aratus for Rapid Precise Determination of rogenation on a Micro and R.B. Wetherill Laboratory, Furdue University, Lajayette, Ind. 47907 THEDETERMINATION OF UNSATURATION on a micro scale is a problem frequently encountered by organic chemists. A number of devices for the determination of hydrogen uptake have been described. A commonly used procedure allows the determination of as little as 10-4 mole with accuracies of 510% ( I ) . Other devices have been described which allow analyses upon one-half to one-third this amount with accuracies of 1% (2, 3). A technique for achieving rapid, precise analysis of unsaturation on a semimicro scale was recently described (4). Often there is a need for analyses on a micro and ultramicro scale; therefore, a new apparatus and analytical procedure were developed for determining unsaturation in the range of 5 X 10-6 mole with an accuracy of 0.5 to 1.0%. Ultimately, this apparatus has been utilized for analyses of as little as 5 X 10-8 mole of unsaturation with reasonably satisfactory accuracy. The apparatus is shown in Figure 1. (A commercial model of this apparatus is currently available from Delmar Scientific Laboratories, 317 Madison St., Maywood, Ill, 60154.) Its unusual compactness allows it to be immersed in a small, simple water bath to provide adequate temperature control. It utilizes hydrogen from sodium borohydride, thus making unnecessary the bulky hydrogen cylinder that is the usual source of hydrogen. Finally, the catalysts are prepared by the reduction with sodium borohydride of metal salts in a hydrogen atmosphere. This provides highly active catalysts which are in essential equilibrium with the hydrogen atmosphere, thereby avoiding the usually long equilibration stage, These advantages make possible successive analyses of high precision with unusual speed. For example, analyses in triplicate usually require only 30 to 60 minutes, including the preparation of the catalyst. The apparatus is flushed with hydrogen generated in the generator bulb by the injection of sodium borohydride solution into acid. This provides hydrogen free of catalyst poisons, etc. (5, 6). The catalyst is prepared in situ in the reactor flask by the action of sodium borohydride upon a platinum salt in isopropyl alcohol in the presence of high surface area carbon (7, 8). The solution of the unsaturate in isopropyl alcohol or ethyl acetate is Ehen injected into the 1 Present address, Department of Chemistry, Stanford University, Palo Alto, Calif.
(1) S. Siggia, “Quantitative Organic Analysis via Functional Groups,” Wiley, New York, 1949. (2) I. B. Johns and E. J. Seiferle, IND.ENG.CHEM.,ANAL.ED., 13, 841 (1941). (3) A. F.Colson, Analyst, 79,2981 (1954). (4) C. A. Brown, S . C. Sethi, and H. C. Brown, ANAL.CHBM.,39, 823 (1967). ( 5 ) H. C. Brown and C. A. Brown, J. Am. Chem. SOC.,84, 1494 (1962). (6) 6. A. Brown and H. 6.Brown, Ibid.,p. 2829. (7) H. 6. Brown and 6. A. Brown, Ibid.,p. 2827. (8) H. C. Brown and C. A. Brown, Tetrahedron, Suppl. 8, Part I, 149 (1966).
