1384
ANALYTICAL CHEMISTRY
admitted, so the sample will diffuse through the solution. Final mixing is obtained by gentle stirring with the capillary. Too vigorous stirring releases the dissolved gases. A check on the degree of mixing can be obtained by moving the capillary to various positions up and down the column of solution to see if there is any variation in the diffusion current from one position to another. The dissolved oxygen content valuzs obtained for various petroleum fractions and organic chemicals are shown in Table I, expressed as milligrams of oxygen per liter of solution a t 25” C. The finished gasoline contained less dissolved oxygen than the original gasoline components, perhaps because the finished gasolines contained oxidation inhibitors. The samples of catalytic feed stocks had been saturated n-ith air previous to analysis. Additional data, showing the repeatability of the polarographic procedure for dissolved oxygen n-hen applied to various hydrocarbons, are given in Table 11.
The accuracy of the polarographic method is not known, as there was no suitable method for preparing a synthetic sample of known dissolved oxygen content. The repeatability of the method is about *2 mg. per liter in the range of 20 to 40 mg. prr liter of dissolved oxygen. LITERATURE ClTED
(1) Biidgeman, 0. C., and .Ildiich, E. W,, S A E Journal, 27, S o . I , 9 3 (1930). (2) Craig, W. A, Petroleum World, 42 (3), 43 (1946). (3) Gemant, A., Trans. Faraday SOC.,32, 694 (1936). (4) Hooper, J. H., Proc. Sni. Petroleum Inst., 28 (III), 31 (1948). ( 5 ) Leeds & Korthrup, “Bibliography of Polarographic Litelature,” E-90(1) (1950). (6) Markham, A. E., and Kobe, K. A., Chem. Rem., 28, 519 (1941).
RECEIVED April 20, 1951.
Infrared Spectrophotometric Determination of Oil and Phenols in Water R. G . SIRIARD, ICHIRO H4SEGAW-4, WlLLIAM BANDAKUK’, A N D C. E. HEADINGTON T h e Atlantic Refining Co., Philadelphia, Pa.
I
NCREASISG activity in the field of stream pollution abate-
ment has accelerated the search for more sensitive and accurate analytical methods in measuring the contaminants of industrial effluent waters. For many years the petroleum industry has been concerned with the elimination of both hydrocarbon oil and phenols from its effluent waters. h number of analytical methods are now in use, but none has both the sensitivity and accuracy desired for future pollution abatement programs. The Gibbs method (S) and the Scott modification thereof (k’), probably the best knoJYn method available for phenol determination in xater, are based on reaction with 2,6-dibromoquinone chloroimide to form a colored indophenol. The more recent method of Lykken, Treseder, and Zahn (6) employs a modification of Stoughton’s so-called nitrosophenol method (8). This method is based on the formation of a nitrosophenol which rearranges in ammoniacal solution to form a colored quinoid radical. Existing methods for the determination of oil in water are gravimetric or volumetric techniques employing extraction or distillation (1,6) and are limited to concentrations above 1, and preferably 10, p.p.m. This paper presents the results of a ctudy of the determination of oil and phenols by means of infrared absorption. From this study the authors have evolved an infrared spectrophotometric method which is sensitive to 0.1 p.p.m. of oil and 10 parts, or less, per billion of phenols. rlccuracy, although not as good as might be desired, is believed to be better than that obtainable by existing methods a t the lower concentrations and is not affected by the volatility of the material being determined. The method for determining phenols is based on bromination of the phenols in the water sample, followed by extraction with carbon tetrachloride and measurement of the optical density a t 2.84 microns, The absorption a t 2.84 microns is due to the 0-H vibration when intramolecular hydrogen bonding takes place between the hydrogen of the hydroxyl group and a bromine atom in the benzene ring ( 7 , 9) in a position adjacent to the hydroxyl group. The 0-H vibration, u-hich normally occurs at ca. 1
Present address, 1618 Broadway, Camden, S . J.
2.79 microns when there is no hydrogen bonding, is shifted to the longer wave lengths when hydrogen bonding occurs. In t h e case under consideration the magnitude of the shift is constant as long as the bromine substitution is ortho to the OH group, making the 2.84-micron band suitable for analytical purposes. Early in the investigation, attempts were made to extract the phenols: directly from the w-ater, but the affinity of the Iolver homologs for water made this impossible with any solvent which was transparent in the spectral interval employed for both the phenols and oil determinations. Oil is also determined in the carbon tetrachloride extract from the phenol extraction by optical density measurements in the 3.4micron region, rvhere absorption is due to the carbon-hydrogen stretching vibrations in CH,, CH,, and CH groups.
