Determination of Permephilitv nf A rrriciil iirsl Spray Coatings t c C. L. COMAR' AND E. J. MILLER, Michigan Agricult respect to all the variables: rather.. emphasis LS been placed . upon study of the precision of permeability measuremenis as a function of experimental technique with a view towards arbitrary strtndardimtian of conditions. It was desirable that the eaperimental conditions be easily maintained and readily capable of duplication, and that factors which might cause uncertainties in the results be controlled. Such considerations led to a satisfactory procedure.
A simple empirical method capable of yielding reproduoible permeability m-surements on films formed from wax and oil-spray emulsions ofthe oilin-water type is desorihed. The experimental conditions a r e easily duplieated. For the type of film studied a linear relationship exists between moisture impedance and film thickness. Advantages in the use of paraffin wax are demonstrated and the role of bentonite in the emulsion is indicated.
Experimental PERMEABILITY MEASUREMEW.A critical survey of procedures used for this ty of measurement up to 1937 has been
&
and the more recent experimental published by Carson methods have been reviewed by Lishmund and Siddle (9). In the method reported here the material to be tested is fastened over the mouth of a cup containing water and the loss of moisture through the sample to a drier atmosphere is determined by weighing. The Payne permeability cup (ZO), which is uvdlable oommeroially, was used for the measurements. It was found convenient, however, to replace the usual clamps with steel ones, since under constant usage the thread8 on the originals were easily stripped. Desiccators of the type shown in Figure 1, with an inside diameter of 250 mm., were used. The cups are supported within the desiccator bya. l.Q-cm. (0.75-inch) plywood board which acoommodates four cu s and has B receptacle for desiccant in the center (see Figure 1). %e cup support has an outside diameterof 233 mm.; the diameter of the center reoeptaole, which is made of metal, is and it is 7 to 8 mm. deep. The holes far the cups are 40 100mm. in diameter and are s y m e t r i c d l y placed, the center of each hole being 25 mm. from the outside edge. The relation between cup posit%n and the surface of the desiccant is then essentially
I
N THE course of studies on the development and use of emulsions far spraying plant materials to retard desiccation,
it b e c m e necemry to determine the efficiency of filmsformed from suoh spray materials in reducing the passage of water vapor, Information on the interrelationships of film perme ahility, film thickness, and the chemical oomposition of the emulsion is indispensable for the formulation of spray materials hsving desirable physical charaoteristios. Methods described in the litemture were not directly applicable to the type of m a t e d under consideration here from the point of view of preparation of the test film, preoision of the measurements, and reproducibility of experimental conditions. It ws8 therefore necessary to develop a more sstiafaotory procedure. Examination of the literature (S), a8 well as lahoratory experience, has indicated the many difficulties encountered in obtaining reproducible values from what appears to be a simple physied measurement. Uncertainties may be due to irregularities in the test film or to lack of consideration of the factors involved in the permeability measurement. In the method herein presented these difficulties have for the most part been eliminated by an emoting uniformity in procedure, The method is a simple one, snd although designed for a specifickind of surhce covering, it should be generally applicable to measurements on various types of coatings, perhaps with slight mcdification of detail in some cases. The apparatus required is readily availshle to most Iahoratories and if all precautions in technique arc observed there should he no difficulty in obtaining reliable comparative measurements. The passge of water vapor through B film bas been conaidered as an ordinary di5usion process, in which case the following relation obtains:
...--..""n.+ F r a w ~1.
where M = mass of water vwor that passes through the film A = area of film surface 1 =time P, - P, = water va or presslue differential across the film = film thiotness IC = constant for the film material
PERMEABILITY APPLEATUS
,bsi**.n+.
z
n.rmo*hili,."
ml".
