University of Bucharest, Faculty of Chemistry, Department of Analytical Chemistry, Panduri, Bucharest, Romania,

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1 Department of Physcal Chemstry 4-1 Regna Elsabeta Blvd, Dstrct 3, Bucharest phone: ; fax: ISSN: ARS DOCENDI PUBLISHING HOUSE Sos. Pandur 90, Dstrct 5 Bucharest phone/fax: e-mal: arsdocend@yahoo.com VALIDATION OF AN ANALYTICAL QUANTITATIVE DETERMINATION METHOD OF CHLORIDE ANION FROM DRINKING AND SURFACE WATER, USING DIRECT POTENTIOMETRY WITH CHLORIDE-SELECTIVE ELECTRODE (I) I.G. Tnase, Dana-Elena Popa and Mhaela Buleandr abstract The purpose of ths study was to elaborate, develop and valdate an analytcal method for quanttatve determnaton of chlorde anon from drnkng and surface water usng drect potentometry wth Cl selectve electrode. The quanttatve determnatons of chlorde on realzed n ths way are quck and relatvely cheap. It wll be proved that such a method s accurate, precse, lnear, senstve and selectve, presentng an adequate lnear workng range. In ths frst part wll be proved that the method presents an approprate workng range and that s lnear. key words valdaton, analytcal method, on selectve electrodes, potentometry. Introducton The possblty of quanttatve determnaton of Cl - from drnkng and surface water usng drect potentometry wth Cl selectve electrode consttutes an opportunty for routne measurements / tests. Analytcal method valdaton s a necessary evl for the qualty assurance and producton processes safety, but represents the frst step or the frst level of qualty assurance wthn a testng and measurement analytcal laboratory. Valdaton s realzed accordng to natonal and nternatonal standards (ISO 9000, ISO 1705), but also accordng to BPL-OECD norms regardng good laboratory practce wthn testng and measurement n analytcal laboratory (GLP) (HG 63/00 modfed wth HG 66/006 and HG 448/007). For chlorde anon determnaton from waters are used, wth good results, both gravmetrc and volumetrc analyss methods, as well as nstrumental analyss methods. Classcal methods presume gravmetrc determnaton by nsoluble chlorde precptaton, or volumetrc ttraton [1 4]. From nstrumental methods can be remnded UV-VIS molecular absorpton spectrometry (mercury tocyanate method ntroduced by Japanese scentsts [5 7] n 195, mercury chloroanylate [8,9]), atomc absorpton spectrometry (ndrect method) [10,11] and drect and ndrect potentometry [1 18]. Unversty of Bucharest, Faculty of Chemstry, Department of Analytcal Chemstry, 90-9 Pandur, Bucharest, Romana, e-mal: tanase.on@gmal.com Analele Unverst dn Bucuret Chme, Anul XVI (sere nou), vol. II, pag. 5 3 Copyrght 007 Analele Unverst dn Bucuret

2 6 I.G. TNASE D.-E. POPA M. BULEANDR Elaboraton, development and valdaton of a quanttatve determnaton method for Cl - usng drect potentometry wll prove that ths method s quck and cheap and shows some performance characterstcs lke accuracy, precson, lnearty, sensblty, applcablty and an adequate lnear workng range. It was chosen ths way because the level of chlorde concentraton n drnkng and surface waters fts relatvely well wth the workng range of the Cl selectve electrode. It s known that Qualty norms that surface waters used to obtan potable water must comply wth NPTA 013 (HG 100/00 modfed wth HG 66/005 and HG 567/006) accept a maxmum chlorde level of 00 mg. L 1. In ths paper t wll be presented only the second part of the method valdaton: characterzaton of the performance parameters of the method. The proposed analytcal method makes possble quanttatve determnaton of chlorde on from drnkng and surface waters by drect potentometry wth Cl selectve electrode as workng electrode. Drect potentometry ( = 0) s a well known technque, that can supply a determnaton method of a on concentraton (caton or anon) by measurng emf (electromotve force) of a potentometrc cell wth lqud juncton, made of an workng electrode (chlorde onselectve electrode) and a reference electrode [19,0]. Expermental All the used reagents had analytcal purty qualty. A ppm chlorde stock soluton (WTW Germany, NIST (Natonal Insttute of Standards and Technology) traceable) was used to obtan standard solutons. KNO 3 was used to obtan the nert electrolyte soluton (1 mol. L -1 ). For standard and workng solutons preparaton adequate volumes of stock standard soluton were measured and then they were put n volumetrc flasks. Tr-dstlled water was used for all solutons preparaton and for vessel cleanng. The necessary measurements for quanttatve determnaton of chlorde on from drnkng and surface waters have been carred out usng a ph/mv-metre WTW Inolab 740, wth 6 pont calbraton; chlorde-selectve electrode as workng electrode; R503/D-WTW reference electrode; 50, 100, 500, 1000 ml volumetrc flasks (A class); 100 ml Berzelus glasses; 1,, 5, 10 ml ppettes (A class). In order to prepare the standard solutons, n 50 ml volumetrc flasks were put: fxed volumes (5 ml) of 1 M KNO 3 soluton, ncreasng volumes of standard chlorde soluton (100 ppm) and tr-dstlled water. Drnkng or surface water samples were prepared as follows: n a 50 ml volumetrc flask was added 5 ml from the water sample, 5 ml nert electrolyte soluton (1 mol L -1 KNO 3 ) and tr-dstlled water. The samples were decanted nto the potentometrc cell and emf was measured. Results and dscussons Accordng to ICH (Internatonal Conference of Harmonzaton) recommendatons [1,] for analytcal methods valdaton, the followng performance parameters must be taken n consderaton: selectvty / specfcty, workng range, lnearty, precson, accuracy,

