Titrations: Water hardness

Classification hardness in mg/L hardness in mmol/L hardness in dGH/°dH hardness in gpg
Soft 0–60 0–0.60 0.3-3.00 0-3.50
Moderately hard 61–120 0.61–1.20 3.72-6.75 3.56-7.01
Hard 121–180 1.21–1.80 6.78–10.08 7.06-10.51
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.