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o
ANALYTICAL CHEMISTRY
reactor flask and stirring is begun. Water is added automatically by the mercury valve of the generator (6, 9) to replace the hydrogen as it is consumed. The moles of unsaturation are then calculated from the volume of liquid added to the system with the aid of a simple equation (see Experimental section). A variety of olefins and dienes were subjected to analysis to test the applicability of the apparatus and procedure. RESULTS
The hydrogenation of distilled 1-octene (Phillips Petroleum Co., 99%) was used for a study of the range and accuracy of the method. The results are summarized in Table I. In general, on quantities of 1.0 x 10-4 to 1.0 x 10-6 mole an accuracy of 1% or better was realized. Analyses of smaller amounts appeared to yield results accurate to about 3 %. With 1.0 X 10-6 mole or less, the uncertainty in reading the buret becomes appreciable. Four olefins, a conjugated diene, a nonconjugated diene, and a fatty acid ester were similarly analyzed using 2.0 X 10-5 mole of unsaturation. The results were again within 1% of the expected value. The results are summarized in Table 11. DISCUSSION
The results obtained with this new apparatus and procedure indicate that it is a convenient and accurate method for the determination of unsaturation on a microscale. Other catalysts than platinum may be used, rates of hydrogenation may be followed to determine if more than one type of unsaturation is present, and the products may be isolated if desired. Thus, the apparatus is of use not only as an analytical tool, but also to the synthetic organic chemist, especially one involved in the isolation and identification of natural products. It is simple to operate, It requires no permanent set-up, but may be readily stored in a cabinet, and used in any convenient location providing only power for the stirrer. Thus it should prove highly convenient and useful for analyses, preparations, and rate measurements a n a micro and ultramicro scale. EXPERIMENTAL
Procedure. In the hydrogenation flask, place 0.1 gram of activated carbon and 2.0 cc of 0.010M chloroplatinic acid in isopropyl alcohol (solution A). Insert stirring bar in the flask and attach it to the apparatus. Place 1.0 cc of 50% aqueous acetic acid (solution E) in the generator. Fill the buret to the zero mark. Insert it in the apparatus. With the buret in place, open the stopcock, place the index finger over the hole in the rubber bulb, and apply gentle pressure to expell the air from the buret tip. Bring the water level in the buret back to zero and close the stopcock. Seal the
(9) @. A. Brown and H. C. Brown, J. Org. Chem., 131,3898 (1966).
1.
a
RUBBER BULB
Table I. Hydrogenation of Various Amounts of 1-Octene 1-Octene, mmoles
Unsaturation found, mmoies"
Std. dev., mmoles
1 2 3
0.1000
0.1003
0.1000
0.0004 0.0005
4
0,0500
5
0.0200
6
0.0100 0.0100~ 0. O05Ob
0.1001 0.0499 0.0499 0,0202 0.0099
Run BURET
TRANSPARENT WATER BATH
7
..>. :.',:
....
.... . .!. ..,.',. . .., .... . .... ..;. .... ..::... .... ,. ...... ..... . .. ... .. ..:
0.0500
8
0.0003 0.0005
0.0003 0. oO04
0.0101
o.Oo01
0.0048
0.0001
Average of four replicate determinations in each run. The generator was filled to two-thirds capacity to reduce free space and thereby magnify the effect of pressure variations. a
b
(
Table 11. Hydrogenation of Various Unsaturated Compounds Compound
Figure 1. Apparatus for micro and ultramicro hydrogenations system with serum stopples and immerse it in the bath (adjusted to any convenient temperature, generally 25.0 O C). Introduce a hydrogen atmosphere by injecting into the generator over 30 seconds 2.0 ml of 2.OM sodium borohydride in 1.OMaqueous sodium hydroxide (solution C). Inject with the point of the hypodermic needle of the syringe in the center of the generator bulb. Begin stirring the hydrogenation flask (stirrer setting about 3). Generate the catalyst by injecting 2.0 cc of 0.1M sodium borohydride solution in isopropyl alcohol (solution B) into the hydrogenation flask. After 15 seconds, introduce 0.5 cc of 10 % hydrochloric acid in isopropyl alcohol (solution D). Alternatively, 0.1 cc of glacial acetic acid may be used. (The syringe must be washed immediately after injection of the acid, as corrosion products from the needle can poison future catalysts.) Allow the apparatus to equilibrate with the bath at 25 O C for 10 minutes The pressure in the system should be adjusted so that the mercury in the capillary tube of the bubbler is at the upper red mark. If the mercury is above the desired level (pressure too low), it may be brought down by squeezing in water from the buret. If it is below the desired level (pressure too high), remove some water from the generator bulb with a syringe (keeping the system closed) until the pressure is as desired. Inject a sample of the unknown in isopropyl alcohol (0.1 to 0.4M, 0.100 to 0.250 cc of sample). Usually an amount of sample equivalent to 0.05 mmole (1.0 cc) of hydrogen absorbed is used. Open the buret stopcock after injection. When reaction has stopped, add water from the buret to bring the mercury thread in the bubbler to the upper red mark. (The behavior of the mercury in the capillary of the bubbler provides a sensitive indication of the absorption of hydrogen, and the end of the reaction is indicated when the level of the mercury shows no change over a reasonable time,) Allow the apparatus and bath to equilibrate for 5 minutes. Readjust the mercury thread to the upper red mark by adding more water from the buret, if necessary. Take the buret reading and reset the buret to zero.
