Determination of Carbon Dioxide in Water Improved Evolution Method FRANK E. CLARKE, U . S . Naval Engineering Experiment Station, Annapolis, M d . This paper describes a simplified procedure and improved apparatus for abcurate determination of carbon dioxide in industrial waters. Carbon dioxide is evolved and circulated in a closed system and absorbed in barium hydroxide solution, and the residual barium hydroxide is titrated with hydrochloric acid. The apparatus is durable and readily available. The precision and accuracy are excellent and approximately constant throughout the range of carbon dioxide concentrations encountered in industrial waters. The time required for a single determination approaches that of less accurate routine control methods.
A
NALYSES of industrial waters frequently require accurate determinations of both free and combined carbon dioxide in the presence of interfering contaminants and in concentrations of total carbon dioxide ranging from a fraction of a part to 100 or more parts per million. Typical examples of problems which require reliable carbon dioxide evaluations are the correlation of corrosivity of condensate lines with concentration of carbon dioxide in the condensate, determination of exact ratios of potential alkalinity to hardness in high-pressure-boiler feedwater, and measurement of carbonate-hydroxide ratios in boiler waters. One investigator (a) has expressed the opinion that valuable information concerning condensate-line corrosion can be obtained by measuring carbon dioxide concentrations as low as 0.25 p.p.m. The limitations of titration procedures for determining carbon dioxide have been well established. Even the excellent procedure published by McKinney and -4morosi (6) suffers from the fact that it does not yield satisfactory evaluation in colored waters or in waters containing high concentrations of ferrous ion or volatile contaminants such as ammonia, hydrogen sulfide, and sulfur dioxide. Constantino (3) resorted to the important improvement of circulating evolved carbon dioxide in a closed system containing a barium hydroxide absorber and titrating the residual barium hydroxide with acid. This plan later was used by Lescoeur and Manjean (4)and by Partridge and Schroeder ( 7 ) , who incorporated it in an improved method specifically designed for analysis of boiler waters. The last' variation was adopted by the American Society for Testing Materials as a standard method for determination of carbon dioxide in industrial waters (1). The evolution method described in this paper is an outgrowth of that proposed by Partridge and Schroeder. DEmLOPMENT OF METHOD
The improved evolution method was developed to meet the need for a simple, rugged laboratory apparatus capable of yielding rapid, accurate determinations of carbon dioxide in all types of industrial waters and in the concentrations normally encountered. Existing evolution methods employed circulating pumps which were neither durable nor readily available and which lacked provisions for working in a completely closed system. The last limitation is particularly objectionable. When the sample is heated, the pressure in the closed system increases. If the system cannot function a t this higher pressure, part of the contained gases must be vented with the loss of a portion of the carbon dioxide. These deficiencies were overcome, through the use of a reliable, commercially available circulating pump and a rubber expansion bladde?.
ber bellows and valves and has a corrosion-resisting interior. The pump is available for operation on direct current circuits of 6, 12, 24, and 32 volts, and can be operated satisfactorily from alternating current circuits by use of a rectifier. Though designed for circulating fluids such as gasoline, it will pump gases against liquid heads far in excess of those required in this determination. The data presented herein were obtained with a 6 volt pump operated from a storage battery. Pressure rise in the closed system was limited by reduction of free air space to a minimum and incorporation of an expansion bladder. A 300-ml. evolution flask provided sufficient space above the 200-ml. liquid level for boiling and air scrubbing but contributed no excess air t o increase the system pressure. The expansion bladder was made from a 45-cm. (18-inch) length of Gooch-crucible tubing, sealed a t both ends with No. 10 rubber stoppers and fitted with Drechsd bottle-type, glass inlet and outlet tubes. This bladder was installed between the absorption tube and the pump inlet, the point of minimum pressure in the system. This combination of minimum volume and expansion bladder provided a completely satisfactory closed system. Preliminary trials revealed that the relatively rapid circulation provided by the pump tended t o carry over a fog of acid-bearing condensate through the water-jacketed condenser. Some of this fog condensed in the circulating line before reaching the absorption tube. As a result, slugs of acidic condensate periodically yielded erratic results. This carry-over and the accompanying errors were eliminated completely by substitution of a double condenser of the Friedrichs type and installation of a chromicsulfuric acid trap on the outlet of the condenser. It was later discovered that this trap removed certain interfering contaminants from the gas stream (ammonia and sulfur dioxide) and that supplementary traps could be inserted in series for removing other contaminants, such as hydrogen sulfide, without reducing the effectiveness of the pump. A barium hydroxide concentration of approximately 0.03 N was selected for absorbing the quantities of carbon dioxide concerned (150 p.p.m. or less). , The concentration of the titratin acid (hydrochloric) was increased to 0.04 N to sharpen the en! point. This also ensured that a single buretful of acid would suffice for 50 ml. of barium hydroxide in blank determinations a t the beginning of each run or with extremely low concentrations of carbon dioxide. The phenolphthalein indicator solution was added to the absorption tube rather than to the barium hydroxide or hydrochloric acid reservoirs to prevent deposition of the indicator on the burets and conse uent poor draining and erratic results. Sulfuric acid was usedlor liberating carbon dioxide in the evolution flask. Samples were heated t o boiling in the evolution flask before starting the pump to avoid cooling the water with circulating air and thus lengthening the period required to ensure maximum dissolution of the carbon dioxide. Circulation for 5 minutes after the sample began boiling was found to yield complete absorption of carbon dioxide. A Bunsen burner proved t o be more practicable than an electric heater for heating the evolution flask, since it permitted both rapid initial heating to boiling and low heating to maintain ebullition but prevent priming during the circulating period. The details of the procedure evolved by this investigation are given below. APPARATUS
An electric fuel pump was selected after trial and rejection of several types. This small pump is equipped with synthetic rub-
The apparatus is illustrated in Figure 1.
