Determination of Low Concentrations of Oxygen in Gas JOSEPH A . SHAW, hlellon Institute, Pittsburgh, Penna.
A method is described for determining in gases concentrations of oxygen as low as several thousandths of 1 per cent. A special type of flask is used. The other equipment and chemicals are likely to be found in most chemical laboratories. About 2 hours are required to obtain results, but during most of this period no
personal attention is needed. As compared with previously published methods, this procedure is an improvement with respect to accuracy, delicacy, and facility of manipulation. It is especially adapted to analysis of gases containing highly unsaturated or easily condensable hydrocarbons.
0
device, two 50-ml. tall-form Xessler tubes, three 10-ml. graduated pipets, three 5-ml. graduated pipets, one 50-ml. volumetric flask, one 100-ml. volumetric flask, and one 250-ml. volumetric flask. CHEMICAL SOLUTIOSS REQUIRED.Sodium hydroxide solution, 50 per cent by weight; concentrated c. P. hydrochloric acid; approximately N hydrochloric acid (8.7 mi. of hydrochloric acid solution diluted to 100 ml.). Ferrous ammonium sulfate reagent solution (15 grams of ferrous ammonium sulfate hexahydrate and 8.7 ml. of concentrated hydrochloric acid diluted to 1 liter with water). Potassium thiocyanate solution (10 grams of potassium thiocyanate diluted to l liter). Standard solution of ferric iron 10.86 gram of ferric ammonium alum dissolved in water and diluted to 100 ml.; 10 ml. of this solution and 1.0 ml. of concentrated hydrochloric arid are diluted to 1 liter. This solution contains 0.01 gram of ferric iron per liter and 1 ml. of the solution is equivalent to 0.001 ml. of dry oxygen a t normal temperature and pressure or 0.00105 ml. of oxygen at 60” F. and 76.2 cm. (30 inches) of mercury.] Sulfuric acid solution, 20 per cent by volume, and potassium hydroxide solution, 30 per cent by weight. Place the two test-tube scrubbers on the samPROCEDURE. pling line, charging the first one with 25 ml. of 20 per cent sulfuric acid and the second with 30 ml. of 30 per cent potassium hydroxide solution. This amount of alkali is sufficient to purify a t least 140 liters (5 cubic feet) of coke-oven gas having a carbon dioxide concentration of 3 per cent. Pass a stread of gas through this train at a rate of from 28.3 to 56.6 liters (1 to 2 cubic feet) per hour, and purge with a t least 5.66 liters (0.2 cubic feet) of gas. At the end of this time, with the gas still flowing, attach to the outlet of the train a Shaw sulfur flask (the volume of which has been determined by water displacement) charged with 15 ml. of the ferrous ammonium sulfate reagent solution and purge the flask with a t least 14 liters (0.5 cubic feet) of gas. At the end of this period close the outlet valve of the flask and then the inlet valve and remove from the sampling line. Adjust the pressure by opening the outlet valve momentarily and take temperature and barometer readings. With a graduated pipet lace 25 drops of a 50 per cent sodium hydroxide solution in the runnel neck of the flask, cool the gas space slightly in water, allow the sodium hydroxide to flow slowly into the flask through the split port in the stopper, and wash the residual caustic into the flask with two portions of water of 1 to 2 ml. each. It is important that no air shall be sucked in with these reagent solutions. Now place. the flask in a mechanical shaker and agitate vigorously for one hour. At the end of this time chill the flask in cold water or evacuate thoroughly if unsaturated hydrocarbons are suspected, place exactly 10 ml. of concentrated hydrochloric acid in the funnel top, and draw it into the flask, wash the residual acid in with two washings of water, and shake the whole thoroughly to mix. If the ferric hydroside does not go into solution more or less immediately, place the flask in hot water for a while, with the vent in the stopper open, provided the flask has not been previously well evacuated. When all the iron salt is in solution, cool the contents of the flask, wash into a volumetric flask of suitable size, and dilute to the mark, after adding enough hydrochloric acid to produce a solution having a free acid concentration of approximately normal strength. For this purpose it is sufficient to consider that the
F THE various procedures proposed for the determina-
tion of low concentrations of oxygen in gas, the methods receiving the most attention seem to fall in three classesnamely, absorption, electrical, and oxidation-reduction. There are serious objections to all these methods. Moreover, certain industrial developments in recent months have greatly increased the need for reliable determinations of low concentrations of oxygen in gas and therefore work has been done t o evolve a procedure of adequate accuracy and simplicity. The first group of methods, absorption, represents a more or less routine procedure to the gas chemist, but for low concentrations of oxygen, a few tenths per cent or less, the results obtainable are of very doubtful value, especially for gases containing a large percentage of easily condensable hydrocarbons, such as gases produced in the petroleum and synthetic rubber industries. The electrical methods usually involve a considerable amount of expensive apparatus and at present are better adapted tQ recording devices than to general analytical work. The chemical procedures may be exemplified b y the Lubberger-Wunsch method (1) and the method of Steinkampf (S), which is a modification of the former and appears in the “Gas Chemists’ Handbook”. The Lubberger-Ti’unsch method is almost prohibitively cumbersome and Steinkampf has remedied this difficulty at the expense of delicacy. Furthermore, both methods depend upon the production of a measurable amount of free iodine in the gas phase and obviously certain sorts of hydrocarbon gases now frequently met with industrially could not be analyzed b y either procedure. T h e writer recently described, in connection with hydrogen sulfide analysis, a special type of flask ( 2 ) that since that time has proved adaptable t o the determination of oxygen in gas. This flask lends itself to a procedure that substantially eliminates difficulties encountered with other methods.
