Therefore, w > rn for any time when weight loss occurs. Potential errors due to this are serious for polytetrafluoroethylene a t certain stages of decomposition (4). To measure this effect, film samples of polymer were subjected to thermogravimetry while supported both above and below the sample pan. Results of these experiments showed that molecular drag played an important role. T o approximate the magnitude of molecular drag, 7-mg. films of polymer were decomposed with linear programmed temperature rise while being supported at the top surface of the reference pan, and a b v e , but not touching the sample pan, with no sample in the sample pan. Results of these experiments are shown in Figure 2. The results indicate that errors are caused by lift effects caused by molecules which migrate to regions below the
pan as they are pumped p a t pan and support wire. With sample placed on the reference pan, more molecules would have found their way to this region. Improved design could probably reduce such errors and faster pumping could reduce the tendency of molecules to migrate helow the pan. Increasing the space between the furnace tube and the pan could be helpful, as could reducing the surface area of the pan when possible. Concurrent rapid pumping from both the top and bottom of the furnace tube could be beneficial. Because it was not possible to reprw duce molecular lift effectswhich occurred during experiments designed to memure momentum transfer effects, the experiments described herein were not able to help in the evaluation of momentum transfer. Momentum transfer effects can be calculated in some cases.
LITERATURE CITED
(1) Cahn, L,,sehultz, H,, cHEM 35, 1729 (1963). (2) C??tiss,xr">;""" C . F., ,Z,L University ^,^of Wis.
~"-
munieation, 1963. (3) Duval, C., "Inorganic Thermogravi-
metric Analysis," Elsevier, Amsterdam, 1963.
(4) Friedman, H. L., U. S. Air Force
Materials Lab. Rept. ML-TDR-64.274, August 1964. (5) "Vacuum Microbalance Techniques," M. .l. Kata. ed.. Vol. 1 . Plenum Press.
HENRYL. FRIEDMAN Space Sciences Laboratory General Electric Co. King of Prussia, Pa. WORKperformed under auspices of U. S. Air Force Materials Laboratory under contract ?To. AF 33(657)-11300.
Elimination of Carbonates from Aqueous Solutions Prior to Organic Carbon Determination C. E. Van Hall,
Dennis Earth, and V. A. Stenger, Special Services laboratory, The Dow Chemical Co., Midland, Mich.
N THE DETERMINATION
bustion-infrared method ( l o ) ,the result obtained for total carbon includes inorganic ss well m organic carbon. Because carbon dioxide and carbonate are not considered as pollutants in waste water analysis and are not included in methods based upon the consumption of a n oxidant, it is desirable to eliminate , I ~ c-... . ~ ~ - ~ ~ n L . 1-2 L-. 1L^ _-__:-I me.= irom TBUIM r e p ' MU U J YLK ~ n p u method., Procedures for avoiding the effect of carbonate are also of interest in connection with other methods of analysis for total carbon. Several possibilities exist: carbonate may be precipitated with barium hydroxide (3); a separatedetermination of carbon dioxide may be made (6, 8,9)and a correction applied; or carbon dioxide may be expelled by acidification and boiling or purging with an inert gas ( 5 ) . The last route seemed most promising in regard to simplicity and rapidity. Displacement of carbon dioxide with a carrier gas raises questions as to the most suitable conditions (acidity, flow rate, time) and the possibility of losing volatile organic matter. Although the method has been used, experimental data in the literature are meager (5, 10). Preliminary mention hss been made (7) of some work done under contract with the U. S. Public Health Service. Removal of carbon dioxide is accomplished ~~~~
grams of long-fiber asbestos in a porcelain dish with a solution of 20 grams of cobalt nitrate in 50 ml. of water. The mixture is evaporated to dryness, placed in a cool muffle, and gradually heated to 950" C. After an hour at 950" C., it is cooled and any large lumps are broken up. About a gram is added to the combustion tube in small amounts with forceps or tweezers to provide a loosely packed wad 4 to 6 em.
of carbon com-
I pounds in water by the rapid com-
~
in . ..
.._^_
Fiaure 1 .
