measured amount of oxygen-free benzene containing a known amount of water sufficient to produce an absorbance change of a t least 0.2. Shake the comparator tube for 3 minutes to ensure complete reaction and record the decrease in absorbance. Since the calibration curve must be prepared a t constant volume, correct the absorbance reading for the amount of standard or sample injected. Repeat the above procedure using aliquots of sample. Calculate the active hydrogen content, expressed as HzO, from the calibration curve. The greatest precision for this method has been achieved when working between 1.5 and 0.8 absorbance units; accordingly, we recommend that all work be done within this range. DISCUSSION
The data published by Mitchen (8) show that the red isoquinoline complex formed from 10.5 mg. of Et2AlH will produce a change of 1.00 absorbance unit in a 1-cm. cell. Calibration data (Table I) show that 1.15 mg. of water decrease the absorbance of the complex by 1.00 unit. The combined data indicate that the EtzAIH reacts with water in a ratio of 2 moles of EtzAlH for each mole of H20 present. Diethylaluminum hydride reacts also with dissolved oxygen, but in a 4:l mole ratio. Therefore, to obtain a determination for water when oxygen is present it becomes necessary to make an independent determination of the oxygen. Henderson's method (1) is convenient for determining the amount of such oxygen in the sample. Briefly, this procedure is as follows: The same spectrophotometer and comparator tube are used except that the cell has a line etched on it to indicate the 3-ml. level. To the comparator tube add 100 ml. of benzene followed by
15 ml. of n-butanol (used as an emulsion breaker). Flush the air space in the tube above the liquid with nitrogen and place a sleeve-type serum stopper over the mouth of the tube, securing it tightly with rubber bands. Add 10 ml. of reduced copper solution and 10 ml. of concentrated ammonium hydroxide. Shake the comparator tube with its contents for 3 minutes, then draw off the aqueous phase with a hypodermic syringe. Again add 10 ml. of reduced copper solution and 10 ml. of concentrated ammonium hydroxide. Shake the contents of the tube again for 3 minutes and draw off all but 3 ml. of the aqueous phase. Read this blank a t a wave length of 640 mp. Add about 10 ml. of the sample to be determined, mix the contents thoroughly for 3 minutes by shaking the tube vigorously, and read the increase in absorbance at 640 mp. A calibration curve may be prepared by adding known amounts of air via a hypodermic syringe and plotting micrograms of oxygen us. the difference in absorbance from the blank.
Data in Table I1 show the results obtained from the determination of moisture in several organic solvents as compared to the Karl Fischer method. Experience indicates that the precision and accuracy for the combined oxygen and moisture determinations are in the A595 range. No difference in sensitivity was obtained in the water calibration when xylene was used in the place of benzene. Therefore, it is believed that any hydrocarbon or aromatic solvent not containing active hydrogen may be substituted for benzene as the reaction solvent. The chemistry of this method indicates that it is particularly applicable to the determination of moisture in metal alkyls and in solvents to be used with aluminum alkyl catalysts or Grignard reagents.
Table I. Calibration Data for Reaction of Water with EtzAlH
(25-ml. volume, 1.0-cm. light path, 460 mp)
H20 Added, Mg.
Decrease in Absorbance
0.100 0.100 0.170 0.170 0.260 0.425 0.425 0.637 0.800 0.850
0.075 0.080 0.140 0.151 0.250 0.365 0.360 0.555 0. 700 0.740
Table II. Comparison of Moisture Values for Several Organic Solvents
Oxygen This Karl Correction" Method* FischerC (as Corrected Method, P.P.M. P.P.M. for 0 2 , H20) P.P.M. H20 HtO
Solvent Xylene 44 215, 217 215 Xylene 64 229, 242 225 Benzene 55 433, 410 433 Dimethyl carbitol 51 637, 639 639 1-Hexene 119 232, 236 4 Oxygen determined by reaction with cuprous ion in ammoniacal solution by Henderson's method ( I ) . b Used 1-ml. sample. c Used 25-ml. sample.
