A selective and sensitive procedure for the fast and indirect determination of chloride by the gas-diffusion FIA method has been optimized and applied for the determination of chloride in water samples which contained different amounts of the analyte. The examined samples were: seawater, tap water and very pure water from the water-steam system of the power plant "Nikola Tesla B" in Obrenovac. Application of an amperometric detector (model LC-17A; BAS, West Lafayette, USA) enables the detection limit to be decreased down to 0.05 mu mol/dm(3) of chloride, which col responds to 35.5 pg, and adjustment of the acceptor flow rate and direction inside the gas-diffusion unit. In this way the optimized FIA system has excellent repeatability. For 5 mu mol/dm(3) it was found to be 1.11% (n = 5). The throughput of this method is 60 samples per hour.Selektivan i osetljiv postupak za brzo, indirektno određivanje hlorida gasno-difuzionom metodom otpimizovan je i primenjen za određivanje hlorida u uzorcima voda sa vrlo različitim sadržajem ove vrste: morskoj, vodi za piće i vrlo čistim vodama u sistemu voda-para TE "Nikola Tesna B" u Obrenovcu. Upotreba amperometrijskog detektora tipa LC-17A omogućava snižavanje granice detekcije na 0.05 μmol/dm3 hlorida (što za uzorak zapremine 0.2 cm3 iznosi 35.5 pg) i podešavanje brzine i smera toka u gasno-difuzionoj jedinici. Ovako optimizovan protočni sistem ima odličnu reproduktivnost. Za hloridni standard koncentracije 5 μmol/dm3 relativna standardna devijacija iznosi 1.11 % (n = 5)

Lolić, Aleksandar

Nikolić-Mandić, Snežana D.

Polić, Predrag S.

