V O L U M E 2 8 , NO. 9 , S E P T E M B E R 1 9 5 6 staff of the Memorial Hospital, Worcester, Mass., made available pooled human sera for these experiments. LITERATURE CITED (1) ilntweiler, H. J., Kolloid-2. 115, 130 (1949). (2) Brice, B. A, Halwer, XI., J . O p t . Soc. Amer. 41, 1033 (1951). (3) Calvet, E., Chevalerias, R., J . chim. p h y s . 43, 37 (1946).
(4) Forsberg, R., Svensson, H., Optica Acta 1 , 90 (1954). (5) Kegeles, G., J . Am. Chem. SOC.69, 1302 (1947). (6) Kegeles, G., U. S. Patent (applied for, 1956). (7) Kegeles, G., Sober, H. h.,ANAL.CHEM.24, 654 (1952). (8) Labhart, H., Staub, H., Helv. Chim. Acta 30, 1954 (1947). (9) Longsworth, L. G., h - 4 ~ CHEM. . 23, 346 (1951). (10) Longsworth, L. G., J . Am. Chem. Soc. 61, 529 (1939).
1463 (11) Philpot, J. St. L., Sature 141, 283 (1938). (12) Philpot, J. St. L., Cook, G. H., Research (London) 1, 234 (1948). (13) Svedberg, T., Pedersen, K. O., "The Ultracentrifuge," p. 4 5 , Fig. 8, Clarendon Press, Oxford, 1940. (14) Svensson, H., Acta Chem. Scand. 4 , 399 (1950). (15) Svensson, H., J . O p t . Soc. Bmer. 44, 140 (1954). (16) Svensson, H., Kolloid-2. 87, 180 (1939). (17) Svensson, H., Optica Acta 1 , 25 (1954). (18) Svensson, H., Forsherg, R., J . O p t . SOC.Amer. 44, 414 (1954). (19) Svensson, H., Olhagen, B., Sei. Tools, L K B Instruments J . 1, K O . 2, 9 (1954). (20) Tiselius, A, Trans. Faraday SOC. 33, 524 (1937). (21) Toepler, A , Pogg. Ann. 131, 33 (1867). (22) Wiener, O., Ann. Phys. Chem. S. F . 49, 105 (1893).
RECEIVED for rel-iew March 21. 1956. Accepted hIay 29, 1956.
Colorimetric Determination of Phenolic Resins M.
H. SWANN
and
D. J. WElL
P a i n t and Chemical Laboratory, A b e r d e e n Proving Ground,
A modification of the nitrous acid test for free phenols is presented for phenol-aldehyde resins in varnishes and other coatings. An intense yellow color is de\eloped with phenolic resins; the color is specific and can be used for quantitative measurement if the nature of the raw materials is known. The phenolic content of resins of unknown origin can be estimated.
R""'"
TS obtained by phenol-aldehj de condensation are Pidely used, usually modified by dispersion in other resins or reacted \T ith fatty acids. They impart desirable properties such as chemical and water resistance, exterior durability, and rapid drying to coating materials. These resins appear in such a large variety of chemical compositions that significant chemical analysis has been considered improbable. ilt the present time, tests for these resins are practically limited to qualitative identification and measurement of free phenol. S o quantitative measurement of phenolic resins has been proposed, and no completely satisfactory qualitative test is available. The most rn idely used qualitative test, the indophenol test of Gibbs ( 2 ) ) is very sensitive but indicates free phenols only. Most phenolic resins contain some free phenol and give a positive test u i t h Gibbs reagent, although a fen- are completely free of uncombined phenol. The recent extensive use of free phenols such as p-tert-amylphenol as antiskinning agents in enamels has caused considerable difficulty in distinguishing qualitatively b e h e e n agents of this type and phenolic resin modifications. T o prevent such interference, the test is conducted on the dried vehicle film. Because the resulting film is insoluble, it is pyrolytically decomposed and the fumes are dissolved in water. The qualitative test with Gibbs reagent is then applied to this water solution. Although this test appears to eliminate interference from the volatile antiskinning agents, it gives positive results n ith resins of nonpheriolic origin or with resins not intended for the scope of the test. For example, polystyrene, epoxy resins, and coatings plasticized IT-ith tricresyl phosphate shorn positive results by this technique. As a result, this qualitative test is significant only when negative. A variety of free phenols, cresols, xylenols, and aldehydes is used in making phenolic-type plastics, adhesives, and resins. Each phenolic raw material may appear in a variety of substituted forms, including many isomeric forms. There are so many potential different resins of this type that significant chem-
Md.
