| HIRANUMA APPLICATION DATA | Automatic Titrator | Data No. | D3 | Nov. 14,2018 |
| Environment | Calcium hardness in tap water |
1. Abstract
There are determination methods for hardness in tap water as follows: (1) Total hardness defined as the sum of calcium and magnesium ion converted to mg/L as calcium carbonate (CaCO3), (2) Calcium hardness determined by calcium ion concentration.
The measurement method for calcium hardness is similar to that of total hardness. Adjust the pH of sample water to higher than pH 12 with sodium hydroxide to mask the reaction of magnesium and EDTA. Start titration with 2-hydroxy-1-(2-hydroxy-4-sulfo-1-naphthylazo)-3-naphthoic acid (NN) indicator. Its color changes from red to blue.
| CaCO₃ + Na₂EDTA → CaEDTA + Na₂CO₃ |
This report introduces an example of calcium hardness measurement for tap water with photometric titration method using EDTA standard solution according to the Standard Methods for the Examination of Water.
2. Configuration of instruments and Reagents
| (1) | Configuration of instruments | ||
| Main unit | : | Hiranuma Automatic Titrator COM Series (M type photometric unit for photometric titration with 650 nm optical filter) |
|
| (2) | Reagents | ||
| Titrant | : | 0.01 mol/L EDTA standard solution | |
| Buffer solution | : | 28 % [w/v] Sodium hydroxide | |
| Indicator | : | NN Indicator Mix 0.5 g of NN and 50 g of powdered potassium sulfate and grind them until uniform. |
|
3. Measurement procedure
| (1) | Dispense 100 mL of sample into a 200 mL beaker with volumetric pipette. |
| (2) | Add 9 mL of 28 % [w/v] sodium hydroxide. |
| (3) | Add 0.1 g of NN indicator. |
| (4) | Immerse photometric probe and start titration with 0.01 mol/L EDTA standard solution. |
4. Measurement conditions and results
Examples of titration conditions
Measurement results
| Number of measurement |
Size (mL) |
Titrant volume(mL) |
Calcium Hardness(ppm) |
|---|---|---|---|
| 1 | 100 | 3.989 | 39.930 |
| 2 | 100 | 4.003 | 40.070 |
| 3 | 100 | 3.992 | 39.960 |
| Statistic calculation |
Avg. | 40.0 ppm | |
| SD | 0.074 ppm | ||
| RSD | 0.18 % | ||
Example of titration curve
5. Note
| (1) | Measurement The calcium hardness in tap water can accurately be determined by photometric titration method using photometric probe with NN indicator. |
| (2) | Magnesium hardness Magnesium hardness can be calculated by subtracting the calcium hardness from total hardness determined by other method. (Refer to the application data-D2 for the detail of total hardness.) Magnesium hardness (CaCO₃ mg/L) = Total hardness (CaCO₃ mg/L) ‐ Calcium hardness (CaCO₃ mg/L) |
| Keywords： | Tap water, Calcium hardness, Magnesium hardness, Photometric titration, EDTA standard solution |
*Some measurement would not be possible depending on optional configuration of system.
