V O L U M E 2 6 , NO. 7, J U L Y 1 9 5 4 Table I. Sample
1199
Spectrographic Determination of Germanium on Low-Rank Coal Samples Ashing Temp., C.
Ashing Time, Hr.
Ash,
%
Ge in Ash, % a (Spectrographic)
Gradual Heating of Samples 200 4 68.80 3.90 4 500 4 3.87 800 3.80 2 1000 99.94 4 200 4 1.26 500 1.17 4 800 1.28 2 1000 4 85.44 200 4 6.71 400 4 6.78 500 6.70 4 800 1000 2 6.62
E
E
1.12
o:oio
0.013 0.012 , . . b
0.60 0.60 0.60
0.56
Rapid Heating of Samples 4 3.83 800 4 1.09 800 4 6.66 800
R 0
R 0
1,
1'10 1 12
1.10 0.010
0.59
Rapid Heating of Samples in Wide Platinum Dishes 1.13 1000 1 3.70 0.018 1000 1 1.28 0.62 1000 1 6.62
Rapid Heating of Samples in J. L. Smith Crucibles (Small Surface .4rea) 2 3.70 1.10 R 1000 2 1.28 0.012 0 1000 2 6.62 0.61 E 1000 Calculated on baais of ash prepared a t 500° C. b No spectrographic germanium results obtained, owing t o high organic content remaining in sample.
Low-rank coal samples weighing 1 gram were placed in a muffle furnace a t 25' C., and the temperature was gradually increased to 200° C. and held for 4 hours. The samples were cooled and weighed, and a portion removed for spectrographic determinations of germanium. ,4similar procedure was repeated a t 400", 500°, 800", and 1000" C. for sample E; and a t 500°, SOO", and 1000" C. for samples R and 0. Because heating these coal samples to 500' C. and 1000" C. yielded approximately the same weight of ash, the spectrographic comparisons for germanium were made on 10-mg. samples of ash. Gradual heating indicated no detectable loss of germanium even a t the final high temperature. Loss of Germanium by Rapid Heating. The procedure consisted in rapidly heating the -80-mesh coal samples iu porcelain
crucibles over Bunsen burners for 45 minutes, final temperature a t 700 to 800' C., then transferring to a muffle furnace already a t 800' C., and holding the samples a t this temperature for 3.25 hours. The samples were cooled and weighed, and the germanium content was determined spectrographically. The data obtained by this test indicated no detectable loss of germanium. Large versus Small Surface Areas. This experiment was to determine the loss of germanium when ashing was accomplished by the rapid heating of samples in widemouthed containers, where large surface areas were exposed to the air, and also samples with small surface areas. If a large surface area of sample is exposed to air, it is to be expected that, even with rapid heating, any compounds of germanium would be oxidized rapidly t o germanium dioxide which ia not volatile, and therefore, no loss of germanium would occur. Accordingly, 1-gram samples of similar particle size were spread in thin layers in 260-ml. platinum dishes and placed in the muffle furnace at 1000" C. for 1 hour. The results indicate no loss of germanium under these conditions. To test for loss of germanium from samples heated under smallsurface-area conditions, 3 to 5 grams of the coal samples were dried a t 100" C., packed in J. L. Smith crucibles, and placed in a muffle furnace a t 1000" C. for 1 hour. rl violent evolution of gases was observed which persisted for several minutes. Then the samples were transferred to platinum dishes and heated for another hour a t 1000' C. There was no detectable loss of germanium under these experimental conditions. The results of all experiments are given in Table I. LITERATURE CITED (1) Ahrens, L. H., "Spectrochemical Analysis," p 216, Cambridge, Mass., Addison-Wesley Press, 1950. ( 2 ) Crossley, H. E., "Occurrence a n d Significance of Certain Minor Constituents of Coal," See. 3, thesis. University of London, 1949. (3) D e p t . Sci. Ind. Research. London. Fitel Research 1049-50, (1951). (4) Goldschmidt, V. M., and Peters. C., Abhartdl. Ges. Wiss. Gatingen, Math.-physik. KI., 3, 141 (1933). ( 5 ) Morgan, G., and Davies. G. R., J . SOC.Chem. I d . ,56, i l i - 2 1 (1937). (6) Rotynskii, U. M . , Compf. r e n d . mad. sc1'. C.R.S.S., 40, 198-201 (1943). RECEIVED for review October 14, 1953. Accepted hlarch 23, 1954. Publication authorined by the Director, C . S.Geological Survey.
