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Institute of Paper Science and Technology a Tanta, Georgia Institute of Paper Science and Technology A tanta, Georgia IPST Technical Paper Series Number 825 Rapid and Direct Pulp Kappa Number Determination Using Spectrophotometry X.S. Chai and J.Y. Zhu November 1999 Subm itted to Journal of Pulp and Paper Science Copyright@ 1999 by the Institute of Paper Science and Technology For Members Only INSTITUTE OF PAPER SCIENCE AND TECHNOLOGY PURPOSE AND MISSIONS The Institute of Paper Science and Technology is an independent graduate school, research organization, and information center for science and technology mainly concerned with manufacture and uses of pulp, paper, paperboard, and other forest products and byproducts. Established in 1929, the Institute provides research and information services to the wood, fiber, and allied industries in a unique partnership between education and business. The Institute is supported by 52 North American companies. The purpose of the Institute is fulfilled through four missions, which are: . top rovide a multidisciplinary education to students who advance the science and technology of the industry and who rise into leadership positions within the industry; l to conduct and foster research that creates knowledge to satisfy the technological needs of the industry; l to serve as a key global resou rce for the acquisition, assessment, and dissemination of industry information, P roviding critically im portant information to decision-makers at all. levels of the industry; and l to aggressively seek out technological opportunities and facilitate the transfer and implementation of those technologies in collaboration with industry partners. ACCREDITATION The Institute of Paper Science and Technology is accredited by the Commission on Colleges of the Southern Association of Colleges and Schools to award the Master of Science and Doctor of Philosophy degrees. NOTICE AND DISCLAIMER The Institute of Paper Science and Technology (IPST) has provided a high standard of professional service and has put forth its best efforts within the time and funds available for this project. The information and conclusions are advisory and are intended only for internal use by any company who may receive this report. Each company must decide for itself the best approach to solving any problems it may have and how, or whether, this reported information should be considered in its approach. IPST does not recommend particular products, procedures, materials, or service. These are included only in the interest of completeness within a laboratory context and budgetary constraint. Actual products, procedures, materials, and services used may differ and are peculiar to the operations of each company. In no event shall IPST or its employees and agents have any obligation or liability for damages including, but not limited to, consequential damages arising out of or in connection with any company’s use of or inability to use the reported information. IPST provides no warranty or guaranty of results. The Institute of Paper Science and Technology assures equal opportunity to all qualified persons without regard to race, color, religion, sex, national origin, age, disability, marital status, or Vietnam era veterans status in the admission to, participation in, treatment of, or employment in the programs and activities which the Institute operates. To be published in J Pulp & Paper Sci., 25( 1l), 1999 (in press) Also Presentedat the 1Uh ISWPC Main Symposium, June 7-10, 1999,Yokohama, Japan Rapid and Direct Pulp Kappa Number Determination Using Spectrophotometry X.S. Chai andJ.Y. Zhu’ Institute of Paper Scienceand Technology 500 ltih Street, N. W. Atlanta, GA 30318 (404) 894 -5310, (404) 894-5752(Fa ABSTRACT A spectrophotometricmethod has been developed for rapid pulp kappanumber measurements under strongly acidic reactionconditions. Pulp kappanurnber is derived from the ratio of the absorptionspectral intensities at a specificwavelength measured at the beginningand end of the reactionof the pulp with permanganate.Therefore, no calibration is necessary.Strong acidificationis usedto preventMnO, precipitationand interference with the permanganateabsorption spectrum. A methodis proposedto determinethe end of the pulp oxidationreactions with permanganateusing the reactionkinetics andrate derived from the time-dependentspectrophotometric measurements of the reactingsolution. It was found that the pulp oxidationwas completedin lessthan 3 minutes. The measuredkappa numbersof three pulp samplesof two wood species,with andwithout bleaching,agreed well with thoseobtained by the current TAPPI standardmethod using thiosulfate titration. The presentmethod is simple,rapid, andaccurate. Key Words: Kappanumber; pulp; spectrophotometry;UWVis absorption; lignin