The petroleum industry has long been concerned with elimination of both hydrocarbon oil and phenols from its effluent waters. None of the analytical methods currently in use has both the sensitivity and accuracy desired, and as part of the petroleum industry’s pollution-abatement program, a sensitive, accurate infrared method has been devised for determining small amounts of oil and phenols in water. The method for phenols is based on bromination of the phenols, extraction of resulting bromides from water with carbon tetrachloride, and measurement of optical density of extract at 2.84 microns where absorption is due to 0-H vibration when intramolecular hydrogen bonding of the bromine and hydroxyl groups takes place. The oil determination is based on optical density measurements of the same extract at the CH,, CH,, and CH stretching frequencies in the region of 3.4 microns. The method for oil is sensitive to 0.1 p.p.m. Less than 10 parts per billion of phenols can be determined with reasonable accuracy.
1385
V O L U M E 23, NO. 10, O C T O B E R 1 9 6 1 Table I .
\ ariation of Minimum Detectable Concentra-
tion with Sample and Solvent Volumes Carbon Tetrachloride Used hdmpie
Taken, 111
._..
1000 2000
:moo
20 ml.
______ ~
Oil
0.2
0.1
0.6
0.1
0.005 0.003
0.3 0.2
0.1
100 ml.
60 In1 -
-
Phenol Oil Phenol Oil Rlinimiini Detectable Concn., P.P.M. 0.03 0 013
o
01
1.0 0.5
0.3
Phenol
The sample is taken in a screw-top bottle having a capacity 30 to 50% greater than the volume of sample itself, and it is very important that the extraction with carbon tetrachloride be carried out in the container in which the sample is taken. It is advisable to place some mark on the outside of the sample container, prior to use, which can be used to judge the approximate volume of its contents.
0.05 0.025 0.02
SPECIAL A P P A R A T U S
T h e apparatus employed by the authors was a Perkiri-Elmer Model 12.4 infrared spectrophotomet'er equipped with a 72" lithium fluoride prisni and a West8ern Electric Type 1'652 airI ,;irked and blackoned thermistor bolometer. Input radiat'ion is interrupted a t 15 cycles per second and the bolometer output :iniplified with a Western Electric KS-10281 amplifier with out[)ut recorded by a Bro\\-n Electronik recorder. The cells m r e 30-niiii. demountable glass cells (Fisher-Porter Co., Hatboro, Pa., (~'ntalogTo. 95, Illustration KO. 4) equipped with quartz wintlo\vs. The cells and quartz windonx were polished optically H a t and for t,hat reason no cement xas required to make the cells liquid tight,. The apparatus emplo>-etl for extraction was a rec,ipi,ocating shaking machine with a horizontal stroke of 1.5 inches at a speed of approximately 100 strokes per minute (A. f l . Thoma? Co., Catalog So. 8917--1). The spectrographic equipment used bj- the authors for this Iiurpose may not be available, because of its cost, in smallel, Ia1,oi~atories. There is reason to believe, holvever, that a small iiistrumcnt could be built to do this qmific job a t a cost much iuitler that of a conventional infrared spectrophotometer. As a nixtter of fact, the method has been developed with this in mind. For this reason, wave lengths in the same general region of the ipec'trum have been selectrtl and a lithium fluoride prism has been used for determinations of both oil and phenols. Actually the oil can be determined esactly as indicated below on an infrared apectrophotometer equipped with a rock salt prism. The lithium fluoride prism was usrd by the authors for the sole pur11oseof determining the phenols on the same instrument.
Determination of Phenols and Oil. For each liter of sample taken, add 100 grams of potassium bromide (see Table I1 for the effect of potassium bromide), 25 ml. of potassium bromate solution containing 12 grams per liter, and 80 ml. of hydrochloric acid (1 part of concentrated acid to 3 parts water). Cap the bottle and shake the contents for 5 minutes. Then add 30 ml. of 10% sodium thiosulfate solution for each liter of sample and add the volume of carbon tetrachloride selected from Table I. Shake the bottle and contents for 15 minutes on the shaking machine described above, or one that mill give agitation equivalent to it when operated a t 100 strokes per minute. After shaking, dran 25 nil. of the carbon tetrachloride layer (only I5 ml. are necessary) off the bottom of the bottle by means of a pipet and shake vigorously for 5 minutes in a separatory funnel Tvith 125 1111. of a 2y0 solution of sodium bicarbonate. After the bicarbonate eytraction, withdraw the carbon tetrachloride layer, filter once through paper to remove suspended water, place in the infrared cell, and scan for phenols from 2.6 to 2.9 microns and for oil from 3.2 to 3.6 microns. If the per cent transmittance in either region is less than 2O%, dilute mith carbon tetrachloride to throw t h f value between 20 and 80% before a final measurement is made Measure the volume of the water layer from the carbon tetrachloride extraction in a graduate and record for use in the calrulation of resnlts.
