estimating the intersection point and the positioning of the sample cell in the Dewar flask. It should be noted that the correction factors for tryptophan and tyrosine are 1.00 because the molar absorptivities are quite small at the wavelengths being used and the solutions are dilute. The measured interisity ratio of the two components of a mixture is a function of the wavelength of excitation, of the wavelength at which emission is measured, and of the sensitivity of measurement of the individual components. This assumes that the decay times of the two components are sufficiently different that an analytically useful logarithmic decay curve results. Note that the mixture of tryptophan-tyrosine is easily resolvable even though the ratio of decay times is only 4.6. The concentration ratio of the components in a two-component mixture can vary considerably if the analyst is willing to sacrifice sensitivity and accuracy of measurement of one or both of the com2onents. Several other uses of the measurement system should be mentioned. Phosphorescence decay times greater than 0.2 second can be simply measured by measuring the slope of the
logarithmic decay curve for the component in concern. It is also simple to check the purity of any phosphorescent molecule by verifying the linearity of the logarithmic decay curve. It should, however, be pointed out that non-exponential decays have been observed for some pure organic phosphors (8-10). For example, the authors have failed to obtain exponential decays for high purity benzyl alcohol and toluene in absolute ethanol.
RECEIVED for review November 10,1966. Accepted February 1,1967. This research was carried out as part of a study on the phosphorimetric analysis of drugs in blood and urine supported by a grant from the U.S. Public Health Service (GM11373-03). (8) T. Azumi and S. P. McGlynn, J. Chem. Phys., 39, 1186 (1963). (9) G. Von Forster, Ibid.,40, 2059 (1964). (IO) H. Sternlicht, G. C. Nieman, and G. W.Robinson, Ibid.,38, 1326(1963).
An Instrumental Method for Rapid Determination of Carbonate and Total Carbon in Solutions C. E. Van Hall and V. A. Stenger Analytical Science Laboratory, The Dow Chemical Co., Midland, Mich.
Apparatus is described with which carbon dioxide or carbonate in solutions at levels equivalent to 2 to 100 mg of carbon per liter can be determined in about two minutes. The technique consists of injecting a micro sample into a heated tube ( B O 0 C) containing an acidtreated packing. The evolved carbon dioxide is swept through an infrared stream analyzer for measurement. The equipment has lieen incorporated into a dual unit so that both total carbon and carbonate can be determined with the same infrared analyzer, recorder, and carrier gas supply.
FORTHE DETERMINATION of organic carbon in aqueous systems it is customary either to determine total carbon and apply a correction for inorganic carbon, or to eliminate carbon dioxide prior to the analysis (1). The elimination procedure is simple and useful when there is no danger of losing volatile organic matter, but this is not always the case. Direct procedures for carbon dioxide or carbonate determination are usually rather slow arid require moderately large samples. One of the more rapid of these is a 10-minute method described by Pobiner (2) in which carbonate is liberated from an acidified sample by evacuation and determined by measuring the pressure and infrared absorbance of the evolved gas. With a 50-ml sample tne method is reported to be sensitive to about 100 ppm Kz COS. A faster and more sensitive procedure for determining carbonate or carbon dioxide would be very useful in the field of water analysis. The authors have now developed a method in which 10 to 50 p1 of the solution to be analyzed is injected into a tube heated to a temperature below that at which
organic matter would be oxidized. An acid-treated packing within the tube serves to release carbon dioxide from carbonates. The liberated gas is swept through a nondispersive type infrared stream analyzer, the output of which is recorded. Carbon dioxide is indicated as a peak whose height is proportional to the concentration. Experience with this method has shown it to be quite satisfactory and specific. Because of similarities in the apparatus required, the new method is well suited for use in conjunction with the total carbon analysis system previously described (3). A compact dual-furnace unit can be constructed in which the same infrared analyzer and recorder are employed for both carbonate and total carbon determinations. Air, supplied by a small pump and suitably purified, serves as the carrier gas for carbon dioxide. It has been found possible also to substitute purified air for oxygen in the determination of carbon, thus avoiding the use of cylinder oxygen. The same gas stream is directed into either of the reaction tubes and then into the analyzer by means of a four-way valve. With this dual unit both total and carbonate carbon can be determined in about 5 minutes. Carbonate or carbon dioxide alone can be determined in either aqueous or nonaqueous systems in about 2 minutes. EXPERIMENTAL
Apparatus. The apparatus assembly is shown in Figure 1. The tube furnace for carbon dioxide evolution is operated in parallel with that for complete combustion, resulting in two trains feeding a single analyzer. The air supply for the
(1) C. E. Van Hall, D. Barth, and V. A. Stenger, ANAL.CHEM.,
37, 769 (1965). (2) H. Pobiner, Ibid.,34,1378 (1962).
