Hypochlorites Bleaching Process | Bleaching with Hypochlorites

Bleaching with Hypochlorites
Bleaching Powder
Calcium hypochlorite is usually a white or grayish-white powder, but it is also available as tablets. It is a strong oxidizer and is considered more stable than sodium hypochlorite. It is also believed to provide more chlorine.

Sodium Hypochlorite

Sodium Hypochlorite is a greenish-yellow liquid commonly referred to as "Bleach." The chemical compound formula for Sodium Hypochlorite is NaOCl.Sodium Hypochlorite is prepared by reacting dilute caustic soda solution with liquid or gaseous chlorine, accompanied by cooling. It is used extensively as a bleaching agent in the textile, detergents, and paper and pulp industries.

The active ingredients in hypochlorite bleaches vary with pH. At pH <>2 is the main component in solution; at pH 4 to 6, HOCl is the dominant species; at pH > 9, OCl − is the only component present. It is the hypochlorite ion in basic solution that is the active ingredient in household bleach, which is typically about 5 to 6 percent NaOCl. The OCl − ion oxidizes chromophores in colored materials, and is itself reduced to chloride and hydroxide ions.

Hypochlorite Bleaching Process
The conditions of the bleaching agent varies according to the result required, the concentration depends upon the fabric quality, degree of whiteness required, types of machine and next operation.

Quantity Required:-
Normally 2.5-3.0 gpl of available chlorine is sufficient for good bleaching , but it is necessary to optimize it on a possible lower value for safe bleaching process.

Analysis Method

Determination of available Chlorine.
Method : Iodometric(Standard)

Apparatus :
  • Erlemeyer flask(100 ml) -I
  • Burette stand -I
  • Pipette(50 ml) -I
  • Spatula -I
  • Measuring Cylinder -I
  • Micro burette(10ml.) -I
  • Measuring Cylinder. -I
  • Sodium Thio Sulphate Solution(.1N)
  • Glacial acetic acid
  • Pottassium Iodide(KI crystal)
  • Starch indicator solution.
Typical Bleaching Recipe for Hypochlorites

Wetting agent                                        0.3-0.5g/l
Caustic Soda                                        0.5-1.0g/l
Soda ash                                              2.0-4.0g/l
Sodium hypochloride solution                5.0-15.0ml/l
Temperature                                         25-60C
Time                                                     30-60 mins
pH                                                       10.5 
M:L                                                      1:10

Procedure :-
1) Collect sample solution ( Sodium hypo chlorite ) in an air-tight bottle.

a) Keep sample solution away from excessive light and do not agitate the sample solution.
b) Start chlorine determination IMMEDIATELY after sampling. ( i.e. do not store sample to be analysed for chlorine ).]

2) Introduce 10ml of glacial acetic acid and about 0.5 g of Pottassium Iodide (Spatula full ) crystal into a 100ml erlemeyer flask.
3) Withdraw y ml of sample solution with bulb pipette and introduce into above Solution.
4) Swirl the flask to well mix the solution.
5) Titrate with 0.1N Sodium Thio Sulphate Solution from upto a light Yellow Colour.
6) Add 1 ml of starch indicator solution and continue to titrate until the last drop of titrant causes the solution to turn colourless.

% active chlorine = 3.545 x N x V/y (i;e 3.545 x N x V divided by y)

Where :
V = Volume of (N ) Na2S2O3
N = Normality of Na2S2O3
y = Sample amount

Important Factors for Bleaching:

pH :-
General PH range is 10-11 or 10.5-11.5 during bleaching if PH reaches 9 then it is a danger level and at 7 PH the bleaching is worse and causes extreme damage to the cloth.The PH value is maintained by adding sod ash or by buffering agent.

The suggested temperature for hypochlorite bleaching is 37-40oC. Generally reaction is accelerated with increases of temperature.

Water Quality:-
Water for bleaching should be soft and even hard water can be used but should be free from Cu++ and Fe++.

Effect of Metals:-
The bleaching equipment should be made of stainless steel , to avoid the catalytic degradation of the cellulose in the presence of copper and iron.

Substrate Preparation
The substrate must be pre scoured in the presence of chelating agents , it should be free from rust spots and traces of metallic impurities when bleaching with hypochlorites.

Bleaching Time :-
The time factor depends upon the following consideration.

(a) Concentration.
(b) PH value.
(c) Degree of Whiteness.
(d) The type of machine used in bleaching.

Roughly for normal machine the time is 2-3 hrs is required for completion of bleaching process antichlor and proper neutralization treatments should be followed by proper wash for removal of reagents after hypochlorite bleaching process.

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Mazharul Islam Kiron is a textile consultant and researcher on online business promotion. He is working with one European textile machinery company as a country agent. He is also a contributor of Wikipedia.

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