Microanalysis of Gases Acetylene, Benzene, and Some Procedure Modifications F. E. BLACET, A. L. SELLERS, AND W. J. BLAEDEL University of California at Los Angeles, Los Angeles, Calif.
A method of analysis for acetylene in the presence of propylene and carbon monoxide uses as the absorbing reagent solid mercuric cyanide and potassium hydroxide. In the microanalysis for benzene vapor both fuming sulfuric acid and ammoniacal nickel cyanide are satisfactory reagents. A new combustion coil for the burning of gases is described, and a change in the method of preparing a cupric oxide-potassium hydroxide reagent for hydrogen is given.
hand. The difficulty was ultimately overcome by using a solid bead of mercuric cyanide and potassium hydroxide. Powdered mercuric cyanide was moistened with a minimum amount of 6 N potassium hydroxide and worked into a smooth paste, which was then molded into a bead in a platinum loop. The paste was dried over a warm electric coil (care being taken not to decompose the mercuric cyanide) and then placed in the gas samde. This reagent was found not to react to a measurable extent with either propylene or carbon monoxide during an exposure of 1.5 hours. Several known mixtures of acetylene, propylene, and - _. - . -. A carbon monoxide were made and analyzed for acetylene with satisfactory results. A single bead of absorbent and a time of 20 minutes were found sufficient for complete reaction in every analysis. Table I is typical of results which were obtained. I n this case approximately equal volumes of the three gases were mixed. A complete analysis would require the subsequent introduction of concentrated sulfuric acid to remove ethylene and silver oxide to absorb carbon monoxide, a procedure which offers no difficulties.
S
IKCE the appearance of the fourth paper of this series there have accumulated in this laboratory some new methods of analysis and modifications of old procedures for the microanalysis of gases.
Acetylene Acetylene can be determined quantitatively in the presence of ethylene and saturated hydrocarbons by using a bead of solid cuprous chloride and potassium hydroxide as the reagent (2). However, in a recent research problem of this laboratory it was important to analyze for acetylene in the presence of carbon monoxide and propylene. The cuprous chloride potassium hydroxide reagent reacts slowly with carbon monoxide, the solid silver oxide normally used to remove carbon monoxide reacts rapidly with acetylene, and sulfuric acid, being the common reagent for absorbing unsaturated hydrocarbons, will combine with both propylene and acetylene. Obviously a new reagent was needed, preferably one which would remove acetylene and not react with the other two gases. Treadwell and Tauber (6) recommended a solution made by dissolving 20 grams of mercuric cyanide in 100 cc. of 2 N sodium hydroxide. The authors found that such a solution introduced in a gas sample by means of a sintered-glass bead (2) absorbs acetylene quantitatively in 10 minutes from a 50 per cent mixture with nitrogen. The results compare favorably with those obtained by using either cuprous chloridepotassium hydroxide or sulfuric acid as the absorbent. Unfortunately, this solution also absorbs propylene t o an appreciable extent and could not be used for the problem at
Benzene
FIGURE 1. COMB U S T I O N COIL I n studying the feasibility of analyzI N O P E R A T I N G ing for benzene vapor in small volumes, POSITION it was found that two of the recognized macroanalvtical reagents can be used with results which appear satisfactory. 0;e is fuming sulfuric acid followed by a solid potassium hydroxide bead, and the TABLEI. AXALYSESFOR ACETYLENEIN THE PRESENCE OF PROPYLEKE A N D CARBON MONOXIDE Determination
T'olume of Sample Cu. mm.
1
45.44
4
40.75 37.66 40.82 4 2 61
2 3 5
,
Theoretical
Acetylene-------Determined
%
70
33.10 33.10 33.10 33.10 33.10
33.09 32.98 33,l5 33.11 32.80 33.03
-0.01 -0 12 +o 0.5
Ar.
