Platinized Silica Gel as an Oxidation Catalyst in Gas Analysis I. Oxidation of Hydrogen and Carbon Monoxide KENNETHA. KOBEAND ELMER J. ARVESON Department of Chemical Engineering, University of Washington, Seattle, Wash.
I
K THE analysis of those mixtures of gases that most
commonly occur in technical practice, the determination of the majority of the constituents may be accomplished by the absorption of the gases in a chemical reagent. This method is not applicable to the determination of hydrogen or hydrocarbon gases of the methane series. I n gas mixtures or gas residues that contain hydrogen and only one hydrocarbon of the methane series, both gases may satisfactorily be determined by burning the gases simultaneously in the combustion pipet (d), and measuring the contraction and volume of carbon dioxide formed. When hydrogen, methane, and ethane are present, the determination cannot be made (5) unless the hydrogen is first removed. For these reasons a satisfactory method of fractional combustion of hydrogen in the presence of hydrocarbons has long been sought and many different procedures have been suggested. In technical gas analysis a t the present time there is but one method in use for the fractional combustion of hydrogen, the copper oxide method in which copper oxide oxidizes the hydrogen ( I ) . Although hydrogen, carbon monoxide, and methane can be determined by one combustion ( 4 the , hydrogen and carbon monoxide may be oxidized by copper oxide at 300' C. without oxidation of the methane. Catalytic methods for the fractional combustion of hydrogen and carbon monoxide have been known since the work of Henry in 1825 (S),Hempel (2), and Nesmjelow (8) who used palladium on asbestos as the catalyst. Although used to a slight extent, it has been entirely replaced by the copper oxide method. It is apparent that the unpopularity of this method is due to the very great care that must be employed in the analysis to secure check results, the low degree of selectivity of the ,catalysts, and the unwillingness of the analyst to prepare his own catalysts which might vary greatly in their degree of reactivity. If a commercial catalyst of uniform activity and a high degree of selectivity can be found, the method of catalytic oxidation has numerous advantages. For this reason the platinized silica gel produced by the Silica Gel Corporation for the oxidation of sulfur dioxide to trioxide was investigated as an oxidation catalyst in gas analysis.
APPARATUS The apparatus used in this ihvestigation was the U. S. Steel Corporation gas apparatus (7). Mercury was used as the confining liquid and acid saturated sodium sulfate solution as the flushing liquid. The fuel gas buret furnished with the apparatus w&s used for all measurements. The platinized silica gel containing 0.075 per cent platinum was sized to pass 14 mesh and be retained on 20 mesh. About one gram of the catalyst was placed in a tube of the same dimensions as the standard copper oxide tube used with the apparatus. This catalyst tube replaced the copper oxide tube in the experiments using catalytic oxidation.
mately 30-cc. samples. The procedure for using the catalyst tube was exactly the same as that used for the copper oxide method for determining hydrogen (8) using 100-cc. samples. I n the first experiments, the catalyst tube was heated to a temperature of 125" C. It was found that after one passage of the gas through the tube most of the hydrogen was oxidized, and after four passes the hydrogen was completely removed and no further diminution in volume took place on further passage through the tube. I n a second series of experiments the temperature was lowered and it was found possible to remove the hydrogen completely a t a heater temperature of 80" to 85" C. The results are shown in Table I. TABLEI. DETERMINATION OF HYDROGEN (Gas mixture:" hydrogen, oxygen, nitrogen)
Analysis Hydrogen. percent
1
2
ANALYSISBY EXPLOSION METHOD 3 4 A v . 5 6 7
A
v
16.3 16.6 16.1 16.3 16.3 14.4 14.5 14.5 14 5 ANALYSIS BY CATALYET T w a METHOD Analysis 1 2 3 Av. 4 5 Av. Temperature of heater 125 125 125 80-85 90 . Number of passes 4 4 6 6 4 16.6 16.5 16 7 1616 14.7 14.7 14 7 Hydrogen, per cent 6 59 per oent oxygen in mixture.
.
..
TABLE11. DETERMINATION OF CARBON MONOXIDE (Gas mixture: oarbon monoxide, oxygen, nitrogen)
ANALYSIS BY EXPLOSION METHOD Anal ysis Carbon monoxide, per oent
1 15.8
2 19.0
-4NALYElS BY C.4TALYS.r
Analysis Tem erature of heater Numger of passes Carbon monoxide, per cent
1 300 6
19.0
2 300
3 Av. 18.9 18.9 TUBEMETHOD Av.
6
19.0
..
