2393
Anal. Chem. WQ2, 64, 2393-2397
Flow Determination of Dissolved Inorganic Carbon Using the Alternate Washing System Equipped with a Potentiometric Gas Electrode Hirokazu Hara,1 Yohzoh Okabe, and Tomoko Kitagawa Department of Chemistry, Faculty of Education, Shiga University, Otsu, Shiga 520, Japan
The alternate washlng system equipped wlth a potentlometric carbon dloxlde sendtlve gas electrode was prepared for the determination of dissolved lnorganlc carbon. The delsgn of thk system Is based on the alternate lntroductlon of a sample solution and a washlng stream Into a flow cell by means of a four-way valve controlledby a microcomputer. Thls enables dmpk, qulck, and selective determlnatlon of dlssolved lnorganlc carbon In dkcrete samples. The llmlt of llnear response was ca. 2 X lo4 M. Determlnatlon at pH 5 Is proposedbecause the Interference from varlour volatlle weak aclds such as nltrlte, suiflde, or hydrogen sulflte can be much reduced wlthout sacrlflclng sensltlvlty. Thk system was applM to natural water analyds, and the results were compared wkh those of alkallnlty.
Recently, we developed a new design of a flow analysis system called the alternate washing method.10 With this system, we demonstrated that the selectivity of an ammonia gas electrode for some volatile amines was improved because the so-called pseudoequilibrium potentials were measured. In this paper, this method was used in the determination of DIC using acarbon dioxide gas electrode. The interference from six inorganic and organic acids was examined and compared with the results from the conventional continuousflow method. Although the pH of a sample solution was found to be the most important factor in reducing the interference, the alternate washing principle was also effective in reducing the interference in some cases. The system was applied to the analysis of natural water samples.
EXPERIMENTAL SECTION INTRODUCTION Dissolved inorganic carbon (DIC) is one of the major anionic components in natural waters in addition to sulfate and chloride. Although its form can be varied according to the sample pH, bicarbonate is believed to be the major component in natural waters having neutral or slightly alkaline pH values. The concentration of DIC is related to the activity of photosynthesis of phytoplanktons or the respiratory activity of microorganism. The equilibrium concentration of DIC in pure water is about 1.5 X M (1M = 1mol dm-3) due to the dissolution of carbon dioxide from the air,l while the concentration of DIC in river water lies in the range of (2-20) X 10-4 M in the Shiga prefecture. Carbon dioxide sensitive gas electrodes have been utilized for the determination of DIC in various samples such as power station water,2 sea water: pond water used for a growth experiment of phytoplanktons,4 and others."' Though this type of gas sensor has a perfect selectivity against ionic species due to the microporous hydrophobic membrane, some volatile inorganic and organic acids are reported to cause interference. The interference has been examined from the viewpoint of a steady-state model* or a dynamic response.9 (1)Kolthoff, I. M.; Sandel, E. B.; Meehan, E. J.; Bruckenstein, S. Quantitative Chemical Analysis, 4th ed.; Macmillan: London, 1969; p 781. (2) Midgley, D. Analyst 1976, 100, 386-399. (3) Nagashima, K.; Washio, Y.;Suzuki, S. Bunseki Kagaku 1984,33, T108-Tl12. (4) Kawai, T.; Miyamoto, K.; Umezawa, Y. Bunseki Kagaku 1990,39, 649-653. (5) Takano, S.; Kondoh, Y.; Ohtauka, H. Anal. Chem. 1985,57,15231526. (6) Collison, M. E.; Aebli, G. V. ; Petty, J.; Meyerhoff, M. E. Anal. Chem. 1989,61, 2365-2372. (7) Nikolelis, D. P.; Krull, U. J. Analyst 1990, 115, 883-888. (8)Lopez, M. E. Anal. Chem. 1984,56, 2360-2366. (9) Mod, W. E.;Mostert, I. A.; Simon, W. Anal. Chem. 1985,57,11221126.
