Continuous-flow method for the determination of total inorganic carbonate in water

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  • Analytica Chimica Acta, 284 (1993) 167-171 Elsevier Science Publishers B.V., Amsterdam

    167

    Continuous-flow method for the determination of total inorganic carbonate in water

    Toyoaki Aoki

    Laboratory of Environmental Chemistry, College of Engineering, University of Osaka Prefecture, Gakuen-cho, Sakai 593 (Japan)

    Yoshiko Fujimaru, Yuko Oka and Kimiko Fujie

    Laboratory of Environmental Science, Department of Natural Science, Osaka Womens University, Daisen-cho, Sakai 590 (Japan)

    (Received 18th February 1993; revised manuscript received 12th July 1993)

    Abstract

    A double-tube separation system with an inner tube of microporous PTFE and an outer tube of PTFE is proposed for the continuous determination of total inorganic carbonate (TIC) in natural waters. Carbon dioxide produced by mixing a sample with 0.5 M sulphuric acid in the outer tube permeates through the microporous PTFE wall and dissolved in 5 mM sodium hydroxide solution in the inner tube. The inner stream passes two electrical conductivity (EC) detectors, one before and the other after the double-tube separator, and TIC in the sample is measured as the difference in the signals obtained from the two detectors. The relative electrical conductivity was proportional to the concentration of TIC in the range 5 x lo--5 X lo- M. The limit of detection (signal-to-noise ratio = 3) was 1.0 X 10e5 M. The time required for a 98% response was 2 min. Sulphide interfered but was completely decomposed by using chromate. The method was applied to the determination of TIC in lake waters.

    Keywords: Flow system; Carbonate; Membrane separation; Waters

    Inorganic carbonates play a major role in regu- lating the chemical composition of natural waters. Therefore, the distribution of carbonates needs to be accurately quantified in order to under- stand their effects on natural waters [l-3].

    There are many methods for the determina- tion of total inorganic carbonate (TIC) in water. However, they are batch methods and time con- suming, and are not suitable for in situ measure- ments [4,5].

    Correspondence to: T. Aoki, Laboratory of Environmental Chemistry, College of Engineering, University of Osaka Pre- fecture, Gakuen-cho, Sakai 593 (Japan).

    Carlson [6] reported a continuous-flow method for the determination of dissolved carbon dioxide by a membrane separation. The method was based on the transfer of carbon dioxide by diffusion through silicone-rubber hollow fibres into a flow- ing stream of deionized water, followed by elec- trical conductivity (EC) detection.

    We previously reported continuous-flow meth- ods for the determination of free chlorine [7], ammonia [8] and total trihalomethanes [9] in wa- ter by membrane separation with a microporous PTFE membrane. In this paper, the application of this technique to the determination of TIC with EC detection is presented. The sensitivity of this method is about 100 times greater than that reported by Carlson [6].

    0003-2670/93/$06.00 0 1993 - Elsevier Science Publishers B.V. All rights reserved

  • 168 T. Aoki et aL /AnaL Chim. Acta 284 (1993) 167-171

    EXPERIMENTAL

    Reagents All reagents were of analytical-reagent grade.

    Distilled, deionized water was used in the prepa- ration of all solutions. A stock standard solution of TIC was prepared from sodium carbonate, which was dried at 410C. Working standard solu- tions were prepared by serial dilution of the stock standard solution just before use.

    Apparatus and procedure A schematic diagram of the apparatus used in

    the determination of TIC is shown in Fig. 1. A sample (9 was mixed with sulphuric acid (H) and then allowed to flow into the outer tube (T2) of the membrane separation unit (Iv%). The construc- tion of the separation unit was as described previ- ously [7]. Molecular carbon dioxide, liberated by mixing the sample with sulphuric acid, permeates through a microporous membrane (Tl) to dis- solve in a stream of sodium hydroxide solution (R) whose electrical conductivity was measured with two EC detectors (Dl, D2), one before and the other after the membrane separator. TIC in a sample was determined from the difference in the two EC values. In this system, conductivity decreased with increase in TIC owing to neutral- ization of sodium hydroxide by absorption of car- bon dioxide in the inner tube.

