Instrumental methods of analysis

Transcription

1 Instrumental methds f analysis 1. Intrductin The analytical methds fr determinatin f pllutants in envirnment have cnsiderably increased t cpe with the cmplexity envirnmental prblems. A variety f advanced instruments have been develped t supprt the rutine mnitring, especially water and wastewater treatment prcesses. These instruments culd assist analytical measurement t be undertaken immediately at surce and t be accessed the recrding ut ff the place f actual measurement. The instruments are ften develped n the basis f either ptical r electrical methds. Only the electrical methds f analysis are discussed in these labratries. Electrical methds f analysis rely upn the relatinships between electrical and chemical phenmena. The principles f chemical analysis included the ptentimetric and plargraphic analysis. When the electrical methds analysis culd detect the electrical ptential via the electrdes, this is named ptentimetric analysis. In case, the suitable electrdes are emplyed int the slutin and then a small measured vltage is applied. The analytical measurements are made based n the assumptin f the current culd flw differently in the different cmpsitins f the slutin, this is called plargraphic analysis. 2. Ptentimetric analysis The ptentimetric analysis is the ne f simple methdlgy. The cncentratin f inic species in slutin can be cnverted frm the relative ptential f an electrde. The specific electrde and the reference electrde are designed in rder t crrespnd the ptential measuring f the specific in f interest. A simple example f ptentimetric analysis is the electrchemical cell. The electrical energy cnsumed by ne mle f chemical is zef, where z is the number f electrnequivalents per mle, F is the Faraday r Clumbs per equivalent and E is the emf f the cell in vlts. The reactin can be ccurred when E is psitive, thus the relatin between free energy and electrical energy can be frmulated as fllws. Δ G = zfe (1) The chemical reactin take place at electrchemical cell can be simplified as fllws. aa bb ze cc dd The relatinship between cell emf and cncentratin f reactants and prducts can be expressed using Nerst Equatin as fllws. (2)

2 E RT zf c d { C} { D} { A} a { B} b = E Ln (3) If the reactin has reached the equilibrium cnditin, there is n current flw, and the emf f cell becmes zer. The ptential at the standard-cell r reference electrde is yielded as fllws. E ΔG RT = = LnK (4) zf zf Fr instance, the aqueus cncentratin f chlride can be detected using the simple electrchemical cell as presented in Figure 1. E Ag AgCl film Saturated calmel electrde M Cl - Sample Figure 1 Cell assembly fr ptentimetric analysis fr chlride (Sawyer et al. 1994) A saturated calmel electrde is intrduced as the reference electrde and cnnected with the chlride measurement electrde-a silver-silver chlride electrde. The reactin at the measuring r indicating electrde can be written as: The ptential f the half cell can be determined as fllwing: The ptential f the cmplete cell is: E AgCl e Ag Cl (5) [ ] RT ECl = ECl Ln Cl (6) zf [ ] RT = ECl ESCE = ECl ESCE Ln Cl zf cell (7) The ptentimetric analysis can be applied t measure many inic species.

3 3. Plargraphic analysis The plargraphic analysis is the analytical methd that relies n the relatinship between vltage applied acrss tw electrdes and current flw. If the cnventinal electrde is applied, this methdlgy can be termed as vltammetry. When a special inert material is inserted int the electrde resulting in an inert electrde, this measurement is named as plargraphy. Hwever, the plargraphic analysis is nt recmmended in Standard Methds, as it requires the specific electrde that may cntain sme inert hazardus material and many supprting measuring devices. The simple electrical circuit fr plargraphic analysis is given in Figure 2. 3 Vlts - Vltmeter Micrammeter I E Saturated Calmel electrde Inert electrde Sample Figure 2 Electrical circuit fr plargraphic analysis (Sawyer et al. 1994) Fr instance, the electrdes are immersed in the slutin and the applied vltage is given thrugh the circuit. The vltage varies acrss the electrdes frm 0 t 3 vlts. The indicating electrde ptential relies n analyte cncentratin. The exact vltage is recrded via a vltmeter. Since the applied vltage is increased cntinually, the current will clse t zer, until the presence f in reductin. The electrlytic reductin begins at the inert electrde, and the increase f vltage brings a sharp increase f current. At this pint, the in depletin ccurs near the inert electrde and fresh ins reach the electrde due t diffusin. When the diffusin rate reaches the limit f current at electrde, this current is a prprtinal t the cncentratin f the ins that are being reduced.