(3) C. E. Van Hall, J. Safranko, and V. A. Stenger, ANAL.CHEM., 35, 315 (1963).
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503
Figure 1. Schematic diagram of dual analyzer system is provided by a Cole-Parmer Dyna-Vac pump, Model 4K (1) with a capacity of 600 cubic incheslminute at 18 psi. The output of the pump is controlled with a Conoflow pressure regulator, Model H-10 (2). This is necessary to prevent the full pressure of the pump from possibly rupturing a tube. The air is passed through a combustion tube (3) to oxidize any organic matter to carbon dioxide. This tube, shown in Figure 2, is maintained at 950" C in the furnace that contains the combustion tube (11) used for determining total carbon. Tube (3) is slightly smaller than the regular combustion tube (3) in order to fit beside it within the furnace. The air is then passed through a fritted glass scrubber (4) containing 2M sodium hydroxide solution to remove carbon dioxide originally present or formed by the combustion of organic impurities. The purified air is then fed to two identical regulating systems, each containing the following: a Conoflow regulator, Model H-10 (5) for coarse flow adjustment; a pressure gauge (6), 0-30 lb, to indicate the pressure in the system; a Nupro needle valve (7) for fine flow adjustment; a Brooks Shorate flowmeter (8), 0-200 cc/minute, to measure the flowrate; and a Kimble check valve No. 38006 (9) to reduce sample blowback. It is likely that equipment of other manufacturers can be substituted for some of that specified here. The purified air enters the carbonate evolution tube (12) through the vertical arm of a syringe adapter (10) shown in Figure 2. The adapter consists of a No. 18 stainless-steel needle cemented in the horizontal arm of a tee with epoxy cement. The needle acts as a holder and guide for the sampling syringe, usually a Hamilton 705 N-LT or SK 148, the latter having an all-metal Luer tip and a 0.010-inch i.d. needle with a 90" (square) end. Between injections the sample port is closed with a size-000 cork stopper. Tubes (3) and (11) each contain cobalt oxide-impregnated asbestos (2) in loosely packed wads to a length of 15 cm in tube (3) and 6 cm in tube (11). The carbonate evolution tube (12) is identical dimensionally with tube (ll), but is packed with 6- to 12-mesh quartz chips. The chips are placed against a small wad of quartz wool or asbestos to a length oi 10 cm. The packing is wetted with 85% phosphoric acid. This is accomplished by pouring an excess of phosphoric acid into the packed tube while holding it in a vertical position, and allowing the excess to drain out. The temperature of each tube furnace is regulated by means of an Assembly Products, Inc., controlling pyrometer Model 504
ANALYTICAL CHEMISTRY
1602 L (13) with the associated control module 915A and thermocouple. The carbonate-carbon furnace (14) is maintained at 150" C and the total carbon furnace (15) at 950" C. Air condensers (16) are affixed to tubes (11) and (12) by means of ball joints secured with spring clips. The ball joints are lubricated with Kel-F stopcock grease. Each condenser drains into a small U-tube reservoir which can be emptied through a stopcock (17). The gas outlets of the condensers are connected with a four-way stopcock (18), Fischer & Porter No. 791-797, which permits the selection of either system for analysis. The gas stream being monitored is passed through a Hoke filter, Model S-541 (19), and thence to the infrared analyzer (20). The stream from the other tube is bypassed to the atmosphere at the valve. A Beckman infrared analyzer, Model IR 215, fitted with 13.3-cm cells sensitized for carbon dioxide, is employed to monitor the gas stream from either system. The cell compartment is thermostatically controlled at 55" C. The output of the analyzer is recorded on a Sargent Model MR or SR Recorder (21), using the 0- to 5-mV range. Reagents. Sodium carbonate-sodium bicarbonate standard solution is prepared by accurately weighing 1.400 grams of A.C.S. grade sodium bicarbonate and 1.767 grams of primary standard grade sodium carbonate into a 1-liter volumetric flask, diluting to volume with distilled water, and mixing thoroughly. One milliliter = 400 pg of carbon. Solutions of other compounds used in this study were prepared from the purest grade of chemicals available, generaliy A.C.S. grade. Impregnated asbestos for the combustion tubes is prepared as previously described ( I ) . Procedure for Instrument Adjustment. The infrared analyzer is turned on, and allowed sufficient warmup time for stable, drift-free operation. A minimum time of 4 hours is necessary; if daily use is anticipated the analyzer should be left on continuously. With the Aowrate adjusted to 90 mlt minute, the air stream passing through the evolution tube at 150" C, the recorder set to the 5-mV range, and a chart speed of 0.33-inch per minute, the amplifier gain is adjusted so that a 20-pl sample of the 100 mg/liter standard gives a peak height of approximately half-scale deflection. At this level of gain the noise level should be less than 0.5 of full scale. Preparation of Standard Curve. A series of standard solutions is prepared containing 20, 40, 60, 80, and 100 mg/ liter of carbon by pipetting 5 , 10, 15, 20, and 25-ml aliquots of the standard solution into 100-ml volumetric flasks. Each
CARRIER GAS COMBUSTION TUBE 11 -0
FUSE0 S I L I C U 1.0 cm 0.0. SYRINGE ADAPTER
NO 1.9 ss NEEDLE
cm.
