Analysis of Bottled Water

 

II. Calcium and Magnesium (Hardness) – Form B

 

Hardness is the measure of the concentration of calcium and magnesium salts in water.  The degree of hardness depends on the type and amount of impurities present in the water.  Hardness caused by carbonates and bicarbonates is called carbonate hardness, while hardness caused by all others (chlorides, sulfates, nitrates) is called noncarbonated hardness.  Hardness enters a water supply when calcium and magnesium salts are dissolved by ground water.  Carbonate hardness is the result of rain water dissolving limestone, i.e., calcium and magnesium carbonate.  Hardness also depends on the amount of carbon dioxide in solution.  Carbon dioxide influences the solubility of the impurities that cause hardness.  Hardness reacts chemically with soap and the higher the hardness, the more soap is required to form lather. 

Sources:

http://www.tpub.com/utilities/59.htm

http://www.yorkshirewater.com/yourhome/inf_hard.html

http://www.aqualink.com/columns/k-love3.html

 

 

Ca2+ + H2EDTA2- → Ca(EDTA)2- + 2H+

 

Procedure:

 

            Standard

·        Place 20 drops of 500ppm CaCO3 in a watch glass

·        Add two drops of pH 10 buffer and one drop of calmagite indicator

o       Stir with toothpick (The solution should be red)

·        Titrate with 0.005 M EDTA one drop at a time

o       Stir with toothpick after each drop

o       Keep track of the number of drops added

o       Stop when a blue color develops

·        Record number of drops of EDTA added and repeat four more times

Bottled water

·        Place 20 drops of bottled water in a watch glass

·        Add two drops of pH 10 buffer and one drop of calmagite indicator

o       Stir with toothpick (The solution should be red)

·        Titrate with 0.005 M EDTA one drop at a time

o       Stir with toothpick after each drop

o       Keep track of the number of drops added

o       Stop when a blue color develops

·        Record number of drops of EDTA added

·        Repeat procedure four more times - maintain precision within one drop

·        Calculate the amount of Ca and Hardness in ppm and record data below

·        Transfer all your data to the class data sheet - Form F

 

 

Standard = 500 ppm CaCO3

Titration

#1

#2

#3

#4

#5

Average

# of Drops of water

20

20

20

20

20

EDTA drops

Drops of EDTA

 

 

 

 

 

 

 

Water Samples

Sample 1:

#1

#2

#3

#4

#5

Average

# of Drops of water

20

20

20

20

20

EDTA drops

Drops of EDTA

 

 

 

 

 

 

 

Sample 2:

#1

#2

#3

#4

#5

Average

# of Drops of water

20

20

20

20

20

EDTA drops

Drops of EDTA

 

 

 

 

 

 

 

Sample 1:

#1

#2

#3

#4

#5

Average

# of Drops of water

20

20

20

20

20

EDTA drops

Drops of EDTA

 

 

 

 

 

 

 

Sample 1:

#1

#2

#3

#4

#5

Average

# of Drops of water

20

20

20

20

20

EDTA drops

Drops of EDTA

 

 

 

 

 

 

 

ppm of Ca =[ (500 ppm CaCO3 Standard) (Av. Drops of EDTA needed to titrate water sample) ÷

                                    (Av. Drops of EDTA needed for the standard)] (0.40 Ca in CaCO3)

 

ppm of Hardness = (500 ppm CaCO3 Standard) (Av. Drops of EDTA needed to titrate water  sample) ÷ (Av. Drops of EDTA needed for the standard)

 

Ca+ and Hardness Data

Type and Brand of water

Ca+  (ppm)

Hardness (ppm)

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Levels of Hardness (ppm CaCO3)

0-100

Soft

100-200

Moderate

200-300

Hard

300-500

Very Hard

500-1,000

Extremely Hard

 

Questions

 

  1. Was there a significant difference in hardness among the water samples?  Was one sample really hard or soft?
  2. In these titrations, there may be an uncertainty of at least one drop in identifying the exact end point.  If you use 20 drops of EDTA solution in a titration, what percent uncertainty in the hardness is contributed by adding one extra drop?

% error = [(# of drops you are in error)/(# of drops used)] x 100

  1. Is very soft water the same as very pure water? Explain.