Anal. Chem. 1989, 6 1 , 1841-1846
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Determination of Total Inorganic Carbon in Aqueous Samples with a Flame Infrared Emission Detector S. Wayne Kubala, David C. Tilotta, Marianna A. Busch, and Kenneth W. Busch* Department of Chemistry, Baylor University, Waco, Texas 76798
A specially designed system, using a flame infrared emission (FIRE) detector, was developed to permit the determination of total inorganic carbon (TIC) in water samples. Carbon dioxide, released from l.OmL samples upon acidification, was purged from solution with He and exclted in a hydrogedair flame. The carbon dioxide emission intensity at 4.42 pm (2264 cm-') was monitored by use of a lead selenide detector in conjunction with a 4.4-pm band-pass optical filter. Peak emission intensity measurements from multiple injections of 1.0-mL water samples gave a relative standard deviation of 1.35 %. The average relative standard deviation of the TIC measured with the FIRE system was found to be 0.89%. The accuracy of the FIRE technique was determined by comparing the FIRE-TIC results with those obtained by total alkalinity titrations for five natural water samples and was found to be 1.22%. The detection limit for the system was found to be equivalent to 3.05 X lo-, mM Na,CO, and calibration curves were linear up to 50 mM Na,CO,. Elevated levels of purgeable organic compounds were found to be a potential interference in the determination of TIC with the FIRE system.
INTRODUCTION Carbonate (C032-),bicarbonate (HC03-), and dissolved carbon dioxide (H2C03*)are found in natural water sources and originate primarily from soluble minerals, the action of microorganisms, and the atmosphere, which contains carbon dioxide from many sources including photosynthesis and the combustion of fossil fuels ( I ) . The sum of these three carbon-containing species, CT CT = [HzC03*] + [HC03-] [CO:-] (1)
+
(where [H2C03*]represents the total analytical concentration of dissolved COz,whether hydrated or not) is generally referred to as total inorganic carbon (TIC). The presence of soluble carbonates and bicarbonates increases the capacity of water to neutralize acid and, thus, affects the pH and mineral content of the water. Since the concentrations of carbonate and bicarbonate also determine the scale-forming, corrosive, and coagulation/flocculation properties of water ( 2 ) ,the measurement of TIC is of fundamental importance in many environmental and industrial areas including oceanography ( 3 , 4 ) ,water resource management ( 5 ) ,and water/wastewater treatment (2, 6). Numerous analytical methods have been developed for the determination of TIC in water samples. Direct methods for TIC include chromatographic separation procedures (7, 81, spectrophotometric titrations (9--11),and nondispersive infrared absorption measurements of evolved COz gas (12,13). However, since these direct methods tend to be time-consuming, tedious, and/or expensive, the most commonly used method for TIC determination is indirect and involves the measurement of the total alkalinity present in the solution. Total alkalinity is defined as the quantitative capacity of water to neutralize strong acid (6). If it is assumed that the only Brmsted bases in the water sample are hydroxide,
carbonate, and bicarbonate (as is the case for many surface waters), then the total solution alkalinity will be given by [total alkalinity] = CT(a1
+ 2a2) + [OH-] - [H+]
(2)
where 01, and a2represent the fraction of total inorganic carbon present as bicarbonate and carbonate, respectively [HC 03-1 a1=--
[H30+1K1
-
CT
[H,0+l2
K1K2
[co32-1 -
a2=--
CT
+ K1[H30+] + K1K2 +
[H30+l2 K1[H30+]
+ K1K2
(3) (4)
and K , and Kz are the first and second dissociation constants of carbonic acid. Previously developed methods for total alkalinity determination include measurement of the specific conductance of the solution (14) and titration of the water sample to a preselected end point using a strong acid and an indicator such as methyl orange or a potentiometer (5,15). For natural water samples, the preselected end point may vary between 5.1 and 4.5 pH units and depends on a number of factors including the concentration of COZ expected at the final stage of the titration (5). For some complex industrial wastewater samples, however, the titration end point may be as low as 3.7 pH units (5). Knowledge of the solution alkalinity (i.e., milliequivalents of acid added to the sample to obtain the desired pH) and the pH of the solution prior to titration allows the TIC to be determined from the equilibrium relationships between COz, HC03-, and C032-or from a graphical representation of these relationships, as in a Deffeyes diagram (16). In order to maintain standards of taste, odor, and levels of dissolved minerals, the EPA has established a primary drinking water regulation that requires monitoring of total alkalinity by every municipal water treatment plant in the country ( 2 ) . Solution alkalinity is also routinely monitored in many applications where the corrosive and