sulfate, 0.05 p.p.ni. hexavalent chromium, 5 1i.p.m. fluoridf., 1 p.1i.m. boron. and 50 p.1i.m. nitrates The nitrates do react mith the chiomotropic reagent to produce a yellow color which obscures the purple color when the test i s performed visually. (This difficulty may be obviated by reduction of the ni trates with two d r o p of concentrated HC‘1 and 1 ml. of concentrated HzS04 1)Pr milliliter of sainple prior to the addition of the solid chromotropic acid to the aliquot of the refluxed sample.) S o n e of these ions interferes when the teit is performed spectrophotometrically. Saturally, any compounds which sillit off formaldehvde w o n addition of Hi?SO,,such as hexamethylenetetramine, forinaldoxime, and some of the oxymethylene compounds, would show a Ilositive reaction. .h much as 500 p , p , m , ethyl alcohol and 500 1i.p.m. hydrazine do not interfere. Monomethyl hydrazine gives; a positive reaction much the same as U D M H and this procedure might be applicable in its detection. FACTORS ;\FFI;CTING REACTIOX. The quality of the chromotropic acid is quite important. The technical grade 4,5 - dihydroxy - 2,T - naphthalenedisulfonir arid does no{,dissolve readily in the sulfuric acid and gives high readings caused by interference a t 580 mp. Thc practical grade is entirely satisfactory and in acid solution is free from residue or interfering color production. Refluxing time is (critical. Shorter
refluxing periods than 16 to 20 hours do not give complete reaction. Reproducible result’s have been obtained on standards that were refluxed from 20 to 86 hours. The ratio of IJDMH t’o HCHO has been found to remain constant using the equipment described. Where the oxidation is effected with a deficiency of air, and the CDRIH only partly converted to HCHO, the factor required t,o calculate V D M H from HCHO, in parts per million, is greater than two, and depends upon the equipment used in t,he determination. Thus, if the ullage were small and t,he amount of available oxygen less than stochiometric, the amount of U D M H oxidized to HCHO would be greatly reduced. Heating a t 60” to TO” C. for 10 minutes is necessary for full color development. DISCUSSION
The oxidation of unsymmetrical dimethylhydrazine is very complicated and has not yet been explained; there are no readily available published methods for the determination of U D M H in the parts per million range. This uncomplicated procedure defines a situation within which C D M H is converted quantitatively and reproducibly to formaldehyde. The well known formaldehyde-chromotropic acid reaction has been postulated by Feigl (3) to consist of condensation of the phenolic chromotropic acid with form-
aldehyde followed by an oxidation to t h e p-quinoidal compound. ACKNOWLEDGMENT
The author thanks E. F. C. Cain and T. S. Lee of this laboratory for their helpful suggestions. Many analyses were performed by analysts P. E. Miller and W. H. Gross, also of this laboratory. LITERATURE CITED
(1) Audrieth, L. F., Ogg, B. A,, “Chemistry of Hydrazine,” pp. 126, 226,
Wiley, Kew York, 1951. ( 2 ) Carpino, L. A., J . Am. Chem. SOC. 79,4472 (1957). (3) Feigl, F., “Spot Tests in Organic
Analysis,” 5th ed., p. 331, Elsevier, New York, 1956. (4) McBride, W. R., Kruse, H. W., J . A m . Chem. SOC.79, 572 (1957). (5) Noller, C. K., “Chemistry of Organic Compounds,” p. 219, W. B. Saunders Co., Philadelphia, 1951. (6) Overberger, C. G., Lombardino, J. G., Hiskey, 13. G., J . Am. Chem. SOC. 79, 6430 (1957). (7) Scot,t, W. W.,“Standard Nethods of Chemical Analysis,” 5th ed., p. 2149, Van Sostrand, Sew York, 1939. (8) Snell, F. I)., Snell, C. T., “Colorimetric Methods of Analysis,” 3rd ed., p. 256, Van Xostrand, Yew York, 1953. (9) Westvaco Chlor-Alkali, Food 11achinery and Chemical Corp., South Charleston, W. Va., Bull. SC 443-3 (September 1956). RECEIVEDMay 31, 1961. Resubmitted December 30, 1963. Accepted July 10, 1964.