oxidation;permanganate. l Author to whom correspondenceshould be addressed 1 Accepted for Publication by J. Pulp & Paper Sci., 25( 1l), 1999 (in press) BACKGROUND The kappanumber of pulp is one of the importantparameters in pulp manufacturing becauseit relatesto the bleachability,or degreeof delignification,of the pulp. The kappa numberis definedas the volume of 0.02 mole/L (0.1 normality) potassiumpermanganate solutionconsumed by one gram of moisture-freepulp in an acidic mediumthrough the following reactions,depending on the acidity of the medium: MI@‘; + 8H+ +5e E”=+lslV >ib2+ + 4H,o (1 a> MnO~ + 4H’ + 3e E”=+1*6gv) it&O, + 2H,O (lb) The current, commonlyused method for kappanumber measurements, such as TAPPI Test Method T236 cm-85 [l], was first proposedin 1934by Wiles [2] and later developedin the 1950’sbased on researchby Watson[3], Valeur and Torngren[4], and Tasmanand Berzins [S]. The methodcalculates the volume of 0.02 mole/L (0.1 N) potassiumperrnanganate consumed by one gram of moisture-freepulp by the difference betweenthe initial andthe final excessvolumes of potassiumpermanganate after a lo- minute reactionat 25OCunder weakly acidic conditions. The final excessvolume of potassiumpermanganate after the lo-minute oxidationreaction is determinedby titrimetry using a standardthiosulfate solution after addingan excessof potassiumiodide to the slurry to react with the excesspertnanganate to form iodine. The key assumptionsof this methodare: (1) the perrnanganateconsumed was mainly due to reactionwith lignin, and the rapid oxidationwas completedin 10 minutes; (2) the effects of reactiontemperature, time, andthe variationin excesspermanganate volume are not significant and canbe correctedby a non-constantcorrection factor as tabulatedin the TAPPI method [l]; and (3) the thiosulphatetitration methodis accurate.The disadvantages of this method are: (1) thiosulfatetitration of iodine is tediousbecause a blank experiment (without pulp, andusing exactly the sameprocedures) is neededto reducethe experimental error in the titration due to the high volatility of iodine; (2) the thiosulfatetitration requires that the reactionbe kept underweakly acidic conditions,which causeslarge variations in kappameasurements as reportedby Valeur and Torngren[4]; (3) the lOominuteoxidation Accepted for Publication by J. Pulp & Paper Sci., 25( 1 1), 1999 (in press) reaction time is arbitrary and can causeerrors in determiningkappa numbers for various pulps, becausepermanganate can be both oxidizedand decomposed by other organic materialsin pulp as explainedby Tasmanand Berzins[5]; the actualoxidation time is much shorterthan 10 minutes,as found by Li andGellerstedt [6] and in the presentstudy; (4) the methodrequires that the volume of consumedpermanganate after the lo-minute reaction shouldbe around50% of the initial volume to obtain goodmeasurements; this is impossible becausethe final excesspermanganate volume can only be known after the kappanumber has beendetermined; (5) the non-constantcorrection factor is purely empirical,derived through experimentalcalibration, and cancause errors; and (6) compoundsother than lignin in a pulp, suchas a significant amountof hexenuronicacid in bleachedpulps, canbe oxidizedby permanganatein the first lo-minute reaction. As a result, the existing kappa test methodshave many shortcomingsand the kappanumber itself is not scientificallywell defined;however, the kappanumber of a pulp obtainedby thesemethods has significant practical importance. A commerciallyavailable instrument, based on thesemethods, has beendeveloped (KAPPAMAT, Kebo Lab, Sweden). Spectroscopicmethods can provide direct and instantaneousmeasurement of chemicalconcentrations in reactions. Spectroscopicmethods using W andFTIR absorptionto determinepulp kappanumber have been reported [7,8]. In thesemethods, the linear relationshipbetween the lignin contentof the pulp and the light absorptionwas used to determinepulp kappanumber. Unfortunately,the perrnanganateconsumption by different types of lignin is different. Therefore,calibration is requiredfor a specificpulp sampleusing the traditional methodof kappanumber determination [l]. By the time the presentstudy was completed,Li and Gellerstedt[6] had reportedthe resultsof a study on the kinetics andmechanism of pulp-permanganatereaction under conditionssuggested in standardkappa testing methods. They directly measuredthe permanganateconcentration in lignin-permanganatereaction solutions using W/Vis spectrophotometry. Accordingto Li and Gellerstedt[6] and Stewart [9], both reactions(la) and (lb) can take placeunder conditions suggested in commonkappa number test methods, suchas TAPPI Test Method - T236 cm-85 [ 1] and SCAN-Cl Test Method [lo], where the dominantreaction is an overall conversionof permanganateto MnO, accordingto
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