Gasometric Method for Determination of Hydrogen in Carbon W. G. GULDNER and A. L. BEACH
&I1
M
Telephone Laboratories, Murray Hill,
N. J.
ICROCHEJIICAL determinations of hydrogen in organic compounds are usually performed by some modification of the gravimetric Pregl method. It was found here, however, that this method does not provide data of adequate precision for hydrogen in carbon samples with low hydrogen content. Because of the need for correlating the chemical and physical properties of carbon prepared by a variety of methods, it became necessary to develop a new technique which would provide precise determinations of hydrogen in carbon. In most cases, the amounts of hydrogen in carbon were too small to be measured by conventional procedures. This low pressure gasometric method described has provided data for the range of 0.0004 to 3.5% hydrogen in carbon. This method involves the combustion of the carbon in a low pressure of oxygen while the hydrogen is converted to water, the
separation of the water vapor from the carbon dioxide and excess oxygen by selective freezing, and the measurement of the water vapor in the gas phase by means of a calibrated closed-end manometer. Recently, ?;aughton and Frodyma (2) used a similar technique in measuring carbon dioxide and water vapor in their modified Pregl apparatus for determining carbon and hydrogen in organic materials. This technique has also been used by Holowchak and Wear ( I ) for the determination of oxygen aa carbon dioxide in organic materials. APPARATUS
This apparatus, developed solely for determining hydrogen in carbon, is shown in Figure 1. In general, the apparatus consists of a purification train for the oxygen used in combustion of the sample, a combustion furnace employing induction heat-
ANALYTICAL CHEMISTRY
1200
ing, a series of calibrated closed-end manometers to cover the range of hydrogen being analyzed, and a vacuum source comprising a single-stage mercury diffusion pump, P, backed by a mechanical oil pump (Cenco Megavac, Central Scientific Co.). Purification of Oxygen. The oxygen is carefully admitted fyom a tank to the evacuated system by a diaphragm reducing valve, through stopcock SI and the palladiumized alumina catalyst in F1, and a double liquid nitrogen trap, T I . The catalyst is preconditioned by heating in a vacuum for a t least 0.5 hour a t 1000° C., and is operated near room temperature during the purification. The catalyst is held within the furnace area in a clear quartz tube which is connected to the borosilicate glass system by means of a seven-step seal. The oxygen does not liquefy in the liquid nitrogen trap because the pressure is never much above 5 cm. Oxygen is admitted to the combustion furnace through the capillary (2 X 30 mm.) mercury cutoff, CI, so designed to facilitate the admission of gas under pressure to an evacuated system. The oxygen flows over the sample, through the freezing traps, T2, and out through another capillary cutoff, C4, to the pumping station. The traps are maintained a t -78" C. with a dry ice-Cellosolve acetate mixture in order to freeze out the water formed from combustion of the hydrogen in the sample. Combustion Furnace. The combustion furnace, F,, is similar in design to one previously described in an apparatus for determining carbon in metals (3). It differs in that the oxygen is admitted through the tube used for loading the samples, so as to sweep out carbon dioxide and water. The sample is burned in a flow of oxygen in order to obtain complete combustion. The platinum crucible is of standard form with reinforced rim and has a volume of about 20 ml. A bail of 0.050-inch platinum wire has been welded to the crucible to provide a means of sus ension in the furnace. The crucible is suspended from the gLss tripod by platinum wire. The carbon samples are usually in the form of granules and are weighed in platinum bags made of 150-mesh gauze, 10 mm. high, and 7 mm. in diameter. The bags are closed by pinching the top edges together and can be used over again. The platinum crucibles
and bags mere obtained from Baker and Co., Inc., Newark, S . J. As shown in Figure 1, the samples are inserted into the glass side arms, H , which are then sealed off. This permits precombustion of the platinum crucible to obtain a very low blank. The sample bags are then selectively pushed out of the side arm by means of a glass-enclosed iron slug operated by the magnet, R . During the combustion of the sample, the hydrogen is converted to water which is frozen out in traps 2'2. The high frequency unit, used for heating the platinum crucible, is a vacuum tube oscillator with less than 2 kw. output and operates a t approximately 500 kc. This unit was constructed by the \Vestern Electric Co. The induction coil and the glass furnace are air cooled by means of a small blower. As shown, mercury cutoffs are employed throughout the system, except for S 1 and Sg, which are so positioned that they are protected by a liquid nitrogen trap and diffusion pump, respectively. All stopcocks are carefully lubricated with Apiezon grease L. Trap Tt should be maintained a t liquid nitrogen temperature throughout the analysis. Analysis System. The hydrogen is measured as water vapor by a closed-end mercury manometer. Two manometers, J I , and Mg, are employed, each of a different volume to facilitate the measurement over the expected range under study. The volume of each manometer is calibrated by weighing with mercury before it is attached t o the system. The volume of a given manometer may be changed by raising the mercury level to any one of the three calibrated marks. The two manometers used for most of this work have had volumes of 130, 140, and 150 and 20, 30, and 40 ml., respectively. For higher precision in the very low range of hydrogen content, the larger manometer was replaced with one having a volume of 2, 3, 4, and 5 ml. The McLeod gage, G, is used only for checking the degree of vacuum mm. with prior t o an analysis. It has a sensitivity 2.3 X a range up to 0.3 mm. PROCEDURE
