Controlling Factors in Identification of Microscopic Chloride particles with Sensitized Gelatin Films FRANCES
D. PIDGEON‘
Cloud Physics Project,
University of Chicago, Chicago 37,
.4n extensive investigation of the use and the limitations of the sensitized gelatin film for the identifioation of microscopic chloride particles included s t u d y of the effects due to variations i n the preparation and thickness of the sensitized gelatin film, methods of sample eolleetion, temperature, aitd examination techniques. r . 1h e relation of particle size to halo size and growth and fading rates for sodium chloride particles are presented.
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ELY’S chemical method for the identification of individual chloride particles ( 1 ) has rwmtly been applied to the airborne particle study carried out by the Cloud Physics Project of the University of Chicago. Questions arising during the evaluation af data from this study necessitated a critical investigation of the scope and limitations of the technique. Seely’s technique is basically a modified spot test, involving a medium of gelatin and glycerol sensitized with mercurous flnosilicate. When a chloride particle is brought into contact with the sensitized gelatin by impaction, sedimentation, or otherwide, a reaction occurs farming a characteristic spot or “halo,” reeognizahle under the dark-field microscope or, when very large, h y the eye. Such “particles” need not he in the solid state hut, as in the case of natural hygroscopic nuclei generally, may range from droplets of dilute solution to dry crystals. In order to interpret the data collected from field measurements, it became necessary to know the growth and fading rates of the hrtlos, and the relation of halo size t o particle size a8 a function of both the temperature and the film thickness in a range of halo sizes extending to hundreds of microns. (The term “particle aiee” 48 used in this paper refers to the equivalent dry sise of the halide in the particle, whatever the composition of the particle may be.) Effects of a variation in examination technique on fading, detection, and identification of halos were also investigated. This included examination by means of a light microscope with and without crossed polaroids and with a cardioid dark-field condenser or Abbe-type condenser containing a dark-field stop. 3
EXPERIMENTAL PROCEDURE
Materials and Apparatus. The sensitized gelatin was prepared by the method presented by Seely (1 ), then spread in a uniform film an a glass or plastic base by means of sn applicator. Until time for use, these slides were stared a t a temperature helow -20’ C. t o prevent softening of the gelatin due to hydrolysis and/or water absorption from the atmosphere. For the chlarideparticles, reagent grade sodium chloride was ground in an agate mortar to a. fine powder. Mercurous fluosilicate, c.P., obtained through chemical supply houses is not of high purity. A rea ent of greater pdrity can be prepared by a metfod desoribed by Seely. As the variations in the purity of this reagent might have a n effect on the resulting halo size, two independently prepared reagents were employed in the sensiiisition of &e gelatin. O n e ~ w a sprepared in this laboratory and the other was obtained from Seelv. Beaause of the limited amount of the latter sample, only particles in the size range from 60 to 600 microns were investigated. Investigation Procedure. The sensitized gelatin was prepared at several times during 1
111. the investigation and stored in the freeeer to different ages prior
t o its use. The gelatin films used varied in thickness from 17 to 200 microns. Halo measurements were made a t room temperatures from 25” to 38’ C. and, in the oilse of partioles over 60 microns in diameter, again a t a temperature from 10‘ to 14” C. The total particle size range covered in this work was from 1.5 to 850 microns. In order to cover this entire range and to study the efiect of collection method on the halo Characteristics, three different collection techniques were used. Particles were embedded directly in the sensitized film by impaction or micromanipulation, OT deposited on the surface of the film by sedimentation or micromanipulation. Aeeordidg to the method of collection, the dry size of the pal,. tides used was determined by direct examination of the individual particle immediately after i t had come in contac.t, with the gelatin or, in the case of the smaller size range, a parallel collection was made on a slide coated with silicone grease. These sizes, however obtained, could then he related to such other factors as the final size of the halos and their fading rates. RESULTS
Characteristics of Halos. The halos formed by chloride particles on t,he surfltce of the sensitized gelatin me composed of relatively insoluble mercurous chloride. Growth occurs as the chloride ion diffuses outward from the particle. In t,he ease of examination without crossed polaroids, the centers of sodium chloride halos over a few microns in diameter contain fairly dense “embryos” of feathery crystals which presumably result from sodium ions precipitating as fluosilicate, These crystals vanish under crossed polaroids. Figure 1 represents an identical field examined with and without crossed polaroids. Sodium chloride halos in tho sine range helow 15 micvons in diameter usudly appear as Tyndall blue with dark-field illumination. With crossed poleroids, halo8 from 10 to 90 microns in diameter retain some blue tint. As the size increases, the halos appear whiter. Halos 500 or more microns in diameter often have a center nucleus which is slightly smaller than the size of the original sodium chloride nuoleus. This nucleus consists of a shell of the original particle, apparently composed of sodium fluosilicate from which the chloride has diffused outward during growth.
