Determination of Active Hydrogen of Sterically Hindered Hydroxyl Groups with Diborane SIR: In the manometric determination of active hydrogen with diborane, Martin and Jay (3) state the reagent is “especially valuable for the determination of sterically hindered hydroxyl compounds. . . not amenable to analysis by various anhydride or acid chloride esterification techniques.” It is also stated that “certain sterically hindered or tertiary hydroxyl groups present problems rvith all the common techniques.” It should probably be emphasized that there are a t least three reliable esterification methods (4-6)for tertiary hydroxyl groups. It also does not appear to be clear from the paper what sterically hindered hydroxyl groups cannot be determined by acidcatalyzed acetylation (1, 7 ) , other then tertiary alcohols. Undoubtedly, the most reliable method for tertiary alcohols is the elegant 3,5-dinitrobenzoyl chloride method of Robinson, Cundiff, and Markunas ( 6 ) . Both Pesez (5) and Mesnard and Bertucat (4)have utilized anhydrides for tertiary alcohols. $dmittedly, tn-o of these methods require 24 hours, a problem which the rapid diborane method overcomes. I t has recently been shown (2) that as long as the side reactions occur reproducibly, tertiary alcohols such as
tert-butyl alcohol can be determined colorimetrically after acetylation by the ferric hydroxamate method. It might have been helpful had the diborane method been applied to a more common tertiary alcohol, instead of triphenylsilanol, for comparison with an acylation method using an aqueous titration. It is well known that silyl acetates hydrolyze almost instantly in water, and that triphenylsilanol behaves quite differently than even the corresponding triphenylmethanol as regards the strength of the oxygen-hydrogen bond (1). Triphenylsilanol is quite acidic compared to triphenylmethanol and might be expected to react more readily with diborane than triphenylmethanol, tert-butyl alcohol, etc. It also dehydrates to a siloxane, not a carbonium ion. Fritz and Schenk (1) also attempted to determine triphenylsilanol using a nonaqueous finish t o avoid hydrolysis of the acetate and obtained an unpublished result of 10970, not much different from 91 or 95% (4). Finally, the diborane method was applied to only one phenol, bis-phenol A, which is not sterically hindered and should be easily acetylated by conventional techniques. No reaction of diborane was mentioned with extremely hindered phenols, such as 2,g-di-tert-
butyl-4-methylphenol, which is easily determined by acid-catalyzed acetylation. Since this phenol can be determined by other manometric active hydrogen methods, it is very likely that the diborane method would be applicable. The diborane method deserves to take its rightful place with the Grignard and lithium aluminum hydride methods, and its potential could be greatly expanded by applying it to the analysis of such sterically hindered compounds as 2-tert-butylcyclohexanol ( I ) , tertbutyl alcohol (e), tert-butyl hydroperoxide ( I ) , tert-butyl mercaptan ( 7 ) , and the 2,6-di-tert-butylphenols ( 7 ) . LITERATURE CITED
( 1 ) Fritz, J. H., Schenk, G. H., ANAL. CHEM.31,1808(1959). ( 2 ) Gutnikov. G.. Schenk. G. H.. Zbid.. ’ 34. 1316 11962): ( 3 ) arti in; F. E., Jay, R. R., Zbid., 34, 1007 (1962). (4) Mesnard, P., Rertucat, M., Bull. SOC. Chim. France 1959,307. (5) Pesez, M., Zbid., 1954, 1237. ( 6 ) Robinson, W. T., Cundiff, R. H., Markunas, P. C., ANAL.CHEW33, 1030 11961). (7) Schenk, G. H., Fritz, J. H., Zbid., 32,987 (1960). \ - - - - I
GEORGE H. SCHENK Department of Chemistry Wayne State University Detroit 2, Mich.
