1. Hardness of Water
1st Step: The calcium ion coordinates with the indicator (Eriochrome Black T).
H2In Ca2+ ↔ CaIn H1+
2nd Step: The EDTA chelates the calcium ion and releases the indicator.
EDTA(aq) + CaIn–(aq) + 2H+(aq) → H2In–(aq) + CaEDTA(aq)
pink blue

2. Sample Calculations: 1st Standardization of EDTA solution.
EDTA(aq) + CaIn–(aq) + 2H+(aq) → H2In–(aq) + CaEDTA(aq)
pink blue
? Mol/L
10.00 mL
38.00 mL
It took mL of EDTA solution to titrate 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 g/mol)
10.00 mL
CaCO3 sol’n
38.00 x 10-3 L
EDTA sol’n
The mean (average) of your three trials will be used for your calculations.

3. The mean (average) of your three trials will be used for your calculations.
Example: Suppose the three calculated values of your standards were , , and Calculate the relative precision in parts per thousand (ppt).
1st find the mean: ( )/3 =
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 = | | =
Trial 2: n = Trial 3: n =
Estimated precision (ppt)

4. Calculating Water Hardness (parts per million)
Water hardness ppm ≈ mg CaCO ≈ g =
1 L CaCO3soln g
Example: mL of an unknown sample was titrated with M Na2EDTA solution. If it took mL of M Na2EDTA solution to reach the endpoint, what was the “hardness of the water”?
EDTA(aq) + CaIn–(aq) + 2H+(aq) → H2In–(aq) + CaEDTA(aq)
pink blue
18.25 mL
25.00 mL
mol L
EDTA
18.25 m L
= mgCaCO3
LCaCO3
sol’n
194.6
0.025 L
CaCO3 sol’n
Sample’s water hardness = ppm
Calculate your average hardness and your experimental precision from the three trials

5. Eriochrome black T