Analyst, February, 1973, Vol. 98,PP. 103-106 103 Determination of Nitro and Nitroso Compounds by Thermometric Titrimetry BY L. S. BARK AND P. BATE (Department of Chemistry and Applied Chemistry, University of Salford, Salford, Lancashire, M6 4WT) Some nitro and nitroso compounds have been determined by reduction with a known and excess amount of titanium(II1) chloride solution and subsequent thermometric titration of the excess of the titanium(II1) with an iron(II1) solution. The accuracy is f 1 per cent. for 0.4 mequiv of sample and the time taken for a single titration is less than 2 minutes. The over-all time taken for a single determination is about 20 minutes. THE various methods that have been proposed for the determination of nitro and nitroso compounds have been reviewed by Ashworth.lS2 In many of these methods, metal ions are used to reduce the functional group; titanium(II1) appears to have been the most widely used of these ions and tin(I1) and chromium(II1) have been used to a lesser extent. Titan- ium(II1) is preferred to tin(I1) because of the stronger reducing power of the former and the fact that the reductions carried out with its use are generally much more rapides The first method in which titanium(II1) chloride was used for determining nitro com- pounds was reported by Knecht4 in 1903. The method involved the addition of a 100 to 200 per cent.excess of the titanium(II1) reagent to the sample and boiling of the solution for 5 minutes while a stream of carbon dioxide was passed through the solution so as to prevent oxidation of the unreacted titanium(II1) solution.The unreacted titanium(II1) was then titrated with a standard solution of ammonium iron(II1) sulphate. Most of the early methods involved the heating of the reaction mixture in a strongly acidic medium, In some instances, chlorination of the nucleus occurred at the high temperatures that were used. However, Kolthoff and Robinson5 found that in most instances reduction could be carried out at room temperature within a few minutes if a buffer such as sodium citrate was used. Ma and Earleye,' used sodium acetate in preference to sodium citrate because of the higher pH and lower blank readings obtained with sodium acetate, and determined a variety of nitrogen compounds that contained reducible groups. Tiwari and Sharma8s9 preferred to use titanium(II1) sulphate instead of the previously used titanium(II1) chloride, and determined several nitro compounds in the presence and absence of a buffer.In most instances the excess of titanium(II1) reagent has been determined by using ammonium iron(II1) sulphate with thiocyanate as indicator, although other reagents, such as methylene blue, Crystal scarlet and Acid green, have also been used. In all these methods a visual indication of the end-point is used. This method of end-point determination is not always satisfactory, especially when highly coloured samples are involved. The advantages of thermometric titrations over visual titrations have been discussed previously,1° especially for industrial materials and highly coloured samples.However, up to the present time, no methods have been reported in which a thermometric indication of the end-point is used for the determination of the excess of titanium(II1) reagent in the determination of nitro and nitroso compounds. Such a method is reported in this paper. EXPERIMENTAL APPARATUS- Electrical circz4it-This has been described previously10 and is a simple Wheatstone bridge incorporating a thermistor sensor as one of the arms of the bridge. Delivery b.ure#es-The titrant was delivered from a multiroller peristaltic pump (LKB Perspex Pump 10200), delivering 0.3 to 0.4 ml min-l. The delivery rate was monitored by gravimet ry . The titanium( 111) chloride solution was dispensed from a Radiometer Autoburette ABUll with a 25-ml burette so that a known volume of solution could be added accurately.0 SAC and the authors,104 [Analyst, Vol. 98 Nitrogen Puuri$cation train-The nitrogen was passed from a cylinder through two Drechsel bottles, the first containing a reagent to remove oxygen and the second distilled water, into the reaction vessel. An alkaline solution of pyrogallol was tried in the first bottle, but it became spent after only a few days. A solution of chromium(I1) chloride was therefore used, which was found to be much more efficient as the reagent was still useful after 3 months. Reaction $ask-A three-necked, pear-shaped flask of 25 ml capacity was used; one of the necks contained the thermistor, another the burette tip and the centre neck a glass tube drawn out into a capillary through which the nitrogen passed into the solution.The reaction flask was surrounded with a block of expanded polystyrene, which was built around a Metrohm magnetic stirrer. The top part of the flask was covered with layers of cotton-wool so that the whole flask was insulated from its surroundings. The