ical analysis may ne11 be considered impossible; however, oilsoluble phenol condensate resins for use in paints and varnishes are obtained from alkyl or aryl derivatives of phenol, the former possessing three or more carbon atoms. p-tert-Aniylphenol, p-tert-butylphenol, and p-phenylphenol are .ividely used; resins made from the latter excel in desirable properties and have the distinction of responding t o specific qualitative tests. Therefore, they can be distinguished from other phenolic resins and analyzed by the procedure described with a high degree of accuracy in any coating solution in which they are used. The procedure is based on the nitrous acid test ( 3 )for phenols. This test, usually conducted in aqueous medium for free phenols, produces an intense yellow color when applied t o phenolic resins dissolved in a 71-at'er-insoluble solvent such as butyl acetate or I-hexanol. It is highly reproducible and specific as no other resin interferes. Epoxy resins, which use bisphenols as starting materials, do not develop color in this test. PROCEDURE
Quantitative Determination. A small sample of resin, varnish, or enamel vehicle is carefully Tveighed, dissolved in n-butyl acetate and diluted to definite volume. For some heat-hardening resins which are not soluble 1-hexanol is substituted throughout the test, but butyl acetate is preferred for its cleaner and faster separations. From the nonvolatile composition of the sample, a n aliquot (not exceeding 40 ml.) estimated t o contain not more than 6 mg. of phenolic resin is transferred to a 250-ml. Erlenmeyer flask. Butyl acetate is then added t o bring the total volume to 40 ml. Ten milliliters of 10 to 1 sulfuric acid (about 3 . 6 s ) is added, followed b y 2 ml. of a freshly prepared 10% aqueous solution of sodium nitrite. All of these volumes may be approximate. The flask is covered with a vented stopper and placed in a water ba,th a t 70" C. for 1 hour. Gentle agitation is applied several times during this period. The sample is then cooled to room temperature and transferred t o a Pepsrator!: funnel with water. The lower aqueous layer is removed and the solvent layer washed twice with 11-ater using gentle agitation. The solvent layer is then filtered through paper dampened with solvent into a 5O-ml. volumetric flask and diluted to volume. Colorimetric comparison is made a t 425 mh. I n most colorimeters, water may be used in the blank cell. Graphs of knon-n samples are prepared in like manner, developing color individually on each known fraction. The phenolic resin content of the sample aliquot can be computed as desired. Qualitative Identification of P-Phenylphenol-Formaldehyde Resins. Because this colorimetric method can be used with high accuracy on resins of k n o n phenolic origin, and because p phenyl-phenolformaldehyde resins can be readily distinguished from other types, the specific qualitative tests are given in detail. FERRIC CHLORIDE.T o 1 ml. of the resin solution in a test tube is added 10 ml. of methyl isobutyl ketone, followed by 10
1464
ANALYTICAL CHEMISTRY
drops of ferric chloride reagent (2% solution in pyridine). Alcoholic potassium hydroxide is then added dropwise and the sample examined after each drop. If no green color forms by the addition of 10 drops of alkali, this resin is absent. CONCENTRATED SULFURIC ACID. Dried films of p-phenylphenol-formaldehyde resins develop an intense green color when dissolved in cold concentrated sulfuric acid, but this may be obscured by dark colors formed if large quantities of other resinous materials are present.
I
80
Figure 1.
I
I
70 60 50 % TRANSMITTANCE
The resins and phenols absorb closely a t 425 mp; therefore, this wave length was used throughout the investigation. Absorbances were measured on the Bausch I%Lomb Spectronic 20 colorimeter with 0.5-inch cells. The choice of a wave length a t which the resins and their corresponding phenols have similar absorbances is important to prevent error due to varying amounts of free phenols in the resins. The usefulness of the method would appear a t first to be limited to the analysis of resins of known origin or to resins that can be identified qualitatively-Le., p-phenylphenol-formaldehyde. For high accuracy this is probably the case. But consideration of the relatively few phenols which produce oil-soluble resins for coatings and the relative similarity of color developed with the resins shown in Figure 1 indicates that the method appears to have possibilities for estimating resin content based on average color development of a variety of resins. One phenolic modified alkyd, stated by the manufacturer to contain 3.5 to 4.0% phenol modification, analyzed 3.9% by this method based on the average described. Phenolic antiskinning agents produce color by this method, but these materials are used in paints in small quantities (0.2 t o 0.6% on the nonvolatile basis) and their interference is not serious in quantitative estimations.
