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Turnmill Street, London, E.C. A Prospectus containing full particulars of the Lecture and Laboratory Courses qualifying for Degrees in Chemistry will be forwarded on application to the REGISTRAR. Applications for admission to the Research Laboratories should be made to the DIRECTOR of the Chemical Laboratories. MICA. Telephone No. 2248 Avenue. THE CHEMICAL NEWS. VOLUME CXII. EDITED BY SIR WILLIAM CROOKES, O.M., D.Sc., Pres.R.S., &c. No. 2901.-JULY 2, 1915. A MODIFIED REDUCTION METHOD the end-point by testing a few drops of the solution under FOR THE VOLUMETRIC DETERMINATION OF going titration with a saturated solution of mercuric NITRO COMPOUNDS. By A. J. BERRY, M.A., and C. K. COLWELL, B.A. NITRO Compounds are usually determined volumetrically by reduction to amino compounds by excess of a suitable reducing agent, the excess of which is determined by titration in some convenient manner. The reducing agents usually employed for this purpose are stannous chloride or titanous chloride in acid solution. When stannous chloride is employed in presence of hydrochloric acid the reduction proceeds in accordance with the equation R. NO2 +3SnCl2 +6HCl = R.NH2+3SnCl4+2H20. The excess of stannous chloride is usually determined in an aliquot portion of the solution by titration with iodine after adding excess of sodium bicarbonate and potassium sodium tartrate. In the course of some experiments on the quantitative reduction of nitro compounds to amino compounds we have applied a method originally due to Weil (CHEMICAL NEWS, 1871, xxiii., 49) for the volumetric determination of copper to determine the excess of stannous halide. This method depends upon the fact that a solution of a cupric salt strongly acidified with hydrochloric acid is reduced quantitatively to the cuprous condition with simultaneous formation of an equivalent amount of stannic salt in accordance with the equation 2CuCl2+SnCl2 = Cu2Cl2 + SnCl4. It is clear that this method might be applied in the reciprocal way; that is, it should be possible to employ a standard solution of a cupric salt to determine a solution of stannous halide, and, consequently, to determine a nitro compound by difference. The results which we have obtained show that the method is capable of yielding satisfactory results. In our earlier experiments the general procedure was as follows:-A definite quantity of the solution of the nitro compound was measured out, acidified strongly with hydrochloric acid, and a known excess of a standard solution of stannous chloride added, and the resulting mixture boiled gently for a quarter of an hour, a slow current of carbon dioxide being passed through fhe solution all the time. The liquid was then titrated at the boiling-point with a standard solution of copper sulphate strongly acidified with hydrochloric acid. The colour of the cupric solution was of course discharged by reduction to the cuprous condition, but it was found necessary to determine chloride. The end-point was taken as that point at which the solution ceased to give a turbidity with mercuric chloride. As the employment of an external indicator is a someexperiments. Etard and Lebeau (CHEMICAL NEWS, 1890, what tedious process we modified the procedure in our later Ixi., 137) have pointed out that a solution of a cupric salt acidified with excess of hydrobromic acid possesses a deep brownish violet colour, which becomes perfectly colourless on reduction with a stannous salt. The colour change is employment of an external indicator is quite unnecessary. much more striking than with hydrochloric acid, and the In our experiments we added a measured quantity of a strong solution of potassium bromide to the solution of the nitro compound which was being reduced by stannous for fifteen minutes in an atmosphere of carbon dioxide, chloride and hydrochloric acid. After boiling the solution we titrated the excess of the stannous salt in solution by means of a strongly acid solution of copper sulphate conreaction the liquid assumed an orange-yellow colour, and taining potassium bromide. At the end-point of the tion was compared with that of a dilute solution of potasto obtain strictly consistent results the colour of the solusium dichromate. This second method was found to be much more satisfactory to work than the first, and as far as our experience goes, slightly more accurate also. In both methods the stannous chloride solution was always titrated against the standard copper solution at the time of carrying out a determination. Examples. 1. Paranitrophenol. 1938 grm. of this substance was weighed out, dissolved in water, and the solution diluted to 250 cc. Aliquot portions of the solution were titrated with the following results: 25 cc. of the solution with 25 cc. of stannous chloride required 31 cc. of copper solution. 25 cc. of stannous chloride solution alone required 17.6 cc. of copper solu. tion. Hence 134 cc. of the copper solution correspond to 25 cc. of the solution of paranitrophenol. The copper solution contained 39 2 grms. of copper (metal) per litre. From this it follows that the weight of paranitrophenol in the 250 cc. of solution is 1913 grm. This experiment was carried out with the aid of mercuric chloride for determining the end-point. 2. Picric Acid.-2.985 grms. of this substance were weighed out, dissolved in water, and the resulting solution diluted to 500 cc. This determination was carried out with the addition of 35 cc. of a 50 per cent solution of potassium bromide to every 25 cc. of the solution of 2 Volumetric Determination of Polythionic Acids by Fotassium Iodate. CHEMICAL NEWS, July 2, 1915 stannous chloride, and a considerable excess of hydro- | acid mentioned above). According to the equation of the chloric acid was, as usual, present in every titration. The reaction · Na2SO4 + K2SO4 + 2IC1 + 6H20 standard copper solution consisted of go grms. of copper 2KIO3+ Na2S2O3.5H2O+2HCl= 25 cc. of the picric acid with 25 cc. of the stannous solution required 314 cc. of copper solution. 