    Unless mentioned otherwise, the flow-rates of the sample, hydrogensulphate solution and sodium hydroxide solution was adjusted to 5.1, 1.5 and 1.5 ml min-, respectively, with a peri- staltic pump (P; Master Flex type). The system,

    C

    R W

    S

    .,.._._................_.._...,............. Fig. 1. Schematic diagram of continuous-flow system for de- termination of TIC in water. S = sample; H = 0.5 M sulphuric acid; R = 5 mM sodium hydroxide; M = membrane separation unit; Tl = microporous PTFE tube; T2 = PTFE, C = thermostat; D = electrical conductivity detector; P = peristaltic pump; W = waste.

    except for the pump (P), was thermostated (0 at 5C. The length of the separation unit was 50 cm. Tl was made of microporous PTFF (1.8 mm o.d., 1.0 mm i.d., maximum pore size 3.5 km) (Japan GoreTex) and T2 was made of PTFE (4.0 mm o.d., 3.0 mm id.). The electrodes of the conduc- tivity cells were made of stainless steel and the gap between the electrodes of each cell was 1.0 mm.

    RESULTS AND DISCUSSION

    Effect of pH on permeation Inorganic carbonate exists in natural waters as

    H&O, (CO,), HCO, and CO:-. The pH of natural water is usually in the range 6-8. In this range, carbonate exists primarily in the anionic form, HCO;, which cannot permeate through the microporous PTFE membrane. Therefore, the conversion of HCO; into H&O, (CO,) was studied by measuring the change in the conduc- tivity at various pH values (adjusted with 0.1 M phosphate buffer solution) of sample solutions fed into the system as shown in Fig. 1.

    The relative conductivity increased with de- crease in the pH of the sample solutions and became constant below pH 3, as shown in Fig. 2. The distribution of HCO; and H&O, is also shown in Fig. 2 as a function of pH. The equilib- rium constant used for the calculation was taken from SillCn and Martell [lo]. The theoretical dis- tribution agrees very closely with those obtained experimentally. In subsequent experiments, sam- ple solutions were acidified to pH < 3 by addition of 0.5 M H,SO,. When sea water was used as the sample, the pH of a mixture with 0.5 M H,SO, was 1.05.

    Effect of sodium hydmaide The sensitivity of the method depends to a

    great extent on the concentration of sodium hy- droxide flowing in the inner tube. Figure 3 shows the relationships between concentration of TIC and concentration of sodium hydroxide. When the concentration of sodium hydroxide decreases, the relative conductivity starts to level off at lower concentrations of TIC. This may be due to

  • T. Aoki et at! /Anal. Chin Acta 284 (1993) 167-171 169

    al-

    0 2 a 6 8 10 PH

    Fig. 2. Relative conductivity and calculated distribution (dotted line) of inorganic carbonate species in aqueous solution as a function of PH. 0 = 0.3 mM TIC n = 0.6 mM TIC.

    the consumption of sodium hydroxide by neutral- ization with a large amount of carbon dioxide for higher concentrations of TIC. On the other hand, the precision became worse with increase in con- centration of sodium hydroxide. The relative standard deviations (n = 5) at 3 mM TIC were 1.0% at 2.5 mM, 1.5% at 5.0 mM, 5.6% at 7.5 mM and 5.4% at 10.0 mM sodium hydroxide. As a compromise, the concentration of sodium hy- droxide was selected as 5.0 mM. Under these

    10

    0 0 I kc, 3 4 5 6

    m&I Fig. 3. Effect of concentration of NaOH in inner tube on calibration graphs for TIC. 0 = 2.5 mM NaOH, + = 5.0 mM NaOH; W = 7.5 mM NaOH, 0 = 10 mM NaOH.

    conditions, TIC in natural waters can usually be determined by this method.

    Effect of temperature The relative conductivity increased with in-

    crease in temperature of the thermostat (C). In the temperature range 5-25C at 0.5 mM TIC, the change in conductivity was 1.4 $I cm- C-l and corresponded to 1.33% of the signal re- sponse. This is probably due to the decrease in the solubility and increase in the permeation rate of carbon dioxide through the membrane separa- tor with increase in temperature. Therefore, it is necessary to keep the temperature of the system constant. At high temperatures, however, many air bubbles were produced which entered the EC detector causing a fluctuation of the baseline. Consequently, the system was held at 5C which was low compared with the usual water tempera: ture.

