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DETERMINATION OF HYDROGEN SULPHIDE IN NATURAL GAS
Hydrogen sulphide is shown qualitatively by means of moist lead acetate paper. If present in a gas mixture, the quantity is determined by absorption in a standard solution of iodine and by titration of the excess of iodine with a standard solution of sodium thiosulphate.
DETERMINATION OF OXYGEN IN NATURAL GAS
Work at the bureau's laboratory has shown that an analyst may fall into error in attempting to determine the oxygen content of some natural gas if he uses alkaline pyrogallate solution or phosphorus for the absorption.
USE OF ALKALINE PYROGALLATE
Oxygen is completely removed from air when a fresh solution of alkaline pyrogallate is used; it can be removed in 2 minutes when the absorption is performed with ordinary Orsat pipettes and the gas mixture is passed back and forth between the burette and pipette to keep the glass rods in the latter covered with fresh solution. This procedure was followed in an attempt to determine the oxygen content of natural gases having the following known compositions:
The percentages of absorption obtained in alkaline pyrogallate solution were as follows: No. 1, 0.13 per cent; No. 2, 0.15 per cent; No. 3, 0.10 per cent.
A greater absorption in alkaline pyrogallate solution was obtained when natural gas containing higher members of the paraffin series was treated with the solution. The percentages of absorption in alkaline pyrogallate solution were as follows: No. 1, 0.32 per cent; No. 2, 0.36 per cent.
USE OF PHOSPHORUS
A large enough proportion of the higher-paraffin hydrocarbons in a gas mixture prevents oxidation of phosphorus by oxygen, which
may be present in the mixture. To illustrate this, a mixture of air and natural gas which contained the following constituents was used:
When this mixture was passed into a pipette containing phosphorus, no reduction in volume of the mixture occurred, even after 1 hour's contact at 20° C. Winkler 59 has called attention to the fact that petroleum vapor prevents the oxidizing action of oxygen on phosphorus. A natural gas that did not contain such a large proportion of the higher paraffins did not prevent this oxidizing action. This gas had the following composition :
Composition of natural gas with small proportion of higher paraffins
Various percentages of air were mixed with this gas and removal of the oxygen with phosphorus was attempted. Oxidation was found to be always complete after 10 minutes. In other words, the action was greatly retarded. The gas mixture had to be passed back and forth between the burette and pipette. The reaction was not complete when the gas mixture was simply passed into the pipette and allowed to stand there.
As explained elsewhere, the analyses of natural gas cited in this work do not show the proportion of higher-paraffin hydrocarbons present in the mixtures; only an indication is given by the proportion of methane and ethane present.
IMPORTANCE OF DETERMINING OXYGEN
The determination of oxygen in natural gas is important because of the statement sometimes made that natural gas used for city consumption may contain air that has leaked into the pipe lines at the compressing stations which force the gas to points of consumption. Natural gas that issues under very high pressure and is trans
69 Winkler, Clemens, Handbook of technical gas analysis. 2d Eng. ed., trans. by George Lunge, 1902, p. 69.
ported through pipe lines does not, however, seem to contain enough higher-paraffin hydrocarbons to affect materially the accuracy of an analysis for oxygen, if alkaline pyrogallate solution is used for the determination of oxygen and if absorptions of 0.20 per cent or under are disregarded as being due to slight absorption of paraffins.
The Bureau of Mines has not detected in the natural gas used in Pittsburgh oxygen in quantities exceeding the error of the apparatus used, although it has been making analyses of the gas for 17 years. The error has never been over 0.20 per cent. Neither has the bureau positively identified oxygen in quantities exceeding the experimental error, in samples received from the wells. The authors can not say that oxygen never exists in gas mains, but they have known of gas mains in a declining field, where the gas was being pumped, to contain considerable air.
MANGANOUS HYDROXIDE METHOD
Because alkaline pyrogallate solution and phosphorus are unsatisfactory for the determination of oxygen the authors have adopted a method used by Phillips 60 for qualitative testing for oxygen and suggested by him for quantitative determination. The method, which is a modification of that of Winkler for the determination of dissolved oxygen in water, is based on the use of precipitated manganous hydroxide in water. Manganous hydroxide, which is white, absorbs oxygen strongly and changes to manganic hydroxide, which is brown. When a little potassium iodide solution and then sulphuric acid are added, the oxide of manganese redissolves, liberating iodine, which is recognizable by its color, even when only the most minute traces of oxygen are present. By means of a standard solution of sodium thiosulphate the free iodine may be determined.
The reactions taking place in the process and data required for the calculation follow:
2Mn(OH)3+3H2SO:+2KI=2MnSO4+K.80+6H,0+21 Hence, 32 grams of oxygen are equivalent to 507.68 grams of iodine. In using N/40 sodium thiosulphate solution for the estimation of the liberated iodine, 1 c. c. of the solution is equivalent to 1.397 c. c. of oxygen measured at 0° C. and 760 mm. pressure; or 1 c. c. of the solution is equivalent to 0.14 per cent of oxygen in a gas mixture, if only 100 c. c. of the sample is taken for the determina
Bo Phillips, F. C., Researches upon the phenomena of oxidation and chemical properties of gases : Am. Chem. Jour., vol. 16, 1894, p. 358.
tion. If larger quantities of the sample are used, the delicacy of . the test is much increased.
ANALYSIS OF SMALL SAMPLE The following scheme is used in the analysis by the authors when only a small quantity, say 100 c. C. of sample, is available for the test: Draw about 15 c. c. of a 10 per cent solution of sodium hydroxide and 25 c. c. of a 10 per cent manganese chloride solution into a 150-c. c. burette with a 100-c. c. bulb at the top. Then draw 100 c. c. of the gas sample into the burette and shake for about 15 minutes with the solutions. Prolonged contact between the gas mixture and the solutions is essential for complete oxidation. Draw a few cubic centimeters of a 10 per cent solution of potassium iodide and 5 per cent sulphuric acid solution into the burette. Transfer the mixture to a flask and titrate immediately with the sodium thiosulphate solution.
When large quantities of the sample are available for the test, bubble the gas mixture slowly through the solutions of sodium hydroxide and manganese chloride. The rate should not exceed 5 C. C. of gas per minute, and at least three solutions should be used to insure a quantitative test.
The following analyses show the results of some experimental work performed by the authors with this method:
Analyses to test the manganous hydroxide method
Oxygen taken, C. C.
Oxygen found, c. c. -------0.76 ------- .85
HEATING VALUE OF NATURAL GAS In calculating the heating value of natural gas, the values given in the following table are used :