Apparatus. The body of an ammonia gas electrode (Orion 95-12) equipped with a flow-through cap (Orion 95-12-25) and a PTFE membrane (Orion 95-12-04)was used as a carbon dioxide gas sensor. The internal filling solution was a mixture of 0.001 M sodium bicarbonate and 0.019 M potassium chloride and was saturated with silver chloride. For the measurement of a sulfide solution, silver chloride was not added in order to avoid the reaction between a silver ion and dihydrogen sulfide. Potentials at 25 1O C were measured by a digital ion meter (Orion 701A) and were transmitted every 0.6 s into a microcomputer (NEC PC880l MkII) through an 8-bit parallel 1/0interface.10 The general I/O port of the microcomputer was used to send an onoff signal to an automatic four-way valve (Kusano Kagakukikai Model KAV-4L) and two pinch valves (Takasago Electric Inc., PK-0305-NC) to control the switchover. The potential values were also recorded by an analog pen recorder (Rikadenki Model R11). Theresponse time,washingtime,and their sum (measuring time) were also measured by a microcomputer and recorded with a printer together with measured potentials. Three peristaltic pumps were used to deliver the sample and buffer solutions (Atto Model SJl2llH x2) and the wash solution, Le., 0.01 M sodium hydroxide (Atto Model AC2110). In the continuous-flow measurement, the sample and buffer solutions were mixed in a small mixing chamber before transfer into the flow-through cap. Reagents. Guaranteed reagent-gradereagentswere purchased from Nacalai Tesque (Kyoto). Ultrapure water prepared with a Milli-Q Lab0 purificiation system (Millipore) was used for the preparation of dilute standard bicarbonate solutions (0.5,1,and 2 X 104 M). To control the pH of a sample solution, 0.14 M sulfuric acid was used for pH 2 and a mixture of citric acid and disodium hydrogen phosphate (McIlvaine's buffer") was used to keep the pH at 3, 4, 5, and 6. The actual pH after mixing with sodium bicarbonate standard solutions was found to be 1.96,3.07,4.06, 5.07, and 6.05 on the average. The concentrations of citric acid and Na2HP04are 0.048 and 0.103 M at pH 5 after mixing with (10) Hara, H.; Motoike, A.; Okazaki, S. Anal. Chem. 1987,59, 19951999. (11)Mashiko, Y. Theory and measurement of pH; Tokyo kagaku dojin: Tokyo, 1967; p 67.
0003-2700/92/0364-2393$03.00/0 0 1992 American Chemical Society
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ANALYTICAL CHEMISTRY, VOL. 64, NO. 20, OCTOBER 15, 1992
a sample solution, which was high enough to keep the pH of all the sample solutions within +0.06. Scheme of Alternate Washing System. The scheme and operation principle of the system was essentially identical to that described in detail in a previous paper.1° A sample solution premixed with a buffer and a wash solution was alternately introduced into the flow cell via a four-way valve. The system was modified so that the wash solution returned into the reservoir via the pinch valve at the measuring stage of a sample solution. The flow rates of sample,buffer, and wash solutionswere adjusted to 8.0,0.6,and 15.6 mL min-l so as to get the maximum sample throughput. The switchover of the four-way valve from sample to wash was programed to occur when the potentialfiit decreased below the predetermined value, Le., the measured potential at 5 X 10-6 M minus 10 mV. The switchover of the four-way valve from sample to wash occurred when a potential difference within fO.1 mV was observed more than 40 times between two successive measurements. The number 40 was determined by the preliminary experimentsat pH 5 so that the measured potentials reached at least 97 % of the fiial equilibriumpotentialin the concentration M. The time difference between range of 2 X 10-4 and 1 X the switchovers from wash to sample was defined as the response time, that from sample to wash was defined as the washing time, and the sum, the measuring time, was the time required for one measurement. Selectivity. The interferencefrom nitrite, hydrogen sulfite, sulfide, acetate, formate, and benzoate was evaluated by the selectivity coefficient calculated from the following equation:
Pt= [(lo'E1-Ed'a - 1) x lo-31/ci (1) where Ei and Eb are the measured potentials for a M sodium bicarbonate solution with and without an interferent of concentration Ci. The slope S was calculated from the linear portion of a calibrationcurve, i.e., 2 X lo4 to 1 X 1P2M. Ifthe difference between Ei and Eb was equal to or less htan f l . O mV, the value of K W t was defined to be zero. Alkalinity. The pH 4.8 alkalinity was measured according to the testing method for industrial water.12 A 100-mL portion of sample solutions was titrated with sulfuric acid to pH 4.8.The pH 4.3 alkalinity was also measured for compari~on.~~