solution was stirred magnetically at a constant rate with a polythene-covered stirring bar in a conventional manner. PRELIMINARY INVESTIGATIONS- A study was made of the reaction conditions that influence the shape of the enthalpo- grams obtained when titanium(II1) solutions were titrated with iron(II1) ions [from am- monium iron(II1) sulphate]. Studies were made of solvent effects and the effects of variations in the acidity of the system. SoZvent efects-In many instances an organic solvent has to be used to dissolve the nitro or nitroso compound.It is advantageous to use a solvent that is completely miscible with the aqueous solutions that are subsequently used. Hence only ethanol, acetone and glacial acetic acid were investigated, each of which showed different thermal effects when diluted with solutions of ammonium iron(II1) sulphate during the titration of the excess of the titanium(II1) ions. The heats of dilution and mixing of the titrant solution with the aqueous organic solution of the titrand, together with the heat of reaction of the titanium(II1) and the iron(II1) ions, determine the shape of the enthalpogram produced. This shape may affect the accuracy with which the end-point can be determined. Enthalpograms were obtained by using the three solvents under identical conditions : aliquots of the solvent (5 ml) under investigation were used, and also of the titanium(II1) chloride solution (5 ml of 0.08 M solution), sodium acetate solution (7 ml of 2.5 M solution) and hydrochloric acid (1 ml of 50 per cent.V/V acid). The graphs obtained with glacial acetic acid and acetone had very rounded end-points and accurate determinations were not possible. All further work was therefore carried out only with ethanol. E$ects of variation in the acidity of the system-It had been found previously6 that titanium(II1) chloride solutions containing less than 10 per cent. V/V of hydrochloric acid BARK AND BATE: DETERMINATION OF NITRO AND NITROSO h 4 b Volume of titrant added Fig. 1. Enthalpograms for titration of titan- ium(II1) solutions under different conditions with 0.8 M ammonium iron(II1) sulphate solutionFebruary, 19731 COMPOUNDS BY THERMOMETRIC TITRIMETRY 105 were readily oxidised when stored in the usual manner.It had also been recommended6 that the acid is added to the sample after reduction of the organic group and before oxidation of the excess of the titanium(II1) ion. Thus enthalpograms were obtained by using different concentrations of hydrochloric acid in the titrand. The concentration of the acid in the titrant was adjusted to 25 per cent. V/V so that at all stages of the titration the titrand was acidic and hence decomposition of the titanium(II1) by oxidation would be minimised. Mixtures of 5 ml of ethanol, 5 ml of 0-08 M titanium(II1) chloride solution, 7 ml of 2.5 M sodium acetate solution and 5 ml of different dilutions of concentrated hydrochloric acid (5 + 0, 4 + 1, 3 + 2, 2 + 3, 1 + 4 and 0 + 5 ) were titrated with 0.8 M ammonium iron(II1) sulphate solution and the enthalpograms obtained are shown schematically in Fig.1. Enthalpogram 1 is characteristic of the solution that contains no added hydrochloric acid, enthalpogram 4 that of solutions that contain 4 ml of 11.5 M hydrochloric acid, while enthalpograms 2 and 3 are characteristic of solutions that have acid concentrations inter- mediate between those for enthalpograms 1 and 4. For each of the enthalpograms, the line BC represents the sum of the heats of reaction between iron(II1) and titanium(II1) and also the heat of dilution and mixing of the titrant acid in the various mixtures.The heat of dilution and mixing is shown by the line CD. In practice, when obtaining an enthalpogram of type 2 by using the 0.16 M titanium(II1) solution prepared from the titanium( 111) chloride solution obtainable commercially, which contains approximately 15 per cent. m/V of titanium(III), it was found that the acid con- centration of the original concentrated solution of titanium(II1) was sufficient to ensure that on dilution to 0.16 M titanium(II1) the diluted solution contained sufficient acid to give type 2 enthalpograms without the addition of further acid. REAGENTS- Titanium(I1I) chloride solution, 0.16 M-Titanium(II1) chloride (76 ml of a 15 per cent. m/V solution) was added to 50 ml of concentrated hydrochloric acid (sp.gr. 1.16) and the mixture diluted to 500 ml with distilled water. This diluted solution was then added to the reservoir of the Autoburette and 1OOg of zinc amalgam were added. The solution was left for 24 hours before use so as to attain a stable and reproducible state. Iron(II1) solution, 0.8 M-Ammonium iron(II1) sulphate (195 g) was dissolved in a mixture of 125 ml of concentrated hydrochloric acid and 300 ml of distilled water. The solu- tion was filtered and the filtrate was made up to 500 ml with distilled water and standardised by an iodimetric method. Zinc