The tared platinum bags are filled with the carbon sample, crimped shut, and reweighed. The weight of sample used is of the order of 0.1 gram. Twelve such samples have been sealed
Figure 1. Apparatus for Determination of Hydrogen in Carbon
V O L U M E 2 6 , NO. 7, J U L Y 1 9 5 4
1201
into the system a t one time, six in each side arm. It was found essential to use the platinum mesh bags, because the carbon has a high static charge which causes the particles to jump and stick to the glass walls. Also, during the firing, some of the sample would be swept from the crucible as a result of the oxygen flow and the rapid evolution of carbon dioxide gas due to combustion. In addition, it was found that the platinum catalyzes the combustion of the sample. The apparatus between stopcocks Si and Ss is evacuated. After the evacuation, the resistance furnace, Pi, is turned on until a temperature of 1000" C. is obtained. This serves to remove sorbed gases from the catalyst. This step is required only after exposure of the catalyst to air. Liquid nitrogen is placed about the trap, T3,the stopcock, SI,is opened to pump for 2 to 3 minutes to flush out the oxygen line, and then the resistance furnace is shut off and allowed to cool to room temperature. After the temperature of the catalyst is less than 500" C., the mercury in cutoff C1 is raised by opening S pto air. Evacuation of the remainder of the system is continued until a vacuum of the order of 2.3 X 10-5 mm. is obtained. The time required to outgas the system and the carbon samples may be as long as 7 hours, depending largely on the nature of the carbon granules. To make an analysis, the level of C1 is adjusted to 5 cm. above the capillary and C4 is adjusted to 2.5 cm. This differential is chosen to eliminate any danger of back diffusion of gas from the combustion furnace to the purification train. The mercury is raised in M p and G and about 30 cm. in M I in order to serve as a pressure gage. The Dewar flask containing the dry ice mixture is raised about traps T z , liquid nitrogen is raised about Ti, and the sample is injected into the crucible by means of the bar magnet. The stopcock, Si, is partly opened, until a flow is obtained that registers a pressure of 4 cm. of mercury on the manometer. The crucible is heated by induction to 1000° C. for 10 minutes, which is more than ample time for combustion of the sample. During this combustion, the water vapor is condensed in the dry ice traps and the noncondensable gases, mainly oxygen and carbon dioxide, are swept out to pump. The induction unit and the oxygen flow are shut off and the mercury in the capillary cutoff, Cl, is raised. Evacuation of the system is continued by pumping through the capillary in Cd to prevent the loss of water by mechanical sweep through traps, until the pressure as indicated by the manometer is nil. Then, Cz is raised to cut off the furnace, C4 is lowered below the cutoff, and the evacuation is continued for a period of 10 minutes, followed by a final check of the pressure with the McLeod gage. The mercury is lowered in Mi and raised in C,. The water is transferred from the traps by removing the dry ice from 2'2 and placing it about the side arm of manometers MI or M 2 , depending on the amount of water vapor to be measured. With the water,vapor condensed in a given manometer, the mercury is raised to a given calibration mark and the dry ice removed. After the water vapor has reached room temperature, 25' C., the pressure is measured a t the three calibrated volumes. However, if the vapor pressure of the water is approaching that a t room temperature, it may be observed that the product PV does not check a t any two volumes. This is because the water vapor is in both the gas and liquid phase. Then, in order to make a quantitative measurement, part of this water vapor must be returned to T 2 ,by placing dry ice about the traps, or transferred to a manometer of larger volume. In many cases, measurements have been made in several portions, each portion in turn being removed by opening to the pump. After analysis of each sample, a pumping time of about 2 to 3 hours is required to remove the water vapor from the glass system. In a like manner, a blank is determined on the platinum crucible and associated apparatus prior to the combustion of any sample. The side arms provide a means of multiple loading, so that as soon as one sample has been measured and the water removed, the next sample can be injected into the crucible without opening the system to the atmosphere. As many as twelve samples have been determined with a single loading of the side arms. Knowing the volume of the manometer a t each of the calibrated points and the pressure due to the water vapor, the percentage of hydrogen can be calculated, plVl being the values obtained as a blank.
