116 Analyst, February, 1973, Vol. 98, PP. 116-121 Thin-layer Chromatography of Simple Urea - Formaldehyde - Methanol Reaction Products Part 11.” Quantitative Aspects BY P. R. LUDLAM (The Borden Chemical Company ( U . K . ) Limited, North Baddesley, Southampton, SO6 9ZB) A thin-layer chromatographic method for the determination of urea, monomethylolurea and dimethylolurea in urea - formaldehyde resins is described. The resin is treated with methanolic boron trifluoride, whereby the methylol compounds are quantitatively converted into their respective methyl ethers. Subsequent application of chromatography on silica gel plates that have been previously treated with ammonia vapour in order to prevent reaction of the urea derivatives results in the effective separation of the components of low relative molecular mass, thus allowing their concentrations to be determined.Examples are given illustrating the change in concen- tration of urea derivatives of low relative molecular mass with time in urea - formaldehyde formulations. CURRENTLY many forms of urea - formaldehyde compositions are being produced that contain, to a greater or lesser extent, urea - formaldehyde condensation products of low relative molecular mass. Chromatography of urea - formaldehyde resins for the purpose of separating, identifying and determining these components of low relative molecular mass can throw some light on the nature of a particular material, its age and its method of manufacture. The direct chromatographic determination of urea, monomethylolurea (hydroxymethyl- urea) and dimethylolurea (dihydroxymethylurea) is fraught with many difficulties.Although it is possible both to apply the material to the chromatographic plate so that it is ready for development without significantly altering the composition of the mixture and to develop the chromatogram in such a way that there is a satisfactory separation of some of the compounds of low relative molecular mass, it is found that monomethylolurea and dimethylol- urea can be almost totally obscured by the unresolved fraction of high relative molecular mass, which extends from the origin to the region of monomethylolurea. Further, the fact that monomethylolurea is coincidental with methylenediurea is another major difficulty encountered in the direct chromatography of this type of mixture.Similarly, monomethylol- methylenediurea interferes in the determination of dimethylolurea because of its proximity on the chromatographic p1ate.l Another major drawback to any direct chromatographic method is the inherent instability of monomethylolurea and to a lesser extent dimethylolurea, both in solution and in the solid state. Consequently, the solid reference materials would have to be purified every few days and the standard solutions prepared daily. It is fortunate that most of these difficulties can be overcome by converting the alcohol groups into their methyl ethers. Serious errors can, of course, be introduced into the determination if significant amounts of methyl ethers are present in the mixture before etherification. This problem, however, is not usual but if any doubts exist about the ether concentration, the sample should be checked by running a direct chromatogram prior to carrying out quantitative chromatography.Both monomethylolurea and dimethylolurea ethers have RF values greater than that of urea and are therefore effectively separated from the “tail” of the resin, and, as shown in Part I of this paper,l no compounds have so far been investigated that have RF values of a similar order. If, however, a considerable amount of monomethylolmethylenediurea is present, then interference with urea will result and measurement of the urea concentration will have to be made by direct chromatography, i.e., without ether formation. Only very * For Part I of this series, see p. 107.