1THEGEOCHEMISTRYOFNATURALWATERSREDOXREACTIONSANDPROCESSES-IICHAPTER5-Kehew(2023)N-O2-H2OSYSTEM2N-O2-H2OSYSTEM3Preliminarymappingofspeciesinpe-pHspacefortheN-O2-H2Osystem.Moreoxidizedspeciesareathighpeandmorereducedspeciesatlowpe.Nitrite(NO2-)isametastablespecies.4NH30/NH4+BOUNDARYAsbefore,wewriteareactionbetweenthespeciesNH3°+H+

NH4+Bydefinition,theboundarybetweenthesetwospeciesiswhereso5Gr°=Gf°NH4+

-Gf°NH3oGr°=(-79.4)-(-26.5)=-52.9kJmol-1SothisisaverticallineatpH=9.27.6Weplotthefirstboundaryfromuppertolowerwaterstabilitylimit,becauseatthispoint,wedonotknowwheretheotherboundarieswillintersectit.7N2(g)/NH4+BOUNDARYWewritethereaction?N2+4H++3e-

NH4+Toplotthisboundary,wehavetofixbothNaq

mNH4+andpN2.ForNaqwechoose10-3molL-1,whichisnearthedrinkingwaterstandardfornitratenitrogen.ForpN2wechoosetheatmosphericvalueof0.77atm.8Gr°=Gf°NH4+

-?Gf°N2Gr°=(-79.4)-?(0)=-79.4kJmol-19TheN2/NH4+boundaryintersectstheNH4+/NH3°boundaryatpe=-6.78andpH=9.27.ThefieldofNH4+isnowtotallyenclosedbyboundariesintersectingatangles<180°.ThenextlogicalboundarytocalculateistheN2/NH3°boundary.10N2(g)/NH3oBOUNDARYWenowwritethereaction?N2+3H++3e-

NH3°WechooseNaq

mNH3°=10-3molL-1andpN2=0.77atmasbefore.11Gr°=Gf°NH3°-?Gf°N2Gr°=(-26.5)-?(0)=-26.5kJmol-112NowtheNH3°fieldistotallyenclosed.WewouldsuspectthatthenextboundarytocalculateistheN2(g)/NO3-boundary.13N2(g)/NO3-BOUNDARYStartingwiththereactionNO3-+6H++5e-

?N2+3H2OTobeconsistent,wechooseNaq

mNO3-=10-3molL-1andpN2=0.77atmasbefore.14Gr°=3Gf°H3O+?Gf°N2-Gf°NO3-

Gr°=3(-237.1)+?(0)-(-110.8)=-600.5kJmol-115FinalEh-pHdiagramfortheN-O2-H2Osystem.NotethatNO3-shouldbepresentinsignificantquantitiesonlyinwaterscontainingfreeoxygen.Ammoniumionandammoniawillbepresentonlyinveryreducingwaters.16ANYWATERCONTAININGSIGNIFICANTFe2+SHOULDREDUCENO3-!Wecanseethisfromthepe-pHdiagrams.Wecanalsoseethisfromasimplecalculation.Wefirstwritethefollowingreaction5Fe2++NO3-+6H+

5Fe3++?N2+3H2OGr°=3Gf°H3O+5Gf°Fe3+

-Gf°NO3-

-5Gf°Fe2+

Gr°=3(-237.1)+5(-16.7)-(-110.8)-5(-90.0)=-234.0kJmol-117Letusassumethatforagivenwater,thepHwasmeasuredtobe6,pN2=0.77atm,and18NO2-/NO3-BOUNDARYDenitrificationofNO3-toN2proceedsviaseveralintermediatesteps,nitrite(NO2-)beingthefirstintermediate.WenextcalculatetheNO2-/NO3-boundary.NO3-+2H++2e-

NO2-+H2OWeassumeaNO3-=aNO2-.19Gr°=Gf°H2O+Gf°NO2--Gf°NO3-

Gr°=(-237.1)+(-37.2)-(-110.8)=-163.5kJmol-120ThedottedlineistheNO3-/NO2-boundary.Itisdrawnasadottedlinebecauseitismetastable.WecanseethisbecauseNO3-isreducedattheNO3-/N2(g)boundaryatapeabovetheNO3-/NO2-boundary.Thus,N