RESULTS AND DISCUSSION Optimization of t h e System. The effect of the flow rate of sample solutions on the measuring time was examined with M bicarbonate solutions at pH 2. The 1X M and 1 X flow rates were varied from 4 to 8 mL min-l; the ratio of flow rates of the sample and wash solutions were kept constant. M was almost independent of The response time at 1 X the flow rate, while the response time at 1X loa M decreased with increasing flow rate. The washing time at 1 X 1C2 M decreased with increasing flow rate, while the washing time a t 1 X lo4 M was independent of the flow rate. Consequently, the measuring time became minimum at the highest flow rate tested at both concentrations. Nagashima et al. recommended using a 0.1 M sodium hydroxide solution as a wash solution in their flow system for the DIC measurement in sea water.3 In our experiment, distilled water was not effective as a wash solution probably because of the slow dissolution rate of carbon dioxide. Though the blank level of DIC in ultrapure water was less than that in distilled water, the washing time became rather long in comparison with that of a sodium hydroxide solution. The best concentration of sodium hydroxide was determined to be 0.01 M based on ease of treatment and experimental cost. As an internal filling solution, three compositions were M NaHC03 + 1 X M KCI, (2) 1 X tested: (1) 1 X M KCl, and (3) 1 X M M NaHC03 + 1.9 X ~~~
~
~
(12) Japan IndustrialStandards: TeatingMethods for IndustrialWater K0101, 1986. (13) Han'ya, T.; Ogura, N. Examination of water quality, 2nd ed.; Maruzen: Tokyo, 1985; p 244.
-2001 I
1
-4
I
L
I o ~ ( C N-3~ H C O ~ / M ) - 2
Flguro 1. pH dependence of the calibration curves. The curves were measured at pH 2 (O),3(0), 4 (X), 5, and 6. The smell trlengle shows the Nernstian slope.
NaHC03 + 1.99 X 1C2 M KC1. In order to stabilize the potential of the internal silver/silver chloride reference electrode, these solutions were saturated with silver chloride. In the calibration graphs at pH 2, the abnormally low peak M bicarbonate solution was observed with potential a t internal filling solution 1, probably due to the very slow response. Although the response speed was normal, the slope of the calibration curve was rather small (48-53 mV decade-1) with solution 3. A Nernstian slope was obtained with solution 2, which was used for further experiments. pH Depdendence of the Calibration Graph. The form of DIC changes according to the sample pH. Below pH 4.4, the DIC should mainly exist as carbonic acid (or aqueous carbon dioxide). The ratio of bicarbonate increases at higher pH values, but this is believed to cause a decrease in ~ensitivity.~J~ Figure 1 shows the pH dependence of the calibration curves. At pH 2-4, the three calibration curves were almost identical. The linear response range was from 1X M down to 2 X lC4M. At pH 5, a linear response down to 2 X lC4M was also obtained; however, the potentials at each concentration decreased 2-6 mV. This linear response range was sufficient for natural water analysis. At pH 6, the potentials decreased so much that a linear response was observed only down to 5 X M. Further experiments were performed a t pH 5, considering the improved selectivity as shown later. Performance of t h e System at p H 6. Figure 2 shows the dynamic response time curves for standard sodium bicarbonate solutions. A potential undershoot was observed in the washing process in the low concentration ranges. The reproducibility of two peaks was usually within 1 mV. The reproducibility of the mean value of two peaks for two runs was 0.9, 1.5, 1.9, 3.1,3.2, and 3.8 mV for 30, 10,5,2, 1,0.5 X 10-4M solutions, respectively. Thisresult shows the existence of carry over, which seems to be a weak point of the gas electrode. It is usually the best way to reduce carry over by repeating the measurement of the same solution.1° For example, the error caused by carry over from 1 X loe2to 1 X M was +15% on the average of 15 independent data (range: 9.9-20.4%), and five repeated measurements of 1 X 10-3 M were enough to eliminate carry over. (14) Fiedler, U.; Hansen, E. H.; RbiEka, J. Anal. Chim. Acta 1976,74, 423-435.
