636
A N A L Y T I C A L CHEMISTRY
in temperature. The last traces of hydrogen were removed, and constancy of weight was attained, by continuous pumping for 2 to 3 hours a t 900" C. That complete removal of hydrogen was achieved by this procedure Fas shown by the fact that the residue from one determination yielded less than 0.001% by n-eight of hydrogen when subjected to vacuum fusion a t 1700" C. RESULTS
Seven runs were performed with the purified decaborane. Two n-ere unsuccessful because the weld sealing the tube burst open before pyrolysis was complete. I n Table I are shown the data for the remaining five runs. The mean of the five atomic ratios of hydrogen to boron is 1.400 with a standard deviation, sm, of 0.002, the standard deviation, s, of the individual runs being 0.004.
Sm =
Table I. Decaborane 232.37 226.29 258.62 239.31 223,42
S
-
l/n
= 0.0°20
Ultimate Analysis of Decaborane Wt. in hIg. Hydrogen 26.73 26.20 29.81 27.64 25.73
Boron 205.64 200,09 228.81 211.67 197.70 Mean (i)=
4-hour period a t 200' to 210' C., and overnight heating a t 300' C. led to 63% pyrolysis. The considerable amount of decomposition noted in these experiments a t 200" C. is in contrast to Stock's report ( 2 ) that on heating for 48 hours a t 200" C. in an evacuated sealed tube the decomposition still remained very slight. The more rapid decomposition noted in this investigation may have been due either to the rapid removal of hydrogen from thepyrolysis zone (which would imply reversible steps in the decomposition) or to catalytic activity on the part of the palladium. NATURE OF THE PYROLYSIS RESIDUE
In all of the runs there was some evidence of alloying between the boron and the palladium. Shiny patches appeared on the outer surface of some of the capsules, and the palladium, after prolonged heating. became very brittle. In all cases the residue remaining after the pyrolysis consisted of large (0.5 to 1.0 mm.), hard, shiny black granules, many of them imbedded in the wall of the tube. X-ray diffraction patterns of two of the residue8 showed only a few diffuse rings, indicating an amorphous material. The palladium contents of residues 1 and 3 (Table I ) were 4.9 and 4.5 weight per cent, respectively, or approximately one atom of palladium per 200 atoms of boron. The density of residue from the fifth sample was 2.38 grams per ml.
Atomic Ratio.
ACKNOWLEDGMENT
H/B
The authors express their appreciation to A. E. Xewkirk for the sample of purified decaborane, to TV. R. Grams for his cooperation in developing the cold welding procedure, to Miss M. J. Ferguson for the weighings, to R. S.McDonald for the vacuum fusion, and to Mrs. B. Decker for the x-ray diffraction work.
1.395 1.406
1.398
1.402
1.397 1.400
LITERATURE CITED
The mean (a), 1,400, corresponds exactly to the formula B,,H,,, and sm is comparable to the level of accuracy (one part per thousand) to 1% hich the atomic weight of boron is reported. In all of the runs hydrogen evolution did not become noticeable until a temperature of about 200" C. had been reached, a t which point a rapid pressure increase was observed. In one experiment 29% of the total hydrogen was evolved during a
(1) Kasper, J.
S.,Lucht, C. M.,and Harker, D., Acta C ~ u s t .3, , 436
(1950). (2) Stock, A,, "Hydrides of Boron and Silicon," p. 83, Ithaca, S . Y . ,
Cornel1 University Press, 1933. (3) Stock, A , , and Pohland, E., Ber. deut. chem. Ges., 62B, 90 (1929). RECEIVED for review Kovember 17, 1952. Accepted December 12, 1952. Presented before the Division of Physical and Inorganic Chemistry at the 122nd Meeting of the - ~ M E R X C CHEsfIcAx. AX SOCIETY, Atlantic City, X. J.
