ANALYTICAL CHEMISTRY
1758 (5) Ibid., 164, 792 (1949). 16) Clark, G. L., Kaye, W. I., and Parks, T. D., IND. ENO.CHEM.,
ANAL.ED.,18,310 (1946). (7) Ewing, G. W., and Parsons, T., ANAL.CHEM.,20,423 (1948). (8) Friedman, H., Electronics, 18, 132 (1945). (9) Iddles, € -4., I. and Jackson, C. E., IND. ENG.CHEM.,ANAL. ED., 6,454 (1934). (10) Lange, J. J. de, and Houtnian, J. P. UT., Rec. trav. chim. PaysBas, 65,891 (1916).
(11) Rice, R. G., Keller, G. L., and Kirchner, J. G., ANAL. CEEM., 23,194 (1951) (12) Roberts, J. D., and Green, IND. ENG.C H m f . , ASAL. ED., 18, 335 (1946). (13) Roberts, 6. D., and Green, C.. J . Am. Chem. Soc., 68,214 (1946). (14) Schroeder, S B., Ann. K . Y . Acad. Sei., 49,204 (1948). (15) Strain, H. H., J . Am. Chem. Soc., 57, 758 (1935). (16) T h i t o , J. V , Jr., ANAL.CREM.,20,726 11948). RECEIVED March 25, 1949.
c.,
Determination of Small Concentrations of Carbonyl Compounds by a Differential pH Method HARRY R. ROE AND JOHN MITCHELL, JR. Polychemicals Department, E. I . du Pont de Nemours & Co., Inc., Wilmington, Del.
A procedure was required for rapid determination of small concentrations of active carbonyl compounds. The method developed involves measurement of the pH of 0.5 iV aqueous hydroxylamine hydrochloride before and after addition of the sample. The decrease in pH is a direct function of the amount of carbonyl compound in the sample. Calibration by a standard working curve permits accurate quantitative analyses. This 5-minute procedure has been applied successfully to 0.0 to 0.023 millimole per milliliter of aldehydes and methyl ketones in benzene, dioxane, methanol, or water solution.
liter. The p H of this solution should be 3.05 Z!Z 0.05. (Both Eastman Kodak Co. 340 and Fisher Scientific Co. H330 hydroxylamine hydrochloride were found satisfactory. Undoubtedly other sources of the C.P. quality chemical are available.) APPARATUS
A Beckman pH meter, Laboratory Model G, equipped with standard glass electrode So. 290 and standard calomel electrode No. 270 was used in this work. Other pH meters of equivalent quality are satisfactory. The meter should be standardized against a known buffer solution having a pH in or near the working range of the analysis (pH 2 , 3 , or 4). PROCEDURE
A
LDEHYDES and ketones react with hydroxylamine hydrochloride, forming oximes and releasing hydrochloric acid, Under selected conditions, the decrease in pH resulting from this reaction is a direct measure of carbonyl concentration. Huckabay, Sewton, and Metler (2) employed oximation with hydroxylamine hydrochloride for the determination of traces of acetone in liquefied gases by making a direct titration of the released hydrochloric acid. Later Byrne (1) described a method for the determination of traces of acetone in aqueous solutions by measuring the resulting change in pH. Studies made by the authors have shown that, with certain modifications, the differential pH principle has a much broader application. Rapid determinations may be madeof small concentrations of many carbonyl compounds in benzene, dioxane, methanol, or water solution. The procedure involves the controlled reaction of the sample with 0.5 N hydroxylamine hydrochloride solution. I n the range 0 t o 0.023 millimole of active carbonyl per milliliter the resulting decrease in pH is sufficiently marked to permit a direct correlation with carbonyl concentration. The time required for a single analysis is about 5 minutes. Results are reproducible to &2% relative.
One milliliter of 0.5 N hydroxylaniine hydrochloride is added to 10 ml. of distilled water in a 50-ml. beaker. The pH meter is adjusted to the temperature of the resulting solution and the pH is determined. This “blank pH” should be in the range 3.65 to 3.75. Then 10 ml. of the sample, containing no more than 0.23 millimole of aldehyde or ketone, are added, and the solution is mixed thoroughly and allowed to stand for 5 minutes a t room temperature. When the carbonyl content of a xater-immiscible sample, such as benzene, is being determined, stirring should be continued for the entire 5-minute reaction period in order to assure quantitative transfer of the carbonyl to the aqueous phase. At the end of this time, the temperature of the solution is checked and, if necessary, the pH meter is again adjusted, and the p H determined. The net decrease in pH observed is referred to a standard working curve (see Figure 1 ) to give a quantitative estimation of carbonyl.
