266
ANALYTICAL EDITION
Vol. 3, No. 3
A Dry Method of Microanalysis of Gases' Francis E. Blacet and Philip A. Leighton DEPARTMENT OF CHEMISTRY. STANFORD UNIVERSITY, CALIF,
HE increasing imporAn apparatus for microanalysis of gases has been sealing process. T o the belltance in photochemidescribed and a method worked out for the analysis of shaped lourer end is attached cal work of analyzing gas samples of the order of 25 to 100 CU.mm. without a short length of very heavy the use of liquid reagents. gas samples of the order of rubber pressure tubing. I n 0.1 cc. or less has caused the Yellow phosphorus has been used to remove oxygen, s e t t i n g UP the buret, it is authors to spend some time in fused potassium hydroxide t o remove carbon dioxide, first inverted and filled comthe study of micro methods. and fused phosphorus pentoxide to remove water vapletely with mercury and s, par. These absorbents were used also in the analysis It was desired to avoid the plug sealed in ths free end elaborate methods of Langby combustion of hydrogen, carbon monoxide, and of t h e p r e s s u r e t u b i n g . nxd~ane. muir ( 4 ) and Ryder (6). When in position for use, the The apparatus of Guye and The results of the analysis of a number of different r u b b e r t u b i n g fits in the samples are given. These indicate that the degree of Germann (3) has the serious jaws of an especially heavy disadvantage of c o n t a i n i n g precision to be expected by this method is of the same buret clamp equipped with many s t o p c o c k s i n w h i c h order of magnitude as that ordinarily attained in a finely t h r e a d e d screw, B. appreciable loss of gas may macroanalysis of gases. By means of this screw the occur. Reeve (5) has promercury level in t h e b u r e t posed a much simpler apparatus for the determination of can be changed with precision. This device was preferred to carbon dioxide by means of fused potassium hydroxide. the steel screw and hard rubber manipulator proposed by Christiansen (1) employed a similar device but used the liquid Christiansen ( I ) , for, owing to the pressure developed in a reagents of macroanalysis. buret of this length, it was found impossible to prevent the The method of liquid reagents has the advantage of wider escape of mercury past the screw. applicability a t the present time because in macroanalysis By means of the telescoped brass tubes C and D and the they have been used almost exclusively and much of this machined screw E, everything on the table, F, can be moved accumulated knowledge can be used in micro methods. How- up or down with perfect control, C: is a mercury reservoir ever, the solubility of the sample in the various reagents and of 7 cm. diameter. H and J are the containers for the gas washing solutions may cause an error of serious magnitude samples and are also used for the processes of absorption and when very small quantities of gases are involved. This is explosion, as will be explained later. They have a capacity because the total volume of the various solutions with which of approximately 2 cc. each. I n the diagram only two the gas must come in contact is proportionately much greater of these are shown, but, in fact, four of them are symmetrithan in macroanalysis. Also, in using a capillary buret it cally attached by means of circular steel springs to the rehas been found very difficult if not impossible to free the walls volving table M . K is a brass tube which acts as a guide entirely of a film of water solution by replacement with for accurately placing and supporting an absorbent holder, L. mercury. This film must be dealt with in volume measurements and unless great care is exercised, upon the introduction Methods of Procedure of another solution, it may be the cause of a precipitate forming in the buret, which may be very hard to remove. TheoretiFor simplicity in explaining the method of using this apcally, a t least, the use of solids as absorbents in conjunction paratus, let it be supposed that it is desired to determine the with the microburet would eliminate these objections. The percentage of oxygen in dry air. First the containers H and authors have investigated this method and have been able to J are filled with mercury. This is done by placing a glass analyze with some degree of precision samples of the general tube, which has been drawn out to a capillary and properly order of magnitude of 25 to 100 cu. mm. for water vapor, bent, so that its end will go down into the reservoir G and carbon dioxide, oxygen, carbon monoxide, hydrogen, and up to the top on the inside of the containers. If the containers methane entirely without the use of water solutions. are clean and have a top of smooth curvature all the air can be drawn out with ease. The high surface tension of Apparatus mercury facilitates this, Several hundred cubic millimeters Figure 1 is a diagram drawn to scale of the apparatus em- of the air are introduced into H . This constitutes the sample ployed. A is a water-jacketed microburet. This is a capil- for analysis upon which several determinations presumably are lary tube of approximately 0.5 mm. in diameter and accurately to be made. If a Toepler pump is used for the purpose of ruled in millimeter divisions for a length of 45 cm. It was Cali- introducing samples, its outlet is bent in the same manner as brated by means of weighing