Determination of 1,Z-Propylene Glycol in Ethylene Glycol Mixtures CHARLES B. JORDAN AND VIRGIL 0. HATCH Paint and Chemical Laboratory, Aberdeen Proving Ground, Md. preparations of ethylene glycol, appreciable Ipresent. amounts of 1,Ppropylene glycol and other glycols are often The procedure described in this paper is applicable to ?\'
cOM?ZERCIAL
the determination of l,2-propylene glycol in ethylene glycol mixtures. I t is a quantitative chemical method utilizing standard laboratory equipment. It is based on the formation of iodoform from 1,2-propylene glvcol and the subsequent quantitative determination of iodoform volumetrically. -4nalyses of samples of knoa n composition varying in lj2-propy1ene glycol content from 1 to 100% showed it to be a reliable and accurate procedure. When used in conjunction with other analyses, this procedure is applicable to the analysis of permanent-type antifreeze solutions. Commonly used inhibitors or inorganic impurities do not interfere. It is also applicable to aqueous solutions of glycols without any modifications. Most of the previous work involved the periodate splitting of vicinal glycols (3, 6, 9 ) ) with subsequent determination, by one method or another, of the acetaldehyde or formaldehyde formed by the splitting. Reinke and Luce (12) determined the acetaldehyde in the presence of the formaldehyde by the reaction of the
latter with a slight excess of glycine and reaction of the acetaldehyde with sodium bisulfite. The reaction of some of the acetaldehyde with glycine limits the accuracy of their method. Warshowsky and Elving (14) and Cannon and Jackson (1) used a polarographic analysis of the aldehydes. Mitchell (11) used an optical crystallographic method for measuring the aldehydes. Other analytical methods involving the use of the Beckman spectrophotometer ( 2 ) and the infrared spectrophotometer have been investigated with varying results. All analytical instrumentation methods Tvere more complicated and gave greater errors than the proposed method. The qualitative iodoform reaction was considered applicable to the problem. This method had, a t an early date, been used in the gravimetric determination of acetone in methanol ( 7 ) and ethyl alcohol in water (8). Messinger (IO)and later Houghton ( 5 ) refined the procedure and used it in acetone determinations. Hatcher and Mueller (4)used the procedure to determine acetaldehyde. However, when these applications were used in the analysis of 1,2-propylene glycol in ethylene glycol mixtures, low and inconsistent results were obtained.
V O L U M E 25, NO. 4, A P R I L 1 9 5 3 An adaptation of the iodoform reaction, based on the selective formation of iodoform from 1,2-propylene glycol, was developed. Direct drying and weighing of iodoform is impractical due to its volatility, so the iodoform is determined quantitatively by a unique application of the Volhard (13) method of determining, volumetrically, the iodide ion. RE4GEYTS
Sitric acid, concentrated, C.P. Iodine solution, 4 h ' aqueous, prepared by dissolving 508 grams of resublimed iodine and 1000 grams of potassium iodide in enough distilled Tvater to make 1 liter of solution. Sodium hydroxide solution, 2.5 A: in methanol, prepared by dissolving 100 grams of reagent grade sodium hydroxide in enough redistilled methanol (free from ethyl alcohol) to make 1 liter of solution. The solution is filtered before using. Sodium hydroxide solution, 10% aqueous, prepared by dissolving 100 grams of reagent grade sodium hydroxide in 900 grams of distilled water. Sodium thiosulfate-sodium hydroxide solution, prepared by mixing 4 volunies of 2 it' aqueous sodium thiosulfate with 1 volume of 2 aqueous sodium hydroxide. Isopropyl ether, C.P. Benzene, C . P . Silver nitrate solution, 0.1 .Yin ethyl alcohol, prepared by dissolving 17 grams of reagent grade silver nitrate in enough absolute ethvl alcohol to make 1 liter of solution. ?;Iethanol, absolute. Ferric alum indicator, prepared by dissolving 35 grams of ferric ammoniuni sulfate in 100 ml. of distilled water and adding enough 6 S nitric acid to cause the disappearance of the hronn color. Potassiuni thiocyanate solution, standardized 0.1 it' aqueous, prepared by dissolving 9.7 grams of potassium thiocyanate in enough distilled water to make 1 liter of solution, standardized with pule silver nitrate. PROCEDURE
