Direct Determination of Total Oxygen in Oils MAURICEE. MARKS,'Research Laboratory, General Electric Co., Schenectady, N. Y.
I
N THE process of some experiments on oxidized oils, i t
became necessary to find a simple means of obtaining their oxygen content which ranged from 0.5 to 15 per cent, usually being about 3 per cent. The problem was to find a satisfactory method to take care of the sulfur and nitrogen, which was about 0.1 per cent, and give good accuracy with such small amounts of oxygen. The direct determination of oxygen by hydrogenation has been investigated by many chemists (1,2, 4, 7-10, 12-16). For the most part the compounds analyzed contained no elements other than carbon, hydrogen, and oxygen. On the other hand, crude oils contain as much as 2 to 3 per cent of sulfur and nitrogen, depending upon the source (3). Upon refining, most of the sulfur and nitrogen is removed, and in highly refined oils, such as cable and transformer oils, the sulfur Bnd nitrogen content runs well below 0.1 per cent. TABLEI. RUNSMADEWITH TRIONAL (69.3 mg. of sulfur passed over catalyst before runs were made)
TAKEN Mg. 181.2 188.7 201.6 154.2 149.9
SULFURCONTENT -OXYGENCalculated Mg .
Found
%
%
48.0 49.9 53.2 41.0 39.6
26.51 26.00 28.90 26.50 21.00
26.41 26.41 26.41 26.41 26.41
Catalyst inactive
Calculated
The ter Meulen method for the direct determination of oxygen in nitrogen-containing compounds has been investigated and modified recently (IS). The modified method consists of passing the vapors of the organic substance in an atmosphere of hydrogen over a very active thoria-promoted nickel catalyst, thereby converting all oxygen to water which is retained by weighed tubes of sodium hydroxide pellets, while the ammonia gas passes through without being absorbed. When sulfur as well as nitrogen is present, this absorbent cannot be used because hydrogen sulfide is formed and would be retained by the sodium hydroxide. Sulfur also poisons the nickel, probably with the formation of nickel sulfide, gradually destroying the activity of the catalyst (6, 11). TABLE11. RUNSMADEWITH A MIXTUREOF TRIONAL AND SUCCINIC ACID TAKEN Ma. 56.3 63.4 52.8 50.1
SULFUR CONTENT~ O X Y Q E N Calculated Found Calculated Mg % %
.
2.09 2.20 2.10 2.00
50.40 50.70 49.90 49 80
50.25 50.50 50.00 49.90
was chosen as WATER being most likely not to absorb either ammonia or hydrogen sulfide, since it is relatively chemically inert (6). This desiccant was prepared from Plaster of Paris by mixing with water to a paste and heating at 250" c*for 3 hours. It was then crushed, screened to pass four-mesh and be held by ten, and again heated a t 250' C. It Was tWted by Passing Over it hydrogen sulfide which had first been bubbled through ammonium hydroxide solution. Qualitative tests for ammonia and hydrogen sulfide were negative. Ammonia Was tested by odor and by action on litmus, and hydrogen sulfide was tested by alkaline lead acetate. That carbon dioxide was not 1
Present address, Rutgers University, New Brunswick, N. J.