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V O L U M E 19, NO. 1 1
890 The relatively small diameter of the absorption tube, H , was selected to provide maximum practicable depth for 50 ml. of barium hydroxide solution. The length of the tube is necessary to allow for titrating the barium hydroxide without flooding the outlet tube. The liquid trap contains concentrated chromicsulfuric acid and can be supplemented with additional liquid traps for removing unusual contaminants. The scrubbing solution should be replaced whenever it becomes cloudy, intolerably diluted, or discolored. The assembly of apparatus can be supported from two ring stands. For permanent installation, an inverted U made from 1.25-em. (0.5-inch) rod of suitable length, with the open end welded to a heavy base, provides a compact and stable support. I t is convenient to support the stock bottles of barium hydroxide solution, sulfuric acid, and hydrochloric acid on a shelf above and behind the apparatus.
ample for most industrial waters. If the alkalinity of the sample exceeds 10 equivalents per million (0.01 S), introduce methyl orange indicator with the sample, titrate t o the end point of the indicator, and add an excess of 5 ml. of sulfuric acid before heating the sample. The pump must be operated during the titration. CALCULATIOh
Calculate total carbon dioxide by the use of the following equation :
1
REAGENTS
Barium hydroxide sclution, approximately 0.03 S,prepared by dissolving 5 grams of C.P. barium hydroxide octahydrate in 1 liter of carbon dioxide-free distilled water. Sulfuric acid, approximately 0.25 N,prepared by dissolving 7 ml. of C.P. sulfuric acid (specific gravity 1.84) in 1 liter of carbon dioxide-free distilled water. Hydrochloric acid, approximately 0.04 N , prepared by dissolving C.P. hydrochloric acid (specific gravity 1.19) in carbon dioxide-free distilled water and accurately standardizing the solution against sodium hydroxide which in turn is standardized against potassium acid phthalate. Phenolphthalein indicator, 0.57, alcoholic solution, prepared by dissolving 0.5 gram of C.P. phenolphthalein in 50 ml. of 95% ethyl alcohol and diluting the solution to 100 ml. with distilled water. The stock bottles for hydrochloric acid, sulfuric acid, and barium hydroxide are equipped with hscarite-protected aspirator bulbs and are attached t o their burets with Ascarite-protected siphons which automatically zero the burets.