25 cc. of the stannous solution alone required 14'0 cc. of the copper solution. From this it is clear that 17:4 cc. of the copper solution correspond to 25 cc. of the solution of picric acid. The weight of picric acid in the 500 cc. of solution is equal to 2:944 grms. It should be pointed out that our experiments demonstrate the applicability of this modified reduction method to the determination of the nitro groups in picric acid. Altmann (Fourn. Prakt. Chem. 1901, lxiii., 380) quoted an experiment in which he failed to obtain satisfactory results with picric acid, and concluded that this substance could not be determined by reduction with stannous chloride. The experiments of Knecht and Miss Hibbert (Ber., 1903, xxxvi., 1549) have, however, shown that picric acid as well as other nitro compounds may be accurately determined by means of titanous chloride. 3. Paranitrophenol.—1*122 grm. of this substance was weighed out, dissolved in water, and the solution diluted The acid solution of stannous chloride emto 250 cc. ployed for reduction contained potassium bromide in solution, and the same copper solution as that employed for the determination of the picric acid in No. 2 was employed for the titrations. The following results were obtained : 25 cc. of the solution of paranitrophenol with 25 cc. of stannous halide required 13.65 cc. of copper solution. 25 cc. of the solution of stannous halide alone required 21.05 cc. of copper solution. The difference, 7'4 cc. of copper solution, clearly corresponds to the quantity of paranitrophenol in the 25 cc. of solution. From this it follows that the weight of para. nitrophenol in the 250 cc. of solution is 114 grm. The few examples which have been quoted are sufficient to demonstrate the possibilities of the method. The experiments were all carried out with measuring vessels which had not been calibrated, and it will be seen that the magr.itude of the experimental error was of the order of 1 or 2 per cent. Greater accuracy could doubtless be obtained with more accurate measuring vessels. Downing College Laboratory, Cambridge. THE VOLUMETRIC DETERMINATION OF I CC. = 0.002068 grm. of Na2S2O3.5H2O. The titrations were made in glass-stoppered bottles in exactly the same manner as in the determination of sulphurous acid. It is important to titrate as soon as possible after the thiosulphate is brought in contact with the hydrochloric acid, which should not be warmer than 18° C.; also it is best to dissolve the thiosulphate in a little water previous to adding the hydrochloric acid. For convenience several solutions of sodium thiosulphate of various strengths were prepared and standardised by titration with known amounts of pure iodine in the usual manner, observing all precautions. The results obtained are given in Table I. potassium iodate solution which had the value I cc. = The titrations given in Table II. were made with a 0.005300 grms. of Na2S2O3.5H2O. The results in both tables show a satisfactory agreement among themselves and with the actual amounts present. A volumetric method for the determination of tetrathionic acid has been described by E. Abel (Zeit. Anorg. Chem., Ixxiv., 395), which is based upon adding an excess of standard iodine solution to the solution of the tetrathionate, which has been made strongly alkaline. Then the solution is acidified with hydrochloric acid, and the POLYTHIONIC ACIDS BY POTASSIUM IODATE. liberated iodine is titrated with a standard solution of By GEORGE S. JAMIESON. Sheffield Chemical Laboratory, Yale University. In a recent paper (Am. Journ. Sci., 1914, xxxviii., 166) from this laboratory it was shown that sulphurous acid could be titrated with a solution of potassium iodate in the presence of 15 to 20 per cent of actual hydrochloric acid and a small volume of an immiscible solvent, such as chloroform, according to the general method of L. W. Andrews (Fourn. Am. Chem. Soc., 1903, 25, 756). It has been found that thiosulphuric and tetrathionic acids can be titrated in the same manner, while dithionic acid, on account of its stability, cannot be determined by this method. In order to test the method for the titration of thiosulphates, a solution containing 3.567 grms. of normal potassium iodate in 1000 cc. was used. (This solution was previously used for the determination of sulphurous sodium thiosulphate in the usual manner. The iodate method has the advantage in requiring only a single standard solution. In order to test the method sodium tetrathionate was prepared according to the method of F. Kessler (Pogg. Ann., 1848, lxxiv., 255), by grinding sodium thiosulphate with a slight excess of indine in the presence of a few cc. of water. When the sodium thiosulphate was entirely dissolved, two volumes of absolute alcohol were added. After the solution had stood for two hours the sodium tetrathionate crystals were filtered by suction and washed with 98 per cent alcohol until all the iodine and sodium iodide were removed. The sodium tetrathionate was found to contain some sulphate, which was determined by the following method:-Weighed portions of the salt were dissolved in water and acidified with I grm. of tartaric acid. Barium chloride was added to the solution, and when the precipitate had settled, it was filtered and determined in the usual manner. The results obtained are given in Table III. |