absorbed was shown by passing this gas over the calcium sulfate and then testing with barium hydroxide. CATALYST.According to Kelber (6), the nickel catalyst can be made more resistant to sulfur by reducing a t a higher temperature-namely, 450" instead of 400" C, When this was done, the catalyst remained active in the presence of large amounts of sulfur. I n Table I are given the results obtained using trional as a test substance. More than 250 mg. of sulfur were passed over the catalyst before it failed. About ten analyses can be made without putting in a new charge of catalyst if the sulfur content is no higher than that of refined oils. The carbon should be burned off the cracking surface with air after about 8 grams of oil have passed over it. It is better to oxidize the carbon after each day's work and then reduce the catalyst again overnight. Reduction is complete in 10 hours a t the high temperature used. Two to three runs can be made in one day. Upon oxidation the nickel sulfide goes to nickel sulfate which gradually accumulates and finally causes destruction of the catalyst. For this reason the catalyst should be tested periodically with a known compound such as succinic acid. METHODAND APPARATUS The method finally used was the same as that of Russell and Marks (IS),except that calcium sulfate was used throughout the system to replace the sodium hydroxide pellets, and the catalyst was reduced a t 450' instead of 400" C. Reduction of the catalyst a t this higher temperature is doubtless in large measure responsible for the low blanks obtained, usually being below 0.1 mg. per 0.5 hour and seldom above 0.3 mg. TABLE111. ANALYSESOF OXIDIZED OILS SUBSTANCE
Run No. 1 oxidized oil Highly oxidiaed refrigerator oil
Run No. 11 oxidized oil Run No. 20 oxidized oil
TAKEN PERCENTAQE AVERAQE Grams 2.1687 2,1252 0.2379 0.2084 0.2042 0.1993 0.2430 1.1533 2.1938 1.7884 2.0263 1.7924
2.20 2.10 11.75 11.90 11.89 11.99 11.85 1.53 1.51 1.50 1.55 1.50
2.16 11.87 1.52 1.515
Because of the large combustion tube used (15 mm. inside diameter) special methods were necessary for keeping air out of the system when the sample was introduced. The quartz combustion tube was drawn out a t the absorption end and a short length of rubber tubing used for connecting to the first tube. This rubber tube was closed with a pinchclamp before removing the first absorption tube. Before making a run, the sample was put in and the tube flushed out for 15 minutes, to remove any air which might have entered from that end. A quartz boat was used in this work, since would be attacked by the sulfur in the oils, The boat was easily cleaned from adhering carbon by heating in an oxygenhydrogen flame, In making a' run the rate of hydrogenation of the carbonoxygen compounds was controlled by watching the narrow as watercondensed here, tube at the absorption end, heating of the sample was discontinued until this water had passed into the absorption tube. of oils volatile at around 500 c., it was necesthe sary to introduce them into the heated portion after the
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March 15, 1935
ANALYTICAL EDITION
system had been flushed out without removing the rubber stopper. The boat was put in the cold part of the tube while it was being flushed out, and then pushed into the heating zone by means of a glass rod, running in a glass collar in the rubber stopper a t the sample end of the combustion tube. A short piece of rubber tubing, forming an air-tight sleeve between the glass collar and the rod, was found to be especially convenient when successive runs were to be made, because it was not necessary to wait for the sample-heating furnace to cool. Oil vapors have a tendency to diffuse back into the cool part of the combustion tube and condense on the rubber stopper, ruining the analysis. By very slow heating and close watching of the sample, the diffusion can be reduced to a minimum. The portion of the tube before entering the furnace was made about 15 cm. (6 inches) long, so that any oil diffusing back would condense there and could be flamed out. A clear quartz tube is essential in order that the progress of the heating may be followed. Diffusion could probably be avoided by the use of a plug put in after the boat had been placed in the tube. This would increase the velocity of the hydrogen a t that point and therefore retard the oil vapor diffusion, but it seems inconvenient and not necessary if sufficient care is taken in the heating of the sample. RESULTS The results obtained are shown in Tables I to 111. I n order to test the method, several runs were made with mixtures of succinic acid and trional which as aliphatic compounds should rather closely approximate petroleum oils in their behavior. Enough trional was added to make the weight of sulfur the same as would be present in 2 grams of oil containing 0.1 per cent of sulfur. The results are shown in Table 11. The accuracy is about that to be expected from this type of method. Analyses made on several types of oxidized oils are presented in Table 111. A good reproducibility is evident and as good an accuracy as was obtained with the known compound may be reasonably expected.