drops of mercury and was found the tip of the buret and placed permanently in position in the to have a volume of approximately 1 cubic millimeter for 4 reservoir and a t the same height as the buret tip. The rotatlinear millimeters. Thus the total capacity is about 112 ing table M is mounted on an adjustable arm so that the concu. mm. The upper end is bent as shown and ends in tainers can not only rotate completely around the central a capillary of small external diameter to facilitate the shaft but may be moved t o any desired position within intake and discharge of gases. The tip of this capillary the reservoir. Accordingly, the outlet for the Toepler pump is ground and fire-polished to a radius of curvature somewhat may be a t the back side of the reservoir and a container can ' less than that of the top of the gas holders H and J . The be moved over its tip and filled. lower end of the buret is sealed to a larger tube containing a Thenext stepis to obtain a known volume for analysis. By trap to insure the exclusion from the capillary of impurities means of the screw B all of the air in the buret is replaced by and chance solid material which may be introduced in the mercury. The tip must be under the surface of the mercury in G while this is done, otherwise when it is submerged it will 1 Received March 7, 1931.
T
July 15, 1931
INDUSTRIAL AND ENGINEERING CHEMIXTRY
invariably carry a small air bubble with it. H is now placed directly over the tip, and by lowering the table F, the tip is brought into the gas sample. By means of B a portion of the sample is drawn into the buret. The volume ordinarily taken varies from 25 to 100 cu. mm. depending upon the total amount available and the subsequent treatment to which it is to be subjected. The table is then raised so that the level of the mercury in the reservoir is on the level with the uppermost calibration mark in the water-jacketed part of the buret. By further unscrewing B, the gas, followed by a thread of mercury, is brought into the calibrated portion for measurement of its volume. When its upper level is a t the same height as the surface of the mercury in the reservoir, it is then theoretically under atmospheric pressure. However, the apparent pressure in the buret may be considerably in error, owing to the fact that mercury has a tendency to stick to clean dry glass. If no precautions are taken two subsequent volumes on the same gas sample may be obtained which vary as much as 1 per cent from one another. To reduce this source of error two expedients are used. First, the buret is gently tapped with the finger as the gas is being brought to the desired position. Second, five independent readings are always recorded and the average taken in calculating the volume, These precautions reduce the probable error in all cases below 0.2 per cent. After the volume readings for the run have been recorded along with temperature and barometric readings, container J is brought over the buret tip and the gas in the buret expelled into it. The gas is now ready to have the oxygen absorber introduced. The removable holder L has a platinum loop sealed in its tip. In this loop is fused a small bead of yellow phosphorus. The fusion may ordinarily be done by placing a fresh piece of phosphorus in the loop by means of forceps and then holding it over a warm resistance coil or even in the sunlight. By quickly removing when fusion occurs and placing it under the mercury, spontaneous combustion may be avoided. In any case it should not be allowed to remain in the air any length of time, for the moisture of the air along with oxygen causes a film of phosphoric acid to coat the bead, thus making it a much slower absorber. As shown in Figure 1, the holder is placed in the guide K . The container J is brought into position and the phosphorus introduced into it. I n this operation care must be taken not to allow the glass of the holder L to touch the walls of the container. It has been found that a cold tube introduced into mercury in this way is coated with a film of air, part of which will be imparted to the walls of J if contact is made between the two glass surfaces. On this account the guide is necessary for the introduction of the absorber up the center of the container. The height of K is also arranged so that only the phosphorus bead and a little platinum wire enters the gas bubble. Fifteen minutes has been found sufficient time to allow for complete absorption in practically all cases. However, absorption t o constant volume is practiced. After absorption has occurred and the absorbent removed, the tip of the buret is brought to the top of the container (more accurately, the top of the container is brought to the buret tip) and the residual gas taken into the buret. With the proper manipulation of B, E, and M , this operation presents no practical difficulties. If the entire gas bubble is not drawn into the buret intact the first time, it need only be driven back into the container and the process repeated. The experimenter soon acquires skill in this regard. The volume of the residual gas is now measured and the temperature and the barometric pressure again recorded. The absorption process is repeated for 5 minutes more, and if no further reduction in volume occurs, all data necessary for the simple standard method of calculation of the determination are complete.