4 4. Types f electrdes The meters can detect a ptential f electrchemical cells and then cvert t cncentratin. The electrdes are the mst imprtant device t fabricate the meters. Tw electrdes are generally referred as reference and indicating electrde. The basic reference electrdes are Hydrgen, silver-silver chlride and Calmel electrdes. The hydrgen electrde is used t detect the hydrgen activity. The flwing stream f gas is required during the perating f hydrgen electrde. The ptential at hydrgen electrde is assigned at value f exactly zer at all temperature. The Calmel electrde is used widely because f its ease f preparatin and cnstancy f ptential. It has a calmel half cell that cntains mercury and calmel (mercury (I) chlride) and this cell is cvered with ptassium chlride slutin with the cncentratins f 0.1, 1M r saturated. The inner tube cnsisting f HgHg 2 Cl 2 and it is cnnected with the uter tube cntain KCl via a small pening. The cntact surface f electrde is made thrugh a prus fibre wick r fritted disc at the bttm f the uter tube. The ptentials f Calmel reference electrdes filled with KCl at the cncentratins f 0.1, 1.0 M and saturated at 25 C are , and Vlts, respectively. The silver-silver chlride electrde is fabricated frm a silver wire r silver plated platinum electrde immersed in saturated KCl slutin. The cntact surface is made f a prus material. Hwever, this electrde is n lnger used and it is nt mentined here. The standard hydrgen and Calmel electrdes are presented in Figure 3. (a) (b) Figure 3 Reference electrdes (a) hydrgen, (b) Calmel reference electrdes (HP training 2002)

5 The hydrgen, Calmel electrdes can perfrm the reactins as fllws. H 2 ( g ) Hg Cl 2 2e 2 2H 2e ( s) ( l ) 2Hg 2Cl (8) The indicating electrde can be fabricated t be metal, redx, membrane r crystalline and liquid electrdes. Fr metal electrde type I, the electrde is cmpsted with a metal in cntact with its ins. Fr example, cpper electrde dipped in cpper slutin (Cu 2 ), the reactin ccurred at the indicating electrde is: Cu 2 2e Cu s therefre, the half cell ptential can be describe as fllws. ( ) RT 1 E = E Ln 2 zf [ Cu ] At the temperature f 25 C, the values f R, and F, are substituted in Eq.10, yields: 2 The graphical plt between E and [ Cu ] E = E Lg 2 z [ Cu ] (9) (10) (11) Lg has slpe The slpe cnstant is applied t cnvert the measured ptential t the cncentratin. The metal electrde type II, this electrde cntains anin, which frms sparingly sluble precipitate with electrde metal. Fr example, the silver-silver idide electrde, the reactins presented n electrde can be described as fllws. AgI Ag ( s) Ag e I Ag ( s) (12) AgI e Ag I ( s ) ( s) Thus, E = EAgI. 059Lg[ I ] The slpe f graphical plt between E and [ I ] 0 (13) Lg is The redx electrde is a nn-reactive metal electrde, such as platinum electrde and the electrde ptential depends n relative cncentratin f reduced and xidised species in equilibrium. Fr example irn electrde, which cnducts the reactin as fllws. Accrding t Nerst s Equatin, yields: 3 2 Fe e Fe (14)

6 2 [ Fe ] [ Fe ] E = 0.77 Lg (15) 3 1 The membrane electrde is cnsisted f membrane sensitive t in f interest. The in cncentratin at electrde is cnstant but the ptential depend n cncentratin in the slutin. Fr example, hydrgen electrde is membrane electrde, the ptential and cncentratin f hydrgen in can be written as fllws. where [ H ] and [ ] and K is cnstant. a e E E indicating indicating = K ph [ H a ] [ H ] = k 0.059Lg e (16) H are the internal and external electrde analyte cncentratin, respectively, References: APHA (1992), Standard Methds fr the Examinatin f Water and Wastewater, Washingtn DC, APHA. Hydrlgy prject (2002), Ptentimetric Analysis, Training mdule #WQ-32, 06/11/02, New Delhi, India. Sawyer C.N., McCarty P.L. and Perkin, G.F. (1994), Instrumental methds f analysis, Chemistry fr Envirnmental Engineering, 4 th eds., New Yrk, McGraw-Hill, pp