Applications of Wide-Diameter Open Tubular Columns in Gas Chromatography SIR: Two types of columns are normally used for separation of mixtures by gas chromatography: packed-solvent dispersed on solid support, and open tubular-solyent coated on column ~ 1 1 s . Packed columns are most frequently used. Thermal conductivity detectors, which respond to all compounds and do not destroy the sample, are usually used with these columns. Separation of mistures can be achieved on relatively large samples. This permits collection of fractions for subsequent identification by spectrometry or other techniques. Of the open tubular columns, those having capillary dimensions are most frequently used. They have a high separation efficiency but because sample size is limited to 10-6 mg.), collecting fractions for subsequent examination is impractical. Other disadvantages of the small sample size requirement are that sample splitting
devices and ionization detectors are needed. Detectors which are based on ionization properties do not have so wide an application as thermal conductivity detectors because they do not respond t o all materials. Flame ionization detectors are relatively insensitive to air, water, and other inorganic gases. The use of large-diameter tubular columns was first mentioned by Golay (2) in his original paper. The following year R. P. W. Scott (4) reported on the successful use of nylon tubes of 0,010-, 0.020-, and 0.10-inch i.d. Zlatkis and Kaufman (5) followed Scott’s paper in reporting columns of 0.020-, 0,034-, and 0.066-inch i.d. Both Scott and Zlatkis used (argon) ionization detectors. The first reported use of largediameter open tubular columns with thermal conductivity detectors was given by Kovats at a meeting of the English Gas Chromatography Discus-
sion Group in Birmingham, England, in 1961 (3). Recently Ettre ( I ) presented a paper in which he showed some of their practical advantages. Prompted by these reports, a study n-as undertaken to investigate largediameter open tubular columns which might retain many of the advantages of the packed column and still approach the separating efficiencyof a capillary. EXPERIMENTAL
The columns used in this study are 250 feet of standard ‘/&ich copper tubing with an inside diameter of 0.065 inch. The columns are coiled around a 11/4inch mandel and the ends are fitted with Swage-locks for use in a F & kl Model 500 gas chromatographic unit. The over-all height of the column is 8 inches and 3 inches in diameter. The walls of the column are coated with a solution containing 10 to 15% VOL 35, NO. 4, APRIL 1963
593
I
'
'
I
AROMATIC BLEND
c
o-XYLENE
0 -I
I
p t m XYLENE
I w Y
9
J !
L
)
I
,
TIME (MIN.) Figure 2. alcohols
by weight of the solute dissolved in a n appropriate solvent. Approximately 15 ml. of this solution is added through a 2-foot length of '/a-inch 0.d. polyethylene attached to the coil. $ . slight nitrogen pressure is applied to force the solvent through the column. The coating solvents investigated S 300, were Surfonic TD 300, Surfonic i and Carbowax 20111. The Surfonics are nonionic surfactants manufactured by the Jefferson Chemical Co., Houston, Texas. Surfonic T D 300 is made by reacting tridecyl alcohol with approximately 30 moles of ethylene oxide It is a good polar solvent and can be operated to temperatures up to 175" C. Surfonic IT 300 is a nonyl phenolethylene oxide adduct with about 30 moles of ethylene oxide condensed with each mole of nonyl phenol. It is also a good polar solvent and has been operated to 200' C. without sign of column bleeding. Carbowax 2011 is a polyethylene glycol having an average molecular weight of 20,000. It is a hard, flaky material and has an upper temperature limit of a little over 200" C. All three solvents are suitable but Carbowax 20M cannot be used below 100' C. At low temperatures, Carbowax 2011 solidifies and gives poor separations.