267
For the removal of moisture, a bead of fused phosphorus pentoxide is used. This may be prepared over a glowing electric coil. A flame should not be used because of the moisture generated by it. Carbon dioxide is removed by means of fused potassium hydroxide. This reagent is prepared in the same way as the phosphorus pentoxide, except that after the bead is formed it must be allowed to collect moisture from the air until it presents a shining surface before placing under the mercufy of the reservoir. This moisture is essential for the rapid absorption of the carbon dioxide. When potassium hydroxide was introduced while still very hot, it was found that as much as 2 hours was required before all the carbon dioxide was taken up.
.a
Figure 1-Diagram of Microburet and Manipulator
The use of these solid reagents in the quantitative determination of water vapor, oxygen, and carbon dioxide opens up the possibility of their use also in the analysis of the combustible gases hydrogen, carbon monoxide, and the simple hydrocarbons. Some combustions have been made in this laboratory with fair success. The apparatus used, in addition to that which has been described above, consists of the
A N A L Y T I C A L EDITION
268
of Analysis
Table I-Results GASESANALYZED
Oxygen in air. Vol. sample, cu. mm. Percentage Oz Carbon dioxide from oxalic acid: Vol. sample, cu. mm. ' Percentage COa Carbon dioxide in unknown: VOI. sampk, cu. mm. Percenfage COz Electrolytic hydrogen: Vol sample cu. mm. Vol oxygen'for combustion, cu. m Percentage Ha Methane from cnlcium acetate and soda lime. Vol. sample, cu. mm. Vol. oxygen for combustion, CII. mm Perrentage CH4 Mixture of hydrogen and mrthane: Vol sample, cu. mm. Vol oxygen for combustion, cu. mm. Percentage Hz Percentage CHI Mixture of hydrogen and carbon monoxide: Vol. sample, CII. mm. Vol oxygen for combustion. cu. mm Percentage HZ Percentage CO
Vol. 3, No. 3
DETERMINATIONS
MEAN
Av. DEVIATION THEORETICAL PROM THEORETICAL MINUS MEAN MEAN
1
2
3
4
%
%
%
%
75 40 20.8
96.39 20.7
102.60 20.6
100.61 20.8
20.75
0.08
20 9 ( 2 )
-0 15
88.16 , 49.6
79 81 49.7
79.16 49.4
100.90 49.5
49.52
0.08
50 .'O
-0.5
75.89 27.0
84.92 26.9
75.42 26 9
87 68 27.0
26.95
0.06
63.66 96.00 100.2
66 45 97.25 100 1
68 39 106.80 100.4
67 76 110 40 100 4
100 3
0 1
100.0
4-0 3
26 11 100
+
30 48 100 97 1
+
26 05 100 96 7
+
30 17 100 98 5
97.4
0.6
100.0 (?)
-2.6
58 40
55 53 108 10 49 0 51.6
55 95 106 97 61 5 49.3
108 06
50 1 49.7
10
50 4 4 8 . 3 (?)