7 Labratry I: ph and cnductivity meters 1.1 ph meter A. General cnsideratin ph is the measurement f the degree f the acid r alkaline cnditin in the slutin. The hydrgen in is measured in rder t determine the hydrgen in activity. The hydrgen electrde is a suitable device fr ph measurement. The hydrgen ins in pure water are 10-7 ml/l. H 2 O H OH (1) Since the reactin achieves the equilibrium cnditin, the rate cnstant (K) can be written as fllws. { H }{ OH } K = (2) H O { } The cncentratin f water is very stable, the degree f inizatin (K w ) can be assumed as fllws. 2 { H }{ } K w = OH (3) Fr the pure water at 25 C, the inisatin rate is equals t 10-14, with cncentratins f H and OH - equal t 10-7 ml/l. When an acid is added, the numbers f hydrgen ins must be increased but the hydrxide ins must be decreased. Likewise, if a base is added, the hydrgen ins must be decreased but the hydrxide ins must be increased. The ph measurement always express the hydrgen-in activity, the ph scale ranges frm 0 t 14. The slutin with ph 7 at 25 C is the abslute neutral slutin. The ph can be presented as fllws. The ph scale is given in Figure 1.1. ph = Lg { H } r ph = Lg 1 { H } (4) Acid range Neutral Alkaline range ph scale Figure 1.1 ph scale (Sawyer et al. 1994)

8 B. Measurement f ph ph meter is made f a single glass membrane electrde and calmel reference electrde as presented in Figure 2. The hydrgen electrde is set up a value f zer and ptentials related t the HCl electrlyte. Fr the calmel reference electrde, the KCl electrlyte is filled as a reference slutin. Figure 1.2 Glass electrde fr ph measurement (HP training 2002) The glass electrde is made f a strip f nn-reactive metal in cntact with bth the slutin and a gas stream. The hydrgen electrde is used as the standard and it cnsists f a strip f nnreactive material and immersed in 1.0 N f hydrgen ins and bath with stream f hydrgen gas under 1 atm f pressure. When the hydrgen in reaches the electrde, it carries an electrn and is reduced t hydrgen gas. The half reactin is frmulated as fllws. 1 H e H 2( g ) 2 (5) The thin glass membrane can respnd t ph due t in exchange reactins. The hydrgen ins in the slutin are exchanged with mnvalent catin in the glass lattice. The ptential at electrde represents the activity f hydrgen in in the slutin, H a and hydrgen within electrde, the relatinship between the ptential and cncentratin at the indicating electrde can be described as fllws. E E indicating k 0.059Lg = K ph [ H a ] [ H ] indicating = e H e, (6)

9 At neutral ph (ph 7), the ptential at prbe is 0 vlts. Fr the basic ph (ph>7), the ptential at prbe ranges frm 0 t 0.41 vlts, and at the acid ph (ph<7), the ptential at prbe varies frm-0.41 t-0 vlts. C. Analytical prcedure C.1 Material and apparatus 1. ph meter 2. Multimeter 3. Magnetic stirrer 4. Deinised water 5. HCl with apprximately ph at 2, 3, 4, 5 and 6 6. NaOH with apprximately ph at 8, 9, 10, 11 and 12 C.2 Testing prcedure 1. Clean ph prbe with deinised water, rub the ph prbe with tissue paper. 2. Immerse ph prbe int the slutin, read the ph value. 3. Place multi-meter int the slutin, read the electrical ptential acrss the electrde. 4. D graphical plt t find the relatinship between E and ph and [H ] tgether. 1.2 Cnductivity meter A. General cnsideratin Cnductivity is the ability f a material t cnduct electric current that is the inverse f resistivity. The cnductivity can be detected using cnductivity meter, which is very simple instrument. It is fabricated using tw metal plates. As sn as the meter is immersed int slutin, a ptential is applied acrss the plates, and the current passed thrugh the slutin is measured. The cnductivity can be calculated using Ohm s law. ( amps) ( vlts) I G = (6) E where G is cnductivity, I is electrical current and E is ptential. In sme cases, the cnductivity may nt crrelate directly t cncentratin, and the linear relatinship between cnductivity and cncentratin may nt be btained. The examples are prvided in Figure 1.3.