UNPOLARIZED LIGHT
POLARIZED LIGHT
Figure 1. Characteristics of Sodium Chloride Halos Halos 70,80, and 1W -mons
ia diametu, wnmined by Abbe light-fieldcondenser oonfsining dark-field qtop
Present address. Box 776, Bartlesville. Okh.
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Relations of Particle to Halo Size. Particles of sodium chloride were impacted, sedimented, or placed upon gelatin coated slides. The relation of particle to halo size was found :is described in the previous section. The growth factor vas found to he inde~)cndentof the method of collection, the scrisitized film thickness, temperature variations within the range of 10" to 38" C., the sensitized gelatin mixture (as long as the same mercurous fluosilicate reagent is used i n the preparation of the gelatin), and the age of the sensitized gelatin film. The use of the two separately prepared mercurous fluosilicate reagents in the preparation of the - .seiisitized gelatin crratrd a very significant I , ' 0 20 40 60 80 100 I20 I40 160 iB0 200 2 X , 240 260 280 3W 320 540 360 effect, apparently due to a difference in TIME (MINUTES1 purity oi the two reagents. Figure 3. Growth Rates of Three Typical Halos Tlw relation of particle hue to halo size is Resulting from NaCl particles 90. 120, and 170 microns in diameter pre~entctdin Figure 2 and compared n-ith remlts of Seely's ( 1 ) investigations. Seely included only a particle size range from 0.5 to 1.5 microns in diameter. In the case of both gelatin niisI ' tures, the log of particle size is found t o be a smooth function / of the log of halo size over t,he entire particle size range in/' vestigated. The results obtained coilverge on Seely's values in - 5 the lowest size range. The spread in points encountered in the results may be due to the following:
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The relation of halo size varies directly with the particle volume. The plot of log of particle diameter versus log of halo diameter interprets this relationship only if the particle diameter takes into account the three dimensions of the particlr. The diameter used during this investigation is the diameter of a circle of area equal to that of the projected area of the sodium chloride particle. As particles under 60 microns in diameter were collected on slides coated with silicone grease, a tacky medium, random orientation was obtained. As the number of particles measured increases, the diameters will more nearly approach that of the
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/ R E L A T I O N OF P A R T I C L E SIZE TO HALO SIZE
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Time Required for Complete Growth of Sodium Chloride Halos
actual particle. The error is the least for measurements in this particle size range under 60 microns, because a comparatively large number of particles lvere measured. In the size range over 60 microns individual particles \vere measured. The short time lapse between the depositing of the part,icles on the film and the actual measurement may introduce a small error in particle diameters. .4s only a small number of particles were present in the upper and lower ranges of each fract,ion under BO microns in diameter, slight displacements in t,he relationship of particle to halo size may be expected. (Results in the upper and lower 5% were not used, for this reason.)