rnd lnaded deaierator
Dowflake (a teobnicd grade of ealoium chloride obtainable from the Daw Chemical Company) was found satisfactory as t h e ami*o.+ino solmi T i k ..~ n O_-___. peq~~ i. n ~ me . y fresh desiccant in t h e . center of the cup support for each run and it is recommended that the Dawflake for this purpase be stored in small completely killed and tightly &sed bottles, each containing enough desiccant for the center compartment (about 57 grams). The Dawflake in the lower part of the desiccator is changed ahout once a week; this desiccant is eontsined in a met$ rece tacle for convenience m d cannot he seen in the illustration. $he desiccators themselves are kept in an air thermostat, at 25 0.4' C. The thermostst is near the balance and is constructed sc that t h e ~
I n actual prsctioe, however, Equation 1 is too simple to express the relationship between the variahles. This mstter has been reeognieed and discussed by many workers (1.8,4,8,9). Since the interest here was primarily in obtaining reliable comparirisom between the permeabilities of various samples, no attempt was made to establish the validity of Equation 1 with I
!~~~I.~_
1~~~
-."""_.. ~
~
__
_I
./
f
Prssent sddrem, University of Florida, Gainenville. Fla.
137
738
INDUSTRIAL AND ENGINEERING CHEMISTRY
deeiccators need not be removed from it for the placing or removal of the cups. Preliminary trials indicated that reproducible resulta could be obtained only if experimental conditions were constant. For this reason the following schedule was strictly maintained. Unless otherwise noted all measurements were made on the basis of a riod of 150 minutes; for the type of material studied no $g&nt error was introduced by lack of attainment of the steady state of moisture interchange. I
20
I 0
I
I
I
I
OPEN CUP GLASS CLOTH
I
I
I
I
I
I
I
I
I
P
Vol. 15, No. 12
which the film was to be formed, and then spun for 30 seconds. Experiment indicated that the film thickness did not vary with the time of spinning between 10 and 500 seconds. The film waa allowed to dry in air before measurements were made or before a second coat was a plied. The thickness orthe film support was measured with a micrometer before spinning, and after the film had dried another measurement was made; the film thickness was then obtained by difference with an estimated accuracy of *3 microns. The thickness value for each film was the average of three measurements halfway between the center and the outer edge of the test disk. All reported results represent the average of values obtained from 3 disks. In the case of viscous solutions the films were thicker at the center, but this was usually not significant. The film weight was determined by difference from the wei h t of the supporting disk and the weight of the disk plus the driecffilm. the values represent the weight of film covering the total area oi the test disk (24sq. cm.).
TABLE I. VARIATION O F MOISTURE IMPEDANCE, FILM THICKNESS, AND FILMWEIQHTWITH SPEEDOF SPINNING I
l l l r l r r r l l l l l l 20 40 60 80 100 I20 I40 TIME (MINUTES)
l
FIGURE2. PERMEABILITY OF GLASSCLOTHAND BONDPAPER
Speed R. p . m. 2270 1760
0.43
1070
0.55 0.59 0.70
1390
880
Before the determination is started, distilled water, the small bottles of desiccant and the cups are brought to 25' C. For each run, four sampies of the film are used, three for the actual measurements and one for the counte A 5-ml. sam le of distilled water.is piptted into each of %:cups. Since alpcupa are of equal dunensions, the distance between the water surface and the film support 18 always the same. A film disk, with the coated side towards the dry atmos here is placed between the flanges of each of the four cups anathe danges are then clamped together tightly. The cup containing no water is used at^ the counterpoise. Fresh Dowflake is then placed in the center rece tacle of the cup support inaide the desiccator. starting a t 0 minutes the first cup is weighed, a t 1 minute the second and a t 2 minutes the third; a t 4 minutes the four cups are placed in the desiccator and the cover is replaced. A t 154 minutes the cups are removed from the desiccator; at 156 minutes the 6rst cup is weighed again and the others a t 157 and 158 minutes, respectively. The cups, then, have been exposed to the dry atmosphere for 150 minutes and to the air for 6 minutes; the only difference in cup treatment is that the first is in the air 4 minutes before being in the desiccator and 2 minutes afterward, thr second cup 3 before