At pH 10.0, the complexes formed by acidic chromium blue K with calcium and magnesium have the same maximum absorption wavelengths and are essentially identical for εMg and εCa.Under certain conditions, the colour shade of the formed complexes was proportional to the total content of calcium and magnesium ions.
I. Blanks and sample preparation
a. When the total hardness concentration is (total_hardness_L) 0.10 to 5.00 mg/L:
1. Preparation of blank samples
Take 20.0 mL of distilled water in the cuvette, add 1.0 mL of Reagent I, then add 2.0 mL of Reagent II, shake well with stopper and leave for 5 min.
2. Sample preparation
Take 20.0 mL of the sample to be tested in a clean cuvette, add 1.0 mL of Reagent I, then 2.0 mL of Reagent II, shake well with stopper and leave for 5 min.
b. When the total hardness concentration is (total_hardness_H) 5.0 to 50.00 mg/L:
1. Preparation of blank samples
Take 20.0 mL of distilled water in the cuvette, add 1.0 mL of Reagent I, then add 2.0 mL of Reagent II, shake well with stopper and leave for 5 min.
2. Sample preparation
Take 2.0 mL of the sample to be measured in the cuvette, make up 18.0 mL of distilled water, add 1.0 mL of Reagent I, then add 2.0 mL of Reagent II, shake well with stopper, and leave for 5 min.
c. When the total hardness concentration is (total_hardness_HH) 50 to 500 mg/L:
1. Preparation of blank samples
Take 20.0 mL of distilled water in a cuvette, add 1.0 mL of Reagent I, then add 2.0 mL of Reagent I. Shake well with a stopper and leave for 5 min.
2. Sample preparation
Take 0.2 mL of the sample to be measured in the cuvette, make up 19.8 mL of distilled water, add 1.0 mL of Reagent I, then add 2.0 mL of Reagent II, shake well with a stopper, and leave it for 5 min.
II. Equipment start-up：The unit is connected to the power cord and the switch is turned on.
III. Blank measurements
Under the main menu, click "Curve Selection" and select the corresponding curve.Then wipe the blank cuvette to be measured, put the cuvette into the colourimetric hole, under the main menu, click on "Blank Measurement", wait until the absorbance value is stable, press the "Confirm" button to confirm, the instrument will be zeroed automatically.
IV. Sample measurements
Clean the cuvette of the sample to be measured and put it into the colourimetric hole, under the main menu, click "Sample Measurement", the instrument will enter the sample measurement interface and directly display the total hardness concentration value (mg/L), and record the readings after the display value is stable.
V. Post-testing
After the measurement is completed, the glass instruments such as the colourimetric tubes are cleaned and dried and set aside.Switch off the equipment and disconnect the power supply.
VI. Precautions
1. Under the test conditions of this method, 10 mg of SO42-, CO32-, NO3-, Cl- do not interfere with the determination; common metal ions in water have a closed effect on complex black T; in the presence of triethanolamine (1+5) 2 mL, 0.5 mg of Al3+, 0.3 mg of Fe3+ and Fe2+ can be allowed to exist.
2. During the experiment, the glassware and colourimetric tubes used were cleaned with sulphuric acid if necessary.
3. Before the prepared sample is tested on the machine, make sure that the outer wall of the colourimetric tube should be wiped clean and there should be no water drops or hand marks.
4. Measurement range:
0.10-5.00mg/L，5.0-50.0mg/L，50-500mg/L。
When the total hardness in a water sample exceeds this range, it needs to be diluted by a corresponding factor and tested again.