3 ANALYTICAL DETN. OF CHLORIDE FROM DRINKING AND SURFACE WATER 7 detecton lmt, quantfcaton lmt, robustness. From these, n ths frst part wll be dscussed the workng range and the lnearty. Before establshng the workng range, the chlorde-selectve electrode response s followed up on a wde concentraton range ( mol. L 1 ), n aqueous medum of 0.1 mol. L 1 KNO 3. The expermental results are graphcally represented n Fg. 1. They emphasze that the response curve of chlorde-selectve electrode presents a sgmod form; there s a large enough concentraton range for whch the dependence s lnear; ths can be used for quanttatve determnaton of chlorde on. 0,35 0,3 0,5 E(V) 0, 0,15 0,1 0, Fg. 1 The response curve E lg c for chlorde-selectve electrode 1. Workng concentraton range Cl lg c Cl-500-WTW, n 0.1 mol L -1 KNO 3 The workng concentraton range, or, shortly, workng range, s specfc for determnaton of each analyte and s establshed to confrm the fact that the developed analytcal procedure provdes an acceptable lnearty, accuracy and precson degree when appled to samples ncludng the analyzed speces nsde or at the lmts of the specfed range. In the workng concentratons range, the analytcal response may be or may not be lnear and, n order to prove ths fact, t s necessary to realze several calbraton ponts (more then 6 dfferent concentratons of the analyte, preferable 10-11). If the relatonshp between the nstrument response and analyte concentraton s not perfectly lnear (t may be an exponental relatonshp), the analytcal procedure can be used, but the calbraton curve has to be reported on daly bass. Establshng the workng concentraton range can be done n two ways: usng mult-ponts calbraton or graphcally: representng E/lgc=f(lg( lgc)).

4 8 I.G. TNASE D.-E. POPA M. BULEANDR If the workng range s establshed by calbraton [3], then s worth mentonng that every calbraton experment begns wth selecton of a prelmnary workng range. The nformatve values are presented n Table 1 (for the lowest concentraton were made 10 replcates and for the hghest concentraton were also made 10 replcates). Informatve concentraton values were equdstantly dstrbuted on the entre establshed prelmnary workng range. j Table 1 Calbraton data for establshng the workng range used for quanttatve determnaton of chlorde from drnkng and surface waters by drect potentometry wth chlorde-selectve electrode (prelmnary range , mol L 1 ) x (mol L 1 ) y,1 y, y,3 y,4 y,5 y,6 y,7 y,8 y,9 y, y 1 = s 1 = s 1 = y 10 = s 10 = 0.01 s 10 = For establshng the dsperson homogenety, 10 repeated determnatons (replcates) were done for the lowest and for the hghest concentraton (x 1 and x 10 ). For each of these seres were obtaned 10 nformatve values y j. The next step conssts n the dsperson homogenety test usng the two concentraton levels x 1 and x 10 to calculate the dspersons s 1 and s 10 accordng to the formula: wth y - average value of e.m.f. 10 j1 y y s, (1) n 1 For = 1 and = 10 there were obtaned: y 1= ; s 1 = ; s 1 = ; y 10 = ; s 10 = 0.001; s 10 = j