19:0
Thus in the catalyst tube method it is not necessary to heat the tube to a temperature greater than 100' C, in order to oxidize all of the hydrogen, whereas in the copper oxide method a temperature of 300" C. is required. It was also found that the gas could be passed over the catalyst a t a greater rate than with the copper oxide. A velocity of 30 to 50 cc. per minute was used, while a maximum rate of 20 cc. per minute is recommended with copper oxide. In order to test the nature of the catalytic effect, a mixture of hydrogen and oxygen was passed over silica gel under the same conditions as with the platinized silica gel. No appreciable oxidation took place even a t temperatures as high as 350" C., thus showing that the oxidation catalyst is the platinum supported on the silica gel.
DETERMINATION OF CARBONMONOXIDE
DETERMINATION OF HYDROQEN A mixture of hydrogen, oxygen, and nitrogen was prepared and analyzed by the explosion method, using approxi-
A mixture of carbon monoxide, oxygen, and nitrogen was prepared and analyzed by the explosion method. When using the catalyst tube it was found necessary to raise the heater temperature to at least 300" C. in order to oxidize the carbon monoxide a t the same rate a t which hydrogen was oxidized. This is the same temperature used in the copper
110
March 15, 1933
INDUSTRIAL AND ENGINEERING CHEMISTRY
oxide method, but the rate of flow is much faster than in that method. The results obtained by the explosion and catalyst tube methods check well and therefore indicate the suitability of the latter method. The results are shown in Table 11.
DETERMINATION OF HYDROGEN IN CITYGAS The results obtained were next applied to technical gas analysis using the city gas, which is a carbureted water gas mixed with coke-oven gas. The usual method of analysis by absorption of carbon dioxide, illuminants, oxygen, and carbon monoxide was used, but hydrogen was determined by passage through the catalyst tube a t a temperature of 100" C. Methane was not determined in all cases, as it required additional time. Analyses of the gas are given in Table 111. TABLE111. ANALYSIS OF SEATTLE CITYGAS Carbon dioxide Illuminants Oxypen Carbon monoxide Hydronen Methane and ethane Nitrogen
%
%
5.7 7.1 0.1 13.4 27.9 18.3 26.5
5.6 7.4 0.1 13.1 27.9 19.3 27.0
% 5.7 7.2 0.2 12.9 28.0 1 0 . 3 (approx.) 26.7
A check of the hydrogen determination by the copper oxide method and the catalyst tube method was then made. The residues from four 100-cc. gas samples after absorption, containing only hydrogen, methane, ethane, and nitrogen, were mixed together to give a uniform mixture. Two copper oxide analyses and two analyses using the catalyst tube were made. The results are shown in Table IV. TABLEIV. DETERMINATION OF HYDROGEN COPPER OXrDE
Analuaia Sample Temperature of heater Number of passea Hydrogen, per cent
1
METHOD
53 300
2 86.1 300
34: 5
34:4
CATALTBT TWBE METHOD 1 2 49.3 100 8 34.6
53.9 100 8 34.6
Thus it is seen that the catalyst tube method has as high a degree of accuracy as the copper oxide method. Though a very slightly greater amount of hydrogen was found by the catalyst tube method, it is believed that this is no error of this method. Rather the copper oxide tube may be a t fault, for this reaction is a heterogeneous reaction in which the copper oxide is consumed, so that a t the end of the oxidation but a very small amount of copper oxide may remain which may be insufficient to give an oxidation a t an appreciable rate. The catalyst, however, is unchanged during the reaction and as long as an excess of oxygen is present the oxidation will go to completion on the catalytic surface.
CARBON MOKOXIDE AS A CATALYST POISON After the catalyst was found successful in determining hydrogen from the residue after carbon monoxide had been removed, it was decided to try to oxidize the hydrogen without first removing the carbon monoxide. When the gas residue containing carbon monoxide, hydrogen, and hydrocarbons was passed through the catalyst tube a t 100" C. no reduction in volume of the gas took place. The catalyst was found now to be inactive toward hydrogen-oxygen mixtures a t 100" C., showing that poisoning of the catalyst had occurred. The same results were again obtained with a fresh charge of catalyst. Since carbon monoxide was oxidized when passed over the catalyst a t a temperature of 300" C. an attempt was made to reactivate the catalyst by heating it to this temperature and passing air through the tube. This procedure caused an oxidation of the adsorbed carbon monoxide which was acting as the poison and the catalyst was
111
found to be completely reactivated by this treatment, so that hydrogen could be determined as readily as before the poisoning had occurred.
