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62 6 3 64 6 5 66 67 68 Bromophenol blue mdlCator solution DIssolve 0 1 g bromophenol blue m 100 ml 50-percent ethanol Chrommm standard solutlOn I, 1 0 ml = 100 ug Cr DIssolve 0 2829 g pure, drIed K2Cr207 m demmeralIzed water and dIlute to 1000 ml Chrommm standard solutlOn II, 1 0 m1 = 10 0 ug Cr DIlute 100 ml chrommm standard solutlOn I to 1000 ml with demmeralIzed water Chrommm standard solutlOn III, 1 0 ml = 0 10 ug Cr DIlute 10 0 ml chrommm standard solution II to 1000 ml wIth demmeralIzed water Methyl Isobutyl ketone (MIBK) SodmmhydrOXIdesolutlOn,1M DIssolve40gNaOHmdemmeralIzedwateranddIlute to 1lIter SulfurIC aCid, 012 M Slowly add 65 ml concentrated H2S04 (sp gr 184) to demmeralIzed water and dIlute to 1 lIter 7 Procedure 71 Pipet a volume of sample contammg less than 25 ug chromIUm (100 ml maxImum) mto a 200 ml volumetrIc flask, and adjust the volume to approxImately 100 ml 72 Prepare a blank and suffiCient standards, and adjust the volume ofeach to approximately lOOml 73 Add2dropsbromophenolbluemdlCatorsolution (ThepHadjustmentto24mayalso be made With a pH meter mstead of usmg an mdlcator ) 74 Adjust the pH by addition of 1 M NaOH solution dropwlse untIl a blue color persists Add 012 M H2S04 dropWlse untIl the blue color Just disappears m both the standards and sample Then add 2 0 ml of0 12 M H2S04 m excess The pH at thiS pomt should be 24 75 Add 5 0 ml APDC solution and mix The pH should then be approximately 2 8 7.6 Add 10 0 ml MIBK and shake vigorously for 3 mmutes 77 Allow the layers to separate and add demmeralIzed water untIl the ketone layer IS completely m the neck ofthe flask 78 Aspirate the ketone layer and record the scale readmg for each sample and standard agamst the blank Repeat, and average the duplIcate results 8. CalculatlOns 6 8 1 Determme the ug/l Cr+ m each sample from a plot of scale readmgs of standards A workmg curve must be prepared WIth each set of samples Report CrH concentratlOns as follows Less than 10 ug/l, nearest ug/l, 10 ug/l and above, two slgmficant figures 9 PreCISion and Accuracy 9 1 In a smgle laboratory (EMSL), usmg the (PDCA) extraction procedure and Cm?mnatl OhiO tap water spIked at a concentratlOn of 50 ug Cr+6/l the standard devlatlOn was ±2.6 WIthameanrecoveryof96% 2184-2PDF Image | Methods for Chemical Analysis of Water and Wastes
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