Diffusion cell
Millimeter rule shown
readily, but losses of certain volatile compounds can occur. Therefore a diffusion technique was developed whereby volatile materials can he kept confined. This paper includes detailed procedures from the contract work, as well as some more recent experimental results. EXPERIMENTAL
Apparatus. The comhustion-infrared analyzer previously described (10) and a modified model constructed by neckman Instruments, Inc., were both used in this study. One change was made in thecomhustion tube packing-asbestos impregnated with cobalt oxide (1s) was substituted for the asbestos-platinum gauze system originally used. The new packing is prepared by treating 15
1mwt.h
With ".. thir "
.....
n a ~ l r i ~, na r_"
nn
platinum gauze is needed. The diffusion apparatus (Figure 1) is essentially an enlarged Conway cell (2) modified to permit sampling of liquid and vapor phases with syringes. The cell is made from 70-mm. o.d. borosilicate glass tubing closed nt the ends to form a compartment 6 em. tall. From the top, a 40-mm. 0.d. reservoir with a 9-mm. rim is suspended by a rod, so that the distance between the bottom of the cell and that of the inner reservoir is 3.0 em. One neck of 15-mm. diameter and two of 8-mm. are provided, the latter being fitted with No. 18 SS syringe needles (sealed in with epoxy resin) which accommodate Hamilton No. 1001 and No. 705 N syringes for sampling vapor and liquid. The larger neck is closed with a cork or rubber stopper wrapped in aluminum foil. The total internal volume of the cell is determined by calibration with water. An ordinary reducing valve and a Brooks flowmeter are used to control cylinder nitrogen for purging. Gas is dispersed in samples through a Corning No. 39533 EC glass frit bubbler tube. VOL. 37. NO. 6. MAY 1965
769
Reagents. Hydrochloric acid, concentrated, ACS reagent grade. Sodium hydroxide, 2001, solution of ACS reagent grade material in water . Salts used in the “standard water” and the other compounds tested were the purest grade available, usually ACS reagents. The standard water (11) had a p H of 9.0 and contained the following ion concentrations, expressed in milligrams per liter: 60 Ca, 25 Mg, 130 Na, 15 K, 20 NH,, 13 NOs, 169 c1, 200 HCOs, 100 SO,,50 sios, 25 P o 4 . Total carbon calculated from the bicarbonate added was 39.4 mg. per liter. That found by the combustion-infrared method was 41 mg. per liter, the difference probably representing carbonate impurities in the other salts used. Procedure for Purging Method. If t h e sample contains suspended solids, it should be blended or otherwise homogenized to produce a uniform dispersion. Dilute the mixture if necessary so t h a t the organic carbon content is not greater t h a n 150 mg. per liter. Place 50 ml. in a 100-ml. low-form beaker and acidify to a p H between 2 and 3 . Generally 0.1 ml. of concentrated hydrochloric acid is sufficient for natural waters or municipal wastes. Purge with nitrogen for 3 to 5 minutes at 200 to 400 ml. per minute. Immediately analyze the solution for residual (nonvolatile) carbon by the combustion-infrared method (IO), using a 2 0 4 . sample. To find the initial carbonate plus volatile carbon content, subtract the result from the total carbon content as determined on a separate portion of the unpurged solution. Procedure for Diffusion Method. Place 2 ml. of 20y0 sodium hydroxide solution in the upper reservoir of a clean, dry, diffusion cell. Insert a magnetic stirring bar into t h e lower part of the cell and close the small necks with the appropriate syringes. With precautions t o avoid losing volatile organic compounds, prepare the sample as specified in the preceding method and measure 50 ml. into the cell. Add 0.1 ml. of concentrated hydrochloric acid, or enough to bring the p H to about 2, and stopper the large neck. Place the cell on the stirring stand and operate the stirrer for a t least 20 minutes. The magnetic bar should rotate three or more times per second,
V , = vapor syringe sample volume,
7
40
P1.
V a = volume of liquid in cell, ml. V , = volume of the overhead vapor space in ml., equal to total volume of cell-(volume of caustic solution volume of stirrer bar volume of sample solution).
