'I'he procedures based on the modification of pH or on the addition of gum ghatti are simple and can be used for routine work. The procedure based on isobutyl alcohol extraction is slightly more complicated, but may be used with advantage when a greater sensitivity is desired. LITERATURE CITED
(1) Cawley, L. P., Spear, F. E., Kendall,
R., Tech. Bull. Registry M e d . Technol. 29, 111 (1959).
(2) Guidotti, G., Colombo, J. P., FOB, P. P., Abstracts of Papers, 137th
(9) Teller, J. D., Abstracts of Papers,
130th Meeting, BCS, Atlantic City, J., September 1956, p. 69C.
Meeting, BCS, Cleveland, Ohio, Bpril 1960, p. 4c. (3) Huggett, A. St. G., Sixon, D. A., Lancet 273,368 (1957). (4) Jacobsen, L. N.,Scand. J . Clin. Lab. Invest. 12, 76 (1960). ( 5 ) McComb, R. B., Yushok, W. D., J . Franklin Inst. 265,417 (1958). (6) Selson, N., J . Bid. Chem. 153, 375 (1944). ( 7 ) Saifer, -4., Gerstenfeld, S., J. Lab. Clin. Med. 51, 448 (1558). (8) Somogyi, M., J . Bid. Chem. 160, 69 (1945).
?;.
GUIDOGUIDOTTI JEAN-PIERRE COLOMBO PIEROP. Foi, Department of Physiology and Pharmacology The Chicago Medical School Chicano 12. Ill. 0
,
AIDEDby grant A-552 from the National Institute of Arthritie and Metabolic Diseases, U. S. Public Health Service.
Spectrophotometric Determination of Hydroxyl Groups in Poly(propy1ene Glycols) SIR: Methods for the determination of hydroxyl content in poly(propy1ene glycols) by infrared spectroscopy have been published ( 2 , 3 ) . The method described needs no construction of standard working curves because it is, in effect, a direct spectrophotometric titration of the hydroxy compound with a standard solution of acetyl chloride. PROCEDURE AND EQUIPMENT
Equal samples of poly(propy1ene glycol), about 10 grams each (differing by not more than 10 mg.), are transferred into I5O-ml. iodine flasks, each of
x
~-
-.
I
I
Figure Near-infrared spectra of poIy(propyiene glycol) after reaction with acetyl chloride Absorbance measurements made at polnts on dotted line Background at point of arrow
which contains 25 to 35 grams of zinc metal (c.P. grade, 20-mesh granules). Sufficient toluene is added in each flask SO that the samples are quantitatively diluted to 40 ml. upon subsequent addition of different and definite amounts of 0.6M acetyl chloride in toluene solution. The toluene and the acetyl chloride solution are most accurately added with burets connected to the respective vessels for easy refilling and delivery. Six or seven samples are sufficient for a determination. Amounts of acetylating reagent are chosen to obtain absorbances between 0.2 and 0.8. The flasks are firmly stoppered, gently shaken, and placed in a 37" =t1' C. oil bath so that only their bases contact the oil. After 40 to 45 minutes they are removed from the bath and cooled to about the temperature of the cell compartment of the spectrophotometer. Spectra of each sample are recorded with a Beckman DK-2 spectrophotometer. A silica cell is employed with 0.52-mm. path. The settings are: reference, air; speed, 5/2X; scale, 0-1; sensitivity, 0.5; period, 0.2. As the temperature affects absorption by hydroxyl, care must be taken to keep constant temperature as far as feasible during scanning of samples for each determination (3). Allowing the sample in the cell to achieve thermal equilibrium with the compartment (about 2 minutes) yields reproducible and usable spectra. If a constanttemperature cell holder is available, it should be preferred. Absorbances are measured a t 2.87 to 2.88 microns, as shown in Figure 1. To determine the background absorbance, an excess of ca. 5 and 8 ml. of acetyl chloride solution is added t o each of two samples, respectively. Total absorbance is plotted us. milliliters of acetyl chloride solution. From the plot the number of milliliters of acetyl chloride solution required to react with the sample is obtained, as shown in Figure 2.
Then
where M W
= =
molarity of acetyl chloride and weight of sample
As any water in the sample is assumed to react completely with the acetyl chloride, correction for water is made as follows: (Mi. X M
-
%Ha0 X W/1.8)56.1 W = hydroxyl No.
The mater content is determined by the Karl Fischer method. It is assumed that the acetic acid formed by the hydrolysis of acetyl chloride does not react with the hydroxy compound, under the experimental conditions. For samples containing up to 0.27'0 water such correction yielded results in good agree-
0.6 -
ig a s 2 8 -
1
0.4
i 0.3
Figure 2. Typical spectrophotometric plot of poly(propy1ene glycol) titrated with acetyl chloride VOL. 33, NO. 1, JANUARY 1961
153
ment with results obtained by the phthalic anhydride method (1). A fair indication of the precision and repeatability of the method may be had from the example: Hydroxyl S o . Analyst A
59.9 60.8 61.4 59.6 61.1 61.2
Analyst B
The hydroxyl number of this sample determined by the phthalic anhydride method above was 60.6. The average molecular weights of the samples tested, as given by the manufacturer, were between ea. 1750 and 3500.
