mercurimetric titrations of the thiols; also to John Pritchett who determined the purity of some of the phenols by alkalimetric titration. They thank the following companies for samples: ilmerican Cyanamid, Don, Chemical, General Aniline & Film, Pennsalt Chemicals. and Pitt-Consol Chemical. LITERATURE CITED
(1 ) Fritz, J. S., Hammond, G. S., “Quanti-
tative Organic Analysis,” p. 261, Kiley, hTewYork. 1957. (2) Fritz, J.’ S., Moye, A. J., Richard, RI. J., A N A L . CHEM.29, 1685 (1957). (3) Fritz, J. S., Schenk, G. H., Zbid., 31, 1808 (1959). (4)Hall. H. K., J . Phus. Chem. 60, 63 (1956). (5) Mehlenbacher, V. C., “Organic Analysis,” T’ol. 1,- pp. 1-38, Interecience, New York, 1933. (6) Mesnard, P., Bertucat, M., Bull. SOC. chim. France 1959, 307.
( 7 ) Pesez, M., Zbid., 1954, 1237. (8) Siggia, S., “Quantitative Organic Analysis via Functional Groups,” p. 9,
Wiley, New York, 1954.
(9) van der Heijde, H. B., A4nal. Chim. Acta 16, 392 (1957).
RECEIVED for revievc- December 17, 1959. Accepted March 29, 1960. Contribution No. 829. Work performed in the Ames Laboratory of the U. S. Atomic Energy Commission.
Spectrophotometric Determination of Hydroquinone in the Presence of Its Monomethyl Ether Application to Acrylic Monomers RENE J. LACOSTE, JOHN R. COVINGTON, and GIN0 J. FRISONE Rohm & Haas Co., Philadelphia, Pa.
)A spectrophotometric procedure for the determination of trace amounts of hydroquinone in acrylic acid inhibited with the monomethyl ether of hydroquinone is based on a reaction between hydroquinone and an amine. Various conditions affecting the reaction have been investigated. Ten parts per million of hydroquinone can be determined in the presence of 1000 p.p.m. o f monomethyl ether. Slight modification might increase the sensitivity tenfold. Extension to other acrylic monomers i s discussed.
A
against polymerization by the monomethyl ether of hydroquinone (MEHQ) may be contaminated with trace quantities of hydroquinone. Because polymerization of the monomer is affected by t h e kind and amount of inhibitor present, a maximum of 10 p.p.m. of hydroquinone was specified. A rapid procedure for the determination of hydroquinone in this concentration in the presence of 1000 p.p.m. of MEHQ was needed. A survey of the literature revealed no analytical procedures capable of determining hydroquinone in the presence of such a large excess of MEHQ. A direct polarographic determination fails because the N E H Q wave interferes with the hydroquinone wave. A direct spectrophotometric measurement in the ultraviolet region fails because the absorption band due to M E H Q obscures t h a t due to hydroquinone. JVhen a n amine is added to hydro-
990
CRYLIC MONOMERS inhibited
ANALYTICAL CHEMISTRY
quinone-inhibited acrylic monomers, a blue-violet coloration is observed ( S ) , which was traced t o the presence of hydroquinone in the system. This observation forms the basis of the procedure. MEHQ does not give colored reaction products with the amines. Hydroquinone can be determined in acrylic acid inhibited with 0.1% MEI-IQ with a sensitivity of 10 p.p.m. and a precision to about ~ 4 ~ 5 %relative. Indications are that the procedure is applicable to other acrylic monomers. The sensitivity limit might be extended with some modification in the method. REAGENTS AND APPARATUS
Hydroquinone, Eastman Organic Chemicals, melting point 172-4’ C. Methyl ether of hydroquinone (MEHQ), . Tennessee Eastman Co., melting point 55-7’ C., white crystals. n-Butylamine, Matheson Coleman & Bell, redistilled in this laboratory. Beckman DK-1 spectrophotometer, 5-cm. silica cells. RECOMMENDED PROCEDURE
This procedure is written specifically for glacial acrylic acid. However, experimental data indicate that with little or no modification it can be extended to other monomers. Pipet 1.0 ml. of monomer into a 100ml. volumetric flask containing 50 ml. of water. Add 20 ml. of n-butylamine and swirl t o mix. Make to volume with water, mix, and immediately pour into a 250-ml. beaker. Allow the solution to stand exposed to air for 15 t o 20 minutes. Measure the absorbance a t 345
mp us. a 20% n-butylamine aqueous solution. Similarly prepare a calibration curve with known amounts of hydroquinone. EXPERIMENTAL
Visible Spectrum. An absorption spectrum from 400 to 800 mp was obtained of the colored reaction product formed with n-butylamine and hydroquinone. The absorbance maximum is at 540 mp (Figure 1). Under controlled conditions, the absorbance-concentration relation a t this wave length is linear. To have an absorbance of 0.10 or more in a 10-em. absorption cell, however, a minimum of about 1 p.p.m. of hydroquinone must be present in solution. This would be the concentration in a 10% solution of acrylic acid containing 10 p.p.m. of hydroquinone. Measurements in the visible spectrum would involve a preliminary separation of the acid from the hydroquinone, because the presence of more than 1% acrylic acid interferes with color development. Investigation of the ultraviolet absorption spectrum of this colored reaction product revealed a stronger absorption band at 346 mp which eliminates the need for separation. Ultraviolet Spectrum. The ultraviolet absorption maximum a t 345 mp is approximately 20 times stronger than the visible one at 540 mp (Figures 1 and 2), and therefore, 0.1 p.p.m. of hydroquinone can be measured in a 5-cm. absorption cell, This corresponds to the concentration in a 1% solution of acrylic acid containing 10 p.p.m. of
0 6, 1
\ \ \
0 6 ~
\
'\A
a
m 03 ET 0 v)
m
B 1
I
I
I
I
I
I
400
450
500
550
600
650
700
WAVE LENGTH ( m p )
Figure 1.
