Water Hardness Lab
BACKGROUND:
In some regions of the country, water is considered "hard." Hard water contains high levels of dissolved calcium and magnesium ions (Ca+2 & Mg+2). These alkaline earth metal cations precipitate (form a solid) with soaps, forming "soap scum." Since much of the soap precipitates and is thus removed from the water, there is less available to form lather. The amount of Ca2+ and Mg2+ cations can be determined using a chelating agent and an indicator. Chelating agents (also known as sequestering agents) are chemical compounds that react with metal ions to form a stable, water-soluble complex.
We will use a method called titration to determine the Ca2+ and Mg2+ content of a standard and several unknown samples. The chelating agent we'll use is ethylenediaminetetraacetic acid (EDTA), and the indicator we'll use is calmagite. EDTA chelates divalent cations, removing them from solution. The calmagite indicator will change from a muddy green to a clear blue color when all of the Ca2+ and Mg2+ have been removed from the solution.
MATERIALS:
- 2.0 mM Na4EDTA
- 0.50 mg/mL calcium carbonate (CaCO3)
- 0.1% calmagite
- pH 10 buffer
- deionized water
- 3 water samples
- a 24-well plate
- 3 test tubes
- a test tube rack
- disposable pipets
PROCEDURE:
Set-up
1. Obtain 3 clean, dry test tubes and label them "EDTA," "CaCO3," and "buffer."
2. Fill each ~1/3 full with the appropriate solution.
3. Obtain a new 24-well plate and several disposable pipets.
Titration of the Color Standard and Reference Standard
1. Add 20 drops of distilled water to well A1.
2. Add 2 drops of pH 10 buffer then 1 drop of calmagite indicator (in that order). This is the color that you're aiming to achieve in the titrations.
3. Add 10 drops of 500 mg/L CaCO3 standard to wells A3-A5, then add 2 drops of pH 10 buffer and 1 drop of calmagite indicator (in that order).
4. Add EDTA solution, drop-wise, stirring briefly after each drop, until the sample turns the same blue color as the color standard.
5. Record the number of drops added to each well in Table 2.
Titration of Unknown Samples
1. To wells B3-B5 add 10 drops of "unknown 1," add 10 drops of "unknown 2" to well C3-C5, and 10 drops of "unknown 3" to wells D3-D5.
2. Add 2 drops of pH 10 buffer and 1 drop of calmagite indicator (in that order) to each of the above 9 wells.
3. One well at a time, add EDTA solution, drop-wise, stirring or agitating briefly after each drop, until the sample turns the same blue color as the color standard.
4. Record the number of drops added to each well in Table 3.
Equations:
Eq 1: (0.5 mg/mL CaCO3 * Av drops EDTA to titrate unknown) / Av drops EDTA to titrate standard = mg/mL unknown
Eq 2: (mg hardness of unknown * 1000 mL) / 1 mL unknown = mg hardness of unknown / 1 L unknown
Table 1: Water Hardness Terminology
Ca+2 concentration
Description
>300 mg/L
Very Hard
150 to 300 mg/L
Hard
50 to 150 mg/L
Moderately Hard
<50 mg/L
Soft
TABLE 2: RAW DATA - Record your data in the table below.
Trial 1
Trial 2
Trial 3
mL of CaCO3 reference standard
0.5
0.5
0.5
drops of EDTA
30
30
20
Average drops of EDTA
mL of unknown #1
0.5
0.5
0.5
drops of EDTA
31
17
17
Average drops of EDTA
mL of unknown #2
0.5
0.5
0.5
drops of EDTA
30
30
37
Average drops of EDTA
mL of unknown #3
0.5
0.5
0.5
drops of EDTA
35
40
33
Average drops of EDTA
TABLE 3: CALCULATED DATA:
Unknowns
Unknown 1
Unknown 2
Unknown 3
Eq
Av drops EDTA (from Table 1)
mg hardness per 1 mL sample
1
mg hardness per 1 L sample
2
hardness rating (Ref table)
LABORATORY QUESTIONS: Answer the following questions in complete sentences or complete thoughts. Show your work and report all answers with correct units.
1. Calcium carbonate (CaCO3), the standard used in this lab, is the major component of chalk, limestone, marble, and the shells of aquatic animals. Before being dissolved in water, what type of solid is solid CaCO3?
2. Do you think pond water would be more or less hard than bottled water? Explain your answer.
3. The molecular formula of ethylenediaminetetraacetic acid (EDTA) is C10H16N2O8. Calculate the molar mass.
4. Calcium carbonate will react with a strong acid according to the following balanced equation. Balance the equation.
CaCO3 (s) + 2 HCl (aq) → CaCl2 (aq) + H2O (l) + CO2 (g)
5. Consider a sample of water ice - frozen H2O. What type of solid is water ice? What type or types of intermolecular force (IMF) is/are present in a sample of water ice?