Test Method To Check The Concentration of Caustic & Silica in Peroxide Bleach Bath

Test Method To Check The Concentration of Caustic & Silica in Peroxide Bleach Bath

IntroductionHydrogen Peroxide is most widely used Bleaching Agent for more than 85% of Textile Substrate.Process involves the the treatment of material with a solution of H2O2 in alkaline media.The active bleaching agent is HO2-The colored substances in the material are oxidized. It produces a stable white color, and bleached fabrics are highly hydrophillic.It produces Non Toxic reaction products.

What is Cross Checking ?It is process in which we examine the concentration of caustic and silica in peroxide bleach bath at regular intervals of time so as to make the the process more efficient and decrease the degradation of material under treatment.Why It is Required ?H2O2 H+ + HO2-The above reaction is activated in alkaline media and is reversible. Alkali shifts the reaction to right hand by absorbing the proton.H+ + HO2- HO2- + H2O (in presence of OH- )An increase in amount of alkali will increase the rate of formation of HO2- , thereby leading to degradation of material & a decrease will result in decreased bleaching efficiency.The pH range preferred is 10.4 to 10.8 , becauseIt is Safe &Rate of formation of HO2- is equal to rate of its consumption for bleaching.Effect of pH on Decomposition of H2O2

pH% decomposition of H2O2

10.4711.113.511.819.012.225.012.659.2Why Silica for stabilization?The HO2- is very unstable & it can be stabilized by inserting a cation with high charge density like Mg , but it suffers a problem of precipitation in strongly alkaline media. Therefore silica is used which makes six-chain structure for stabilization.

Note: Silica also controls the decomposition of H2O26Test Method 1Principle : Silica reacts with Sodium Fluoride in the presence of water to form caustic soda as under:

SiO2+ 6NaF + 2 H2O Na2SiF6+ 4NaOH

Reagents Required H2SO4: 0.1 N Standardized with NaOH Sodium Fluoride Methyl Red Methylene BlueIndicator to be Used0.1 gm of Methylene Blue is dissolved in 100 ml of Distilled Water.0.2 gm of Methylene Red is dissolved in 100 ml of Ethyl Alcohol.The above two solutions are mixed and this solution is used as Indicator.The end point is shown by change in color from Green PurpleProcedureTake 10 ml of the peroxide solution in 250 ml flask. Add 100 ml distilled water Add five drops of Indicator & titrate with N/5 Sulphuric Acid Solution Let the reading be A Add 0.1 gm of Sodium Fluoride Shake for 1 Minute Titrate again Let reading be B

Continued.Also find the volume of N/5 Sulphuric Acid required to neutralised 1 gm of Sodium Fluoride. Let this reading be C

Calculations Percent NaOH : (A-X) x N x 0.040 x100 10 Percent Silica: {B-(A+C)} x N x 0.060 x 100 4 x 10 X : {B-(A+C)} x 60 x D 4 x 31Where , N = Normality of acid D = Ratio of Na2O:SiO2 of the sodium silicate used in the bath. This test method is very accurate in estimating the alkali in peroxide bath except the case that Phosphates & Borates are present which affect the test accuracy. Test Method 2 : AATCC 98-2002This test method determines the total alkali content of bleach bath containing Hydrogen Peroxide.

Principle: A weighed specimen of the bleach bath is titrated with a standardized solution of Sulphuric Acid to a Phenol Red end-point . The total alkali content is expressed as %NaOH.Reagents Required Phenol Red i.e. , Phenol- Sulphonphthalein. H2SO4 (0.1 N)

Caution : Water should not be added to acid. Acid should be added to water for dilution.ProcedureWeigh 10gm of specimen from bleach bath & add 20 ml of water to it.Add 2-3 drops of Phenol Red solution. Titrate against 0.1N H2SO4 to a Greenish Yellow end-point. Let this reading be Y Repeat the experiment to get three concordant readings.

Calculations% Total Alkali, as NaOH: Y x N x 0.040 x 100 W Where, N = normality of H2SO4, 0.040 = milliequivalent weight of NaOH , W = mass of specimenNew Developments in Determination Caustic & SilicaNumerous attempts have been made to replace the manual titration by an automatic titration where end point was not determined by visual color change but via a potential change at redoxelectrodes.Such systems were based on : Calorimetric Potentiometric Conductometric methods of determination. The main drawbacks in these systems were: High Investment Cost Long Response Time( of order of minutes)FIA SystemThe Flow Injection Analysis(FIA) was developed which have a short response time (In seconds). This system is based on spectrophotometric detection.A constant flow of the bleaching solution(Flow) is mixed with a constant flow of solution of known Concentration of Caustic(Injection). This solution is detected and a correct pH is adusted and liquor is fed to machine. The process is continuous and adjust the pH after regular intervals of time. Sensors

ConclusionTherefore, We can say that the pH control is the most important parameter in Peroxide Bleaching. We have to control this parameter by checking the amount of alkali present in bath at regular intervals. This can be done either by titration method or by Automatic Sensing Instruments. With FIA system , the concentration of Hydrogen Peroxide as well as pH can be determined over complete pH area , although the setup cost is very high.

References Process & Quality by ATIRA AATCC Testing Manual Analytical Electrochemistry in Textiles by P. WestbroekThank You