Sunday, 15 May 2016

Water Analysis : To Estimate The Total Hardness Of Water

INTRODUCTION:
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
Hardness Range
(mg/l  as CaCO3)
Degree of Hardness
0-75
Soft
75-150
Hoderatory hard
150-300
Hard
7300
Very hard
Reactions:
Ca(HCO) + Heat   ----------> CaCO (pp+ )+CO+HO
Mg(HCO)+Heat ------------> MgCO (pp+  )+CO+HO
 
AIM: ESTIMATION OF TOTAL HARDNESS
PRINCIPLE:
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
REQUIREMENT:
  •        Buffer Solution
-          NHCl 16.9 gm
-          Con. NHOH 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
PROCEDURE:
-          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
CALCULATION:
Hardness as MgCaCO/l =  ( C * X *1000) / ml of Sample

Where, c = ml of titration (EDTA)
              X = MgCaCO3 equivalent to ml EDTA titration = 1 ml

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