642 Analyst, October, 1967, Vol. 92, pp. 642-644 The Determination of 4-Aminobiphenyl in Refined Aniline BY D. A. REILLY (Inzpevial Chemical Industvies Limited, Dyestuffs Division, Blackley, Manchester 9) A method has been developed for the determination of from 2 mg per litre, upwards, of 4-aminobiphenyl in refined aniline. The bulk of the aniline is separated by distilling the sample in the presence of paraffin wax, which retains 75 per cent. of the 4-aminobiphenyl. An acid extract of the distilla- tion residue is diazotised and coupled with sodium 2-naphthol 3, B-disul- phonate (R-salt) . The dyestuffs resulting from 4-aminobiphenyl and residual aniline are separated by circular-paper chromatography, and the former is determined by visual comparison of the corresponding ring on the chromatogram with standards prepared from aniline containing known concentrations of 4-aminobiphenyl.A METHOD has been published by Stagg and Reed1 for the determination of 4-aminobiphenyl in technical diphenylamine. When applied to the determination of this impurity in refined aniline, the procedure was found to be unsatisfactory because the concentration of R-salt was too low in relation to the weight of aniline taken, which resulted in the production of 4-aminobiphenyl from the diazotised aniline at the coupling stage, thus giving falsely high values of up to 1 per cent. for its concentration in the aniline. Norman and Vaughan2 made similar criticisms and modified the method by increasing the concentration of R-salt 10-fold thereby eliminating the production of 4-aminobiphenyl at the coupling stage.Their modified method would detect as little as 20 mg per litre of 4-aminobiphenyl in aniline. EXPERIMENTAL A more sensitive method has been developed in this laboratory, in which a mixture of 100 ml of aniline and 1 g of paraffin wax was distilled until only 0.5 to 1 g of the aniline remained in the distillation flask. Under these conditions about 75 per cent. of any 4-amino- biphenyl present was retained in the distillation residue. The residue was dissolved in toluene and amines were extracted with dilute hydrochloric acid. A portion of the acid extract was diazotised and coupled with R-salt, by using a concentration of R-salt twice that recom- mended by Stagg and Reed for diphenylamine. The coupled solution was subjected to circular- paper chromatography and the intensity of the ring due to 4-aminobiphenyl compared visually with the rings on chromatograms prepared from high quality aniline containing known added concentrations of 4-aminobiphenyl. METHOD APPARATUS- Distillation unit-This consisted of a 250-ml, short-necked, round-bottomed flask, with a B24 joint, sloping recovery bend and a long (about 80 cm) air condenser.100-ml Separating funnels with short stems and ground-glass stoppers. Plates for circular-paper chromatography-Two squares of plate glass about 25 x 25 cm x 0-5 cm thick, one of them with a central hole about 1 cm in diameter. Solution reservoirs for chromatographic plates-Capable of holding 20 ml of liquid. Six-inch straight absorption tubes of the type normally used to hold calcium chloride or soda lime are suitable.The stem of each passes through a rubber bung that fits tightly into the hole in the upper of the pair of chromatographic plates (see Procedure). REAGENTS- Whatman No. 1 jilter-papers, 24 cm in diameter. Parafin wax, melting-point 51" to 54" C. Toluene, laboratory-reagent grade. Hydyochloric acid, N.REILLY 643 4-Amino biphenyl. Sodium nitrite sol.ution, 0.5 N. Sodium carbonate solution, M. R-salt solution, approximately 0.05 N-weigh into a beaker 18.0 & 0.5 g of R-salt (di- sodium-2-naphthol-3,6-disulphonat e, M. W. 348) of analytical-reagent grade, dissolve it in 500 ml of hot water and make the solution slightly alkaline to Brilliant Yellow with sodium carbonate solution (M).Cool to room temperature, filter into a 1-litre standard flask, dilute to the mark with water at 20” C and mix well. Store in an amber bottle. PROCEDURE- Weigh 1.0 * 0.1 g of paraffin wax plus a small piece of porous pot into the distillation flask. Add 100 1 ml of the sample, measured carefully in a clean, dry measuring cylinder. Attach the recovery bend, wrapped in asbestos tape or string, and the air condenser. Heat the flask, preferably electrically, and maintain a fairly rapid rate of distillation until the flow of aniline vapour into the condenser is seen to diminish and slight fuming is visible in the flask. Turn off the heat and allow the flask to cool until it can be held comfortably in the hand, but do not allow the residue