LITERATURE CITED
(1) Henderson, S. R., unpublished work.
This is a colorimetric procedure based upon oxidation of cuprous ion in ammoniacal solution. (2) Henderson, S. R., Snyder, L. J., ANAL.CHEM. 31,2113 (1959). (3) Mitchen, J. H., Ibid., 33,1331 (1961).
RECEIVEDfor review March 29, 1961. Accepted June 5, 1961.
Spectrophotometric Determination of Dialkylaluminum Hydride and Trialkylaluminum J. H. MITCHEN Research and Development Department, Ethyl Corp., Baton Rouge, l a .
b The intense red color formed when isoquinoline is added in excess to a dialkylaluminum hydride is the basis of a method for determining dialkylaluminum hydride in trialkylaluminum. The absorbance i s measured at 4 6 0 mp. The greater strength of the trialkylaluminum complex with isoquinoline forms the basis of an extension of the method to the determination of trialkylaluminum, even though this complex does not absorb at 4 6 0 mp.
The procedure is simple and requires no complicated equipment. This method is believed to provide the most rapid simple determination of the compounds currently in use. Compounds of the type R,AICI3-n are believed to interfere with the determination of trialkylaluminum but not with dialkylaluminum hydride. In general, the method is accurate to within of the amount present.
T
HE direct analysis of alkyl aluminum compounds is difficult, and most methods depend on the measurement of various gases evolved when the sample is hydrolyzed or treated with other compounds containing active hydrogen. Methods involving gas evolution are usually very time consuming since the gases must then be analyzed by conventional means, using gas-liquid chromatography or mass spectrometry. I n addition, it is often difVOL 33, NO. 10, SEPTEMBER 1961
1331
ficult or impossible to obtain accurate information on dialkylaluminum hydride content from gas evolutions. For example, triisobutylaluminum decomposes under ordinary storage conditions. The products are diisobutylaluminum hydride and isobutylene, which remain dissolved in the triisobutylaluminum. When triisobutylaluminum is hydrolyzed, there is often thermal decomposition to the same products. Thus one never knows whether the decomposition had already occurred or whether the hydrolysis was faulty. Bonitz (1) reported a yellow complex of triethylaluminum with isoquinoline, and both a colorless 1:l mole ratio complex and a red 1:2 mole ratio complex of diethylaluminurn hydride with isoquinoline. He also reported the potentiometric titration of triethylaluminum and diethylaluminurn hydride with isoquinoline, as did Farina, Donati, and Ragazzini (8). Vaillant (5) has reported the use of the triethylaluminum-isoquinoline product as an indicator for visual titrations, and attributes the red color to a 1:2 complex of triethylaluminum and isoquinoline rather than diethylaluminurn hydride and isoquinoline. Our work shows conclusiveIy that the red color is due to diethylaluminum hydride as reported by Bonitz. Neumann (4) reported the determination of dialkylaluminum hydride using benzylideneaniline. He also reported, and we have confirmed, a constant molal absorbance for a number of dialkylalumin u n hydride complexes with isoquinoline. In this paper we report a simplified method based on the red 1:2 dialkylaluminum hydride : isoquinoline complex for the determination of either dialkylaluminum hydride or trialkylaluminum or mixtures of the two. Theoretically, it should be possible to titrate triethylaluminum and diethylaluminum hydride spectrophotometrically. The absorbance a t 460 mp should remain constant with addition of isoquinoline until both the 1 : l complex with triethylaluminum and the 1: 1 complex with diethylaluminum hydride are formed, since both are stronger than the 1:2 diethylaluminum hydride: isoquinoline complex. As the diethylaluminum hydride 1:2 complex is formed, the absorbance should increase until the 1:2 complex formation is complete, and then remain constant as additional isoquinoline is added. This hypothesis was tested by adding a standard solution of isoquinoline in benzene to a solution of triethylaluminum in benzene in a circulating cell in a Beckman DU spectrophotometer. The absorbance reached a maximum much too soon and then decNased as more isoquinoline was added. This suggested that ev-n due t c im-
1332
ANALYTICAL CHEMETRY
purities in the titrant were present, and that these same errors might explain some of the difficulty we had experienced in attempting to duplicate Bonitz’s potentiometric titrations. Very careful attention to contamination would minimize this error. However, a spectrophotometric method in which the sample is added to an excess of isoquinoline would be preferable, since this would allow f0.r inactivation of the impurities in the analytical apparatus and reagents before the actual determination. APPARATUS AND REAGENTS
A Beckman Model DU spectrophotometer was used for this study, with the cell compartment modified as described by Henderson and Snyder (3). The sample cell was similar to the one they describe except that a rubber serum cap was used in place of the ground glass stopper. Sample transfers were made using a hypodermic syringe
+SI-.