Iodoform Formation. Accurately weigh 2 to 3 drops (0.07 to 0.10 gram) of the sample to be tested into a 500-ml. Florence flask. Add 0.5 ml. of concentrated nitric acid, and heat at 50' C. for 20 minutes or until dense brown nitrogen dioxide fumes appear. Stopper the flask, and let i t stand for 10 minutes. Add 7 ml. of 4 S iodine solution, stopper, and swirl for 5 minutes. Add 10 nil. of 2.5 A' alcoholic sodium hydroxide, drop by drop, at such a rate that the addition takes 10 minutes. Swirl the flask during the addition. .4dd 5 ml. more of iodine solution. Then add 10 nil. of the alcoholic sodium hydroxide solution, drop by drop, with constant swirling, taking another 5 to 10 minutes for the addition. .4dd 5 ml. more of the iodine solution. .4dd a third 10-nil. portion of the alcoholic sodium hydroxide, in the same manner as above. Heat the flask for 3 to 4 minutes a t 50" C. Rapidly add 15 ml. of 10% aqueous sodium hydroxide solution. Add, in 1- to 2-ml. portions, the sodium thiosulfatesodium hydroxide mixture until a portion causes no change in the canary yellow color. Add 150 ml. of distilled water which has been heated to 35' to 40" C. Let the mixture stand 30 minutes. If yellon- iodoform crystals appear, filter them. Wash the residue with a small amount of distilled water. Combine the wash water and filtrate. If, upon the addition of the 150 ml. of distilled water above, no iodoform crystals appear in the reaction mixture, treat the mixture, from this point, in the same manner as the filtrate. Iodoform Determination in Residue. I n a 500-ml. Erlenmeyer flask, dissolve the residue with 75 ml. of isopropyl ether. .4dd exactly 25 ml. of 0.1 N alcoholic silver nitrate. Add 75 ml. of benzene and 30 ml. of absolute methanol. Clamp the flask upright over a steam bath, and direct a stream of dry air into the flask at such a rate that the solvents do not visibly boil. When approximately half of the mixture has been evaporated, add another 25-ml. portion of alcoholic silver nitrate. Continue the evaporation. When the contents of the flask are dry, remove the air and continue to heat on the steam bath for 30 minutes. Remove the flask, add 100 ml. of distilled water, 3 ml. of ferric alum indicator, and titrate with standardized 0.1 S potassium thiocyanate solution. Iodoform Determination in Filtrate (or Reaction Mixture If No Crystals Form). Extract with three 25-ml. portions of benzene, Place the extractions in a 500-ml. Erlenmeyer flask. Add 75 ml. of isopropyl ether and 25 ml. of alcoholic silver nitrate, place on a steam bath, and proceed as above through the potassium thiocyanate titration.
637 Blank Determination. Place 75 ml. of isopropyl ether, 75 ml. of benzene, 30 ml. of methanol, and 25 ml. of alcoholic silver nitrate in a 500-ml. Erlenmeyer flask, place on a steam bath, and proceed through the potassium thiocyanate titration. Calculations. The formula for calculating % 1,2-propylene glycol is: ( B - ,4) X N X 0.076 X 1.39 X 100 % 1,2-propylene glycol = 3 x B, ml. of standard potassium thiocyanate solution required for a blank containing the same amount of alcoholic silver nitrate as was added to the total sample. A , ml. of standard potassium thiocyanate solution required for sample. N , normalitjf of potassium thiocyanate solution. W , weight of sample.
w
DlSCUSSION
The addition of concentrated nitric acid to the original sample was thoroughly investigated. I t n a s noted that yields of iodoform were greatly increased by this addition. The exact chemical reaction which takes place is not known. I t is believed to be a combination oxidation and nitration of the 1,2-propylene glycol. If this is the case, explosive mixtures are formed, which are considered impotent as long as they are not isolated and are present in such small quantities. Larger quantities of nitric acid than the 0.5 ml. specified decreape thP yield of iodoform. Smaller quantities are imprartical.
Table I.
Determinations of 1,2-Propylene Glycol i n Ethylene Glycol Mixtures c , Calculated
a, 70 12-Propylene
Glycol in Sample" 100 100 100 66.7 66.7
31 5
31.5 8 9 8.9 4.4 4.4 0 0 14.9b 14.96
b , % 1.2-Propylen? Glycol Found 72.9 72.0 i3.2 477 . 88 4 4 7 .6 47 22.5 22.5 6.2 6.5 3.2 3.3
%
I .P-Propyiene Glycol (Using Factor 1.39) 101.2 100.0 101.5 66.4 HH 1
31 3 31 3 8 6 9 0 4.4 4.6
0.2 0.2
0.3 0.3
10.4
14.5
Error, 5, (c
-
a)
+1.2 0.0 +1.3 - 0 . 3.?
