Hardness of Water
1st Step: The calcium ion coordinates with the indicator (Eriochrome Black T).
H2In- + Ca2+ ↔ CaIn- + 2H2+
2nd Step: The EDTA chelates the calcium ion and releases the indicator.
EDTA(aq) + CaIn–(aq) + 2H+(aq) → H2In–(aq) + CaEDTA(aq)
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Sample Calculations: 1st Standardization of EDTA solution.
EDTA(aq) + CaIn–(aq) + 2H+(aq) → H2In–(aq) + CaEDTA(aq)
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? Mol/L
10.00 mL
38.00 mL
It took 38.00 mL of EDTA solution to titrate 10.00 mL of standardized calcium
ion stock solution (1.000 g CaCO3/L solution). Determine the molarity of the
EDTA solution. (The molar mass of CaCO3 is 100.1 g/mol)
10.00 mL
CaCO3 sol’n
10-3
38.00 x
L
EDTA sol’n
1.000g
1mol
1mol
CaCO3 EDTA
100.1g 1mol
CaCO3 sol ' n CaCO3 CaCO3
CaCO3
1000mL
=
0.002629
mol EDTA
L EDTA sol'n
The mean (average) of your three trials will be used for your calculations.
The mean (average) of your three trials will be used for your calculations.
Example: Suppose the three calculated values of your standards were
0.002629, 0.002711, and 0.002649. Calculate the relative precision in parts per
thousand (ppt).
1st find the mean: (0.002629 + 0.002711 + 0.002649)/3 = 0.002663
2nd: Find the absolute value of each trial’s deviation from the mean.
absolute deviation for trial n:
𝑛 = |Na2EDTA]mean − [Na2EDTA]n|
Trial 1: n = |0.002663 - 0.002629| = 0.000034
Trial 2: n = 0.000048
Trial 3: n = 0.000014
 (0.000034+0.000048+0.000014) 


3

 x1000  12.02 ppt
Estimated precision (ppt) 
0.002663
Calculating Water Hardness (parts per million)
Water hardness ppm
≈ mg CaCO3
≈ 10-3g
1 L CaCO3soln
1000g
= 10-6
Example: 25.00 mL of an unknown sample was titrated with 0.002663 M
Na2EDTA solution. If it took 18.25 mL of 0.002663 M Na2EDTA solution to reach
the endpoint, what was the “hardness of the water”?
EDTA(aq) + CaIn–(aq) + 2H+(aq) → H2In–(aq) + CaEDTA(aq)
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18.25 mL
25.00 mL
0.002663mol
L
EDTA
18.25 m L 0.002663 mol EDTA 1 mol CaCO3 100.1 g CaCO3 = 194.6 mgCaCO3
L EDTA
0.025 L
CaCO3 sol’n
or =
1 mol EDTA 1 mol CaCO3
LCaCO3
sol’n
18.25 mL Na 2 EDTA sol'n 0.002663mol Na 2 EDTA 1 mol CaCO3 100.1 g CaCO 3
0.02500 L CaCO3 sol ' n
L Na 2 EDTA sol'n
1 mol Na 2EDTA 1 mol CaCO3
Sample’s water hardness = 194.6 ppm
Calculate your average hardness and your experimental precision from the three trials
Eriochrome black T