.-$e-WEIGHT OF SAMPLE
Figure 1. Absorbance changes in triethylaluminum determinations
fitted with a 22-gage needle. Sample weights were obtained by weighing the hypodermic syringe before and after sample transfer. Dry benzene, distilled over PzOs, is used as solvent. Solutions of freshly distilled isoquinoline in benzene are used to develop the color; a 5% solution is used for determining &ALH, and a 2% solution when RBAl is also to be determined. A solution of diethylaluminum hydride in benzene is required to establish the blank unless the sample itself has enough diethylaluminum hydride as an impurity. The diethylaluminum hydride need not be pure, but should have a high hydride content. Diethylaluminum hydride should be used to establish the initial absorbance for all types of &AlH compounds, since 8 stable blank is difficult to achieve with the higher alkyl aluminum hydrides. This instability results from the slower rate of reaction of the higher aluminum alkyls with water and oxygen in the system. The stability and rate of formation of their isoquinoline complexes, however, compare favorably with those of triethylaluminum.
PROCEDURE
The highly reactive nature of the aluminum alkyls must always be an important consideration in any analytical work, both from the standpoint of safety to the analyst and of accuracy of the results. All equipment must be thoroughly cleaned and oven dried. The sample must never be exposed to oxygen or water. All open vessel transfers must be made in a dry nitrogen atmosphere. Add 25 ml. of a solution of isoquinoline in benzene to the absorption cell, and stopper the cell with a serum cap after flushing out the air with nitrogen. Add a few drops of the diethylaluminum hydride reagent and mix it well until a stable color having an absorbance of approximately 0.3 to 0.5 a t 460 mp is obtained. The absorbance should not change more than 0.01 in 3 to 5 minutes. To determine dialkylaluminum hydride, add a weighed portion of sample, previously diluted with dry benzene if necessary, note the volume added, and record the absorbance. The increase in absorbance is due to dialkylaluminum hydride and is calculated from a standard curve or formula, If it is desired to determine trialkylaluminum in this same sample, continue the addition of sample until the absorbance passes through a maximum. It is not necessary to measure the maximum. At the point of maximum absorbance all the isoquinoline is bound up in the red 1:2 dialkylaluminum hydride :isoquinoline complex and in the trialkylduminum 1:1 complex. Thereafter, additional sample destroys the red complex in favor of the 1:1 complexes, decreasing the absorbance a t 460 ma. Continue to add sample until a convenient value for measurement is reached, such as 1.5, for example. Record the absorbance and the volume of sample added. Add a second weighed portion of sample to the cell, and record the absorbance and volume added. All absorbance values should be corrected to the absorbance equivalent a t the original volume (25 ml.). The approximate volume of each addition may be obtained by using the graduations on the syringe. More accurate corrections may be made by using the specific gravity and weight of the sample or sample solution. The decrease in absorbance is due both to dialkylaluminum hydride and trialkylaluminum. Because the curve is calibrated as dilkylaluminum hydride, the decrease in color observed is that which would be produced by the dialkylaluminum hydride (already determined) plus the dialkylal’uminum hydride equivalent of the trialkylaluminum present in the sample. This is shown graphically in Figure 1. For illustration, it is assumed in Figure 1 that the sample size (SI)used to determine the E t A l H (original color increase produced) is identical with the sample size (SZ) used to determine AlEts. Let 2 represent milligrams of
E t d H and y represent milligrams of AlEt, in the weighed samples: Then A,-A, represents the color due to z mg. of E t J l H . The dotted line from Az to As represents additional unweighed sample which complexes all the isoquinoline a t the maximum and then decreases the color as the stronger AIEta and Et&H 1 : l complexes are formed at the expense of the 1:2 hydride complex. A3-A.4 represents the further decrease due to the second weighed sample (SI)containing z mg. of E t d H and y mg. of AlEts. The difference, (A3-AJ - (A2-AJ, is the Et2AlH equivalent of y mg. of AlEt3.