COMBUSTION OF HYDROCARBONS A mixture of methane, oxygen, and nitrogen was prepared and passed over the catalyst a t a temperature of 305" C. No carbon dioxide was found in the products. A hydrogen, methane, and oxygen mixture was prepared and passed over the catalyst a t 100" C. for determination of the hydrogen. No carbon dioxide was found in the products. These data, taken with those of Table IV, show that hydrogen can be determined by catalytic oxidation a t 100" C. in the presence of the hydrocarbons in city gas. An attempt was made to determine the possibility of oxidizing hydrogen and carbon monoxide together a t 300" C. in the presence of the methane hydrocarbons in city gas. Some of the hydrocarbon residues of the city gas after carbon monoxide and hydrogen removal were passed over the catalyst with oxygen. At a temperature above 300" C. many passes gave only a fraction of a cubic centimeter of carbon dioxide. This seemed to indicate that the ethane and any other higher hydrocarbons which might be present were oxidized to some extent. The small amount of carbon dioxide formed eliminated the possibility of determining hydrogen and carbon monoxide in the gas residue by passage over the catalyst a t 300" C. It was, however, found possible to oxidize both hydrogen and carbon monoxide a t the same time a t a catalyst temperature of 300" C., so that it should be possible to determine the composition of gases containing only hydrogen, carbon monoxide, and methane when the gas is mixed with sufficient oxygen and then passed through the catalyst tube a t a temperature of 300" C. The oxidation of hydrocarbons is being studied further. ADVANTAGES OF CATALYST TUBEMETHOD In the copper oxide method as the amount of hydrogen in the gas decreases, the rate of reaction falls off because of the decreased hydrogen and the fact that the surface of the copper oxide is reduced. The amount of hydrogen removed in the last few passes over the copper oxide is very small, so that the analyst may stop before all the hydrogen is completely removed, while with the catalyst tube the number of passes is considerably less, so that all hydrogen is removed. This fact makes the catalyst tube method much more rapid than the copper oxide method. Also the catalyst is unaffected by the reaction, while the copper must be reoxidized to copper oxide, a saving in time corresponding to one determination. The copper oxide soon loses its activity and must be discarded, while the platinized silica gel has an indefinite life, Though the catalyst is susceptible to poisoning by carbon monoxide a t 100" C., it is easily restored to its original activity. The removal of carbon monoxide by absorption before the hydrogen oxidation offers no difficulty. With the copper oxide method the size of sample is definitely limited by the amount of copper oxide present to oxidize the hydrogen. However, with the catalyst a sample of any size may be taken, since oxidation is effected by the oxygen or air added and the mere presence of the catalyst is all that is necessary. The use of larger samples gives a greater degree of accuracy to the method. Lastly, the use of a standard commercial catalyst of uniform activity releases the analyst from the duty of preparing his own catalyst, which may have a variable activity.
SUMMARY 1. Hydrogen is quantitatively oxidized by oxygen over a commercial platinized silica gel a t a temperature of 100" C. 2. Carbon monoxide is a catalyst poison a t 100" C. but is
112
ANALYTICAL EDITION
oxidized a t a temperature of 300" C., the catalyst being completely reactivated. Mixtures of hydrogen and carbon monoxide can thus be oxidized a t 300" C. 3. Hydrogen in a hydrogen-hydrocarbon mixture may be determined by fractional combustion with oxygen by passing the gases from four to six times at a rate of 30 to 50 cc. per minute over the catalyst a t a temperature of 100" C. 4. Further studies are being made on the extent of oxidation of methane and higher hydrocarbons under the conditions required for carbon monoxide oxidation.
Vol. 5 , No. 2
LITERATURE CITED (1) Burrell and Oberfell, J. IND: ENG.CHEM.,8, 228 (1916). (2) Hempel, 2. angew. Chem., 25, 1841 (1912). (3) Henry, Ann. Philosophy, 25, 428 (1825). (4) Kobe, IND.ENQ.CHEM.,Anal. Ed., 3, 159 (1931). (5) Ibid., 3,262 (1931). (6) Nesmjelow, Z. anal. Chem., 48, 232 (1909). (7) U.S. Steel Corp., "Methods for Sampling and Analysis of Gasses'' Carnegie Steel Co., Pittsburgh, 1927. (8) Ibid., p. 29. RECEIVED October 18, 1932.
Volumetric Methods of Estimating Nitrites RAYMOND D. COOLAND JOHN H. YOE, Cobb Chemical Laboratory, University of Virginia, University, Va.