1-Octene Cyclohexene Cyclooctene 2,4,4-Trimethyl-lpentene 1,3-Cyclooctadiene 1,5-Cyclooctadiene Ethyl oleate
Amount, mmoles
Unsaturation found, mmoles"
Std. dev., mmoles
0.0200b 0,0200~ 0,0200~
0.0201 0.0200 0.0200
0,0002 0.0002
0.0200b
0.0198 0.0197 0 0198
0. O1OOb~,c 0 .O1OOb~c
0.0200b
0.0201
0.0003 0.0001
0,0002 0.0002 0.0001
, Average of four determinations in each case. See Table 1. 0 Contains 0.0200 mmole of unsaturation. b
Inject another sample and repeat fhe above procedure. For higher accuracy it is desirable that four or five successive determinations should be made on one compound, the first result being discarded. The moles of hydrogen absorbed are calculated as follows: mmoles Hz =
+
273 (V, VI) (P - 45) 22.4 760 ( T + 273)
V , = Vol of water added from the buret, in ml VI = Vol of sample injected, in cc P = Atmospheric pressure in mm of mercury T = Temperature, "C Experimental Solutions. A. 0.010M CHLOROPLATINIC ALCOHOL.Dissolve 0.1 gram of chloroACIDIN ISOPROPYL platinic acid (40% metal by weight, HJtC ' 18) in 20 cc of dry isopropyl alcohol. B. 0.10M SODIUMBOROHYDRIDEIN DRY ISOPROPYL ALCOHOL.Dissolve 0.4 gram of sodium borohydride, (98 %) in 100 cc of dry isopropyl alcohol (F). Allow any sediment to settle, decant the clear liquid, and store at 0" C protected from moisture. Do not keep over one week. C. 2.OM SODIUMBOROHYDRIDE IN 1.OM AQUEOUS SoDIUM HYDROXIDE. Dissolve 4.0 grams of sodium hydroxide pellets and 8.0 grams of sodium borohydride in 80 cc Qf water and dilute to 100 cc. D. 10% HYDROCHLORIC ACID M ISOPROPYL ALCOHOL. Dilute 5 cc of concentrated hydrochloric acid to 50 cc with isopropyl alcohol. Replace this solution every one to two days. E. 50% AQUEOUSACETICACLD. Mix 20 cc of glacial acetic acid and 20 cc of water. VOL. 39, NO. 14, DECEMBER 1967
1883
F. DRY I~OPROPYL ALCOHOL. Reflux 1 liter of isoPropyl alcohol with 15 grams of sodium borohydride for 30 minutes and distill. Protect from atmospheric moisture.
the development of this apparatus, and Frederick R. Jensen of the University of California, Berkeley, in whose laboratory part of the experimental work was performed.
ACKNQ WLEDGMENT
RECEIVED for review April 12, 1967. Accepted September 6, 1967. Study assisted in part by a grant from Parke, Davis and Co. This is paper IV in a series on Catalytic Hydrogenation.
