We have confirmed the need to adhere to a fixed sequence and time schedule for color development in order to give reproducible results. Thus, it is important to add the rhodamine-B immediately after completing the oxidation to antimony(V), to avoid the rapid hydrolysis of antimony(V) to nonreactive hydroxychloride complexes. The maximum color intensity is developed shortly after the phases are separated, and the absorbance should be measured within five minutes.
Applications. The recommended method has been applied t o a series of commercial grades of arsenic and arsenic oxides. The results shown in Table 111 indicate the satisfactory application of the method to these classes of high-arsenic materials.
RECEIVED for review February 5, 1968. 1968.
Accepted April 4,
Potentiometric Titrations of Cyanuric Acid and Melamine in Dimethylsulfoxide Raul Morales E. I . du Pont de Nemours and Co., Wilmington, Del. CYANURIC ACID and melamine are of significant industrial importance for a wide variety of uses. Along with a host of other related products, both of the s-triazines are produced by heating urea or urea-derived products. Some of the related compounds formed during the thermal processes are so chemically similar (see structures below) that usual methods of analyses for the subject compounds are often not free from interferences. These compounds are normally determined by tedious volumetric or gravimetric procedures (1-8) of poor precision and accuracy. Other approaches, such as the polarographic studies performed by Suchy (9) and Malik (10) on cyanuric acid make no mention of interferences from likely contaminants and are therefore difficult to evaluate for analytical purposes.
CYANURIC ACID
MELAMINE (2, 4, 6-TRIAMINOS-TRIAZINE)
BIURET
AMMELINE
UREA
AMMELIDE
(1) R. N. Marek (to Olin Mathieson Chemical Corp.), U. S. Patent 2,986,452 (May 30, 1961). (2) G. Ostrozovich and A . Nemes, Chem. Absrr., 55, 162836 (1961). (3) . , S. N. Kazarnovskis and 0. I. Lebedev, Chem. Absrr., 52, 135250 (1958). (4) A. A. Korinfskv. Zacodsk. Lab., 12. 418 (19461. . , (5) G. Widmer, Runsrsroffe, 46, 359 (1956). (6) L. Nebbia, F. Guerrieri, and B. Pagoni, Chim. Ind., 39, 81 (1957). (7) J. R. Steele, J. H. Glover, and H. W. Hodgson, J . Appl. Chem., 2, 296 (1952). (8) E. Doehlemann, Angew. Chem., 66,606 (1954). (9) K. Suchy, Chem. Absrr., 46,10959e (1952). (10) W. W. Malik, A. A. Khan, and B. Haque, Nafurwissenchafren, 48, 47 (1961). 1148
ANALYTICAL CHEMISTRY
The first ionization pK,’s for cyanuric acid, ammelide, and ammeline are 6.5, 6.9, and 9.4, respectively (11). These compounds are therefore weak acids and differentiation between ammelide and cyanuric acid by a titration procedure would seem not possible. A search of the literature for analytical investigations of s-triazines in nonaqueous solvents yielded only the limited work done by Kreshkov (12) on melamine, pKb -9 (13), and ammeline. The obvious advantages of nonaqueous titrimetry over those analytical procedures already cited for the triazines apparently have been overlooked. This paper describes the potentiometric titration, in dimethylsulfoxide (DMSO), of cyanuric acid and melamine in the presence of various possible contaminants. Kolthoff and Reddy (14) and Barnes and Mann (15) enumerate the various desirable properties of DMSO as a solvent for the titration of moderately weak acids and salts. This study also indicates that the solvent may be used satisfactorily for the titration of weak bases. EXPERIMENTAL
All titrations were performed with an L & N Model 7664 p H meter with recorder output. The titration vessel consisted of a 40-ml glass bottle (5-cm diameter) fitted with a plastic cap with openings drilled for the electrodes. Beckman micro electrodes, glass, and methanolic saturated KC1, made up the rest of the titration system. The titrations were carried out with the aid of a magnetic stirrer in 40 ml of solvent, pure DMSO for melamine and a 4 : l benzene:DMSO solution for cyanuric acid. A dry COz free nitrogen blanket was maintained over the solvent throughout the titration. The solvent “blank” was found to be negligible in both systems. A 0.1N solution of perchloric acid in dioxane was used for the titration of melamine. The acid was standardized against potassium acid phthalate dissolved in anhydrous glacial acetic acid. A 0.1N solution of tetra-n-butylammo(11) R. C. Hirt e t a l , J. Chem. Eng. Dam, 6, 610 (1961). (12) A. P. Kreshkov, A. N. Yarovenko, and V. N. Nevska)a,Zhur. Anal. Khim., 21, 350 (1966). (13) J. K. Dixon, N. T. Woodberry, and G. W. Costa, J . Am. Chem. Soc., 69, 599 (1947). (14) I . M . Kolthoff and T. B. Reddy, Znorg. Chem., 1, 189 (1962). (15) K. K. Barnes and C. K. Mann, ANAL,CHEM., 36,2502 (1964).
