Water hardness

Classificationhardness in mg/Lhardness in mmol/Lhardness in dGH/°dHhardness in gpg
Soft0–600–0.600.3-3.000-3.50
Moderately hard61–1200.61–1.203.72-6.753.56-7.01
Hard121–1801.21–1.806.78–10.087.06-10.51
Very hard≥ 181≥ 1.81≥ 10.14≥ 10.57

Table from Wikipedia

EDTA Titrimetric Method (adapted from APHA Standard Method 2340 C. Titrimetric Method)

Indicator dye is Calmagite. This dye changes color depending on pH.
pH below 9.1, dye is a red color. Measure max. at 520 nm
pH 9.1-11.4, dye is a blue color. Measure at 610 nm.

Reagents

 

 Catalog #

 Cost

 Shelf life

Water hardness buffer LC268101 $24.10 for 500 mL 6 months
0.1% calmagite indicator solution LC128507 $14 for 125 mL 24 months
0.01M EDTA titrant LC138501 $13.80 for 500 mL 24 months
1,000 ppm calcium carbonate standard, 
(1 mL = 1 mg CaCO3)
 LC127007 $21.50 for 125 mL 24 months


Calibration Curve
  • Dilute the CaCO3 standard to yield a range between 10 to 200 mg/L (i.e. from soft to very hard)
  • Add 0.4 mL buffer and mix
  • Add 0.4 mL indicator dye and mix

  ppm CaCO3 Vol CaCO3 stock (mL)Vol dH2O (mL)  Vol buffer (mL)Vol calmagite (mL)
 1 10 ppm 0.10 9.15 0.40 0.40
 2 25 ppm 0.25 8.95 0.40 0.40
 3 50 ppm 0.50 8.70 0.40 0.40
 4 75 ppm 0.75 8.45 0.40 0.40
 5 100 ppm 1.00 8.20 0.40 0.40
 6 125 ppm 1.25 7.95 0.40 0.40
 7 150 ppm 1.50 7.70 0.40 0.40
 8 175 ppm 1.75  7.45 0.40 0.40
 9 200 ppm 2.00 7.20 0.40 0.40

Method 

  • Transfer 2 mL into a cuvette with a stir bar
  • Place cuvette in colorimeter
  • Place colorimeter onto the stir plate base and switch the stir-plate on
  • Start colorimeter plotting program and select the red led (625 nm) and calibrate.
  • Click measure and enter 0.00 mL volume in the table
  • Slowly add EDTA titrant. Initally add 50 uL drops, then once absorbance starts to change, add in 10uL steps. After each addition of EDTA click measure. Once absorbance stops changing (absorbance around 0.9 units) stop adding EDTA. Switch off stir plate.

Calculating titration endpoint volume

We recently wrote a python program to calculate titration endpoint volume. (Still in early development days). The colorimeter titration program finds the endpoint and corresponding EDTA volume. 
Below is an example titration with endpoint.




Subpages (1): Experiment 1
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Joanne Long,
Jul 6, 2014, 6:18 PM
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Joanne Long,
Jul 19, 2014, 6:27 PM
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