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76 Pipette 10 0 ml of sulfunc aCid solutIOn (6 3) mto each tube and mix by sWlr1mg Allow tubes to come to thermal eqUlhbnum m the cold bath Be sure that temperatures have eqUlhbrated m all tubes before contmumg 77 Add 05 ml brucme-sulfamhc aCid reagent (64) to each tube (except the mterference control tubes, 7 3) and carefully mix by sWlrhng, then place the rack oftubes m the 100°C water bath for exactly 25 mmutes Caution Immersionofthetuberackmtothebathshouldnotdecreasethetemperatureof the bath more than 1 to 2°C In order to keep this temperature decrease to an absolute mmlmum, flow ofbath water between the tubes should not be restncted by crowdmg too many tubes mto the rack If color development m the standards reveals discrepancies m the procedure, the operator should repeat the procedure after revlewmg the temperature control steps 78 Remove rack of tubes from the hot water bath and Immerse m the cold water bath and allow to reach thermal eqUlhbnum (20-25°C) 79 Read absorbance agamst the reagent blank at 410 nm usmg a 1 cm or longer cell 8 CalculatIOn 81 Obtam a standard curve by plottmg the absorbance of standards run by the above procedure agamst mg NOr N/1 (The color reactIOn does not always follow Beer's law) 82 Subtract the absorbance of the sample without the brucme-sulfamhc reagent from the absorbance of the sample contammg brucme-sulfamhc aCid and determme mg N03-N/l Multiply by an appropnate dl1utlon factor If less than 10 ml of sample IS taken 9 PrecIsIOn and Accuracy 9 1 Twenty-seven analysts m fifteen laboratones analyzed natural water samples contammg exact mcrements of morgamc mtrate, with the followmg results Increment as Nitrogen, Nitrate mg N/hter 016 019 108 124 (FWPCA Method Study 2, Nutnent Analyses) BIbliography PreCISIOn as Standard DevI~tlQn mg N/hter Accuracy as Bias, BlaS, % mg Nlllter -679 -001 +830 +002 +412 +004 +282 +004 0092 0083 0245 0214 1 Standard Methods for the Exammatton of Water and Wastewater, 14th EditIOn, p 427, Method 419D (1975) 2 Annual Book of ASTM Standards, Part 31, "Water", Standard D 992-71, P 363 (1976) 3 Jenkms, D, and Medsken, L, "A Brucme Method for the Determmatton ofNitrate m Ocean, Estuarine, and Fresh Waters", Anal Chem , 36, P 610, (1964) 352 1-3PDF Image | Methods for Chemical Analysis of Water and Wastes
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