estimation of chlorine

7/30/2019 estimation of chlorine

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Hypochlorites are salts, chiefly important as powerful oxidizing and bleaching agents; not, however, when pure, butwhen 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 PowderCalcium 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 strongoxidizer and is considered more stable than sodium hypochlorite. It is also believed toprovide more chlorine.

Bleachingpowder is partially soluble in water, the hypochlorite dissolving and beingresponsible for oxidizing and bleaching powers.Available chlorine is a measure of theoxidizing power of the hypochlorite ion.

Chemical Properties of Bleaching Powder

Reaction with WaterCa(OC)Cl + H2O = Ca(OH)2 + Cl2With HCl:Ca(OCl)Cl + 2HCl = CaCl2 + Cl2 + H2OWith CO2:2Ca(OCl)Cl + CO2 + H2O = CaCO3 + CaCl2 +2HOCl

Sodium HypochloritesSodium Hypochlorite is a greenish-yellow liquid commonly referred to as "Bleach." Thechemical compound formula for Sodium Hypochlorite is NaOCl.Sodium Hypochlorite isprepared 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 inhypochlorite bleachesvary with pH. At pH 2 is themain 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 theactive 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 tochloride and hydroxide ions.

Hypochlorite Bleaching ProcessThe conditions of the bleaching agent varies according to the result required, theconcentration depends upon the fabric quality, degree of whiteness required, types ofmachine and next operation.

Quantity Required:Normally 2.5-3.0 gpl of available chlorine is sufficient for good bleaching , but it isnecessary to optimize it on a possible lower value for safe bleaching process.
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pH :General PH range is 10-11 or 10.5-11.5 during bleaching if PH reaches 9 then it is adanger level and at 7 PH the bleaching is worse and causes extreme damage to thecloth.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 isaccelerated with increases of temperature.

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

Effect of MetalsThe bleaching equipment should be made of stainless steel , to avoid the catalytic

degradation of the cellulose in the presence of copper and iron.

Substrate PreparationThe substrate must be pre scoured in the presence of chelating agents , it should befree 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 bleachingprocess. Antichlor and proper neutralization treatments should be followed by properwash for removal of reagents after hypochlorite bleaching process.

Testing of Available Chlorine in Bleaching PowderTwo methods are in common use for the determination of the available chlorine. In thefirst, the hypochlorite solution or suspension is treated with an excess of a solution ofpotassium iodide, and strongly acidified with acetic acid:

ClO- + 2I- + 2H+ = Cl- + I2 + H2O

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


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

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

Working Procedure:1.Available chlorine in bleach liquor (iodometric method)2.Sodium Hydroxide and Sodium Carbonate Determination3.Available chlorine in Bleaching Powder

1.Available chlorine in bleach liquor (iodometricmethod)Reagents Required1.70 gpl solution of sodium thiosulphate

2.10 % potassium iodide solution3.400 gpl acetic acid solution4.Starch indicator solution

ProcedureTake 10 ml of bleach liquor solution in a conical flask , add 20 ml KI solution and 20 mlacetic acid solution , the color will turn to amber brown , shake it well and then titrate itwith 70 gpl sodium thiosulphate solution to a light brown color , now add few drops ofstarch 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 CarbonateDeterminationTake 10 ml of sample and add equal(of av. chlorine gpl) ml of 70 gpl sod.thiosulphate toneutralise the chlorine , shake well and titrate the content with N/2 sulphuric acid usingphenolphthalien indicator.

End point: Pink to color less. caustic gpl will be equal to ml of sulphuric acid consumed.

3.Available Chlorine in Bleaching PowderAbout 2 gm of sample , accurately weighed , is pasted up with a little water and madeup 500 ml .Take 50 ml of this , add 20 ml of 10% KI , and solution is acidified with diluteacetic acid , titrate it with 0.1 N Sodium thiosulphate using starch indicator , to acolorless end point.

1 ml of 0.1 Sod. thiosulphate = 0.003546 g chlorine. other tests of bleaching powder arealkalinity and chlorates. for detailed analytical procedure.


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estimation of chlorine