I
I
40
Color developed by phenolformaldehyde resins
Heat-hardening b, c , d , f, 0. Son-heat-hardening h. p-Phenylphenol-formaldehyde a , e.
MILLON'SREAGENT ( 1 ) . The reagent is prepared by dissolving a small amount of mercury in an equal weight of concentrated nitric acid and diluting with an equal volume of water. When heated t o boiling in the presence of p-phenylphenol-formaldehyde resin, or preferably the fumes from its pyrolysis, a purple color is formed. Other phenolic resins may form red colors that should not be confused. RESULTS
To test the analytical possibilities of the procedure, color was developed and measured with a large variety of phenolic resins, mostly of unknown origin. Among this group were heatrhardening and non-heat-hardening resins, as well as heat-reactive and non-heat-reactive types. A variety of substituted phenols, many of which are used as raw materials for resin manufacture, was similarly tested. The color developed with resins is shown in Figure 1, and with phenols in Figure 2. The curves were obtained by plotting known weights (1 to 6 mg.) of each against the per cent transmittance a t 425 mp. Because the origin of a few of the resins was known, comparison of the color produced by resins and their corresponding phenol was of particular interest. This comparison revealed that the color developed was similar but not identical, and each showed an individual spectrum in the visible wave-length region. At approximately 422.5 mp each resin and its corresponding phenol absorbed identically. At longer wave lengths the color produced by resins absorbs more strongly, while a t shorter wave lengths the phenols show stronger absorption. This is illustrated in the graph of Figure 3. Most colorimeters do not provide for the use of the exact wave length of 422.5 mp; some are graduated a t 5-mp intervals so that 420 or 425 mp could be used. The 425-mp wave length is available on practically all types of colorimetric instruments.
1
I
80
Figure 2. a.
b. c.
d.
e.
I
I
I
70 60 50 'A TRANSMITTANCE
I
40
Color developed by some substituted phenols
m-Cresol o-Cresol p-tert-Amylphenol p-tert-Butylphenol Bisphenol A
f. 8.
h. 1.
p-Cresol tert-Butylcresol 2,4-Diamylpheno p-Phenylphenol
Considerable interest and discussion have been centered around the chemistry of the reaction between oxidizing oils, such as tung or linseed, and p-phenylphenol-formaldehyde resins ( 3 , 6). When oil-reactive phenolic resins are heated with drying oils which contain some conjugated unsaturation, there is elimination of water and an abnormal viscosity increase. It is generally believed that a form of diene synthesis takes place. It is also believed that the reaction is greatest with tung oil and less with linseed. When known varnishes of both types were prepared on a laboratory scale and analyzed for p-phenylphenol-formaldehyde, low results were obtained with both types of varnishes, but the residual phenolic resin content was much lower in the linseed varnish than in the tung oil varnish. The usual techniques, which involve higher reaction temperature for the linseed varnish, were used for the preparations. To demonstrate that
V O L U M E 28, NO. 9, S E P T E M B E R 1 9 5 6
1465
Table I. Analysis of Varnishes Made from Drying Oils and p-Phenylphenol-Formaldehyde Resins Sample _ Resin, _ 5% _ ~ No.
-; 3 4 5 6
Oil
Tung Tung Linseed Linseed (same conditions as for t u n g oil) Paraffin oil (nonreactive) Glyptal 2585 (phenol-modified alkyd)
Present 33.3 33.4 32.8 36.5 33.3 3 . .5 t o 4 . 0
Found 29.2 29.0 22.5 36.5 34.0 3.9 (resin c , Figure 1)
heat degradation of the phenolic resin was not the reason for this difference, an inert oil such as paraffin oil was substituted for the drying oil and the resulting product analyzed for phenolic resin content, in which case 100% recovery was obtained. Some analytical results are shown in Table I. Saponification has been known to cause reversal of some DielsAlder type additions and products of diene synthesis, but saponification of the linseed varnish (sample 3, Table I), followed by solvent extraction and analysis for p-phenylphenol-formaldehyde resin, gave results identical to those shown. The color developed with phenols by the action of nitrous acid is believed to be due to the formation of p-nitrosophenol, which condenses to form other complex colored compounds ( 4 ) .