scale-forming tendencies of water must be controlled. It should be borne in mind, however, that because alkalinity is an aggregate property of the water, alkalinity determinations are subject to many interferences, and the results can be strictly related to TIC only when the entire chemical composition of the water is known. Furthermore, since proper selection of the end-point pH requires prior knowledge of the water composition, the great variation in sample characteristics introduces a considerable uncertainty in the final results. For example, the flocculating properties of clay-based drilling muds used in petroleum recovery is highly dependent on TIC, but alkalinity titrations provide only a very approximate idea of the true TIC content of such complex samples (17). Thus, there is a real need for a direct method of TIC determination which is quick, simple, inexpensive, reproducible, and not subject to the uncertainties resulting from sample variability. This paper reports the development of a new method for the direct determination of TIC based on the principle of flame infrared emission (FIRE) detection first described by Busch and Hudson (18, 19). In essence, the FIRE-TIC instrument consists of two commercially available purge devices coupled to a FIRE detector. Water samples are acidified in
0003-2700/89/0361-1841$01.50/00 1989 American Chemical Society
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ANALYTICAL CHEMISTRY, VOL. 61, NO. 17, SEPTEMBER 1, 1989
A
I
I
B
Flgure 1. Schematic diagram of FIRE instrument for determination of total inorganic carbon in water samples: purge section (lA), FIRE detector (1B); He, helium cylinder; R, regulator; M, flowmeter: V,, V,, three-way valves; S, rubber septum, G, glass frit; P,, sample purge device; P,, reference purge device; H,, hydrogen cylinder: Air, air cylinder; B, burner; L, CaF, lens; C, chopper; F, band-pass filter; D, PbSe detector; PS, power supply; PA, preamplifier; LIA, lock-in amplifier; R / I , recorder-integrator.
the purge device to convert bicarbonate and carbonate to carbon dioxide. The total COz is then purged from the sample cell by using an inert carrier gas and introduced into a lowbackground hydrogenlair flame where vibrational excitation of the COz molecule occurs. Direct determination of total inorganic carbon, as carbon dioxide, is accomplished by monitoring the 4.42-pm infrared emission intensity from the excited COz molecule. Previous work in this laboratory has demonstrated that the 4.42-pm asymmetric stretching vibration of carbon dioxide is analytically useful due to its relatively high emission intensity and its freedom from other strong overlapping infrared emission bands produced by the hydrogenlair flame (18, 19). The FIRE detector consists of a miniature capillary-head hydrogenlair burner, an optical collection lens, a band-pass filter to isolate the 4.42-wm COz emission band, and a PbSe photoconductive detector.
EXPERIMENTAL SECTION schematic diagram of the FIRE-TIC instrument, shown in Figure 1, has been divided into two principal sections for purposes of discussion. Figure 1A shows the CO, generation and purging portion of the apparatus, and Figure 1B shows the FIRE detection system. Purge Apparatus. Two purge devices (Model 991710, Wheaton Scientific, Millville, NJ) were connected together in parallel with 3.2 mm 0.d. polyethylene tubing and two, three-way valves (Model B-42XS4, Whitey Co., Highland Heights, OH). Each purge device consisted of a 5-mL, demountable glass tube, equipped with a fritted glass disk, and an inlet and outlet opening for the purge gas. A third opening at the top of the purge device was fitted with a rubber septum (Model 212,4334, Aldrich Co., Milwaukee, WI) and used for sample introduction. The purge tube with its associated frit was attached to the purge device with plastic screw-caps and could be disconnected for cleaning between sample injections. Water samples and sulfuric acid aliquots were injected into the purge tubes with a glass syringe (water sample syringe, Model 1002, Hamilton Co., Reno, NV; acid syringe, Model 2300, Becton-Dickinson & Co., Rutherford, NJ). The pressure of the helium purge gas was regulated at 0.75 atm with a three-stage regulator constructed by coupling a single-stage regulator (Model 6200, Rexarc Co., West Alexandria, OH) to a conventional dual-stage unit (Model 2068, Rexarc). The helium flow rate was monitored with a Brooks Instrument flowmeter (Model 1110-05FlAlA Brooks Instrument Division, Emerson Electric Co., Hatfield, PA) corrected for helium. The output of the helium flowmeter was connected to the purge device with 6.4 mm 0.d. polyethylene tubing and Swagelok fittings (Crawford Fitting Co., Solon, OH). FIRE Detection System. The components of the FIRE detection system were mounted on a 39.5 x 29.1 x 2.0 cm aluminum plate and consisted of a hydrogen/air burner, collection lens, Apparatus. A