'Table 11.
Effect of Addition of Potassiuni Bromide on Phenol Determination
IiBr Added. Grama 6
Phenol Present.
P.P.11. 1 1 !r
Phenol Foiind. KP.11, 8.0
Recovery.
% 67
63 96
100
6.8
92
REAGENTS
Tlw carbon tetrachloride used for extraction should be uniI'0i.m in composition from lot to lot; otherir-ise, calibrations ma!-
tiot, be reliable when a different lot of carbon tetrachloride is uwd. I n general, the carbon tetrachloride should be free from (;--I3 bands in the 3.4-micron region for cell thicknesses up to 20 mni., and except for a broad absorption band a t 3.28 microns, -1iould have no other absorption bands. Generally, a cut within I lie distillation range 76.1" to 76.3" C. will be satisfactory-for vzample, Eastman Kodak Co. s sulfur-free grade has, in most r:tws, been found to meet, these requirements. However, each Iiatc.11 used should he examined carefully to determine its suit:iliility, particularly in t,he 3.4-micron region. The spectrometer calibrations require pure 2,4,6-tribromophenol, ASTM iso-octane, ASThI n-cetane, and pure benzene. The determination of phenols requires potassium bromide, potassium bromate solution containing 12 grams per liter, 10% +citliunithiosulfate solution, 2% sodium bicarbonate solution, and ,concentrated hydrochloric acid diluted with 3 parts of water.
,Is CH, groups are to be measured on this solution, the stopcock in the separatory funnel must be absolutely free of stopcock grease. Any lubrication nrcessary for the stopcock is provided hy the solution. Calibrations and Calculations. Calibration of the spectrometer for phenols is effected at. 2.84 microns with a series of known solutions of 2,1,6-trihromophenol in carbon tetrachloride cont.aining weighed amounts of the compound equivalent to concentrations of 5 to 100 p.p.m. of phenol. The calculation of the phenol absorption a t 2.84 microns employs the base-line method of Heigl, Bell, and JVhite ( 4 ) . By this method, the baseline optical density is calculated from the equation: DB
ANALYTICAL PROCEDURE
Sampling. The size of sample taken for the determination and the volume of carbon tetrachloride used to extract it may be varied to meet the sensitivity requirements shown in Table I. \\-henever the phenol concentration is known to be high, it is prefrrable to extract 1000 ml. of sample with 100 ml. of carbon tetrachloride because this gives a large excess of carbon tetrac.hloride ext,ract for cases m-here emulsions are encountered and reduces to a minimum the amount of potassium bromide that niust be added to the water sample. As 15 ml. are required to fill I he cell, no values are given in the tahle for carbon tetrachloride volumes below 20 ml.
= log,o
IBlI
where
DB = base-line optical density I = distance on the recorded spectrum from the zero lint. to the 2.84-micron absorption peak I R = distance from the zero line to a straight line, the baseline, joining two spect,ral points located a t 2.63 and 2.92 microns. l e and I are both measured a t the same wave length-2.84 microns. The calibration for oils may be established by using a blend of 3 7 . 5 7 iso-octane, 37.57, cetane, and 25% benzene as the standard, and plotting the sum of the optical densities a t 3.50, 3.42. and 3.38 microns against concentrations containing 5 to 100
ANALYTICAL CHEMISTRY
1386 Table 111. Absorption Maxima of Brominated Phenols Compound 2,4,6-Trihromophenol Brominated o-cresol Brominated m-cresol
Brominated 1,3,5-xylenol Brominated p-tert-amylphenol Brominated nonylphenol
Table IV.