Visible absorption spectrum
4 p.p.m. hydroquinone, 20% aqueous n-butylamine vs. n-butylamine E . Blank, 20% n-butylamine vs. H t 0 , 1 0-cm. cell A.
hydroquinone. Thus, no preliminary separation of the acid from the hydroquinone is necessary, because 1% MEHQ-inhibited acrylic acid does not interfere in the reaction. Type of Amine. Hydroquinone was added t o various amines in a n aqueous medium. Of the amines which gave a color, n-butylamine was chosen for this study because of its availability (Table I). Effect of Amine Concentration. Using a n aqueous hydroquinone solution, the absorbance of the reaction product a t 345 mp was measured as a function of n-butylamine concentration over the range of 0 to 1 0 0 ~ (Table o 11). Absorbance was maximum between 10 and 3Oy0amine, followed by a slight decrease in intensity a t higher concentrations. This latter observation was checked with duplicate experiments and ZOyGamine was used for the reaction because this is the middle of the maximum range. Effects of Oxygen and Time. I n the complete absence of air (for a total of 48 hours), hydroquinone does
Table 1. Color Reactions of Amine with 4 P.P.M. of Hydroquinone Type of Amine (20% Present) Color PH n-Butylamine Blue-violet 12.3 Isobutylamine Blue-violet 12.3 sec-Butylamine Xone ... tertButylamine None ... Cyclohexylamine Blue-violet 12.2 Aniline Red 11.5 Ethylenediamine Pale yellow 12.2 Diethylamine Pale yellow 12.2 Diisopropylamine None 12.2 Diphenylamine None ...
C
I
I
I
300
310
320
I
I
330
340
350
360
L E N G T H ( m p L)
WAVE
20% aqueous
Figure 2.
Ultraviolet absorption spectrum
0.4 p.pm hydroquinone in 1% aqueous acrylic acid and 20% n-butylamine vs. 20% aqueous n-butylamine A. 0.001% MEHQ B. No MEHQ C. Blank, 1% uninhibited acrylic acid, 20% aqueous n-butylamine vs. H20, 5-cm. silica cell
not form the colored reaction product with the amine, but benzoquinone does. Condensation reactions of benzoquinone and amines t h a t give colored reaction products have been described (1, 2, 4 ) . Benzoquinone reacts with amines according to the following equation: 0
+ RNH
Table II. Effect of Amine Concentration (0.4 p.p.m. hydroquinone)
HZO __f
B
from 5 to 30 minutes, and then after 60 minutes. Absorbance was maximum after 10 minutes, and remained essentially unchsnged up to 60 minutes (Table 111). A precipitate begins to form after 60 minutes.
%
or ale.
%
Amine Absorbance Amine Absorbance 0 0.000 30 0.470
0
1
0,320
10
0.470 0.470
20
d
+ HtO
After 48 hours, the deaerated solution of hydroquinone and amine was poured into a beaker and exposed to air. A blue-violet color developed immediately a t the surface of the solution and within a short time appeared throughout. This suggests that the hydroquinone is air-oxidized to quinone before reacting with the amine, and is consistent with the well known air oxidation reaction of hydroquinone under basic conditions. An aqueous hydroquinone solution was reacted with 20y0 n-butylamine, poured into a 250-ml. beaker, and exposed to air. The absorbance was measured a t 345 me a t 5-minute intervals
50 70 100
0.370 0.370 0.420
Table 111. Effect of Time (0.4 p.p.m. hydroquinone) Time, AbsorbTime, AbsorbMin. ance hlin. ance 0.450
5 10
0.472
Table IV.
15 20-GO
0.480 0.470
Effect of Acid Concentration
(0.4 p.p.m. hydroquinone)
Acrylic Acid Concn., % 0.0 0.5 1.0 1.5 2.0 5.0
Absorbance 0.480 0.482 0.475 0.515 0.515 0.520
VOL. 32, NO. 8 , JULY 1960
991
Temperature Effects. A change of temperature had little effect on absorbance. When a n aqueous hydroquinone ( 4 p.p.m.) solution and t h e 20% n-butylamine reagent were cooled t o 18" C. and then reacted, t h e absorbance was 0.490; when reacted at 25' C., i t was 0.475, and when heated t o 39" C. and reacted, 0.480. Reagent Purification. Uninhibited glacial acrylic acid was added t o a 20% aqueous undistilled n-butylamine solution. Blank values were high and variable. With distilled amine, there was a negligible absorbance in the -
V,
Hydroquinone, P.P.M.