nium hydroxide (TBAH) in benzene-methanol (16) was used for the titration of cyanuric acid. The reagent was standardized against benzoic acid in DMSO. All other chemicals were commercial reagent grade and used as received. DISCUSSION
Initially, various nonaqueous solvents were investigated for the determination of cyanuric acid. These included n-butylamine, DMSO, benzene, and a 1 : 4 solution of the last two, respectively. Although butylamine and D M S O produced satisfactory results with cyanuric acid alone, in the presence of contaminants the analytical results were erratic. Benzene was found t o be a poor solvent for the dissolution of cyanuric acid but when dissolved with DMSO, in the ratio already stated, the acid dissolved completely. The solubility of ammelide in the mixed solvent proved t o be small and, therefore, provided an ideal situation for the determination of cyanuric acid in the presence of ammelide. The potentiometric titration of cyanuric acid in the prescribed mixed solvent, yielded a standard deviation of 0.40 o n the basis of 9 runs and sample sizes ranging from 0.04 t o 0.08 gram. The inflection points of the titration curves were quite sharp having potential changes of about 300 mV. The proposed reaction taking place is (11):
Typical determinations of cyanuric acid in synthetic mixtures are shown in Table I. With the exception of binary mixtures containing melamine and biuret, the results are in agreement with theory. Biuret is slightly soluble in the mixed solvent and apparently also titrates. F o r the binary samples containing melamine, the results are markedly low and seem t o indicate a reaction between the two triazines with one molecule of cyanuric acid reacting with two of the amine. The resulting product probably precipitates from solution during the titration and therefore is not acted upon by the stronger base, TBAH. The present study showed the determination of melamine by nonaqueous titrimetry to be simple and reliable in the presence of many possible contaminants. Using the described procedure, a standard deviation of 0 . 4 6 z absolute was obtained with eight samples of 0.04-0.08 gram. The average per cent found was 99.4%;. The titration curves showed the end point to be unambiguous with a potential break of about 165 mV. The proposed reaction taking place (11) is :
H 2 N y N y " z +
H+4
NYN
zNYNY"2 H / N Y N
"2
/:\H
Table I. Determination of Cyanuric Acid in Synthetic Mixtures Sample composition
Added,
Cyanuric acid Ammelide Cyanuric acid Ammeline Cyanuric acid Melamine Cyanuric acid Urea Cyanuric acid Ammelide Melamine Cyanuric acid Ammelide Biuret Cyanuric acid Ammelide Urea
76.7 23.3 73.6 26.4 73.3 26.7 76.3 23.7 52.4 23.2 24.4 72.1 14.4 13.5 58.8 21.4 19.8
z
Found,
d,
Z
77.3
0.6
73.6
0
58.5
-14.9
75.2
-1.1
48.5
-3.9
73.7
1.6
59.9
0.9
Table 11. Determination of Melamine in Synthetic Mixtures Sample composition Melamine Ammelide Melamine Ammeline Melamine Cyanuric acid Melamine Urea Melamine Urea Cyanuric acid Melamine Ammelide Ammeline
Added, % 69.6 30.4 74.2 25.8 75.2 24.8 74.9 25.1 59.8 19.8 20.4 66.1 17.3 16.6
Found, 69.2 76.5
z
4
-0.4 2.3
73.8
-1.4
74.2
-0.7
59.2
-0.6
66.7
0.6
results to be low; however, the interference is minimal when compared to results obtained with the benzene/DMSO system. Data obtained with the tertiary systems are in excellent agreement with theory. This investigation indicates the feasibility of the quantitative determination of cyanuric acid and melamine by nonaqueous titrimetry. I n spite of the fact that the levels of interferences studied here are generally greater than those encountered in practice, the methods prove to be capable of great accuracy. No evidence of decomposition was found for DMSO during the titrations and neither were similar distortions in the titration curves noted beyond the end point as seen by Barnes (15).
The effect of possible interferences in the titration of melamine is tabulated in Table 11. Ammeline being so structurally similar to melamine, interferes slightly. Partial reaction of cyanuric acid with the basic triazine causes the
The author thanks J. J. Kirkland for his helpful suggestions in the preparation of this manuscript.
(16) R.. H. Cundiff and P. C. Markunas, ANAL.CHEM.,28, 792 (1956).
RECEIVED for review December 20, 1967. Accepted March 27, 1968.
ACKNOWLEDGMENT
VOL. 40, NO. 7,JUNE 1968
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