ANALYTICAL CHEMISTRY, VOL. 64, -50
t
I
nn
I
'I
IO
0
20
30
40
50
Time/min
F l p m 2. The potentialresponsecuve for sodlum bicarbonatestandard solutions at pH 5. The concentrations of bicarbonate corresponding to each pair of peeks are increasing In the order of 5 X 1, 2, 5 X lo4, 1,3 X 10" and 1 X lo-* M, then decreasing in the reverse M bicarbonate was order. The first peak measured for 5 X necessary for the determination of the base potentlai (shown by the broken line).
Table I. Dynamic Response Characteristics of the System at pH 5 response washing measuring time, 8 time, s time, s PJaHC031, M meana stddev meana stddev meana stddev lo-' 10-3 10-2
72.2 76.1 75.2
7.38 6.99 7.90
20.1 39.9 68.3
5.92 13.4 19.1
92.4 116.1 143.9
11.4 18.7 24.0
Resulta of 12 independent measurements. Two peaks of each concentration were used for calculation.
Table 11. Repeatability of 20 Successive Measurements at PH 5 [NaHCOsl, M found (mean) 2 X lo-' 2.03 X 1Oa 5 X lo-' 5.34 X lo4 1.07 x 10-3 10-3 3X 3.14 X l W 3 1.01 X 1 t 2 10-2
amt added
% re1
std dev
std dev
0.0297 X lo4
1.5 1.7 0.9 2.3 1.7
0.0888 X lo4
0.0096 x 10-3 0.0724 X 0.0169 X
% error 1.5 6.8 6.5 4.7 0.6
Table I summarizes the response time, washing time, and measuring time from 12 independent measurements. Interestingly, the concentration dependence of the response time was insignificant, while the washing time strongly depended on the concentration of bicarbonate. As a result, the measuringtime became longer as the concentration increased. It can be concluded that the measuring time for one sample was 1.5-3 min in the concentration range examined. (It is noteworthy that the measuring time of a sample of a certain concentration depends on the concentration of the base solution.) Table I1 gives the results of 20 successive measurements performed after one set of a calibration run from 5 X 10-6 to 1 X 10-2 M (and several repeated measurements of 1 X 10-3 M for 2 and 5 X 1o-L M solutions in order to reduce the effect of carry over). Concentration of each peak was calculated by the equation of the regression line obtained from the linear M). As judged part of a calibration graph (2 X lo-'to 1 X from the relative standard deviation of 20 peaks, the repeatability is fairly good. However, a positive drift in peak potentials was observed at 3 X M. Carry over is also the M. reason of the positive error at 5 X 10"' and 1 X
NO. 20, OCTOBER 15, 1992 2995
Nonetheless,the error was within 7 % in all cases listed, which was not very bad, considering the logarithmic character of the sensor. Selectivity. Lopez studied the selectivity of the potentiometric carbon dioxide gas-sensing electrode? Her important conclusion was that the selectivity is governed primarily by the acidity rather than the volatility of the interferenta. Among the interferenta she tested, six substances which are sources of the possible interference in natural water analysis were selected. Table I11 gives the apparent selectivity coefficienta for six interferenta at pH 2 and 5 by the continuous flow method and the alternate washing method. The reproducibility of these values is rather poor as judged from the standard deviation, partly because they are rather sensitive to the slight variation of the slope of a calibration graph. Nevertheless, several conclusions can be deduced from the table. First, the pH is of primary importance relative to the selectivity coefficient obtained by both methods. Only benzoate showed little interference at both pHs, probably because of the low vapor pressure of benzoic acid? The percentage of undissociated species versus the total concentration can be calculated using the PKa values given in the table. At pH 2, over 95% of these ions exists in undissociated form except HSO3- (about 37%). On the contrary, the 37 5% ,and 5%for nitrite, percentage decreased to 2 %,O.M%, hydrogen sulfite, acetate, and formate at pH 5. This is the reason for the remarkabledecrease in the selectivity coefficient at pH 5. Only sulfide exists in