REAGENT
Hydroxylamine hydrochloride, containing no free hydrochloric acid, is used. The quality of the hydroxylamine hydrochloride may be checked in the following manner: Five grams of the reagent are dissolved in 25 ml. of water and a few drops of bromophenol blue indicator solution are added. (If no free hydrochloric acid is present, the solution will be neutral to the indicator.) If free hydrochloric acid is present, as shown by the development of a yellow color, the solution should be neutralized with standard 0.5 N sodium hydroxide solution and the titer noted. A 0.5 N hydroxylamine hydrochloride solution is used in the analysis. I t is prepared by dissolving 35 grams of C.P. hydroxylamine hydrochloride, NH,OH.HCI, in water, adding the calculated amount of 0.5 N sodium hydroxide to neutralize exactly any free hydrochloric acid, and diluting the resulting solution to 1
C)-DIOXANE OR BENZENE SOLUTION @-METHANOL OR WATER SOLUTION
0.003 o.dos
o.do9
0.012
0015
O.dl8
O.d21
CONCENTRATION ( r n M / r n l )
Figure 1. Analytical Data for Acetone or Butyraldehyde in Various Media
V O L U M E 2 3 , NO. 12, D E C E M B E R 1 9 5 1 Table I. Compound Formaldehyde Acetaldehyde Propionaldehyde Butyraldehyde Isovaleraldehyde 2-Ethyl hexanal Salicvl aldehvde Ben;aldehydk 8 Crotonaldehyde 9 Heptaldehyde 10 11 Vanillin 12 Furfural 13 Acetone Methvl ethyl ke14 tone 15 Methyl n-butyl ketone 16 Methyl n-amyl ketone 17 Methyl n-decyl ketone 18 Acetophenone 19 Diethyl ketone 20 E t h y l n-amyl ketine 21 ProDvl n-amvl ketone 22 Diacetyl 23 Acetyl acetone 24 Acetonyl acetone 25 Benzil a 0,012millimole of carbonyl
120. 1 2 3 4 5 6 7
1759
Analytical Data for Carbonyl Compounds"
Methanol Solution Blank Final A PH PH PH 3.68 2.56 1.12 2.54 3.70 1.16 1.15 2.53 3.68 2.53 3.68 1.15 2.52 3.67 1 15 2.51 3.67 1 16 1.19 2.50 3.69 2.56 3.70 1.14 1.16 2.52 3.68 1.14 2.54 3.68 1.17 2.53 3.70 2.51 1.18 3.69 2.53 1.16 3.69 1.15 2.54 3.69
3.70
2.54
1.16
3.70
2.55
1.15
3.69
2.53
1.16
3.66 3.69 3.67
3.32 2.60 2.77
0.34
3.70
2.84
0 86
3.68 3.68 3.68 3.67 conipound
2.51 2.45 2.44 3.50 per rnl. of
1.17 1.23
1.09
0.90
1.24
0.17 solution.
Blank PH
Water Solution Final PH
3 70
2:53
3 68
2:52
Dioxane Solution Blank Final PH PH
2 .'44
..
.. ..
2:i3
3 70
2'53 ..
3:73
.. .. ..
2:41 , .
.. .. ..
3 70
Benzene Solution A Blank Final PH PH PH
..
3171
..
..
..
2:43
1126
..
.. ..
EXPERIMENTAL
The present investigation was prompted by the need for a routine method to determine trace amounts of free carbonyl compounds in methanol. For this reason emphasis was placed on the analyses of methanolic solutions having carbonyl conrentrations of less than 0.023 millimole per milliliter (approximately 0.1 weight % calculated as acetaldehyde). I n Table I differential pH data are listed for methanolic s o h tions of 25 aldehydes and ketones, a t a fixed concentration of 0.012 millimole per milliliter. A feR- results necessary to show trends are included for water, dioxane, and benzene solutions. I t is evident from the data presented in Table I that the differential pH method may be applied successfully to the determination of small quantities of a wide variety of aldehydes and methyl ketones. For any one solvent, the A pH values were nearly constant for equimolar concentrations of most of the compounds of these types studied. Comparable results xere obtained with a particular solvent type. The A pH values obtained with the two polar solvents, methanol and water, are in good agreement, as are those obtained with the two nonpolar solvents, benzene and dioxane. I n the latter case the values are slightly higher. Pome of the compounds listed in Table I did not give comparable results on an equivalent basis. The diketone, diacetyl, 22), reacted as if it contained but a single CH3COCOCH3(KO. carbonyl group, while two other diketones, acetyl acetone, CH&OCH&OCHg ( S o . 23), and acetonyl acetone, CHICOCH2CHzCOCH3 (No. 24), gave slightly higher results indicating that possibly there was some reaction of the second ketone group. Acetophenone (KO. 18), benzil (KO. 25), and the ethyl and propyl ketones (Nos. 19, 20, and 21) appeared to react incompletely in the 5-minute period allowed for the analysis. I n order to determine whether these carbonyl compounds follow the same general curve (concentration LS. A pH), the A
..