LCOJO~-ASBESTOS 15cm
TOTAL CARBON COMBUSTION
TUBE
"VYCOR"
FU 1.6
CARBONATE EVOLUTION TUBE S 10/30 "VYCOR"
Figure 2. Combustion tubes and syringe adapter solution is diluted to volume with distilled water and mixed. An additional flask is filled with the same distilled water for use as a blank. Twenty microliters of each of the standards and the blank are introduced into the evolution tube successively at about 3-minute intervals and the peak height is determined in scale divisions for each injection. The actual sampling technique is as follows: the syringe is rinsed several times with the solution to be analyzed, filled, and adjusted to 20 pl. The excess is wiped off with soft paper tissue, taking care that no lint adheres to the needle. The plug is removed from the syringe holder, the sarnple syringe inserted, and the sample injected into the tube with a single, rapid movement of the index finger. The syringe is left in the holder until the flow rate returns to normal, then replaced with the plug. All samples should be run in duplicate. The results are corrected for the blank and a standard curve is prepared in terms of mg per liter of substance sought us. peak height on rectangular coordinate graph paper. Analysis of Samples. If the material to be analyzed contains suspended solids it should be homogenized or blended thoroughly so that a uniform sample can be obtained with a syringe, A 20-4 sample of the solution to be analyzed is injected into the evolution tube and the height of the resulting carbon dioxide peak is measured. From the peak height and the standard curve, the concentration of carbonate carbon in the sample is determined. Total carbon, if desired, is determined similarly by turning the four-way valve and injecting a sample into the combustion tube after the base line has become readjusted (3). RESULTS AND DISCUSSION
Duplicate analyses were made of six separately prepared solutions, each containing sodium carbonate and acetic acid in proportions corresponding to 50 mg each of carbonate carbon and organic carbon per liter. For the 12 determinations of total carbon at the 100 mg per liter level, the mean was 100.3 and the standard deviation 0.74. For direct carbonate carbon at the 50 mg per liter level the mean was 49.6 and the standard deviation 0.35. By averaging the duplicates on each solution and subtracting carbonate from total carbon to obtain average values for organic carbon, the mean of the resulting six averages was 50.7 and the standard deviation was 0.58. For comparison, duplicate portions of each solution were acidified and purged with nitrogen ( I ) , then analyzed for the
remaining carbon (nonvolatile organic). The mean was 50.4 with a standard deviation of 0.65. Several experiments were performed to establish the optimum operating conditions for the carbonate method. It was found that the peak height obtained is only slightly dependent on flowrate in the range from 40 to 200 ml/minute. For normal operation a rate of 100 ml/minute is recommended. A rapid increase in peak height was observed with an increase in tube temperature up to about 150" C, but only a slight temperature dependence was found thereafter. Higher temperatures should be avoided to prevent the formation of a cement-like deposit in the tube. The standard solution containing equimolar quantities of sodium carbonate and sodium bicarbonate was chosen for calibration purposes, as it is more stable with respect to loss or gain of carbon dioxide than a solution of either component alone. The calibration curve for carbonate carbon is a nearly straight line resembling that for total carbon (3), but the two do not necessarily coincide. Usually the carbonate curve lies slightly below the other. In initial experiments on carbonate determination, prepurified nitrogen was used as a carrier gas. It was thought that air or any gas containing oxygen should be excluded to avoid oxidation of organic matter during analysis. When purified air was substituted, however, it appeared to work equally well. Apparently the presence of oxygen is not detrimental when the temperature is low enough. The use of purified air also appeared to be satisfactory in the total carbon method, for which oxygen had previously been employed. No difference in response was noted as a result of the change. Applications in Water Analysis. The proposed method has been found applicable for the determination of carbonate in most of the aqueous