washed. The filtrate and washings were combined and diluted in a volumetric flask. The same procedure was then followed as for the phosphor bronze alloy. The synthetic sample of lead, cadmium, and copper was dissolved with dilute nitric acid, and the resulting solution was diluted to 200 ml. in a volumetric flask. Aliquot portions of 20 ml. were taken, diluted with water to about 60 ml., and introduced onto the ion exchange column. The cadmium was eluted with 130 ml. of 0.5M hydrochloric acid and the lead with 145 ml. of 0.6M hydrobromic acid. The assays
of the alloys and the results of the analyses are given in Table IV. LITERATURE CITED
(1) Barnard, A. J., Broad, W. C., Fla-
schka, H., “The EDTA Titration; Nature and Methods of End Point Detection,” .J. T. Baker Chemical Co., Phillipsburg, N. J., 1957. (2) Erkelens, P. C. van. AnaZ. Chim. Acta 2 5 . 42 (1961). (3) Fritz,‘ J. S., Abbinli, J. E., Payne, 31. A , , ANAL.CHEX 33, 1381 (1961). (4) Fritz, J. S., Garralda, B. B., Ibid.,34, 102 (1962). ( 5 ) Fritz, J. S., Garralda, B. B., Karralicr, S. K., Ibid.,33, 882 (1961).
(6) Fritz, J. S.,Rettig, T., Ibid.,34, 1562 (1962).
(7) Klement, R., Sandmann, H., 2.Anal.
Chem. 145, 325 (1955). (8) Korbl, J., Pribil, R., Chemist-Analyst 45, 102 (1956). (9) Minami, E., Ishimoi, T., J . Chem. SOC. Japan 74, 378 (1953). (10) Nelson, F., Kraus, II. A., J . Am. Chem. Soc. 76, 5916 (1984). (11) Nelson, F., Rush, R. lM., Kraus, K. A., Ibid., 82, 339 (1960). (12) Sill, C. W., Peterson, H. E., ANAL. CHEY.24, 1175 (1952). ( 1 3 ) Tsintsevich, E. P., Nazarova, G . E., Zavodslz. Lab. 23, 1068 (1957). RECEIVED for rcview December 17, 196%. Accepted March 7, 1963.
Colorimetric Reagents for the Analysis of Aluminum Alkyls D. F.
HAGEN and W.
D. LESLIE
Research and Development Department/ Continental Oil Co., P onca City, Okla.
-
b Triphenylmethane type indicators can be used to determine the equivalence points in the titration of aluminum alkyls with electron-donor reagents such as aromatic arnines, ethers, and alcohols. Deactivated species, such as dialkylaluminum alkoxides, do not form stable complexes with these reagents, and the “activity” determination i s a direct measure of the purity of the alkyl sample. Small amounts of the dialkyl hydride destroy the triphenylmethane-type indicators, rendering them irreversible in the titration. W e have found a number of compounds which form highly colored reversible complexes with aluminum alkyls and can be used as indicators for the volumetric determination of “activity.” The intensity of the color i s greatly increased when two or more azo-methine linkages are present in the indicator molecule. Reversibility of the alkyl indicator complex i s dependent on the position of the coordination sites and the relative basicity of the indicator with respect to the titrant.
