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E§ ~ E A ~ T l CLARK E ~ l BRICKER,2 and DAVID GARVINa Qe~~r~m ofeChemistry, n~ Princeton University, Princeton, N. 1. A method involving the oxidation organic compounds in water by btion with ultraviolet developed for the determination of trace amounts of organic material in wafer. Ferric sulfate is .used as a photosensitizes during the photolysis. The carbon dioxide obtained from the oxidation of the organic material present in the water is collected in a cold trap at liquid nitrogen temperature and then transferred to a mass spectrometer for analysis.
the radiation; peroxide was formed by the reaction of hydrogen atoms with dissolved oxygen and hydroxyl destroyed the peroxide. Thus, any organic material which consumed hydroxyl radicals in being oxidized enhanced the peroxide yield. As soon as the organic material was consumed, a constant rate of peroxide production ensued. A plot of the micromoles of hydrogen peroxide formed us. time yielded a straight line; the time corresponding to a change in the slope of this line which was coincident with the constant rate of formation was an extremely sensitive measure of the organic material present. This indirect determination of trace organic materials apparently has not been developed further. The purpose of this investigation was to deveIop a method which would be suitable for the determination of 1 p.p.m, or less of any carbon-containing compounds in water. The proposed method involves the oxidation of the organic material in the water to carbon dioxide by exhaustive irradiation with ultraviolet light. Ferric sulfate is added to the water as a photosensitizer. Following irradiation, the carbon dioxide is condensed in a cold trap a t liquid nitrogen temperature and transferred to a mass spectrometer for analysis. Although no attempt was made to elucidate the mechanism of the reaction, it is likely that hydroxyl radicals are the reactive intermediates in the oxidation of the organic material. The hydroxyl radicals may be produced by direct photodecomposition of water by ultraviolet light of 2000 A. or shorter. Dainton has pointed out that iron ions may induce this decomposition with 2537-A. radiation (3). Equations which may represent the over-all stoichiometry of the reactions for the photolytic oxidation of isopropyl alcohol, for example, we:
HERE appears to be a t present no sensitive analytical method for the determination of trace amounts of organic impurities in water, Although classical wet combustion methods continue to be studied and improved and sensitive tests for specific compounds and classes of compounds have been developed (8, 7, 8), no general method exists for the determination of 1 p.p.m. or less of total carbonaceous materials in water. Probably the most sensitive existing method for this purpose is applicable in the range of 5 p.p.m. and involves an aqueous chromic-sulfuric acid oxidation in series with a cupric oxide vapor phase oxidation a t 950’ C., followed by absorption on Ascarite and weighing of the carbon dioxide produced (6). Allen and coworkers (1) studied the decomposition of water purified by various means by fast neutrons and gamma radiation. Carbon dioxide was produced by the irradiation of even the most carefukly purified water. These authors concluded that the carbon dioxide did not come solely from organic impurities in the water but presumably from the walls of the reaction vessel FS well. This concluaion was reached because different silica ampoules in which the experiments were performed produced varying amounts of carbon Hi0 Rv He *OH dioxide. Allen and Holroyd (9)have reported Ha H e He that the initial rapid rate of peroxide (CIJs)&HOH -I-18(*OH)e production in aerated water when ir13Hn0 3COn with Cot0 rdiatian appears e to organic ~ ~ u ~in tthe~ e The s actual reactions involved in these water. The water was photolytic oxidations are undoubtedly numerous and complicated and, if
+
+
p
+
considering the final balanced equation, proceed with extremely low quantum efficiency. PURIFICATION OF WATER
To establish the validity of this method, it was necessary to obtain “clean” water to which a known amount of organic material could be added, photolyzed, and analyzed for carbon dioxide. Many attempts have been made to obtain water free from organic contamination. Fricke and Hart (4) reported purifying water from a Barnstead still by distilling from alkaline permanganate, then from sulfuric acid-chromate solution, and finally passing the vapor with oxygen through a silica vessel a t 900’ C. The original water from the still yielded 10 to 20 Qmoles of carbon dioxide and hydrogen per liter when exhaustively irradiated. The yield from the purified water fell to several micromoles per liter. These authors report that the water may be further purified by irradiation with x-rays and this suggestion that water may best be purified by irradiation has been the basis for most of the purification methods since then. EXPERIMENTAL
Apparatus. The vacuum system employed in this study is shown schematically in Figure 1. The bulk of the system is made from 10-mm. 0.d. borosilicate glass tubing. Flask A is a 50-ml. silica vessel with a flat bottom to permit use of a glasscovered magnetic stirring bar and is joined to the remainder of the system by a quartz-borosilicate graded seal. Trap G for condensing carbon dioxide with liquid nitrogen is merely a Ushaped piece of tubing. Traps B and J are used to condense water vapor and to isolate the system from the vacuum pump, respectively. The vacuum pump is connected beyond stopcock M , and tank nitrogen, which is passed through Ascarite and a calcium chloride drying chamber, is introduced through
0
Figure 1 .