Absorption Maximum, Microns 2.84 2.84 2.85 2.84 2.85 2.83 2.83 2.84 2.85 2.83 2.84
Analysis of Known Solutions of Phenols Phenol Added. P.P.M. 3.5 1.2 3.5 6.3 2.6
Phenol Found, P.P.M. 3.5
Recovery Wt. % Mole 100 100 76 87 0.91 69 79 2.4 62 71 3.9 65 85 1.7 3.3 6.4 52 68 4.0 5.4 74 96 46 74 2.6 5.6 4.1 48 ' 84 8.6 2.3 4.7 49 87 30 70 4.0 1.2 30.0 16.0 81 53 3.7 46 71 8.0 1.0 0.6 60 64 67 71 4.17 2.8 10.4 6.1 59 63 a These acids were found to contain 31% phenol, 42% cresols, 207, xylenols, 3 % higher phenols, and 4% mercaptans.
Compound Used Phenol o-Cresol m-Cresol p-Cresol 1,2,5-Xylenol 1,3,4-Xylenol 1,3,5-Xylenol p-tert-Butyl phenol p-tert-Amyl phenol 4-Phenylphenol o-Non lphenol I-Napgthol 2-Naphthol Cresylic acids"
cresols, and five of the six possible xylenols are not in this catrgory, it is not believed to be a serious shortcoming. Since organic acids show a small amount of infrared absorption a t 2.84 microns (approximately one tenth that of phenol), it is advisable to remove the acids from the carbon tetrachloride extract by contact with sodium bicarbonate solution. The data in Table V show the effects of acid interference and its removal by the bicarbonate treatment. It is apparent that if a determination of acids is desired, the bicarbonate solution may be acidified and the free acids extracted with fresh carbon tetrachloride and determined by the infrared absorption a t 3.4 microns. Here one is faced with the limitation encountered in the attempt to determine the phenols by direct extraction-the fact that the lower homologs cannot be quantitatively extracted from water with carbon tetrachloride. An alternative way to determine organic acids would be to utilize the infrared absorption a t 3.4 microns before and after the bicarbonate extraction. The difference in absorption would then be a measure of the concentration. This method will give approximately correct results because higher homologs of the organic acids show infrared absorp tion very similar to that of hydrocarbons in the 3.4-micron region. Table V.
Removal of Acid Interference by Bicarbonate Treatment
Mixture Prepared Water, phenol, and cyclohexane caproic acid
p.p.m. in carbon tetrachloride. For these measurements the cell-in, cell-out method is used. In both the phenols and oil determinations, the ratio of the volumes of carbon tetrachloride to water is multiplied by the results obtained on the spectrophotometer in order to convert the answer to the original sample bases. DlSCUSSlON OF PHENOL METHOD
In the case of most radiation ahsorption methods utilizing other spectral regions, the individual phenols show absorption maxima a t different wave lengths. The advantage of using the absorption due to hydrogen bonding is shown in Table 111 where it will be observed that the brominated derivatives of the eleven phenols that were measured have their maximum absorp tion a t practically identical wave lengths. Table IV shows that the magnitude of the absorption varies somewhat with the phenol present and in general decreases with increasing molecular weight. On a molal basis, there is considerably better agreement among the phenols. The authors used 2,4,6-tribromophenol, because it is easily prepared in the pure form. Using phenol as the reference standard, the per cent recoveries that may be expected are shown in the fourth column of Table IV. If some specific phenolic compound is to be determined (in the absence of other phenols), the results obtained with the phenol calibration may be multiplied by the ratio of the molecular weight of the compound sought to that of phenol to yield the recoveries shown in column 5 of Table IV or, if better accuracy is desired, the calibration can be made with the phenol being determined. Obviously the latter can also be employed with mixtures of phenols. One weakness of the method lies in the fact that the 2.84-micron band utilized is due to an intramolecular hydrogen bonded 0-H vibration which occurs only in those compounds in which a bromine is substituted in a position ortho with respect to the hydroxyl group. Phenols, therefore, which are already suhstituted in both the 1 and 5 positions, or in which steric hindrance prevents substitution of bromine in either of these positions, will not respond to this method. However, as phenol, the three
Phenol Added, P.P.M.
Naphthenic Acid Added, P.P.M.
Phenol Found NaHCOa Before NaHCOa After wash, p.p.m.
wash, p.p.m.
Water, phenol, and mixed naphthenic acids
The data in Table V also give an idea of the reproducibility and accuracy of the phenol method. The ultimate sensitivity of the method is shown in Tables V and VI, where good recoveries were obtained on concentrations as low as 10 parts per billion on both phenol and o-cresol. In order to do this, it was necessary to reduce the quantity of carbon tetrachloride from 100 to 10 nd. per liter of sample. The data in Tables V and VI give no indication that the method is near its lower limit, and the authors are of the opinion that by employing larger samples the sensitivity might be extended down to 5 or possibly fewer parts per billion. Table VI. Compound Phenol" o-Cresolh
a b
Sensitivity of Phenol Method Phenol Present, P.P.M. 0.01 0.01
0.012 0.031 0,049 0.061
Phenol Found, P.P.M. 0.008 0.006
0.016 0.034 0.054 0.078
Recovery,
% 80 60 133 110 111 128
Phenol calibration used. o-Cresol calibration used.