and 3 afterward, and the third cup 2 brfore and 4 afterward. With very impermeable materials it may be necessary to use a longer time period; in this case the exact timing becomes of less importance. The desiccator should not be opened during the run. In all cnaes determinations were made in triplicate and the values usually agreed within * 3 per cent. It was necesstary to use a counterpoise identical with the loaded cups except for the water, to eliminate an extraneous changes in weight. Experiment showed that the iakage between the sample and the rim of the cell or through the cut edges of the film support was so small as to be within the limits of experimental error for the type of material studied and no correction was applied. When measurements are made on relatively impermeable materials it may be necessary to use gaskets or other means of reducing rim and edge leakage. PREPARATION OF FILMS. Satislacto tree films could not be prepared and a supporting material haTto be employed. Glass cloth (IO) and various types of paper were tried; the most consistent results were obtained with bond paper (Howard Bond, white, Sub. 24) and this was used for the work reported here. The water-marked parts of the sheet were not used. The doctor blade technique could not be used because of the buckling of the film support caused by water absorption from the emulsion, and spraying or dipping yielded irregular films. Satisfactory films were pre ared by the spinning method suggested by and Gardner (6). The apparatus conGardner and Sward sisted essentially of a 7.5-cm. circular spinning table powered by a 0.125-homepower motor. The Rpeed could be varied by means of pulleys in steps from 430 to 3480 r. p. m. The film support waa eecured to the spinning table, covered with the solution from
6)
(Oil emulsion) Moisture Impedance Film Thickness Hr.sq. cm./ma. Microns
Film Weight
Mg*
49 62 82
0.49
94 120
119 158 207 228
295
Experimental Results The Payne permeability cup is designed to present an area of 10 sq. cm. for the passage of the water vapor. Although it has not been shown that the permeability is linearly proportional to the area, the results in this paper have been calculated on the basis of 1 sq. cm. for convenience in expression; since all data were obtained under the same conditions, no error in comparative results should be introduced by this procedure. Likewise the results have been calculated on the basis of water loss per hour, although unless otherwise noted the actual time period for the measurement was 150 minutes. The permeability is expressed therzfore as milligrams of water vapor permeating through 1 sq. cm. area of film and supporting material in 1 hour under the specified experimental conditions. Where the resulta have been expressed as the reciprocal of the permeability, or the moisture impedance, the units are hour square centimetcrs per milligram. The reciprocal relation between moisture impedance and moisture permeability is analogous to that between electrical resistance and electrical conductivity. In any system, then, the impedance values should be additive while the permeabilities
vs. THICKNESS vs. WEIGHT 40 80 EO 160 200 240 280 FILM THICKNESS (MICRONS) OR FILM WEIGHT tMG)
IMPEDANCE us. FILM F I ~ U R3.E MOIRTURE THICKNESS AND FILMWEIQHT Oil smubion
ANALYTICAL EDITION
Decedmr lS, 1943
139
obtain a threefold increase in the impedance over the valuw obtained when less than 16 per cent oil was used. Increasing (Oil emuhiom) the oil content from 31 to 47 per cent gave over a fourfold inMoisture Impedanoe Oil crease in the impedance, but this range is not of practical interwt % Hr. 8q. cm./mg. because sprays containing such high percentages of oil tend to 0.23 9 cause injury to the plants. 0.23 16 0.75 31 Table I11 shows the effect of incorporating small amounb 8.47 47 of paraffin wax in an oil emulsion; it required only 0.7 per cent wax to increase the impedance fourfold, a more economical means of obtaining a greater impedance than the use of higher percentages of oil. There was an increase. in the impedance OP W a ON M o r s IMPIDDANCID, ~ Fm TABLE111. EFFECT per unit film thickness due to the presence of the wax. THICKNESS, AND FILMWEIGHT
TABLE 11. OIL CONTENT m MOISTIJR~ IMPIDDMCM
(Oil emuhiom)
Oil and Wax ¶ l . 6 % oil
No WBX
20.9 oil o . ? E WBX
Moisture Im edance dnit Film Pekhicknes.
Mointure Imwdenoe HI. 8;. em./mg. 0.26
Film Thirknesa Micron# 10
Fi!m Weight 26
0.026
1.04
ie
36
0.065
1.18
20
44
0.057
1.25
20
38
0.068
30-
-
M;.
