Analyst, November 1996, Vol. 121 (1617-1619) 1617

Catalytic Determination of Dissolved Inorganic Carbon in Natural Waters by Flow Injection Spectrop hotomet ry

Nelson Maniasso, Sandra Sato, Maria F. Gine and Antonio 0. Jacintho* Centro de Energia Nuclear na Agricultura, Universidade de Sa8 Paulo, P.O. Box 96, 13400-970, Piracicaba SP, Brazil

A flow injection procedure for the spectrophotometric determination of inorganic dissolved carbon in natural waters is described. The method is based on the ability of the hydrogencarbonate anion to catalyse the slow reaction of EDTA with Cr"' ions of an aged aqueous solution. The proposed system handles up to 36 samples per hour with hydrogencarbonate contents ranging from 10 to 300 mg 1-1, with RSDs better than 1%. The results for waters agree well with those obtained by titration.

Keywords: Catalytic. analysis; dissolved inorganic carbon; flow? injection; water analysis; spectrophotometry

Introduction The content of dissolved inorganic carbon (DIC) in surface waters has been used as an indication of air pollution, water quality and degree of dissolution of chemical species such as silicates and carbonates. 1 Therefore, DIC determination is of considerable interest in environmental, hydrological and geo- logical studies. The official procedure for evaluating DIC uses titration to determine the total alkalinity.2 Potentiometric procedures employing a C02-selective electrode as the detector, based on the evolution of carbon dioxide from the sample bulk, has been reported.3 Nevertheless, other volatile inorganic and organic species are also detected. In order to overcome these interferences, a laborious proposal using consecutive washings of the electrode membrane and proper control of pH was described earlier.3 These analytical procedures are time con- suming, so when several samples need to be analysed a faster procedure is required. For this purpose, spectrophotometric or conductometric flow injection methods based on the on-line generation of carbon dioxide, followed by separation through a gas diffusion device inserted into the analytical path, have been proposed.s.6

The reaction of Cr"' in an aged aqueous solution with EDTA is characterized by slow kinetics and is catalysed by inorganic carbon.7 The spectrophotometric determination of DIC based on the catalytic effect was proposed earlier.8.9 In this paper, a flow injection procedure to determine DIC in natural waters by a catalytic spectrophotometric reaction is presented.

Experimental

Apparatus The equipment consisted of a peristaltic pump (Ismatec MP- 13 GJ-4) provided with Tygon pumping tubes, a water-bath with temperature control (Fanen 147), a spectrophotometer (Femto 432) furnished with a flow cell (190 pl inner volume, 14 mm optical path) and a strip-chart recorder (Radiometer REC 61).

* To whom correspondence should be addressed.

The flow network consisted of a manual injector with one commutation section machined in Perspex,'o Y-shaped joint points and a reaction coil made of PTFE tubing (200 cm X 0.3 mm id, wall thickness < 0.2 mm). Others flow lines were made of polyethylene tubing (0.8 mm id).

Reagents

All chemicals were of analytical-reagent grade and freshly distilled, de-ionized water was used throughout.

A stock standard solution of 1000 mg 1-1 HC03- was prepared by dissolving 1.377 g of NaHC03 in water and diluting to 1000 ml. This solution was stored at about 5" C.

Working standard solutions containing 10.0-300.0 mg 1 of HC03- were prepared by diluting the stock standard solution with water. These solutions were prepared daily before use.

Chromium(m) stock standard solution (6000 mg 1- I ) was prepared by dissolving 6.0 g of finely powdered metallic Cr in an Erlenmeyer flask by adding slowly 42 ml of concentrated hydrochloric acid and warming up to complete the dissolution. After cooling, the volume was diluted to 1000 ml with water. Working standard solutions were prepared by dilution of the stock standard solution. All these solutions were left to rest for 10 d before use.

Stock standard solutions of 1000 mg 1-I N03-, C1-, S04*-, FP, P043- and SiO32- were prepared from the sodium salts and stock standard solutions of 1000 mg 1- K+, Ca2+, Mg2+, Mn2+, Al3+, Cu2+ and Zn*+ from the chloride salts by dissolution in water.

