Measurement of Acid Neutralizing Capacity

Transcription

1 40 Measuremen of Acid Neuralizing Capaciy Inroducion Acid neuralizing capaciy (ANC) is a measure of he abiliy of waer o neuralize acid inpus. Lakes wih high ANC (such as Cayuga Lake) can mainain a neural ph even wih some acid rain inpu whereas lakes wih an ANC less han he acid inpu will no mainain a neural ph. In he Adirondack region of New York Sae, lakes ypically receive large inpus of acids during he spring haw when he accumulaed winer snow mels and runs off ino he lakes. The ANC of Adirondack lakes is no always sufficien o neuralize hese inpus. Theory The ANC for a ypical carbonae-conaining sample is defined as: ANC = [HCO ] 2[CO ] [OH ] - [H ] This equaion can be derived from a charge balance if ANC is considered o be he caion conribued by a srong base iran and if oher ions presen do no conribue significanly. Deerminaion of ANC or Alkaliniy involves deerminaion of an equivalence poin. The equivalence poin is defined as he poin in he iraion where iran volume ha has been added equals he "equivalen" volume (V e ). The equivalen volume is defined as: V N V = s s e N where: N s = normaliy (in his case Alkaliniy or ANC) of sample, equivalens/l V s = volume of sample, liers N = normaliy of iran, equivalens/l. 4.2 The iraion procedure involves incremenally adding known volumes of sandardized normaliy srong acid (or base) o a known volume of unknown normaliy base (or acid). When enough acid (or base) has been added o equal he amoun of base (or acid) in he unknown soluion we are a he "equivalence" poin. (Noe: he poin a which we add exacly an equivalen or soichiomeric amoun of iran is he equivalence poin. Experimenally, he poin a which we esimae o be he equivalence poin is called he iraion endpoin). There are several mehods for deermining V e (or he equivalence poin ph) from iraion daa (iran volume versus ph). The shape of he iraion curve (V versus ph) can reveal V e. I can be shown ha one inflecion poin occurs a V = Ve. In he case of monoproic acids, here is only one inflecion in he ph range of ineres. Therefore, an effecive mehod o find he equivalence volume is o plo he iraion curve and find he inflecion poin. Alernaely, plo he firs derivaive of he iraion plo and look for a maximum. CEE 453: Laboraory Research in Environmenal Engineering Fall 2006

2 41 Gran Plo Anoher mehod o find he ANC of an unknown soluion is he Gran plo echnique. When an ANC deerminaion is being made, iraion wih a srong acid is used o "cancel" he iniial ANC so ha a he equivalence poin he sample ANC is zero. The Gran plo echnique is based on he fac ha furher iraion will resul in an increase in he number of moles of H equal o he number of moles of H added. Thus afer he equivalence poin has been reach he number of moles of H added equals he number of moles of H in soluion. ( ) ( ) N V V = V V H 4.3 e s Solving for he hydrogen ion concenraion: ( e) ( V V ) N V V H = s Equaion 4.4 can be solved direcly for he equivalen volume. ( ) H V Ve = V 4.5 N Equaion 4.5 is valid if enough iran has been added o neuralize he ANC. A beer measure of he equivalen volume can be obained by rearranging equaion 4.4 so ha linear regression on muliple iran volume - ph daa pairs can be used. ( ) We define F 1 (Firs Gran funcion) as: V F 1 = [H ] 4.7 If F 1 is ploed as a funcion of V he resul is a sraigh line wih slope = N and abscissa inercep of V e (Figure 4-1). The ANC is readily obained given he equivalen volume. A he equivalence p: s e s s s 4.4 V NV NV e H V = 4.6 V V V ANC = V N 4.8 Equaion 4.8 can be rearranged o obain ANC as a funcion of he equivalen volume Gran Funcion Gran Funcion Linear Region y = 9.57E-04x E-03 R 2 = 9.99E Volume of Tiran (ml) Figure 4-1. Gran plo from iraion of a weak base wih 0.05 N acid. C = moles of carbonae and sample volume is 48 ml. The equivalen volume is 4.8 ml. From equaion 4.9 he ANC is 5 meq/l. Measuremen of Acid Neuralizing Capaciy