Table 1.
Standardization Data Showing Absorbance of Et2AIH-2-lsoquinoline
(25-ml. volume, 0.5-cm. light path, 460 mr) 1st Determination mg.
Absorbance
5.4 12.15 18.90 25.65 3’2.40 41.84 48.59 55.34
0.217 0.522 0.885 1.228 1.560 2.050 2.380 2.69
-7.8 -1.7 -0.8 $0.8 +0.6 f1.2 $1.3 f0.7
EkAlH, mg.
Absorbance
6.75 13.50 20.25 27.00 33.75 40.50 47.25 54.00
0.328 0.624 0.952 1.200 1.622 1.945 2.290 2.60
% Dev. from curve f2.4 -0.3 -0.1 -1.6 0.0 fO.1 f0.4
-0.3
Precision of Spectrophotometric Method
Table II.
EXPERIMENTAL
Standard Curve. A standard curve was prepared by a reversal of the working procedure. Weighed increments of isoquinoline were added to a solution of the 1: 1 complex in 25 ml. of benzene. Since accurate addition of small amounts of isoquinoline necessitated the use of a dilute solution of isoquinoline in benzene, the volumes added were of significant magnitude. Therefore, the absorbance value for each addition was corrected for the dilution effect. This calibration technique was duplicated to obtain two curves independently. The results are shown in Table I. Any moisture in the isoquinoline solution will introduce an error in the standardization, just as in the titration mentioned previously. However, only the amount of moisture present in the increment necessary to produce the desired absorbance change is involved, whereas in the titration, the moisture present in all the isoquinoline necessary for the entire sample will react with the dialkylaluminum hydride. Water in the isoquinoline solution can be reduced to a very low level by passing it through a column of Molecular Sieves. The data shown in Table I indicate close adherence to Beer’s law, and the method’s relative simplicity indicates that it is practical. The column headed “Et&H” for each determination is actually the diethylaluminum hydride equivalent of the total isoquinoline added at that point. It is better to standardize by adding increments of diethylaluminum hydride to isoquinoline if a pure sample is available. This was done in our laboratories. To standardize with pure diethylaluminum hydride, simply employ the working procedure and calculate the absorbance factor from the known diethylaluminum hydride content. No special purification steps are necessary for the isoquinoline since the procedure effects purification in the closed cell. The absorbance factor obtained by diethylaluminum hydride standardization was about 1% higher than that
2nd Determination
% Dev. from curve
EkAlH,
(0.5,-om. cell)
wt. 7 0
EkAlH 90.3 93.3 89.3 91.4 94.2 94.2 89.0 96.9
Av. 92.3
1st Analyst, Sample I
Dev. from mean
2nd AnalyBt, Sample I1
Wt. % EkAlH
Dev. from mean
-2.0 +1 .o -3.0 -0.9 fl.9 fl.9 -3.3 f4.6
83.2 84.2 79.2 78.1 82.1
f1.8 $2.8 -2.2 -3.3 f0.7
2 . 3 (2.5% rel.)