-0.3 -1 4
58 106 54 49
61 104 52 49
51 3 49 0
2 2 0 5
50 4 49 6
+o
97 2
106 55
51 9 48 7
58 57 93 56 49 0
48 3
35 11
0
2
a
ordinary Ruhmkorff coil of gas analysis and special electrodes constructed in the following manner: A piece of glass tubing having an external diameter of 2 mm. and about 13 cm. in length is placed within a 4-mm. tube of 12 cm. in length so that at one end the tubes are flush with one another. Then two platinum wires about 14 cm. long are placed one inside the 2-mm. tube and the other in the space between the inner and outertube in such away that both extend about 1mm. beyond the flush ends of the tubes. These ends are then carefully fused, closing the tubes and leaving two insulated electrodes about 1 mm. apart. These electrodes are broken off at the surface leaving an end of perfectly smooth curvature. Finally, the two concentric tubes are bent together in the same shape as the holder L of Figure 1, bringing the opposite ends of the wires in an accessible position for contact. I n combustion work a third gas holder is used to store the oxygen. When the proper mixtures are ready for explosion the electrodes are placed at the top of the gas bubble to minimize the possibility of oxidizing the mercury. This brings the glass of the electrode holder above the mercury surface and undoubtedly introduces a small error due to the exchange of gases on the surface. An alternative method would be to place electrodes through the top of the gas holder, but because the smooth curvature of this- is essential for the complete removal of gas, this method was not used. Discussion of Results I n Table I are given a number of experimental results which were obtained by the procedure which has been described. It will be observed that the volumes used for analysis are from 500 to 1000 times less than thwe used in macroanalysis. It is possible to use a capillary buret of smaller diameter, and consequently to use smaller volumes for a determination, but it was felt that this would disproportionately increase the errors due to surface forces. Smaller volumes for analysis are not necessary in most photochemical work where highintensity light sources are used. It was found that the volume of oxygen necessary for complete combustion was at least double that theoretically required for the reaction. This need not be measured accurately in the analysis of a single hydrocarbon, such as methane, when the percentage is determined by absorption of the carbon dioxide produced in the explosion. I n the
12 72 9 5
+
58 78
48 2
48 1
-0
9
6
analysis of a mixture of combustible gases, on the other hand, the exact volume of oxygen added must be known. I n macroanalysis the percentage of one combustible gas may be determined directly by obtaining the contraction in volume, assuming that the water vapor produced condenses until its vapor pressure at that temperature is reached and a correction made for this vapor pressure. That method was found not to apply in this work. In the combustion of hydrogen, for example, far more than enough water is formed than is theoretically necessary to saturate the residual oxygen. Nevertheless, consistent results could never be obtained until the water was removed by means of phosphorus pentoxide and the contraction in volume then read. Doubtless this was due to the surface of the glass adsorbing moisture to such an extent that the normal vapor pressure was never reached. Although every gas used in these combustions was dried over either phosphorus pentoxide or calcium chloride, no difficulty which could be attributed to the lack of moisture was experienced in getting an explosion. The glass in this case probably furnished the moisture which is generally conceded necessary to catalyze the reaction. I n the mixtures of hydrogen and carbon monoxide no spark was used for the explosion. Attempts to explode this mixture quantitatively with the gases a t room temperature were unsuccessful. However, upon increasing the temperature to approximately 300" C. the combustion was spontaneous and complete. The technic of this is simple. With the gases in a container ready for explosion, the flame of a minute gas and compressed-air torch is directed down on the top of the container, The gas bubble rapidly expands and, when its volume is approximately doubled, explosion occurs. Preliminary experiments with carbon monoxide and methane indicate that the carbon monoxide in this mixture may be determined successfully a t 300" C. but that spontaneous explosion does not occur. Note-Since submitting this article the authors have succeeded in selectively absorbing carbon monoxide in the presence of other combustihle gases by using especially prepared solid silver oxide, The details of this method will be given in a subsequent paper.
The methane sample of Table I was prepared from calcium acetate and soda lime, washed with boiled water, and dried over phosphorus pentoxide, which is admittedly not a very satisfactory procedure. It is felt that the mean value of 97.4
July 15, 1931
INDUSTRIAL AND ENGINEERING CHEMISTRY
per cent is probably nearer to the true value than is the theoretical 100 per cent. This idea is substantiated by the fact that, if 97.4 per cent is taken in calculating the theoretical per cent of methane in the hydrogen and methane mixture, it gives a value of 49.6 per cent, which is only 0.1 per cent lower than the mean of 49.7 per cent experimentally determined for this. The theoretical purities of the other gases are considered to be correct. The authors believe that Table I is representative of the degree of precision which is to be expected from this type of microanalysis. It is of the same general order as that of macroanalysis.
269
Acknolyledgment
The first named author was the holder of the du Pont fellowship of the Department of Chemistry while this research was in progress and wishes to express his appreciation for this substantial aid. Literature Cited (1) Christiansen, J . A m Chem. Soc., 47, 109 (1925); 2. anal. Chem , 80, 435 (1930). (2) Dennis, “Gas Analysis,” p. 371, Macmillan, 1920. (3) Gbye and Germann, J . chim. phys., 14, 194 (1916). (4) Langmuir, J . Am. Chem. Soc ,34,1310 (1912). ( 5 ) Reeve, J. Chem. Soc., 126, 1946 (1924). (6) Ryder, J . Am. Chem. Soc., 40,1656 (1918).