to solidify. Detach the flask, add about 20 ml of toluene, swirl it gently and pour into a 100-ml separating funnel.Rinse out the flask with further portions of toluene and add these to the funnel, making a total volume of about 80 ml. Add 15 ml of N hydrochloric acid to the separating funnel, stopper and shake it for 1 minute. Allow the layers to separate and run off the lower aqueous layer into a second 100-ml separating funnel. Repeat the extraction of the toluene layer with two further 15-ml portions of N hydrochloric acid, again running the separated acid layers into the second separating funnel. Add 15ml of toluene to the combined acid layers, shake the mixture for 1 minute and allow it to separate. Run the acid layer into a 50-ml standard flask, dilute to the mark with N hydrochloric acid, and mix well.This is the test solution. Transfer by pipette a suitable portion, not more than 0-5m1, of the test solution into a 6 x 1-inch test-tube and add from a dropping pipette sufficient N hydrochloric acid to make the total volume 5 ml. Add 0.5 ml of sodium nitrite solution, mix by gentle swirling and allow to stand for 3 minutes at room temperature. Pour the contents of the tube into a second 6 x 1-inch test-tube containing 4ml of R-salt solution and 6ml of M sodium carbonate solution, mix by gentle swirling and allow to stand for 10 minutes at room temperature. With a cork borer cut a hole about 1 cm in diameter in the centre of ten sheets of Whatman No. 1 filter-paper, 24 cm in diameter, placed one above the other. Lay the pad of papers on the lower of a pair of chromatographic plates placed horizontally, and place the upper plate on top of the pad so that the hole in it is over the hole in the papers.Place two diametrically opposed weights on the upper plate (either Winchester quart bottles full of water or 5-inch lengths of mild steel bar of 2-inch square cross-section are suitable). Insert the solution reser- voir tightly into the hole in the upper plate. After the 10 minutes’ coupling period, add 10 ml of N sodium hydroxide to the coupled solution and stir until a clear solution is obtained. Pour this into the solution reservoir and allow it to soak completely into the pad of paper. Remove the solution reservoir, rinse free from coloured solution and replace it. Fill the reservoir with N sodium hydroxide and allow the chromatogram to develop until the solution front reaches the edge of the paper.Detach the solution reservoir, remove the paper pad and hang it up to dry. When dry, compare the chromatogram visually with standard chromatograms obtained with mixtures of aniline (free from 4-aminobiphenyl) with known concentrations of 4-aminobiphenyl. The concen- trations to be added will depend on the concentrations expected in the sample, but for an “unknown” sample 2, 5, 10 and 20mg per litre should suffice. RESULTS Three samples of aniline recently manufactured by I.C.I. Ltd., gave no visible pink ring when tested by this method. The lowest concentration of added 4-aminobiphenyl that gave a visible ring was 2 mg per litre. Concentrations of 2, 5, 10 and 20 mg per litre gave rings that could be readily distinguished from each other, and more closely spaced standards644 REILLY could probably be used.Comparison of the rings obtained from known weights of 4-amino- biphenyl dissolved directly in N hydrochloric acid and then diazotised, coupled and chromato- graphed, as described above, with those obtained from equivalent concentrations of 4-amino- biphenyl added to aniline, showed that about 75 per cent. of the 4-aminobiphenyl originally present in the aniline was retained in the distillation residue. Results for the recovery of 4-aminobiphenyl from the distillation residue are as follows- 4-Aminobiphenyl added to aniline, mg per litre . . 22 22 10 10 5 Recovered from distillation residue, equivalent mg per litre . . . . . . . . .. . . 17 to 18 17to 18 7 to 8 7 to 8 slightly less than 5 At the low concentrations present in good quality aniline, visual comparison of the test ring with standards is capable of giving sufficiently precise results. If greater precision at concentrations above 20 mg per litre is needed, then the rings can be eluted from the chromatogram by the technique of Stagg and Reed,l and the dyestuff present in the eluate measured spectrophotometrically. It is evident that no 4-aminobiphenyl is produced at the coupling stage under the conditions used. REFERENCES 1. 2. Stagg, H. E., and Reed, R. H., A+aalyst, 1957, 82, 503. Norman, O., and Vaughan, G. .4., Ibid., 1966, 91, 653. Received March 23rd, 1967