The author thanks Herbert C . Brown of Purdue University who provided suggestions and laboratory space during
u r ~ ~ ~ iTitration ~ ~ t rof ~Fluoride c with Neodyrniurn(lll) John E. Roberts Department of Chemistry, University of Massachusetts, Amherst, Mass. 01002 1958, Brandt and Duswalt ( I ) investigated the Photometric end-point detection when fluoride was titrated with calcium or thorium ions. While the results were satisfactory for simple systems, the presence of phosphate or sulfate interfered seriously. Mention was made that titrations with cerium(II1) were also satisfactory, but no further details or supporting evidence were offered. The titration of fluoride with thorium (IV) has long been known to be nonstoichiometric requiring standardization against known fluoride under carefully controlled conditions. The lanthanide(lI1) cations precipitate fluoride quantitatively even in solutions of low pH. The solubility products of the lanthanide fluorides ( ~ 1 O - ~ 5lead ) to an equilibrium concentration of fluoride which is at least an order of magnitude smaller than that from calcium fluoride. Furthermore, in contrast to thorium fluoride, the lanthanide trifluorides are stoichiometric. The method reported here utilizes neodymium(II1) as the precipitant for fluoride in acidic solution with photometric determination of the equivalence point. Simple equipment is used and either macro- or micro-scale operation is satisfactory. IN
‘ EXPERIMENTAL
The solutions used were 0.1000M sodium fluoride, approximately 0.2M neodymium nitrate prepared from freshly ignited 99.99 % Nd& and a 50 % v/v water solution of polyethylene glycol 400. A seeding reagent was prepared by suspending enough precipitated and washed NdF3 in water to give a strong turbidity. Equipment used was as follows: Photometric end points were detected with a Fisher Electrophotometer 11. This was modified for magnetic stirring by mounting an airdriven magnetic stirrer (G. F. Smith Chemical Co.) below the cell compartment. This arrangement was inexpensive and had a sensitivity commensurate with turbidimetric end points. Two sizes of cells were used. A short length of 33 mm borosilicate glass tubing sealed and flattened at one end was satisfactory for 50-ml total volumes. For titrations on a smaller scale, the 25-ml cells supplied with the instrument were used. Regular 5-ml burets were used with the large cell; a Benedetti-Pichler style horizontal buret (2) was used for small scale operations with the 25-ml cell.
(1) W. W. Brandt and A. A. Duswalt, ANAL.CHEM.,30, 1120 (1958). ( 2 ) A. A. Benedetti-Pichler, “Introduction to the Microtechnique of inorganic Analysis,” Wiley, New York, 1942, p. 2 5 6 8
1884
e
ANALYTICAL CHEMISTRY
Development of Procedure. INDUCTION PERIOD. When an acidic solution of fluoride was titrated with neodymium(II1) solution, the plotted results showed the expected two straight lines which could be extrapolated to an intersection at the equivalence point. The graph showed some curvature in the vicinity of the end point and also at the beginning of the titration, an induction period. This latter effect, apparently due either to solubility or precipitation-nucleation rate effects, was eliminated by addition initially of enough suspended NdF3 to produce a noticeable turbidity. This was zeroed out instrumentally before titration. While this effect did not affect accuracy for large samples, with 1-mg samples correct results were obtzined only after addition of the seeding reagent. PROTECTIVE COLLOID.During titrations with continuous stirring, variations in turbidity were occasionally noticed and were apparently related to particle size changes and coagulation of the precipitate. Addition of a protective colloid eliminated this effect. While Gum Arabic was effective, ethylene glycol 400 gave better stability of the suspension and was not susceptible to organic growths. INCIDENT LIGHT. The incident light color had only minor effects on the titration. A green filter (525 mp) yielded the steadiest most reproducible readings. Although aqueous neodymium(II1) absorbs strongly in the visible spectrum, this was never observed in this work; a much narrower band pass would be required to detect this effect. EFFECTOF pH. Titrations were carried out at acidities ranging from 0.1M H N 0 3 to pH 4.5. At pH 4.5 (acetate buffer) slightly high results were found suggesting either that acetate exerts a solubility effect on the precipitate or that some mixed complex in which the F:Nd ratio is less than 3 :1 is being formed. At pH 2.9 (monochloroacetate buffer) the results were satisfactory although in this case (as in some others in which a sizeable salt concentration existed) the initial induction period was not completely eliminated by introducing solid NdF3 before titration. In this connection, microscopic examination of the precipitate showed that the NdFa formed from chloroacetate buffered solutions had a particle size ranging from 3 to 15 microns (Martin’s diameter) whereas that from nitric acid solutions consisted of particles uniformly less than 1 micron. SOLVENT.Addition of up to 50% by volume of ethanol or acetone had no measurable effect on the titrations. INTERFERENCES. Ions which form insoluble compounds with Nd(lI1) under acidic conditions will interfere ; notable is oxalate. Ions which form insoluble compounds or stable complexes with fluoride will interfere; in this group are iron(III), aluminum(IPI), borate, caicium(II), thorium(IV), and perhaps others which were not investigated. It is especially noteworthy, however, that neither phosphate nor