Acid Value of Cellulose Fatty Acid Esters and
Rapid Analysis of Certain Cellulose Acetates’ T. F. Murray, Jr., C. J. Staud, and H. LeB. Gray EASTMAN KODAK Co., ROCHESTER, N Y.
Some of the more important methods to be found in and Singer (14) published a INCE cellulose acetate the literature for the determination of the acetyl value modification of it. was first prepared, and of acetylated compounds have been critically reviewed. Cross and B e v a n (1) proe s p e c i a l l y when celluThe method of Eberstadt has been found to be basically posed the use of sodium ethlose acetates became of techthe best suited for work with acetylated cellulose, but oxide for saponification, pernical importance, the question it has been modified slightly in the !nterests of economy mitting the sample to stand of determining the acid values of material and time. a t room temperature for 12 of the esters p r o d u c e d has A method is described for the determination of acetic hours. Wood b r i d g e ( I 5 ) been the subject of numerous acid and formic acid in the presence of each other and found that best results were investigations. The methods in the presence of one or more acids not volatile with obtained on standing for 16 employed have fallen into two steam. hours, and &fork (Q), on atgeneral classes : acid hydrolyA method is proposed for the analysis of cellulose tempting to use higher temsis and saponification with acetate by means of which the saponification time is peratures to increase the rate alkali. hour, and the time of pretreatcut from 24 hours to of saponification, found that T h e a c i d hydrolysis was ment is cut from 30 to 15 minutes. The results obthe effect of sodium ethylate p r o p o s e d b y O s t in 1906 tained by the method are in close agreement with those as ordinarily used a t refluxing ( I O ) , and in the same year obtained by the slower method. temperatures, was to produce by G r e e n a n d P e r k i n (4) Suggestions are made for a method of precipitating acid groups in the cellulose who added alcohol, removed the cellulose acetate in order to render it readily soluble residue, thus causing high apthe ethyl ester by distillain warm pyridine. parent acetic acid values. He t i o n , and saponified it with Some of the limitations of the pyridine method are proposed the use of a 0.5 N alkali. indicated. The acid method has two solution of sodium hydroxide major disadvantages: It is in which the solvent is a mixtime-consuming and its accuracy is subject to question. The ture of equal parts of water and ethyl alcohol. various processes calling for the distillation of ethyl acetate I n 1909, too, Eberstadt (2), working under Knoevenagel, instead of acetic acid (3,4) were devised to increase the speed prepared a dissertation in which he showed that preliminary of the determination. Inaccuracies were due to the large swelling of the acetate in alcohol-water or alcohol-acetone amount of distillate that was required and the indefiniteness mixtures increased its porosity and facilitated saponification of the end point of the hydrolysis and distillation. Fre- with 0.5 N potassium hydroxide. After that time, Knoevenaquently the action of the strong acid on the cellulose residue gel (6) and his pupils did considerable work on the analysis produced formic acid which distilled with and was titrated of cellulose acetates. T o r i (IS) compared the methods of Green and Perkin, Ost, as acetic acid. Sulfuric acid was commonly used to hydrolyze the cellulose ester, and the organic matter often resulted in Woodbridge, Barthelemy, Eberstadt, and Barnett. He the reduction of part of it to sulfur dioxide, causing high concludes that Eberstadt’s method is best in principle, but he recommends saponification with 1 N alkali for 1 hour a t results. Hess (6) continued to support the acid hydrolysis method room temperature, a period which has been found to be far and with some of his pupils has devised an elaborate system too short for some cellulose acetates. for buffering his sulfuric acid before distilling, and then Kruger (8) again reviewed the literature on the determinacarrying out the distillation under reduced pressure and also tion of the acetyl values of cellulose acetates and pointed employing steam and hydrogen (6). There were, however, out that for technical purposes the alkaline saponification is objections to the method, and the following year Weltzien favored. Technical control in the production of cellulose acetate 1 Received March 7. 1931. Communication 461 from the Rodak Remade it necessary to select a method which would first give search Laboratories.
S