Determination of Low Concentration of Oxygen in Gas Briefly, the method consists in shaking a sample of gas in the special flask with ferrous hydroxide freshly precipitated by sodium hydroxide from an acidified solution of ferrous ammonium sulfate previously thoroughly purged of its dissolved oxygen. The solution is then acidified and when the ferric salts are dissolved the solution is diluted to a known volume and the ferric iron formed is determined colorimetrically against potassium thiocyanate by a specified procedure. The ferric iron formed is a measure of the oxygen in the gas. APPARATUS REQUIRED.Two test-tube gas scrubbers, 20 x 2.5 cm. (8 X 1inch), 1 Shaw sulfur flask (S), 1 mechanical shaking 891
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25 drops of 50 per cent sodium hydroxide neutralized 2.0 ml. of tHe 10 ml. of hydrochloric acid added and that 8.7 ml. of concentrated hydrochloric acid diluted to 100 ml. will yield an acid solution of normal strength. Then, if the solution of ferric iron mentioned is to be diluted to 100 ml., first add 0.7 ml. more of concentrated hydrochloric acid, or 13.7 ml. of acid if it is to be diluted to 250-ml. volume. A normal concentration of acid in this solution has no inherent virtue, but it is a concentration of the desired order of magnitude and a convenient means of keeping a reasonably accurate knowledge of the amount of acid present in the solution. This fact is of importance in the subsequent color comparison, as in the standard and the unknown tubes the acid concentrations must be substantially equal. Th@ solution is now ready for the colorimetric ferric iron determination. COLORIMETRIC COMPARISON.In each of two Kessler tubes place 10 ml. of the potassium thiocyanate solution. Into one tube put an aliquot of the sample sufficient to develop a color equivalent to that produced by not more than 5 ml. of the standard solution. If necessary, add sufficient N hydrochloric acid solution to produce at least 2.5 ml. of normal acid in the tube and dilute to 20 ml. with water. To the other tube add sufficient N hydrochloric acid to equal the total amount of acid in the first tube and follow with a measured amount of iron standard to nearly equal the color in the unknown tube. Dilute to 20 ml. with water and adjust the colors to match with additional standard solution as required. The milliliters of standard used times the dilution factor equals the total milliliters of standard required for the sample. From this figure it is necessary to deduct a solution blank, which is determined as described below. SOLUTION BLANK. Place 25 drops of 50 per cent sodium hydroxide and 15 to 20 ml. of water in a 50-ml. volumetric flask, add exactly 10 ml. of concentrated hydrochloric acid, and cool the solution. Add 15 ml. of the ferrous ammonium sulfate reagent aolution and dilute to the mark. To each of two Xessler tubes add 10 ml. of the potassium thiocyanate solution. To one of these tubes add 5 ml. of the blank solution and dilute t o 25 ml. with water. To the other tube add 9.2 ml. of the N hydrochloric acid and enough standard iron solution nearly to match the color in the first tube. Dilute to 25 ml. nith water and add sufficient additional standard to make the colors match. Then ten times the milliliters of standard solution equal the milliliters of total solution blank. In the writer's experience the total solution blank, using a fresh ferrous ammonium sulfate reagent solution, is in the order of magnitude of 15 ml. of standard solution. Under the specified procedure for oxygen determination the reagent solution is diluted finally to known volume and an aliquot placed in the comparison tubes. It therefore follows that this blank of 15 ml. is'to be deducted from the milliliters of standard solution calculated to be required for the known volume to which the reagent solution has been diluted. For a gas containing 0.1 per cent oxygen this is approximately 100 ml. The blank is therefore about ode seventh of the volume of standard solution required for the comparison. This ratio will, of course, vary with change in oxygen content of the gas. CALCULATIOKS. (Total ml. of standard - ml. for solution blank) X 0.1 ml. of gas sample (N. T. P.) If gas is corrected to standard conditions, substitute 0.105 for
0.1 in the numerator.