@ SAC and the author.LUDLAM 117 occasionally is this approach necessary. The stability of the ethers of both mono- and di- methylolurea is much greater than that of the base compounds, which makes it practical to prepare fairly large amounts of the pure compounds, and the standard solutions in methanol can be used for many months without noticeable deterioration occurring. The conversion of the methylol compounds into their methyl ethers must, of course, proceed quantitatively. For all commercial products so far examined the method given below has proved satisfactory. However, should the composition under test be highly alkaline or strongly buffered, it is probable that etherification will not be complete. In this event, trial adjustments of the concentration of boron trifluoride will enable the optimum methylating conditions to be determined and allow quantitative conversion to be achieved.EXPERIMENTAL CONVERSION OF METHYLOL COMPOUNDS INTO THEIR METHYL ETHERS- Initial experiments showed that with 1 per cent. of boron trifluoride or hydrogen chloride in methanol, monomethylolurea and dimethylolurea could be quantitatively converted into their ethers within 1 minute. However, after 3 minutes small amounts of mono- and di- methylolurea had been regenerated in the solution. This effect was more noticeable after 5 minutes, by which time compounds of high relative molecular mass were being precipitated out of solution. It was obvious that the concentration of the catalyst would have to be controlled very carefully, and at this stage it was decided to concentrate on boron trifluoride as the catalyst as it was considered that buffers, which occur in commercial compositions, may have less effect on the catalytic activity of boron trifluoride than that of hydrogen chloride.Further experiments with methanolic boron trifluoride solutions showed that a 0.16 per cent. solution would quantitatively convert mono- and dimethylolurea into their ethers after 15 minutes and that the solution was stable for a further 20 minutes, thus giving adequate time for sample preparation and manipulation. EFFECT OF BUFFERS ON THE ETHERIFICATION PROCESS- Sodium formate, disodium hydrogen orthophosphate and triethanolamine are three buffers that can either be formed in, or added to, commercial urea - formaldehyde resins.The effect of such compounds on the etherification of a mixture of mono- and dimethylolurea is shown in Table I. The efficiency of a particular set of methylating conditions is indicated by the amount of monomethylolurea that remains. TABLE I EFFECT OF BUFFERS ON ETHERIFICATION Ratio of Buffer in sample, per cent. Monomethylolurea, per cent., A BF, in methyl- c \ remaining after- anol, reagent to ethanol- 2 6 10 16 20 meth- ating Tri- f A \ per cent. sample HCOONa Na,HPO,. 12H,O amine minutes minutes minutes minutes minutes - - - 0.16 6: 1 1.8 0 0 100 - 100 0-26 3: 1 0-25 0 0 - 100 - - 10 10 10 - 10 0 1 0 - (10 <lo <10 0 0 0.6 10 <lo - - 10 0 0 0.66 10 - 0-60 3: 1 0.26 0 0 <10 (10 <lo - - 0.66 0 0 - 60 0 0 1.8 100 0.60 0 0 20 10 0.66 0 0 80 - - - - 60 - - - - < 10 - < 10 - As can be seen in Table I, ether formation appears to be mostly affected by sodium formate.However, the concentration in urea - formaldehyde resins is not likely to exceed 0.6 per cent. and etherification with a 0-5 per cent. solution of boron trifluoride in methanol for 15 minutes is likely to prove effective in most instances. Should the buffer content be unusually high, then the presence of monomethylolurea will be observed on the developed chromatogram, and further experiments will be necessary so as to ascertain the most suitable methylating conditions for that particular composition.118 LUDLAM : THIN-LAYER CHROMATOGRAPHY OF SIMPLE [Analyst, VOl. 98 EFFECT OF WATER ON THE ETHERIFICATION PROCESS- Experiments on the methylation of mono- and dimethylolurea, in which the methanol to water ratio varied from 2: 1 to 6: 1, showed that in the presence of a large percentage of water the conversion of a methylol compound into its ether was very poor, much of the original material remaining unchanged. At ratios of 5: 1 or above, the reaction became quantitative.To allow for possible variations in the water content of the sample, a methanol to sample ratio of 4: 1 was adopted for the method. Thus, with 35 per cent. of water in the sample, the over-all methanol