undissociated form even at pH 5. The decrease in interference from sulfide at pH 5 cannot be explained by the difference in the degree of dissociation at each pH. Morf et al. reported that the sharp increase and the subsequent gradual decrease in potential could be expected after the step change from a M COZsample to a M solution of H2S with a COZgas electrode having a silicone rubber membrane.g However, the time response curve was monotonous in every continuous-flow measurement including the case of sulfide, although the rapid increase in potential was observed in some cases. Further examination will be necessary to understand the interfering behavior of sulfide when the microporous PTFE membrane is used. When the results of the alternate washing method (AWM) are compared with those of the continuous-flow method (CFM),the interference can be said to decrease at both pHs, except for the cases when the interference is very large. In the AWM, the pseudoequilibrium potential, which was usually smaller than the equilibrium potential obtained by the CFM, was measured. It is natural that the apparent selectivity coefficient of the AWM was usually smaller than that of the CFM. This is the significant feature of the AWM: the ability to measure a signal that is transient but very near to the true equilibriumfor a standard bicarbonate solution and relatively far from the equilibrium for solutions containing the interfering substance. In the AWM, the interference at pH 5 was almost completely removed for all interferents tested except sulfide at least in the concentration range below lo-' M. The observed sequence of the interference at pH 2 was as follows: hydrogen sulfite > nitrite > sulfide > formate > acetate > benzoate. This sequence is in accordancewith that of the pK, only for hydrogen sulfite and nitrite and for formate and acetate. This result seems to be in conflict with the conclusion of Lopez, at least for sulfide and benzoate. However, part of the discrepancy with the resulta of Lopez may be ascribed to differences in the experimental conditions such as the concentration of NaHC03 in the internal filling solution (0.01 M was used), the concentration of the inter-
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ANALYTICAL CHEMISTRY, VOL. 84, NO. 20, OCTOBER 15, 1992
Table 111. Selectivity Coefficient for Volatile Interferents apparent selectivity coefficiento at pH 2 interferent nitrite
uK.
concn,M
3.29
lo+
hydrogen sulfite
1.76
sulfide
7.0
acetate
4.76
formate benzoate
AWMb 0 1000f 100 (n = 4) 8OOf 100 (n = 3) 20 f 10 (n = 6) 5000 f 700 (n = 3) 4000 f 400 (n = 4) 0 2 f 0.4 (n = 6) 300f 100 (n = 4)
10-4 10-3 10-6 10-4 10-3 10-6 10-4 10-3 10-4 10-3
3.75
1
4.21
10-3 10-4 10-3
~
0 4
co.1 0 co.1 0 0
at pH 5 CFMe
700f200(n=4) 1000&200(n=4) 8OOf 200 (n = 4) 2000f 1000 (n = 3) 5000 f lo00 (n = 3) 4000*500(n=3) 30 f 10 (n = 4) 100 f 40 (n = 4) 300&100(n=6) 2 f 1 (n = 3) 20 f 6 (n = 4) 3 f 1 (n = 4) 100 f 50 (n = 4) 1 f 0.8 (n = 3) 0.3 f 0.2(n = 3)
CFMc
AWMb 0 0 0.1f 0.05 (n = 4) 0 0 -0.1 f 0.02 (n = 6) 0 0.6f 0.1(n = 4) 0.3f 0.05 (n = 6) 0 0 0
~ _ _ _
0 1 f 0.9 (n = 4) 10 f 2 (n = 4) 0 0 4 f 2 (n = 4) 10 t 1 (n = 3) 2 f 2 (n = 4) 0.6 f 0.4 (n = 6) 0 1 f 0.4 (n = 4) 0 0 0 0
0 0
0
M sodium bicarbonate. Mean f SD is shown with number of data. * Alternate washing method.
Mixed solution method was used for Continuous-flow method. (I
,O0-I
150 -
PH3 1
\
100-
Timelmin
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. E
2oo
w
a
t
0 30
0
T i m e t min
I
3
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L PH
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I
5
6
Figwe3. pHdependenceof the Interference from nitrite. The potential dlfference between the two M blcarbonate solutions with and without lo4, lo-', and M sodium nitrite (curves 1, 2, and 3, respectively) Is plotted against the pH of a sample solution after mlxlng with a buffer stream. The error bar corresponds to the standard deviatlon of four Independent data of the continuous flow method.