    At 5C, the transfer rate of carbon dioxide produced from TIC in the sample solution to the sodium hydroxide solution (5 mM) was 47 f 3% in the TIC concentration range 0.1-2.0 mM.

    Analytical performance The sensitivity of the method depends to a

    great extent on the dimensions of the separation unit (M). If the length of the microporous PTFE tube was shorter, response time was shorter but sensitivity was lower. For a tube length of 50 cm, it took 2.5 min after the sample solution began to flow into the system to reach a constant relative conductivity. The time required to reach 98% of the constant signal was 2.0 min. The relative conductivity was linearly proportional to TIC concentration from 5 x 10V5 to 5 x 10e3 M. The relative standard deviations (n = 5) were 3.4% at 0.1 mM TIC and 1.8% at 0.6 mM TIC. The detection limit (signal-to-noise ratio = 3) was 0.01 mM.

    Typical daily changes in the calibration graph in the TIC concentration range 0.2-1.0 mM was as follows:

    (Jan. 21, 1993) y = 7.56x - 0.06 (R = 0.98)

    (Jan. 23, 1993) y = 7.57x - 0.03 (R = 0.99)

    (Jan. 25, 1993) y = 7.07x - 0.02 (R = 0.98)

  • 170

    TABLE 1

    Results obtained with sodium hydroxide solution and distilled water as the flowing stream in the inner tube

    TIC (mMI Relative conductivity

    NaOH solution a Distilled water b

    0.3 142 1.5 0.6 290 2.8 0.9 380 3.5 1.2 490 4.2 1.5 580 4.9

    a In this system, TIC was determined in samples under the conditions of 5 mM NaOH as the flowing stream, 0.5 M H,SO, as acid and temperature 5C. b The conditions were the same as for the NaOH system except for the use of distilled water instead of NaOH solution.

    where y, x and R are the relative signal re- sponse, concentration of TIC and correlation co- efficient, respectively. The linearity of the calibra- tion graphs was satisfactory.

    As mentioned, Carlson [6] determined TIC using a membrane separation system that was based on the transfer of carbon dioxide by diffu- sion through silicone-rubber hollow fibres into a flowing stream of deionized water. With the pres- ent system, TIC was determined in samples by using distilled water instead of sodium hydroxide solution in the inner tube (Tl). The results are compared with those obtained by using sodium hydroxide solution in Table 1. The sensitivity of the present method was about 100 times larger than that using distilled water for the flowing stream in. the inner tube.

    Intetference studies Table 2 lists levels of interference with 0.3 mM

    TIC for various species that occur in natural waters. The concentrations of each species in this study were much higher than those in natural waters. These species did not interfere with the determination of TIC in this method, as none of them could permeate through the microporous PTFE membrane.

    Under anaerobic conditions in natural waters, sulphide is produced by sulphate reduction, which occurs not only as an assimilation process but also as a respiratory process [ill. Sulphide is

    T. Aoki et al. /Anal. Chim. Acta 284 (1993) 167-l 71

    TABLE 2

    Interference of foreign species with the determination of TIC=

    Species

    Sodium chloride Sodium nitrate Sodium sulphate Ammonium chloride Glycine Sucrose Citric acid Acetic acid

    Concentration of species (mMI

    1 2 3

    97 97 98 102 98 97 97 97 102 96 98 97 97 101 100

    105 99 101 104 100 100 101 98 99

    a Concentration of TIC = 0.3 mM. Results are recovery (%I compared with TIC alone.

    converted into volatile H,S in acidic media and can pass as such through a microporous mem- brane.