-n i R -0.2
-0.2 -0.3 +o. 1 0.0 t o .2
+o
3 +O. 3
-0.4
10.7 14.9 0.0 a "Chemically Pure" propylene glycol was fractionally distilled twice; only the center cut from each fractionation was used. All propylene glycol used had a boiling point of 188.4' C. b Samples taken from commercial antifreeze mixtures.
The strength and order of addition of the iodine and sodium hydroxide solutions are critical. Weaker aqueous solutions gave from 0 to 60% conversion to iodoform. T o obtain reproducible yields the methyl alcoholic sodium hydroxide must be used, and the alternate addition of the iodine and sodium hvdroxide must be carried out as described in the procedure. The use of the sodium thiosulfate-sodium hydroxide solution Tvas necessaiy to obtain reproducibility. Sodium hydroxide or sodium thiosulfate used individually gave lower iodoform yields. The mixture of the two chemicals solved the problem of forming the iodoform and removing excess iodine from the solution. Slanv methods of iodoform determination were tried, but the adaptation of the T'olhard volumetric procedure proved most successful. Starting Tvith pure iodoform, consistent yields between 99.7 and 100% were obtained by using the procedure outlined. A possible equation for the reaction of iodoform a i t h alcoholic silver nitrate is: CHI3
+ 3BgSOa + 3ROH
+
3dgI
+ 3"03
+ HC(OR)a
R may be C2Hjfrom the ethyl alcohol, CHI fromthemethanol, or H from any moisture present. The orthoformate formed usually decomposes, and does not exist as such.
638
A N A L Y T I C A L CHEMISTRY
Due to interfering reactions, 1 0 0 ~ yields o of iodoform based on known l,2-propylene glycol were not obtained. 'By varying certain conditions, yields up to 90% of the 1,2-propylene glycol in the sample were obtained. These yields were not reproducible and varied according to the yo l,2-propylene glycol in the sample. The highest reproducible percentage found for the range of 1 to 100% 1,2-propylene glycol was 72 f 1.5%. This led to the choice of the factor 1.39 (reciprocal of 0.72). See Table I. Variations from the procedure as outlined will cause a change in the factor. Impurities present in the sample containing the grouping CH3CO--, adjoined to a hydrogen atom or to a carbon atom which does not carry active hydrogen atoms or groups which provide an excessive amount of steric hindrance, will interfere. Also, compounds which may be oxidized to compounds containing the above grouping by the reagents will interfere. A few specific examples are ethyl alcohol, isopropyl alcohol, acetaldehyde, acetone, methyl ethyl ketone, 2,3-butylene glycol, 1,3butylene glycol, 2-methyl-2,4-pentandiol, and all straight chain polymers of 1,2-propylene glycol. Based on the potential wide field of application of the iodoform reaction, it may be possible to extend the procedure to other determinations involving these compounds.
ACKNOWLEDG.MENT
,
The authors gratefully acknowledge the laboratory assistance rendered by Richard G. Bradyhouse I11 during the development of this procedure. LITERATURE CITED
CHEX,,24, 1053 (1) Cannon, W. A,, and Jackson, L. C., .INAL. (1952). (2) Dal Nogare, Stephen, Sorris, T O., and Mitchell, J., Jr., -bar.. CHEM.,23, 1473 (1951). (3) Fleury, P., and Boisson, S., Compt. rend.. 204, 1264 (1937). (4) Hatcher, W. H., and Mueller, W, H., T r a n s Roy. Soc Can., [3j 23, Section 111,35-44 (1929). (5) Houghton, C. O., IND.ENG.CHEM.,SAL. ED.,9, 167 (1937). (6) Jackson, E. L., and Hudson, C. S., J . Am. Chem. Soc., 59, 994 (1937). (7) Kramer, G., Ber. deut. Chem. G e s . , 13, 1000 (1880). (8) Lieben, -4., Ann., Bpeciai Vol., 7, 377 (1870). (9) Malaprade, L., Compt. rend., 186, 382 (1928). (10) Messinger, J., Be?. deut. chem. Ges., 21, 3366 (1888). (11) Mitchell, J., Jr., i i ~ 7 . 4 ~CHEM., . 21, 448 (1949). (12) Reinke, R. C., and Luce, E. E.,TND. ENG.CHEM.,ANAL.ED., 18, 244 (1946). 113) Volhard, J., Ann. Chern. Pharm., 198 (1879). (14) Warshowsky, B., and Elring, P. J., ISD. ENG. CHEM., ANAL. ED., 18, 253 (1946). .