10 Figure 1.3 Relatinship between cnductivity and inic cncentratin (Cle-Parmer 2009) The unit f cnductivity can be simply presented in a unit f cnductance-siemen (S) frmally called mh. The gemetry f cell can affect the cnductivity values, in rder t cmpensate, the measurement is presented in a term f specific cnductin that is in a unit f S/cm. The specific cnductivity, C can be calculated as: G L C = (7) A where G is the measured cnductivity, and L is length f the clumn liquid between the electrde and A is area f electrde. K is defined as the electrde cnstant (cm -1 ) and equals t L/A. The parameter is given in Figure 1.4. Figure 1.4 Scheme f cnductivity electrde (Cle-Parmer 2009) The range f measurement depends n the cell cnstant as given in Table 1.1.

11 Table 1.1 Optimum cnductivity ranges fr different cell cnstants (Cle-Parmer 2009) Cell cnstant, K (cm -1 ) Optimum cnductivity range (μs/cm) 0.5 t t t 200,000 Cnsequently, the knwn cncentratin f inic substances can be applied t estimate the specific cnductivity f the slutin as fllws. where k zi ( i )( mm i ) zi ( λi )( mm ) = i z i is abslute value f the charge f the i-th in, the i-th in and λ, i λi λ (8) are equivalent cnductance f the i-th in. The cnductance f cmmn inic cnstituents in water is given in Table 1.2. mm i is the millimlar cncentratin f H 1/2Ca 2 1/2Mg 2 Na K NH 4 1/2Fe 2 1/3Fe 3 Table 1.2 Equivalent cnductance, λ and λ fr ins in water at 25 C (APHA 1992) Catin λ (mh-cm -2 /equivalent) Anin λ (mh-cm -2 /equivalent) OH - - HCO 3 2-1/2CO 3 2-1/2SO 4 Cl - Ac - F - - NO 3 - H 2 PO 4 2-1/2HPO The inic strength, IS in Mlar unit and the mnvalent in activity cefficient, y can be estimated as fllws. ( ) 2 z i mm i IS = (9) / 2 1 / 2 0.5[ IS ( 1 IS ) 0.3IS y = 10 ] (10) Then the specific cnductivity can be estimated as fllws. C 2 cal = k y (11)

12 Furthermre, the standard temperature fr cnductivity measurement is at 25 C. In case, the slutin temperature is nt exactly at 25 C, the measured cnductivity can be justified as fllws. where Gt and Gtcal t tcal [ ( t t )] G = G 1 α (12) cal are the cnductivity at any temperature t in C and at calibratin temperature tcal in C and α is the temperature cefficient at t cal in C. The temperature cefficients fr cmmn slutin are given in Table 1.3. Table 1.3 Values f α fr cmmn slutins (Cle-Parmer 2009) Substance at 25 C Cncentratin (%wt) α Substance at 25 C Cncentratin (%wt) α HCl KCl NaCl H 2 SO 4 HF HNO B. Measurement f cnductivity Cnductivity meter ften cntained tw electrdes, which are cmmnly fabricated frm platinum metal. The ptential between the inner pair f electrdes and their distance are measured and reprted as specific cnductivity. The pssible specific cnductivity fr cmmn slutin is given in Table 1.4. Table 1.4 Specific cnductivity at 25 C in general slutins (Radimeter analytical 2009) Slutin C (μs/cm) Slutin C (ms/cm) Pure water Industrial wastewater 5 Deinised water 1 Seawater 50 Rain water 50 1 M NaCl 85 Drinking water M HCl 332 C. Analytical prcedure C.1 Material and apparatus 1. Cnductivity meter 2. Thermmeter 3. Magnetic stirrer

13 4. Deinised water 5. KCl with cncentratins f 0.01, 0.1, 0.5 and 1.0 M 6. NaCl with cncentratin f 0.01, 0.1, 0.5 and 1.0 M. 7. HCl with cncentratins f 0.1, 0.5, 2 and 6 M. C.2 Testing prcedure 1. Rinse cnductivity prbe with deinised water, use tissue paper t absrb the film f water at the prbe. 2. Immerse the prbe and thermmeter int the slutin, recrd the specific cnductivity value and temperature. 3. Calculate the specific cnductivity at standard temperature (25 C) and plt find the relatinship between C and cncentratin. 4. Cmpare the measured specific cnductivity f KCl and HCl t the calculated values. D. Reprt D.1 ph measurement Slutin ph E (vlts) [H ] Slutin ph E (vlts) [H ] D.2 Cnductivity measurement Slutin Measured C Temp C Calibrated Slutin Measured C Temp C Calibrated (μs/cm) C (μs/cm) (μs/cm) C (μs/cm)

