How are the multipliers derived for the hardness buret titration?

Document ID

Document ID TE283

Version

Version 4.0

Status

Status Published

Published Date

Published Date 07/10/2018
Question
How are the multipliers derived for the hardness buret titration?
Summary
Multipliers for the hardness buret titration
Answer
Using Normality (equivalents/L):

At endpoint, equivalents of EDTA = equivalents of CaCO3.
Equivalents/L of CaCO3 = conc (g/L)/equivalent weight.
The equivalent weight of CaCO3 = ½ MW CaCO3 = 50 g/equivalent (in a complex-formation reaction, the equiv weight is that weight which reacts with or provides one mole of the reacting cation if it is univalent, or one-half mole if divalent, one-third mole if trivalent, etc.).
Each mL EDTA used = 0.00002 equivalents:
0.001 L x 0.02 equivalent EDTA/L = 0.00002 equivalent EDTA = equivalent CaCO3.

Convert to mg CaCO3 and divide by sample volume to get sample concentration:
0.00002 equivalent CaCO3 x 50 g/equivalent = 0.001 g or 1 mg CaCO3. Example: Sample volume is 50 mL: 1 mg/0.05 L = 20 mg/L. Therefore, 1 mL of 0.02 N EDTA = 20 mg/L CaCO3 if 50 mL sample volume is taken; multiplier is 20

Using Molarity and a 0.02 N EDTA solution:
0.020 N EDTA = 0.01 M EDTA
At endpoint, moles EDTA = moles Ca or CaCO3
Each mL EDTA used = 0.00001 moles CaCO3:0.001 L x 0.01 mol EDTA/L = 0.00001 moles EDTA = moles Ca or CaCO3.

Convert to mg CaCO3 and divide by sample volume to get sample concentration:
0.00001 mol CaCO3 x 100 g CaCO3/mol = 0.001 g or 1 mg CaCO3. Example: Sample volume is 50 mL: 1 mg/0.05 L = 20 mg/L. Therefore 1 mL 0.02 N EDTA = 20 mg/L CaCO3 if 50 mL sample volume is taken; multiplier is 20.

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