A shift of this kind, due to a change in the activity of one or more potentialcontrolling species, will be encountered frequently in solutions that are recycled in continuous plant processes. The differential system is essentially independent of these displacements because it automatically seeks the shoulder of the titration curve, Its use under such circumstances advantageous.
be
ACKNOWLEDGMENT
(3) MacInnes, D. A., 2. physik. Chem.
The writer thanks Marjorie IT. Eastwood for laboratory work in evaluation Of the COntinUOUS titrator and W. A. Morgan for recommending the detectorcontroller system which was used.
(4) 1.hIacInnes, A , , J . Am, Chem. D. A., Sot.Cowperthwaik, 5j, j55 (1931), (5) MacInnes, D. A., Dole, M., Zbid., 51,
LITERATURE CITED
(1) cox, D. c., J . Am. Chem. &c. 47, 2138 (1925). (2) Hart, Porter, ZSA Journal 4, 472 (1957).
l3OA! 217 (lg2V.
1119 (1929). (6) MacInnes, D. A., Jones, J. T., Zbid. 48, 2831 (1926). (7) Milton Roy Co., data sheet A-58-2.
RECEIVEDfor review March 6, 1961. Accepted May 15, 1961. Division of Analytical Chemistry, 138th Meeting, ACS, New York, N. Y., September 1961.
A Spectrophotometric Method for the Determination of Moisture and Active Hydrogen T. G. MUNGALL and J. H. MITCHEN Research and Development Departmenf, Ethyl Corp., Baton Rouge, l a . ,A new colorimetric procedure permits the rapid determination of moisture in many liquids, such as metal alkyls, where the Karl Fischer method is inapplicable. Microgram quantities of moisture can be accurately determined, and normally the entire procedure takes about 10 minutes. This method is based on the fact that water or other forms of active hydrogen destroy the intense red color of the diethylaluminum hydride-2-isoquinoline complex. Accordingly, it can also be used to determine the total reactivity based on water and/or active hydrogen.
0
i t is necessary to determine the inertness of solvents to be used in systems containing highly reactive materials, such as aluminum alkyls or Grignard reagents. Existing techniques for measuring moisture or other forms of active hydrogen in these solvents are cumbersome and frequently are inapplicable. For example, the Karl Fischer method requires special techniques and great care to obtain accuracy in the few parts per million range; furthermore, it is not sensitive to hydroxyl groups or acids, and cannot be used with unsaturates without modification. Gas evolution techniques based on Grignard reagents likewise are not easily adapted to determining low moisture concentrations; corrections for vapor pressure and solubility of the evolved gas in the solvent are particularly troublesome. This paper describes a new procedure for determining moisture and active hydrogen. It is based upon the reaction used by Mitchen (3) for the spectrophotometric determination of diethylaluminum hydride. The reaction and color measurement are carried out in FTEN
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ANALYTICAL CHEMISTRY
the same vessel, which eliminates most of the manipulative and sample handling difficulties of conventional techniques, enabling completion of the analysis in 10 minutes. Because the method is sensitive to microgram quantities of moisture or active hydrogen, a suitable choice of sample volumes will permit determinations over a wide range. Dissolved oxygen in the sample is a source of positive error, but this can be measured and corrected for. Organic functional groups that react with diethylaluminum hydride, such as aldehydes and ketones, constitute an interference. APPARATUS AND REAGENTS
The method employs a Beckman DU spectrophotometer with a modified cell compartment and a 200-ml. comparator tube fitted with a 1-cm. cell ( 2 ) . A 2.0-cc. hypodermic syringe is used to deliver measured amounts of standard or sample into the comparator tube. Benzene. Especially dry benzene is prepared by distilling benzene from a mixture of 1% triethylaluminum and reagent grade benzene. Standard Solutions of Water in Benzene. Oxygen-free benzene is prepared by passing nitrogen over hot reduced copper turnings and then through dry benzene. Several hours’ bubbling time a t a flow rate of approximately 50 cc. per min. is required to decrease the oxygen concentration in 500 ml. of benzene from 40 to 2 p.p.m. For calibration, water is added to the benzene by bubbling the nitrogen through a water scrubber located upstream from the oxygen removal system. Bubbling time with the water-saturated nitrogen may be varied to obtain several different water concentrations which are then determined by the Karl Fischer method. To avoid contamination by oxygen or water prior to use, these standards are covered with Sani-Tab
caps and stored in a water- and oxygenfree dry box; even with these precautions they should be used within 2 hours after preparation. 10% Isoquinoline Solution. Ten milliliters of freshly distilled isoquinoline is diluted to 100 ml. with dry benzene. Diethylaluminum Hydride. It is not necessary to have pure diethylaluminum hydride for development of the red isoquinoline complex. However, for best results the purity of this reagent should be a t least 50%. Reduced Copper Solution. A solution of 5 grams of cupric sulfate pentahydrate and 75 grams of ammonium chloride in 500 ml. of 501, ammonium hydroxide is reduced by storing it over copper wire in a container fitted with a serum-type rubber stopper. Reduction is evidenced when the solution in contact with the copper loses its dark blue color. It is not necessary for the solution to become completely colorless before it is used PROCEDURE
To prepare a suitable blank, place a sleeve-type serum stopper over the mouth of a dry, nitrogen-flushed, 200ml. comparator tube and secure it tightly with rubber bands. Introduce 20 ml. of dry benzene followed by 5 ml. of 10% isoquinoline solution via a hypodermic syringe. Add diethylaluminum hydride until a red isoquimline-Et2A1H complex is formed with an absorbance reading between 1.00 and 1.50 a t 460-mp wave length. Shake the comparator tube with its contents for 3 minutes prior to reading the spectrophotometer to ensure that all oxygen and water in the reagents have been reacted. It may become necessary a t this point to readjust the absorbance of the mixture to the above-mentioned limits by adding a few more drops of EtzAlH. Xext, prepare a calibration curve by injecting through the serum stopper a
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
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