A
LARGE number of procedures for the volumetric estimation of nitrites, or nitrous acid, are to be found in the literature. Because of the considerable difference of opinion as to the accuracy of these various methods, and the lack of a critical systematic study of them, a careful comparison has been made of a number of the procedures, in order to determine their relative values and see which might be expected to give accurate results when used by the average analyst under ordinary conditions. Recalibrated precision volumetric ware and the best grade of c. P. chemicalswere employed throughout the investigation. Conductivity water was used for all solutions, which were protected from light, blanks were run on all the reagents, and the usual end-point and temperature corrections were made. Standard solutions were made either from analyzed reagents of known purity, or the solutions were standardized against certified analytical standards, as, for example, Bureau of Standards sodium .oxalate for permanganate. Standardizations, comparisons, and blanks were made under the same conditions as the determinations. Samples of 50 to 100 mg. of sodium nitrite (10 to 20 cc. of a solution containing 5.0000 grams of recrystallized c. P. sodium nitrite per liter) were used for individual determinations.
PERMANGANATE METHODS Direct titration of nitrite in strongly acid solution with potassium permanganate proved unsatisfactory. When the neutral nitrite solution was acidified a distinct odor was noticeable, indicating a loss of nitrous acid, and the results were invariably low. The process was also very slow because of gradual fading of the pink coloration near the end point. Decolorization was hastened by heating the solution to 50" C. near the end of the titration, but the loss of nitrous acid on acidifying the nitrite solution still introduced an error and the results were always low. In an attempt to overcome the loss of nitrous acid when the solution was strongly acidified, permanganate was added to a neutral sodium nitrite solution until it became pink; it was then made slightly acidic and the titration was completed without further addition of acid. While the results obtained were nearer the theoretical than those with the preceding procedure, they were still low, giving a constant error of - 1.4 per cent when 50 mg. of sodium nitrite were present, and a practically constant error of -2.2 per cent with 100 mg. of sodium nitrite. When the slightly acidic solution was strongly acidified just before reaching the end point (6), a nearly constant error of - 1.3 per cent resulted with 50 mg. of sodium nitrite, and -1.4 per cent with 100 mg.
A procedure suggested by Adie and Wood (1) gave results with an average error of +0.1 per cent (procedure A, Table I). The amount of permanganate required to react with a measured quantity of the sample was determined approximately by adding standard potassium permanganate to a strongly acidified solution of nitrite. For the exact determination, standard permanganate solution was added to the neutral nitrite, diluted to 100 cc., to within 1 cc. of the amount found in the preliminary approximate analysis. Then 10 cc. of 6 N sulfuric acid were added and the titration was completed. Lunge (17) claimed that if the titration process is reversed, so that the nitrite is added to acid permanganate, the nitrogen trioxide set free from every drop of the nitrite is immediately oxidized before it can decompose into nitric oxide and nitrogen pentoxide. On adding the nitrite solution to permanganate, 0.6 N with sulfuric acid, an average error of -0.4 per cent resulted (procedure B, Table I). However, decolorization of the permanganate takes place very slowly near the end point, and the large errors obtained in some of the determinations were probably due to the titrations being carried out too fast, with a consequent over-stepping of the end point. In order to hasten the decolorization near the end point and thereby shorten the time of titration, determinations were made with the permanganate heated to 30" to 40" C. as recommended by Lunge (17) a t different rates of titration. The results obtained indicate the necessity of carrying out the titration slowly, especially near the end point, which still appears slowly in spite of the heating, although it is faster than a t room temperature. When the titrations were carried out as fast as possible, the average error amounted to -0.5 per cent. At a moderate rate of titration the error decreased to -0.1 per cent, and when the titration was conducted very slowly near the end an average error of +0.1 per cent was obtained (procedure C, Table I). TABLEI. SUMMARY OF RESULTS OF SATISFACTORY METHODS
PROC~DURE A B C, fast
c moderrtte c:slow
D, 3-36% excess KMn04 D, 127-132% excess
EG
H
I J
Maximum
Minimum
Arithmetical mean
%
%
% +0.1
-0.7
fO.0 -0.1 -0.3
$0.2 -0.7
-0.4 -0.5
f0,6 +0.3
2to.o *0.2
-0.1 $0.1
4-0.4
10.0 +0.4
+0.3 +0.6 +0.3
AO.0
+0.2 +0.2
+o.s
+0.4 i-0.4
4-0.3 +0.8 -0.4 +0.5
+0.2
*o.o *o.o *o.o
+0.1
t",:", +0.1
Average deviation arithmetical mean from
% 10.1
f0.2 f0.2 fO. 4 f0.2
AO.1 2c0.2 *0.1 *0.2 *0.1 10.2
f0.2 10.2
