V O L U M E 2 6 , NO. 8, A U G U S T 1 9 5 4 which mill be widely distributed, will further the usefulness of this rapid graphical method for setting up two-component analyses even when pure standard samples are not readily available to the user. However, caution must be exercised in the use of data in such a manner, and a realistic view of the effects on the accuracy of the analysis must be taken. ACKNOWLEDGMENT
The assistance Of Iiarl preparation Of knoTyn acknowledged.
J'
''
and Davis in the and Of the figures is gratefullv
1273 LITERATURE CITED
Hirt, R. C., and Gisclard, J. B., AXAL.CHEM.,23, 185 (1951). ( 2 ) Hirt, R. C., and King, F. T., Ibid., 24, 1545 (1952). (3) Hirt, R. C., King, F. T., and Schmitt, R. G., I b i d . , 26, 1273 (1954). (4)
Hiskey, C. F., Rabinowitz, J., and Young, I. G., Ibid., 22, 1464 (1950).
(5) Hughes, H. K., Ibid., 24, 1349 (1962). (6) Kuentzel, L. E., Ibid., 23, 1413 (1951). (7) Shurcliff, W, .I.,and Steams, E. I., J . O p t . SOC.Amer., 39, 72 (1949). RECEIVED for review September 16, 1953. Accepted February 11, 1954. Presented a t the Pittsbureh Conference on Analvtical Chemistrv a n d Applied Spectroscopy, Pittsburgh, P a . . March 6 , 1953.
Detection and Estimation of Melamine in Wet-St rength Paper by UIt raviolet Spedrophotometry R. C. HIRT, F. T. KING', and R. G. SCHMITT Stamford Research Laboratories, American Cyanamid Co., Stamford, Conn.
The use of melamine resins to impart wet strength to paper makes desirable a rapid method for the detection and estimation of the melamine content of paper samples. A spectrophotometric method has been developed which makes use of the strong absorption of the melamine ion at 235 mp. The resin is effectively extracted from the paper and hydrolyzed to melamine by refluxing cut-up paper samples in 0.1.V hydrochloric acid.
A
RAPID METHOD for the detection and estimation of the melamine content of paper samples is desirable in using melamine resins t o impart wet strength to paper. The strong absorption of melamine (2,4,6-triamino-s-triazine) in the ultraviolet (1, 6) near 235 mp makes possible the detection and estimation of melamine by a spectrophotometric method of high sensitivity. The relatively large change in the spectrum of melamine between neutral and acetic conditions has been utilized for the determination of the ionization constant (3),and serves as an additional means of identifying melamine in solution. Refluxing cut-up paper samples in 0 . 1 S hydrochloric acid effectively extracts and hydrolyzes the resin t o the melaminum ion, which is then meawred spectrophotometrically. APPARATUS AND MATERIALS
X Cary automatic recording spectrophotometer, Model 11, To. 6 i , and fused quartz cells of various light path lengths were used for examination of the extracts in the range from 260 to 220 mp. Appropriate adjudment of cell length and/or concentration was made in order to obtain absorbance readings a t the analytical wa"e lengths in the accurate range of the spectrophotometer. An all-glass flask and reflux condenser assembly was used for both the extraction of paper samples and for the hydrolysis experiments. The melamine (1, 6) and ammeline ( 4 ) samples were of high purity and identical with thoqe used in previous investigations The resin sample was a commercial trimethylolmelamine, a partially condensed resin, called Parez Resin 607 ( 2 ) . SPECTROPHOTOVETRIC METHODS
The ultraviolet spectra of melamine, ammeline, and trimethylolmelamine in acid solution have been presented in connection with the absorbance-ratio method ( 5 ) . The strong band of the melaminum ion near 235 m p has an absorptivity of 81.0 and may be utilized in a simple one-component analysis in the absence of
* Present address, Chemistry Department, Kansas S t a t e College, M a n hattan. K a n
interfering materials. A detectability of 4 -( may be obtained with an observed absorbance of 0.100 (in a 100-mm. cell 31 ml. in volume), if no other absorbing materials are present. Because other ultraviolet absorbing materials are extracted from paper samples by the refluxing with 0.1S hydrochloric acid, some type of background correction is desirable. Examination of the extracts of various paper samples did not yield any reproducible value that could be used as a simple subtractive background correction. However, because the absorption of melamine falls off so rapidly toryard longer wave lengths, becoming only as high a t 260 mp as at 235 mp, all the observed absorption at 260 mp can be taken as due t o absorbing materials other than melamine. If it is assumed that such absorption does not decrease from 260 t o 235 mp, the absorbance a t 260 mp may be subtracted from that a t 235 mp, t o give a first approximation of a background correction. Because ammeline, the acid hydrolysis product of melamine, may be present, the analytical x a v e length used should be 237 mp (rather than 235 mp), which is the isoabsorptive ( 5 ) wave length for melamine and ammeline. The absorptivity at 237 mp is 79.0. HYDROLYSIS OF MELA\.lINE AND OF PAREZ RESIN 607
