Determination of Small Amounts of Aromatic Hydrocarbons in Aqueous Solutions H. E. MORRIS, R. E. STILES,
AND
W. H. LANE, Monsanto Chemi:al Company, Texas City, Texas estimated by comparing Lvith a sample pwpared at the same tinic and under exactly similar conditions from an appropriate quantity of an aqueous solution of known concentration of the aromatic hydrocarbon in question. For example, when styrene is being determined it is convenient to have a t hand a standard solution of styrene in water. By trial and error the exact amount of the styrene solution of known concentration required to give a perfect color match with the unknown is determined. The styrene concentration in the unknown may then be calculated. This procedure is actually applied t o the determination of the styrene concentration in an aqueous unknown as follows: A standard styrene-in-water solution was made up containing 15 p.p.ni. of styrene. Thrre trials were made in which ( a ) 5.0 ml. of' the unknown were compared with 25 mi. of the standard, ( b ) 3.0 ml. of the unknown were compared with 50 ml. of the standard, and finally ( c ) 3.0 ml. of the unknoim were compared Rith 60 ml. of the standard. The last mentioned gave a perfect color match between the unknown and the standard, so the concentration of styrene in the unknown was calculated to be 60/3 X 15 = 300p.p.m.
A method is described which employs formaldehyde-sulfuric acid reagent for the determination of small amounts (Ito 500 p.p.m.) of aromatic hydrocarbons in aqueous solutions with an accuracy of 10%. The sensitivity of the test is 0.0001 gram of aromatic hydrocarbon.
*
T
HE determination of aromatic hydrocarbons in plant
effluents or other aqueous solutions is a problem frequently encountered in laboratories. -4reasonably accurate method foi, such analyses depends upon the development of a brown coloration when concentrated sulfuric acid containing a small amount of formaldehyde is brought in contact with an aromatic hydrocarbon under certain specific conditions. Sulfuric acid containing formaldehyde has been used for detecting small amounts of benzene in air (5,5) a brown color being developed when the benzene-containing air is bubbled through the reagent. Toluene and coal-tar naphthas also give the test. Naphthalene is said to interfere by producing a black film. Thiophene and unsaturated hydrocarbons also interfere but can ht. removed by bubbling the air through sulfuric acid first. An extenpion of this method has been reported (4, 5 ) , in which the degree of color development in the reagent is used in connection with a color standard to make a quantitative estimate of thtb amount of aromatic hydrocarbons present.
SCOPE AND APPLICATION
r-eing the method outlined above, concentrations of aromatic hydrocarbons in viater ranging fpom 1 to 10 p,p.m. may be determined with an accuracy of * l p.p.m. However, for higher concentrations it is necessary to dilute the sample with distilled water until it does fall within this range and then multiply the ansxer by a dilution factor. The inherent error of the method is multiplied by the dilution factor. Thus, for a sample containing 50 p.p,m. of an aromatic hydrocarbon, a fivefold dilution is recommended; and the final answer \ d l be 50 * 5 p.p.m. Since the test is capable of detecting 1 p.p.m. of aromatic hydrocarbon in a 100-ml. aqueous sample, the sensitivity is actually 0.0001 gram of aromatic hydrocarbon. The formaldehyde-sulfuric acid.reagent has been applied to aqueous samples containing benzene, toluene, ethylbenzene, and styrene alone or in combination and in concentrations ranging from 1 to 500 p,p.m. It has been found that these aromatic hydrocarbons give different degrees of discoloration of the formaldehyde-sulfuric acid reagent : hence for the most accurate work it, is necessary that t,he identity of the aromatic hydrocarbon be known. This is not necessary if only a rough estimate of concentration is desired. For routine control work it was found desirable to have available a set of permanent color standards to eliminate the necessity of running a standard along with the unknown. Such a set of color standards was prepared by mixing appropriate quantities of colored inorganic salt solutions to obtain color matches with known samples containing 1, 2 , 5 , and 10 p.p.m. of aromatic hydrocarbon. The partition coefficient of aromatic hydrocarbons between water and carbon tetrachloride is such that one extraction with carbon tetrachloride gives a complete removal from the water layer even in thosecascs where the aromatic content is very high. Extraction reagents other than carbon tetrachloride may be used to removr the aromatic hydrocarbon from aqueous solution. (Dilution of the reagent prevents its direct application to aqueous solutions.) Diethyl ether results i n a much lower sensitivity of the test; this very fact might he of value if the use requirements Lvere such that high aromatic concentrations were frequently encountered.