optical chopper, optical band-pass filter, and PbSe detector. The associated electronics included a preamplifier, lock-in amplifier, and recorder/integrator. The premixed hydrogen/air, capillary-head burner was fabricated from an aluminum block and has been described previously (19). The burner head consisted of a circular array of six stainless-steel capillary tubes surrounding a seventh capillary, each having a 0.6 mm i.d. Fuel and oxidant gases were premixed within the burner body and introduced through the six outer capillaries. The center capillary was connected to the outlet valve of the purge assembly with a modified Swagelok male connector and 3.2 mm 0.d. Teflon tubing. With this design, the sample and purge gases could be introduced directly into the flame without premixing with the fuel and oxidant gases. The burner was enclosed with an aluminum shield and chimney assembly to minimize signal fluctuations resulting from air drafts and changes in ambient temperature conditions. As in the case of the helium purge gas, pressure regulation of the fuel and oxidant gases was accomplished through three-stage regulation. A hydrogen dual-stage regulator (Model 2067HY, Rexarc) was coupled to a hydrogen-line regulator (Model 6203, Rexarc), and an air dual-stage regulator (Model 2067A2,Rexarc) was coupled to an air-line regulator (Model 6200, Rexarc). The hydrogen and air flow rates were controlled with variable area flowmeters (Models 53216-06 and 53216-16, respectively, ColeParmer Instrument Co., Chicago, IL). Fuel and oxidant gases were used without purification to remove residual carbon dioxide which might be present. The optimum flow rates for hydrogen and air were determined to be 324 and 754 mL/min, respectively. These flow rates correspond to a fuel-to-air ratio of 3:7 and were chosen because they produced the lowest flame background noise signal without flame lift-off from the burner head. The infrared emission from the flame was collected and directed onto the detector by using an F/2,5-cm focal-length, CaFz lens (part 43150, Oriel Corp., Stratford, CT). A laboratory-constructed chopper modulated the infrared radiation at 575 Hz. The 4.42-pm COz emission band was isolated optically by means of a 4.4-wm optical band-pass filter (part 58300, Oriel) placed immediately before the detector. A PbSe photoconductive cell (part P2038-01, Hamamatsu Corp., San Jose, CA), operated at a 30-V bias potential, served as the infrared detector. The preamplifier circuit used with the PbSe detector has been previously described (18). The preamplified signal was demodulated by a lock-in amplifier (Model 3962, Ithaco Corp., Ithaca, NY) and recorded on a plotter/integrator (Model HP3396A, Hewlett-Packard Corp., North Hollywood, CA). A 1-s time constant was employed in the lock-in amplifier for all measurements. Reagents. All chemicals were A.C.S. reagent grade and were used without further purification. Stock solutions of 4.0 and 10.0 mM NaZCO3(Mallinckrodt, Inc., St. Louis, MO) were prepared by dissolving primary standard Na2CO3,dried at 110 "C for 24 h, in deionized water. The laboratory deionized water was further conditioned by passing it through two mixed-bed deionizing columns. Standard Na2C03solutions, having concentrations of 0.1, 0.5, 1.0, 2.0, 4.0, 5.0, 8.0, and 10 mM, were prepared immediately before use by diluting aliquots of the stock solutions to the appropriate volumes. Procedure. Natural water samples were obtained from several local sources and stored according to standard procedures ( 5 ) . For alkalinity-determined TIC, the water samples were titrated with 0.0219 N H,S04 to an end point of 4.8 pH units. A Fisher Accumet Model 825MP pH meter was used for all measurements. For TIC determinations performed with the FIRE-TIC system, one of the purge devices (Figure 1A) served as the sample chamber while the other served as the reference chamber. As part of the warm-up procedure, He purge gas was directed through the sample purge tube by using the dual, three-way valve system, and a 2-mL volume of deionized water was injected onto the frit of the reference purge tube with a syringe. The He gas was then rerouted through the reference purge tube, the flame lit, and the FIRE-TIC instrument allowed t o warm up until a stable base line was obtained on the chart recorder. When the instrument had stabilized, a 0.5-mL volume of 3 M H$04 (Mallinckrodt) was introduced onto the frit of the sample
ANALYTICAL CHEMISTRY. VOL. 61. NO. 17. SEPTEMBER 1. 1989 3.5
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F l p 0 3. Ftmtoqaph of the FIRE-TIC insbunmt showhg (he drhney and enclosure used to shield the flame. The front plate of me encburelchimney assembly has been removed to show me buner and
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noUn 2. Fovier-tramfarhframdJpectra fran 3000 to loo0 an-'. (A)and(C)areemission Jpectra planed on (he same &We intensny scala (not cwected for instrument response). (A) Flame infrared emission fran a hyaogenlalfflame contakhg carbon dioxide showhg the strong CO, asymmetric stretching vibration at 2264 cm-'. (8) Transmission spectrum of me band-pass finer used In the FIRE detector (maximum transmission at 4.42 am 75% T.
