V O L U M E 2 5 , NO. 10, O C T O B E R 1 9 5 3 felt that in favorable cases concentrations as low3 as 10-8 should be detectable in a suitably designed mass spectrometer tube. Order-of-magnitude estimates of concentration can be computed from ion currents integrated over time, even though individual ionization cross sections are not known. I n the course of this work it has become increasingly clear that proper choice of the crucible material is of the utmost importance. Chemical side reactions between sample and crucible may significantly change, or even completely inhibit, the process of evaporation. rlnalysis of two samples, which had been treated by the gradient furnare method, has established that a number of Group 11, IT', and V impurities are present in Eagle-Picher germanium. ACKNOWLEDGRlEhT
Thc enriched germanium samples, including resistivity measurements and weighings, vere prepared through the courtesy of S. 11, Christian, who has shown much interest in the present work. It is a pleasure to acknowledge the help rendered by -4.W.Vance and C. C. Shumard who designed and engineered the magnet current regulator, in itself a major project. AT. .I. Colacello has been very helpful in the design and mounting of the various types of evaporators. .issociates within the Physical Research Laboratory, RC.l Laboiatories, Princeton, N.J., have
1535 contributed greatly t o this study by stimulating discuqsions and constructive suggestions. LITERATURE CITED C o n n , W ,JI.,J . O p t . SOC.Amcr., 41,445 (1951). D e m p s t e r , A. J., Rev. Sci. Instr., 7, 46 (1936). Dibeler, T. H., Mohler, F. L., a n d Reese, R. 11..J . Research S a t l . Bur. Standards, 38, 617 (1947). DuRridge, L. A , , a n d Brown, H. B., Reo. Sci. Irisf.. 4, 531" (1933). Goldberg, E. A., RCA Re?., 11, 296 (1950). Gorman, J. G.. Jones, E. J., a n d Hippie, J. .i,,~ \ S I L . C k E k r . , 23, 438 (1951). R i c k a m , W.XI., P h y s . Ret., 74, 1222-1 (1948). H i c k a m , T, lI,,private communication. Honig, R. E., J . A p p l . Phys., 16, 646 (19451. Honig, R . E., J . C h e m Phys., 21, 573L (19531. Massey, €1. S. W., and B u r h o p , E. H. S., "Electronic and Ionic . I m p a r t Phenomena," p. 38, London. Oxford L-niversity Press, 1952. N e t z g e r , F., Hela. P h y s . Acta, 16,323 (19131. Plumlee, R. H., private c o m m u n i c a t i o n Sidgwick, S . V., "Chemical Elements and Their Compounds," p . 759, London, Oxford L-niversity Press, 1950. Stranski, I. K., a n d Wolff, G., 2. S n t w f o r s c h . . 4A, 21 (1949).
RECEIVED for review February 7, 1953. .Iccepted J u l y 14, 1953. Presented a t the Pittsburgh Conference on Analytical Chemistry and Applied Spectroscopy, Pittsburgh, Pa., March 1953.
Determining Rubber Hydrocarbon in Rubber-Bearing Plants J4T1ES W. TIEEKS, RGTH V. CROOK, CLAY E. P I R D O , .JR., AXD FREDERICK E. C L I R K C'. S . Natural Rubber Research Station, Salinas, Calg. The conFentiona1 method consists of comminution of tissue to pass a 3-mm. hammer mill screen followed b? extraction with water, alcohol, and benzene, respectively. The major disadvantages of the method are: (1) all of the rubber is not extracted because the method of comnlinution does not rupture all of the plant cells; (2) the benzene extract may contain impurities; (3) the extract is susceptible to oxidation during dr?ing; and (1) a minimum of 3 days is required to complete an analysis. To overcome the disadvantages of the older method, comminution is carried out by crushing and sheeting through corrugated and smooth rolls so that all of the plant cells are ruptured. Benzene extraction is carried out by mechanically shaking with pebbles and trichloroacetic acid. Extraction can be effected in as short a time as 2 minutes and a complete analysis can be made in less than a day. The new method represents an improvement in accuracy and precision as well as in time sayed.