I n the butanone method for benzene in air (6)the benzene is first nitrated to form m-dinitrobenzene, which is subsequently estimated colorimetrically with thr, aid of butanonc. This method is both tedious and lengthy. Another method ( 5 ) is also based on the nitration of benzene and subsequent separation of the m-dinitrobenzene from the nitrating acids by steam distillation. The amount of nr-dinitrobenzene present is then determined by titration with standard titanous chloride solution. This method is also lengthy. I n the oxidation method for benzene in air ( 1 , 2 ) the saniplc is treated with hydrogen peroxide in the presence of ferrous sulfate, and the depth of the brown coloration which devrlops is taken as a measure of the amount of benzene present. Certain organic materials, particularly the more water-soluble solvents, interfere and must be removed before the test is carried out. Of all these methods, the one employing the formaldehydesulfuric acid reagent seems to offerthe best combination of speed, sensitivity, and ease of applicability to aqueous snlutions cnntaining aromatic hydrocarbons. REAGENTS
Forrnaltiehyde-sulfuric acid reagent, prepared by mixing 1.0 nil. of 37y0U.S.P. formaldehyde with 100 ml. of C.P. concentrated sulfuric acid. C.P. carbon tetrachloride (a technical grade may be used if it is washed with sulfuric acid until it remains colorless in contact with the reagent). METHOD
The formaldehyde-sulfuric acid reagent is used as follows for determining the aromatic hydrocarbon content of aqueous samples: 100 ml. of the aqueous sample to be analyzed are shaken in a separatory funnel with 25 ml. of carbon tetrachloride. After separating, the carbon tetrachloride layer is carefully withdrawn and added to 5 ml. of the formaldehyde-sulfuric acid reagent in a 200-ml. flask. After vigorous shaking for 1 minute, the reactants are allowed to stand for 5 minutes, whereupon a brown discoloration of the acid layer develops. The depth of this discoloration changes on standing; hence it is important that it be observed a t some standard interval of time after shaking. The quantity of aromatic hydrocarbon present is
EFFECT OF FORMALDEHYDE CONCENTRATION IN REAGENT
I t has been recommended (4, 5 ) that the formaldehyde concentration be 5 ml. of 37% formaldehyde per 100 ml. of concentrated sulfuric acid. In order to determine t,he effect of formaldehyde 294
ANALYTICAL EDITION
May, 1946
concentration, tests were made in which formaldehyde concentrations of 0.5, 1.0, 2.0, 3.0, 4.0, and 5.0 ml. per 100 ml. of concentrated sulfuric acid were employed on solutions containing 2, 4, and 6 p.p.m. of ethylbenzene. I n all cases the low formaldehyde concentrations gave from two to three times the degree of color development obtained with the strong concentrations. Thus, for greatest sensitivity it is recommended that the formaldehyde concentration be kept low. Obviously one formaldehyde concentration must be accepted and maintained as standard to prevent any possible variation from this source.
