remain firmly bound in solution as ethylenediamine hydrochloride. After cooling in ice, the solution was filtered under suction and the filtrate was titrated with hydrochloric acid. Partially carbonated sodium hydroxide solutions were also analyzed by this method. DISCUSSION
Successive analyses for carbon dioxide were reproducible to within better than 0.2%. Sources of error include those usually associated with volumetric measurements as well as the negligible error caused by barium carbonate solubility near 0 "C [solubility product 7 x 10-9 (g ion/l.)2 at 16 "C] (4). On this basis, the maximum relative error of a determination is =k0.5%. The time spent in one determination of both the amine and carbon dioxide concentration of a single sample was about 15 minutes and somewhat less when multiple samples were analyzed. Apart from its usefulness in carbon dioxide analysis, the method can be extended to the determination of any acid gas in solution, existing in the free state or as a thermally unstable compound. The sole requirement is that a suitable metal salt be used to form an insoluble precipitate with the gas. For (4) Handbook of Chemistry and Physics, 48th Edition, Chemical Rubber Publishing Co., Cleveland, Ohio, 1967.
example, hydrogen sulfide in amines and hydroxides can be determined in a completely analogous manner, using the chloride of cadmium, lead, manganese, or mercury, the sulfides of these being insoluble. The method can also be used to determine both carbon dioxide and hydrogen sulfide if these are present together. Carbon dioxide can be selectively precipitated with barium chloride (barium sulfide being soluble) and removed from solution. Both gases form insoluble precipitates with any of the above metals allowing the determination of total acid gas; hence, no complication arises from the presence of both gases. A further advantage of the technique is the removal of the suspended iron and sludge commonly found in gas sweetening units, which would otherwise prevent the use of visual indicators for titration. The inherent precision and rapidity of this procedure combined with complete freedom from the need for any special equipment recommend the method for the determination of acid gases both singly and as mixtures.
RECEIVED for review March 14, 1969. Accepted May 15, 1969. One of the authors (R. H. W.) gratefully acknowledges the financial support of the National Research Council of Canada.
Determination of Carbon Dioxide in Aqueous Solutions under Pressure Paul B. Stewart, Prem K. Munjal,' and Frank QuiringZ Sea Water Conversion Laboratory, and Department of Mechanical Engineering, University of California, Berkeley, Calif. 94720
IN A RECENTLY COMPLETED STUDY of the solubility of carbon dioxide in sea water and sea water concentrates ( I , 2), standard inorganic analytical chemical procedures did not prove to be applicable. This was mainly because the samples were usually under pressure, from 5 to 50 atmospheres; the smallest sample that could be obtained conveniently was approximately 10 ml in volume; and the pressure in the sample container effectively removed the possibility of taking aliquots. The method and apparatus described in this paper are further developments of the work of Etienne and Mather (3),developments to accommodate the original method to the large sample size and increased quantity of COz (in most cases), and plumbing improvements. The C02 in the sample is volatilized by boiling in the presence of strong mineral acid, H2S04,and sweeping the boiling solution with COz-free air. The gas thus evolved is absorbed in a known excess of strong base, Ba(OH)2,and the excess base then back titrated with standard acid. Both the desorption of the COz from the sample and the subsequent absorption in the base are slow processes requiring much care to be carried out quantitatively. 1 Present address, System Development Corp., 2500 Colorado Ave., Santa Monica, Calif. 90406. Present address, Clayton High School, Clayton, Mo. (1) Prem K. Munjal, Ph.D. Dissertation, University of California, Berkeley, Calif., University Microfilms Publication No. 67-5126, Ann Arbor, Mich., 1966. (2) Paul B. Stewart and Prem K. Munjal, Sea Water Conversion Lab. Report 69-2, University of California, Berkeley, Caif., 1969. (3) F. S. Etienne and E. R. Mather, J. Assoc. OBc. Agr. Chem., 39, 844 (1956). 1710
ANALYTICAL CHEMISTRY
Sample Container. The Monel sample tubes, 10-ml capacity, and the '/*-inch needle valves on each end are both manufactured by Hoke, Inc., Englewood, N. J. The needle valves required lapping-in with a diamond lapping compound and special lapping tools, fabricated in university shops for this job, to ensure leak-tightness at the higher pressures. Apparatus. The apparatus developed is shown schematically in Figure 1. Capacities of the various glassware components are given in Table I. All glassware connections from the sample tube through the displacement flask to the absorbers were standard ground glass stopcocks (valves in Figure 1) or ground-glass ball joints. These ground-glass fittings were lapped in to assure tightness and sealed with vacuum lubricant. Reagents. The necessary reagents are solutions of Ba(0H)z and hydrochloric acid, O.lN, 0.2N, 0.3N, and 0.4N. The HCI solutions used were prepared from Bio-Rad standardized capsules, the Ba(OH)2 solutions from crystalline analytical reagent grade material, standardized against the HCI. The Ba(OH)z solutions were stored in borosilicate glass bottles, 4-liter capacity, from which they were syphoned to burets as needed; both storage bottles and burets were fitted with soda-lime tubes to prevent contact with C02 in the air. Procedure. All glassware components of analytical train are first disassembled as required, and thoroughly cleaned, with hydrochloric acid if necessary to dissolve carbonate, and rinsed with distilled water. The various units then have the following quantities of solutions put in them: 3-neck flask, about 100 ml of approximately 0.5N H2S04 (a 2-fold excess); washer column, distilled water, filled; absorber T3, Ba(OH)2, 20 ml of known concn; absorber Tz, Ba(OH)Z, 50