A,
B
0
7
PROCEDURE
Determination of Blank. Boil vigorously 400 ml. of distilled water for a t least 15 minutes to remove dissolved carbon dioxide. Add 4 or 5 drops of phenolphthalein indicator to the clean absorption tube, H , measure int'o it exactly 50 ml. of barium hydroxide solution, and place the tube securely on its stopper as shown in Figure 1. Pour approximately 200 ml. of the freshly boiled distilled water into the evolution flask, G, fix the flask securely on its stopper, support it Kith the ring, I , and add 15 ml. of sulfuric acid from buret B. Start the cooling water flowing in the condenser, C, light the burner, and when the sample in the evolution flask begins to boil, start air circulat'ion with the pump, D. . Starting and stopping the pump several times in rapid succession at the beginning of the circulation will prevent priming while equalizing pressures. Continue the boiling and air circulation for 5 minutes and then, without shutting off either the burner or the pump, titrate the barium hydroxide in the absorption tube with hydrochloric acid from buret A . For accurate results the end point of t'he titration must be approached slowly or it will be overrun. Circulation should be continued for several minutes' after the end point has been reached to ensure that the pink color of the indicator does not reappear. Record the milliliters of standard hydrochloric acid required for the titration as W . The blank should be determined each day and should be repeated until duplicate determinations agree within 0.1 ml. using 0.04 N hydrochloric acid. Determination of Carbon Dioxide. Prepare the absorption tube as in the determination of the blank and measure 200.0 ml. of sample into the evolution flask. With the absorption tube and evolution flask in place, acidify and heat the sample; circulate the confined gas; and titrate the residual barium hydroxide by the exact procedure prescribed for determination of the blank. Record the milliliters of standard hydrochloric acid required in the titration as S. The titrations should agree within 0.1 ml. using 0.04 N acid. If the carbon dioxide content of the sample exceeds the absorption capacity of 50 ml. of barium hydroxide (165 p.p.m. for 0.03 -V barium hydroxide), a proportionately smaller sample should be taken and appropriate adjustments in the calculations should be made. Very lo\$; concentrations of carbon dioxide can be analyzed accurately by using a larger sample in an appropriately larger evolution flask. Unless the evolution flask is filled to within 100 t o 150 ml. of its capacity, excessive pressure will develop in the system. Enlarging the bladder to relieve such pressure increases the time required for complete absorption. A fixed quantity of sulfuric acid (15 ml.) is prescribed for all samples in the interest of speed and simplicity. This quantit,y is
50 1
Ill UI
4
I
Figure 1.
Improved Evolution Apparatus
Buret, 50 m l . (HCl) Buret, 50 m l . (HzSOa) C. Double Friedrichs condenser (made t o order by Ace Glass Co., Vineland, N. J. Any efficient colidenser c a n be used) D . Electric fuel p u m p (Autopulse Model 500 p u m p , Autopulse Corp.Lz8?1Brooklyn Ave., Detroit, mlcn.) E . Expansion bladder, 45-cm. (18-inch) length of 5-cm. (2-inch) Gooch crucible tubing o n No. 10 rubber stoppers, smaller diameter outermost F . Heat barrier, 2.5-cm. (1-inch) Transite board G. Erlenmeyer evolution flask, 300 m l . H . Absorption tube, 3 X 30 o m . heavy-walled test tube I . Ring support, removable a n d adjustable i n height J . Liquid trap K. Rubber hands for maintaining air-tight seals L . Bunaen burner
A. B.
N O V E M B E R 1947
891
Table I. Analysis of Known Solutions by Improved Evolution Method Known Concentration Carbon Dioxide, P.P.RI. 0.5 1.0 2.0 5.0 1 0 .0 25.0 50.0 100.0
Carbon Dioxide Found, P . P . M . Run 1 Run 2 Run 3 Mean 0.60 0.53 0.30 0.69 1.09 2.00 4.84 10.22 25.09 49 54 100.05
0.99 1.85 4.94 10.07 25.10 50.50 99,99
1.19 1.95 4.99 9.93 25.35 50.25 100.09
1.09 1.93 4.92 10.07
25.18 50.10 10.0.04
Table 111. Reproducibility of Determinations" (Summary of eight values) Blank without Blank with Value
Acid Trap Acid Trap M i . HCi/BO M1. B a ( O H ) *
Mean 33.70 Range 0.59 Standard 0.18 deviation Two blank determinations made with solutions.
33.73 0.08 0.03
Known 50 P.P.M. Solution, Con, Found P.p.m 49.87 0.96 0.32
different barium hydroxide stock
Table 11. Analysis of Contaminated Solutions Containing 50 P.P.M. of Carbon Dioxide Contaminant, 10 P P A I . Concentration 90% HzS "3
Carbon Dioxide Found, P P . M . Run 1 Run 2 Mean 50 76 50.62 50.47 49.90 50.86
49.67 50.32
49.79 50.59
where
N W
=
analysis of natural waters. Duplicate consecutive values for each contaminated solution are shown in Table 11.