Buffer solutions (pH, 4.0, 4.50, 4.75, 5.0 and 5.50) of 0.10, 0.25 and 0.50 mol 1-1 sodium acetate were prepared, adjusting the pH with acetic acid.

A 0.3 mol 1-1 EDTA solution was prepared from its sodium salt by dissolution in water.

Water samples from different locations (river, well and ground water) were collected by completely filling the collec- tion bottles to avoid air contact and were stored in a refrigerator at 5 "C. The same care was taken with the standard solutions.

Flow Injection System

The flow diagram designed to study the experimental variables is depicted in Fig. 1. The injection device is shown in the loading position where the sample is being aspirated by the peristaltic pump to fill the loop (L), attached on the sliding bar (central part) of the injector. By displacing the sliding bar to the other resting position (shaded area), the sampling loop L is placed in the path of the carrier stream (C). Under this condition, the sample inside loop L is transported by the carrier stream to the detector. The buffer solution at Rl, the Cr"' solution at R2 and EDTA reagent at R3 are added to the sample zone at the confluence points x, y and z, respectively. While the zone sample is transported through the reaction coil B3, which is

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1618 Analyst, November 1996, Vol. 121

inside a water-bath at 45 OC, mixing between solutions occurs, allowing the development of the chemical reaction.

Results and Discussion The main response employed in this work to determine hydrogencarbonate in natural waters was based on the differ- ence of the analytical signals generated before and after the catalytic action caused by the DTC on the reaction between Cr"' and EDTA. The kinetics of formation of the corresponding complex were affected by the age of the Cr"I solution, the temperature and pH. The difference between the signals of the catalysed and not catalysed reactions was higher with aged Cr"' solutions, thus enhancing the method sensitivity. In preliminary tests, attempts to produce Cr"' by on-line CrvT reduction failed because it was observed that reaction with EDTA occurred instantaneously even in absence of hydrogencarbonate. The effect of the age of the Cr"' solution was studied by using 3000 mg 1-1 Cr"' solutions resting from 12 h to 15 d. As shown in Fig. 2, the signals related to both types of reaction (catalysed and uncatalysed) were stable when the Cr"' solution remained at rest for 3 10 d, and similar behaviour occurred when 100, 200 and 300 mg 1-1 HC03- working solutions were used. Presumably, 10 d is the shortest time required to complete the aquo complex formation. Considering these results, CrlI1 solutions aged for 3 10 d were used in subsequent experiments. Chromium solutions that had rested for 4 months were still useful for experimentation.

-W

Te R, R3 W

Fig. 1 Flow diagram of the system used for measuring dissolved inorganic carbon. S = aspirated sample (2.0 ml min-I); L = 150 cm (750 pl) sampling loop; C = 30 mg I-' Si03-* sample carrier stream (1.6 ml min-I); R I = 0.5 mol 1-l H.Ac-Na.Ac buffer solution (0.4 ml min-I); R2 = 3000 mg I-' Cr"' reagent (0.4 ml min-I); R3 = 0.3 mol 1-1 EDTA reagent (0.4 ml min-I); B1 and B2 = 10 cm and B3 - 200 cm coils; I = optional stream to test interferences (2.0 ml min-I). The spec- trophotometer is set at 540 nm.

.6 I o 100 mg 1-1 o 200 mg 1-1 I 300 mg 1-l Base line

0 0 1 2 3 4 5 6 7 8 9 1 0 1 1 1 2 1 3 1 4 1 5

Time/d

Fig. 2 different hydrogencarbonate concentrations.

Evaluation of the age of the Crlll solution on the catalytic effect for

~~

The best pH for development of the chemical reaction was defined after testing the various acetate buffer solutions by adding them to the sample at confluence point x. The results caused by variations in sample acidity are given in Table 1. Buffer solution of pH 5 was selected to attain better sensitiv- ity.