3 42 ANC V N e = 4.9 V s ph Measuremens The ph can be measured eiher as aciviy ({H } as measured approximaely by ph meer) or molar concenraion ([H ]). The choice only affecs he slope of F 1 since [H ] = {H }/γ. V V { H } V Ve F 1 = [H ] = = Ν 4.10 γ where γ is he aciviy correcion facor and he slope is N /V 0. If H concenraion is used hen γ Ν where he slope is. V V V V F = {H } = γ N 4.11 s e 1 V s (This analysis assumes ha he aciviy correcion facor doesn' change appreciably during he iraion). There are many oher Gran funcions ha can be derived. For example, one can be derived for Acidiy or he concenraion of a single weak or srong acid or base. To faciliae daa generaion and subsequen Gran plo consrucion and analysis ph versus iran volume can be read direcly ino a compuer, ha can be programmed o analyze he daa using he Gran, plo heory. The program generaes he Gran funcion for all daa and hen sysemaically eliminaes daa unil he Gran funcion (plo) is as linear as possible. The line is hen exrapolaed o he abscissa o find he equivalen volume. ANC Deerminaion for Samples wih ph < 4 Afer he equivalence poin has been reached (adding more acid han ANC = 0) he only significan erms in equaion 4.1 are H and ANC H >> H 2 CO3 OH 4.12 When he ph is 2 ph unis or more below he pks of he bases in he sysem he only species conribuing significanly o ANC is he hydrogen ion (equaion4.12) and hus he ANC is simply ANC = - [H ] 4.13 For a sample conaining only carbonaes, if he ph is below 4 he ANC is approximaely equal o -[H ] and no iraion is necessary. CEE 453: Laboraory Research in Environmenal Engineering Fall 2006

4 43 Tiraion Techniques Operaionally, he firs few iran volumes can be relaively large incremens since he imporan daa lies a ph values less han ha of he equivalence poin (approximaely ph = 4.5 for an Alkaliniy iraion). As he ph is lowered by addiion of acid he ionic srengh of he soluion increases and he aciviy of he hydrogen ion deviaes from he hydrogen ion concenraion This effec is significan below ph 3 and hus he effecive linear range is generally beween ph 4.5 and ph 3.0. The maximum incremenal iran volume ( V a ) ha will yield n poins in his linear region is obained as follows. If V s» V hen equaion 4.3 reduces o ( V Ve) N [H] 4.14 Le [H ] e be he concenraion of hydrogen ions a he equivalence poin and [H ] f be he final concenraion of hydrogen ions a he end of he iraion. ( Ve Ve) ( Vf Ve) N = [H ] e - [H ] f 4.15 Thus he volume of acid added o go from [H ] e o [H ] f is V - V f e ([ ] [ ] ) H f H e = 4.16 N To obain n daa poins beween [H ] e - [H ] f requires he incremenal iran volume ( V ) be 1/n imes he volume of acid added beween he equivalence poin and he final iran volume. Thus by subsiuing n V, and ypical hydrogen ion concenraions of [H ] e = and [H ] f = ino equaion 4.16 he maximum incremenal iran volume is obained. ( ) V 4.17 nn nn Procedure Calibrae he ph Probe Calibrae he ph probe using 3 sandards (ph = 4, 7, and 10). Deermine ANC of a Known Sandard Do he following procedure wice. The firs ime irae as fas as possible and learn how he sofware works. Then repea he procedure wih he goal of making an accurae ANC measuremen and creaing an accurae iraion curve by using 0.25 ml iran incremens hroughou he enire iraion. Remember ha he bigges source of error for his lab will likely be poor pipee echniques. Measuremen of Acid Neuralizing Capaciy