normality of hydrochloric acid
50 ml. of barium hydroxide solution (blank) S = milliliters of hydrochloric acid required for titration of residual barium hydroxide solution after absorption of carbon dioxide evolved from the sample (carbon dioxide) = milliliters of hydrochloric acid required for titration of
The proportions of carbonate, bicarbonate, and free carbon dioxide can be calculated from the total carbon dioxide and pH by the method proposed by McKinney ( 5 ) . EXPERIMENTAL DATA
The precision and accuracy of this procedure were evaluated using solutions of known carbon dioxide concentrations which ranged from 0.5 to 100 p.p.m. (Table I). The test concentrations shown n-ere obtained by pipetting the required volumes of pure sodium carbonate stock solution into the evolution flask and diluting t o 200 ml. with carbon dioxide-free water. Pure sodium carbonate was obtained by heating recrystallized sodium bicarbonate overnight in an oven thermostatically controlled a t 265" C. The stock solution was prepared t o contain 1000 p.p.m. of carbon dioxide by. dissolving the pure sodium carbonate in carbon dioxide-free distilled water. This stock was stored in an Ascarite-protected bottle equipped with an automatic buret. Concentrated stock was employed t o reduce the possibility of error from absorption of atmospheric carbon dioxide which frequently results if dilute stocks are used. The water used for preparing the sodium carbonate stock, for determining the blanks, and for diluting the test samples was prepared by boiling laboratory-distilled water for 20 minutes and was stored in an Ascarite-protected bottle with an automatic siphon. I n each case the triplicate values shown in Table I indicate three consecutive runs. When the study summarized in Table I was completed, the ability of the procedure to yield accurate carbon dioxide evaluations in the presence of common contaminants was determined. Standard carbonate solutions of the type described above, containing 50 p.p.m. of carbon dioxide and 10 p.p.m. of selected contaminants, were employed. Preliminary tests made r i t h the regular concentrated chromic-sulfuric acid trap revealed that no error resulted from the presence of sulfur dioxide or ammonia but consistently high results were obtained for samples contaminated with hydrogen sulfide. Tests with lead acetate paper in different parts of the apparatus revealed that hydrogen sulfide was neither completely oxidized by the liquid trap nor completely absorbed in the barium hydroxide. T o overcome the error due to this contaminant, an additional trap (not shown in Figure 1) containing 0.1 S iodine solution was inserted between the condenser and the regular acid trap. The fact that ammonia gas probably was not liberated from the acidified evolution flask is appreciated; however, this same condition would exist in the
DISCUSSION OF RESULTS
I t can be seen from Table I that the precision and accuracy of this procedure are excellent and approximately constant a t all concentrations from 0.5 to 100 p.p.m. Even a t the concentration of 50 p.p.m. where the greatest dispersion occurred, the maximum deviationfromthemeanwas0.5 p.p.m. Table IIIshoivsthat eight determinations a t this concentration yielded a standard deviation of only 0.32 p.p.ni. The extremely low standard deviation shown for the acid-scrubbed blank determinations confirms the general soundness of the procedure. Table I11 also indicates that a considerable portion of the already small deviation obtained in the sample runs resulted from unavoidable analytical errors, such as errors in pipetting the concentrated sodium carbonate stock. The importance of properly t,rapping entrained acid is shown by the wide standard deviat,ion recorded in Table I11 for the blank determinations made without the acid trap. Durability of Apparatus. During the investigation described in this paper, more than 400 separate determinations were made with the apparatus over a &month period. During this time there m-as no evidence of fatigue or failure of the pump or deterioration of the expansion bladder. Such performance probably would provide for at least a year of trouble-free carbon dioxide analysis in the average laboratory. COI\-CLUSIONS
The improved evolution method discussed in this paper is capable of accurately determining carbon dioxide in all concentrations normally encountered in industrial waters despite the presence of common colored or volatile contaminants. The equipment required is inexpensive, readily available, and durable, and is designed for operation in a completely closed system. An inexperienced technician can learn to apply the method satisfactorily in a few minutes; a single determination can be completed in approximately 15 minutes. Thus the method approaches the speed of less reliable rout,ine control methods while yielding the high precision required of a referee method. ACKNOWLEDGMENTS
Acknowledgment is made t,o C. Herbert Pund, Jr., who collected the major portion of the data required for the evaluation of this method and to Robert C. ildams for helpful suggestions concerning the preparation of this paper. LITERATURE CITED
(1) Am. Yoc. Testing Materials, Standard D51343. ( 2 ) Collins, L. F., Detroit Edison Co., private communication. (3) Constantino, A , , Atti accad. Lincei, 28, 11, 118-21 (1919). (4) Lescoeur, L., and Manjean, S., Bull. SOC. chim. bid., 10, 523-3;
(1928). (5) McKinney, D. S.,IND. ENG.CHEM..i l N a L . ED.,3, 192-7 (1931). (6) McKinney, D. S., and Amorosi, A. M., I b i d . , 16, 315 (1944). (7) Partridge, E. P., and Schroeder, W. C., I bid., 4, 271-84 (1932). RECEIVED May 20, 1947. The opinions expressed in this paper are those of the author and are not necessarily official opinions of the U. S. N . Engineering Experiment Station or the Kavy Department.
