Bleaching With Hypochlorites

Hypochlorites

Hypochlorites are salts, chiefly important as powerful oxidizing and bleaching agents; not, however, when pure, but when containing chlorides. The chief hypochlorites, or at least the complex substances which contain hypochlorites, are bleaching-powder, and the bleach liquors made with potash and caustic soda/soda.

Bleaching Powder

Calcium hypochlorite is a chemical compound with formula Ca(ClO)_2,Molecular weight: 143 .Calcium hypochlorite is usually a white or grayish-white powder free from lumps , contains about 35-37% active chlorine, 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.

Bleaching powder is partially soluble in water, the hypochlorite dissolving and being responsible for oxidizing and bleaching powers.Available chlorine is a measure of the oxidizing power of the hypochlorite ion.

Chemical properties of bleaching powder

Reaction with Water

Ca(OC)Cl + H₂O = Ca(OH)₂ + Cl₂
With HCl:
Ca(OCl)Cl + 2HCl = CaCl₂ + Cl₂ + H₂O
With CO₂:
2Ca(OCl)Cl + CO₂ + H₂O = CaCO₃ + CaCl₂ +2HOCl

Sodium Hypochlorites

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,cl>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.

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.

Temperature:-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.

Testing of Available Chlorine in bleaching powder

Two methods are in common use for the determination of the available chlorine. In the first, the hypochlorite solution or suspension is treated with an excess of a solution of potassium iodide, and strongly acidified with acetic acid:

ClO^- + 2I^- + 2H⁺ = Cl^- + I₂ + H₂O

The liberated iodine is titrated with standard sodium thiosulphate solution. The solution should not be strongly acidified with hydrochloric acid, for the little calcium chlorate which is usually present, by virtue of the decomposition of the hypochlorite, will react slowly with the potassium iodide and liberate iodine:

ClO₃^- +6I^- + 6H⁺ = Cl^- + 3I₂ + 3H₂O

In the second method, the hypochlorite solution or suspension is titrated against standard sodium arsenite solution; this is best done by adding an excess of the arsenite solution and then back-titrating with standard iodine solution.

Procedure

1.Available chlorine in bleach liquor (iodometric method)

Reagents required
1.70 gpl solution of sodium thiosulphate
2.10 % potassium iodide solution
3.400 gpl acetic acid solution
4.Starch indicator solution
Procedure
Take 10 ml of bleach liquor solution in a conical flask , add 20 ml KI solution and 20 ml acetic acid solution , the color will turn to amber brown , shake it well and then titrate it with 70 gpl sodium thiosulphate solution to a light brown color , now add few drops of starch indicator and titrate it to a colorless end point.
Result:- gpl of available chlorine is equal to  ml of thiosulphate consumed.
2.Sodium Hydroxide and Sodium Carbonate Determination
Take 10 ml of sample and add equal(of av. chlorine gpl) ml of 70 gpl sod.thiosulphate to neutralise the chlorine , shake well and titrate the content with N/2 sulphuric acid using phenolphthalien indicator.
End point:-Pink to color less.
caustic Gpl will be equal to ml of sulphuric acid consumed.
3.Available chlorine in Bleaching Powder
About 2 gm of sample , accurately weighed , is pasted up with a little water and made up 500 ml .Take 50 ml of this , add 20 ml of 10% KI , and solution is acidified with dilute acetic acid , titrate it with 0.1 N Sodium thiosulphate using starch indicator , to a colorless end point.
1 ml of 0.1 Sod. thiosulphate = 0.003546 g chlorine.
other tests of bleaching powder are alkalinity and chlorates.
for detailed analytical procedure click here