%TRANSMITTANCE
Figure 3.
Spectra of substituted phenols and their formaldehyde condensates a. b. c. d.
p-Phenylphenol-formaldehyde resin p-Phenylphenol p-tert-Butylphenol-formaldehyde resin p-tert-Butylphenol
tion conditions while others are completely unsaponifiable even under drastic conditions. To analyze materials of unknown phenolic resins origin such .YR phenol modified alkyds, it is recommended that the estimation be made by comparing the color developed by the unknown to a representative resin such as c of Figure 1. In choosing this resin it should be noted that curve h for p-phenylphenol-formaldehyde resin is not considered because this resin produces more color than any of the others and can be identified as such when present. The phenol-modified alkyd shown in Table I was the only resin solution of this type available with known composition.
DISCUSSION
To analyze pigmented coatings for phenolic resin content, a clear pigment-free vehicle is necessary. Black and olive-drab coatings are not usually clean enough after ordinary centrifuging to permit satisfactory analysis. For these materials it is necessary to dilute about 50% with toluene and add 5 to 10 grams of dry powdered calcium hydroxide to each 100 ml. of paint before centrifuging. While this technique may affect some analytical results, such as acid number determinations it has not shown any undesirable effect on the phenolic resin analysis. Prior to the development of the colorimetric method the only indication of the extent of phenolic modification of alkyd resins, for example, was from the determination of unsaponifiable content. This test is obviously affected by a number of other resinous modifications, and varies with different types of phenolic resins. Some of these saponify readily under ordinary saponifica-
LITERATURE CITED
(1) F e d , F., “Qualitative Analysis by Spot Tests,” 2nd ed.. p. 276,
Nordemsnn, New York, 1939.
(2) Gibbs, H. D., J. BioZ. Chem. 72, 649 (1927). (3) AIorrell, R. S., “Synthetic Resins and Allied Plastics,” 3rd ed , p. 314, Oxford University Press, London, 1951. Snell, F. D., Snell, C. T., “Colorimetric Methods of Analysi-, ’ vol. 11, p. 372, Van Nostrand, New York, 1937. (5) Sprengling, G. R., J. Am. Chem. SOC.74, 2937 (1952).
(4)
RECEIVEDfor review February 10, 1956.
Accepted M a y 28, 1956
Radioassay of Tagged Sulfate Impurity in Cellulose Nitrate S A M U E L HELF,
T. C. C A S T O R I N A , C. G. WHITE,
and R. J. G R A Y B U S H
Chemical Research Laboratory, Picatinny Arsenal, Dover,
Two methods of sample preparation and counting are discussed for the application of radiotracer techniques, using radiosulfur, to the quantitative assay of sulfate impurity in samples of cellulose nitrate. One method entails the preparation of uniform films and the determination of the counting rate with a windowless flow counter. The other method involves solution of a fixed weight of cellulose nitrate in a liquid phosphor medium and measurement of beta activity with a coincidence liquid scintillation counter.
I
N T H E course of a study on the purification and stabilization of cellulose nitrate, it was necessary to analyze a large number of samples for total sulfate content. The sulfate, considered an
N. J. undesirable impurity, is imparted to the nitrated polymer from the sulfuric acid used in the mixed nitrating acid to esterify the cellulose; it is believed to exist as both strongly occluded free acid and chemically combined sulfate ester. The usual methods for total sulfate determination in cellulose nitrate and other cellulose esters have been surveyed by Hoffpauir and Guthrie ( 1 ) . Essentially all of these techniques involve the wet combustion or decomposition of samples in a strong oxidizing or hydrolytic medium and subsequent gravimetric determination of sulfate as the barium salt. Hoffpauir and Guthrie concluded that “when the sulfate content of cellulose nitrate and other esters is very low, these methods require the use of large samples to provide sufficient amounts of the barium sulfate precipitate for convenient manipulation and the decomposition becomes lengthy and tedious.”