Ultraviolet Spectrophotometric Determination of an Indicator (1s a Measure of Basic Impurities in Dimethylf o rmamide H. E. Z A U G G and A. D. SCHAEFER Organic Chemisfry Deparfment, Research Division, Abbotf Laboratories, Norfh Chicago, 111.
b Using 3-phenyl-2-benzofuranone as an indicator, a method is reported for the quantitative spectrophotometric determination of basic impurities in dimethylformamide in the concentraA protion range lo-* to 10-W. cedure for removing basic impurities from dimethylformami’de is detailed and the results of stability measurements on purified solvent are reported. Unsuccessful attempts to employ various phenols as indicators are outlined and a reason is given for the ineffectiveness of p-nitrophenol as an indicator, despite the fact that iit is a stronger acid than 3-phenyl-2-benzofuranone in aqueous solution.
T
HE HIGH SOLVENT capacity of dimethylformamide combined with its transparency in the visible and nearultraviolet region of the spectrum has accounted for its serviceability as a solvent in spectrophotometric work. Unfortunately, it tends to accumulate and retain appreciable quantities of basic impurities (probably mainly dimethylamine) ( 4 ) which can complicate the spectra of even weakly acidic substances. This paper describes a procedure for removing these impurities from dimethylformamide and a method for their quantitative determination in the concentration range t o 10-6N using 3-phenyl-2-benzofuranone (I) as a
spectrophotonietricz indicator baqed on the following reaction:
I
I1
+
+
B:
(+)
B:H
3-Phenyl-Bbenzofuranone i h readily synthesized in one step from phenol and mandelic acid ( 1 ) . I n its acid form I , it does not absorb above 300 nip. I t s VOL. 36, NO.
1 1 , OCTOBER 1964
2121
Table I. Results with Solutions of Known Dimethylamine Concentration in Dimethylformamide
Indica-
x
I 36 x 10-5.w 1 82 2 50 3 73 4.03 4.03 4.88 4.92 5.97 6.19 7.58 8.05 9.83 a The same solution was examined with different amounts of indicator. 50 1 25 3 50 6 40 4 24.7" 50.0" 50 8 50 0 51 4 50 2 49.7 50.2 50.2
1 45 2 00 2 57 3 73 4.30 4.30 4 85 4 88 6 03 6 13 7 11 7.67 9.38
10-5x
conjugate base 11, however, absorbs 3 6 strongly at 353.5 mp ( ~ ~ ~ ~ X2 .lo4) in dimethylformamide. EXPERIMENTAL
Apparatus. T h e absorbance meaeurements were made in 1.000-cm. silica cells using a Cary 11 recording spec t ro p ho t o meter . Materials. The 3-phenyl-2-benzofuranone (I), m.p. 114-16@ C., was prepared by the method of Bistrzycki and Flatau ( I ) and rec 95y0 ethanol to a co point. I t was dried under vacuum a t 75" C. and stored in a tightly stoppered bottle. The commercial anhydrous dimethylamine used was of 99.0% minimum purity. The dimethylformamide (DMF) was purified by the following modification of methods reported previously (3, 4 ) . To 2 liters of reagent grade dimethylforniamide \vas added -100 ml. of reagent grade benzene and the mixture was distilled at atmospheric pressure until the vapor temperature reached 130@C. The undistilled liquid was transferred to two carefully dried 1-liter bottles which previously had been flushed with dry nitrogen. To each bottle was added 20 to 30 grams of phosphorus pentoside, and after closing with Teflon covered rubber stoppers, both were shaken mechanically for 3 to 4 hours. After allowing the phosphorus pentoxide to settle, the supernatant liquid was decanted into two other dryl nitrogenfilled bottles, 30 grams of potassium hydroxide 1)ellet.q were added to each, and shaking was resumed for 1 to 2 hours. The supernatant liquid was once again decanted into a 3-liter, sidearm round bottom flask (filled with nitrogen) and distilled under vacuum (20 to 25 mm.) in an atmosphere of dry nitrogen. The side-arm of the boiling flask was equipped with a finely drawn glass capillary tube to which a nitrogen filled rubber balloon was fastened.
