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The Precipitation of Calcium and Magnesium From Sea Water

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  441 ThePrecipitationofCalciumandMagnesiumfromSeaWater. By LaurenceIrving, NationalResearchFellowintheBiologicalSciences U.S.A. With2FiguresintheText. ApPARENTLYalciumcarbonateisprecipitatedincertainpartsoftheoceanbyprocesseswhichareinorganicinsofarasthatthecalciumdoesnotfirstformatrueconstituentoforganisms(Clarke,1920,p.128).Theconditionsgoverningsolubilityofcalciuminsea-waterhavebeenreviewedbyJohnstonandWilliamson(1916)withtheconclusionthatsurfacelayersoftheoceanareapproximatelysaturated,andthatslightnaturalchanges,particularlyincarbondioxidetension,mightsufficetocauseprecipitation.Themolarconcentrationofmagnesiuminsea-waterisaboutfivetimesthatofcalcium.Inmarinesedimentsmagnesiumcarbonateisacon-stituentwhichvariesinproportionaccordingtotheorganicremainsproducingthem(OlarkeandWheeler,1922).Itisafrequentconstituentofshells,butmuchlessabundantthancalcium.Themodeofformation ofmagnesium-containingdepositsisoftenobscure. . Magnesiumhydroxideisprecipitatedfromsea-waterbyadditionofsmallamountsofalkali.Infact,bothmagnesiumandcalciumareinadelicateequilibriumwhereslightchangesinalkalinityandcarbondioxidetensionmaycauseprecipitation.Thedelicacyoftheseequilibriaandthenearnessoftheirpointsofmaximumsensitivitytonaturalconditionsmakecalciumandmagnesiumparticularlysubjecttochangesinducedbyorganisms.Likewisetheyaretwoofthemostimportantelementsbiologically,bothinamountandspecificeffect.Itisthereforesignificanttoexaminethesolubilityconditionsforbothelementstogether,andtodeterminetheconditionsfortheirprecipitation. *IwishtoexpresstoDr.Allenmyappreciationfortheextensionoflaboratoryfacilitiesandmaterialandhelpfulpersonalinterest,andtoDr.AtkinsandMr.Harveyformanysuggestionsandpracticalhelp. Journal of the Marine Biological Association 14(2) 1926  442 LAURENCEIRVING. Thesolubilityproductconstantsforthetwocarbonatesare,at16degrees-(1)Kl\IgC03 =1.4 X10-4(Johnston,1915).(2)KCaCOa=0.98X10-8 Thesolubilityproductconstantforcalciteispracticallyequaltothatforcalciumcarbonate(JohnstonandWilliamson,1916).Insea-wateratpH8theexcessbase,whichispracticallyameasureof[HCO 3]isabout2.5X10-3N.Fromtheequation(3)[CO~]=k~[HCO~] [CO;o]=1.35 X10-4From(2)[Ca++][Co~]=0'98x10-8and,from(2)and(3)[Ca++]=0,73X10-3representingthesolubilitylimitforCaC03atpH8insea-water.bysubstitutionByactualdetermination[Ca]=1X10-2Therefore,sea-wateratpH8issupersaturatedwithCaCO;;.Thesolubilityproductconstantsformagnesiumhydroxideandcalciumhydroxideare (4)KMg(OH)2=1.2X10-11KCa(OH).=4.1X10-6(Johnston,1915).  Insea-water[Mg]=0'05.Substitutingin(4)'1.2X10-11=1.6X10-5[OH-] = J -5X10-2pH9.2isattainedbyphotosynthesisexperimentallyandprobablynaturally(Atkins,1922),and UZm canapparentlyproduceapHcloseto10.Itthereforeappearsthatmagnesiumhydroxidemightbeprecipitatedundernaturalconditions.Inthesecalculationsnoallowanceismadeforactivityfactorsorfortheinfluenceofneutralsalts.AdditionofalkalitopH10isknowntoproduceaprecipitateinsea-water.QualitativeexaminationofsuchaprecipitateshowedCO2,Ca,andMg.  CALCIUMANDMAGNESIUMFROMSEA-WATER. 443Afterpreliminarytests,thetwoseriesreportedinTables1and2weremadeinwhichgradedamountsofNaOHandNa2C03,respectively,wereaddedtosamplesofsea-water.Afteratleast24hoursshaking,themixtureswerefilteredandasampleoffiltrateanalysedforCabyMcCrud-den's(1909-10)methodwithF..Mn04'Theresidueintheflaskswa~washedintothefilterwithasmallamountof70 alcohol,andthefilterwashedoncewithalcohol.TheresiduewasdissolvedinstandardizedH2SO4andtitratedwithNaOH.ThedifferencebetweenH2SO4andNaOHwasequivalentto([Mg]+[Ca])intheprecipitate.[Ca]beingdeterminedinthefiltrate,[Mg]=([Mg]+[Ca])-[Ca]. TABLE1.ADDITIONOFNAOHTO200ML.SEA-WATER. Inprecipitate. NumberNaOHaddedpHCa.Mg.   total   total grammoleculesgrammoleculesgrammolecules Ca.Mg.psrlitsr.perliter.perliter.110.002099,30.00020100120'006259,50,00030.00015141.5130.017410.00,000390.001611916140.052310.50.0003919150.15611.50,00083387b0.27811.60.001250,008575786Sea-water0.002120.01TABLE2.ADDITIONOFNA2COaTO200ML.SEA-WATER.Na2C03 Inprecipitate. Number.--2addedpH - Ca. Mg. total total grammoleculesgrammoleculesgrammolecules Ca.Mg.perliter.perliter.perliter.18 0.00285 9.100,000600,8190.008558,50.0008750,000053.80,5200.02599,30.00170,00005810,5210,074010.00.009170,0004954,0220.13410.60.001830.0042864017b0.43811.00,0019391Sea-water0.002120.010  444 LAURENCEIRVING. ThisprocedurefordeterminationwasworkedoutwithsuggestionsfromMr.H.W.Harvey.Qualitativetestsontheamountof70 alcoholusedforwashingshowedonlytracesofCaandMg.Willstiitter'smethodwasnotnoteduntillater.Itissimilar,butusesalcoholandacetonetitrationstoeffectaseparation. pH \\ 9 «:i u jO 5 -or ~ Ca. I~ 10~'5 10 ..... c 10 ~ ~ ~  025 05 075 Na ONAdded in ltAMMOLECULES erUT~[ FIG.1.Titrationofsea-waterwithNaOH(partofcurvefrompointsgiveninTable1). ColorimetricpHdeterminationsandMgprecipitatedareconsistentwithmanypotentiometrictitrationsmadein1924.Fig.1representsthelowerpartofthecurvedrawnfrompointsgiveninTable1,showingtherelationofpercentCaandMgprecipitatedandpHagainstNaOHadded.Fig.2showsdatafromTable2forCaandMgprecipitationandpHagainstNa2C03.ItisconspicuousthateitherwithNa2C03orNaOH,CaexceedsMgintheprecipitate.Na2C03precipitatesmuchmoreCa,andrelativelylittleMguptopH10.A
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