The influence of temperature on the catalytic performance of DIC on Cr"'-EDTA complex formation was evaluated by submerging the reactor coil B3 in a water-bath with controlled heating. Increasing the water-bath temperature from 30 to 55 "C in steps of 5 "C produced increased signals up to 50 OC, so this temperature was selected. The temperature effects were eval- uated by using standard solutions of 100, 200 and 300 mg 1-1

HC03- and a 15 d aged Cr"' solution. In addition to the above-mentioned parameters, when a flow

injection system is employed to handle the sample and reagent solutions, other variables such as the carrier stream flow rate, the sample residence time, the analytical path length and the sample volume injected, have considerable effects on the analytical signal. These parameters were optimized, consider- ing the compromise between sample dispersion and residence time in order to enhance the sensitivity. The effect of the sample volume was studied by injecting 250, 500, 750 and 1000 pl aliquots into the sampling loop (L), inserted on the injector (Fig. l), and the maximum signal was attained with 750 pl.

The addition of sodium chloride at concentrations up to 4% (m/v) to the buffer solutions at Rlverified that there was no effect of ionic strength on the reaction kinetics.

The order of addition of Crnl, EDTA and buffer solutions to the sample zone was apparently not significant, as verified by all permutations, but the previous mixture of these solutions is not recommended to prevent the slow reaction and the possibility of a catalytic effect from C02 in the air.

Coils of 10 cm for mixing the buffer and Cr"' solutions with the sample bulk presented minimum dispersion and good precision. Reaction coil lengths of 100, 200, 300 and 400 cm were tested. The best signal difference between the catalysed and uncatalysed reactions was attained when using the 200 cm coil.

The influence of the anions NO3-, C1-, S042-, F-, H2PO4- and Si032- and the cations K+, Ca2+, Mg2+, Mn2+, AP+, Cu2+ and Zn2+ on the method was studied by adding them one at a time through the confluence point xg, as indicated by the dashed line (I) in Fig. 1. This interference test indicated a significant effect of Si032- on the analyte signals, an increase of approximately 28% (Table 2). To overcome this interference, a solution of 30 mg 1-1 Si032- was used instead of the carrier solution. This silicate concentration is higher than those usually found in natural waters, and thus the interference caused by different concentrations of Si032- in the sample were mini- mized as the blank and analyte signals increased proportion- ally.

Different concentrations of the Crrrr solution (2500, 3000, 4000 and 6000 mg 1-1 Cr"' and the EDTA solution (0.1,0.2 and 0.3 mol I-' EDTA) were tested using univariate design experiments. Optimum results were obtained when 0.3 moll-'

Table 1 Influence of pH on the Cr"I-EDTA reaction. The values are absorbances corresponding to concentrations of HC03- from 0 (baseline) to 300 mg 1-1

PH Baseline 100.0 200.0 300.0 4.0 0.166 0.212 0.262 0.345 4.5 0.209 0.342 0.457 0.566 5.0 0.356 0.732 1.005 1.241 5.5 0.381 0.791 1.016 1.192

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Analyst, November 1996, Vol. 121 1619

EDTA solution was used, considering signal repeatibility and the linearity of the calibration graph. When the various Crrrr concentrations were used, the absorbance signals related to the uncatalysed reaction increased from 0.156 to 0.383, as shown in Table 3. Constructing the calibration graphs with data from Table 3, the best linearity was found when using 3000 mg I-' Cr"' for hydrogencarbonate concentrations up to 300 mg 1-1,

hence this solution was adopted throughout. Once the main experimental variables had been defined,

freshly collected water samples were analysed. As shown in the

Table 2 Effect of potential interferent ions on the Cr"'-EDTA reaction when a 45 mg HC03- I-' standard solution is employed with absorbance 0.377

Tested Interferent Salt concentra- ion employed tions/mg 1- I Absorbance

NO3- Cl- SO4*- F- HP04*- Si03*- K+ Ca*+ Mg2+ Mn*+

cu*+ Zn*+

~ 1 3 +

NaN03 NaCl Na2S04 NaF Na2HP04 Na2Si03 K CI CaC12 MgC12 MnCI2 AICl3 CUCl* ZnCI2

200.0 200.0 300.0

1 .o I .o

25.0 50.0

200.0 2.5 2.5 2.5 2.5 2.5

0.376 0.376 0.378 0.378 0.377 0.480 0.377 0.377 0.377 0.376 0.376 0.376 0.376

Table 3 Effect of Cr"' solution concentration on the HC03- catalytic effect. The values are absorbances for HCO3- concentrations from 0 (baseline) to 300 mg 1-'