solutions tested. In conjunction with the total carbon method, it enables one to derive a more accurate value for organic carbon by avoiding two common causes for error in the carbon dioxide elimination procedure ( I ) , the loss of volatile organic substances and the coagulation of suspended solids upon acidification. Table I presents results obtained with a series of industrial secondary effluent sewage samples. The suspected presence of volatile organic matter was confirmed by the data; the difference between total volatile carbon lost by purging and carbonate carbon found by the direct method yields the volatile organic carbon. Table I1 contains data acquired on miscellaneous water samVOL 39, NO. 4, APRIL 1967
505
Figure 3. Interference of sodium citrate at 2OO0C Flow rate = 90 ml per minute
CARBON, 100 mg /liter (FROM NazC031
~~
CARBON, 100 mg /liter (FROM NazC031+ 100 mq /liter FROM SODIUM CITRATE
~~
CARBON, 100 mg /liter (FROM NazC03)
~~
Table I. Carbon in Secondary Effluent Samples, mg per liter Total, direct
Carbonate, direct
Organic, difference
39.5 31.7 39.6 45.6 49.2 37.1 30.3 37.3 42.4 49.2
21.1 20.0 21.1 20.9 21.9 22.8 20.0 23.3 23.0 23.8
18.4 11.7 17.5 24.7 27.5 14.3 10.3 14.0 19.4 25.4
Nonvolatile, Volatile acidified organic, and purged difference 14.7 8.4 14.7 21.4 23.7 11.7 7.0 11.5 16.2 21.2
3.7 3.3 2.8 3.3 3.8 2.6 3.3 2.5 3.2 4.2
ples. Except in the case of the river water, the values reported for volatile organic carbon are probably not significant but represent the differential errors of the methods. Results for residual carbon following the acid-purging technique tend to be low for the reasons given above. This causes the results for volatile organic carbon to be high. Interferences. Table I11 contains the results of interference tests obtained with several organic substances that could decompose to form carbon dioxide, either by direct decomposition o r acid hydrolysis. Only tartronic acid (hydroxymalonic acid) was found to interfere at the recommended operating temperature of 150" C, whereas several interfered at 175" C or higher. The type of interference usually encountered is illustrated in Figure 3, showing a slower release of carbon dioxide from the organic substance following the
Table 11. Analyses of Miscellaneous Water Samples Carbon found, mg per liter Sample Lake Huron water Cooling tower water Municipal water River water
Total, direct 20.8 9.5 8.6 46.5
Carbonate, direct
Organic, difference
Nonvolatile, acidified and purged
17.7
3.1 9.5 1.6 11.8
2.7 8.9 1.4 10.5
nil 7.0 34.7
Volatile organic, difference 0.4 0.6 0.2 1.3
Table 111. Interfering Substances Interference at indicated temperature 150" C 175" C 2o0° c None Slight Definite Definite ' Slight
Concn., grams/liter 225" C Substance Urea 0.5000 Definite Oxalic acid 1 ,0506 DL-Malic acid 1.1174 Malonic acid 1 .ON6 Definite Citric acid 1.6010 Tartronic acid 1.oooO Definite 2.8525 None Sucrose Sodium nitrate 0.1000 Sodium nitrite 0.05oO ' Substances showing no interference at temperatures tested: calcium acetate, formic acid, methanol, butyric acid, o-phthalic acid, benzoic acid, chloroform, acetic acid, picric acid, sodium bisulfite, sodium chloride. '6
506
ANALYTICAL CHEMISTRY
main peak. This does not prevent accurate measurements of carbonate carbon provided sufficient time is allowed between analyses. An unstable or drifting base line was observed with substances like malic acid, which decompose more slowly. Nitric and nitrous acids o r their salts were found to interfere at a temperature of 175’ C or higher. Larger concentrations than those indicated in Table I11 will interfere at 150’ C. Either of these compounds produces a sharp peak that coincides with the normal carbon dioxide peak. It is probably caused by nitrogen dioxide or nitric acid, both of which absorb infrared radiation in the carbon dioxide region. It is unlikely, however, that concentrations greater than those tested will be encountered in the analysis of natural waters. Those substances which gave no interference at the temperatures tested are included in Table I11 also. ~~
ACKNOWLEDGMENT
The authors thank William E. Hatton for his valuable technical assistance in the assembly of the apparatus used in this work. A patent application has been filed on certain features of the apparatus and procedure described herein. It is intended that an instrument incorporating these features will be made commercially available in the near future.