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The method has given good results with sulfur compounds even when small amounts of oxygen are involved. It is hoped a t some later date to deal with still other sulfur compounds using this method of oxygen analysis, as it is believed that it should be of rather general applicability. ACKNOWLEDGMENT The author wishes to thank Charles Van Brunt of the General Electric Company’s Research Laboratory and W. Walker Russell of Brown University for their helpful criticism of the manuscript. He also wishes to thank Charles Van Brunt for his advice and suggestions in the development of this method. LITERATURE CITED (1) Beek, F. van, and Ward, S. de, Brennstof-Chem., 12, 402-3 (1931). (2) Boswell, M. C., J. Am. Chem. SOC., 35,284-90 (1913). (3) Day, D. T., “Handbook of the Petroleum Industry,” Vol. I, pp. 525 and 629, New York, John Wiley & Sons, 1922. (4) Dolch, M., and Will, H., Brennstof-Chem., 12, 141-6, 166-9 (1931). (5) Hammond and Withrow, IND. ENQ.CHBM, 2 5 , 6 5 3 , 1 1 1 2 (1933). (6) Kelber, Ber., 49, 1868 (1916). (7) Meulen, H. ter, Brennstof-Chem., 12,401-2 (1931). ( 8 ) Meulen, H. ter, Bull SOC. chim., 49, 1097-1106 (1931). (9) Meulen, H. ter, Chem. Weekblad, 23, 348-9 (1926); 27, 18 (1930). (10) Meulen, H. ter, Rec. trav. chim., 41, 509-14 (1922), 43, 899-904 (1924). (11) Rideal, E. K., and Taylor, H. S., “Catalysis in Theory and Practice,” p. 128, London, Macmillan & Co., 1926. (12) Russell, W. W., and Fulton, J. W., IND. ENG.CEEM.,Anal. Ed., 5, 384-6 (1933). (13) Russell, W. W., and Marks, M. E., Ibid., 6, 381-2 (1934). (14) Schuster, F., Gas- u. Wasserfach, 73, 549-51 (1930); Het Ga8, 52, 56-7 (1932). (15) Wanklyn and Frank, Phil. Mag., (4) 26, 554 (1863). (16) Will, H., Brennstof-Chem., 12, 423-4 (1931).
RECEIVED December 21, 1834.
Determination of Free Sulfur in Rubber A. F. HARDMAN AND H. E. BARBEHENN, T h e Kelly-SpringGeld Tire Co., Cumberland, Md. H E determination of free sulfur in vulcanized rubber has always been an important analytical operation. It has been accomplished in the past by extraction and direct weighing (2) ; by conversion into thiocyanate and titrating with silver nitrate (2, ??, 8); by conversion into thiosulfate and titration with iodine (1); by reduction with tin, hydrochloric and acetic acid, and titration of the hydrogen sulfide with iodine (6); and by oxidation and gravimetric estimation as barium sulfate. Under the last method, oxidizing materials employed have been nitric acid and potassium chlorate (d), liquid bromine ( I O ) , potassium permanganate ( d ) , and nitric and perchloric acids with bromine (7). The oxidation and gravimetric methods are on the whole satisfactory, that of Tuttle (10) being generally preferred as most convenient. The volumetric methods proposed are all lacking in some essential features, usually arising from slow or incomplete reactions or unsatisfactory end points. The chief objection to the gravimetric methods is in the large amount of time consumed in oxidation, precipitation, digestion, liltration, preparation of crucibles, and weighing. A minor objection is that the oxidation methods do not differentiate between true free sulfur and sulfur in organic compounds which may be present in the acetone extract (6). The volumetric method described below avoids all these
T
objections. It is rapid, accurate, and distinguishes between true free sulfur and organically combined sulfur. It should be useful not only in the rubber laboratory but wherever small amounts of free sulfur must be accurately determined. VOLUMETRIC METHOD It was recently observed in this laboratory that when a clean surface of metallic copper is exposed to sulfur dissolved in acetone, the sulfur is quickly and quantitatively absorbed with the formation of a black film of cuprous sulfide. When no interfering substances are dissolved in the acetone in addition to the sulfur, the copper may be weighed before and after reacting with the solution and the sulfur obtained by the difference in weights. However, in the extract of a rubber sample, almost invariably there will be found interfering materials such as the acidic softeners usually employed, certain accelerators and antioxidants, and even the natural resins of the rubber itself. Certain of these are absorbed or react sufficiently with the copper so that high results are obtained by direct weighing. An indirect method, therefore, must be used. Such a method was developed, based on the discovery that hot, concentrated hydrochloric acid attacks and completely removes the film of sulfide from the copper with the quantitative evolution of hydrogen sulfide. The latter is
