31 3
V O L U M E 19, NO. 5, M A Y 1 9 4 7 ponent, enable the andyst to diffrt~~nti:rtc~ iirtneeti tlic coloi,.: of this class. ACKNOWLEDGMEVT
The authors Irish to esprt’ss their appreciation t o their director,
IT.C. Bainbridge, for his encouragement, and to Virgini:i Haase for her assistance in hringing the project to completion. LITERATURE CITED
(1) Battegay, Laiigjahr, a n d Rettig. Chimie et Iritiiistrir, 11, 453-5 (1924).
(2)
Clieronis, N. l)., and Koeck, M.,J . Chem. Education, 20, 48s
(1943). (3) Grandmougin, Bcr., 39,2494, 3661, 3929 (1906). (4) Knecht. E., J . SOC.Dyers Colourists, 45, 133 (1929). (’)
Koch, L., Milligan R . F., and Zuckerman S., IKD.ENG. CHEM., .\N.nzene,drying the residue in a n atmosphere of carbon dioxidt,, and weighing. Fox (4)determined rubber in guayulc by extracting first with acetone, then with benzene. He then prccaipitated the rubber from benzene x i t h alcohol, and dried and iwigkied the precipitate. Whittelsey (15) deterniincd rubber in guayulr by extracting the sample with wat.er and alcohol to rvinove the rubber. He evaporated the carbon tetrachloride on a -tram bath and boiled the rubber film n-ith alcohol and xater t>o r(movc any resin or water-solubles extracted with the rubber. After pouring off the water and alcohol he dried the film and weighed it as rubber. Hall and Goodspeed (6) and Hall and Long f;) in t,heir survey of the rubber content of Xorth American plant,s extracted t,he samples for 3 hours with boiling acetone and thrn for 3 hours with boiling benzene. The acetonc extract was
314
ANALYTICAL CHEMISTRY
evaporated and the residue weighed as resin; similarly the benzene extract residue was weighed as rubber. In 1033, Spence and Caldwell (14) published the results of their nork on the determination of rubber in guayule. They found rarhon tetrachloride unsuitable for extracting rubber in analytical procedures, and used benzene instead. Their procedure consists essentially of the following steps: 1. Pass sample twice through differential laboratory mill rolls set as tightly as possible. 2. Pass successively through a Universal grinder and corn mill. After each stage of grinding, quarter sample by a Jones riffler. 3. Weigh samples into special porcelain thimbles and boil in 1% sulfuric acid for 3 hours. 4. Autoclave a t 13.6 kg. (30 pounds) pressure for 3 hours. 5 . Extract for 12 hours with acetone. 6. Dry acetone from thimbles in vacuum oven. 8. Add antioxidant, evaporate the benzene on a n ater bath, dry, and weigh residue as rubber. EXPERIMENT&L
Early in these studies it was found that with efficient grinding of the guayule sample, strength of acid in which sample was
boiled, length of boiling, and time and pressure of autoclaving were not critical, and the procedure was modified as to the following conditions, primarily t o shorten the time, so that more samples would be handled with available equipment and personnel: 1. Boil in 2% sulfuric acid for 1.5 hours. 2. Autoclave at 6.8 kg. (15 pounds) pressure for 1.5 hours. 3. Extract for only 8 hours with acetone.
This modified procedure was used as standard for making studies of the method reported in this paper, only one qtep at a time being modified PREPARATION OF SAMPLE
One of the difficulties about any method of analysis for rubber in plant material is getting the rubber into solution. In guayule the rubber occurs within individual plant cells and not in latex tubes as in most other rubber-bearing plants. I t was found practically impossible to dissolve the last traces of rubber, even with 36 hours’ extraction with benzene, subsequent staining nearly always showing a trace of rubber. However, when finely ground samples were used, the amount of residual rubber after extraction appeared to be so small that it would have no appreciable effect on the percentage of rubber determined. Repeated tests have shown that within limits the finer the sample is ground the more rubber can be extracted. In Table I are given figures showing the influence of fineness of grinding on percentage of rubber obtained. (For explanation of significant difference, see 1.2.) Fineness of grinding was not compensated for by increasinp the extraction time. This is show-n in Table 11.
Table 1.
Influence of Fineness of Grinding on Rubber Extraction
(Samples ground through Wiley laboratory mill) Rubber Percentage Sample 6-mm. 4-mm. 3-mm. 2-mm. s o (R-) screen screen screen screen 3488 4.22 4.25 4.52 3 81 3.22 3493 3.68 4.04 3.71 3.30 3495 3.76 3.62 3.94 4.22 3548 4.65 5.02 4.93 3.93 3551 4.22 4.82 4.48 3.71 3556 4.08 4.32 4.30 3552 3.84 4.18 4.17 3.90 3.83 3553 4.31 4.26 4.42 3555 4.03 4.97 4.93 4.79 3 82 3487 4.11 4.04 4.33 Mean 3.77 4.17 4.27 4.46 Differencerequiredforsignificancebetween means: P