T
HE
ALUMINUM
ALKYL
MOLECULE
is electron-deficient and can be considered a Lewis acid in complex formation reactions with electron-donating reagents. Oxidation or hydrolysis of one of the aluminum-carbon bonds results in a reduction in the electronseeking character of the entire molecule. Dialkylaluminum hydride, dial kylaluminum halide, and trialkylaluminum are active species, while the dialkylaluminum alkoxide and tetraalkyl814
ANALYTICAL CHEMISTRY
Figure 1. Model DB
Activity cell for Beckman
aluminum oxide are inactive. Dialkylaluminum hydrides are unique in that they form 2 :1 as well as 1: 1 basehydride complexes with certain nitrogen compounds. I n this study a large variety of aluminum alkyls have been analyzed and it has been found that volumetric methods using visual indicators can be employed for the majority of these samples with good accuracy and speed. Bonitz (1) has used the red-colored 2: 1 isoquinoline-dialkylaluminum hydride complex as the end-point indicator in a volumetric isoquinoline titration for alkyl activity. Dialkylaluminum hydride must be added to samples Yhich do not contain hydride, however, so that a n endpoint can be obtained. The red-colored 2 : 1 isoquinoline-dialkylaluminum hydride complex is less stable than the yellow 1: 1 complexes formed with the aluminum-trialkyls, dialkyl halides, or dialkyl hydrides. LIitchen ( 2 ) utilized these stability
relationships for the complete analysis of dialkyl hydride-trialkyl mixtures. However, this method is applicable to trialkyls only when dialkylaluminum hydride is added to the sample to function as an indicator. A modification of this method is used in our laboratory to determine the hydride content and we employ a photometric titration t o obtain the total hydride plus trialkyl content. Wadelin (4)has also reported a photometric titration utilizing the indicator properties of dialkylaluminum hydride. Razuvaev (3) reported that methyl violet, gentian violet, and crystal violet act as visual indicators in the volumetric determination of activity. Hydride-containing samples, however, destroy these indicators with evolution of hydrogen. Investigations of triphenylmethane-type indicators in our laboratory show that reversibility occurs only when the nitrogen atoms of the dye are not completely methylated. We have also found certain azinetype indicators to be more stable to hydride-containing samples. Difunctional reagents yield highly colored complexes with aluminum trialkyls and eliminate the necessity of adding hydride to obtain an end point. EXPERIMENTAL
Apparatus. absorbance measurements mere made using a borosilicate cell with a 0.1-cm. path length and a Beckman Model DB spectrophotometer. This cell, illustrated in Figure 1, has a total capacity of 35 cc. and is equipped with a small septum port for the addition of reagents and sample. Titrations are performed using
Weighed quantities of sample are then injected into the cell, and after thorough mixing spectral scans or absorbance measurements are made a t 460 mp. The per cent of AIRzH can be determined from a standard curve or as described by Mitchen ( 2 ) . However, it is not necessary to dilute the sample prior to analysis when our cell is used. The total hydride and trialkyl content can be accurately determined on the above solution by using a photometric isoquinoline titration. The sample is injected into the cell with mixing until a slight excess is present as shown by the disappearance of the red-colored 2 :1 isoquinoline-AlRtH complex to form the more stable 1 : l species. The sample syringe is weighed to obtain the weight of sample added, and the absorbance of the 1:l complex is determined. Increments of standard isoquinoline are added to the cell via the needle-tipped buret, and the solution's absorbance is determined a t 460 mp for each increment. Volumes of titrant are plotted us. absorbance readings as illustrated in Figure 3. The intersection of the lines denoting 1 : l and 2 : 1 complex formation is taken as the end point, B. The weight of sample a t this point contains the same number of mmoles of active aluminum (A1R8 A1R2H) as the number of mmoles of isoquinoline added to the cell. VISUALACTIVITYTITRATIONS. A 4ounce bottle containing a small stirring bar, 20 to 25 cc. of dry xylene, and allproximately 5 to 10 mg. of the desired indicator is capped with a rubber septum, and the air is removed Kith the argon purge and pressure release systems. After 2 to 3 minutes the argon flow is stopped and the needle-tipped
LINDE L 4 A MOLECULAR SIEVE
PRESSWE RELEASE SYSTDr(
ARGCN INLET
N U J O L BUBBLER
2 OR 4
02. BOTTLE
OR ACTIVITY CELL
Figure 2.
