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5. 4 3 Samples must be preserved wIth 2 ml of 10 N sodmm hydroxIde per hter of sample (pH ~ 12) at the time ofcollectIOn 44 SamplesshouldbeanalyzedasrapIdlyaspossIbleaftercollectIOn IfstorageISreqUlred, the samples should be stored m a refngerator or m an lCe chest filled wIth water and lCe to mamtam temperature at 4°C Interferences 5 1 Interferences are ellmmated or reduced by usmg the dlst111atIOn procedure descnbed m Procedure 81,82 and 8 3 52 SulfIdes adversely affect the colonmetnc and tItraUon procedures Samples that 6. Apparatus 7. 65 Reagents 7.1 Sodmm hydrOXIde solutIOn, 1 25N DIssolve 50 g of NaOH m dIstIlled water, and dIlute 5 3 contam hydrogen sulfIde, metal sulfIdes or other compounds that may produce hydrogen sulfIde durmg the dlst111auon should be dlsulled by the opuonal procedure descnbed m Procedure 8 2 The apparatus for thIS procedure IS shown m FIgure 3 Fatty aCIds WIll dIstIll and form soaps under the alkahne tItratIOn condItIons, makmg the end pomt almost ImpossIble to detect 5.4 HIgh results may be obtamed for samples that contam mtrate and/or mtnte Dunng the disUllation mtrate and mtnte WIll form mtrous aCId whIch WIll react WIth some orgamccompounds to form OXImes These compounds formed WIll decompose under test COndItIOnS to generate HCN The mterference of mtrate and mtnte IS ehmmated b) pretreatment WIth suifamic aCId 53 1 5 3 2 ACIdIfythesampleWIthacetIcaCId(1+9)topH60to70 CautIOn ThIS operatIOn must be performed m the hood and the sample left there untliit can be made alkahne agam after the extractIOn has been performed Extract WIth Iso-octane, hexane, or chloroform (preference m order named) WIth a solvent volume equal to 20% of the sample volume One extractIOn IS usually adequate to reduce the fatty aCIds below the mterference level AVOId multIple extractIOns or a long contact time at low pH m order to keep the loss of HCN at a mInImUm When the extractIOn IS completed, ImmedIately raIse the pH of the sample to above 12 WIth NaOH solutIOn 6 1 6 2 6 3 64 to 1 lIter WIth dIstIlled water Reflux dIstIllatIOn apparatus such as shown m FIgure 1 or FIgure 2 The bOIhng flask shouldbeof1hterSIzeWIthinlettubeandprOVISIonforcondenser Thegasabsorbermay be a Fisher-MIlhgan scrubber Microburet, 5 0 ml (for tItratIOn) Spectrophotometer sUltable for measurements at 578 nm or 620 nm WIth a 1 0 cm cell or larger Reflux dlsullauon apparatus for sulfide removal as shown m Figure 3 The bOllmg flask same as 6 1 The sulfIde scrubber may be a Wheaton Bubber #709682 With 29/42 Joints, size 100 ml The aIr mlet tube should not be fntted The cyamde absorptIOn vessel should be the same as the sulfIde scrubber The aIr mlet tube should be frItted Flow metel, such as Lab Crest with stamless steel float (FIsher 11-164-50) 3352-2PDF Image | Methods for Chemical Analysis of Water and Wastes
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