A
N ACCURATE method foi thr determination of rubber hydrocarbon in plant material has been sought ever since guayule rubber first became available commercially in 1905. The early workers, WhYtelsey ( I S ) . Fos ( S ) , and Hall and Good-peed ( 6 ) , made a start on the problem through solvent extraction. However, they lacked modern grinding equipment and their choice of solvents usually resulted in rubber films highly contaminated with nonrubber impurities. It remained for Spence and Caldwell ( 1 2 ) t o develop the first method of determining rubber hydrocarbon in guayule shrub Lvhich can be classified as truly quantitative. This was accomplished through careful attention t o shrub comminution, acid-stream hydrolysis, and choice of solvents Their method consists of successive extraction with water, acetone, and benzene. The film remaining after the benzene is evaporated is weighed as rubber hydrocarbon. Nodifications of this triple solvent method have been introduced by IVillits et al. (15) and Holmes and Robbins ( 7 ) . Further modifications have been used in this laboratory. There are t x o major disadvantages of the triple solvent method, either modified or unmodified. The extraction of the
rubber is never complete. Rubber hvdrorarbon can always be detected in the extracted tissue by the staining techniques of IVhittenberger ( 1 4 ) or Haasis ( 5 ) . The method takes too much time; approximately 3 days are required to complete an analysis. The purpose of this paper: is to present a method in tvhich the disadvantages of the triple solvent method are largely overcome. This is achieved by using a different procedure for shrub preparation and a different means of putting the rubber into solution, and determining the ruhher hydrocarhon as ruhtxr hromide rather than R S the less stable ruhber film. REAGElVTS AND EQUIP\IENT
The following reagents are required 1)y the modified triple solvent and shaker met,hods: et,hgl alcohol, 95%: benzene, ,4CS reagent grade; chloroform, ACS reagent grade: and trichloroacetic acid, reagent grade. Brominating solution was prepared 11y dissolving 2 grams of C.P: iodine in 100 ml. of carbon tetrachloride, filtrring, and adding 5 mi. of C.P. bromine to the filtrate. The folloffing equipment, was used in shrub preparation: enrotary silage-type cutter (Paper llachine Co., Shortaville. K.I-.): knife cutter (Ball & Jen-ell, Brooklyn, K , Y , ) ;a forced-air rircu-
ANALYTICAL CHEMISTRY
1536 lation oven; crushing rolls consisting of two 6 X 12 inch powerdriven corrugated rolls turning a t a back roll to front roll ratio of 1.3 to 1; sheeting rolls consisting of two 6 X 12 inch poaerdriven smooth rolls turning a t a back roll to front roll ratio of 1.4 to 1; and a Raymond laboratory hammer mill. ASTM (1) extraction equipment was used in the triple solvent method. Shaker extractions were carried out in a mechanical shaker with a 3-inch horizontal stroke of 135 oscillations per minute. A wooden rack was constructed to accommodate a total of 12 heavy-wall, 250-ml. centrifuge bottles held lengthwise with respect to the shaking motion. After the work had been completed it was found that a shaker which had a 1.75-inch horizontal stroke of 275 oscillations per minute was also satisfactory. An explosionproof centrifuge was used to clarify the benzene-rubber solutions.
Table I.
Effect of Acid Boil on Rubber Hydrocarbon Determinations s o . of
Determinations
Rubber Hydrorarbon, %a
Standard Deviation
4 No significant difference between means. P = 0.01. P denotes "at a probability level of." b Coarse cut shrub waq dried a t 65' C. and hammer milled.
Table 11.
Comparison of Acetone and Ethyl Alcohol Extractiono
Resin Solvent Acetone Ethyl alcohol, 96%
Rubber Hydrocarbon, 13.13 13.22
%b
Standard Deviation Resin, % e 0.31 7.57 0.34 7.93
Standard Deviation 0.10 0.18
5 Samples were extracted with water, resin solvent, and benzene; dry film was weighed. Each valueis the mean of 8 determinations. b KOsignificant differencebetween means, P = 0.01. Difference between meanssignificant, P < 0.01.