Analysis of a mixture of
coated with Surfonic N 300. The injected sample volume was 2 pl, The temperature of the column was 100' C. and the carrier gas flow rate through the column was 20 ml. per minute. The calculated number of theoretical plates for o-xylene is approximately 12,000 when calculated by the standard equation. Large-diameter open tubular columns permit a high carrier gas flow. Because of the small flow resistance of the column, high speed analysis can be easily carried out. Figure 2 shows the analysis of a mixture of Cg to C16 normal alcohols a t 175' C. The flow rate mas 800 cc. per minute. -it this high flow rate, all peaks were resolved and complete analysis i\as made in less than 8 minutes. Sample capacity of large-diameter open tubular columns is considerably higher than with small-bore capillaries. Sample splitting devices are not needed. Sample size generally ranges between 1
1
1
1
1
1
1
1
1
1
1
ISOPROPYL ALCOHOL
-I
to 5 pl. but for trace analysis, as much as 20 ,d.can be used without significantly overloading the column. Figure 3 shons the analysis made on 20 pl. of isopropanol. Note the well resolved water peak. Water generally emerges from a packed gas chromatographic column as an unsymmetrical peak. This peak dissymmetry is believed to be caused by adsorption of the samplr on the solid support of the column. \Then largediameter open tubular columns are used, relatively sharp peaks are obtained. This is illustrated in Figure 4 rrhich shows a chromatogram of a solution of witer, triosane. and methanol obtained on a 150-foot column at a temperature of 75" C. CONCLUSION
Large-diameter tubular columns should find wide application in the analytical field. Because they have high
~
1
1
1
1
1
WATER
t 1
normal
WATER METHANOL TRIOXANE
Figure 1 shows the analysis of an aromatic blend using a 250-foot column 1
C16
Col.: 250-ft. open tubular Col. temp.: 175" C. Sample size: 7-pl. Coating: Surfonic T.D. 300 Flow rate: 800 cc./minute
RESULTS AND DiSCUSSiON
1
Ce to
r '3
W
I Y
a W
I
n
METHANOL
1
0
5
Figure 3.
10
15
20
,
25 30 TIME ( M I N I
35
,
40
45
Analysis of isopropyl alcohol Col.: 250-ft. open tubular Col. temp.: 60' C. Sample size: 20-pl. Coating: Surfonic T.D. 300 Flow rate: 1 8 cc./minute
594
,
ANALYTICAL CHEMISTRY
I
I h l
I
TIME WIN.)
Figure 4. Chromatogram of solution of water, trioxane, and methanol Col.: 150-ft. open tubular Col. temp.: 70' C. Sample size: 1-1.11. Coating: Surfonic T.D. 300 Flow rate: 2 0 ml./minute
1
separating efficiency and can use a relatively large size sample, they are particularly suitable for use when coupled with a time-of-flight mass spectrometer. We are a t present using this combined technique in our laboratories for identifying gas chromatographic fractions from their mass spectra. S o problems in sensitivity have been encountered with these open tubular columns. The thickness and uniformity of the coating material on the walls of the column is believed to be critical. Undoubtedly, they play an important part
in affecting column efficiency. Techniques for coating these columns should be investigated. Large-diameter open tubular columns can never attain the efficiency of a small-bore capillary, but it has been demonstrated that it can give better resolution and perform an analysis faster than the conventional packed column. Because there is essentially no pressure gradient in large-bore columns, longer columns can probably be employed to improve separating efficiency. This area should be explored.
LITERATURE CITED
(1) Ettre, L. S., Pittsburgh Conference
on Analytical Chemistry and Applied Spectroscopy, Pittsburgh, Pa., March 5, 1962. ( 2 ) Golay, M. J. E., “Gas Chromatography,” V. J. Coates, H. J. Noebels, I. S. Fagerson, eds., pp. 1-1 1, Academic Press, Xew York, 1958. (3) Hawkes, S. J., A’ature 190, 867 (1961). (4) Scott, R. P. W., Ibid., 183, 1753-4 (1959). (5) Zlatkis, A.. Kaufman, H. R., Ibid., 184, 2010 (1959). E. R. QUIR.4M Esso Research and Engineering Co. Linden, ?i. J.
Analysis of Molybdovanadophosphoric Acids and Their Ammonium Salts SIR: In connection nith a research program on the composition, structure, and properties of the heteropoly acids of niolybdenum and tungsten, it became necessary t o develop a procedure for tlle accurate analysis of the molybdovanadophosphoric acids and their ammonium salt.. These are acids in which the familiar cage arrangement of molybdenum atoms about a central phosphorus atom, as in dodecaniolybdophosphoric acid, is presumed to have been modified by substitution of one or more molybdenum atoms by an equal number of atoms of vanadium (I). M‘ith this type of compound, the usual gravimetric procedures for the determination of molybdenum and of phosphorus were subject to interference from the presence of vanadium, and the titration of vanadium n-ith permanganate was made completely infeasible by the strong color of the heteropoly anion. These problems have been overcome by an adaptation of the method of Klement ( 2 ) for ion exchange separation of molybdenum from heavy metals. X dilute sulfuric acid solution of the heteropoly acid is treated with citric acid to complex the anion, the vanadium is reduced with sulfur dioxide to the cationic vanadyl (VO)+2 state, and the solution is passed through a column of a strong cation exchange resin in the hydrogen form. The reduced molybdophosphate anion passes through unchanged, and after reoxidation mith nitric acid, the phosphorus and molybdenum are readily determined by standard gravimetric procedures. The vanadium retained on the column is 91, and eluted with sulfuric acid (1 subsequently determined by titration with standard potassium permanganate.
