Hardness is define as the concentration of multivalent cationas in solution originally, water hardness as understood to be a measure of the capacity of precipitate soap. Soap is chiefly precipitate by calcium & magnesium ion present in water
Other polyvalent cations also may get precipitate. So, they often are in complex form frequently with organic constituents and have a role in water hardness. Total hardness is defined as sum of calcium & magnesium concentration. Both are expressed as calcium carbonate in/l.
When hardness is numerically greater then the sum of carbonate and bi-carbonate alkalinity, that amount of hardness equivalent to total alkalinity is called carbonate hardness. The amount hardness in exams of these is called non-carbonate hardness. When the hardness remain in the range of 100, 200, 300… mg/l depending on the lower 7 treatment to which water has been subjected
According to international standard ISO (10500:1991), hardness of drinking water=< 300 mg/l to 600 mg/l
(mg/l as CaCO3)
Degree of Hardness
Ca(HCO₃)₂ + Heat ----------> CaCO₃ (pp+ )+CO₂+H₂O
Mg(HCO₃)₂+Heat ------------> MgCO₃ (pp+ )+CO₂+H₂O
AIM: ESTIMATION OF TOTAL HARDNESS
In alkaline condition, EDTA or its sodium salt reacts with calcium & magnesium to form soluble chelate complex. Ca & Mg ions develop wine red color small amount of dye (Erichrome Black T) EBT is added under alkaline condition. When EDTA is added as a titrate, the Ca & Mg will be complexed with EDTA resulting in pharp change from wine red to blue which indicates end-point of titration.
M+2 + EBT --------> M – EBT Complex
M - EBT + EDTA ---> M –EDTA Complex + EBT (blue color)
Where, M = Ca & Mg
- Buffer Solution
- NH₄Cl 16.9 gm
- Con. NH₄OH 143 ml
- EDTA 1.179 gm
- MgSO4 0.78 gm
- D/W (final volume) 250 ml
- Erichrome Black T Dye
- 0.095 gm dye + 10 gm NaCl
- Std. EDTA solution (0.01 M)
- EDTA 3.723 gm
- D/W 1000 ml
- Dilute 25 ml sample to about 50 ml with D/W
- Add 1 or 2 ml of buffer solution usually 1 ml will be sufficient to give the PH around 10.0
- Add a pinch of EBT and titrate with std. EDTA solution till wine red color changes to blue. Note down the volume of EDTA consumed (A).
- Run a reagent blank as with D/W as per above method.
- Note down the amount of EDTA (B).
- Calculate the volume of EDTA requirement by sample
Hardness as MgCaCO₃/l = ( C * X *1000) / ml of Sample
Where, c = ml of titration (EDTA)
X = MgCaCO3 equivalent to ml EDTA titration = 1 ml