RECEIVED for review September 22,1966. Accepted February 1, 1967. Division of Analytical Chemistry, 152nd Meeting, ACS, New York, N. Y., September 1966. ~~
~
Transfer Coeff icielnts and Heterogeneous Rate Constants for Cadmium(l1:) in Various Supporting Electrolytes Evaluated by Phase-Angle Measurements with an A.C. Polarograph J. K. Frischmann and Andrew Timnick Departm -nt of Chemistry, Michigan State University, East Lansing, Mich. 48823
Transfer coefficient!; evaluated for the reduction of l.OmM Cd(ll) at the dropping mercury electrode at 24O & lo C for the following supporting electrolytes, 0.5M Na2S04,1.0M Na,S04, 0.5M H2S04,1.OM H2SOI, 1.OM NaC104, 1.0mM HCIO.I, 1.OM KNOI, 1.OM HCI, 1.OM NaCI, 1.OM KCI were: 0.21, 0.20, 0.27, 0.24, 0.31, 0.26, 0.50, 0.42, 0.46,0.44, respectively and the values for the heterogeneous rate constants, kr, for the same order of the above supporting electrolytes were: 0.076, 0.063, 0.14, 0.12,0.34,0.25,0.63,0.94,1.2,1.2, cm per sec, respectively. Straight line cot Q plots, with intercepts cot Q = 1, extrapolated to zero frequency, were obtained through the frequency rangci 30-1100 cps for all supporting electrolytes containing Cd(ll). The heterogeneous rate constants decrease with double-layer capacitance of supporting electrolyte salts and this correlation arallels the decrease in adsorbability of the electroyte anions on the mercury electrode.
P
DURING RECENT YEARS a number of methods have been developed for the evaluation of the heterogeneous rate constant, kh, and the transfer coefficient, CY,for an electrode reaction. The newer methods are usually tested by measuring these electrode kinetic parameters for the reaction of Cd(I1). Numerous studies of the reduction of Cd(I1) present in various supporting electrolytes by various methods and different investigators have yielded a range of values for a and k h (1-10). (1) H. H. Bauer and P. J. :Eking, ANAL.CHEM.,30,341 (1958). (2) H. H. Bauer and P. ,J. Elving, J. Am. Chem. SOC.,82, 2094 (1960). ( 3 ) H. H. Bauer and P. J. Elving, ANAL.CHEM., 30, 334 (1958). (4) B. Breyer and H. H. E:auer, Australian J. Chem., 9,425 (1956). (5) T. Biegler and H. Laitinen, ANAL.CHEM.,37,572 (1965). (6) W. F. Head, Anal. Chr’m. Acra, 23, 294 (1960). (7) P. J. Lingane and J. H. Christie, J . Electroanal. Chem., 10, 284 (1965). (8) R. S. Nicholson, ANAL.CHEM.,37, 1351 (1965). (9) Y.Okinaka, T a h t a , 11, 203 (1964). (10) N. Tanaka and R. Tamamushi, Electrochim. Acta, 9, 963 (1964).
Recently, Bauer (11) has presented an explanation for the range of values of a and kh reported for aqueous Cd(I1). The original equations for the a.c. wave were derived for diffusion to a plane electrode by Matsuda (12) and more recently Delmastro and Smith (13) derived equations for diffusion to a spherical electrode. From these equations it is apparent that the electrode kinetic parameters can be evaluated by alternating current measurements, but the work involved is tedious. A more direct approach is through the measurement of the phase angle between the faradaic alternating current and the applied a.c. potential. This study was undertaken to evaluate a and kh for the reduction at the DME of Cd(I1) present in various supporting electrolytes by phase-angle measurements. THEORY
On the basis of Matsuda’s theory (14)for a diffusion and charge transfer controlled (quasi-reversible) electrode process, O+ne$R Tamamushi and Tanaka (15) showed that the phase angle, 4, can be expressed as
where D = DoaDRa
(11) (12) (13) (14)
H. H. Bauer, J. Electroanal. Chem., 12,64 (1966). H. Matsuda, 2.Elecktrochem., 61, 489 (1957). J. D. Delmastro and D. E. Smith, ANAL.CHEM.,38,169 (1966). H. Matsuda, Z. Elecktrochem., 62,977 (1958). (15) R. Tamamushi and N. Tanaka, 2.Phys. Chem., N.F., 11, 89
(1959). VOL. 39, NO. 4, APRIL 1967
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