295 LITERATURE CITED
(1) Cook, W. A., and Ficklen, J. B., IXD.EXG.CHEM.,.4N.4L. ED.,4, 406 (1932). (2) Cook, W. 8 . ,and Ficklen, J. B., J . Ind. Hyg., 17,41 (1935). (3) Dept. Sci. Ind. Research, "Methods for Detection of Toxic Gases in Industry, Benzene Vapour", Leaflet 4,p. 43, London, H. M . Stationery Office, 1938. (4) Ficklen, J. B.,"Manual of Industrial Health Hazards", p . 43, Lancaster, Pa., Science Press Printing GO., 1940. (5) Jacobs, M. B., "Analytical Chemistry of Industrial Poisons, Hazards, and Solvents", pp. 399-415,.New York, Interscience Publishers, 1941.
Determination of the Solubility of Styrene in W a t e r and OF W a t e r in Styrene W. H. LANE, Monsanto Chemical Company, Texas City, Texas
,
The Karl Fischer reagent and cloud point observations have been applied to the problem of determining the solubility of water in styrene between 6" and 51 ' C. The formaldehyde-sulfuric acid reagent as well as cloud point observations were utilized for determining the solubility of styrene in water between 7 ' and 65" C., and data are reported.
'
Table
I.
Solubility of Water in Styrene Solubility of Water in Styrene
Temperature
c. 6 25 31 40 51
D
ATA on the solubility oi styrene in water and of water
in styrene and the effect of temperature on these mutual solubilities can be of considerable value when it is necessary to compute organic losses by way of aqueous plant effluent streams. Since no such data are available in the literature, the present investigation was undertaken.
70 Karl Fischer Reagent 0,032 0.066 0,084 0.101 0.123
14 27 34 40 45
Cloud Poirir 0,040
g:0.100 K8 0.120
SOLUBILITY OF WATER IN STYRENE
The Karl Fischer reagent ( 2 ) v a s applied successfully to the problem of determining water in styrene. The data shown in Table I and Figure 1 are average values of two determinations. The reproduciI6O bility of the method is ex5 cellent, as the difference 5 between duplicate determinations never exceeded 3% 5 of the average value. Equi$ 080 librium conditions weie obtained by shaking a large 5 \ample of styrene contain0040 ing a slight excess of mater a t a given temperature and then allowing it to stand in a water bath a t this temTEMPERATURE I N "C perature for 24 hours to enFigure 1, Solubility of Water'in sure complete separation of Styrene the two phases before withdrawal of a sample of the styrene phase for titration of the water present vith the Karl Fischer reagent. The styrene phase was always clear, and it did not become cloudy even when cooled considerably from a higher temperature a t which saturation conditions had existed. Cloud points never occurred when an excess of both phases was present. Several precautions must be observed in applying the Karl Fischer reagent as described above. Samples of styrene stored for several days over anhydrous calcium sulfate showed negligibly small blanks. Since styrene polymerizes readily, fresh samples were used for each determination. The usual polymerization inhibitor, p,tert-butylcatechol, interferes even in the small
amounts (10 p.p.m.) normally employed. The presence of this inhibitor in dry styrene caused rather high blanks, so styrene samples free of p,tert-butylcatechol 17-ere used in all the solubility determinations. I n spite of the fact that during the determination of the solubility of water in styrene by means of the Karl Fischer reagent no cloudiness or visual evidence of phase separation was ever observed (perhaps because of the presence of an excess of both phases), under slightly different conditions it was possible to observe a cloud point and to make use of this phenomenon for solubility determinations. The technique employed was the same as that described below. The data shown in Table I and Figure 1 are averages of two or more determinations. Duplicate determinations gave cloud points agreeing within 1" C., except in the case of 0.12070 water in styrene, where the agreement n as within about 3" C. Cloud points at 0.04070 water were very faint. SOLUBILITY OF STYRENE IN WATER
z 5
,Figure 2.
Solubility of Styrene in Water
The solubility oi styrene in water was determined by means of the formaldehyde- ul f u r ic acid reagent ( 1 ) and cloud point obqcrvations. (Table I1 and Figure 2 ) . The value shoiyn for 7 " C. by formaldehyde-sul f uri c acid reagent is the meanof threedeter-
