1 THEGEOCHEMISTRYOFNATURALWATERS REDOXREACTIONSANDPROCESSES IICHAPTER5 Kehew 2001 N O2 H2OSYSTEM 2 N O2 H2OSYSTEM 3 Preliminarymappingofspeciesinpe pHspacefortheN O2 H2Osystem Moreoxidizedspeciesareathighpeandmorereducedspeciesatlowpe Nitrite NO2 isametastablespecies 4 NH30 NH4 BOUNDARY Asbefore wewriteareactionbetweenthespeciesNH3 H NH4 Bydefinition theboundarybetweenthesetwospeciesiswhereso 5 Gr Gf NH4 Gf NH3o Gr 79 4 26 5 52 9kJmol 1SothisisaverticallineatpH 9 27 6 Weplotthefirstboundaryfromuppertolowerwaterstabilitylimit becauseatthispoint wedonotknowwheretheotherboundarieswillintersectit 7 N2 g NH4 BOUNDARY Wewritethereaction N2 4H 3e NH4 Toplotthisboundary wehavetofixboth Naq mNH4 andpN2 For Naqwechoose10 3molL 1 whichisnearthedrinkingwaterstandardfornitratenitrogen ForpN2wechoosetheatmosphericvalueof0 77atm 8 Gr Gf NH4 Gf N2 Gr 79 4 0 79 4kJmol 1 9 TheN2 NH4 boundaryintersectstheNH4 NH3 boundaryatpe 6 78andpH 9 27 ThefieldofNH4 isnowtotallyenclosedbyboundariesintersectingatangles 180 ThenextlogicalboundarytocalculateistheN2 NH3 boundary 10 N2 g NH3oBOUNDARY Wenowwritethereaction N2 3H 3e NH3 Wechoose Naq mNH3 10 3molL 1andpN2 0 77atmasbefore 11 Gr Gf NH3 Gf N2 Gr 26 5 0 26 5kJmol 1 12 NowtheNH3 fieldistotallyenclosed WewouldsuspectthatthenextboundarytocalculateistheN2 g NO3 boundary 13 N2 g NO3 BOUNDARY StartingwiththereactionNO3 6H 5e N2 3H2OTobeconsistent wechoose Naq mNO3 10 3molL 1andpN2 0 77atmasbefore 14 Gr 3 Gf H3O Gf N2 Gf NO3 Gr 3 237 1 0 110 8 600 5kJmol 1 15 FinalEh pHdiagramfortheN O2 H2Osystem NotethatNO3 shouldbepresentinsignificantquantitiesonlyinwaterscontainingfreeoxygen Ammoniumionandammoniawillbepresentonlyinveryreducingwaters 16 ANYWATERCONTAININGSIGNIFICANTFe2 SHOULDREDUCENO3 Wecanseethisfromthepe pHdiagrams Wecanalsoseethisfromasimplecalculation Wefirstwritethefollowingreaction5Fe2 NO3 6H 5Fe3 N2 3H2O Gr 3 Gf H3O 5 Gf Fe3 Gf NO3 5 Gf Fe2 Gr 3 237 1 5 16 7 110 8 5 90 0 234 0kJmol 1 17 Letusassumethatforagivenwater thepHwasmeasuredtobe6 pN2 0 77atm and 18 NO2 NO3 BOUNDARY DenitrificationofNO3 toN2proceedsviaseveralintermediatesteps nitrite NO2 beingthefirstintermediate WenextcalculatetheNO2 NO3 boundary NO3 2H 2e NO2 H2OWeassumeaNO3 aNO2 19 Gr Gf H2O Gf NO2 Gf NO3 Gr 237 1 37 2 110 8 163 5kJmol 1 20 ThedottedlineistheNO3 NO2 boundary Itisdrawnasadottedlinebecauseitismetastable WecanseethisbecauseNO3 isreducedattheNO3 N2 g boundaryatapeabovetheNO3 NO2 boundary Thus NO3 isnotavailabletoreacttoformNO2 Thus nitriteisameta