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calculating final concentrations 2012

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 How  to Write a Paper  in  Journal  Style and  Format  Dept. of  Biology, Bates College 1 Rev. 5‐10‐2012 Calculating final,  or working,  concentrations  for  journal ‐style scientific papers. Stephanie Richards, Ph.D. Bates College, Department of  Biology When you read a Material and Methods section, you will notice that authors use two types of  notations when referring to the concentration of  reagents they are using: a  final  concentration in moles or in weight/volume (i.e. grams/ml), or, in percent (%). When reporting the formulations of  complex solutions, authors may use both methods to report final concentrations depending on the particular reagent used and how its concentration is most usually reported. Furthermore, the concentrations you read are the final, or working, concentrations of  reagents used in the experiments, NOT the starting, or stock, concentrations stored in the laboratory. You do not need to report the starting, or stock, concentrations because they do not matter as long as the final concentration is given. You may trust that another scientist wishing to repeat your method will be able to do so from the final concentrations you report. Example 1: You set up an experiment  that requires a final concentration of  0.1 M Tris in your tubes, in a total volume of  2 ml. You have a 1 M stock solution in your laboratory.  How much of  the stock solution will you need in each tube? There are several ways to solve this problem, but they all use basic algebra. One method is this: C 1 V 1  = C 2 V 2  C 1  = concentration of  the stock solution = 1 M V 1  = the volume of  the stock solution you need; this is your unknown that you solve for C 2  = 0.1 M Tris, the final concentration V 2  = 2 ml, the volume of  the reaction V 1  = (0.1 M)*(2 ml) / 1 M V 1  = 0.2 ml Each reaction tube will contain 0.2 ml of  1 M Tris + 1.8 ml of  other soluble reagents = 2 ml total. The final concentration of  the Tris in this volume will be 0.1 M. Now, if  your partner has a stock solution of  0.5 M Tris instead of  1 M Tris and makes the tubes, he would still want the final concentration of  Tris to be 0.1 M Tris, but would use 0.4 ml of  0.5 M Tris + 1.6 ml of  other soluble reactions in each tube. It will not matter whether you used 1 M or 0.5 M Tris to accomplish the correct final concentration; the final concentration is the only concentration reported. 

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Calculating Final Concentrations 2012

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  • HowtoWriteaPaperinJournalStyleandFormatDept.ofBiology,BatesCollege

    1 Rev.5102012

    Calculatingfinal,orworking,concentrationsforjournalstylescientificpapers.StephanieRichards,Ph.D.

    BatesCollege,DepartmentofBiologyWhenyoureadaMaterialandMethodssection,youwillnoticethatauthorsusetwotypesofnotationswhenreferringtotheconcentrationofreagentstheyareusing:afinalconcentrationinmolesorinweight/volume(i.e.grams/ml),or,inpercent(%).Whenreportingtheformulationsofcomplexsolutions,authorsmayusebothmethodstoreportfinalconcentrationsdependingontheparticularreagentusedandhowitsconcentrationismostusuallyreported.Furthermore,theconcentrationsyoureadarethefinal,orworking,concentrationsofreagentsusedintheexperiments,NOTthestarting,orstock,concentrationsstoredinthelaboratory.Youdonotneedtoreportthestarting,orstock,concentrationsbecausetheydonotmatteraslongasthefinalconcentrationisgiven.Youmaytrustthatanotherscientistwishingtorepeatyourmethodwillbeabletodosofromthefinalconcentrationsyoureport.Example1:Yousetupanexperimentthatrequiresafinalconcentrationof0.1MTrisinyourtubes,inatotalvolumeof2ml.Youhavea1Mstocksolutioninyourlaboratory.Howmuchofthestocksolutionwillyouneedineachtube?Thereareseveralwaystosolvethisproblem,buttheyallusebasicalgebra.Onemethodisthis:

    C1V1=C2V2C1=concentrationofthestocksolution=1MV1=thevolumeofthestocksolutionyouneed;thisisyourunknownthatyousolveforC2=0.1MTris,thefinalconcentrationV2=2ml,thevolumeofthereactionV1=(0.1M)*(2ml)/1MV1=0.2ml

    Eachreactiontubewillcontain0.2mlof1MTris+1.8mlofothersolublereagents=2mltotal.ThefinalconcentrationoftheTrisinthisvolumewillbe0.1M.Now,ifyourpartnerhasastocksolutionof0.5MTrisinsteadof1MTrisandmakesthetubes,hewouldstillwantthefinalconcentrationofTristobe0.1MTris,butwoulduse0.4mlof0.5MTris+1.6mlofothersolublereactionsineachtube.Itwillnotmatterwhetheryouused1Mor0.5MTristoaccomplishthecorrectfinalconcentration;thefinalconcentrationistheonlyconcentrationreported.