it reacts, to some extent, with the acid formed, its presence favors the forward reaction. The reaction, as written, proceeds t o completion stoichiometrically. This was verified by titrating reacted samples with sodium hydroxide (4) for any residual acetyl chloride. Values of 507, of the blanks were obtained when various deficient amounts of acetyl chloride were allowed t o react with the sample. A plot of net absorbance-Le., total minus b a c k g r o u n d u s . moles of acetyl chloride reacted with the hydroxyl groups of the sample indicated the two quantities 15 ere linearly related. If reaction times are prolonged, side reactions are possible: ROH
+ HC1
ZmCl1
RC1
+ HnO
I,
The reaction of acetylation can be symbolically shown as
CHs-&-OR
0
0 CH~-&-OH
-C1
+ ROH
370
0
/I
CHIC--OR The zinc acts as a catalyst
+ HCl
(4). Since
LITERATURE CITED
(1) Aerojet-General Corp., Aausa, Calif., “Determination of the Molecular Weight or Hydroxyl Number of Organic Hydroxyl Compounds by the Phthalic
+ H20 I
ACKNOWLEDGMENT
The encouragement extended by the management for this study is gratefully acknowledged. Thanks are also given to D. A. Xole for his assistance in instrumentation.
(1)
0 DISCUSSION
she\$- any appreciable increase in absorbance. This may be plausibly explained on the basis that Reaction 2 must b e preceded by Reaction 1. The Lucastype reagent, ZnC12-HC1, produced in situ may react with available hydroxyls. K h e n sufficient acetyl chloride is present, all the hydroxyls are esterified; thus, neither Reaction 1nor 2 is possible.
+ ROH
(2)
An increase in absorption was noticed in the region 3.5 to 2.5 microns when deficient amounts of acetyl chloride were allowed to react with the sample overnight. Samples that reacted with some excess of acetyl chloride did not
Anhydride-Pyridine Method,” Aerojet Laboratory Standard, ALS-430.02 (Dec. 8, 1955). (2) Burns, E. A,, Muraca, R. F., ANAL. CHEM.31,397 (1959). (3) Hilton, C. L., Ibzd., 31, 1610 (1959). (4) Kyriacou, D., I b d , 32, 291 (1960). DEMETRIOS KYRIACOU Analytical Chemistry Department Aerojet-General Corp. Sacramento, Calif.
Colorimetric Determination of Aluminum 8-Quinolinolate in Chloroform SIR: The colorimetric determination of aluminum 8-quinolinolate in chloroform has been widely used. I n attempting to use this method, an instability of the solutions was found which we call to the attention of other workers. Afoeller (3) has used analytical reagent chloroform containing 1% ethyl alcohol by volume and extraction a t p H 4.3 to 4.6 with absorbance measurement at 395 mp; in later work Moeller (4, 5 ) reported that chloroform solutions of aluminum 8-quinolinolate are light sensitive, characterized by a decrease in absorbance in the 390- to 401-mp region and the appearance of a peak in the 315- to 318-mp region. The rate of photochemical decomposition increased as the size and equivalent weight of the central metal ion increased. The aluminum 8-quinolinolate should therefore be relatively stable. According to Lloeller and Cohen ( 5 ) , photochemical decomposition may be prevented by preparing solutions under red light and keeping them either under red light or in darkness. Margcrum, Sprain, and Banks (2) determined aluminum in thorium by extraction with 1% 8quinolinol in reagent grade chloroform and reported serious absorbance errors in solutions exposed to sunlight, but 154
ANALYTICAL CHEMISTRY
unexposed solutions were stable for a day or more. EXPERIMENTAL
Absorbance measurements were made on a Beckman Model DU using 1.00em. stoppered silica cells corrected for mismatch. The aluminum 8-quinolinolate was precipitated from aluminum chloride solution with 8-quinolinol by the procedure given in Kolthoff and Sandell (1). , Aluminum 8-quinolinolate wasalso precipitated from 5/6 basic aluminum chloride solution, in which case a prior nitric acid digestion and ammonia neutralization was used before the conventional method (6). 8-Quinolinol was an Eastman Organic chemical. Aluminum chloride was supplied by a 32’ B6. solution containing less than 50 pap.m. of Fe, obtained from the Reheis Co., Berkeley Heights, N. J. The 5/6 basic aluminum chloride, Chlorhydrol (Reheis Co.), was supplied as a concentrated solution containing 12.5% Al, 8.37% C1, 20 p.p.m. of Fe, and less than 5 p.p.m. of heavy metals such as Pb. All other reagents were reagent grade with the exception of chloroform (Table I).
flasks which were diluted to volume with chloroform under red light, and the absorbance was measured as indicated. The results are tabulated in Table 1. Solutions containing 5.0 nig. of aluminum 8-quinolinolate per 100 ml. of U.S.P. chloroform were aged for 24 hours, in one case flushed and stored under 02,and in another with XIatheson prepurified N2 (8 p.p.m. of 02). iibsorbance measurements a t 390 mp were as follows (hours, atmosphere, absorbance): 0, air, 0.684; 24, Nz dark, 0.668; 24, I\j2 light, 0.462; 24, O2dark, 0.665; 24, O2 light, 0.402. Addition of further 95% ethyl alcohol (lye by yolume) to the U.S.P.chloroform gave somewhat more rapid aging results than those given in Table I. Addition of 0.01% (by weight) of Eastman B H T (2,6-di-tert-butyl-4methylphenol) to either U.S.P. or analytical reagent chloroform solutions gave aging results identical to those in Table I. Although only measurements a t 390 mp are given here, the maximum absorbance slowly shifts, on aging, from 388 to 382 mp in the 24 hours aged under O2and light. DISCUSSION
PROCEDURE
The aluminum 8-quinohnolate was weighed into clean dry volumetric
The above results show a dark decomposition reaction for chloroform