+
Reagent dispensing and titration apparatus
septum while still hot. Air is removed the apparatus illustraced in Figure 2. from the cell by inserting the argon Standard solutions are transferred to the purge and pressure release needles absorbance cell or reaction flask with through the septum and passing argon the 50-cc. needle-tipped buret which is partially filled with Linde KO. 4A through the cell until it has cooled to Molecular Sieves to re nove last traces room temperature. Twenty-five milliliters of 0.200M standardized reagent of water from the reagent. Titration (isoquinoline) are transferred to the vessels are 4-ounce bottles capped with cell via the needle-tipped buret. rubber septums, and stirring is done magnetically. A specktl sample syringe was constructed by using a smoothbore barrel and a machined Teflon plunger which requires no lubrication. Reagents. Isoquinoline was purified by distillation under a nitrogen I I atmosphere. Pyridine was ACS grade AIR, , A l R z H AIRzH I (99.9yo purity) from Matheson, Coleman and Bell, Norwood (Cincinnati), FORMING I :I COMPLEX +2:1 , I ,EXCESS I SOQUlNOLlNE Ohio. I Standard solutions of reagents such I I as isoquinoline, pyridine, and hexyl alcohol were prepared by weighing into xylene and passing through molecu0.6 lar sieve prior to us':. Analysis indicated the presence of less than 7 p.p.m. of water after this treatment. Xylene, used as the titration solvent, was dried with calcium hydride and m a then redistilled under a nitrogen at0.4 mosphere. Concentration effects were noted for solvents OF higher vapor pressure such as benzer e. Indicators were dissolved in xylene when soluble or addcd as solids to 0.2 the titration vessel prior to degassing. Argon was used for degassing cells after passage through a, molecular sieve column to reduce the moisture content. Procedures. PHOTOMETRIC METHOD FOR DIALKYLHYDRIDE AND TRIALKYL CONTENT. The AIRrH content of 0 6: I :7 219 30 organic aluminum compounds can A B C be determined using the cell shown in ML OF I S O Q U I N O L I N E AOOEO ( 0 . 2 M ) Figure 1. The cell is dried in an Figure 3. Photometric titration of activity oven and capped with :t silicone rubber
I
I
k-+' A
I
A
VOL. 35, NO. 7,JUNE 1963
815
buret is inserted through the septum. Three or four drops of sample are added to the bottle via the sample syringe to remove last traces of water or oxygen and to solubilize the indicator. After waiting several minutes for the system to stabilize, the solution is titrated to the color change of the indicator with a standard solution of pyridine (0.200M) in xylene in the needle-tipped buret. Fifteen to 20 cc. of excess titrant are added to the bottle and then the sample is injected dropwise until the indicator changes to the excess alkyl color. The solution is carefully titrated to the end point and the syringe is reweighed to obtain the weight of sample used between the two end points. The activity is calculated as follows: Mole % ’ (A& NRzH) = mmoles reagent used x 100 total mmoles AI in sample
+
where the total mmoles of A1 is obtained from the per cent A1 and weight of sample injected between the two end points. RESULTS
Pure aluminum alkyl samples are extremely reactive to air and moisture. The precision and accuracy of the analysis depend to a large extent on the protection of the sample prior to and during the analysis. The normal procedure is to use a lubricated glass plunger syringe to facilitate weighing and addition of small quantities of sample. We have found that consider-
Table 1.
able variation in results is due to sample decomposition by reaction or solvation with the lubricant and subsequent seepage of air into the syringe during the analysis. This difficulty has been eliminated by using the Teflon plunger syringe described. Samples have been stored in the syringe for several days without severe decomposition. Table I illustrates the accuracy obtained in deactivation studies in which known amounts of purified n-propyl alcohol were reacted with an excess of trialkylaluminum. The excess AlRa was then titrated to the visual indicator end point kvith a standard pyridine solution. The accuracy of volumetric results has also been evaluated by using a modification of the gaseous ammonia method (5) and the photometric method described earlier. Table I1 illustrates the precision obtained with several of the indicators to be discussed. The aluminum alkyls range from Cz to Cs0 in chain length, and no dependence on molecular &-eight has been observed. Pyridine was used for the values obtained in both tables. It is superior as a titrant because of its availability in pure form and because its alkyl complexes are relatively colorless, thus causing less interference in end-point detection. Isoquinoline gives sharper end points than pyridine, but it must be purified before use. Secondary amines and alcohols are unsatisfactory
Deactivation of AIR3 with ROH
Sample No.
CaH,OH added, mmoles
AlR, deactivated, mmoles
1089-167a 1089-16713 1089-167~ 1089-167d 1089-168a 1089-16813
0.910 1.188 1.265 1.869 1.938 1.997
0.945 1.193 I . 270 1.826 1.937 2.016
Table 11.
