492 MILLER AND PEARMAN : TITRIMETRIC [Vol. 75 Titrimetric Determination of Ethylene BY S. A. MILLER AND F. H. PEARMAN SYNoPsrs-The determination of ethylene in gas mixtures that contain no other unsaturated compounds, by shaking with excess of bromine in acetic acid, and titrating the unreacted bromine, is described. The method is simple and reliable, and gives results accurate to f 0.02 per cent. A METHOD is described for determining the amount of ethylene in gas mixtures containing up to about 7 per cent. of ethylene with an accuracy of & 0.02 per cent. The usual gasometric methods, such as those employing the Bone and Wheeler apparatus, can be relied upon a t best to & 0.1 per cent., and the Orsat apparatus only to & 0.2 per cent. A method was devised which was found to be simple and reliable in operation, capable of being used by the average laboratory assistant, and which gave the desired accuracy.Of the usual gases met with in normal gas analyses, only other unsaturated compounds can interfere. The method depends on the determination of the unreacted bromine remaining after a known volume of gas mixture has been shaken with an excess of bromine solution, under specified conditions. METHOD REAGENTS- Sodium thiosulphate-0.1 N , standardised in the usual way. Potassium iodide--lO per cent. solution, Starch solution-1 per cent. solution. Saturated brine-A solution of commercial salt, boiled and Bromine in acetic acid-5 g. of potassium bromide, 1 ml. of filtered. bromine, 300 ml. of acetic acid. All the reagents (except the brine) were prepared from A.R.grade chemicals. APPARATUS- It consists of two bulbs separated by a tap, with a further tap at each of the other ends of the two bulbs, and a funnel attached to the tap of the small bulb. The volume of the smaller bulb is about 100 ml. and that of the larger bulb 1 litre. The two taps connected to the smaller bulb are of 8 mm. bore. The apparatus is shown in Fig. 1. Fig. I . PROCEDURE- bulb. The larger bulb is filled with saturated salt solution through the funnel and the small The middle tap is then closed, and the apparatus inverted, and used in the normalSept., 19501 DETERMINATIOS OF ETHYLENE 493 manner of collecting a gas sample. After the sample is taken the tube is returned to the original position, and the temperature in the smaller bulb is measured; the barometric pressure is also read.By using a wide bore tap, it is possible to insert the thermometer through the bore of the tap. It is assumed that the temperature is the same in the two bulbs. The smaller bulb is then thoroughly rinsed out three or four times with distilled water to wash away all trace of brine. The bromine in acetic acid reagent is then pipetted into the small bulb, the tip of the pipette being made to pass through the bore of the tap. The quantity of reagent taken is 15 ml. per 1 per cent. of ethylene present in the gas; thus for a gas believed to contain some- thing of the order of 5 per cent. of ethylene, 75 ml. of reagent solution is used. The reagent is conveniently blown into the pipette by means of a blow-ball. No reagent should remain adhering to the funnel or the tap of the bulb, but if any does so, it should be washed in with not more than 1 ml.of distilled water. The middle tap is then opened and the apparatus shaken vigorously for 20 minutes. The middle tap is then closed, the upper tap opened and 20ml. of 10 per cent. potassium iodide added, and washed in with 1 to 2 ml. of water. The upper tap is then closed and the middle tap opened to allow the iodide to run into the larger bulb. The tap is then closed and the apparatus vigorously shaken for a short time and allowed to stand for 5 minutes. The liquor is then run into a 500-ml. iodine flask, together with five rinsings of distilled water, each of 10 to 20 ml. The iodine solution is then titrated with thiosulphate in the usual way.A blank determination is carried out in exactly the same way, using 25 ml. of bromine reagent. CALCULATION OF RESULTS- The tap is then closed. Straightforward calculation shows that if V = Volume of the larger bulb, in litres, P = Barometric pressure in millimetres of mercury, T = Temperature of the gas in degrees C., F = Factor of the thiosulphate ( x 0.1 K ) , a = Titration starting with 75 ml. of bromine solution, b = Blank titration starting with 25 ml. of bromine solution, 273 + T 760 x - per cent. P x (3b -- n ) F x ____ 0.1121 the ethylene present = ~ V 273 REACTION CONDITIONS BROMINE SOLVENT- The use of carbon tetrachloride as a solvent for bromine was not found to be satisfactory. Owing to the immiscibility of the carbon tetrachloride with water, it would be necessary to add water as well as the 10 per cent.potassium iodide solution, with a greater risk of loss of bromine by