are not, just as in an electrical circuit the resistances are additive but the conductances are not. This has been discussed by Edwards (6). Figure 2 presents the rate of lor8 of water vapor from the open cup as compared with that through glass cloth and bond paper. The ratios of the open cup values to those of the g l m cloth agree closely with the numerical values of Payne (10). The permeability value obtained by extrapolation nf the curves to zero time is probably due to the water loss during the &minute exposure to the air. The fact that the curve is linear up to a water vapor lose of almost 20 mg. from the open cup indicates that the efficiency of the desiccant has not been decreased by the absorption of that amount of moisture. Table I presents the moisture impedance, thicknese, and weight of a film formed from an oil-emulsion spray as a function of the spced of spinning. If these data are plotted it is apparent that the film thickness and weight are essentially linear with the moisture impedance. Figure 3 prcsenta the moisture impedance curves of films formed from an oil-emulsion spray. In this case the variation was obtained by changing the dilution of the emulsion rather than the speed of spinning. The linearity is in agreement with tho results of other workers for some types of films under certain conditions (11, 19). Wing ( I d ) has reported the impedance of paint or enamel films to liquid water to be a linear function of film thickness. The moisture impedance obtained by extrapolation to zero thickness or weight agrees closely with the value for the film support alone. This type of information is important, since after linearity between moisture impedance and film thickness has been established for a given emulsion it is possible to interpolate the impedance a t any desired thickness for comparative purposes. Figure 4 presentv the moisture impedance-per cent solids curves for the films forincd from three dserent emulsions. Here also the impedance values approach that of the support material as the lower limit. The relative efficiency of these emulsions in retarding the passage of water vapor is obvious. A t 12 per cent solids, wax emulsion A is about twice as efficient aa emulsion B and 4 times as efficient as the oil emulsion; a t 20 per cent solids, the efficiency ratios are 1.7 and 8.5, respectively. Table I1 presents the moisture impedance of films formed from oil-emulsion sprays as a function of the oil content. It waa necesssry to increase the oil content to about 30 per cent to
25
0
W X EMVLSION A WAX EMULSION B OIL EMULSION
PERCENT SoClDS
F I Q4. ~M o ~ a m mIMPEDANCE va. PER CENT SOLIDS
Table IV shows the effect of adding bentonite to an oilemulsion spray. The bentonite increases the viscosity of the emulsion, and since the films are prepared under standard conditions, it is evident that an increased film thickness results. At the higher bentonite lcvels, however, the impedance per unit film thickness was markedly decreased. At the lower bentonite levels the decreased impedance per unit thicknem was counterbalanced by the increased thickness, so that there was no net change in the impedance values in going from 0 to 2.1 per cent bentonite. Above 2.1 per cent bentonite the film thickness became the predominating factor arid the over-all impedanm values were increased. The formulation of emulsions of given physical propertiee has been facilitated by information obtained from the above type of experiment.
TABLEIV. EFFECTOF BBNTONITE ON MOISTUREIMPIDDANCE AND FILM THICKNESS (Oil emulsions)
Bentonite
% 0.0
0.5 2.1 4.2 5.2
Moisture Impedance Hr. sq. om./mg. 0.21
0.21 0.22 0.28 0.47
Film Thickness Micron8 11
14 19
63 125
Moisture Impedance per Unit Film Thickness
0.019 0.015 0.012
0.0045
0.0038
Acknowledgments The authors wish to acknowledge the assistance of Robert Young, Almo Squitero, Howard Winters, and Anne Mandenberg with various parts of the experimental work. This research waa supported by the Horace H. Rackham Research Endowment
740
INDUSTRIAL AND ENGINEERING CHEMISTRY
of the Michigan State College of Agriculture and Applied Science for studies on the industrial utilization of agricultural products.
Literature Cited (1)
Abrams, A., and Brabender, G. J . , Paper Trade J . , 102,No. 15,
32 (1936). (2) Brabender, G. J., Proc. f a t Food Conf. Inst. Food Tech., 1, 227 (1940). (3) Carson, F.T.,Natl. Bur. Standards, Misc. Pub. M127 (1937). (4) Charch, W.H., and 'Scroggie,A. G., Paper Trade J . , 101,N o . 14, 31 (1935). (5) Edwards, J. D.,IND. ENO.CHEM., ANAL.ED.,25,846 (1933). (6) Gardner, H.A., "Physical and Chemical Examination of Paints,
Vol. 15, No. 12
Varnishes, Lacquers, and Colors", 9th ed., Washington, Institute of Paint and Varnish Research, 1939. (7) Gardner, H.A.,and Sward, G. G., IND.ENQ.CIIEM.,19, 972 (1927). (8) Kline, G. M., J . Research Natl. Bur. StandardP, 18,235 (1937). (9) Lishmund, R. E., and Siddle, F. J., J . Oil Colour Chem. Assoc., 24, 122 (1941). (10) Payne, H. F.,IND. ENQ.CHEM.,ANAL.ED.,11,453(1939). (11) Taylor, R. L.,Hermann, D. B., and Kemp, A. R., IND.ENO. CHEM.,28, 1255 (1936). (12) Wing, H.J., Ibid., 28, 786 (1936). (13) Wray, R.I., and Van Vorst, A.R., Ibid., 25, 842 (1933). P U E L I R Hwith ~ D the permission of the Director of the Experiment Station 8s Journal Article 639 (n. E.).