to water ratio will be of the order of 12: 1. METHOD APPARATUS- over silica gel. Thin-layer chromatographic plates-Merck Kieselgel F,,, (fast running) on glass. Store Microfiif&ks, 1 , 2 and 5 $-Constriction pipettes or Drummond “Microcaps.” Chromatographic tank.MATERIALS- Urea-Analytical-reagent grade. Prepare standard solutions in methanol, 0.4 to 2.0 per cent. m/V in intervals of 0.2 per cent. Dimethylolurea dimethyl ether-Prepare according to the method given in Part 1 .l Prepare standard solutions in methanol equivalent to 0.8 to 3.0 per cent. m/V of dimethylolurea, in intervals of 0.2 per cent. Monomethylolurea monomethyl ether-Prepare according to the method given in Part 1 .I Prepare standard solutions in methanol equivalent to 0.8 to 3.0 per cent. m/V of mono- methylolurea, in intervals of 0.2 per cent. These solutions are stable for at least 6 months. Eluting solvent-Mix 44 ml of ethyl acetate with 3 ml of methanol and 3 ml of ammonia solution of sp. gr. 0.88.Allow the mixture to stand for 1 hour before use. Prepare freshly each day. Chlorine-As supplied in a cylinder (BDH Chemicals Ltd.) . Spray reagent-Dissolve 5 g of o-toluidine in 95 ml of acetic acid. Boron tri$uoride, 0.5 per cent. solution in methanol-Dilute the 14 per cent. solution of boron trifluoride in methanol (BDH Chemicals Ltd.) with methanol. PROCEDURE- To 1 g (or less) of sample, add dropwise with vigorous stirring, 0.5 per cent. of boron trifluoride in methanol so as to obtain a final volume of 5 ml. It is recommended that a pre-calibrated test-tube or a small measuring cylinder is used for the preparation of the sample. A calibrated flask is not suitable owing to the method of sample treatment. Allow the solution to stand for 15 minutes. Place the chromatographic plate in the vapour of the eluting solvent for a few seconds, remove the plate and spot on to it 1 pl of each of the standard solutions.Spot on to the plate 1, 2 and 5 pl of the sample solution. Allow the spots to dry and place the plate in the solvent in the chromatographic tank. It is not necessary to saturate the atmosphere of the tank with solvent vapour. When the solvent has risen almost to the top of the plate, remove the plate from the tank and allow it to dry for 15 minutes at room temperature. Do not dry it at elevated temperatures. Expose the dry plate to chlorine gas either in a separate vessel or by directing chlorine gas from the cylinder directly on to the plate in an efficient fume hood. Allow the excess of chlorine to disperse from the plate for a t least 5 minutes and spray with the 6 per cent.o-toluidine solution. Do not expose the plate unnecessarily to bright light. Evaluate the plate after half an hour. Foreknowledge of the approximate concentrations of urea and mono- and dimethylolurea in a sample can permit a smaller range of standards and sample to be used. The procedure allows urea to be determined at concentrations from 0.5 to 10 per cent. and mono- and dimethylolurea at concentrations from 0.8 to 15 per cent.February, 19731 UREA - FORMALDEHYDE - METHANOL REACTION PRODUCTS. PART 11 119 RESULTS AND DISCUSSION Mixtures of urea and mono- and dimethylolurea, in which the balance was methylene- diurea, were analysed by the method given above (on a 0.25-g sample). The results are given in Table 11.TABLE I1 ANALYSIS OF MIXTURES OF UREA, MONOMETHYLOLUREA AND DIMETHYLOLUREA Monomethylolurea, Dimethylolurea, Urea, per cent. per cent. per cent. - r - Sample* Theoretical Experimental Theoretical Experimental Theoretical Experimental A 16 (i) 16 f 3 26 (4 17 * 3t 37 (i) 36 f 3 (ii) 36 f 3 B 20 20 f 2 30 33 f 3 46 46 f 3 (ii) 15 f 3 (ii) 26 f 3 * In each sample the balance of the content was methylenediurea. t This very low result is possibly due to pre-reaction of the monomethylolurea during the sample There is little doubt that if this type of mixture is ground and left for several hours, preparation stage. reaction will occur even if the sample is dry. A detailed examination of the plate showed that , except for A (i) , no significant amounts of the original methylol compounds remained, nor were any