HC03/mg 1- C P / mg 1-1 Baseline 100.0 200.0 300.0 2500 0.156 0.204 0.262 0.305 3000 0.185 0.248 0.308 0.368 4000 0.232 0.297 0.379 0.441 6000 0.383 0.463 0.555 0.631

Table 4 DIC contents in natural waters determined by the proposed (FI) and the titration [American Public Health Association (APHA)] procedures

DIC/mg 1-'

Water sample Lake River River

Well River

Well Lake

Tap

Tap

Titration (APHA) 28.3 53.0 56.3 11.7 7.0

54.6 28.8 6.8

39.0

FI catalytic 29.2 53.6 55.3 11.4 6.6

54.6 29.9 7.9

38.9

flow diagram, the Cr"' and EDTA solutions flowed continuously through the confluences y and z. As a consequence, the uncatalysed reaction was continuously occurring and the related signal was recorded as the baseline. Routine analysis was performed using the flow system in Fig. 1, attaining a throughput of 36 determinations per hour.

The feasibility of the proposed procedure was ascertained by analysing a set of natural waters and the results are given in Table 4. The accuracy of the results was then assessed by analysing samples by the titrimetric method described earlier.2 By applying the paired t-test to the results obtained by titrimetry, no significant difference was observed at the 95 9% confidence level. In addition, a precision characterized by RSD of less than 1% (n = 9) was calculated for samples with concentrations of about 50 mg 1-1. The precision and sample throughput are considerably better than those of the manual titration procedure, in which both parameters are strongly affected by the difficulty of attaining the titration end-point.

Conclusions The method based on the catalytic effect of hydrogencarbonate ions on the reaction between an aged Cr"' solution and EDTA proved to be a good alternative for the determination of hydrogencarbonate in water samples up to 300 mg 1-I. The main feature of the flow injection automation of this procedure to prevent air contamination during the analysis and the previous uncatalytic reaction between Cr"' and EDTA is due to the fast and closed reaction media. The proposed flow injection procedure provides distinct advantages of increased sample throughput and precision over the standard method of DIC determination.

Partial support from the Conselho Nacional de Desenvol- vimento Cientifico e Tecnol6gico and Fundaqiio de Amparo a Pesquisa is greatly appreciated. The authors express their gratitude to B, F. Reis for his assistance in the development of this project and H. Boylan for language corrections.

References 1

2

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4

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6

7 8 9

10

Manahan, S., Environzental Chemistry, Lewis, Chelsea, MI, 1991,Sth edn. Standard Methods for the Examination oj Water and Wastewater, American Public Health Association, Washington, DC, 1976, 14th edn., pp. 293-302. Takano, S., Kondoh, Y., and Ohtsuka, H., Anal. Chem., 1987, 57, 1523. Hara, H., Okabe, Y., and Kitagawa, T., Anal. Chem., 1992, 64, 2393. Motomizu, S., T8ei, K., Kuwaki, T., and Oshima, M., Anal. Chem., 1987, 59, 2930. Jardim, W. F., Pasquini, C., Guimaraes, J. R., and Faria, L. C., Water. Res., 1990, 24, 351. Beck, M. T., J . Inorg. Nucl. Chem., 1961, 15, 250. Meditach, J., and Barros, E. C., Rev. Quim. Ind., 1978, 47, 7. Perez-Bendito, D., and Silva, M., Kinetic Method7 in Analytical Chemistry, Ellis Horwood, Chichester, 1988, ch. 2, pp. 76-83. Krug, F. J., Rergamin, Fo, H., and Zagatto, E. A. G., Anal. Chim. Acta, 1986,179, 103.

Paper 6103324E Received Muy 13,1996 Accepted June 25, 1996

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