250-mI. beaker. Add 9 ml. of chlorofonn from a buret and 2.5 mi. of brominating solution to the rubber solution. Place the beaker in a water bath a t 25" C. and allow bromination to proceed for 100 + 5 minutes. No direct sunlight should come in contact with the solution in the water bath. At the end of 100 minutes, add approximately 200 ml. of 95% ethyl alcohol to the contents of the beaker. Allow 2 hours for the rubber bromide to settle. Determination of Rubber Hydrocarbon. Filter the rubber bromide into a tared, asbestos Gooch crucible and wash precipitate thoroughly with 95% ethyl alcoho!.o Dry the crucible to constant weight in a vacuum oven a t 60 C. (requires about 1 hour). Cool in a desiccator and weigh. The weight of the rubber bromide when multiplied by the conversion factor (0.300) will be converted to rubber hydrocarbon which, when multiplied by the dilution factor of 6, gives the weight of rubber hydrocarbon in the aliquot of tissue.
MODIFIED TRIPLE SOLVEXT METHOD
The triple solvent method used in this laboratory is sufficiently different from published methods ( 7 , 12, 1.5) to require a detailed description. Preparation of Sample. Immerse lants in boiling water for 15 minutes and shake off leaves. s a s s through a? ensilage chopper and through '/Tinch screen on rotary knlfe cutter. Dry a t 65' C. in a forced-air circulation oven until the moisture is reduced to 2 to 6Tc(4hours). Grind to pass a 5-mm., followed by a 3-mm., screen in laboratory hammer mill and remove a 10gram portion for determination of moisture by drying for 1 hour a t 110" C. Extraction. Transfer a 2-gram sample to a glass extraction thimble in which the perforations have been covered with a plug of borosilicate glass wool. Insert a second plug above the sample, attach the thimble to a block tin condenser, and connect to a 400ml. ASTM ( 1 ) rubber extraction flask containing approximately 100 ml. of water, and extract for 4 hours. Remove water from thimbles by suction. Extract for 16 hours with 100 ml. of 95% ethyl alcohol. If resin is to be determined, evaporate ethyl alcohol on a steam bath and dry extract in vacuum oven (28 to 30 inches vacuum) for 1 hour a t 100' C. Extract for 16 hours with 100 ml. of benzene. Evaporate benzene on a steam bath and dry rubber film in a vacuum oven for 1 hour a t 100' C. Tared flasks are used for benzene extractions and also for ethyl alcohol extractions if resin is to be determined. RECOMMEXDED SHAKER METHOD
Preparation of Sample. Immerse plants in boiling water for 15 minutes and shake off leaves. Pass through an ensilage chopper and through '/?-inch screen on rotary knife cutter. Dry a t GoC. in a forced-air circulation oven until the moisture is reduced to 2 to 6% (4hours). Crush 10 passes between closely set corrugated rolls. Sheet 10 passes between closely set smooth rolls, rolling the sheet and inserting the rolled sheet endwise each time. Remove a 10-gram portion for moisture determination. (If dry, finely divided tissue is available, all steps may be omitted until the sheeting step. Tissue which is low in rubber may powder instead of sheet.) Extraction. Weigh accurately a portion calculated to give a rubber solution of 1.2 to 2.0 mg. per milliliter (1.5 gram for tissue containing 12 to 20y0 rubber hydrocarbon) and place in a heavy-wall, 250-ml. centrifuge bottle. Add approximately 100 grams of 5 to 10 mesh pebbles and exactly 150 ml. of a 1% solution of trichloroacetic acid in benzene. Stopper and place on a mechanical shaker lengthwise with respect to the shaking movement. Shake for 10 minutes and centrifuge 15 minutes a t 2000 r.p.m. Bromination. Pipet a 25-ml. aliquot of the clear supernatant solution (containing 30 to 50 mg. of rubber hydrocarbon) into a
Table 111. Comparison of Extraction Methods and Determinations by Film Weight and Bromination" Analytical Treatment
Rubber Hydrocarbon,
%b
Standard Deviation
a Coarse cut shrub was dried a t 65' C. and hammer milled. b Each value is the mean of 3 determinations. Least significant difference between means, P < 0.01 = 0.22. X o significant difference in variance. P = 0.02.