14 D.3 Graphical plts Frm D.1 E vs ph and E vs [H ] Frm D.2 Calibrated C vs inic cncentratin Example References: APHA (1992), Standard Methds fr the Examinatin f Water and Wastewater, Washingtn DC, APHA. Hydrlgy prject (2002), Ptentimetric Analysis, Training mdule #WQ-32, 06/11/02, New Delhi, India. Sawyer C.N., McCarty P.L. and Perkin, G.F. (1994), Instrumental methds f analysis, Chemistry fr Envirnmental Engineering, 4 th eds., New Yrk, McGraw-Hill, pp Cle-Parmer (2009), Cnductivity, Cle-Parmer Technical Library: [accessed: Octber 2009]. Radimeter analytical (2009), Cnductivity thery and Practice, Radimeter Analytical SAS, Villeurbanne Cedex, France.

15 Labratry II: DO and ORP meters 2.1 DO meter A. General cnsideratin Disslved xygen r DO is the measurement f aqueus xygen cntained in waste r wastewater. Sme gases such as nitrgen, ammnia and carbn dixide can react with water after disslving. Unlikely, disslved xygen des nt react with water. The majr surce generated disslved xygen in water are the atmsphere and phtsynthesis. Waves and turbulent flw can recharge the xygen int water, and aquatic plants can prduce xygen as a by-prduct during phtsynthesis. The disslved xygen cncentratin can be applied t evaluate the cnditin f sewage treatment plants, the river quality and the fish farming. The cncentratin f disslved xygen relies upn the temperature, salinity and atmspheric pressure. The amunt f disslved xygen in water is influenced by temperature, salinity and atmspheric pressure. The clder water can hld mre disslved xygen, and the fresh water can hld mre disslved xygen than saltwater des as well as the amunt f disslved xygen decreased with decreasing atmspheric pressure. Adequate disslved xygen levels are needed in the natural stream purificatin prcesses. If the disslved xygen in water is less than 5.0 mg/l, the aquatic animals are under stress. Whenever the disslved xygen level becmes lwer than the stress, fr example the DO level is 1-2 mg/l fr a few hurs can kill large fish. Nevertheless, the disslved xygen level at the ver saturatin; exceed 110 percent, the fish may suffer frm gas bubble disease. The bld vessels cannt carry the bld due t flw blcking. B. Measurement f disslved xygen A cncentratin f disslved xygen can be detected using DO meter. The DO prbes are fabricated with a variety technique as fllws. - Plargraphic (Clark cell methd) prbe, an external vltage is applied maintaining the ptential difference between the ande and cathde t less than 0.5 vlts. - Galvanic prbe, an external vltage is nt required and the difference vltage between ande and cathde is higher than 0.5 vlts. This methd is mre stable and accurate than the plargraphic prbe. - Optical flurescence prbe, this is suitable fr lng-term measurement in grundwater as it des nt react with any cnstituents. The plargraphic electrde is intrduced in this labratry. The structure f prbe is presented in Figure 2.1.

16 Figure 2.1 D prbe and its structure (HP training 2002) The reactin ccurred at the electrde can be described as fllws. Ande: 2 Ag 2Cl 2AgCl 2e (1) Cathde (Platinum, gld r palladium): Ttal reactin: 1 2 2e O2 H 2O 2OH (2) 1 2 2Ag 2Cl O2 H 2O 2e 2OH 2AgCl 2e (3) The cncentratin f disslved xygen in fresh-pure water at 1 atm (760 mmhg) is given in Table 1. Table 1 Full disslved xygen capacity in fresh water C DO (mg/l) C DO (mg/l) C DO (mg/l)