DISCUSSION OF 0 1 L METHOD
The absorptions at 3.50, 3.42, and 3.38 microns are due t o CH,, CH2, and CH configurations of hydrocarbons. The absorption a t each of the three wave lengths will vary with the relative proportions of CHI, CH,, and CH in the sample. In this particular work, where the composition of the oil to be analyzed varies from day to day and from sample to sample, the sum of the optical densities at 3.50, 3.42, and 3.38 microns gave results which reflected more closely the true value of the oil concentration than a single optical density measurement at, say, 3.42
V O L U M E 23, N O . 10, O C T O B E R 1 9 5 1
1387
microns. This sum is equivalent to the average of three optical densities. The assumption has to be made, of course, that all oils absorb to the same extent a t these frequencies. Table VI1 s h o w the optical density of a number of pure compounds, refinery products, and a series of waste oils collected from a wide variety of locations in a complete petroleum refinery. These oils were taken from areas covering such widely different operations as catalytic cracking, solvent extraction of lubricating oils, sulfuric acid manufacture, synthetic detergent manufacture, patsoliiie treating, and wax processing. The variations shown in T A l e VI1 are not great enough to affect the results seriously when concentrations below 10 p.p.m. are being measured, unless high concentrations of the lower boiling aromatics are present. Probably the most serious shortcoming of t,he method is its inability to detect the lower homologs of t8hearomatic series. As the aromatic CH absorption is relatively weak, it is only aromatics with appreciable paraffin or cycnloparaffin sick chains that exhihit iiormal behavior in the detrrmination. In view of the variation of optical density shown in Table VII, it is obviously not possible to pick a reference standard for the oil determination which will give the correct value for all waste oils. For the best accuracy a sample of the oil that is dispersed in the water, if available from an oil separator in t,he sewer system, should be used for calibration purposes. Where such an oil sample is not available, a mixture consisting of 37.5% isooctane, 37.5% of cetane, and 25% of benzene is suggested as a reference standard which, on the hasis of Table VII, should g h e Satisfactory recoveries on at least the majority of oil samples encountered in a petroleum refinery. The ability of carbon tetrachloride to remove the oil from the water is shown in Table VIII, where the results indicate that 15 minutes of shaking are adequate to remove an average of 98% of the oil. These data were obtained on an average oil from a plant sewer and were calculated by using the optical density of the oil saxnple for the calibration in order to eliminate the error due to deviation of the optical density from that of the cetane-isooct:tne-benzene reference 3tand:ird. The same method of calculation was used to compute the reeults in Table IX showing the sensitivit>yof the method. Here 10 rnl. of carbon tetrachloride per liter of water were used for -~
_ ~ _ _ __ _
Tahle VII.
Infrared Absorption o f CH3, CH?, and CH Groups
~
_
Optical Density of 16 P.P.hI in CClr
_
~
% of Iso-
octane-cetaneSample Esamined Benzene Standard Iso-octane--octane-benzene o.:19n standard“ Waste -. .oils From kerosene treating area 0,424 109 From gasoline treating area 0,384 98 From pipe stills and gasoline desulfurization area 0.374 96 From solvent extraction area 0 415 107 0 318 From thermal cracking area 81 0 355 From catalytic cracking area 91 0 380 From wax refining area 100 0 409 105 From crude oil handling area Refinery products 0 42i Light lubricating oil 110 0 491 Heavy lubricating oil 126 0,38.i Liaht Refuaio crude oil 99 Furnace ol’l from catalytic cracking 0.339 87 74 S o . 6 fuel oil 0.287 107 White oil (medicinal) 0.417 Kerosene 0.440 114 Gasoline base stock 0.387 99 Pure coinpounds Cetane 0 615 158 Iso-octane 0,404 104 157 n-Heptane 0 613 p-Di-tert-butyl cyclohexane 0 396 102 Ethyl cyclohexane 0 543 139 Cyclohexane 0 703 180 I-Tetradecene 0.427 110 Deoene 0.414 106 Diisobut lene 0.208 53 High moreoular weight alkyl henzene 0.165 40 Cumene 0.109 28 Mixed xylenes 0.070 18 Benzene 0.00 0 Blend of 37.5’7 bo-octane (2,2,4-trimethylpentane), 37.5% cetane, and 25% benzene b v vofume.