+3.8 +0.4 +0.4 -0.2 -0.1 +0.9
Methyl
Basic
Phenazin
Neutral
4v. dev.
violet
fuchsin
12368
96.3 96.2
96.3 96.3
96.6 96.2
96.8 96.6
96.6 96.1
96.4
0.2
1236B
98.8 98.3
99.3 99.1
99.2 98.7
99.4 99.1
99.6 98.8
99.0
0.3
1236C
93.1 92.8
93.1 92.7
94.0 93.4
93.3 92.5
93.0 93.0
93.1
0.3
1089-164
73.6
79.1
...
75.1 75.1
79.9 77.5
79.5 78.7
77.3
2.0
1089-169
56.9 57.4
58.0 58.1
Fj6.6 56.9
57.2 56.3
57.4 56.9
57.2
0.5
8 16
0
...
ANALYTICAL CHEMISTRY
violet
Average
A number of approaches have been used in this laboratory for analyzing various aluminum alkyl samples. We have found that a large number of these alkyls can be analyzed more accurately and much faster by the titration procedure utilizing a visual indicator. Two general reactions illustrate the basis of these titrations and are designated as (1) deactivation and (2) displacement reactions. [In
- A1R3] + R’OH
-+
AlR20R‘ [In - AlR3]
+ RH + In
(1)
+ 0: AlR,] + I n ( 2 ) +
[ a :
The indicator must be capable of forming a coordination complex with an active molecule only and it must be reversible upon deactivation of the alkyl, as shown in Equation 1. A stronger Lewis base can be used to displace the indicator (as illustrated with pyridine in Equation 2) if it is able to form a more stable coordination complex with the alkyl. Moderately stable compounds can be formed with reagents containing an active hydrogen, as shown with ammonia (Equation 3).
+ AlRgH + NHP
+ AIR213 Neutral Red
DISCUSSION
AlRr
Comparison of Visual Indicators for Volumetric Analysis
Per cent AlRa Sample NO.
Relative error, %
as titrants because of gas evolution which obscures the end point when low molecular weight alkyls or alkyls containing hydride are being analyzed. Keutral violet indicator is used for the majority of the samples encountered. m7hen certain impurities are present, it may be advantageous to utilize one of the other indicators. Of those shown in Table 11, basic fuchsin is the most readily destroyed by hydride-containing samples.
“3
-+
+ RH AlRzNHz + Hz
AlRzNH,
+
(3) (4)
Aluminum-nitrogen covalent bond formation also occurs by reduction of an azomethine linkage by AlRZH. The formation of the A1--N bond is probably responsible for the irreversible behavior of the indicators when AlRzH is present in large amounts. Investigations of triphenylmetlianetype indicators in this laboratory indicated that other compounds could also function as reversible indicators, providing certain amino groups are not completely methylated. For example, basic fuchsin (a mixture of pararosaniline, rosaniline , and magenta IJ) is reversible, while methyl green (crystal violet containing a seventh methyl group) does not display reversible complex behavior. It has been reported ( 3 ) that crystal violet (hexamethyl pararosaniline) functions as an
ndicator; however, small amounts of methyl violet (pen tamethyl species) may be responsible for the reversible colorations observed. Ethyl violet, Victoria blue 4R, and malachite green are closely related to crystal violet, and none of these compounds give evidence of a reverijible color change with aluminum alkyls. The triphenylmethane-type indica1;ors which are reversible (methyl oiolet and other pararosanilines with lesser degrees of methylation) are unstable with respect to AlRzH and it appears that the hydride-sensitive part of the molecule is also responsible for its ability to act as an indicator. We have observed reversible color changes with 41R3 snd ketones, such :as anthrone, benzil, and Rlichler's ketone , aldehydes, such as p-dimethylaminobenzaldehyde; and a variety of compounds containing cyclic or straight chain azomethine linkages. Much of our work has been concerned with this last category and includes the following compounds: pyraxine, pyrimidines, quinoxalines, phenazines, dipyridyls, di- and tri-quinoline:;, phenanthrolines, yuinaldines, quinolines, phenanthridine, benzalazine, hydrobenzamide, and benzalaniline. Our obser.rations include:
The stability of A1R2H complexes with the difunctional reagents is related to the position of the complexing sites in the electron-dominating molecule. For esample, o-phenanthroline (111); a,a'-dipyridyl (IT); and 6,7dimethyl - 2,3-di(2 - pyridy1)quinosaline (V) are capable of forming complexes with XIRzH which are not readily displaced with a stronger base such as pyridine ; while m-phenanthroline forms intenqely colored AlR2H complexes which are readily decolorized by pyridine.