Apparatus for photolytic oxidation of organic matter in water A. 58-ml. silica flask fl,. J. Cold trapsj B immersed in dry ice-acetone and J in liquid nitrogen C, F H I , M, N. High vacuum oblique 4-mm. bore stopcocks 0. 'hiih vacuum oblique 2-mm. bore stopcock connected to short mercury manometer E, 12/30 standard-taper joint 1. 1 8 f 9 semibalkjoint K. 28f 12 semiball joint G. U-tube immersed in liquid nitrogen to colleet carbon dioxide
stopcock L. The entire apparatus was cleaned with hot nitric-sulfuric acid and rinsed with distilled water. A smooth filmof water on drainage was used as a criterion of cleanliness. Irradiation, A Hanovia mediumpressure mercury vapor lamp was used for i l l radiations and was placed as close as possible to the quartz flask. To improve the efficiency of the radiation, a cylindrical reflecting chamber, constructed frpm a tin can 10 cm. in diameter, was placed around the flask. Photosensitizer. Both ceric and ferric ions were used as photosensitizers. Because no significant difference in effectiveness of these two substances was observed and because ceric ions precipitated from solutions that were not strongly acid, ferric ions were chosen as the more convenient photosensitizer. Most solutions that were photolyzed contained 0.4 ml. of 0.5M Fez(SOr)s.9Hz0 in 20 ml. of solution. Analysis. A Consolidated Engineering Corp., Model 21-620 mass spectrometer fitted with the No. 21-081 metal inlet system was used for all analyses. These analyses were made by comparing the mass 44 peak height observed from the condensed gas in trap G with that peak height from carbon dioxide of known composition. Calibrations a t mass 44 were made with tank carbon dioxide and with known mixtures of carbon dioxide and argon by introducing a known pressure of gas into the mms spectrometer and measuring the height of the mass 44 peak. Over a period of 6 months, the average peak height per millimeter of mercury pressure of carbon dioxide introduced was 36 f 2 divisions. Preliminary Procedure. Preliminary experiments were performed with acidified bicarbonate solutions to determine the recoverability of small amounts of carbon dioxide from solution.
Approximately 20 ml. of freshly boiled distilled water were added through the side arm to flask A and frozen with a dry ice-acetone mixture. The side arm was sealed and the system evacuated. To outgas the water sample, stopcock C was closed until the sample was melted and heated, and then C was opened to evacuate the evolved gases. C was closed and compressed air was admitted through an Ascarite guard tube at stopcock N . The side arm was opened with a slight positive pressure of air in the system to prevent contamination by atmospheric carbon dioxide. -4 small quantity of sodium bicarbonate solution, prepared with boiled distilled water, was added and rinsed down the neck of the flask with about 1 ml. of freshly boiled distilled water. The sample solution was frozen and then 1 ml. of 1M sulfuric acid was added. After stopcock N was closed, the side arm was collapsed shut and the entire system was evacuated. Stopcock C was closed and the sample meited, causing the bicarbonate solution to be acidified. A dry ice-acetone mixture was placed around trap B and liquid nitrogen around trap 6. Stopcock W was closed and C opened. A quiet vacuum distillation of the water from flask A to flask B was effected with heat supplied to the sample flask with warm air from a commercial hair dryer. Periodic amlyses were made by stopping the distillation, closing C, P, and M ! venting the system from M to Ip, and removing the U-tube to the mass spectrometer. In the final experiments of this group, the acid was added with the water and only the bicarbonate solution and rinsings were added after the degassing procedure.
reflux back to A rather than distill into trap B. Approximate1 20 ml. of water and 0.4 ml. of eider 0.5M ferric or ceric sulfate solution were added to the sample flask and outgassed as in the previous procedure. After exhaustive irradiation (at least 6 hours) of the water sample, nitrogen gas was admitted and maintained to slightly greater than atmospheric pressure when the side arm was opened. After 25 or 50 pl. of the appropriate solution of the organic compound had been introduced and the neck rinsed down with three to four t i e s that volume of boiled distilled water, the nitrogen gas, was turned off and the side arm was collapsed below the open end. Stopcock (7 was closed, the magnetic stirrer' burned on, and the irradiation begun. All irradiations were thus performed with a pitrogen atmosphere and with stirring. After the irradiation, flask A was immersed in dry ice-acetone until the sample solution was frozen. The atmosphere in the apparatus was then evacuated through trap G and the contents of this trap were analyzed for carbon dioxide. RESULTS
The results of the recovery of carbon dioxide from acidified bicarbonate solution are shown in Table I. Over a thousandfold range of bicarbonate added, the average per cent recovery of carbon dioxide, assuming complete conversion of bicarbonate to carbon dioxide, was 116% with an average deviation of =k20%. Even though these recoveries may leave something to be desired, it is obvious that the method is sensitive to quantities of carbon dioxide as small as 0.1 pmole. Before known amounts of organic compounds could be photolyzed to determine if they were converted quantitatively to carbon dioxide, it was necessary to know how much carbon dioxide would be obtained from water alone. Samples of boiled distilled water, of water distilled from alkaline permanganate, and of water refluxed with ceric sulfate were treated with sufficient ferric sulfate to make the solution 0.02.W
Table 1.