Journal of the Serbian Chemical Society

English

Faculty of Chemistry Repository - Cherry

J. Serb.Chem.Soc. 66(9)637–646(2001) UDC 543.553:546.132+533.15JSCS – 2893 Original scientific paperOptimization and application of the gas-diffusion flow injectionmethod for the determination of chlorideALEKSANDAR LOLI], SNE@ANA NIKOLI]-MANDI] and PREDRAG POLI]Faculty of Chemistry, University of Belgrade, Studentski trg 16, P. O. Box 158, YU-11001 Belgrade,Yugoslavia(Received 17 July 2000, revised 2 July 2001)A selective and sensitive procedure for the fast and indirect determination ofchloride by the gas-diffusion FIA method has been optimized and applied for the deter-mination of chloride in water samples which contained different amounts of the analyte.The examined samples were: seawater, tap water and very pure water from the wa-ter-steam system of the power plant “Nikola Tesla B” in Obrenovac. Application of anamperometric detector (model LC-17A; BAS, West Lafayette, USA) enables the detec-tion limit to be decreased down to 0.05 mol/dm3 of chloride, which corresponds to 35.5pg, and adjustment of the acceptor flow rate and direction inside the gas-diffusion unit.In this way the optimized FIA system has excellent repeatability. For 5 mol/dm3 it wasfound to be 1.11 % (n = 5). The throughput of this method is 60 samples per hour.Keywords: gas-diffusion flow injection method, amperometric detection, indirect chlo-ride determination.INTRODUCTIONNeutral chlorine does not exist in nature. As a result of its extreme positive redoxpotential and electronegativity, chlorine occurs only as chloride. It can be found in largeamounts in seawater, as well as in sediments bound to cations such as potassium and so-dium. Chlorine can be used as a disinfectant, a water purifier or as a bleaching agent. Itis obvious that the determination of chloride is important. Tracking its concentrationshows the level of water pollution. Therefore, it is not surprising that a number of flowinjection (FI) methods have been developed for this analyte.1–7The quality of water in power plant systems is important, especially for turbinemaintenance. Turbines are very sensitive to the presence of corrosive substances suchas NaOH, NaCl, Na2SO4, organic and inorganic acids, iron and copper oxides, becausethey cause the oxidative burst of metal under pressure. The maximum allowed concen-tration8 of chloride is 10 g/dm3. Deviation from the allowed concentration causes pit-ting corrosion and corrosion strain. Pitting corrosion occurs if:6371. There is increased tension within the material;2. A material sensitive to this kind of corrosion is being used (Cr-Ni steels);3. Impurities that cause this corrosion are present (Cl–, OH–).Increased concentration of chloride in the system water-steam occurs because of:1. Breakthrough of cooling water in the condenser;2. Poorly performed chemical preparation;3. The use of inappropriate cleansing agents for blades during maintenance.Standard methods for tracking the level of chloride in the system are spectro-photometry and ion-chromatography. The spectrophotometric metod is based on the re-action of chloride with mercury(II) thiocyanate.9 The method is used for the concentra-tion range of 5 ppb to 1 ppm. Chloride reacts with mercury(II) thiocyanate formingthiocyanate ion, which forms the red coloured iron(III) thiocyanate complex. Interfer-ences are caused by bromide, iodide, thiosulphate, hydrazine and nitrites. Ion chroma-tography9 is used for the concentration range of 1 to 3 ppb. Strong acids and bases causeinterference. The presence of boric acid can cause deformation of the peaks.EXPERIMENTALReagents and materialsAll the chemicals used were of analytical-reagent grade. The aqueous reagent and standard so-lutions were stored in polyethylene bottles. Deionized water was used throughout. A saturated solu-tion of KMnO4 served as the oxidizing agent. It was prepared by boiling the saturated solution withsubsequent filtering in order to remove MnO2 and any excess of KMnO4 that might be present. Astock solution of 0.1 mol/dm3 NaCl was prepared from BDH (Poole, Dorset, UK) concentrated volu-metric standards, which are certified to have an accuracy within the factor limits of 0.999 and 1.001.Standard Cl- solutions, which, in most of the experiments were made in 3 mol/dm3 H2SO4, were pre-pared by diluting aliquots of the stock solution to the appropriate volumes. Solutions of I-, Br- andBrO3- were prepared by dissolving appropriate amounts of KI (Jugolek, Beograd, Yugoslavia), KBrand KBrO3 (Zorka, [abac, Yugoslavia).Instrumentation and apparatusThe FIAmanifold used for the determination of chloride is illustrated in Fig. 1. Two peristalticpumps were used. One was a Model HPB 5400 (Iskra, Kranj, Slovenia), and the other one was aMoled MS REGLO (Ismatec, Zürich, Switzerland). The injection valve was a Model 5020 (Rheo-dyne, Cotati, CA, USA) equipped with a 200 l sample loop. The gas-diffusion unit was obtainedfrom the Shenyang Film-Projector Factory (Shenyang, China). The PTF