+
2.20 L rn
”
-
-. /LIOUID
V8
where : Co = organic carbon, mg./liter in original solution; C1 = residual carbon, mg./liter in liquid after diffusion; C, = carbon, apparent mg./liter in vapor (uncorrected for sample volume) ;
0.01
4.0
4.4
0.8
nil
+
T o obtain a readable carbon dioxide peak by the combustion-infrared method when analyzing a sample from the vapor space over a dilute solution of volatile organic material, a larger syringe volume must be used than is ordinarily taken with liquids. The carbon concentration, as read from the calibration curve when analyzing a larger sample, must be corrected back to the basis of the aqueous volume used in obtaining the calibration curve, 20 pl. This is accomplished with the factor 2 0 / V , as a multiplier for C,, the apparent concentration. The second factor VD/V8relates the carbon in the vapor volume back to the original solution volume. RESULTS A N D DISCUSSION
“The standard water” was used to simulate, with respect to minerals content only, a typical municipal waste. It was free from organic carbon. Tests of the effectiveness of purging for carbonate removal were carried out by treating the standard water as in the procedure, with various amounts of acid and for definite periods at a nitrogen flow rate of 400 ml. per minute. Results for carbon remaining in the solution, found independently by two observers, are given in Table I. Because carbon dioxide is eliminated rapidly, it is difficult to reproduce results during the early stages of purging. From the dissociation constants of carbonic acid i t can be calculated that all carbonate in a dilute solution is converted to the acid form at a p H of 4.7 or less. The data of Table I indicate that at any p H value below 4, substantially all of the carbonic acid is eliminated by purging for 3 to 5 minutes under the given conditions. The rate of evolution of carbon dioxide seems slightly slower at the lower p H values. If this is so, it may be that excess acidity lowers the rate of dehydration of carbonic acid to carbon dioxide. From a practical standpoint, a p H range of 2 to 3 is recommended. The volume of concentrated hydrochloric acid added to attain a pH of 1.0 would result in a dilution error of 1% on any nonvolatile carbon in the solution. The amount of acid needed for a p H of 2.0 results in an error of less than 0.2y0, which is considered negligible. To find the optimum nitrogen flow rate, 50-ml. samples of standard water adjusted to a pH of 2.0 were purged at various rates. Samples were analyzed for carbon at regular intervals. Data given in Table I1 show that purging for
Table 111.
Table II. Effect of Nitrogen Flow Rate on Carbonate Removal at 25" C.
Flow rate, ml. per min. T?me of, purging, min. 1 2 3 5
Carbon, mg. per liter Taken Founda
Compound 100
200 300 400
Residual carbon,
- mg. per liter 16.9 9.6 3.8
nil
9.6 5.6 4.2 2.3 1.4 0.7
nil nil
nil nil
nil nil
3 minutes at a rate of 200 ml. or more per minute is sufficient to remove dissolved carbon dioxide. Several tests were carried out to determine whether there is a loss of certain volatile compounds during agitation with nitrogen. The compounds were dissolved in standard water or in pure demineralized water free from organic matter (Table 111). Each solution was adjusted to p H 2 and bubbled for 5 minutes at 400 ml. of nitrogen per minute. Except in the cases of acetaldehyde and acetone, compounds of high solubility in water are not vaporized appreciably from these dilute solutions, even though the compounds alone are volatile. On the other hand, substances of low solubility and moderately high vapor pressure are lost rapidly. The accelerated vaporization of a nonpolar compound from water, a logical consequence of Gibbs' adsorption rule and principles of steam distillation, has been pointed out previously (1). The reverse case, involving loss of water from a nonpolar solvent, has also been described (4). The data of Table I11 agree with the findings of Montgomery and Thom ( 5 ) that ethanol, phenol, and formic and acetic acids are not evolved at an appreciable rate when oxygen is passed through their aqueous solutions. Montgomery and Thom pointed out, however, that hydrogen cyanide is removed fairly rapidly a t 20" C. and p H 4.8; they suggested that this can be prevented by treatment with a small excess of silver sulfate solution before acidification. Tests of the diffusion method were performed in two ways. I n one series of experiments, 50 ml. of standard water was carried through the procedure and samples were taken periodically to show the changes of carbon dioxide concentration in liquid and vapor phases. Room temperature was about 25" C., but the solutions became somewhat warmer during magnetic stirring. The results (Figure 2 ) indicate that, under the authors' conditions, carbon dioxide diffuses out of the acidified sample solution and is completely absorbed by the aqueous sodium hydroxide within 20 minutes. In preliminary work ( 7 , 11) a 100-nil. sample was used in a corre-
Recovery of Compounds after Blowing at 25" C.