17 C. Analytical prcedure C.1 Material and apparatus 1. DO meter 2. Thermmeter 3. Magnetic stirrer 4. Beaker 5. Diffuser 6. Deinised water 7. Fresh deaerated water C.2 Testing prcedure 1. Rinse DO prbe with deinised water and blt dry. 2. Place 1000 ml baker n the magnetic stirrer place. Gradually pur the deaerated water in t beaker. Turn n magnetic stirrer. 3. Immerse the prbe and thermmeter int the deaerated water, recrd the initial disslved xygen cncentratin and temperature. 4. Insert the diffuser int water and turn n. Recrd the DO value and calculate the degree f saturatin fr every minute until achieving saturatin. 2.2 ORP meter A. General cnsideratin Oxidatin Reductin Ptential r ORP indicates the rati between the slutin s ability t xidise and reduce. Oxidatin refers t the lss f electrns by an atm, mlecule r in. The basic xidatin prcesses are irn rusting and wd burning, which may either be r may nt be accmpanied by the additin f xygen. As sn as the substances have been xidised, their xidatin states increase. Many cnstituents can exist in a varius xidatin states, fr example 2 sulfur can exhibit xidatin states f -2 ( H 2 S ), 0 ( S ), 4( SO 2 ) and 6( SO 4 ). Reductin refers t the net gain f electrns by an atm, mlecule and in. When a substance is reduced, the xidatin state is decreased. Oxidatin and reductin always present at the same time as the electrns lst in xidatin must have anther substance as a destinatin. Hence, the ORP refers t redx measurement, the reactins ccurred at electrdes can be described in accrding t Nerst half cell ptential as fllws.

18 E = E 2.3RT nf Lg [ Ox] [ ] Re d (3) where E is the electrde ptential measured, E is a vltage specific t the system under analysis, R is the universal gas cnstant, T is the abslute temperature in K, n is the number f electrns invlved in the equilibrium between the xidised and reduced species and F is the Faraday cnstant (96500 culmbs). [] dentes activity f bracketed ins. B. Measurement f ORP The utilised reagent in this labratry is hypchlrus/chlride, which is nrmally used as infectant. The half cell reactin is presented as fllws. HOCl H 2e Cl H 2 O E = 1490mV (4) The ORP relies upn the cncentratins f all substances excluding f water. The ORP f hypchlrus depends n chlride in and ph ( H ). Accrding t Nerst s equatin, the ptential f cell can be determined as fllws. ( ) [ Cl ] 29. Lg [ HOCl] [ H ] E = (5) C. Analytical prcedure C.1 Material and apparatus 1. ORP meter 2. ph meter 3. Magnetic stirrer 4. Beaker 5. Burette and stand 6. Deinised water 7. Bleach slutin C.2 Testing prcedure 1. Rinse ORP prbe with deinised water and blt dry. 2. Place 1000 ml baker n the magnetic stirrer place. Gradually pur the bleach slutin int burrette and hang the burette nt the stand. Turn n magnetic stirrer.

19 3. Immerse the ORP and the ph prbes int the deaerated water, recrd the initial ORP and ph f slutin. 4. Gradually drp the bleach slutin int water. Recrd the values f ORP, ph and vlume f bleach slutin used. Repeat until achieving the equilibrium. D. Reprt D.1 DO measurement Time DO Temp Time DO Temp Time DO Temp (min) (mg/l) ( C) (min) (mg/l) ( C) (min) (mg/l) ( C) D.2 ORP measurement Bleach ORP ph Bleach ORP ph Bleach ORP ph added (mv) added (mv) added (mv) (ml) (ml) (ml)

20 D.3 Graphical plts Frm D.1 time vs DO Frm D.2 bleach added vs ORP References: APHA (1992), Standard Methds fr the Examinatin f Water and Wastewater, Washingtn DC, APHA. Emersn Prcess Management (2008). Applicatin Data Sheet, ADS , Irvine, USA. Hydrlgy prject (2002), Ptentimetric Analysis, Training mdule #WQ-32, 06/11/02, New Delhi, India. McGinnis, D.F. and Little, J.C. (2002). Predicting diffused-bubble xygen transfer rate using the discrete-bubble mdel. Water Research 36, Sawyer C.N., McCarty P.L. and Perkin, G.F. (1994), Instrumental methds f analysis, Chemistry fr Envirnmental Engineering, 4 th eds., New Yrk, McGraw-Hill, pp