membrane that separates thedonor and acceptor stream does not need to be changed for experiments, which last for less than 10hours. All concentrations were made with 0.5 mm i.d. tubing.The flow-through amperometric cell LC-17A (BAS, West Lafayette, USA), shown in Fig. 2,consists of a platinum working electrode and counter electrodes. The working electrode has two sepa-rate electrodes embedded in a Kel-F block. The entire block easily withstands exposure to the sol-vents and temperatures. The reference electrode is an Ag-AgCl with a 3 M NaCl gel filling solution(all electrode potentials refer to this reference electrode). The auxiliary electrode is a solid stainlesssteel block. A gasket defining the flow channel is sandwiched between the auxiliary and the workingelectrode block. The platinum-working electrode was polished occasionally in a specific manner.First, it was rinsed with water followed by methanol, then it was polished with alumina polish slurry,rinsed with distilled water and acetone and finally air dried. The potential to the flow-through638 LOLI], NIKOLI]-MANDI] and POLI]amperometric cell was applied and currents were measured with a Model MA5450 polarograph(Iskra, Kranj, Slovenia); the resulting FI signals were recorded on a Model 61 Servograph (Radiome-DETERMINATION OF CHLORIDE639Fig. 1. FI manifold used for the indirectdetermination of chloride: A– acceptorsolution (0.01 mol dm-3 H2SO4); R –reagent (saturated KMnO4); C – carrier(3 mol dm-3 H2SO4); P – peristalticpump; I – injection valve; MC – mixingcoil; D – diffusion cell; FC – electro-chemical flow-through cell; PO – pote-ntiostat; RE – recorder and W– waste.Fig. 2. The flow through the ampero-metric cell: a) 1 – working electrodeblock; 2 – auxiliary electrode; 3 – lock-ing collar; 4 – miniature reference elec-trode; 5 – gasket; 6 – quick releasemechanism; b) working platinum elec-trode geometry; c) top view.ter, Copenhagen, Denmark) strip-chart recorder. Temperature regulation was achieved with a con-stant-temperature bath, type Messgeraete-Werk Lauda, Germany.The chloride solution, which was injected by the injection valve, is carried by the carrierstream (sulphuric acid solution) and reaches the mixing coil where it is mixed with the reagent (satu-rated potassium permanganate solution) and oxidized to chlorine. The chlorine diffuses through themicro porous membrane in the gas-diffusion unit from the donor into the acceptor stream. The otherproducts of the reaction exit the system as waste. Chlorine, taken by the acceptor stream, reaches theamperometric cell where it is reduced to chloride. The recorder registers the cathode current, which isproportional to the concentration of chloride in the sample.RESULTS AND DISCUSSIONThe effects of several parameters on the performance of the flow injection systemwere studied.The effect of the potential applied at the working platinum electrode was investigatedin the range of +0.20 to +0.70 V versus the Ag-AgCl reference electrode. The hydrody-namic voltammogram for a 2.50 mmol/dm3 sodium chloride standard in 3 mol/dm3sulphuric acid is shown in Fig. 3. As can be seen, the optimum potential is +0.30 V.The effect of sulphuric acid concentration on the peak currents was studied by in-jecting the same chloride standard (2.50 mmol/dm3) but with the concentration of theacid varying between 1.0 to 5.0 mol/dm3. From the obtained data, it can be seen that thecurrent increases exponentially with increasing H2SO4 concentration. A simple equa-tion of the type: I = 1.50 exp(0.84c) (where I is the current in A, and c is the concentra-tion of H2SO4 in mol/dm3) fits the data very well. The corresponding correlation coeffi-cient was found to be 0.9954.640 LOLI], NIKOLI]-MANDI] and POLI]Fig. 3. Hydrodynamic voltammogram for a 0.200 cm3 injection of a 2.50 mmol/dm3 sodium chlo-ride standard.A 0.01 mol/dm3 H2SO4 solution was used as an acceptor.10Theeffectof theacceptor flowrateandflowdirectionwas investigated in therangeof1.6 to 3.0 ml/min. The donor flow rate was constant (2.8 ml/min). The data obtained areshown in Fig. 4. As can be seen, the optimal flow rate was 1.8 ml/min for a parallel flow di-rection of both the acceptor and the donor stream. It was previously determined that an ntimesdecreaseof theacceptor flowrate results inann times increaseof theFIAsignal.11DETERMINATION OF CHLORIDE 641Fig. 4. Acceptor flow rate and direction effect (the circles present parallel flow while the squarespresent counter flow).Fig. 5. Effect of the gasket thickness.The effect of the gasket thickness was studied, as this is an important factor thatdetermines the working solution volume. The results of the study of this effect are pre-sented in Fig. 5. As it can be seen the 0.05 mm thick gasket gave the best result and wasused throughout.The effect of temperature was examined by injecting a 2.50 mmol/dm3 NaClstandard in a 3 mol/dm3 H2SO4 solution, while varying the temperature in the interval20 – 60 ºC. The obtained data show that there is an exponential dependence describedby the