RESULTS AND DISCUSSION
The data shown in Tables I to VI11 were obtained by the modified triple solvent and shaker methods for the most part. Variations from these methods are indicated. Analyses of variance and the experiment on sampling were calculated according to Youden (16). Least significant differences were calculated by the formula given by Johnson (8). Homogeneity of variances was examined by a modified F test (8). Acid-Steam Treatment. Spence and Caldwell(l2) stated that an acid-steam pretreatment of the shrub sample was necessary to release all of the rubber in subsequent extraction. Willits et al. (15) did not investigate pretreatments but used a 2% sulfuric acid boil for 2 hours, followed by 2 hoursQf autoclaving a t a prwsure of 15 pounds per square inch above atmospheric. Holmes and Robbins ( 7 ) found that the acid-steam prrtwntment was not necessary with tissue containing up to 8% rubbc r hydrocarbon n-hich had been hammer milled to pass a 1.5-mm. screen. Shrub containing high rubber hydrocarbon i,g difficult to grind through a 1.5-mm screen because of sticky resin and rubber balls which separate out. Therefore, a 3-mm. screen is used instead of the smaller screen. However, acid-steam pretreatment is not ne:essary with tissue passing a 3-mm. screen, as shown in Table 1, hence this was eliminated from further consideration. In addition to being time-consuming, acid treatment results in shrub subject to more packing and channeling than untreated shrub. The results in Table I were obtained by the modified triple solvent method except acetone was used as the resin solvent. Resin Solvent. Acetone has been used by all previous investigators as the solvent with which t o extract the resin before
V O L U M E 25, NO. 10, O C T O B E R 1 9 5 3
1537
extracting the rubber. Meeks et al. (IO)found that a small fraction of guayule rubber is soluble in acetone. Therefore, 95% ethyl alcohol n a s substituted in the triple solvent method on the premise that low molecular weight rubber is not as soluble in alcohol as in acetone. Table I1 shows that there is no appreciable difference between the rubber hydrocarbon values whether the extraction is made with alcohol or acetone: however, the resin value is higher when the estraction is made with alcohol. Triple Solvent, Double Solvent, and Shaker Extraction. In developing the shaker procedure, it was necessary to know if rubber hydrocarbon determined by bromination is equivalent t o rubber hydrocarbon determined by film weight as used in the triple solvent method. In Table I11 it can be seen that there is no significant difference between bromination and film weight determinations so long as the benzene extraction is preceded by water and alcohol extraction. The shaker extraction yields more rubber hydrocarbon than the triple solvent extraction. Since both extractions were carried out on hammer milled shrub, it was first thought that the shaker estraction was more efficient in releasing rubber from the tissue. However, when the alcohol estraction is omitted from the triple solvent procedure, the rubber hydrocarbon values equal those obtained by shaker extraction. This suggeste that alcohol has e?itracted some of the rubber. Acetone and alcohol are not usually considered t o be rubber solvents, but under these rather drastic conditions an appreciable amount of rubber may be degraded t o a soluble state. Up t o 50% of a rubber film formed by evaporation of benzene guayule rubber solution has been dissolved in either boiling acetone or alcohol (9). Shaker Extraction Time. One of the chief disadvantages of the triple solvent method has been the excessive time consumed in eutraction--4 hours for the water extraction and 16 hours each for alcohol and benzene extraction. It was of interest to determine how long an estraction time was needed with the shaker procedure. The time of shaking is not critical as evidenced by Table IV in which there is no significant difference in results between 2 and 60 minutes of shaking. These results were unexpected because rubber is notoriously slow in dissolving. However, the finely divided rubber-bearing plant material and the violent agitation of the shaker and pebbles combine to give a rapid solution. Shrub Preparation. Hammer milling has not proved too successful as a means of comminuting shrub. a s pointed out above, it is not feasible to hammer mill high rubber content tissue through a screen finer than 3 mm. Some of the gummy tissue ordinarily sticks to the side of the hammer mill. Fine dust low in rubber content may be lost if the mill is not entirely tight. Frequently, little rubber balls are formed in passing tissue through the hammer mill if the shrub is high in rubber, making sampling difficult. -411 of the cells are not ruptured because rubber can always be detected in the extracted tissue by staining (6,14),
Table IV.
