Tuesday, September 8, 2020

CHLORIDE ION IN WATER

Determine the Chloride ion concentration in a water sample

 Chloride-ion in Water

Chloride ion icon image

Chloride ion can be determined by titration of water sample against a standard solution of AgNO3 using Potassium Chromate as indicator, in a neutral or slight alkaline medium. When AgNO3 solution is added to the water sample in presence of K2CrO4,the chlorides present in it are precipitated as AgCl. All the Chlorides are precipitated out even a drop of AgNO3 added in excess gives a red precipitate of silver chromate, this indicates the end point.

REACTION:

 AgNO3+NaCl®AgCl¯ +NaNO3

                        (white ppt.)

AgNO3+ K2CrO4® Ag2CrO4 ¯ +2KNO3

                               (Red ppt)

The PH of the solution should be in between 7-8, because at higher PH, Ag+ ion is precipitated as AgOH.

            Ag++OH-®AgOH

If the PH is lower than 7-8 then the chromate ion is converted to dichromate ion.

   2CrO42-+2H+=Cr2O72-+H2O

       The required PH range can be achieved easily by adding a pinch of pure CaCO3.Excess CaCO3 being insoluble , does not interfere.

      2H++CaCO3® Ca2++CO2+H2O

APPARATUS:

  • Conical flask
  • Burette
  • Measuring cylinder
  • Beaker
  • Dropper
  • Volumetric flask

PROCEDURE:

  • Blank Correction:

i)Take 25 ml distilled water in a beaker and add 3-4 drops of K2CrO4 indicator.

ii)Titrate against (N/50) AgNO3 solution till the brick red colour appears in the solution.

iii)Repeat the Titration.

  • Sample water titration:

i)Take 25 ml of water sample in a beaker and add a drop of methyl orange. If red or pink colour appears, add a pinch of CaCO3, red or pink colour disappear.

ii)Add 2-3 drops of K2CrO4 indicator and titrate against (N/50) AgNO3 solution until the red colour appear in the solution.

iii)Repeat.


OBSERVATION:

  • Table 1: Blank Correction-
Chloride table 1 image


  • Table 2: Sample water-
Chloride table 2 image

CALCULATION :-

We know,

                                                V1S1=V2S2

                                                S2=V1S1/V2        


                                                                                                 V1=Volume of AgNO3 sol.

 S1=Strength of AgNO3 sol. =(N/50)

V2=Volume of water

S2=Strength of water


V1= |Vol. of AgNO3 sol. Of sample water-Vol. of AgNO3 sol. Of distilled water|

Strength of Chloride ion = S2X 35.5 g/l

                                           =S2 X 35.5 X 1000 mg/l

\The Chloride ion concentration in the given sample is ______mg/l.


PRECAUTION:

i)The glass apparatus should be washed with distilled water before Chloride ion test. 

ii)Volume of the indicator should be same in all titration.

iii)The end point should be noted when a tinge of brownish red colour persists.






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