( P mm. X V ml.)
(Pi mm. X V i ml.) weight of sample (7)
c.
=
c. X 0.1084 X 100
% hydrogen
All measurements were made in a temperature-controlled room a t 25' C. If this work is to be carried out a t some other temperature, proper corrections must be applied.
Table I. Source Polymeric
Hydrogen in Carbon
Range of No. of 70 Sample Determi- Hydrogen, Weight, Rlg. nations Average
Anthracite Pyrolytic
74-76 58-77 100-102 101-112 91-99 200-275 267-294
3 3 6 8
3.552 3.172
3
2 2
0.631 0,339 0.0107 0.0027
149-174
8
500 480 500 500 480 480 480 500
2 2 2 2 2 2 2 2
Mean Deviation
Standard Deviation
0.006 0.033 0,008 0,007 0,005 0.0009 0.0005
0.0068 0.0400 0.0101 0,0078 0,0059 0.0009 0.0005
0.207
0.003
0.0031
0.00194 0.00188 0.00160 0.00160 0.00076 0.00070 0.00044 0.00038
0.00003 0.00000 0.00008 0.00008 0.00002 0.00002 0,00003 0,00008
0.00003 0.00000 0.00008 0.00008 0,00002 0.00002 0,00003 0.00008
0.805
EXPERIMENTAL DATA 4 N D DISCUSSION
The data in Table I show that hydrogen has been analyzed in carbon from 0.0004 to 3,5%. The bulk of the samples, however, have a hydrogen content of the order of a fen- tenths of 1% or less. The blank, which is the amount of hydrogen collected as water vapor during the specified combustion cycle, is nil. The water collected is much too small to be measured by means of a manometer. I n view of the small vapor pressure of water a t -78" C., precautions were taken by inserting two pairs of traps, Tz, into the apparatus. I t has been found by experiment that no measurable amount of water vapor is swept through the traps during an analysis. 5.0
I .o
5
PERCENT HYDROGEN I N CARBON 0 PYROLYTIC D ANTHRACITE 0 POLYMERIC
0.5
(u
z
0
23
0.1
yz
0.05
0
e k
3
0.01
V
?
0.005
0.001
0.0005 0.0002 0.0002
Figure 2.
0.001
0.01 0.1 PERCENT HYCROGEN FOUND IN A.M.
1.0
5.0
Comparison of Duplicate Determinations Effect of evacuation time
+4s outlined, the samples were outgassed in a vacuum for a period up to 7 hours. Because of the marked affinity of carbon for most gases, one carbon having 0.63% hydrogen was divided into eight samples; four were analyzed, after a bake a t 200' C. for 2 hours in vacuum, and compared with those which had only the normal vacuum outgassing. The average hydrogen content checked within 0.001%. Based on the data obtained on this sample, it would appear that the outgassing time could be reduced by baking the samples without loss of hydrogen. Also, these data indicate that no measurable water was left on the carbon after a vacuum degassing a t room temperature.