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Relation of Particle Size to Halo Size
Two separately prepared mercurous fluosilicate reagents. Results below 1.5 microns were found by Seely
Growth Rates of Sodium Chloride Halos. The growth rates of sodium chloride halos were investigated for a sodium chloiitle particle size range of 60 to 850 microns in diameter. The individual halos were followed during the period of growth until equilibrium was attained. Figure 3 shows the growth rates of sodium chloride halo? on film thicknesses over 24 microns for three typical sodium chloride particles 90 to 170 microns in diameter. -4very rapid growth takes place during the first 10 minutes x i t h a decreasing rate thereafter. Figure 4 presents the time lapse necessary for the conipl~te
ANALYTICAL CHEMISTRY
1834
No significant dependence of the fading rates of sodium chloride halos on the following variables was observed: the preparation of the sensitized gelatin film, the age of the sensitized gelatin film, or the temperature variation from 20' to 35' C. However, a very significant decrease in fading rates was observed in the cme of halos located near the edges of gelatin films. The films a t these edges were found to be a p proximately 15 microns or less in thickness. In this case observations using the cardioid condenser indicated that after 6 hours halos 20 microns in diameter still had not faded. A further advantage of the use of very thin gelatin films is that the nuisance of background material is minimized. However, as these films are difficult to prepese in a routine fashion, the results cover thicknesses of 40 to 200 microns. It was Figure 5. Characteristic Sodium Chloride Halos 16 Minutes after Collection found immssible to substantiate Seely's findings of homogeneity in the gelatin or to prepare sensitized Examined hy Ahhe light-field condenser ~ontsiningdark-field stop gelatin free from an unidentified crystalline material which made dark-field work, especially with thick films, very difficult. growth of sodium ohloride halos on film thioknesses from 24 to Fading rates for sodium chloride halos are shown in Figures 6 200 microns. The time of complete growth is less than 5 minutes and 7. Figure 6 presents fading rates observed when examins, for chloride halos less than 450 microns in diameter (particle tion is made a t a low magnification with a light-field condenser size less than 40 to 60 microns in diameter). The growth rate in which a dark-field stop has been inserted. Figure 7 presents of these halos was found independent of the method of collecfading rates observed when examination is made a t the higher tion, the sensitized gelatin mixture, the age of the sensitized gela 'resolution obtained with the cardioid condenser. tin film, the mercurous fluosilicrtte reagent, and the temperature A comparison indicates, as expected, that one may reduce the variation. No significant effect was observed on growth rates effect of fading by more than 50% a t a given time by virtue of for the oase of sensitized film thicknesses from 24 to 200 microns. the higher resolution made possible with the cardioid condenser. However, a very definite decrease in growth rate was noted in Observed fading rates decrease with time, until after 8 hours the case of the film thickness of 17 microns or lower. they become very small and after 24 to 32 hours they became Fading of Sodium Chloride Halos. A significant amount negligible. No signifioant fading is observed for halos over 150 of fading is found to occur in sodium ohloride halos. The informicrons in diameter when examined by either dark-field method. mation resulting from this investigation i s much more extensive This limiting diameter is decreased to 60 microns when the than that presented hy Seely ( 1 ) . Indication is found that the cardioid condenser is employed. fading established definite limitations on the application of this identification method for sodium chloride particles, and that it is more pronounced than indicated by Seely. SUMMARY A N D CONCLUSL. When fading occurs, the blue halo previously described graduUsing 8, gelatin film between 24 and 100 microns in diameter, ally becomes fainter until it can no longer be observed. Only none of the parameters investigated appear to affect the relation an embryo of feathery crystals remains. With the cardioid conof particle size t o halo size, the growth or fading rates, or the denser, one sees halos which appear to be completely fsded when visibility of sodium ohloride halos a6 long as the same preparation examined with a light-field condenser containing a dark-field of mercurous fluosilicate reagent is employed in gelatin sensitizas t o n Even with the cardioid condenser the smaller halos eventually fade so completely that they no longer o m be observed. Figure 5,A shows a field examined 16 minutes after the collection of the sodium IY) chloride particles. By careful examination the nay be seen on outer portion of the two halos I no the lower right-hand side, though fading is illmast IU complete. However, the embry08 m y be obm4 served with ease. When examined with crossed polaraids, the .. same sodium chloride halos appear only a8 a solla blue disk (Figure 1,B). No central embryo can be seen. Halos less than 15 microns in diameter, lacking the embryo, appear roughly the same with or without crossed palaroids. F- ----i m w 5->- . R nortravs the pame field as Fieure 5,A , examined 40 with crossed polaroids. Again tht: :photograph was taken 16 minutes after the call.ection of the M "^Ai___ " L L " < A " .."..&"I"" V . . " . , 1. U V U L U l l L rlll"ll"r pa, Y l n r " . U * C I I uy careful examination, one is unable to observe th at the two small halolos revealed a t the right in Figure 5,A had existed. Individual sodium chloride halos 20 M 40 a. M 70 PERCENT OF "ALOE FmED up to 180 microns in diameter w ere observed Figure 6. Fading Rates of Sodium Chloride Halos for up to 32 hours, a t which time tlle fading rate
