break point -experiment

The breakpoint experiment

First by iodometry (thiosulphate) method

1 chlorine solution

Prepared from a source of chlorine such as calcium hypochlorite

 Ca(OCl)­₂

Take little amount of  Ca(OCl)­₂  and dissolve it in appropriate amount of distillated water and let it to precipitate and take the clear solution 

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2) Sodium thiosulphate Na₂S­₂O₃   0.1 N , 0.025 N 

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3) Potassium dichromate K₂Cr₂O₇  0.1 N

4)potassium iodide KI 5% or crystals

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5) Starch indicator (1 g in 100 ml distillated water )

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Procedure :

i) Standardize thiosulphate by dichromate  as the following :

Add 10 ml of dichromate in conical flask ,  add  about 1 gm KI crystals ,

 ,30 ml distillated water ,2 ml glacial acetic acid  , stand for about 6 minutes ,  titrate with thiosulphate solution  until yellow color begin to pale add 1 ml of starch solution  and complete until blue color disappear  .

ii) Standardize chlorine solution to strength 1 g/L. by the same way above replacing dichromate with chlorine solution

in the manner of  1ml = 1 mg Cl.

iii) Take about 10 conical flasks ,  fill it with 500 ml raw water  and  the doses of chlorine as the following :

Close the conical flasks with plaster and place it in dark place for  the staying time of water in  precipitation pool as calculated from equation :

Staying time= Volume of precipitation pool / rate of pumped water

Shake the flasks each 30 minutes distribute the dose in the entire volume

After the time ends  titrate each conical flask with thiosulphate  as the following :

Add 2 ml of glacial acetic acid , 1 g of KI  to each flask, let stand for 6 minutes, titrate with thiosulphate solution until pale yellow then add 1 ml of  starch solution  and complete titration until blue color disappears

Calculations :

 (N*V)_S2O3=(N'*V`)_Cl

    N`=0.025*V_S2O3 /500 …………….(1)

Strength for Cl =N`*Eq.Wt

             = 0.025* V_S2O3*35.5 /500      g/L

            =0.025* V_S2O3*35.5*1000 /500       mg/L

Res Cl   = V_S2O3 *1.775   mg/L

Record your data in table like this :  

Res Cl   = VS2O3*1.775 mg/L	VS2O3   mL Taken in titration	The dose of Cl  mg/l	Series
			1
			2
			3
			4
			5
			6
			7
			8
			9
			10

Draw a plot between the doses of Cl against the Res.Cl such as:

sometimes break point curve appear as

  

the shape of the curve is differ from the conventional shape of break-point because of the trace amounts of ammonia

The first part of curve  Res Cl appear as chloramines so if ammonia present in a trace amount the formation of chloramines decrease and one imagine the  curve  like straight line but the fact that break point greater than the chloramine products and the point that the curve   incline not be clear .

 Whereas the Res.Cl at the breakpoint equals to 2.5 mg/L so the curve must appear in the way that we see above

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