..
.. ..
.. .. ..
.. ..
..
, .
..
..
..
..
.. ..
.. ..
.. .. .. ..
..
.. ..
..
2:60 ..
Standard solutions were prepared by dissolving 23 millimoles of C.P. carbonyl in carbonyl-free solvent (benzene, dioxane, methanol, or water) and diluting to 1 liter. These stock solutions were diluted further for studies a t lower concentratiqns.
..
..
..
Samples containing more than 0.23 millimole of carbonyl per 10 ml. also can be analyzed. In these cases proportionately smaller samples are taken and the volumes adjusted to 10 ml. by the addition of carbonyl-free solvent.
..
..
..
.. ..
.. ..
.. ..
, .
3:iO ..
.. .. ,.
.. ..
2'43
1128
.. ..
.. .. ..
.. .. .. ..
..
2:i5
..
..
.. ..
.. ..
.. .. .. .. .. ..
pH values of varying concentrations of acetone were determined and compared with those of butyraldehyde. The results of these studies in methanol, water, dioxane, and benzene solutions appear in Figure 1. From the data presented in this figure it is concluded that, for a particular type of solvent, but a single standard working curve is needed for the quantitative determination of the carbonyl compounds of the types listed in Table I which give A pH values of 1.14 to 1.18. Carbonyl compounds in methanol or water solutions follow the same concentration-A pH curve, while those in benzene of dioxane solutions follow a similar curve having a slightly greater slope. INTERFERENCES
Inorganic and organic acids interfere with the determination of small concentrations of carbonyl compounds by the differential pH method, causing high results (see Table 11). There is some indication that this interference can be eliminated by properly adjusting the p H of the sample before analysis. Buffering materials will interfere with the method through reaction with some of the hydrochloric acid. I n an analysis the pH drop will be smaller than it should be and, consequently, low results will be obtained. Table 11. Analytical Data for Acetone in the Presence of Acetic Acid Acetone Added, FVt. %
Scid Added, Wt. %
Acetone Found, Wt. %
Error, Wt. %
Esters apparently do not interfere with the analysis. Samples of known carbonyl concentration containing up to 5% of ethyl acetate were analyzed with no significant error. DISCUSSION
In carrying out a carbonyl determination by the differential pH method two conditions are recognized which affect the pH of the sample solution. First there is the pronounced decrease in
1760
ANALYTICAL CHEMISTRY
pH due to the release of hydrochloric acid which constitutes the basis of the analysis, and then there is a small change in pH which accompanies the change in environment resulting from the dilution of the “blank solution” with a nearly equal volume of sample solution. This latter effect constitutes an error in the analysis which amounts to about +0.05 pH unit \There the solvent is water and about -0.02 pH unit where the solvent is methanol, Keither benzene nor dioxane appears to have any effect on the pH. This error is practically negligible when working in the range of 0.01 millimole per milliliter where the total pH change is about 1.16 pH units. If desired, however, a correction may be made by running a blank analysis on 10 ml. of carbonyl-free solvent. A significant “change of environment” error may be introduced in the analysis, if one uses hydroxylamine hydrochloride reagent which contains an appreciable amount of free hydrochloric acid. For example, the use of 0.05 S hydroxylamine hydrochloride reagent containing but 0.3 X 10-6mole of free hydrochloric acid per milliliter resulted in an error of more than twice that normally encountered. The sensitivitv of the analysis was checked using varying concentrations of hydroxylamine hydrochloride. I n the range 0 to 0.023 millimole per milliliter of carbonyl the best results v.-ere obtained xith a 0.5 Ksolution. More concentrated solutions were
less sensitive to small changes in pH while the less concentrated solutions showed poor stability, tending to hydrolyze rapidly to release hydrochloric acid. A 0.5 A! solution is stable for about 3 days. The pH change is not appreciably affected by variations in temperature between 20” and 30” C., provided the pH meter is adjusted for this change. Some heat results from the addition of methanol or dioxane to the aqueous blank solution, causing the temperature to rise as much as 5” C. above room temperature. ’VCTith stirring, the temperature normally returns to room temperature in the 5 minutes allowed for the reaction. This investigation was limited to systems involving benzene, dioxane, methanol, and water. Cndoubtedly analyses may be performed in many other solvent systems. However, for each solvent the standard working curve should be checked by analysis of known concentrations of carbonyl compounds. LITERATURE CITED
(1) Byrne, R . E., A N ~ LCHEM., . 20,1245-56 (1945).