+
EXPERIMENTAL
Ion Exchange Column. The column used was made from a borosilicate glass tube of 14-mm. i d . , containing
a bed of resin approximately 250 mm. in height. The resin was conditioned by passing 100 ml. of sulfuric acid (1 9) through the column, followed by a rinse with 100 ml. of distilled water. The resin used was identified as cation exchange resin AG50JJr-X8, 100- to 200-mesh, hydrogen form, obtained from the Bio-Rad Laboratories. Richmond, Calif. Recommended Procedure. Weigh a sample containing approximately 80 mg. of molybdenum into a 250-ml. beaker. If the sample is a heteropoly acid, it may be dissolved directly in 50 ml. of water; if i t is a n ammonium salt, add just sufficient 6 N sodium hydroxide, drop by drop, to effect dissolution. -4dd 4 ml. of sulfuric 9) and evaporate to dense acid (1 fumes of sulfur trioxide, cool, and dilute to 50 ml. with u-ater. Add 6 grams of citric acid monohydrate and stir until dissolved. Now add 10 ml. of a saturated solution of sulfur dioxide in water and boil gently until the odor of sulfur dioxide can no longer be detected. During the heating period the solution will gradually assume the characteristic deep blue color of the reduced heteropoly anion. Cool the solution to room temperature and pass through the ion euchange column a t a rate not greater than 2 ml. per minute. Wash the column with a total of eight 25-ml. portions of mater, collecting the effluent in a 500-ml. volumetric flask. As the wash water displaces the molybdate solution from the column, the vanadium will become visible as a green band a t the top portion of the resin bed. N o w begin the elution of the vanadium with sulfuric acid solution (1 9), but continue to collect the washings from the column in the same flask with the molybdenum effluent until the leading edge of the green band has been displaced approximately halfway down the resin bed. At this time, replace the flask containing the molybdenum and phosphorus with a second 500-ml. volumetric flask and continue the elution with a solution of sulfuric acid (1 9) until the vanadium has been completely stripped from the column and 100 ml.
+
+
+
+
Table I. Analysis of Synthetic Mixtures of Known Composition Vanadium 410 P V V present, present, present, found, mg. mg. mg. mg. 800 800 800 800
32 32 32 32 32
28.63
28.85
36 .. 81 -~
36-71 _ ._
51.13 61.35 92.03
50.95 61.36 91.68
Molybdenum P h10 present, present, present, my. mg. mg.
found, mg.
... 17
30 30 30 30 30
25 25 25 15 15
497.5 497.5 497.5 497.5 497.5
Phosphorus V M O P present, present, present, mg. mg. mg. 30 30 30 30 30
500 500 500 500 500
24.93 24.93 24.93 14.96 14.96
hl0
496.6 496.0 494.6 495.3 496.6
P found, mg. 24.99 25.05 24.96 14,94 14.78
of clear sulfuric acid solution has passed through the resin bed behind it. Wash the column with four 25-ml. portions of water, continuing to collect the washings in the same flask. The column is no\T ready for a new sample. Dilute the contents of the first flask to the mark and reserve for the determination of molybdenum and phosphorus. Dilute the contents of the second flask to the mark and reserve for the determination of vanadium. For the determination of vanadium, transfer a 200-ml. aliquot of the vanadium effluent solution to a 500-ml. flat-bottom extraction flask and add 10 ml. of concentrated sulfuric acid. Add 5% potassium permanganate solution dropwise until a stable pink color is obtained. Now add approximately 0.03N ferrous sulfate solution in increVOL 35, NO. 4, APRIL 1963
595