5 44 1) Weigh a 100 ml plasic beaker. 2) Add approximaely 50 ml of a 2.5 mm soluion of Na 2 o he beaker. 3) Weigh he beaker again o deermine he exac volume of Na 2 soluion. 4) Place he beaker on he magneic sirrer, add a sir bar and sir slowly. 5) Place boh he ph elecrode and he emperaure probe in he Na 2 soluion. 6) Analyze he sample using Gran plo analysis as deailed a hp://ceeserver.cee.cornell.edu/mw24/sofware/ph_meer.hm) Add 0.05 N HCl (he iran) using a digial pipee in incremens of 0.25 ml. 7) Save he Gran daa o \\Enviro\enviro\Courses\453\acid\neid_gran by selecing. The daa will be saved in a file (ab delimied forma) ha can be opened by any spreadshee program. You will use his daa o plo a iraion curve and o verify ha he Gran echnique accuraely measures he ANC of a sample. 8) Record he ANC and he equivalen volume. If he error is greaer han 2% hen check your pipee echnique using a balance and hen repea he iraion. Deermine ANC of Acid Rain Samples Deermine ANC for all samples colleced from he previous week's lab. Use he same echnique as oulined above (Deermine ANC of known sandard) excep subsiue he samples colleced las week and use iran incremen of 0.1 ml in he linear region. For samples ha have a high ANC you can reduce he analysis ime by adding iran in larger volumes iniially unil he ph approaches 5. If he iniial ph is less han 4.5 no iraion is necessary and equaion 4.13 can be used o calculae he ANC. Record he iniial ph (prior o adding any iran) and iniial sample volume. Afer he Gran plo analysis record he alkaliniy (ANC) and equivalen volume for each sample. There is no need o save he daa o disk. Prelab Quesions 1) Compare he abiliy of Cayuga lake and Wolf pond (an Adirondack lake) o wihsand an acid rain runoff even (from snow mel) ha resuls in 20% of he original lake waer being replaced by acid rain. The acid rain has a ph of 3.5 and is in equilibrium wih he amosphere. The ANC of Cayuga lake is 1.6 meq/l and he ANC of Wolf Pond is 70 µeq/l. Assume ha carbonae species are he primary componen of ANC in boh lakes, and ha hey are in equilibrium wih he amosphere. Wha is he ph of boh bodies of waer afer he acid rain inpu? Remember ha ANC is he conservaive parameer (no ph!). 2) Wha is he ANC of a waer sample conaining only carbonaes and a srong acid ha is a ph 3.2? 3) Why is [H ] no a conserved species? CEE 453: Laboraory Research in Environmenal Engineering Fall 2006