I
RECEIVED for review July 12, 1952. Accepted Xovember 11, 1952.
Direct Determination of Oxygen in Rubber Comparison between Isotopic and Schiitae- Unteraaucher Methods A. D. KIRSHENBAUM AND A . G. STRENG Research Institute of Temple University, Philadelphia 40, P a .
N THE past, oxygen has been determined
as in I other organic compounds, by difference (9).in rubber, This method was time-consuming and was subject to cumulative errors. Only in the past 25 years have serious attempts been made to develop a direct analytical method for the determination of oxygen. The first successful attempt was by ter Meulen (8) who catalytically hydrogenated the oxygen of organic materials to water. Cramer, Sjothun, and Oneacre (2) applied ter Meulen's method of determining oxygen to rubber. This method has the disadvantage of most catalytic methods, in that it requires careful and accurate control and purity of the catalyst. I n 1939 a more convenient method was developed by Schutze (10) and later improved by Unterzaucher (11). This method is based on the decomposition of the organic compound in a stream of pure nitrogen over pure carbon a t a temperature of 1120" C. -411 of the oxygen of the organic compound appears as carbon monoxide. Chambers ( 1 ) and Walton et al. ( l a ) used this method to determine the oxygen content of rubber. An excellent review by Elving and Ligett ( 3 ) summarizes these methods of analyzing for oxygen in organic compounds. All the above-mentioned methods require quantitative separation and quantitative recovery of the oxygen-containing compound or of its reaction products. I n 1946 Grosse, Hindin, and Kirshenbaum ( 5 ) developed an isotopic method for determining the oxygen content of organic compounds. The isotopic method has the fundamental advantage that quantitative results are obtained without any quantitative separation or recovery of any oxygen or oxygen-containing compounds. In this paper the isotopic method is compared directly with the SchutzeUnterzaucher method by determining oxygen in rubber. SCHUTZE-UNTERZAUCHER
METHOD
The sample is vaporized a t 950" to 1000" C. in a stream of pure nitrogen, and the products are carried over carbon heated
a t 1100" to 1150" C., whereby all the oxygen present is converted to carbon monoxide. The latter is then oxidized to carbon dioxide by iodine pentoxide, and the equivalent amount of iodine liberated is determined titrimetrically with sodium thiosulfate. This method has been described in detail in other publications (1, 3, 7 , Q, 10). The rubber sample (30 to 50 mg.) is vaporized at 1000" =I= 25' C. in a stream of nitrogen (10 ml. per minute) and passed over carbon heated at 1150' zk 25' C. in a quartz reaction vessel. The carbon monoxide formed is swept by nitrogen over pure iodine pentoxide heated to 120" C. and thus converted to carbon dioxide. Simultaneously, the iodine pentoside is reduced to iodine, which is determined quantitatively by titrating vith a standard solution of 0.02 N sodium thiosulfate. The nitrogen used is first freed of all traces of oxygen by passing the gas over copper heated at 500" C. ISOTOPIC METHOD
The principle of the isotopic method is an adaptation of the isotopic dilution principle originally introduced hy von Hevesy and Paneth ('7). The sample, together with a known amount of oxygen-18 is heated in a platinum tube to the point of decomposition (800" to 900" C.) for 30 to 60 minutes. At this temperature all the oxygen atoms exchange statistically with the various oxygen-containing compounds which are stable a t these teniperatures (water, carbon dioxide, carbon monoxide, oxygen. and any oxygen in the residue). The ratio of the oxygen isotopes in all of these compounds is identical. The ratio, oxygen-18 to oxygen-16, of a small sample of the gas is determined tTith a high degree of precision by means of modern isotopic mass spectrometers. Knowing the ratio one can easily calculate the original oxygen content from the simple mixture rule, for which the equation is
x%=
b (m - n) a7t
x
100