spots present other than those of urea, methylenediurea and the methyl ethers of mono- and dimethylolurea, thus indicating that side reactions had not occurred.NH,.CO.NH, + CH20 + NH,.CO.NH.CH,.OH by measuring the residual formaldehyde in the reaction mixture. The following experiment was designed to determine the rate constant, k,, by measuring the residual urea and the monomethylolurea formed in the reaction. Urea (log) was added to 30g of 36 per cent. formaldehyde solution (molar ratio of urea to formaldehyde, 1:2*16) at pH 7-0. The temperature was maintained at 25 "C in a water-bath and the pH at 7.0 by the addition of 0.1 N sodium hydroxide solution. After 20 minutes, a sample was analysed by the chromatographic method and the following results were obtained: urea 18, monomethylolurea 10 and dimethylolurea 1 per cent.The rate constant, k,, calculated from the residual urea content is 0.79 x 10-4, which is in good agreement with De Jong and De Jonge's value of 0.6 x The experimental value of the monomethylolurea concentration (10 per cent.) is in good agreement with the value of 10.5 per cent. calculated from the change in urea concentration (26 to 18 per cent.). Two liquid urea - formaldehyde resins, stored at 21 "C, were monitored for a 3-month period in order to determine the variation of free urea and mono- and dimethylolurea with time. It is common practice with compositions of this type to add urea towards the end De Jong and De Jonge2 have determined rate constants for the reaction w a +.' s 8- m ? 5 - 0 > - .- E n 6 1 I I I I I I I t 40 60 80 1 2 4 6 8 1 0 20 Tirne/days (logarithmic scale) 00 Fig.1. Variation of dimethylolurea content with time (resin A)’ O 1 ki a Time/days (logarithmic scale) Fig. 2. Variation of urea (A), monomethyl- olurea (B) and dimethylolurea (C) contents with time (resin B) Solvent front Origin 0 0 0 0 0 0 Q Fig. 3. Thin-layer chromatogram of a methylated urea resin and standards. Spots 1, 2 and 3, 1 p1 of 0.5, 0.75 and 1.0 per cent. urea solutions, respectively; spots 4, 8 and 12, 1 p1 of resin solution (10 per cent.); spots 5, 6 and 7, 1 pl of 0.8, 1.0 and 1.2 per cent. monomethylolurea monomethyl ether solutions, respectively; and spots 9, 10 and 11, 1 pl of 1.0, 1.2 and 1.5 per cent.dimethylolurea dimethyl ether solutions, respectively. Spots a t A are dimethylolurea dimethyl ether; a t B, monomethylolurea monomethyl ether; a t C, dimethylol- methylenediurea dimethyl ether; a t D, urea; and a t E, methylene- diurea. Colours of spots: open spots, yellow; closed spots, blue; and half-closed spots, greenFebruary, 19731 UREA - FORMALDEHYDE - METHANOL REACTION PRODUCTS. PART 11 121 of manufacture to reduce the level of free formaldehyde. One of the resins (A) did not contain “end” urea; the other (B) contained 11.6 per cent. The urea and monomethylolurea contents of resin A were as follows- 1st day 6th day 10th day 20th day 40th day 60th day Urea, per cent. . . .. . . <0.6 <Om6 <Ow6 <Om6 <0.6 <0*6 Monomethylolurea, per cent. . . 1.6 1.0 1.0 0.6 0.6 < 0.5 The dimethylolurea content is shown in Fig. 1. The urea and mono- and dimethylolurea contents of resin B are shown in Fig. 2. A typical chromatogram is illustrated in Fig. 3. Examination of the many chromatograms obtained in the course of this study shows that only two other compounds of low relative molecular mass are present to any significant extent in normal urea - formaldehyde resins. These compounds are methylenediurea, up to approximately 1 per cent., and dimethylolmethylenediurea, perhaps up to 6 per cent. Two minor spots, both of which probably amounted to less than 1 per cent., occur above dimethylolurea dimethyl ether on the chromatogram and it is probable that these are due to compounds formed in the etherification reaction, which so far have not been identified. REFERENCES I thank Mr. J. G. King for his skilful technical assistance. 1. 2. Ludlam, P. R., Analyst, 1973, 98, 107. De Jong, J . I., and De Jonge, J., Reel Trav. Chim. Pays-Bas Belg., 1962, 71, 643. NOTE-Reference 1 is to Part 1 of this series. Received June 21st, 1972 Accepted October 6th, 1972