ferents and the difference in the method used to obtain the selectivitycoefficient (a separate solution method was used% Among the interferents tested, nitrite is most important for river water analysis because its concentration in polluted river waters sometimes exceeds 1 X M in the Shiga prefecture. The effect of the coexistence of nitrite was examined by the continuous-flow method and the results are shown in the following two figures. Figure 3 shows the pH dependence of the increase in potential caused by the coexistence of sodium nitrite in sodium bicarbonate solution. The interference almost disappeared at pH 5 for 1X 1V M nitrite solution (curve 2) even in the continuous-flow measurement. Figure 4 shows the time response curves of a mixture of nitrite and bicarbonate at each pH. In the AWM, the potential value is recorded after 40 occasions in which the potential difference of two successive measurements every 0.6 s was within fO.l mV. This means that the maximum potential change during 24 s should be below 4 mV (i.e., dE dt-l< 10mV mi+) for the switchover. If the rate of potential
PH5
0
0
2
t
W
a
0-
'. I,.- -_-,;, *0°
w
50 -
Timet min
30 Timelmin
:lool a
,
3,:-10 0
30 Time1 min
Flgwe 4. The tlme response curves of lo3 M bkarbonate sdutlon In the presence of lo3, lo4, and M sodlum nltrite (curves 1,2, and 3, respectively) measured by the contlnwus flow methodat each PH.
change is very fast (see curve 1at pH 2,3,or 41, the potential of the horizontal part is also recorded in the AWM as in the CFM. However, potential values much nearer to that of the M bicarbonate solution are measured in the AWM, if the rate of potential change is slower than 10 mV min-l as was the case for curve 3 at pH 2. Thus, the method of potential measurement in the AWM is useful for improving the apparent selectivity. Natural Water Analysis. Table IV shows the results of the DIC measurement for natural water samples. The alkalinities of pH 4.8 and 4.3 are included in the table because these values have often been used as a measure of the DIC in aquatic chemistry. The interferencefrom volatile inorganicand organicspecies was concluded to be negligible because the values with the pH 2 buffer agreed with those with the pH 5 buffer very well
ANALYTICAL CHEMISTRY, VOL. 64, NO. 20, OCTOBER 15, 1992
Table IV. Results of Natural Water Analysis allralinity, X 104 equiv L-1 DICp X le M sample pH2 pH5 pH 4.8 pH4.3 2.28 2.11 2.62 pondwater 2.36 6.04 6.57 6.62 6.89 Lake Biwa 2.81 2.29 2.36 2.39 river 1 2 3 4 5 6
5.68 5.96 7.23 9.92 9.96
5.62 5.93 7.19 9.63 9.91
5.14 5.31 6.49 7.92 8.15
5.62 5.82 6.97 8.47 8.69
a Dissolved inorganic carbon measured by the altemate washing method with the buffer of pH 2 or 5.
(correlation coefficient r = 0.9986). The concentration of nitrite was below 1 X lo” M in all samples. The alkalinity was obtained by titration with a sulfuric acid solution. Both the pH 4.8 alkalinity and the pH 4.3 Alkelinity are often wed as a measure of the concentration of bicarbonate. Although the correlation between the DIC
values measured with the pH 5 buffer and the alkalinity was very high (r = 0.996for the pH 4.8 alkalinity and r = 0.997 for the pH 4.3 alkalinity), the slopes of the regression lines are 0.78 and 0.79,respectively. This result means that the alkalinity is not directly correlated with the DIC measured in this method. On the other hand, Kuwaki et al.16 reported that the DIC values in natural waters measured by a gas diffuionlflow injection analysis with a photometricdetection system agreed well with the resulta obtained by indirect photometric ion chromatography (the error was within f2.2 % for five river water samples). Because their system used also the gas diffusion principle, the DIC values measured in our system may correspond to the totalconcentration of carbonate species. The reasonable interpretatino of the alkalinity is beyond the scope of this paper and will require further studies.
ACKNOWLEDGMENT A part of thiswork was financially supported by Mitaubishi Heavy Industries, Ltd.
RECEIVED for review March
23, 1992. Accepted July 20,
1992. Registry No. Water, 7732-18-5; carbon, 7440-44-0; sodium bicarbonate, 144-55-8.
(15) Kuwaki,T.;TBeiK.;Akiba,M.;Oshima,M.;Motomizu,S.Bunseki
Kogcrku 1987,36, T132-Tl35.
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