Influence of Time of Shaker Extraction on Rubber Hydrocarbon Determinations"
Time, minutes Rubber h drocarbon found, & b
10 17.01
Sample 1 20 17.18
30
40
50
60
17.01
17.02
li.07
16.90
Sample 2 4 6 8 10 12 Time, minutes 2 Rubber hydrocarbon found, % " 17.17 17.27 17.25 17.38 17.29 17.15 4 Each value is the mean of two determinations. 5 Pooled standard deviation = 0.05. No significant difference a t P = 0.01. C Pooled standard deviation = 0.11. No significant difference a t P = 0.01.
For these reasons, crushing and sheeting were investigated as a means of comminuting the shrub before sampling. The results are found in Table V. Crushing and sheeting gave higher rubber hydrocarbon values than hammer milling. However, hammer
milling followed by sheeting is equivalent to crushing and sheeting, provided the difficulties in hammer milling are overcome. Crushing and sheeting are preferred over hammer milling and sheeting on the basis of time saved. It is probable that less crushing and sheeting would be satisfactory, but this point was not investigated in view of the rapidity with which crushing and sheeting can be carried out.
Table V.
Effect of Shrub Preparation on Shaker ExtractionsG
Rubber Standard Shrub Preparation Hydrocarbon, % b Deviation 15.68 0.163 Hammer milled Hammer milled and sheeted 10 times 15.90 0.077 Crushed 10 times and sheeted 10 times l5,94 0,099 a Coarse cut shrub was dried a t 65' C. prior to treatment. b Each value is the mean of four determinations. Least significant difference between means, P < 0.01 = 0.22. S o significant difference in variance, P = 0.02.
Sampling of Sheeted Shrub. Sampling of hammer milled tissue for analysis has always been a problem. Samples taken with a spoon will tend t o be erratic if rubber balls have been formed during hammer milling. This problem is accentuated with high rubber content shrub. If vigorous shaking of a quart jar is used as a method of mixing, the tissue actually may be less uniform after mixing than before, because the h e r particles tend to segregate to the bottom. Rolling and coning on paper folloived by quartering is a better method of obtaining samples than is spooning, but it is time-consuming and does not completely remove the variation due to rubber balls. Sampling is not as great a problem, however, if the tissue has been prepared by sheeting on smooth rolls. Uniform samples ran be obtained by cutting strips from the sheet. Table VI shows the results of a sampling experiment. The variance in determination between samples is only slightly greater than the variance between determinations from the same sample, proving that the sheet was remarkably homogeneous.
Table VI.
a
Sampling of Sheeted Shrub"
Average rubber hydrocarbon content, % Range of rubber hydrocarbon content, % Number of samples Number of determinations per sample Standard deviation, sample means Variance between results from different samples Variance between results from same sample Analyses made by shaker method.