vapourisation at this stage. Both gave similar results with the same reproducibility, but the stability of the stock methanol solution was inferior, which made it necessary to carry out blank determinations with each analvsis. In the case of the glacial acetic acid solution, one or a t most two blanks were sufficient for a day's work. Methanol and glacial acetic acid both appear to be suitable so1vents.l TIME OF REACTION- In the first study of the conditions, a temperature of 0" to 5" C. was thought to be desirable in order to reduce possible substitution, either of the ethylene itself or of the accompanying saturated hydrocarbons, and the amount of bromine used was 125 per cent.of that equivalent to the ethylene present. Mixtures of known constitution were prepared volumetrically, using pure ethylene obtained by fractionation on the Podbielniak apparatus. The mixtures contained 4 to 5 per cent. of ethylene, about 5 per cent. of oxygen and 10 per cent. of carbon dioxide, the rest being nitrogen. The results obtained in a number of preliminary experiments are shown in Table I.494 Time TEMPERATURE MILLER AND PEARMAN TITRIMETRIC TABLE I EFFECT OF TIME ON THE REACTION Excess reagent . . . . 25 per cent. Temperature . . .. 0" to 5°C. of shaking, Ethylene present, Ethylene found, Error, min. /'0 % 0' ,'O 01 5 4-49 3 8 i - 0.62 5 4.76 3.64 - 1.12 7 4.01 3-35 - 0.66 15 4.65 4.5 1 - 0.14 20 4.67 4.56 - 0.11 OF REACTION- [VO~.75 As the reaction was incomplete even in 20 minutes at 0" to 5', the cooling was omitted, and the reaction studied at room temperature. The results obtained are shown in Table 11. TABLE I1 REACTION AT ROOM TEMPERATURE Temperature . . . . 15" to 20" C. Excess reagent . . . . 25 per cent. Time of shaking . . . . 20 minutes Ethylene present, Ethylene found, Error, 0' % / O O / 10 4-06 3.51 4.12 4.83 3.97 3-51 4-03 4.82 7- 0.08 0.00 - 0.09 - 0.01 AMOUNT OF REAGENT- The excess of reagent taken had first been tried at 25 per cent., which seems to have been used in other connections in determinations with bromine and acetic acid.l Experiments were carried out using both 25 and 50 per cent. excess of bromine (calculated for a gas containing 5 per cent.of ethylene). The results obtained are shown in Table 111. TABLE I11 EFFECT OF AMOUNT OF REAGEKT USED Temperature . . * . 20" c. Time .. .. ,, . 20 minutes I > r \ Ethylene Ethylene Ethylene Ethylene present, found, Error, present, found, Error, 25 per cent. excess bromine 50 per cent. excess bromine JL 0' /O 0) 46 4.05 3.9i 5.25 523 - 0.04 - 0.08 % 0' / O % 3.5 1 4.83 5.51 4.84 4.13 3.51 0.00 4-94 4-95 + 0.01 4-82 - 0.01 2.97 3.00 + 0.03 5.29 - 0.22 4.54 4.54 0.00 4.73 - 0.11 4.83 4.82 - 0.01 4.03 - 0.10 Mean error . . .. . . - 0.09 Mean error . . . . . . 0.00 The second group of five results in Table I11 shows the desired level of accuracy in the determination. Attempts were then made to reduce the shaking time to 5 minutes, using the preferred level of 50 per cent. of excess reagent, and a reaction temperature of 20" C.Under these conditions, however, the reaction was incomplete and deficiencies of up to 0.46 per cent. were recorded. Similarly, a determination which was carried out by shaking for 5 minutes and then allowing to stand for 15 minutes also led to a seriously low result.Sept., 19501 DETERMINATION OF ETHYLENE 496; COMPARISON WITH GASOMETRIC DETERMINATIONS- Samples of gas containing ethylene in admixture with (mainly) nitrogen, oxygen and carbon dioxide were analysed titrimetrically and gasometrically with the following results- TABLE IV COMPARISON OF ETHYLENE DETERMINATIONS BY THREE METHODS Ethylene found, per cent. A r -I Bromine method A r \ Orsat, Bone and Wheel,er, Mixture Individuals Mean mean value mean value I 3.95 3-93 3-94 3.6 4.1 I1 3-69 3.63 3-61 316 3-8 I11 5.66 5-72 5.69 5.9 8-8 IV 5.01 4.97 4.99 5.0 5.2 v 4.53 4-57 4.55 4.4 4.9 FURTHER TEST- * Further studies of the reproducibility of the method, and comparison of the results with those obtained gasometrically, were carried out on a gas containing about 90 per cent. of methane, and on another gas conSisting in the main of methqne, hydrogen, carbon molaoxide and nitrogen in roughly equal proportions. The results are shown in Table V, the above mixtures being I and I1 respectively. TABLE V FURTHER COMPARISONS WITH GASOMETRIC METHODS Ethylene found, per cent. A r -I Bromine method Orsat r \ CT~-, Boneand Mixture Individuals Mean Individuals Mean Wheeler I 3.8 1 3.82 3.84 3.82 3.8 3.7 3-75 4.0 I1 0.73 0.74 0.74 0.74 0.6 0.5 0.55 - A The authors wish to thank the Directors of the British Oxygen Co., Ltd. for permission to publish this paper. REFERENCE 1. Morgan, P. W., Ind. Eng. Chm., AnaE. Ed., 1946, 18, 500. THE BRITISH OXYGEN COMPANY LIMITED MORDEN ROAD, LONDON, S.W.19 A V U Y , 1950