function I = 2.67 exp(0.06t) (where I is the current in A, and t is the temperaturein ºC). The correlation coefficient was 0.9986. This finding is interesting, as a change intemperature not only affects the oxidation rate, but also the solubility of the formedchlorine, the diffusion process and the reduction of chlorine at the platinum workingelectrode. However, due to the amperometric cell properties, stable signals are obtainedeven at low temperatures.The detection limit determined by a recommended procedure12 at room tempera-ture and a 3 mol/dm3 H2SO4 solution as the carrier was 0.05 mol/dm3, which corre-sponds to 35.5 pg of chloride (the sample loop volume was 200 l). The throughput ofthis method was 60 samples per hour.The determination of chloride by this technique can be interfered with by ionsthat can be oxidized by potassium permanganate to form neutral molecules, if the neu-tral molecules have a relatively high vapour pressure and a low water solubility so thatthey can diffuse through the PTF membrane into the acceptor stream and if the mole-cules that get into the acceptor stream are electroactive at the working potential.These conditions are fulfilled by iodide, bromide and hydrazine (present in thepower plant water samples).To inhibit the 24.44 % increase of FIAsignal caused by the presence of bromide,the procedure decribed in the work of Kubala13 was used. This iodate pre-treatmentmethod is based on the classical argentimetric procedure:IO3– + 6Br– + 6H+  I– + 3Br2 + 3H2OThe formed bromine is expelled from the solution by heating for 15 min on thewater bath. Iodide does not interfere.Applying this pre-treatment method enables the determination of chloride in thepresence of bromide whose concentration is 1g/dm3 with an insignificant increase of theFI signal, compared to the signal of pure chloride standard. The difference is then –3.84 %(1mmol/dm3)and–3.46%(0.5mmol/dm3).The interferenceofhydrazine (1g/dm3)waseliminated by adding 0.5 mol/dm3 bromate and heating on a water bath for 15 min.The reproducibility of the method is shown in Fig. 6. The relative standard devia-tion for a 2.00 mmol/dm3 chloride solution was found to be 0.61 % (n = 5) whereas for a5.00 mol/dm3 solution it was 1.11 % (n = 5).This method was used for the determination of chloride in seawater, tap water,commercial mineral water and water taken from the water-steam system of the powerplant “Nikola Tesla B” in Obrenovac.642 LOLI], NIKOLI]-MANDI] and POLI]Two calibration runs were screened, one for samples containing higher chlorideconcentrations (seawater) and the other one for lower concentrations. The first one wasobtained by injecting in triplicate a total of five chloride standards between 10 and 100mmol/dm3 in 3 mol/dm3 sulphuric acid (the 0.5 mm thick gasket was used). Thesulphuric acid aided the expulsion of carbonates from the samples. The linear regres-sion equation was I = (8.143  	c + (0.709  0.036) (I is the peak current in A, andc is the concentration of Cl– in mmol/dm3), with a correlation coefficient of 0.9926. Thesecond one was made for tap water, mineral water and water samples from the powerplant. This one was obtained by injecting in triplicate a total of six chloride standardsbetween 5 and 100 mol/dm3 in 3 mol/dm3 sulphuric acid (the 0.05 mm thick gasketwas used here). The linear regression equation was I = (0.0028  0.0001)c + (0.014 ±0.007) (I is the peak current in A, and c is the concentration of Cl– in mol/dm3), with acorrelation coefficient of 0.9961.Two procedures were introduced for the pre-treatment of the power plant samplesbecause of the possible presence of hydrazine. Under the given conditions, hydrazine isoxidized by permanganate to form nitrogen, which can diffuse through the PTF mem-brane. Adding 0.5 mol/dm3 KBrO3 and heating the mixture for 15 min on a water bathwas one procedure and the other one was the standard complement method (the stan-dard complements were 5, 10 and 15 mol/dm3 of chloride standard). Both methodsshowed good agreement with a difference of less than 1 %.The results obtained for all examined samples are shown in Table I. The seawatersamples were taken from the Tivat bay, Montenegro. Obtained results show that thesamples @upa and Krasi}i, taken further away from the coast, have a higher concentra-DETERMINATION OF CHLORIDE 643Fig. 6. Response of the amperometric detectorto: (a) five repetitive injections of a 2.00mmol/dm3 chloride standard; and (b) five re-petitive injections of a 5.0 mol/dm3 chloridestandard.tion of chloride than the Kaliman sample. There are several reasons that can explainthis. The first one is that there are several springs of non-salty water located near thecoast and the second is that the outlets of the local sewage pipes are located further awayfrom the coast, and as it is known that sewage contains certain amounts of chloride, theobtained results are acceptable.TABLE I. Results of the chloride determination in the examined water samplesSample concentrationArgentimetric FI LabelMinakva (commercial mineral