Recovery,
70
SOLUTIONS I N DEMINERALIZED W.4TER Acetaldehyde Acetic acid Acetone Benzene Benzoic acid Butyric acid Cyclohexane Ethanol Formaldehyde Formic acid Methanol Phenol Toluene
99.0 100.0 97.0 99.0 100.0 102 0 45 0 98 0 99.0
80.9(f0.9) 99.9(f0.3) 82.5(f0.5) 0.0 ifO.51 100.2 ( f 0 . 7 j 101.4(f0.6) 1.8( f 0 . 8 ) 95.7 ( f 0 . 3 ) 96.7 ( h 0 . 7 )
97.0 100.0 98.0
99.7(11.2) 2.4 ( 1 0 . 3 )
81.7 99.9 85.1 0.0 100.2 99.4 4.0 97.7 97.7 100.3 96.5 99.7 2.5
SOLUTIONS IN STANDARD WATER Acetone Benzoic acid Ethanol Methanol Phenol 0
25.0 25.0 25.0 25.0 25.0
23.7 ( f O . 7) 23.7 i l t 0 . 7 1 25.0 i f O . 5 ) 24.8 ( f 0 . 4 ) 24.7(10.7)
94.8 94.8 100.0 99.2 98.8
Average of four determinations; range also shown.
spondingly larger cell and a 35-minute period was required. I n the final tests, dilute stock solutions of benzene or acetaldehyde were prepared in carbon dioxide-free water and analyzed for total carbon by the combustion-infrared method. Results represent organic carbon content. The solutions were diluted further with standard water in the diffusion cell, so that carbonate was also present, and the organic contents were calculated from the dilution ratios. The samples were then taken through the diffusion procedure. The data so obtained (Table IV) illustrate the precision that can be obtained with careful work. Using both purging and diffusion methods, it is possible to analyze solutions containing both volatile and nonvolatile organic compounds in addition to inorganic carbonates. Carbon that is nonvolatile under the described conditions is given by the result on the purged sample, after purging to a minimum carbon content. Volatile organic carbon is given by the difference between this minimum and the total carbon found with the diffusion method. Carbonates are found from the total carbon content of the unpurged sample less the total found by diffusion. - i n amine would not be considered as volatile in this procedure as it is held by the acid solution. LITERATURE CITED
(1) Acree, F., Jr., Beroza, M., Bowman, 11. C., J . Agr. Food Chem. 11, 278 (1963). ( 2 ) Conway, E. J., "Microdiffusion
Analysis and Volumetric Error, Van Nostrand, Xew York, 1940. (3) Kieselbach, R., AKAL. CHEM. 26, 1313 (1954).
Table IV.
Analyses of Known Solutions by Diffusion Method
Organic carbon, mg./liter (liquid basis) Found Liauid Vaaor Calcuphke phase Total lated Benzenewater
Acetaldehydewater
6 . 0 4 . 5 10.5 1 0 . 3 5.3 4.4 9.7 9.7 5.7 4 . 4 10.1 9.8 58.5 39.5 98.0 98.2 57.0 38.1 95.1 95.2 56.0 3 9 . 5 95.5 9 5 . 0 9 . 5 0.2 9 . 7 10.0 9.9 0 . 2 10.1 10.0 9 . 8 0 . 2 10.0 10.0 97.3 1 . 0 9 8 . 3 98.4 96.0 1 . 0 97.0 97.6 97.5 1 . 0 98.5 100.0
(4) Meeks,
M. R., Whittier, V. E., Young, C. W., Ibid., 23, 792 (1951). (5) Montgomery, H. A. C., Thom, E.S., Analysc87, 689 (1962). (6) Pobiner, H., ANAL.CHEM.34, 878 (1962). (7) Schaffer, R. B., Analyzer 5 (4), 14 (1964) (Beckman Instruments, Inc.,
Fullerton, Calif.).
(8) Smith, J. B., Gilbert, E. K., Howie, M. P., ANAL.CHEM.26, 667 (1954). (9) Underwood, A. L., Howe, L. H., Ibid., 34, 692 (1962). (10) Van Hall, C. E., Safranko, J., Stenger, V. A., Ibid., 35, 315 (1963). (11) Van Hall, C. E., Stenger, T'. A,, R e p t . P H 86-63-94, to U. S. Publ. Health Serv., Nov. 15, 1963. (12) Vecera, M., Snobl, D., Synek, L., Mikrochim. Acta 1958, p. 9.
WORK initiated and performed in part under contract PH 86-63-94 with the U.S. Public Health Service, Robert A. Taft Sanitary Engineering Center, Cincinnati, Ohio. VOL. 37,
NO. 6, MAY 1965
8
771