During the refluxing with 0 . 1 s hydrochloric acid to remove the resin from the paper sample, hydrolysis of the resin t o melamine occurs, and t o a much lesser extent, hydrolysis of melamine t o ammeline. I n order to study these hydrolysis reactions, a twocomponent analysis of melamine and ammeline was set up, using the giaphical absorbance-ratio method ( 5 ) . Because an isoabsorptive point occurs b e h e e n melamine and ammeline a t 237 mp, this wave length 17-as selected as one of the analytical wave lengths (the other being 226 mp), so that a plot of the ratio of the absorbances at the analytical wave lengths us. fraction of melamine or ammeline would be a straight line. The hydrolysis of the resin of melamine also involves the presence of measurable quantities of ammeline in its later stages, so a three-component analysis, using the graphical absorbance-ratio method, was set up. The details of the three-component application of this method have been described ( 5 ) . -4sample of melamine n a s refluxed in a large excess of 0.1,V hydrochloric acid and aliquots withdrawn and analyzed spectrophotometrically a t suitable time intervals, using the two-component absorbance-ratio method described. The results are shown in Figure 1. h similar study was made on the resin, and the three-component modification of the absorbance-ratio method was used t o obtain the resin content as a function of time, as shown in Figure 1.
ANALYTICAL CHEMISTRY
1274 The hydrolysis of the resin, a t p H = 1, is much more rapid than that of melamine, with a hydrolysis rate constant a t 100" C. of 3.4 X set.-' compared to 2.0 X 10-6 see.-'. Thus, the acid extraction of paper samples effectively hydrolyzes the resin to melamine with a small but measurable amount of the melamine hydrolyzing further to ammeline. By using the isoabsorptive wave length of 237 mp for the calculation of melamine, no error is introduced because of the ammeline present; the total amount of melamine plus ammeline is determined, and because the molecular weights have but one unit difference, this amount may be reported as melamine. PROCEDURE
-4sample of 1 to 2 grams of paper is cut into small pieces (about 1 em. on a side) and weighed on an analytical balance. The weighed sample is placed in a flask of 200- to 300-ml. capacity having a ground-glass joint connection with a water-jacketed reflux condenser (the use of cork or rubber stoppers must be avoided because of contamination of the extract). A volume of 100 ml. of spectroscopically pure 0.1N hydrochloric acid, p H = 1, is added to the flask and refluxed for 1 hour. This is filtered through good quality filter paper or sintered glass and examined in the spectrophotometer. Appropriate adjustment of cell length and/or concentration is made so that the readings a t the analytical wave lengths are in the accurate range of the instrument. The comparison or blank cell is filled with 0.1147 hydrochloric acid. The range from 260 mp to the lower wave-length limit of the instrument is scanned to ensure the observation of the maximum near 235 m p . The concentration of melamine in the paper sample is calculated from the following equation, derived from Beer's law:
where A = the observed absorbance (8) a t the subscript wave length f = the dilution factor (usually 1) V = the volume of HCl in ml. (usually 100) a = the absorptivity (8) (at 237 m, 79.0) b = the cell light path length in mm. TV = the sample weight in grams RECOVERY TESTS
Because no other method for the determination of melamine in wet strength paper was available for checking this spectrophotometric method, comparison to Kjeldahl-nitrogen analyses were used to furnish the knon-n values for recovery tests. Samples of papers containing melamine resin for wet strength were analyzed by the conventional Kjeldahl-nitrogen method and by the spectrophotometric method. The values for per cent nitrogen were calculated as if all the nitrogen content were due to melamine, although some nitrogen may be found in papers not containing melamine resin. The recovery figures for the spectrophotometric method are based on these values. These data are shown in Table I. The maximum difference from the nitrogen-analysis values was observed to be a relative ll%, and the average difference was 5.5%.