OF TESTDATAUSEDAS BASISFOR METHOD TABLEI. SUMMARY
(Data in table involved the deduction of t w o blanks-the customary solution blank and a blank on the nitrogen purified by passing through several unita charged with sodium pyrogallate and chromous chloride) Added Total Oxygen (N. T.P.) Oxygen Added Oxygen Oxygen Date Added Found to Gas Found inGas M1. IU1. % % % 12/10 0.011 0.008purified Nz o:i& 0.595 iOO:o 0.820 0.820 12/11 0,0238 0 . 0 1 7 purified Na 0.143 100:~ o:ik O.'lj'5 0.176 12/14 .. 0.0222 0.0158 ... 0.0222 0.0158 0,0401 0.0415 0.045 103:o 0:oio 0.0415 0.045 103.0 0.030 0.0401 12/16 0.0225 0.0159 ... 0,077 0.071 98.6 0:055 0.078 0.065 , , . (N2 not purified) . . , 0.054 12/27 .. ,
.
.
...
TABLE 11. DETERMINATION OF OXYGEN IN COMPRESSED CLEAX COKE-OVEN GAS (Ammonia and hydrogen sulfide added during sampling) Oxygen Found in Oxygen Unknown Sample Added Found (Column 2 Minus Date 1 2 Column 1) 12/20 12/21
%
%
%
None 0.13 0.13
0.38 0.53 0.49
0.38 0.40 0.36
TABLE111. DETERMINATION OF OXYGENIN PYROLYSIS GAS (Containing 50% of easily condensable unsaturated hydrocarbons) New Method Absorption Method"
a
%
70
0.074 0.161
0,4 0.4
100-ml. gas buret and chromous chloride abaorption solution
Discussion of Method Volatile bases, hydrogen sulfide, hydrogen cyanide, and certain unsaturated organic compounds will interfere with the procedure by reducing the ferric iron if they are present when the solution is rendered acid. All these substances except the unsaturates can be readily removed from the gas by scrubbing it with acid and alkali. The unsaturated hydrocarbons can be taken out by evacuation of the flask just before the addition of the acid. The acid and alkali scrubbing solutions have the further advantage that only neutral oxidizing agents can get into the reaction flask. This fact would seem to eliminate substantially all oxidizing agents other than oxygen and possibly peroxides. A half-hour shaking period did not seem sufficient to complete the removal of oxygen from the sample. The method is theoretically capable of estimating about 0.001 per cent of oxygen, but at such a low concentration the blank becomes relatively very high. Actually check tests were obtained showing 0.008 per cent of oxygen in specially purified nitrogen. Khere the concentration of oxygen in the gas is greater than 1 per cent, some difficulty may be experienced in getting the ferric hydroxide in solution. Such trouble may be minimized by adding greater known volumes of acid, followed by a higher final dilution. Where the ferrous ammonium sulfate reagent solution is not stored in an inert atmosphere it is preferable to make up a fresh solution every 48 hours. When working with a gas having an oxygen concentration of 0.1 per cent or more, one blank determination in an %hour period is satisfactory for most purposes. The author's experience has indicated that an error of not greater than 2 per cent is introduced b y this procedure on such a gas. I n determining very low concentrations of oxygen or where exceptional accuracy is required it is desirable to determine an over-all blank on the procedure, but this step is somewhat cumbersome and the author's work has indicated that for most purposes it is unnecessary. Satisfactory agitation is produced by clamping the flask to a 0.95-cm. (0.375-inch) rod with laboratory fittings, the rod being reciprocally operated a t a 45" angle by a n eccentric having a 10-cm. (4-inch) stroke and traveling at 120 r. p, m. Special care must be taken to make sure that all connection6 are vacuum-tight. While the flask is under vacuum i t is well to maintain a few milliliters of water in the funnel top to prevent air seepage into the flask. After the solution in the flask has been acidified small air leakages are unimportant.
Literature Cited (1) Anon., Gas u. Wasserfach, 72, 525-6 (1929). (2) Shaw, IND.ENG.CEEM.,ANAL.ED.,12, 668 (1940). (3) Steinkampf, Het Gas, 57, 414-15 (1937).
