Visual Titrimetric Determinationof Total Reactivity and l k y ~ a l u ~urn i nand DiaIkylaluminum Donald E. Jordon Research and Development Department, Continental Oil Co., Ponca City, Okla. A rapid, visual, titrimetric method to determine total reactivity, as well as to differentiate aluminum trialkyl (ATA) from aluminum diaikyl hydride (ADAH) moieties in mixed complex systems of aluminum alkyls, is desired. Phenazine used as the indicator forms red to brown complexes with ATA, and green to green-blue complexes with ADAH. The titration utilizes inexpensive, commonly available, laboratory apparatus, and is completed easily in 2 to 4 minutes after initial purging of the titration vessels. The method is applicable to alkyls ranging from CBto at least Ca0separately or in mixtures. Precision is +I% relative standard deviation for both low and high concentrations of ATA and/or ADAH, and accuracy, although unknown, should approach the precision.
A METHOD REQUIRINGonly commonly available, inexpensive laboratory apparatus to determine quantitatively the total reactivity, as well as to differentiate aluminum trialkyl (ATA) and aluminum dialkyl hydride (ADAH) in complex aluminum alkyl systems, would be most desirable. Visual endpoint titration methods for ATA compounds, each with moderate speed, have been reported ( I , 2). In every case, except where ADAH was added to determine the ATA titration end point with isoquinoline ( I ) , ADAH reacted with the titrant or indicator to liberate hydrogen which destroyed the indicating properties of the indicator ( 4 ) . Spectrophotometry has been used to determine total reactivity (activity) and to differentiate ATA and ADAH (2, 3, 5). One method (2), which is an extension of another (3), has been operative in our laboratory for more than five years. Although it is an excellent method, it requires a minimum of 20 to 30 minutes after initial purging to complete an analysis, except with the most ideal samples. Further, moderately expensive equipment, cell modification, and multiple weighings are required to suitably carry out the determination. Gas volumetric (6-8) and thermometric (9, 10) techniques have been used to differentiate ATA and ADAH in mixtures. Both, however, require elaborate and moderately expensive equipment. Gas volumetric methods are severely limited by the length of the hydrocarbon side chain, while thermometric methods are (1) E. Bonitz, Chern. Ber., 88, 742 (1955). (2) D. F. Wagen and W. D. Leslie, ANAL.CHEM., 35, 814 (1963). (3) J. H.Mitchen, ibid., 33, 1331 (1961). (4) G. A. Razuvaev and A. Graevskii, Doklady Akad. Nauk. S.S.S.R. 128, 309 (1959). ( 5 ) C. W. Wadelin, Pittsburgh Conference on Analytical. Chemistry and Applied Spectroscopy, March 1962. (6) D. F. Hagen, J. L. Hoyt, and W. D. Leslie, ANAL.CHEM., 38, 1691 (1966). (7) W. P. Neumann, Ann. Chem., 629, 23 (1960). (8) T. R. Crompton and V. W. Reid, Analysf, 88, 713 (1963). (9) W. L. Everson and E. M. Ramirez, ANAL.CHEW,37, 806 (1965). (10) E. G . Hoffmann and W. Tornau, 2.And. Clzern., 188, 321 ( 1962).