5 ANALYTICAL DETN. OF CHLORIDE FROM DRINKING AND SURFACE WATER 9 Obtaned dspersons were tested usng the F test to examne the sgnfcant dfferences at the workng concentraton range lmts. The F test requres the testng value, PG, determnaton startng wth the formula: that s PG, s PG for s s1 s10, () The obtaned PG value was compared to the table values for the F dstrbuton. Consultng the table contanng values of F (dsperson homogenety test for 1 = = n 1 = 9 freedom degree for dspersons s 1 and s 10 ), t was obtaned F 9;9;0.99 = 5,35. In ths case, PG =,17554 and F 9;9;0,99 = 5,35, so PG F 9;9;0,99 ; the devaton between dspersons s 1 and s 10 wasn t sgnfcant and the workng range was correctly selected: , mol. L -1 (0,5 50 mg. L -1 ) Cl -. Comparson of calculated value PG, wth the table value ddn t showed sgnfcant dfference. Dspersons are homogenous and are allowng the smple (lnear) regresson analyss. In the statstc lnearty test, calbraton data are used to calculate a lnear calbraton functon and also a non-lnear calbraton functon, both of them showng a standard resdual devaton s and s. Dspersons dfference DS can be calculated usng the formula: y 1 for = 1 freedom degrees. y DS N s N 3s y1 y, (3) DS and the dspersons of the non-lnear calbraton functon are subject of an F test n order to examne the sgnfcant dfferences. The PG value requested for F test s calculated usng the formula: DS PG 31, (4) s y Comng back to calculaton, for the lnearty test was obtaned: standard devaton of lnear regresson s = and s = ; standard devaton of a nonlnear y 1 regresson s = and s = y Dsperson dfference was: DS y y 1 N s N 3 s y1 y = Comparng PG calculated value wth F 9;9;0,99 = 5, t was ascertaned that PG F, so the nonlnear functon doesn t supples an mprovement; the calbraton functon that must be used s the lnear functon.

6 30 I.G. TNASE D.-E. POPA M. BULEANDR. Lnearty Lnearty of a quanttatve analytcal method represents ts ablty to obtan results proportonal wth analyte concentraton from the sample. Lnearty can t be expressed, t must be demonstrated drectly on analyte or spked samples, usng at least fve concentratons on workng range. Besdes the vsual evaluaton of the analytcal sgnal as a concentraton functon, t s recommended to do the correspondng statstcal calculatons, lke lnear regresson. Reportng statstcal parameters lke the slope of the calbraton curve, b and ts ntersecton to the orgn, a, resdual values squares amount and correlaton coeffcent s a mandatory condton. Lnear calbraton functon must result from obtaned data, startng from a workng range from x 1 to x 10, as t was establshed startng from uncorrelated measurements for blank samples. Usually, mustn t be ncluded any blank sample value n calbraton experments and n least squares method. Lnear calbraton functon s descrbed by the equaton: y a bx (5) The lnear model s, no doubt, the most mportant one for processng dmensonal data. Over szng the equatons system represents the regresson analyss focussng pont, meanng the determnaton of more coordnate pars then the obtaned ones (x 1, y 1 ) and (x, y ), requested to calculate a and b usng the classc algebra. The least squares model was selected, leadng to a straght lne subject (submtted) to the restrcton: r y y mnm (6) Here, r represents the resdual values startng wth measurement. The estmated parameters of regresson analyss realsed for quanttatve determnaton of chlorde by drect potentometry usng chlorde-selectve electrode are presented n table. Usng the calbraton data, t was calculated a lnear calbraton functon (y = a + bx) and a nonlnear calbraton functon (y = a + bx + cx ). The results acheved when calculatng frst and second degree functons are presented n Table. Table Expermental results and parameters values of the calculated regresson functons c x lg c Cl Cl y x 3 x 4 x y x y x y abx E E+0 6.5E E E E E E E E E E E E E E E+0 7.0E E E E E E+01.89E E E E E E E+01.56E E E E+00.80E E E E E E-01.94E E E E E E E-01.8E E E E E E-0-6.1E E+00.06E-04 y