-
Effect of Shaking Time on Extraction
Table VIII.
cclc
Waste Oil Added, P.P.M.
Used,
10 5 10 5 10 5
100 100 50 50 25 25
Per Cent Recovered after Shaking 1 hour 0.5 hour 0.25 hour
hll.
92 102 100 99 106 99 -4v. 100
94 95 91 95 108 107 98
98 92 92 99 100 99 98
the extraction, making possible the detection of 0.1 p.p.m. of the oil in water. As the blank determination gave a value of 0.04 p.p.m., it is believed that the method is, in this case, approaching its lower limit of sensitivity. The analytical results given in Table X for synthetic mixtures containing both oil and phenols show that these components can be separately determined in one extraction without serious interference. All data given in the tables are on synthetic mixtures. The synthetic oil was prepared by weighing the required amount of oil into the bottle of water, followed by a t least 15 minutes of violent mechanical agitation equivalent t o that used in the extractions. The carbon tetrachloride was then added t o the bottle and the extraction carried out as indicated above. Less agitation was required in preparing the synthetic phenol mixtures owing to the solubility of the phenols in water. Tahle IX.
Sensitivity of Oil Determinationo
Waste Oil Added, P.P.M.
Waste Oil Determined, P.P.hI
1 2 1 2_ . 0.1 0.1 0.0
1 1
I n
0.1 0.1 0.04
Water extracted with 10 ml. of CCIPper liter.
Table X . Sample 1 2 3
Analysis of Synthetic Samples Containing Phenols and Oils Oil Fraction, P.P.M. Phenol, P.P.hI.
By synthesis 3 6 ,Za 29.3b 12.3
By analysis
By synthesis
By analysis
29.5 26.0 13.7
9.2 9.1 9.6
9.3 8.9 5.3c
a Composed of 11.8 p.p.m. cyclohexane caproic acid 13.2 p,p.m. cyclohexane acetic acid and 11.2 p.p.m. lubricating oil base stdck. b Coniposed of i 0 . 4 p.p.m. cyclohexane caproic acid, 9.0 p.p.m. aromatics (benzene, toluene. and xylene), and 9.9 p ~ p . mlubricating . oil base stock. Phenol fraction consisted entirely of p-tert-butylphenol, determined as phenol. Molecular weight correction gives 8.5 p.p.ni. p-tert-butylphenol by analysis.
~~
Although no dat’a are shown on the analysis of actual waste waters, enough work has been done to show that the method is practical and usable, except on an occasional sample t h a t will form a stable emulsion with carbon tetrachloride. These cases are unusual, however, and unless a very stable emulsion is formed, t’he required 15 mi. of carbon tetrachloride can usually be broken out of the emulsion on standing or by filtration if 100 ml. are used for the extraction. LlTERATURE ClTED
(1) A m . P e t r o l e u m I n s t . , “ M a n u a l o n Disposal of Refinery IVastes, Section 1, Water C o n t a i n i n g Oil,” 4 t h e d . , A u g u s t 1949. (2) A m . P u b l i c H e a l t h Assoc., N e w York, N. Y., “ S t a n d a r d M e t h o d s for t h e E x a m i n a t i o n of W a t e r a n d Sewage,” 9 t h ed., p . 216, 1946. ( 3 ) G i b b s , H. D . , J.Bid. Chem., 72, 649 (1927). (4) Heigl, J. J., Bell, hf. F., a n d W h i t e , J. U.. AXIL. CHESI.,19, 293 (1947). (5) K i r s h m a n , H . D., a n d P o m e r o y , R i c h a r d , Ibid., 21, 793 (1949). (6) L y k k e n , Louis, Treseder, R. S.,a n d Z a h n , Victor, IND.Ex-,. C H E M . .ANAL.ED.,18, 103 (1946). (7) Pauling, Linus, “ N a t u r e of t h e Chemical B o n d , ” p. 316 e t s e q . , Cornel1 U n i v e r s i t y Press. 1940. I t h a c a , N. Y., (8) S t o u g h t o n , R. W., J . Bid. Chem., 115, 293-9 (1936). (9) TTulf, 0. R . , Liddel, U r n e r , a n d H e n d r i c k s . S.E . , J . Am. Chem. SOC.,58, 2287 (1936). RECEIVED April 20, 1951.