Determination of Nicotine and Nornicotine in Tobaccos C. V. BOWEN AND W. F. BARTHEL United States Department of Agriculture Bureau of Entomology and Plant Quarantine, Beltsville, Md.
To determine nicotine and nornicotine in tobacco the total steam-volatile alkaloids are isolated by steam distillation from a strongly alkaline solution containing sodium chloride in excess. One aliquant of the distillate is treated with silicotungstic acid precipitating nicotine and nornicotine, and another is treated with nitrous acid, steam-distilled from an alkaline solution, and the nicotine
A
METHOD of analysis for nicotine and nornicotine when present together in solution has been presented by Markwood (6). It WES developed for cases in which the alkaloids are already separated from interfering materials, and no procedure is given for obtaining the alkaloids from tobacco or tobacco products. To separate nicotine and nornicotine from the plant material, a steam distillation or continuous extraction with an organic solvent such as benzene is necessary. In view of the difficulty experienced in completely removing nornicotine from alkaline plant material by repeated extraction with benzene, ether, or gasoline, the method presented here resorts to a steam distillation, which completely separates both the nicotine and nornicotine from the sample, This method is based on the assumption that anabasine or other steam-volatile secondary-amine alkaloids aside from nornicotine are absent; such alkaloids have been found only in traces in Nicotiana tabacum. The steam-volatile secondary-amine alkaloid has been identified as nornicotine by methylation in most of the samples analyzed, The nornicotine is determined as the difference between the total steam-volatile alkaloid and the nicotine; consequently the analysis takes place in two parts. The first is an application to nornicotine of Burkat's method (4) for determining anabasine; it depends upon the complete steam distillation of nicotine and nornicotine from strong alkali solutions saturated with sodium chloride. The second part, which has been proved satisfactory by Markwood (6), ihvolves treating the solution containing both alkaloids with nitrous acid and steam-distilling from an alkaline solution, whereby the secondary amines, including nornicotine,
precipitated as a silicotungstate. The nornicotine is determined by difference. Analysis of tobaccos differing widely in alkaloid content shows the rather common occurrence of nornicotine. Comparison with nicotine determinations by the conventional methods shows the unreliability of the heretofore accepted method of analysis when nornicotine is present.
remain behind as nitrosoamines and the nicotine is obtained in the distillate.
Analysis of Standard Solutions The standard solution of nicotine used for proving the method was repared from a sample of nicotine that had been vacuumdistifed, treated with nitrous acid to remove any secondary amine, and then steamdistilled from an alkaline aqueous solution. The nornicotine used in preparing the standard nornicotine solution was identical with that used by Markwood (6),had been obtained from Robinson Medium Broadleaf, a strain of Maryland tobacco, and formed a picrate melting at ~9O-l9IoC. The absence of nicotine was established by treating a sample with nitrous acid and steam-distilling the a ueous solution after making it basic to phenolphthalein. No Jkaloid was present in
OF NICOTINEAND NORNICOTINE IN TABLEI. RECOVERY MIXTURESBY STEAM DISTILLATION FROM STRONQ
ALKALIAND SODIUMCHLORIDE
Composition of Mixture Nicotine Nornicotine Mg. Ma. 18.2 2.6 14.5 4.5 10.9 6.2 7.7 7.2 4.5 8.2 a
b
Direct pptn. YO.
Siliootungstate Residue" Pptn. of distillate Reoov ery
Yo.
%
97.2 98.6 101.8 98.7 99.4 Av. 99.1 Average of three determinations except where otherwise noted. Average of two determinations. 190.1 172.7 152.2 138.95 118.6b
184.86 170.3 155.0 137.1 117.9