2 122
e
ANALYTICAL CHEMISTRY
(The capillary extended to the bottom of the flask and served to prevent bumping during the distillation). The distillate was redistilled once again in the same manner, and a center cut of about one liter was distilled into an appropriate storage vessel. ;\t the conclusion of the distillation, the vacuum was released with dry nitrogen and the solvent was dispensed from the storage vessel as needed. In this way, dirnethylforniamide could be obtained which contained no titratable acidic impurities and basic impurities in concentrations less than i X 1 0 - 6 A as ~~ measured by the following procedure. Recommended General Procedure. Place 25 to 50 mg. of 3-phenyl-2benzofuranone (I) in a dry nitrogenfilled IO-ml. volumetric flask and fill to the mark with the dimethylformamide to be tested. As soon as the solid has disbolved, measure the absorbance (L1) (1-cm. cell) at 353 to 354 mp. The equivalent concentration of basic impurity is then given by the expression [Base] = 612.36 X lo4. RESULTS A N D DISCUSSION
Beer's Law Adherence. The molar absorptivit'y ( emax 353.5) of 3-phenyl-2benzofuranone a t five concentrations in the range 2 to 11 x 1 0 - ~ vwas determined in dimethylformamide conbining excess dimet'hyla,mine. The maximum observed emax 353.5 was 2.36 X l o 4 and the minimum was 2.21 X lo4 with no indication of any nonrandom variation. Optimum Concentration of Indicator. T h e absorbances of varying concentrations of 3-phenyl-2-benzofuranone from 0.1 mg.110 ml. to 100 mg./lO ml. were measured using a single sample of dimethylformamide cont,aining a 2 X l O-sJI concent,ration of basic impurity. Up to a concentration of 10 mg.110 ml., t,he absorbances increased rapidly with increasing concentration of indicator. A4t this point the increase began to level off and in t,he range 20 to 50 mg.:'lO ml. further increases were negligible. Test of the Method. h sample of dry dimethylformamide was gassed with dry dimethylamine for a n appropriate time and was potentiometrically 0.0115.Y in dimethylamine (50-ml. samples added to 50 ml. of acetone were titrated with 0.100OS aqueous hydrochloric acid). tTsing purified dimethylformamide, which gave no detectahle absorption at 353 to 354 mp in the presence of I, samples of this dimethylamine solution were diluted t,o t,he known molar concentrations listed in the second column of Table I. Using the quantit,y of indicat,or specified in column 1, the dimethylamine concentration was then measured by the recommended procedure \vit,h the results listed in column 3. The exact agreement observed for t,he two runs in which indicator concent,ra~
tion was the only variable suggests that the bulk of the discrepancy between known and found values may have st'emmed from errors in dilution to the known concentrations rather than from errors inherent' in the method, The maximum molar absorptivity (2.36 x lo4) observed for I, rather than the mean, was used in this work because it gave the best agreement between the known and found values. A plot of the found values os. known concentrations gave a line with slope 1.08 i 0.02. The st,andard deviation(,) about the best fitt,ing line was 0.16. Stability of Purified Dimethylformamide. Even when stored under d r y nitrogen, purified dimet'hylformamide slowly accumulates basic impurities. Furthermore, the stability appears to vary slightly wit,h the sample. The following list summarizes the increase in base concent,ration observed for four different samples during the t,ime interval noted in parentheses: 4.79 t'o 11.4 X 10-6M (17 days), 6.36 t o 13.1 X 10-6M (20 days), 7.20 to 8.90 X 10-6JI (5 days), 1.02 to 4.84 x 1 0 - ~ M (21 days). In one case where no base could be det'ected after purification, none was observable aft'er 2 days of storage. Stability of Indicator Solutions. The optical instability of indicator solutions suggest's that, a slow reaction occurs between solvent' and solute. The absorbance increases and the maximum wavelength decreases with time. The following is a typical example of t8he spectral change