    Figure 4 shows calibration graphs for both sulphide and TIC under the same analytical con- ditions. The calibration equations are

    y = 3.25x, + 0.022 (R = 0.99)

    y = 1.88x, + 0.024 (R = 0.99)

    where y, x,, xb and R are the relative signal response, concentration of TIC, concentration of sulphide and correlation coefficient, respectively. Hence the sensitivity for the determination of TIC was 1.73 times larger than that for sulphide.

    u 2- w

    0.0a20.40.6~l.o

    conas.ofTIC or sulfide, mM Fig. 4. Calibration graphs for f 0 I TIC and (+I sulphide. Inner solution in separator and acidic reagent were 5.0 mM NaOH and 0.5 M sulphuric acid, respectively.

  • T. Aoki et al. /Anal Chim. Acta 284 (1993) 167-171 171

    In the determination of TIC in samples con- taining sulphide, it is necessary to decompose the latter. Various oxidizing reagents were tested such as Cr(VI), Mn(VI1) and H,O,, which are non- volatile. Cr(VI) was the best with respect to de- composition rate. Figure 5 shows the decomposi- tion rate of sulphide using Cr(VI) which was dissolved in 0.5 M H,SO, (H). In this case, vari- ous lengths of reaction coils (PTFE, 3.0 mm o.d. and 2.0 mm i.d.1 were inserted before the mem- brane separation unit after mixing the sample with 0.5 M H,SO, containing Cr(VI). When us- ing 0.1 M Cr(VI), 0.6 mM sulphide was com- pletely decomposed in less than 5 min.

    Application Continuous determinations of TIC were per-

    formed at Akanoi, Lake Biwa using the proposed method. At the same time, lake water was taken from the same sampling site and analysed in duplicate within 6 h with a non-dispersive in- frared (NDIR) spectrometer (Shimazu, Model TCSOO) by catalytic combustion. The results are given in Table 3. For all samples, the concentra- tions of TIC obtained by the proposed method were in agreement with those given by the NDIR method, within experimental error.

    Fig. 5. Decomposition of sulphide using chromate. Experi- mental conditions are given in the text. 0 = 0.01 M Cr(VJ); + = 0.05 M @VI); n = 0.10 M CdVI).

    TABLE 3

    Comparison of the proposed and NDIR methods for determi- nation of TIC in lake water

    Date Concentration of TIC (mM)

    Proposed method NDIR method

    5th Aug. 1991 0.58,0.56 0.56,0.55 19th Aug. 1991 0.54,0.55 0.55,0.56 20th Aug. 1991 0.63,0.62 0.65,0.64

    a Samples were taken at Akanoi, Lake Biwa.

    Studies on the changes in TIC concentration in lake water and sea water using this method are in progress.

    The authors thank the Rikoh Kagaku Labs. for the construction of the TIC analyser. We also thank K. Kawamoto, K. Ohta, S. Tatsumi in Valves R&D, Kubota, for financial support. Ac- knowledgement is made to Dr. M. Nakanishi (Centre for Ecological Research, Kyoto Univer- sity) and Dr. M. Kumagai (Lake Biwa Research Institute) for helping to obtain samples at Akanoi, Lake Biwa.

    REFERENCES

    1 J.J. Bisogni, Jr. and S.L. Arroyo, Water Res., 25 (1991) 185.

    2 A. Lerman and W. Stumm, Water Res., 23 (1989) 139. 3 A.L. Hercxeg and R.H. Hesslein, Geochim. Cosmochim.

    Acta, 98 (1984) 837. 4 American Public Health Authority, American Water

    Works Association and Water Pollution Control Federa- tion, Standard Methods for the Examination of Water and Wastewater, APHA, Washington, DC, 17th edn., 1989, 4-13.

    5 J.F. Dye, J. Am. Water Works Assoc., 50 (1958) 812. 6 R.M. Carbon, Anal. Chem., 50 (1978) 1528. 7 T. Aoki and M. Munemori, Anal. Chem., 55 (1983) 1620. 8 T. Aoki, S. Uemura, and M. Munemori, Environ. Sci.

    Technol., 20 (1986) 515. 9 T. Aoki and K. Kawakami, Water Res., 23 (1989) 739.

    10 L.G. Sill&t and A.E. Martell (Eds.), Stability Constants of Metal-Ion Complexes, Chemical Society, London, 1964.

    11 H.L. Ehrlich, Geomicrobiology, Marcel Dekker, Basle, 1990, pp. 449-513.

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