Table I. Capacity of Glassware Components (Figure 1) Capacity rnl Component Letter symbol 500 CO, Absorber TI 250 CO, Absorber T2 40 CO, Absorber T3 150 3-Neck RB flask 250 Liquid trap L1 250 Liquid trap Lz 500 Liquid trap L3 60 Washer column ... Table 11. Proof of Analytical Method
152.23
152.15
4
237.72 383.51
237.60 383.62
5
470.17
470.01
Percentage error 0.06 0.05 0.05 0.03 0.03
6
620.39
620.62
0.04
Sample 1 Figure 1. Assembly of analytical train
ml of known concn; and absorber TI, Ba(OH)2, 100 ml of known concn. The concentration of Ba(0H)Z used depended upon the quantity of COz expected in the sample, and was sufficiently high so that complete absorption should take place in the first absorber (trap). The units are then reassembled, and all connections checked for tightness. The Hoke tube containing the sample is connected in at stopcocks (valves) VZand V3. The cooling water to the condenser is started, stopcocks V6,V,, Vs, V s , Vll, V I Sand , VI6 are then opened, all others are closed, and a vacuum is applied to the system by starting the water flow in the aspirator. The application of the vacuum, and the resultant air flow, must be done very carefully to prevent flow surges. After the flow is established, the vacuum is maintained in the 1-3” Hg range keeping the froth in the absorbers below 1 cm in height. The sample is then transferred to the boiling flask by fully opening stopcocks V4 and V s and using V3as a control valve. The sample is admitted at a sufficiently slow rate so that the bubbling of the air in the flask does not stop. Were this bubbling to cease, there is the probability that some of the H2S04 and COz-sea water mixture would be driven over into the liquid trap LZwhich would spoil the determination. After most of the sample is transferred to the boiling flask and Va is fully open, the air sweeping is continued for approximately one hour. During this period the vacuum must be checked periodically as the air flow resistance gradually increases because of the precipitation of BaCO,. Stopcock V6 is then closed, causing an increase in the system vacuum, and then V I and Vz are opened to sweep any COz remaining in the sample tube into the flask. This sweeping is continued for another hour with periodic checking and adjustment of the applied vacuum. The next stage in transferring the COSto the Ba(OH)z absorber is to boil the contents of the 3-neck flask while continuing the air sweep and the application of vacuum; this again lasts for an hour and periodic checks are necessary to make certain that everything is proceeding smoothly. This is followed by another hour of air sweeping without boiling. This 4-hour period for air sweeping and boiling may well seem to be an unnecessarily long time. Perhaps it is; this was not ascertained as two samples were customarily analyzed at the same time in duplicate sets of equipment, and there were invariably other tasks that were also being performed.
2 3
Wt COP,mg Taken Found 69.08 69.12
The completeness of the removal of COS from the flask is then checked by opening stopcocks VIOand VISand closing Vll, thus bypassing the first two absorbers and sending the air to the third absorber. An absence of a precipitate there indicates the complete transfer of COz. After this check the contents of the second absorber are transferred to the first. This is accomplished by decreasing the vacuum until the air flow is very nearly stopped, and slowly opening stopcock VI,. Absorber TZ is then washed several times with distilled water, introduced by using the rubber aspirator bulb with VI, being opened and closed intermittently. The washings are thus also added to TI, The first absorption trap, TI, is then removed from the assembly by disconnecting ball joints JIand Jz, and its contents are poured into a titration flask. The trap is thoroughly rinsed and the washings are added to the titration flask. The excess Ba(OH)z is then back titrated with standard acid using phenolphthalein as an indicator. Calculation. The weight of COZin the sample is calculated by the equation : COS = [(Vb)(No) - (VaNa)](22.0056) where: COz = wt of C o n ,mg Va,Vb = vol of acid and base respectively, ml
Na,Nb = normality of acid and base, respectively The analytical method and procedures were proved accurate by preparing, in the sample tubes used for the solubility measurement work, samples of synthetic sea water containing weighed quantities of pure COS. These results are given in Table 11. That the solubility measurements gave consistent results checking well with the more reliable values reported in the literature is further verification of the accuracy of the method. RECEIVED for review January 21, 1969. Accepted July 10, 1969. Work was supported by funds allocated by the Water Resources Center, University of California. Frank Quiring’s work was sponsored by a National Science Foundation Research Participation Grant for High School Teachers.
VOL. 41, NO. 12, OCTOBER 1969
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