of a solution 0.02403f in I (times are indicated in parentheses): 353.5 mp, 0.21 (0) ; 353.0, 0.29 (2 hours) ; 352.5, 0.34 (4hours); 351.5, 0.40 (6 hours); 350.0, 0.61 (23 hours). These results serve to emphasize the importance of obtaining the epectrum immediately after the indicator is dissolved in the solvent,. Similar, but, no great'er, instability was encountered with eolut'ions of 3-phenyl-2-benzofuranone in the related solvents, X,?;-dimethylacetamide and N-methyl-2-pyrrolidone. Although procedural details have not been worked out for these two solvent,s, it is likely that 3-phenyl-2-benzofuranonecan be used for t'he spectrophotometric determination of basic impurities in them as well. Other Indicators Tested. The pK. of 3-phenyl-2-benzofuranone is 8.1, as measured by potent>iometrictitrat,ion in various alcohol-water mixtures followed by graphic extrapolation to pure water. This relatively weak acidity (in water) suggested t,hat cert'ain phenols might also serve as spectrophot,ometric indicators. However, at dimethylamine concentrations in dimethylformamide as high as 10-4.1P, there was no indicat'ion that any phenoxide ion was produced from
phenol, p-cresol, p-chlorophenol, or p-methoxyphenol. Even p-nitrophenol (pK, 7.0) in large excess v a s converted to its anion in amount: only fractionally equivalent to the actual quantity of base present. This reversal of acidity bequence (p-nitrol~henolZ J S . I) in going from n ater to dimethj.lforinaniide very likely is a reflection of an ability of dipolar aprotic solvents such as dimethylformamide to solvate (and thus to stabilize) large polarizalble anions better than they solvate smaller anions with more localized charge ( 2 ) . I n the anion I1 it is apparent that the negative charge can be delocalized throughout the nholc molecde, including the two benzene rings (NAIll spectral comparizions of I and 11 bear this out). This results in a much larger, more polarizable anion than is represented by
the less extensively delocalized p nitrophenoxide ion. This explanation receives further support from a consideration of the tranqition energies of IT and p-nitrophenoxide ion in ethanol (aprotic solvent like water) and in diniethylformamide. I n going from ethanol ,A,(, 400 mp) to dimethylformamide ,A,(, 435 mp) the transition energy of the latter decreases by only 5.8 kcal.1 mole. A similar change in environment for the anion I1 results in the much larger decrease of 9.6 kcal./mole (A,,,C2B6oH 316 mp: AmaXD"IF 353.5 mp). Thus, relative to p-nitrophenoxide ion, I1 is more highly solvated in dimethylformamide. Consequently, dissociation (or acidity) of I is increased more than that of p-nitrophenol. (The ultraviolet spectra of both p-nitrophenoxide anion and I1 in either ethanol
or dimethylformamide are unaffected by variation of the cation. Hence, it can be concluded that the observed spectra in these solvents are characteristic of the free solvated anions.) ACKNOW LEDGMENl
The authors are indebted to D. C. Wimer and V. E. Papendick for the potentiometric titrations and pK determinations. LITERATURE CITED
( 1 ) Bistrzycki, .4., Flatau, J., Ber. 28, OS9 (1895); Ihid. 30, 124 (1897). (2) Parker, A. J., Quarterly Revs. 16,
171-6 (1962). ( 3 ) Prue, J. E., Sherrington, P. J., Trans. Faraday SOC.57, 1795 (1961). ( 4 ) Thomas, A . B., Rochow, E. G., J . Am. Chem. SOC.79, 1843 (1957). RECEIVEDfor review >\lay 15, 1964. Accepted July 24, 1964.
S pect ro p hoto metric Dete rminat io n of PerchIorate JAMES S. FRITZ, JANET E. ABBINK, and PATRICIA A. CAMPBELL Institute for Atomic Research and Department of Chemistry, Iowa State University, Ames, Perchlorate in a concentration range of 0.0001 to 0.01 mniole is extracted from aqueous solution by n-butyronitrile as ferrous 1,lO-phenanthroline perchlorate. The cimount of perchlorate is calculated from spectrophotometric measurement of the absorbance of the extract. Moderate to large amounts of chloride, sulfate, or nitrate cause little or no interference. Chlorate is slightly extracted but the interference can b e corrected.