2 158
ANALYTICAL CHEMISTRY
limited to known samples of ATA and ADAH with welldefined heats of reaction. In the work described here, a rapid, visual, titration method is presented whereby ATA and ADAH, either separately or in wide molecular weight and concentration ranges of complex mixtures, are reversibly titrated with pyridine using phenazine as the indicator. After initial purging of the titration vesseI, only 2 to 4 minutes are required to titrate a sample and only commonly available, inexpensive, laboratory equipment is required. EXPERIMENTAL
Apparatus and Reagents. A nitrogen (air free) tank with appropriate pressure gauges was used. The 50-ml buret was equipped with an 18-gauge hypodermic needle delivery tip and was partially filled with 5A or 4A Linde Molecular Sieve. Rubber septums were selected to fit bottle caps and 2-02 narrow-necked bottles. A Kel F or Teflon-covered stirring bar (0.5 to 0.75- X 0.25-inch) was used. The smooth-bore, 1-ml Luer-Lok hypodermic syringes were equipped with Teflon plungers (Hamilton Syringe Co.). The 1.5-inch hypodermic needles were 23 gauge, and the pressure release system was for a Nujol nitrogen gas scrubber. Silicon rubber plugs, &/le- X OS-inch, were obtained from the Kirkhill Rubber Go. Phenazine (J. T. Baker Co. 3T-178) was used to prepare a 0.5% w/v solution in redistilled dry xylene. Xylene was distilled over calcium hydride from an allglass still and only a heart cut was retained. The distilled xylene was stored over Linde Molecular Sieve 4A or 5A. Reagent grade pyridine was 0.2M in dry xylene and was standardized against standard perchloric acid in glacial acetic acid solvent. Procedure. A clean, dry, 2- or 4-02 bottle containing a magnetic stirring bar is rinsed with 5 ml of dry xylene. A second portion (10-20 ml) of dry xylene and 10 drops of phenazine indicator are added to the bottle. The bottle is closed with a septum-lined screw cap, and purged with stirring for 3 to 5 minutes with nitrogen, flowing 100-200 cclrninute. A 20-gauge needle connected to a pressurerelease NujoI scrubber is inserted through the septum into the bottle atmosphere. Another 20-gauge needle connected to a nitrogen inlet is inserted through the septum into the bottle atmosphere and, while stirring, the bottle and contents are purged for 3 to 4 minutes with a nitrogen flow of 100 to 200 cc per minute. The aluminum alkyl is sampled as described by Hagen and Leslie ( 2 ) and added to the dry xylene-indicator solution by carefully uncapping the needle, inserting it through the septum, and depressing the plunger. Then the needle is carefully withdrawn and capped.- The buret is rinsed with standard pyridine, then filled to volume with more standard pyridine. Then the buret needle tip is inserted through the septum and the sample is titrated to the ATA end point (disappearance of brown or change to pure green] and the milliliters are recorded. The titration is continued for ADAH to colorless or until no further change occurs, and the total milliliters are recorded.
Aluminum Determination. Aluminum is determined by controlled deactivation hydrolysis (11) followed by chelometric titration with cyclohexane-diamine tetraacetate (CDTA)(12, 13). Calculation.
% Total reactivity
=
+
millimoles ATA ADAH millimoles A1
% ADAH % ATA
=
millimoles ADAH millimoles A1
=
x
x
100
% Total Reactivity - % ADAH
100 (1) (2)
(3)
or millimoles ATA %ATA = millimoles A1 ~~
x
After establishing that phenazine formed reversible complexes with ATA and ADAH, it was necessary to determine not only the stoichiometry of the displacement of phenazine but to also determine if the ADAH could, in fact, be separated quantitatively from ATA. Of the several amines used in this study, pyridine was found most suitable. Pyridine (VII) quantitatively displaces phenazine (I) from both ATA and ADAH (VI) to form more stable 1 : 1 complexes (VIII).
1
1
AlRn
A I Rn
100 9I
nm
3m
I
DISCUSSION
At the outset of this investigation the primary objective was to establish a simple, rapid method for total reactivity of aluminum alkyl mixtures to supplement the much slower spectrophotometric isoquinoline method (2, 3). In addition to the complexing indicators mentioned by Hagen and Leslie (2), at least a hundred others had been scanned for applicability to ADAH determination. Gas volumetric methods have been studied but were too limited for our system. Thermometric methods, such as described by Everson and Ramirez ( 9 ) and Hoffmann and Tornau (IO), appear too limited for the complex mixtures to be analyzed because of overlapping heats of reaction in the wide molecular weight and concentration ranges, and offer no real advantage over the established method (2). Hagen and Leslie ( 2 ) reported the use of phenazine (I) as an indicator suitable for ATA but not ADAH because of the possible formation of an AI-N bond with ADAH. Recently, we re-evaluated phenazine as an indicator for ADAH because structurally it appeared to fulfill model requirements to prevent alteration by ADAH and form reversible complexes with ADAH. It was also observed that the 1 :1 and 2 :1 (11-111) red to redbrown complexes are due to ATA while the green complex reported earlier is due to ADAH which forms 1 :1 green-blue (IV) and 2 :1 green (V) complexes. In addition, at least currently, we have been unable to separate quantitatively and distinguish the red 2 :1 and 1 :1 complexes, but the green-blue 1:1 ADAH complex can be differentiated from the 2 :1 green ADAH complex.