  • HowtoWriteaPaperinJournalStyleandFormatDept.ofBiology,BatesCollege

    2 Rev.5102012

    Thesameapproachappliesifyouareusingareagentwithconcentrationunitsofweight/volume,suchasmg/ml.Example2:Youaregivenasolutionofamyloseazureat2mg/ml.Youwantyourfinalconcentrationtobe0.5mg/mlina5mlreaction.Howmuchvolumeofthestockamyloseazuresolutionwillyouuseforthat5mlreaction?

    C1=2mg/mlstocksolutionV1=volumeofstocksolutionyouneed;thisisyourunknownyousolveforC2=0.5mg/mlV2=5mlV1=(0.5mg/ml)*(5ml)/2mg/mlV1=1.25ml

    Each5mlreactionwillcontain1.25mlof2mg/mlamyloseazuresolution+3.75mlofotherliquids.Ifyoustartedwithastocksolutionof5mg/mlamyloseazure,thevolumeofstocksolutionyouwouldusewouldchange,whilethefinalconcentrationandfinalvolumewouldnot.Thefinalconcentrationistheonlyconcentrationreported.

    WhatifIamgivenastockconcentrationandthevolumes;howcanIfigureoutthefinalconcentration?Example3:Youhavea2mg/mlstocksolutionofBSA,andyouuse0.2mlofthatBSAinatotalvolumeof1ml.Again,asimplealgebraicapproachworkshere.Identifywhatyouknowandwhatyouneed.Youknowthestockconcentration:2mg/ml=C1Youknowthevolumeofstocksolutionthatyouuse:0.2ml=V1Youknowthefinalvolumeofthesample:1ml=V2Youdonotknowthefinalconcentrationofthesample:C2=unknown

    (0.2ml)*(2mg/ml)=(1ml)*C2C2=(0.2ml)*(2mg/ml)/1mlC2=0.4mg/ml

    ThisisthefinalconcentrationofBSA,0.4mg/ml,thatyouwoulduseinyourreport.

  • HowtoWriteaPaperinJournalStyleandFormatDept.ofBiology,BatesCollege

    3 Rev.5102012

    WhatifIamgivenareagentwithoutconcentrationunits?HowdoIreporthowmuchIused?Hereyouneedtoreporttheproportionofthereagentyouusedaspartofthetotalreaction.Thebestwaytodothisisasafinal%,whichisnotaconcentrationunitbutdoesgivethereaderanideaofhowmuchyouused.Example4:Youhave1cupoforangejuiceconcentrate.Youadd4cupsofwater,foratotalvolumeof5cups.Whatpercentofthemixtureisorangejuice?

    1cuporangejuiceconcentrate/5cupsofsolutionwithorangejuice=0.20.2*100=20%orangejuiceinthefinalsolution

    NOTE:Noticetherearenounitshere;theycanceloutintheequation.Example5:Youuse1mlofbloodinafinalvolumeof10mlofheparinsolution.Whatistheconcentrationofbloodinthefinalsolution?

    1mlblood/10mlfinalvolume=0.10.1 *100=10%ofthesolutionisblood

    __________________________________________________________________________________

    Insummary,whenyouarereportingtheconcentrationsofreagentsyouwill: Reportthefinalconcentrationsused,asyoucalculatethem.Youdonotreportthestartingstock

    concentrations. IfyourstockreagentisgiveninaconcentrationunitsuchasM(moles/liter)orweight/volume(i.e.

    grams/liter),thefinalconcentrationisreportedinsimilarunits. Ifyourstockreagenthasnoconcentrationgiven,youwillreportthefinalamountusedasa

    proportion,mostcommonlyreportedasapercent(%). YouWILLNOTuseaconcentrationunitAND%togethertoreportthefinalamountusedofasingle

    reagent.Forexample,youwillNOTreportusing10%ofa5mg/mlsolution.


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