M
for the quantitative determination of perchlorate have been reviewed recently (6). Generally the methods fall into i.hree categories: reduction to chloride by fusion, reduction to chloride in solution with titanium(" or other reductant, and precipitation as insoluble salts such as tetraphenylarsonium perchlorate. Most of the published methods are either subject to numerous interferences or else they are difficult to make work quantitatively. In 1960, Burns and Muraca ( 2 ) obtained good results for ammonium perchlorate by reduction with titanium(II1) using osmium tetroxide as a catalyst. I n 1962, Johannesson (3) publi:,hed an isotope dilution method for perchlorate in which volatile anions were removed by evaporation. Recently, .Illey and Dykes ( I ) have analyzed ammonium and potassium perchlorates by reduction with titanium hydride in 1 : 3 sulfuric acid followed by titration of total chloride with silver nitrate. ETHODS
Iowa
adjusted to p H 5 with acetic acid or sodium acetate. Pipet exactly 4.00 ml. of n-butyronitrile into the centrifuge tube and shake for a t least 1 minute. Centrifuge briefly to separate layers. Pipet euactly 2.00 nil. of the butyronitrile layer into a 25-ml. volumetric flask and dilute to volume with acetone. (Kear the upper end of the percentage range it is better to use a 50-ml. volumetric flask.) Measure the absorbance of the extracted solution a t 510 mp on a Beckman Spectrophotometer, RIodel B, using 1-em. glass cells. If other common anions are present, obtain the correction factor, if any, from the appropriate graph and adjust the absorbance. Read the amount of EXPERIMENTAL perchlorate from a Beer's law plot Reagents and Solutions. FERROIN. prepared by extracting and measuring the absorbance of known perchlorate A 0.01Jf solution of ferroin, ferrous standards using this same procedure. l,l0-phenanthroline sulfate, was prepared using ferrous sulfate and 1,lOphenant hroline. DEVELOPMENT OF METHOD PERCHLORATE. A standard solution of 0.10OM sodium perchlorate was preSolvent. Most organic solvents do pared. More dilute perchlorate solunot extract ferroin perchlorate from tions were prepared by diluting aliquots aqueous solution. Margerum and of this solution to a definite volume. Banks studied the extraction of iron FOREIGN ,INIONS. Solutions were as the ferrous- 1,lO-phenant hroline prepared using reagent grade sodium, complex from solutions containing perpotassium, or ammonium salts. chlorate ( 4 ) . They found nitrobenzene BUFFER. The p H 5 acetate buffer was approximately 0.1M in sodium to be the best solvent for this extraction. acetate and 0.1JI in acetic acid. However nitrobenzene has a yellow SOLVENTS.n-Butyronitrile was obcolor and is not an ideal solvent for tained from Eastman Chemical Prodextraction work. Recently n-butyroucts and was used without purification. nitrile and benzonitrile have become Reagent grade acetone was used. commercially available. Both of these Procedure. Add a 2-ml. sample consolvents are water-white and either taining 0.0001 to 0.01 mmole of perchloextracts ferroin perchlorate comrate to a glass-stoppered centrifuge pletely. n-Butyronitrile was chosen tube. Add 1 ml. of ferroin solution. Add 3 ml. of acetate buffer which was for the present work because the organic The present determination of perchlorate is based on solvent extraction. Ferrous 1,lO-phenanthroline sulfate (ferroin) is not extracted from aqueous solution into organic liquids. However, if ferroin is added to an aqueous solution containing perchlorate, ferrous 1,lOphenanthroline perchlorate is extracted into n-butyronitrile and the amount of extraction is proportional to the amount of perchlorate present. Perchlorate is determined by measuring the absorbance of extracted ferrous 1 , l O phenanthroline perchlorate spectrophotometrically.
VOL. 36, NO. 1 1. OCTOBER 1964
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