turns purple. The change from red to purple is gradual. Thereafter, add one drop at a time, and stir thoroughly. Continue until the color starts to change from purple to blue. The reaction is slow; allow time for the reaction to take place.

!!!Click here to See Movie.
!!!Click here to See Picture.
!!!Click here to See Movie.
Add a drop of calcium standard to see if the purple color returns.

!!!Click here to See Movie.
!!!Click here to See Movie.
If the solution remains blue, very slowly add the calcium standard dropwise with stirring after each addition. Stop when the solution just turns purple. Add these drops to the number of drops of standard in your calculations.

!!!Click here to See Movie.
!!!Click here to See Movie.
You may add one more drop of EDTA to see the color change one more time before doing the remaining trials.

!!!Click here to See Movie.
Repeat the titration. Subtract 5 drops from the number of drops required for the first calcium standard. Add that amount at once. Stir until no further color change is noted. Continue titrating dropwise. Stir after each addition.

!!!Click here to See Movie.
!!!Click here to See Movie.
Perform three trials. Once you know the approximate amount of EDTA to use, the repeat titrations will go quickly.
Add 50 drops of the water to be tested to one clean well of the plate. Add 2 drops of buffer. Add 1 drop of indicator solution.
Add EDTA, one drop at a time, and stir. Continue until the color starts to change from purple to blue. The reaction is slow; allow time for the reaction to take place (many seconds; not many minutes, but not instantaneous either.)
Use the values from the first titration to adjust the number of drops used in repeat trials. The total number of drops should be kept within a factor of 2 of the number used to titrate the standard. Dilute the standard is necessary.
Add 2 extra drops of buffer for each 30 drops of unknown titrated past the first 30 drops.
Repeat the titration at least twice for the unknown.
Wash hands.
The calcium standard is reported as mg CaCO₃. Use the ratio of the number of drops of EDTA for the known to the unknown to determine the hardness of the unknown sample(s).

Questions

Twenty drops standard solution containing 483 "mg CaCO₃"/L required 24 drops of EDTA for titration. When 45 drops of an unknown are titrated with the same EDTA, 22 drops are used. Find the hardness (mg CaCO₃/L) for the unknown.
Explain the basis of reporting water hardness in terms of mg CaCO₃/L.
Explain the addition of more buffer when "soft" waters are titrated.

Handout Makeup

Name ___________________________ Class _______

Teacher __________________________

SmallScale 051 Determining the Hardness of Water

Watch the movies.

The 20 drops of calcium standard (500 mg CaCO₃/liter) required 20, 20 and 21 drops of EDTA for titration.

H1. 20 drops of an unknown required 5 drops of EDTA. How many drops of this sample should be titrated according to the instructions?
H2. How many drops of buffer must be added to the unknown for this titration?
H3. Two 50-drop portions of a different unknown required 21 and 22 drops of EDTA, respectively. The calcium standard is reported as mg CaCO₃. Use the ratio of the number of drops of EDTA for the known to the unknown to determine the hardness of the unknown sample(s).

Answer the questions.

Curriculum-

Use when discussing water and water treatment. If there is a section on alkaline earth metals, this topic fits well. It is an excellent experiment for applied chemistry. It works with solution stoichiometry and volumetric assays. It also works well when the topic of complex ions is covered.

Safety-

The buffer is toxic and gives off toxic vapors. Wear goggles and apron. Use is a well ventilated laboratory area. Wash hands after the experiment.
Use extreme caution when preparing the buffer solution. The amounts being handled are significant, and inhalation of fumes is a concern.

Time-

Teacher Preparation: 30 minutes

Class Time: 40 minutes

Materials-

6 mL of EDTA -- (Dissolve 0.19 g disodium EDTA dihydrate (Na₂C₁₂H₁₄N₂O₈•2H₂O) in enough water to make 100 mL solution.) If calmagite is the indicator used, add one very small crystal of magnesium salt to the solution. See Lab Hints.
6 mL of 0.005 M Calcium standard -- (Dissolve 0.367 g calcium chloride dihydrate (CaCl₂•2H₂O) in enough water to make 500 mL solution.) This solution has a hardness of 500 mg CaCO₃/liter.
1.5 mL of Buffer, pH 10 -- (In a hood mix 30 mL of concentrated NH₃ with 3.33 g. NH₄Cl; dilute to 100 mL with water.) Provide this solution in 1-mL microtip pipets or pulled plastic transfer pipets. Do not provide in open beakers.
Use either Calmagite or Eriochrome Black T indicator (See Lab Hints.)
- 0.5 mL of Calmagite--0.2% in ethylene glycol(Weigh 0.100g Calmagite. Add Calmagite to 50 mL of ethylene glycol or an ethylene glycol antifreeze.
  - or
- 0.5 mL of Eriochrome Black T -- (Weigh 0.15 g Eriochrome Black T and dissolve in 25 mL of triethanolamine.) (This solution is stable indefinitely. A methanol solution may be used, but it is not stable for more than a few days or weeks.)
Unknown water samples to be tested. The experiment is more interesting if several different samples are available. Local tap water is one obvious possibility. Waters of differing hardness may be available locally. Surface water such as lakes and streams may also be tested. Students may want to bring in their own samples (home, neighbors, relatives).
2 pipets for titration
2 pipets for dispensing indicator and buffer
24-well plate
toothpicks