amalgam-"Mossy" zinc (100 g) was treated with a solution of mercury(I1) chloride (10 g of HgC1, dissolved in a mixture of 5 ml of concentrated hydrochloric acid and 150 ml of water).After 5 minutes, the solution was poured off and the zinc amalgam was washed several times with distilled water. Chromium(I1) chloride solution-This solution was prepared by a previously reported method. l1 Sodium acetate, 2.5 M-Anhydrous sodium acetate (103 g) was dissolved in distilled water and the solution made up to 500 ml. Hydrochloric acid, sp. gr. 1.16. Solvent-E t hanol. Reducible nitrogen compounds-All compounds were purified before use and solutions were made by dissolving an amount of the compound in ethanol such that a 5-ml aliquot of the solution was equivalent to approximately 0.4 molar equivalent of sample. PROPOSED METHOD- A 5-ml aliquot of the nitrogen-containing compound (approximately 0.4 molar equivalent) is transferred by pipette into the reagent flask followed by 7 ml of 2.5 M sodium acetate solution.Nitrogen is passed through the solution for 10 minutes at the rate of 10 to 15 ml min-l while the solution is stirred. After this period, a known amount (approximately 5 ml) of the titanium(II1) chloride solution is added from the Autoburette. After allowing 5 minutes for reduction of the functional group, 4ml of distilled water are added and the burette tip and thermistor inserted in the solution. The nitrogen flow is stopped as the flow has a cooling effect on the solution (probably owing to the evaporation of the solvent). As the flask is virtually sealed, no oxidation of the titanium(II1) solution apparently occurs in the time taken to carry out the titration (1 minute). It was used immediately.106 BARK AND BATE The solution is then titrated with the ammonium iron(II1) sulphate solution.The amount of unreacted titanium(II1) solution is then determined from the enthalpo- gram and hence the amount of reducible substance originally present can be calculated. The titanium(II1) solution is standardised by taking a similar aliquot of organic solvent without the presence of any reducible nitrogen compound and proceeding as above. The results for various nitro and nitroso compounds are given in Table I. TABLE I RESULTS FOR THE DETERMINATION OF NITRO AND NITROSO COMPOUNDS Compound Amount takenlmg Amount found/mg Purity, per cent. Nitrobenzene . . .. . . .. 6.96 6.97 100.1 8.82 8.81 99.8 1,3-Dinitrobenzene . . .. .. 5-18 5.13 99.0 4.14 4.13 99.8 2,4-Dinitrobenzoic acid .. .. 4.68 4.7 1 100.7 5-80 5.82 100-3 4-Nitrobenzoic acid . . .. . . 8.69 8.70 100.1 6.95 6.93 99.7 4-Nitrophenol .. .. .. 8-46 8.49 100.4 6.77 6.78 100.2 3-Nitrophthalic acid .. ,. 12.56 12-47 99-3 10.04 10.07 100.3 4-Nitroaniline .. .. . . 8.50 8.46 99.5 6-80 6.77 99.6 8-Nitroquinoline . . .. .. 10.22 10.18 99-6 8-18 8-15 99.6 4-Nitroacetanilide . . .. .. 10.03 10.00 99.7 8.02 8.06 100.6 1 -Nitroso-2-naphthol .. . . 11.41 11.35 99.5 14.23 14.39 101.1 2-Nitroso- l-naphthol .. . . 13-24 13-20 99.7 10-56 10.63 100.7 4-Nitroso-NN’-diethylaniline . . 14.23 14.14 99.4 11.38 11-34 99.7 N-Nitrosodiphenylamine . . .. 9.47 9-50 100.4 7-57 7.64 99.6 INTERFERENCES- The most troublesome interference in the method is caused by oxygen, but the steps taken to avoid its presence were adequate for the present work.However, any reducible substance will interfere in the determination although ethylenic and acetylenic groups generally are not reduced by titanium(II1) chloride. Ma and Earley6 reported that nitroso compounds could be determined in the presence of nitro compounds by excluding the buffer and carrying out the reduction in strongly acidic media. However, in the present work it was found that nitro compounds interfered in the nitroso determination even if acidic conditions were used. CONCLUSION Some nitro and nitroso compounds have been determined by their reduction with titanium(II1) chloride solution. The accuracy is &l per cent. for 0-4 molar equivalents of sample. The time taken for a single titration is less than 2 minutes and the over-all time taken for a single determination is about 20 minutes. REFERENCES 1. 2. 3. 4. 6. 6. 7. 8. 9. 10. 11. Ashworth, M. R. F., “Titrimetric Organic Analysis. Part I. Direct Methods,” Interscience Pub- - , “Titrimetric Organic Analysis. Part 11. Indirect Methods,” Interscience Publishers, New Glynn, E., Analyst, 1947, 72, 248. Knecht, E., Bey. dt. chem. Ges., 1903, 36, 166. Kolthoff, I. M., and Robinson, C., Recl Trav. Chim. Puys-Bas Belg., 1926, 45, 169. Ma, T. S., and Earley, J. V., Mikrochim. Acta, 1959, 129. -,- , Ibid., 1960, 685. Tiwari, R. D., and Sharma, J. P., 2. analyt. Chem., 1962, 191, 329. -, -, Analyt. Chem., 1963, 35, 1307. Bark, L. S., and Bate, P., Analyst, 1971, 96, 881. Vogel, A. I., “Quantitative Inorganic Analysis,” Longmans Green and Co. Ltd., London, 1964. lishers, New York, 1964. York, 1966. Received August 31st, 1972 Accepted October 6th, 1972