SOP for the determination of Total Hardness
Scope
These SOP is applicable for the determination of total hardness and calcium hardness in water.
Principle of Total Hardness:
This method is depends on ability of EDTA to form stable complexes with calcium and magnesium ions. When the dye Eriochrome black T (EBT) is added to a solution containing calcium and magnesium ions at pH 10.0 a wine red complex is formed. This solution is titrated with standard solution of disodium salt of EDTA, which extracts calcium and magnesium from the dye complex and the dye is changed back to its original blue color. Eriochrome black T is used to indicate the end-point for the titration of calcium and magnesium together.
Calcium Hardness:
Calcium ions in water are sequestered by the addition of EDTA. The end point of the reaction is detected by means of an indicator, Murexide which is dark purple in the absence of calcium but with which calcium forms a light salmon red complex. The optimum pH range is about 10.0±0.1
Reagents
- Buffer Solutions
Dissolve 1.179 gm disodium salt of EDTA (AR) and 780 mg magnesium sulphate or 644 mg Magnesium chloride in 50 ml of distilled water. Add this solution to 16.9 g ammonium chloride and 143 ml concentrated ammonium hydroxide with mixing and dilute to 250 ml with distilled water. Dilute 10 ml of solution to 100 ml with distilled water and check pH value 10.0 ± 0.1.
- Standard Calcium Solution
1.00 ml = 1.00 mg calcium carbonate (CaCO3). Dry analytical grade calcium carbonate (CaCO3) in an oven at 180°C for 1 h. Weigh 1.000 g, suspend it in distilled water and add 1 : 1 hydrochloric acid AR quality, drop wise slowly to dissolve the solid. Use minimum amount of acid. Boil for a few minutes, cool, add a few drops of methyl red indicator and adjust to orange colour with 3 N ammonium hydroxide or 1 : 1 hydrochloric acid. Dilute to 1 000 ml with distilled water.
- EBT
Dissolve 0.5gm of Eriochrome black-T in 100ml solvent grade Methanol
- Standard EDTA Solution
Dissolve 3.723 g EDTA which has been dried overnight in a sulphuric acid desiccator, in demineralized water and dilute to 1 000 ml.
- Sodium Hydroxide (1N)
- Murexide Indicator
It is prepared by mixing 200 mg of Murexide with 100 g of solid sodium chloride and grinding the Mixture to 300 to 425 microns.
Procedure of Total Hardness
- Measure 50ml of the sample into a 250ml conical flask.
- Add 6-7 drops of Ammonia buffer solution mix well then add 2-3 drops of EBT indicator.
- Titrate the sample immediately with 0.02 N EDTA solution with continuous mixing until the color changes from trace of red and purple disappear and solution is clear sky blue in color.
- As the end point is approached, the solution shows some blue coloration but a definite tinge is observed and is discharged at the end point.
Procedure of Calcium Hardness
- Take 50 ml of water sample in a 250-ml conical flask.
- Add 2 ml of 1N Sodium hydroxide solution.
- Add a pinch (0.1gm or 0.2 gm) of Murexide Indicator.
- Titrate against 0.02 N EDTA solutions.
- At the end point, color changes from pink to purple.
Calculation
Total hardness (as CaCO3), mg / litre = (V1-V2) x Normality of EDTA x 50 x 1000/V3
Where – V1 = Volume in ml of EDTA solution used in the titration for sample .
V2= Volume in ml of EDTA solution used in the titration for blank .
V3 = Volume in ml of the sample taken for the test.
Calcium hardness of water (mg/l) = (V1-V2) x Normality of EDTA x 1000 x 50 / V3
Where – V1 = Volume in ml of EDTA solution used in the titration for sample.
V2= Volume in ml of EDTA solution used in the titration for blank.
V3 = Volume in ml of the sample taken for the test.
Magnesium Hardness Calculation: –
Magnesium Hardness (mg/l) = Total Hardness – Calcium Hardness
Safety Guidelines
Proper Safety and health precaution are taken while doing the analysis by using safety goggles, Apron, Safety shoes and Using Fuming Chamber for fume generating chemicals.
References
IS 3025 (P-21): 2009 RA 2019