7 ANALYTICAL DETN. OF CHLORIDE FROM DRINKING AND SURFACE WATER E E E E E E E-04 N= E E E+01.81E E E E-03 N E E+0.47E E E E E-04 Lnear functon y = a + bx Equaton y = 0,0564x + 0,05 Slope sensblty, b b x x y y x x 0, 0564 N 1 Intercept, a a y bx 0, 05 N Resdual standard devaton : s 1 y a bx 1 y N s 0, Standard devaton of the method: sx sy b s x 01 0, Coeffcent of varaton (RSD%): 01 ( s x) Standard devaton of the slope: V x V 01 0, s b s b 0, Standard devaton of the ntercept: s a s a 0, Correlaton coeffcent: R R 0, 9994 Determnaton coeffcent: R R 0, 9989 y 1 Non-lnear functon: y = a + bx + cx N 1 Equaton of the curve y 0, , x 0,00363x QxxQ 3Q Qxx Q 3 Q xxq 4 c coeffcent: c x x y x x c 0, b Q c Q 3 Q x b 0, b coeffcent: xy xx Intercept a: a y b x c x N a 0, Sensblty at the centre of the workng range: E = b + cx E 0, Standard devaton: N s ( y a bx cx ) N 3 y s 0, Standard devaton of the method: sx sy E s 0, x0 Varaton coeffcent (RSD%): Vx sx x V x 0 0, Although the correlaton coeffcent (R) and the determnaton coeffcent (R ) obtaned durng the regresson analyss, are often used to prove the lnearty (R>0.997) [4], ther values are not able to do that; they can only prove f statstcal processed values are correlated (R= ) or uncorrelated (R=0.000). An alternatve approach for usng the correlaton coeffcent to lnearty establshment conssts n dvdng the sample sgnal to the correspondng concentraton ( lgc ) and the y Cl

8 3 I.G. TNASE D.-E. POPA M. BULEANDR graphcal dsplay of these relatve responses versus lg( lgc Cl ). It must be obtaned a horzontal straght lne wth postve devaton at low concentratons and negatve devaton at large concentratons. Concluson In ths frst part of the valdaton study t was proved that the method s lnear and presents an approprate lnear workng range. REFERENCES 1. Wllams, W.J. (1979) Handbook of Anon Determnatons, Butterworths, London, Ptroescu, C. and Gnescu, I. (1980), Analza apelor, Edtura Scrsul Românesc, Craova, Mnescu, S., Cucu, M. and Daconescu, I.L. (1978), Chma santar a medulu, Edtura Medcal, Bucuret, *** SR ISO 997 (001) Caltatea ape. Determnarea connutulu de clorur. Ttrare cu azotat de argnt utlzând cromatul ca ndcator (Metoda Mohr), ASRO, Bucuret 5. Iwask, I., Utsum, S. and Ozawa, T. (195) Bull. Chem. Soc. Japan 5, 6 6. Iwask, I., Utsum, S., Mageno, K. and Ozawa, T. (1956) Bull. Chem. Soc. Japan 9, Zoll, D.M., Fscher, D. and Garner, M.Q. (1956) Anal. Chem. 8, Barney, J.E. and Bertolacn, R.J. (1957) Anal. Chem. 9, Bertolacn, R.J. and Barney, J.E. (1958) Anal. Chem. 30, Rechel, W. and Acs, L. (1969) Anal. Chem. 41, Funuma, H., Kasama, K., Takeuch, K. and Hrono, S. (1970) Japan Analyst 19, Prokopov, T.S. (1970) Anal. Chem. 4, Bladel, W.J., Lews, W.B. and Thomas, J.W. (195) Anal. Chem. 4, Malmstad, H.W. and Wnefronder, J.D. (1959) Anal. Chm. Acta 0, Bshop, E. (1956) Mkrochm. Acta, Bshop, E. (1958) Analyst 83, Bshop, E. and Dhaneshar, R.G. (196) Analyst 87, Florence, T.M. (1971) J. Electroanal. Chem. 31, Tnase, I.G. (000) Tehnc metode electrometrce de analz, Ars Docend, Bucuret, Tnase, I.G. (007) Analz nstrumental. Partea I. Tehnc metode electrometrce de analz, Edtura Unverst dn Bucuret, Bucuret, *** ICH QA 1994 Valdaton of Analytcal Methods (Defntons and Termnology). *** ICH QB 1996 Analytcal Valdaton Methodology 3. *** SR ISO (1999) Caltatea ape. Etalonarea evaluarea metodelor de analz estmarea caracterstclor de performan. Partea 1. Evaluarea statstc a funce lnare de etalonare, IRS, Bucuret 4. Chan C.C. (004) Potency Method valdaton n Analytcal Method valdaton and nstrument Performance Verfcaton, Chan C.C., Lam H., Lee Y.C. and Zhang X.M. (Eds.), Wley Interscence, Hoboken, New Jersey, p. 1-10