81.4
2 . 2 (2.7% rel.)
obtained in the isoquinoline standardization. This indicated that removal of o.xygen and water from the isoquinoline had not been complete, but the resulting error was of no consequence. Precision. Reproducibility data based on valuea obtained by one analyst on one sample and by a second analyst on a second sample are shown in Table 11. The close agreement of the average deviations obtained by the two analysts indicated that the reproducibility of the method was within &3y0 of the amount of diethylaluminum hydride present. Accuracy. Several samples were run by the isoquinoline spectrophotometric method and also by hydrolyzing the sample and analyzing the gas evolved. The close agreement of the two methods is shown by the data summarized in Table 111. In determining the Et&H content by gas analysis, small amounts of ethylene which appear in the evolved ga8 result from thermal decomposition of AlC,Hs groups to AI-H CP&; the AI-H then hydrolyzes to give Hn. Accordingly, the mole per cent cpI.f( was subtracted from the mole per cent H2 and added to the mole per cent CrHs. ks shown in the table, when this is done the agreement between the two methods is usually very good.
+
Table 111. Comparison of Results of lsoquinoline and Gas Analysis Methods
EkAIH. Wt. O/n Gas 160quinoline analysis
Sample 1
28.6
27.7
7
25.0 26.2 28.2
24.8 28.0 29.3 34.9
8
9 10
Table IV.
35.8
Analysis of Alkyl Aluminum Compounds
Sample Sample Sample Wt. % EkAlH found
Wt. % E&.AlH
expected wt. % ALEk found Wt. % Amk expected
1
2
69.6
28.1
58.6
27.6
41.6
72.8
41.5
72.8
Wt. % AIM& found Wt. %. AIMer expected
VOL 33, NO. 10, SEPTEMBER 1961
3
94.7 96.3
1333
Results of the analysis of samples of trialkylaluminums and diethylaluminum hydride are shown in Table IV. Samples 1 and 2 were prepared from known mixtures of AlEts and EtAIH. The purity of each was determined to be "+% by gas evolution, mass spectrometric analysis of the gas, and infrared analysis. The
&Me3 was analyzed in the same manner and the purity was found to be 9€.3%.
(4) Keumann, '8. P., Angew. Chem. 69,
730 (5) Vaillant, M., Chim. Anal. 39, 413 (1957).
LITERATURE CITED
(1) Bonitz, E., Chem. Ber. 88,742 (1955). (2) Farina, M., Donltti, M., Ragazaini, M., Ann. chim. (Rome)48,501 (1958). (3) Henderson, S. R., Snyder, L. J., ANAL.CHEM.31,2113 (1959).
RECEIVED for review August 29, 1960. Resubmitted April 21, 1961. Accepted June 29, 1961. Division of Analytical Chemistry, 138th Meeting, ACS, New York, N. Y., September 1960.
Spectrophotometric Determination of Naphthylamines HANS 0.SPAUSCHUS Major Appliance Laboratories, General Electric Co., Louisville, Ky.
When naphthylamines, or nitrogensubstituted derivatives thereof, are dissolved in certain halogenated solvents and exposed to ultraviolet irradiation, colors rapidly develop. The Beer-Lambert law is obeyed for concentrations in the range of 10 to 100 p.p.m. Irradiated solutions of N-phenyl-1 -naphthylamine in chloroform absorb at 640 mp. Color intensity varies with concentration according to the least-squares equation, p.p.m. of amine = 16.9 68.9 X absorbance. The average standard deviation of the data points from their estimated value according to the correlation line is 0.022 absorbance unit. Other naphthylamines such as 1 N-phenyl-2-, N-methyl-1 and N,N-dimethyl-1-naphthylamine also react with halogenated solvents under the influence of ultraviolet light. These photo-induced reactions should prove generally useful as a basis for both qualitative and quantitative determinations. To determine naphthylamine-type additives in mineral oils, a known excess of chloroform is added to the oil, and color is formed by ultraviolet irradiation of the solution. Thus N-phenyl- 1 -naphthylamine, an oxidation inhibitor for lubricating oils, can be determined simply and rapidly in both new and used oils.