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Examination msde u a i n s daik-field stop
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VOLUME 2 6 , NO. 11, NOVEMBER 1 9 5 4
ameter). The differentiation of the embryos from other spherical particles is, however, difficult. (CARDIOID DARK FIELD CONDENSER) When identification of particles 8 microns or less in diameter is desired, slides must be examined almost immediately after collection. However, if the actual size of the particles over 40 microns in diameter (producing halos over 500 microns in diameter) is of interest, a time lapse will be necessary for complete growth. When examination cannot be undertaken immediately, or the sampling period is long, the method appears to be applicable only to particles over 5.0 microns in diameter. If it is necessary to identify particles as small as 5.0 microns in diameter, examination must be made by means of the cardioid condenser. Any procedure which would enable one to prepare consistently films of the sensitized gelatin having very little visible background would enable I I I I I I 1 more rapid examination techniques with greater 0 0 20 30 40 50 60 70 80 I d , PERCENT OF HALOS FADED ease in identification of chloride halos. Where Figure 7 . Fading Rates of Sodium Chloride Halos samples are collected a t high humidities and temperatures, a less hygroscopic base is desirable. Examination made using cardioid dark-field condenser Under such conditions, the gelatin-glycerol medium mas found to soften in short periods of time. tion. Homver, extremely thin films of 17 microns and less were Polyethylene glycol and gelatin might provide a more suitable found to decrease very significantly both the rate of growth and base in this instance. the rate of fading of these halos. ACKNOWLEDGMENT Because of the background in the sensitized gelatin films, the visibility of small halos is a function of film thickness. Until The author xishes to extend thanks to Guy Goyer and Jamep films of gelatin are prepared consistently with very little visible P. Lodge of this laboratory for their encouragement and advice background, film thicknesses in the range of 40 to 70 microns in carrying out this investigation, and to B. K. Secly, Kew Mexshould be employed. Films of this thickness are not difficult ico Institute of Mining and Technology, Socorro, N. X., for to prepare, and will provide good visibility of halos under 60 the use of a sample of mercurous fluosilicate prepared in his labmicrons in diameter. oratory. Applications to air-borne particle studies seem to be limited LITERATURE CITED by the fading rates of the smaller particles. Examination with crossed polaroids has the advantage of eliminating from the (1) Seely, B. K., ANAL.CHEM., 24,576-9 (1952). field of view many of the foreign particles, but prevents the deRECEIVED for review January 15, 1954. Accepted August 9, 1954 Retection of faded halos. Examination without crossed polaroids search sponsored b y the Geophysics Research Directorate of the Air Force permits the detection of the embryos of faded halos in the size Cambridge Research Center, Air Research and Development Command, range over 15 microns in diameter (particles 1.7 microns in diunder Contract AF19(604)-618 FADING RATES OF N A C L HALOS
Colorimetric Determination of Carbonyl Sulfide in Synthesis Gas L. A . PURSGLOVE' and H. W. WAINWRIGHT Synthesis Gas Branch, Bureau o f Mines, Morgantown,W. Va.
A procedure for determining carbonyl sulfide in synthesis gas is based on hydrolysis of this compound in a dilute base and determination of the sulfide ion by the methylene blue procedure. The method is especially useful in determining the concentration of carbonyl sulfide in synthesis gas containing other common organic sulfur compounds. Carbon disulfide, ethyl mercaptan, and thiophene do not interfere.
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N THE course of research and development on the gasification of coal it has been necessary to develop analytical methods for determining the various components in the synthesis gas produced (10, 11, 15, 19). The more common gases may be determined by the usual Orsat analysis, but the gaseous sulfur 1
Present address, Dow Chemical Co., Midland, Mich
compounds arising from the sulfur in the coal gasified require more complicated analytical procedures. Part of the program of the Bureau of Xines deals n i t h the removal of these sulfur compounds in order to render the gas suitable for the production of liquid fuels, natural gas substitutes, ammonia, and other chemicals ( 1 7 , 18). Pilot plant studies on removing impurities from raw synthesis gas revealed that adsorption on activated carbon is an efficient and economical way of removing all organic sulfur compounds from the gas ( 1 1 ) . Such treatment reduces the total sulfur content of the purified gas to less than 0.01 grain per 100 standard cubic feet. Of the sulfur compounds remaining in ram synthesis gas after hydrogen sulfide has been removed, carbonvl sulfide is, generally, the only one that is a gas under normal conditions ( N . T P ), and is the compound that determines the length of the adsorption period before regeneration of the carbon is necessary. It is obvious, therefore, that a method for determining the carbonyl sulfide content quan-