Recovery of Carbon Dioxide
from Bicarbonate sodium
Bicarbonate Added, pmoles
Carbon Dioxide Found, pmoles
Recovery,
%
Photolytic Procedure. For the photolysis experiments, the procedure and apparatus were modified. Stopcock N was removed and a cold finger was installed above flask A, so that the water vapor from A would
bv. 116
IT, it is apparent that not all organic Table I/.
ioxlde from Irradiation of Qrganic Compounds
Recover
Rate of COa
Production, Added, Iaradia Found, P,P.M. tion, Min. pmoles pmole/Hr.
Carbon
Bubstrate Added, pmole ( CHs)2CHOHr0.65
1.2
Time of
COz
40 1.6 670 0.87 105 0.09 Total 2.56
GO2
Calcd., pmoles
Recovery,
%
2.2 0.08 0.05
_ I
0.68 Blank0 (13.5 hours at 0.05pmole/hr,) COzfrom sample 1.88 KHGeIlrOi, 0.128
0.6
73 64 60 960
0.61 0.29 0.05 0.14 Total 1.09
1.96
96
1,024
88
0.50 0.27 0.05 0.01
Blankb (19 hours at 0.01 pmole/hr.) 0.19 Coli from sample 0.90 ~
( CHs)aCHOBI,0.05
0 09 ~
40 54 700 Total
0.11 0.12
0.11 ___
0.16 0.13 0.01
0.34
Blank5 (13 houra at 0.01 pmole/b.) 0.13 COz from sample 0.21 I _
0.50 0.50 0.22 0.13 0.13 150 Total 1.48
Blanko (21hours at 0.04pmole/b.) 0.84 COSfrom sample 0.84
0.15
140
0.16 0.05 0.05 0.05 0.05
74 0.86
a Water containin 0.02M iron(II1) and before any organic compound was added was exhaustively radiates , On further radiation water continued to give about 0.05 pmole of C o i , phour. ~ b Tbli sample of water after exhaustive radiation continued to give about 0.01 prnole of COSper hour. This sample of water gave approximately 0.04 pmole of GO2 per hour.
in iron, thoroughly outgassed, and then photoly~edfor various lengths of time. These studies showed that boiled distilled water produced a t least 40 pmoles pes liter of carbon dioxide and that & s t i n g the water from alkaline peritp significantly. The majority of the carloon dioxide from these impurities in the water was obtained during the first 3 hours of irradiation and the smaller amounts of carbon dioxide ed on prolonged exto be formed a t a ate. Thus, to obtain the photolysis studies on added organic compounds, boiled distilled water containng 0.02X iron (111) was exhaustively irradiated u n f i dioxide per hour of
water for photolysis studies is to photolyze it. This photolytic method for the determination of small amounts of organic compounds was investigated on three compounds-isopropyl alcohol, potassium acid phthalate, and aoetic acid. The results of these experiments are These results ina t least three organic compounds, when present in very small amounts, can be converted to carbon dioxide by this photolytic process. Virtually all of the carbon dioxide that can be obtained from isopropyl alcohol and from potassium acid phthalate is found during the first 3 hours of radiation. On the other h n d , acetic acid does not appear t o be completely oxidized after 21 hours of ~ ~ d ~ a t even ~ o n rthough, as with the ds, the majority of the L produced during the
ched by Fricke an
In view of the r~~~ shown in Table
compounds are oxidized with the same efficiency in this photolytic process. The fact that the different samples of water, after several hours of radiation showed varying blanks on further exposure to ultraviolet light can be attributed to the presence of traces of compounds in the water which are oxidized a t various rates. This photolytic method for the determination of traces of organic compounds in water should be investigated with more compounds, but it appears that 3 hours of mdiation, with the apparatus and the common ultraviolet source used in this investigation, will oxidize the majority of the organic compound to carbon dioxide. Thus, it should be possible by this technique to get an indication of the content of carbonaceous materials in water to better than *50% in a comparatively short period of time. The studies mentioned above suggest several modifications that might be made in the procedure. The addition of hydrogen peroxide to the ?rater during the “clean-up” phase and also during the oxidation should shorten the irradiation time by providing an increased hydroxyl radical concentration. For the same reason, the presence of an oxygen atmosphere and thereby a solution saturated with oxygen during the radiation. may enhance the rate of the oxidation of carbonaceous matter. Also, any source of ionizing radiation with a sufficient flux (such as neutrons or y-rays from a nuclear reactor) should serve in place of the ultraviolet source. However, except for special applications, the latter modification offers little advantage over the rather simple apparatus used here. ACKNOWLEDGMENT
The financial assistance of AFOSR contract A F 49(638)-467, which supported this study, is gratefully acknowledged. LITERATURE CITED
(5) Love, 5. IC., Thatcher, L. L., ANAL. CWM. 29,722(1957). (6)Picfiardt, W. P., Oemler, -4.N., Mitchell, J. dbid., 27,1784 (1955). (7) Bkougstad, M. w., ~ ~ h m a M. n , J., Ibid., 33, 138R (1961); (8) Thatcher, L. L., Klser, R. T., Ibid., 31,775(1959). mmm for review June 23, 1961. Accepted September 13,1961.