Disposal-

Neutralize the solutions using vinegar. Discard the solutions used in the titrations at the sink with running water.

Lab Hints-

Calmagite responds to Mg²⁺. To be sure that some Mg²⁺ is present, a small amount is added to the EDTA solution. Since the EDTA solution is standardized with known calcium solutions after the magnesium addition, this addition does not change the results. Calmagite is soluble in ethylene glycol. Calmagite dissolved in antifreeze (with the characteristic dye marker) is used in these pictures and movies.
Eriochrome Black T is also a common indicator for this titration. Solutions of Eriochrome Black T tend to be unstable. When dissolved in triethanolamine as a solvent, Eriochrome Black T solutions are stable for a very long time. Pictures and movies with this indicator are also included.

Data Analysis-

The calcium standard required 20, 20 and 21 drops of EDTA for titration. 20 drops of unknown required 9 drops of EDTA. On the basis of this observation, the sample was changed to 50 drops. Two 50-drop portions of unknown required 21 and 22 drops of EDTA, respectively.

mg CaCO₃ = 500 mg CaCO₃ x( 20 drops std/ 50 drops unk ) x ( 21.5 drops EDTA/20.3 drops EDTA)

= 210 mg CaCO₃

Answers-

Q1. Twenty drops standard solution containing 483 "mg CaCO₃"/L required 24 drops of EDTA for titration. When 45 drops of an unknown are titrated with the same EDTA, 22 drops are used. Find the hardness (mg CaCO₃/L) for the unknown.
!!!Click here to See Picture. A2. mg CaCO₃ = 483 mg CaCO₃ x (20 drops std/45 drops unk) x (22 drops EDTA/24 drops EDTA) = 200 mg CaCO₃
Q2. Explain the basis of reporting water hardness in terms of mg CaCO₃/L. A2. A primary source of water hardness comes fro dissolving limestone, CaCO₃.
Q3. Explain the addition of more buffer when "soft" waters are titrated. A3. When the volume is increased, more buffer is needed to keep the pH high.

Makeup Ans.-

H1. 20 drops of an unknown required 5 drops of EDTA.
How many drops of this sample should be titrated according to the instructions? HA1. (20 drops EDTA known/5 drops EDTA unknown) x 20 drops unknown = 80 drops recommended. Value may be 40 to 100 drops according to instructions. Because the well's size sets a limit on the total volume, values between 40 and 80 are practical.
H2. How many drops of buffer are recommended for this titration? HA2. One drop for each 30 drops of sample is recommended. A 80 drop sample would require a total of 3 drops of buffer. A 40 drop sample would require 2 drops of buffer.
H3. Two 50-drop portions of a different unknown required 21 and 22 drops of EDTA, respectively. The calcium standard is reported as mg CaCO₃. Use the ratio of the number of drops of EDTA for the known to the unknown to determine the hardness of the unknown sample(s). HA3. mg CaCO₃ = 500 mg CaCO₃ x (20 drops std/ 50 drops unk) x (21.5 drops EDTA/20.3 drops EDTA) = 210 mg CaCO₃

Literature Data-

Labels used to describe water hardness:

Concentration (mg CaCO₃) Description

0-50
soft

50-100 moderately hard

100-300 hard

> 300 very hard

CoopLearn-

Different groups may be assigned water from different sources to collect and analyze. Groups may then share the results. In some circumstances, using a map to plot results may be of great interest -- say when students in a class use water from two quite different sources.

Key Words 1-

water, water treatment, alkaline earth metals, applied chemistry, complex ions, solution stoichiometry, solutions, endpoint, assay, titration, volumetric,

Elements-

Ca Mg C

Concentration (mg CaCO₃)	Description
0-50	soft
50-100	moderately hard
100-300	hard
> 300	very hard