1202
*
ANALYTICAL CHEMISTRY
For the samples containing up to 0.002% hydrogen, approximately a 500-mg. sample was used, and for those containing more, a correspondingly smaller sample was used. The precision of this method for hydrogen in the range of 0.2 to 3.5% is about 1% of the hydrogen present, and in therange of 0.0004 to O.Ol%, the precision is about 7%. It is difficultto assign a value for the accuracy of this method, because there are no carbon standards available. Attempts to use organic materials which would be suitable for standardizing the apparatus have been found to be unsatisfactory for combustion in this apparatus. These materials, such as benzoic acid, have a fairly high vapor pressure a t room temperature and are lost during the long initial evacuation and conditioning of the system. These materials, when injected into the hot crucible, sublimed in part to the glass walls before combustion was possible. The apparatus and procedure described is suitable for analyzing carbon only. I n view of this, the data have been scrutinized statistically. In most cases, the per cent of hydrogen in one sample was measured in the morning and the duplicate in the afternoon. These duplicate values are plotted in Figure 2. As shown, the scale is the same in the abscissa as in the ordinate and the line is drawn arbitrarily a t 45'. Then, if the value obtained in the morning is the same as in the afternoon, the point, representing a pair of determinations, will fall on the line. If the value obtained in the afternoon ia higher than that obtained in the forenoon, the point will fall above the line. Likewise, the converse is true. -4s shown, the points lie very close to the line over the entire range of hydrogen studied, 0.0004 to 3.5%. I n the insert, these same points, representing pairs of determinations, are more clearly defined and plotted above and below the line regardless of magnitude. For the pyrolytic carbon with low hydrogen content, there are eight points, three above the line, one on the line, and four below. Thus, there is a random distribution of points. However, in the samples of higher hydrogen content, there are 21 points, 16 above, 1 on, and 4 below the line. This behavior, to give higher results in the afternoon than in the forenoon, must be attributed to some assignable cause. After cardul ecrutiny of the procedure, it is believed that this behavior is purely the result of a longer pumping prior to an analysis of the
glass station in the forenoon than in the afternoon. This extended pumping of the glass system continues to remove water vapor. These glass surfaces will then sorb more water formed from combustion of the sample. This belief is further substantiated when the hydrogen values are examined in the low range and when the pumping or evacuation time was about the same between determinations. Although the amount of hydrogen found in the forenoon is lower than the amount found in the afternoon, the mean average and standard deviation of all the forenoon values of hydrogen are not significantly different from those obtained in the afternoon. Because of the magnitude of this difference, this behavior is purely of academic interest and has no practical significance in correlating chemical-physical relationships in carbon. The method described is an example of the contributions of vacuum or low pressure techniques to analytical methods. It provides a means of determining amounts of hydrogen in carbon much too small to be measured by conventional techniques. The features which help to make this a precise method for determining hydrogen in carbon are: I t is a unitized piece of equipment. It has high sensitivity, inherent in vacuum techniques. It has multiple loading of samples with magnetic injection. I t employs induction heating. I t utilizes platinum mesh bags to hold the sample and catalyze the combustion. ACKNOWLEDGMENT
The authors are indebted to JValter A. Shewhart and Margaret
C. Packer of these laboratories for their assistance and suggestions in interpreting the data. LITERITLRE CITED
(1) Holowchak, J., and Wear, G . E. C., ANAL.CHEM.,23, 1404-7 (1951).
(2) N a u g h t o n , J. J., and F r o d y m a , M. M., Ibtd., 22, 711-14 (19.50). (3) Wooben, L. A,, and Guldner, W. G . , IND. ERG.Camif., Ax.4~. ED.,14,845-8 (1942).
RHCEIVED for review August 20, 1953. Bccepted March 5 , 1954. Presented a t the Pittsburgh conference on Analytical Chemistry and Applied Spectroscopy, March 2 to 5 , 1953.
Polarographic Determination of Zinc in Plant Materials 0. N. HINSVARK, WhA. H. HOUFF, S. H. WIllWER, and H. M. SELL Departments o f Horticulture and Agricultural Chemistry, M i c h i g a n State College, East Lansing, M i c h .
T
HIS laboratory's interest in the apparent requirement of zinc for the synthesis of tryptophan and indirectly for the synthesis of indole-3-acetic acid (8) suggested the need for a rapid and accurate method for a quantitative measurement of zinc in plant material#. The published polarographic procedures involve rather laborious separation techniques, with the final measurement still subject to interference of other ions (4, 5, 7 ) . Utilization of the polarographic wave of the zincate ion (6) and the prevention of the coprecipitation of the zincate ion ( 3 ) by the addition of the tetrasodium salt of ethylenediaminetetraacetic acid (sodium Versenate) form the basis of the described method for the polarographic determination of zinc in plant materials.
was circulated by a centripetal pump through the jacketed polarographic cell. The capillary had a flow rate of 1.989 mg. of mercury per second and m*/3t1'6 = 1.569 mg.2'3 sec.1'6 a t -0.1 volt applied potential. REAGENTS
Analytical reagent grade sodium hydroxide was employed in the preparation of the solutions. The Versene obtained from the Bersworth Chemical Co. was recrystallized from hot formamide. A 1M solution of the tetrasodium Versenate was prepared by adding the required amount of sodium hydroxide to this recrystallized acid. All solutions were prepared with zinc-free distilled water. The reference curve was prepared using Fisher's tested purity zinc acetate which was dried a t 100' C. Triple-distilled mercury was used for the electrodes.
APPARATUS
A Sargent Model X X I polarograph with water-jacketed mercury anode cells was used for this investigation. A constant temperature bath was maintained at 30" 0.5" C. from which water
PROCEDURE
Ashing of Plant Material. TWO grams of an appIe leaf sample which had been analyzed for zinc previously by conventional
.