Table I. Type of Paper Sample Facial tissue Specialty wrap Bleached southern kraft, bag Unbleached southern kraft, bag Army map Specialty wrap
Comparison of Kjeldahl-Nitrogen and Spectrophotometric Results
Kjeldahl Nitrogen,
Calculated Resin,
0.25 0.25 0.28 0.28 0.40 0.41
0.65
0.38
0.73
0.42
1.05
0 52 0.53
%
%
Calculated Melamine,
70
Ultraviolet Melamine,
%
Recovery,
%
0.38 0.39 0.40 0.43
100
0 61
0.68 0.64
1111 105
1.36
0.79
0.72
91 92
0.70
1.82
LO5
0.70 1 05
107 106
2 73
1
1.12 1.11 1 65
0.73
57
103 95 102
I05
ADVANTAGES OF THE SPECTROPHOTOMETRIC METHOD
The principal advantage of the spectrophotometric method is the speed with which analyses may be performed. The amount of time spent in scanning the desired spectral range with a recording spectrophotometer or in taking several point readings on a nonrecording instrument, and calculating the per cent melamine is of the order of 10 minutes per sample. The samples may be refluxing while other operations are being carried on. The presence of a maximum near 235 mp in hydrochloric acid, which disappears in alkaline solution, serves as a confirmatory test for melamine. This is easily done by making the solution alkaline and re-examining the spectrum. The band a t 235 mp in acid becomes a slight shoulder in neutral or alkaline solution ( I , 6, 9). If a band is not observed near 235 mp in hydrochloric arid. the presence of melamine should not be reported. The Kjeldahl-nitrogen method does not distinguish between melamine and urea resins, and is interfered with by the presence of nitrogen from casein, glue, amine-softeners, or other nitrogencontaining materials sometimes present in paper. These materials do not have ultraviolet spectra of appreciable intensity, or, like rosin, are not extractable by the acid solution, and hence do not interfere. The background correction using the absorption a t 260 mp fairly Tell compensates for other absorbing materials present. The dye-staining test for melamine ( 7 , I O ) gives only a qualitative indication of melamine present. ACKNOWLEDGMENT
The authors wish to acknowledge the assistance of members of General Analytical Laboratory in the Kjeldahl nitrogen analyses and of C. G. Landes and S. T. Moore in obtaining the wetstrength paper samples. LITERATURE CITED (1) Costa, G. IT., Hirt, R. C., and Salley, D. J., J . Chem. P h y s . , 18, 434 (1950). (2) Dixon, ,J. K., Christopher, G. L. hl., and Salley, D. J., T a p p i , 31. 412 (1948): P a m r T r a d e J . . 127, 455 (1948). Pare, Resin 607 is the registered trade-mark of the American Cyan-
amid Co.
t
i
,
2
3 4 5 6 7 8 9 ' TIME IN HOURS Figure 1. Hydrolysis of Trimethylolmela'0'1
mine and Melamine Refluxed Hydrochloric Acid
in
0.1N
(3) Dixon, J. K., Woodberry, N. T., and Costa, G. W., J . Am. Chem. Soc., 69, 599 (1947). (4) Hirt, R. C., A p p l . Spectroscopy, 6 , No. 2, 15 (1952). . 26. (5) Hirt, R. C., King, F. T., and Schmitt, R. G.. A x ~ LCHEW., 1270 (1954). (6) Hirt, R. C.. and Salley, D. J.,J . Chem. Phus.. 21, 1181 (1953). (7) House. K. R.. and Woodberry, N. T., T a p p i , 37, 255 (1954). (8) Hughes, H. K., ANAL.CHEM.,24, 1349 (1952). (9) Klotz, I. M , , and Askouniq, T., J . Am. Chem. Soc., 69, SO1 (1947). (IO) Stafford, R . W., Thomas, W. AI., Wllliams, E. F., and Woodberry, N. T., T a p p i , 28, 275 (1945) ; Paper Trade J., 120, 155 (1945). RECEIVED for review January 22, 1954. Accepted June 10, 1954.