+
-,
D
-,
commercial applications and some reported physiological effects of 1- and 2-naphthylamines and their nitrogen-substituted derivatives have created a demand for methods of detecting and determining these compounds. Numerous spot test procedures (7, 8, 10) and color reactions with specific reagents (8, 4, 5 ) have been reported. For quantitative determination, primary naphthylamines have been diazotized and coupled and the absorption spectra of the resulting azo dyes measured (1, 3, 6). Analytical methods in which the naphthylamines IVERSE
1334
*
ANALYTICAL CHEMISTRY
produce a color with a diazonium salt have also been reported (9, 13, 14). Methods employing the diazo reaction are complicated and precautions must be exercised to ensure complete conversions. Liebhafsky and Bronk (12) reported that a reaction between N-phenyl-1naphthylamine and Nessler's reagent produced an intense color with an absorption maximum below 400 mp. Unfortunately, the color continues to develop for a t least 16 hours, seriously impairing the utility of this reaction as a basis for an analytical method. The present investigation was pursued when it was observed that naphthylamines, dissolved in certain halogenated solvents and esposed t o ultraviolet radiation, yield colored solutions suitable for spectrophotometric study. Results for solutions of N-phenyl-1-naphthylamine in chloroform established that color intensity is proportional to the concentration of amine. Other naphthylamines, including 1-naphthylamine, N-phenyl-2-naphthylamine, N-methyl-1-naphthylamine,and N,Ndimethyl 1 - naphthylamine, all yield colors when dissolved in selected halogenated solvents and exposed to ultraviolet radiation. Substituted naphthylamines are frequently used as additives for petroleum oils. The present method has been applied to the direct determination of N-phenyl-1-naphthylamine in mineral
-
Oil * EXPERIMENTAL
Reagents and Apparatus.
N-
Phenyl-1-, N-phenyl-2-, and N-methyl1-naphthylamines, Eastman Grade, were obtained from the Eastman Kodak Co. High purity 1-naphthylamine, reagent grade N,N-dimethyl1-naphthylamine, and indicator grade diphenylamine were obtained from the Fisher Scientific Co. All amines were used as received. The chloroform was Mallinckrodt analytical
reagent grade containing 0.75% ethyl alcohol as preservative. The alcohol was removed from the chloroform for some of the experiments by shaking a quantity of chloroform three times with a small volume (5%) of concentrated sulfuric acid, washing four times with an equal volume of distilled water, drying over anhydrous potassium carbonate, and distilling through a small column. The fraction boiling a t 60.3" to 60.4" C. a t 747.1 mm. of Hg was stored in a tightly sealed amber bottle. Other solvents used in the experiments included Transistar grade carbon tetrachloride (Mallinckrodt) ; bromoform (Eastman White Label, stabilized with diphenylamine) ; absolute methanol (analytical reagent grade, Mallinckrodt) ; 1,2-dichloroethane (purified, Fisher Scientific Co.) ; and trichloroethylene (b.p. 86-87 " C., Matheson, Coleman and Bell). Solutions were prepared by weighing the components on an analytical balance, and the concentration is expressed in parts per million by weight. A Blak-Ray Model X4 ultraviolet lamp provided radiation of 3660-A. wave length. This lamp was placed across the open top of a small box, 7 cm. deep. Solutions were irradiated in fused silica absorption cells of IO-mm. light path by placing the cells in the bottom of the box directly under the center of the lamp. Absorption studies in the visible region were made with the Cary recording spectrophotometer, Model 14M, a t scanning speeds of 10 and 50 A. per second. Procedure. A solution of naphthylamine in chloroform was transferred to one of a pair of matched 10-mm. cells. The second cell was filled with the pure solvent, the cells were placed in the sample compartment of the Cary spectrophotometer, and the differential spectrum was recorded from 550 to 700 mp. No absorption peaks were observed and the background absorbance changed a maximum of only 0.02 absorbance unit over the range scanned. The cell containing the solution was irradiated by the ultraviolet lamp for 1 minute. The cell was in-