water) 15.7 0.2 mg/dm3 17.35 mg/dm3Mivela (commercial mineral water) 21.73 0.04 mg/dm3 21.05 0.03 mg/dm3 17 mg/dm3Belgrade tap water 13.32 0.05 g/dm310.370.07 g/dm312.61 0.05 g/dm3 12.240.05 g/dm3Water from the power plant I 1.41 0.06 g/dm3II 0.85 0.03 g/dm3III < 0.355 g/dm3Seawater Kaliman 4620 6 mg/dm3Zupa 99408 mg/dm3Krasi}i 105005 mg/dm3 102308 mg/dm3The samples from the power plant represent condensed purified water before it isbrought onto the turbines. The previously determined conductivities of these samples were0.25 S (I), 0.18 S (II) and 0.08 S (III). It can be concluded that there are some other sub-stances other than chloride that affect the conductivity, most probably silicates.CONCLUSIONSA rapid, indirect gas-diffusion flow injection method with amperometric detec-tion has been developed for the selective determination of chloride. The method isbased on the oxidation of chloride to chlorine by a saturated potassium permanganatesolution. The chlorine diffuses through the micro-porous membrane in a gas-diffusionunit from the donor into the acceptor stream, and is amperometrically quantified on aplatinum working electrode.The effect of the following variables on the performance of the system was stud-ied:1. The potential of the working Pt electrode (+0.30 Vvs.Ag/AgCl was chosen);2. The H2SO4 concentration as the acceptor (0.01 M H2SO4);3. The H2SO4 concentration as the carrier (3 M H2SO4 was chosen);4. The temperature in the 20–60 ºC interval (exponential increase of sensitivity).The major interferent of this method are Br– anions, because they can be oxidized644 LOLI], NIKOLI]-MANDI] and POLI]on-line by permanganate to form molecular species which are reducible at the platinumelectrode at the applied potential. The iodate pre-treatment method was used to expelBr– out of solution. The water samples from the power plant contained small amountsof hydrazine which were expelled by treatment with bromate.The reproducibility of the analytical system was analysed by five repetitive injec-tions of chloride standard. The relative standard deviation for a 2 mM Cl– standard was0.61 % and for a 5 M it was 1.11 %. The detection limit at room temperature was 0.05M Cl– which corresponded to 35.5 pg of Cl–. The sample loop volume was 0.200 ml.The throughput of this method is 60 samples per hour.The method is successfully applied for the determination of chloride (withoutconcentrating or diluting the samples) in seawater, tap water, mineral waters (“Mi-nakva”) and water from the water-steam system of the power plant “Nikola Tesla B” inObrenovac.I Z V O DOPTIMIZACIJA I PRIMENA GASNO-DIFUZIONE PROTO^NE INJEKCIONEMETODE ZA ODRE\IVAWE HLORIDAALEKSANDAR LOLI], SNE@ANA NIKOLI]-MANDI] i PREDRAG POLI]Hemijski fakultet, Univerzitet u Beogradu, Studentski trg 16, p. pr. 158, 11001 BeogradSelektivan i osetqiv postupak za brzo, indirektno odre|ivawe hlorida ga-sno-difuzionom metodom otpimizovan je i primewen za odre|ivawe hlorida u uzo-rcima voda sa vrlo razli~itim sadr`ajem ove vrste: morskoj, vodi za pi}e i vrlo~istim vodama u sistemu voda-para TE "Nikola Tesna B" u Obrenovcu. Upotrebaamperometrijskog detektora tipa LC-17A omogu}ava sni`avawe granice detekcije na0.05 mol/dm3 hlorida ({to za uzorak zapremine 0.2 cm3 iznosi 35.5 pg) i pode{avawebrzine i smera toka u gasno-difuzionoj jedinici. Ovako optimizovan proto~ni sistemima odli~nu reproduktivnost. Za hloridni standard koncentracije 5 mol/dm3 rela-tivna standardna devijacija iznosi 1.11 % (n = 5).(Primqeno 17. jula 2000, revidirano 2. jula 2001)REFERENCES1. H. Satake, S. Ikeda, Nippon Kagaku Kaishi (1990) 3852. F. T. Esmadi, M. Kharoaf, A. S. Attiyat, Analyst (London) 116 (1991) 353. S. Wilke, H. Franzke, H. Mueller, Anal. Chim. Acta 268 (1992) 2854. J. F. Van Staden, C. J. Cornelius, D. Malan, Fresenius’ J. Anal. Chem. 351 (1995) 7585. R. H. Taylor, J. W. Grate, Talanta 42 (1995) 2576. A. M. S. Abdennabi, M. E. Koken, Talanta 46 (1998) 6397. N. M. Holden, J. F. Dowd, A. G. Williams, D. Scholefield, Environ. Monit. Assess. 36 (1995) 2178. Collection of seminar studies, Control and Tracking of the Condition of Metals and the WorkingLife of Thermoblocks of the EPS, Book III, Belgrade, 1995, p. 79 (in Serbian)9. Manual for the Control of the Water Quality in the Water-Steam System in Thermoenergetic Objectsof the EPS, Belgrade, 1994, p. 95/99 (in Serbian)10. S. D. Nikoli}, E. B. Milosavljevi}, J. L. Hendrix, J. H. Nelson, Analyst 116 (1991) 49DETERMINATION OF CHLORIDE 64511. J. Ru`i~ka, E. H. Hansen, Flow Injection Analysis, 2nd ed., Wiley, New York, 1988, pp. 192–20012. Analytical Methods Committee, Analyst 112 (1987) 19913. S. W. Kubala, D. C. Tilotta, M. A. Busch, K. W. Busch, Anal. Chem. 61 (1989) 2785.646 LOLI], NIKOLI]-MANDI] and POLI]

Optimizacija i primena gasno-difuzione protočne injekcione metode za određivanje hlorida

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Optimizacija i primena gasno-difuzione protočne injekcione metode za određivanje hlorida

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