rn YELLOW
A~AH GREEN -BLU E
+
lx
ADAH
A~A. RED-REDBROWN
II
ATA
t
A ~ A
GREEN
P
BROWN
m
WHERE Rn = TRIALKYL OR DlAlKYL HYDRIDE
Phenazine forms very intensely-colored stable 1 :1 and 2 :1 complexes with both ATA and ADAH. In negligible concentrations, however, phenazine is suitable as an indicator, but it must also be remembered that the colors are actually due to stable complexes. A study of the displacement reaction kinetics showed that for up to 30 minutes phenazine could be quantitatively displaced with pyridine. After 30 minutes, the displacement reaction with pyridine was somewhat obscured, indicating the formation of a more stable complex or the possible formation of a true A1-N bond between ATA-ADAH and phenazine although the displacement of phenazine could be accomplished with other complex formers such as alcohols. Several possible phenazine complexes could exist that could explain the increased stability observed in a large excess of ATA and/or ADAH. For example, the nominal 2 : l complex (VI) could polymerize (IX) or interact
t 91
1x
x
within the molecular complex (X). Many other possibilities exist, but it was beyond the scope of this work to attempt to elucidate them. Although phenazine is unconventionally used as an indicator, and the complex is affected by time, no errors in the determination will occur if the suggested method is followed, because once the pyridine complex is formed there is no tendency to disproportionate from pyridine to phenazine, even upon standing. Therefore, several sample additions could be made to the sample bottle and titrated for total (but not hydride) reactivity. Samples containing ADAH and titrated with pyridine are easily differentiated when one sample is added to the reaction chamber. However, if a second sample containing ADAH is added to the same reaction cham(11) D. F. Hagen, D. G. Biechler, W. D. Leslie, and D. E. Jordan, Anal. Chim. Acta, 41, 557 (1968). (12) E. Wannien and A. Ringborn, ibid., 12, 308 (1955). (13) F. Nydahl, Talanta, 4, 141 (1960). VOL. 40, NO. 14, DECEMBER 1968
2151
Table I. Comparison of Results Obtained by the Described Phenazine Differential Titration and the Spectrophotometric ~sQquinQ~ine Procedures for ADAN and Total Reactivity Mole % ADAH Mole % ATAb Mole % reactive Sample Phenazine Isoquinoline Phenazine Isoquinoline Phenazine Isoquinoline A 81.3“ i 0.85 80.8 10.5 11.1 9 1 . 8 ~f 0.92 91.9 I% 65.4 63.0 28.5 30.9 93.9 93.7 e 4.51=i 0.05 4.55 ... .,. ... ... D
E F a 6
58.8 16.1 96.6
...
24.1 33.8 96.8
82.6 56.8 96.9
83.0 55.2 96.8
Five or more replicate analyses. ATA = Total Reactive - ADAH.
ber the hydride-pyridine complex is disproportionated to form ATA-pyridine and ADAH-phenazine by the mixed ATA-ADAH sample added because of the greater stability of ATA for pyridine and ADAW for phenazine. This then means that enough sample must be added to displace all the ADAW from the previous titration before a differentiation of the second sample can be accomplished. It is readily understood why such a system would soon become unwieldy. However, the same basic technique affords an excellent way to determine indirectly very low concentrations (