6 45 Quesions 1) Plo he iraion curve of 2.5 mm Na 2 wih 0.05 N HCl (plo ph as a funcion of iran volume). Label he equivalen volume of iran. Label he 2 regions of he graph where ph changes slowly wih he dominan reacion ha is occurring. Type he chemical reacions in exboxes and place hem on he graph in he ph regions where each reacion is occurring.) Noe ha in a hird region of slow ph change no significan reacions are occurring (added hydrogen ions conribue direcly o change in ph). 2) Prepare a Gran plo using he daa from he iraion curve of he 2.5 mm Na 2. Use linear regression on he linear region or simply draw a sraigh line hrough he linear region of he curve o idenify he equivalen volume. Compare your calculaion of V e wih ha calculaed by he ph meer compuer program. 3) Compare he measured ANC wih he heoreical value for he 2.5 mm Na 2 soluion. Noe ha ANC can be defined as he excess of posiive charges over he anions of srong acids. Thus for he 2.5 mm Na 2 soluion he ANC is 5 meq/l since here are 5 meq/l of Na and no anions of srong acids. 4) Plo he measured ANC of he lake on he same graph as was used o plo he conservaive, volaile, and nonvolaile ANC models (see quesions 2 o 5 on page 36). Did he measured ANC values agree wih he conservaive ANC model? Wriing a High Performance Repor Wriing will be an imporan par of your professional career. Your objecives in wriing may be o presen recommendaions or explain opions o a clien. In ha case you will be elling he clien a sory (or sories) ha you creae o explain he facs (or daa) from your invesigaion. You will wan o explain how he differen pieces of your invesigaion fi ogeher and how hey lead o he recommendaion ha you propose. For he laboraory repor ha you will be wriing in he nex few days imagine he following scenario. Under pressure from energy companies, he EPA has decided o increase allowances of sulfur dioxide and nirous oxide from coal fired generaing plans. Your consuling firm has won a conrac (wih funding from EPA and ARM & HAMMER) o assess he feasibiliy of mainaining he ph of Adirondack lakes in he range of 6.5 o 8.2 given he increased inpu of acid. Your specialy in he consuling firm is aquaic chemisry and your ask has been o develop a model ha describes he response of acidic lakes o remediaion wih he goal of providing guidance on how much sodium bicarbonae o add o he lakes and how frequenly o add i. As you wrie your repor you can weave wha you have learned ino your recommendaions. This repor should incorporae wha you learned from boh he acid lake lab and he Gran analysis. If you wish you could preend ha he resuls ha you obained were for a small Adirondack pond. In weaving your sory you migh consider answering he following quesions. 1) Why is i imporan o know he exen of carbon dioxide exchange wih he amosphere? 2) Why have you chosen o analyze he lake using he echniques ha you used? Measuremen of Acid Neuralizing Capaciy

7 46 3) How do he dissociaion consans for he carbonae sysem affec he abiliy o mainain he lake ph in he desired range? 4) Wha is your recommendaion for how o add he sodium bicarbonae o he lakes and wha complicaions do you anicipae? 5) How would you deermine wheher oher Adirondack lakes are exchanging significan quaniies of carbon dioxide wih he amosphere? 6) Wha monioring program would you propose o access he impac of your remediaion sraegy? 7) Wha services migh you offer o your clien as ways ha you could provide furher guidance while hey implemen his remediaion projec? (You would like hem o offer you a coninuing conrac ) References Sawyer, C.N., P.L. McCary and G.F. Parkin Chemisry for Environmenal Engineering, 4h ed., McGraw-Hill (1994). Pankow, J.F. Aquaic Chemisry Conceps, Lewis Publishers (1991). Morel, F.M.M. and J.G. Hering Principles and Applicaions of Aquaic Chemisry Wiley-Inerscience (1993). Summ, W. and J.J. Morgan Aquaic Chemisry 2nd ed. Wiley Inerscience (1981). CEE 453: Laboraory Research in Environmenal Engineering Fall 2006

8 47 Table 4-1. Reagen lis. Lab Prep Noes Descripion Supplier Caalog number HCl 5.0 N Fisher Scienific LC Buffer-Pac Fisher Scienific SB105 Na 2 Fisher Scienific BP357-1 Table 4-2. Equipmen lis Descripion Supplier Caalog number Accume 50 Fisher Scienific ph meer ph elecrode Fisher Scienific x7 sirrer Fisher Scienific S sirbar 1/2" long Fisher Scienific ml Fisher beaker Fisher Scienific B Seup 1) Prepare 1 L of he known sandard (2.5 mm soluion of Na 2 ). The MW is g/mole. 2.5mM L mg mm = 265 mg Na 2 /L 2) Prepare 1 L of he iran (0.05 N HCl from 5.0 N HCl). Dilue 10 ml of 5.0 N HCl o 1 L. Disribue 100 ml iran o each suden group. 3) Verify ha he ph probes are operaional, sable, and can be calibraed. 4) Verify ha buffers (ph = 4, 7, 10) are disribued o each suden group Measuremen of Acid Neuralizing Capaciy