17.630 17.36-17.75 10 2 0.080 0.0036 0.0029
A n experiment designed specifically to show the error in sampling hammer milled tissue was not carried out. Experience gained in analyzing several thousand samples has indicated that the variation in hammer milled tissue is much greater than in sheeted tissue. A greater precision is shown in Tables V and VI1 for sheeted tissue over tissue which has been hammer milled only, although the difference in variance is not significant a t the 2% level in Table V. It should be emphasized that these estimates of precision were obtained on relatively small numbers of observations. Deresinated Shrub. Shrub which has been deresinated before processing produces a high-quality guayule rubber ( 2 ) . Hoivever, shrub which has been deresinated with acetone or other solvent presents difficulties in analyzing for rubber hydrocarbon by the triple solvent method. Exactly what happens during or after solvent deresination to cause low rubber hydrocarbon values is not certain. The explanation appears to be connected with the fact that the acetone-extracted tissue is allowed to become bonedry before hammer milling simply by allowing the acetone to
ANALYTICAL CHEMISTRY
1538 evaporate, whereas in the case of the unestracted tissue, 3 or 4Y0 moisture remain? after oven drying. One or more of the following possibilities ma>-exist with bone-dry tissues. (1) Gel rubber may be formed by oxidative cross linking. ( 2 ) The cell walls may tiecome hard and impervious and not rupture on hammer milling. (3) The cells may shrink and rubber particles in contact with cell walls may become physically attached to the cell walls.
JVhatever the reasons for the low results obtained Ivith deresinated shrub by the triple solvent method, the situation is improved by using a combination of sheeting and shaker extraction. Table VI1 shows a comparison of the two methods. The shaker method estracts about 16% more rubber hydrocarbon than the triple solvent method. In fact, the values before and after deresination are equal by the triple solvent method, although water solubles and resin totaling about 15% of the dry neight of shrub were removed in the process of deresination.
The average value was 70.03y0 bromine with a standard deviation of 0.16. This value agrees well with the theoretical value of 70.12% and with the value of 69.9% reported for guayule rubber bromide bv Gowans and Clark (4). It may be concluded, then, that the rubber bromide is pure. The higher rubber content resulting from the recommended shaker method is evidently due t o inclusion of alcohol-soluble rubber hydrocarbon not accounted for in the triple solvent method. Trichloroacetic acid serves a dual function in the shaker method. I t solubilizes gel rubber and it decreases the viscosity of the benzene rubber in solution t o the point a t which the benzene insolubles can be readily centrifuged. Impure rubber bromides giving low results are obtained in the absence of trichloroacetic acid. Four rubber bromides obtained by the shaker method, escept for the omission of trichloroacetic acid, anal) zed 69.1 to 69.5% bromine. COYCLUSIONS
Table T'II. .4nalyses of Acetone-Deresinated Shrub by Shaker and Triple Solvent Methods" Rubber Hydrocarbons, % Deresinated shrub
Control Shrub hammer milled analyzed by triple solvent, film method
1 7 . 11 17.57
Shrub hammer milled, sheeted 10 times, analyzed by shaker, bromination method a Each value is the mean of 3 t o 5 determinations. methods differ significantly, P < 0.02.
Pooled Std. Dev
17.23 17.47 20.64 20.64
0.25 0.05
Variances of the t w o
Shrub of Different Rubber Content. Most of the results reported were obtained with shrub having a rubber hydrocarbon content varying from 12 t o 20%. Occasionally shruh of lower rubber hydrocarbon content must be analyzed-e.g., shrub of different age groups, varieties, and hybrids. Data are presented in Table VI11 for four types of tissue analyzed hy four methods. Comparisori of column 2 with 6 shows the recommended shaker procedure yields higher values than the modified t,riple solvent procedure for all four types of shrub. Crushing and sheeting in place of hammer milling significaritly raises the rubber hydrocarbon values with the triple solvent extraction. This was found to be true, also, with shaker estrartion (Table V j . Results equivalent, t o triple solvent extraction are obtained if water and alcohol extractions precede shaker extraction (columns 1 and 5 ) . This again suggests that ruhher is dissolved during hot alcohol estraction.
Table T'III. Analyses of Shrub of Different Rubber Contents by Shaker and Triple Solvent RIethods Rubber Hydrocarbons Found, %"
Strain
.. ...
Age. Teaiq
Crushed and Sheeted Fhaker method, Shaker method, no pretreatH20 and ale. nient extracted
7 22
Triple solvent method
Hammer milled, triple solvent method
.. .