Colored complexes can be formed with A& by the use 3f compounds such :is pyrazine, allowin;; the colorimetric determination of smztll amounts of the alkyl. Complexes of the difunctional compounds with AIRzH are much more intensely colored than those previously reported for moniifunctional compounds, such as isoquinoline and pyridine ( I ) . The azomethine nirogen atoms must be separated by at least one carbon atom. For esample, benzalazine (I) does not give a colc'red complex with AIR& while hyd:.obenzamide (11) forms an extremely intensely colored complex with an absorption maximum a t 5.50 mp. In the structures illustrated below, it is ,assumed that the hydrogen will add to the azomethine linkage and a nitrogen-aluminum covalent bond is formed:
I'eutral red and neutral violet undergo several color changes as the end point is approached in the titration of with pyridine, isoquinoline, or alcohols. Phenazine also gives this color transition and it appears that with excess alkyl a green complex is formed which could be attributed to a quinoidlike compound (VI). As the 81R3 is displaced from the indicator with pyridine, the color changes to red and finally to yellow:
Iv
I11
V
AIR3
AIR3 Green
VI
AlRzH destroys the ability of these indicators to give sharp color changes at the end point, and this may be due to a reaction with the primary amino group or the formation of a covalent 81-N bond, as discussed earlier. These compounds never assume the green coloration but go directly to the red upon addition of AlR2H. Azine dyes in which one of the nitrogens is pentavalent (amethyst violet, magdala red, and phenosafranine) do not function as reversible indicators, and this niay be due to their greater basicity. Oxazines, pyronins, and sulfur compounds, such as methylene blue and dithizone form highly colored solutions with aluminum alkyls; but these appear to be too stable or are destroyed. Dyes containing carboxyl groups, thio linkages, sulfonate groups, or azo dyes, in general, have shown little tendency to act as reversible indicators in the displacement reaction. The titration procedure utilizing visual indicators is applicable to trialkyl samples that contain little or no hydride. Accurate AlRzH determinations are best obtained by the spectrophotometric technique described by Mitchen (Z), although we obtain better results using a 0.1-cm. path length cell, thus eliminating dilution time and errors. Total AIRs) in samples activity (.4&H containing too much 4&H for the indicator titration is best obtained by the photometric titration technique described. One must be cautious in applying these methods to all types of samples, since they are all based on the electron-deficient character of the aluminum alkyl molecule. Any constituent in the sample behaving as a Lewis base may reduce this electronseeking tendency. Certain impurities or reactants are found to reduce the alkyls' activity with respect to isoquinoline but not with respect to gaseous ammonia. The phenazinetype complexes offer a means of determining small amounts of X1R3 colorimetrically and can be used as spot tests for the detection of A1R3 or A1R2H.
+
AIR3
LITERATURE CITED
Red VI1
'ItP
(1) Bonitz, E., Chem. Ber. 88, 742-63 (1956). (2) Mitchen, J. H., AXAL. CHEM. 33, 1331-4 (1961). (3) Razuvaev, G. A,, GraevskiI, Doklady A k a d . X a u k S.S.S.R. 128, 309-11 (1959). (4) Wadelin, C. IT., paper presented a t the Pittsburgh Conference on Analytical Chemistry and Applied Spectroscopy, March 1962. (5) Ziegler, K., Gellert, H. G., Ann. Chinz. 629, 20 (1960).
Yellow VI11
RECEIVED for review December 13, 1962. Accepted March 11, 1963.
f
1
VOL. 35, NO. 7, JUNE 1963
817