(2) Huckabay, W. B., Newton, C. J., and hletler, A . V., Ibid., 19, 8.15-41 (1947). RECEIVEDhiaroh 30, 1951,
Analysis of Technical Pentachlorophenol J. B. LACLAIR California State Department of Agriculture, Sacramento, Calif Existing methods of analyses were inadequate for the proper enforcement of the Agricultural Code of California pertaining to the sale and labeling of products containing pentachlorophenol which are widely used as wood preservatives, fungicides, herbicides, and defoliants. An ultraviolet spectrophotometric method was developed for the quantitative determination of the individual components of technical pentachlorophenol, adaptable to most com-
T
HE use of technical pentachlorophenol as a fungicide and
herbicide has been increasing during the past few years. The task of enforcing the Agricultural Code of California pertaining to labeling and sale of technical pentachlorophenol and its formulations is difficult because of the lack of an accurate, specific method of analysis. A literature search disclosed methods of analysis based on determining pentachlorophenol from total chlorine ( 4 ) , and colorimetric procedures (1, 3,4,6, 7 ) which are not specific for pentachlorophenol, but include many phenolic substances. EXPERIJIE5T4L
Using a Beckman hlodel Dli spectrophotometer, an ultraviolet absorption study of the components of technical pentachlorophenol, separated by vacuum sublimation and compared with the most probable components (Figures 1, 2, and 3), disclosed only pentachlorophenol, 2,3,4,6tetrachlorophenol,and an unidentified, dark brown, high melting, chlorophenol containing 58.3% chlorine (Figure 3), which is probably a polymerization product produced during process of manufacture. An examination of the absorption spectra of pentachlorophenol and tetrachlorophenol (Figure 1) showed that a t 255 millimicrons tetrachlorophenol absorption offered the least interference to
mercial pentachlorophenol products. The use of ultraviolet spectrophotometric methods for the analysis of substituted phenolic compounds has apparently not been developed. By using spectrophotometric methods, in conjunction with appropriate separation procedures, i t is possible to identify and quantitatively determine phenolic compounds which are too closely related structurally for ordinary chemical methods.
pentachlorophenol absorption. The best point for tetrachlorophenol absorption was found a t 285 millimicrons. After determining the extinction coefficients for pentachlorophenol and tetrachlorophenol a t these two wave lengths a series of known mixtures was analyzed (Table I) using these data. Close adherence to the Lambert-Beer law was noted for the concentrations tested.
Table I. Analysis of Prepared Mixtures of Pentachlorophenol and Tetrachlorophenol Pentachlorophenol Added, Found, Recovery, mg. mg. % 96.0 0.48 0.5 104.0 1.04 1.0 100.8 2.52 2.5 102,2 5.11 5.0 99.6 7.47 7.5 94.1 8.47 9.0 100.2 9.52 9.5 0.09 0.0 104 2 10.42 10.0 99.3 2.58 2.6 102.6 5.85 5.7 98.2 9.82 10.0 95.2 14.08 14.8 99.1 1 1 58 11.7 Av. recovery 99.6
:
Added, mg. 9.5 9.0 7.5 5.0 2.5
1 .o
0.5 10.0 0.0 11.4 15.5 11.8 4.2 2.1
Tetrachlorophenol Found, Recovery, mg.
%
9.60 8.99 7.51 5.10 2.50 1.02 0.54 9.87 0.06 11.47 15.88 11.82 4.06 2.06
101.1 99.9 100.1 102.0 100.0 102.0 108.0 98.7 l06:6 102.3 100.e 96.8 98.1 100.8