-4n accurate method for determination of rubber hydrocarbon in rubber-bearing tissue has been developed. The essential features of the method are plant material preparation, benzene estraction by means of a mechaiiicd shaker, and rubber hydrocarbon determinations by bromination of a benzene rubber solution. Crushing and sheeting of rubber-bearing tissue ruptures more cells than does hammer milling, and makes possible a more complete rubber extraction. After sheeting, the rubber in the tissue is more uniformly distributed than after hammer milling. Appreciable quantities of rubber hydrocarbon can be lost during hot alcohol or acetone extraction; therefore, these solvents should be omitted from the rubber hydrocarbon determinations. Resin dissolved in benzene-rubber solution does not interfere in rubber hydrocarbon determination by bromination. A complete analysis can be carried out in less than a day by the recommended shaker method as compared to 3 days by the triple solvent method. The shaker method is applicable to shrub of widely different rubber content. ACKNOW LEDGMENT
The authors are grateful t o I. C. Feustel for helpful suggestions, and to Eleanor C. Taylor, of the Bureau of Plant Industry, Soils, and Agricultural Engineering a t this station, for statistical aesiptance. LITERATURE CITED
( 1 ) A4~n. Sot. Testing Mate~ials,Standards, Rubber Products, Desiznation D 297-50T. D. 1 1 8 . 1 9 5 2 . ( 2 ) Chibb, R. L., Taylor,-E. C . , and Feustel, I. C., I n d i a Rubber W o r l d , 123, 667 (1951). (3) Fox, C. P., J . I n d . Eng. Chem., 1 , 7 3 5 (1909). (4) Gowans. W.J., and Clark, F. E., - 4 ~ 4C~H . E x . , 2 4 , 529 (1952). ( 5 ) Haasis, F. IT., S t a i n Technol., 2 0 , 37 119453. (6) Hall, H. II.,and Goodspeed, T. ('nit,. Cali/. (Berkeley), Pub. Botany, 7 , 216 (1919). ( 7 ) Holmes, R. L., and Robbins, H. TV., ASAL. CHEM.,19, 313 (1947). ( 8 ) Johnson, L. P. V., "Applied Biometrics," pp. 121 and 123, Alinneapolis, Burgess Publishing Co., 1960. (9) Jones, E. P., unpublished report, U. 8. Xatural Rubber Research
N.,
(10)
Station, Salinas, Calif. Jleeks, J. W ,Banigan, T. F., Jr., and Planck. R. W., I n d i a
(11)
Peters, E. D., Rounds, G.
I5 9
Rubber W o r l d , 122, 301 (1950).
-
meansb 9 28 8 45 8 39 a Each valueis the mean of 3 determinations. b Least significant difference between method means, P < 0.01
8 25 =
0.13
Purity of Rubber Bromide. I t was considered possible that the higher results obtained by the shaker method might be due to a contaminated rubber bromide, because the shrub is not extracted with water or alcohol prior t o benzene extraction. Accordingly, eight rubber bromides were analyzed for bromine content by a dry combustion procedure described by Peters e t a l . ( 2 1 ) .
r,,and Agazzi, E. J.. AXAI..CHEM.,
2 4 , 710 (1952).
(12) Bpence, D., and Caldwell, 31. L., ISD. ESG. CHEM.,A s a ~ ED., . 5, 371 (1933); Rubber C'heni. and Technol., 7 , 111 (1934). (13) Whittelsey, T., J . I n d . Eng. Chem., 1 , 245 (1909). (14) Whittenberger, R . T., St(ri?i.Technol., 19, 93 (1944). (15) Willits, C. O., Ogg, C. L., Porter, If-, L., and Swain, 31. L., J . Assoc. Ofic. A g r . Cheniisfs, 29, 370 (1946). (16j Youden. W, J., "Statistical Methods for Chemists," pp. 37, 50, S e w York, John Wiley & Sons, 1951. R E C E I V Efor D review March 1 6 , 1953. Accepted July 21, 1953. Presented before the Division of Rubber Chemistry at the 123rd hfeeting of the A M E R I C S CHEMICAL S O C I E T Y . 1,os Angeles. Calif.