1% acid
VI.
0 7-
a
o 605-
a
04-
030 2-
0
01
I
1
I
02
03
04
0 1 0.5 0 1 0 5
0.130
0 604
Concentration-Absorbance
Monomer l yo ethyl acrylate
quinone, Absorb-
P.P.hl.
0":;o ,
ancea
::!!:
1 % methyl meth-
2 0.4 0.0 0.1 0.4 0.8
0,200 0.400 0,002 0.120 0.390 0.790
1% methacrylic acid
0.0
0.080
acrylate
O.OS5 0 .. 84 0
0.450 0 ,885
In all cases absorbance due to hydroquinone corrected for absorbance obtained in absence of added hydroquinone.
ANALYTICAL CHEMISTRY
, 05
06
07
08
I
I
09
10
ppm HYDROQUINONE
Figure 3.
Concentration vs. absorbance a t 345 mp
5-cm. silica cell, 20% aqueous n-butylamine reference
0 1 % uninhibited ocrylic acid A N o acid
plus 0.001% MEHQ. S o significant differences in absorbance a t 345 mp were observed (Figure 2 ) . Concentration us. Absorbance Relation. From 0.1 to 0.8 p.p.m. of hydroquinone weie added t o a 1% aqueous solution of uninhibited glacial acrylic acid (corresponding t o 10 t o 80 p.p.m. of hydroquinone in the acid). The absorbance-concentration relationship 1% as linear and essentially
Mean Absorbance 0 129 0.575
of Other Monomers Hydro-
992
090 8-
Precision of Measurements in Inhibited Acid and in W a t e r
Sample No acid
Table
IO-
,
(Figure 2). Concentration of Acid. The absorbances of aqueous hydroquinone solutions (0.4 p.p.m.) were obtained with various concentrations of uninhibited glacial acrylic acid. At 5% acid concentration, t h e absorbance is about 8% higher than at 1% (Table IV). However, when t h e same experiment was repeated with inhibited acrylic acid (0.1% hlEHQ), a strong absorption band due t o t h e presence of MEHQ obscured the hydroquinoneamine absorption band a t a n acid concentration in excess of 37,. Effect of MEHQ. The absorption spectrum of a n aqueous hydroquinone solution (0.4 p.p.m.) containing 20y0 n-butylamine was compared t o t h e absorption spectrum of a n aqueous solution containing the same quantity of hydroquinone and n-butylamine
Table
I1
No. of U
Detns.
jzl 8% relative j =1 . 2 % relative 1 2 . 0 7 0relative i1 4% relative
5 5 4 5
identical with t h a t obtained in t h e absence of acrvlic acid from 0.1 t o 0.5 P.P*nl. of hydroquinone. dkbove 0.5 P.P,nl* a negative deviation from Beer's law was observed in the absence of acid (Figure 3 ) . The absorbance values obtained above this level, however, are reproducible. Precision and Sensitivity. Known amounts of hydroquinone were added to acrylic acid containing 0.1% M E H Q . Each sample was replicated 'Vere four to five times and the compared t o those obtained in t h e absence of acid (Table V). Taking a n absorbance of 0.1 as t h e minimum practical measurement for good quantitative the data indicate that as little as 10 pap.m. of hydroquinone in MEHQ-inhibited acrylic
acid can be determined with an estimated precision to i5% relative. The data further indicate that an aqueous calibration curve can be used as a basis for the determination in acid. The sensitivity limit might be eytended by analyzing acid solutions more concentrated than l%, if aqueous solutions of acid are used in the reference beam of the spectrophotometer to suhtract the absorbance due to RIEHQ. Other Monomers. The applicability of this method to other acrylic monomers is shon-n in Table VI. Knon n quantities of hydroquinone were added t o the uninhibited monomers; in each instance, a linear relation exists between t h e concentration of hydroquinone and absorbance. The ielatively high blank absorbance in the methacrylic acid is probably not characteristic of uninhibited acid. Only one sample of this acid vas available a t the time the experiment n as performed. ACKNOWLEDGMENT
The authors express appreciation to
v, p, De XIarco for part of t,he experinlental '\T.Ork. LITERATURE CITED
( 1 ) Xnslow, W. K., Raustrick, H., J . Cheni. SOC.1939, 1446. (2) Cason, J., "Organic Reactions," R. A d a m et al., eds., Vol. IV, pp. 360-1, Wiley, New York, 1948. (3) Frisone, G. J., private communication. (4)Jolles, z. E . , "Chemistry of Carbon
Compounds, E. H. Rodd, ed., Vol. 111-B,p. 700, Elsevier, New York, 1956. RECEIVED for review Kovember 5, 1959. Accepted March 15, 1960. Third Del&ware Valley Regional hleeting, ACS, Philadelphia, Pa., February 1960.