356
Difference
%
70.01 - 0 30
0 10
ANALYTICAL EDITION
JUNE 15, 1940
other is an ammoniacal nickel cyanide solution made according to the detailed directions given by Dennis and McCarthy (4). The latter reagent must be followed by solid phosphorus pentoxide or concentrated sulfuric acid to remove ammonia (1). Gas samples for testing these analytical methods were made by slowly bubbling nitrogen through liquid benzene a t room temperature and then adding approximately 10 per cent more nitrogen to the resulting gas volume. Table I1 gives results which were obtained using the different analytical procedures. The percentages of benzene obtained are about what one would expect from the mode of preparation of the sample, and the fact that the two methods of analysis are in good agreement is taken as evidence that both are satisfactory. TABLE 11. DETERMIXATION OF BENZENE Determination
Volume of Sample Benzene Cu. mm. % B y Fuming Sulfuric Acid
Deviation from Average
%
357
wire and then bent into a loop as shown. Sufficient wire is allowed to extend into the unconstricted portion of A to give a good electrical contact when this portion is filled with mercury. The other end of the wire extends beyond the glass about 2 mm., is bent sharply at B , and is attached to the holder a t D with the aid of a glass bead. The tip of the glass below B should be well rounded and carefully polished to prevent introduction of small air bubbles into the sample. The distance betmareenB and D is about 15 mm. .4t bend B the wire is carefully fled and ground down on a fine whetstone until a diameter is obtained at one point which is from one fourth to one fifth of that which was originally present. This gives a cross section at this point roughly one twentieth that of the rest of the wire, a ratio which may be compared to one third as previously used ( 3 ) . The strong anchorage a t D holds the wire in place and has made possible the substantial reduction in diameter a t B . This in turn has decreased the current necessary to produce the hot spot, and as a consequence, has diminished the operating temperature of the rest of the wire to the point where the danger of mercury oxidation has been greatly reduced, if not completely eliminated. Anchorage D is placed low so that it will be well below the mercury surface in the gas holder, C , at all times, and any small air bubbles which may be trapped at D will not be introduced into the combustion mixture. This type of construction has overcome the difficulties encountered earlier, when attempts were made to attach the wire beyond B to the glass (3).
Hydrogen Reagent Av.
1
2
3
4.51
By Ammoniacal Nickel Cyanide 101.33 4.54 4.57 4.37
96.72 99.05
Av.
4.49
*0.10 +0.05
+0.0s -0.12
t0.08
Modification of Combustion Coil The platinum combustion coil now in use in this laboratory for the burning of gases is more satisfactory than the one previously described (3) and is illustrated, drawn to scale, in Figure 1. The soft-glass holder, A , is a I-mm. capillary with a 2.5-mm. external diameter. During construction of the coil the lower end of this tube is shrunk around the proper length of 24gage platinum
The method of preparing a cupric oxide-potassium hydroxide absorbent for hydrogen has been practically reversed from t h a t described before (1). The bead is prepared now by fusing potrtssium hydroxide in a platinum loop and touching this while molten to powdered cupric oxide, allowing i t to enclose as much of the oxide as i t will and still give a smooth external surface on cooling. This reagent is easier to prepare than the one formerly used and gives just as satisfactory results.
Literature Cited (1) Blacet and MacDonald, IND. ENO. CHEM.,Anal. Ed., 6, 334 (1934). (2) Blacet, MacDonald, and Leighton, Ibid., 5, 272 (1933). (3) Blacet and Volman, Ibid., 9, 44 (1937). (4) Dennis and McCarthy, J. Am. Chem. SOC.,30, 233 (1908). (5) Treadwell and Tauber, H e h . Chim. Acta, 2, 601 (1919). ~~
Microdetermination of Sulfur in Organic Compounds An Absorption Apparatus for Use with the Combustion Method L. T. HALLETT AND J. W. KUIPERS, Kodak Research Laboratories, Rochester, N. Y.
I
T IS well known that organic material must burn a t a much slower rate in the determination of sulfur by the method
of combustion with oxygen gas than in other determinations where combustion is involved. While studying the use of tetrahydroxyquinone as a n indicator for the microvolumetric determination of sulfur (5) i t became apparent t h a t i t would be desirable for routine work to reduce the burning time to 15 minutes, so that the total time for a determination would not be over 30 minutes. The well-known Pregl combustion tube with spiral absorber is not suited to rapid combustion and is not convenient because the tube must be removed from the furnace before the combustion products can be washed out. If a sample is burned rapidly (15 minutes) in the Pregl combustion tube with spiral absorber or in the absorption apparatus designed by Hallett (4) with the oxygen flowing a t the rate of 75 to 100 ml. in 5 minutes, low results are usually
obtained, owing to the loss of sulfur trioxide as mist. This electrically charged mist is formed by the union of sulfur trioxide and water vapor; once formed, i t cannot be absorbed in chemical reagents and so escapes from the absorption train. A study was made, therefore, of the type of absorber that would allow rapid combustion and be convenient for removal of the absorbed combustion products. It was found that hot gases containing sulfur oxides traveling rapidly readily formed the sulfur trioxide mist. Any constriction, therefore, at a point at the end of the combustion tube near the absorber would increase the speed of the gases and the tendency to mist formation.
Apparatus The first type of absorber used (Figure 1) was similar to that previously described (4) but smaller, having a 6-mm. bore and I-mm. wall. The absorption spiral is 8 cm. long with 2 mm.
