nique, a hydrolysis time of approximately 30 minutes only was required to obtain complete recovery of ammonia. I n the chromotropic acid technique a transfer of hydrolyzate is necessary prior to the addition of chromotropic acid reagent, whereas in the nesslerization technique, color reagent is added directly to the hydrolyzate. Finally, time of color development in the latter technique is 10 minutes compared to 30 minutes in the former. I n commercial preparations, there are more likely to be impurities in the form of nitrogen-containing compounds, which upon hydrolysis can be converted into ammonia, than impurities which form formaldehyde on degradation. It is considered, however, that the mild acid hydrolysis conditions of the present procedure are not likely to produce ammonia with most other organic nitrogen compounds that may be encountered, Free ammonia may be detected by carrying out an initial nesslerization on aqueous solutions of both methenamine and methenamine
Table IV. Stability of Methenamine and Methenamine Mandelate in Aqueous Solution
Sample, 7/Ml.
Time in Aqueous Soh- Absorbtion Prior to ance, Nesslerization 465 M p
Methenamine mandelate 18.0 18.0 18.0 18.0 18.0 18.0
18.0
5minutes 15 minutes
30minutes
60minutes 2 hours 4 hours 18 hours
Nil Nil Nil Nil 0.018
11 days
ACKNOWLEDGMENT
The authors extend thanks to Albert W. Jackson for his interest and timely suggestions during the course of this work and to Louis Greenberg for his criticism of the manuscript. Methenamine and methenamine mandelate were provided by the Nepera Chemical Co., Inc., Yonkers, N. Y.
0.044
0.175'
Methenamine 6.2
mandelate. This limitation of specificity in the nesslerization procedure is not a problem in the assay of methenamine mandelate in sensitivity disks.
Nil
a Sam les read against a nesslerized water bknk after 10 minutes' color development time. * A precipitate waa formed in this sample.
LITERATURE CITED
.,
f l ) Buchi. J.. Pharm. Acta Helv. 13. 132 (1938). ' (2 Knight, V., Dra er, J. W., Brady, ! A.. Antibiotics & - A,. Attmore. C Chem'therapy 2,' 615 (1952). (3) MacFadyen, D. .4.! J. Biol. Chem 158, 107 (1945). (4) Slowick, E. F., Kelley, R. S., J. Am. Pharm. Assoc. 31, 15 (1942).
b.
RECEIVED for review February 9, 1959. Accepted July 17, 1959.
A Study of Ebullioscopic Molecular Weight Methods JAMES V. KENNEDY, R. JAY FRIES,' LLOYD J. SULLIVAN,I and CHARLES B. WILLINGHAM Mellon Institute, Pittsburgh
13, Pa.
b The precision and some of the limitations of two ebulliometric techniques, the ebullioscopic constant and the comparative standard method, were evaluated using pure hydrocarbons and a few nonhydrocarbons as solutes. The ebullioscopic or molal boiling point constant, Ka, was independent of molecular weight and solute type a t infinite dilution, and with this constant molecular weights can b e obtained to within 1 to 2% of the true formula weight. The comparative method, with either an internal or an external standard, may yield reliable molecular weights to within 2.5%; however, the variation of & a t finite concentrations is more pronounced in acetone and cyclohexane than in benzene, and appropriate corrections may have to be introduced.
I
N MANY methods for the characteri-
zation of high-boiling petroleum hydrocarbon fractions, experimental molecular weights (9,6) or properties Present address, Lo8 Alamos Scientific Laboratory, LOBAlamos, N. M.
* Preaent
address, Texas Instrumente
Co., Dallas, Tex.
1884
ANALYTICAL CHEMISTRY
which correlate with molecular weight are required (1, 4 ) . Of the number of molecular weight methods derived from the colligative properties of solutions, one of the most common is the ebullioscopic technique which is based upon the elevation of the boiling point of a solvent by a nonvolatile solute. The purpose of this study was to determine experimentally the reliability of two techniques used for aacertaining molecular weights by the ebullioscopic method, employing apparatus developed in this laboratory (7). The first of tlieae techniques is baaed upon the determination of the ebullioscopicconstant of a solvent and the application of this constant for the determination of molecular weights of a wide range of other materials. The second technique involves the measurement of the elevation of the boiling point of a solvent by a solute of unknown molecular weight relative to that found €or a standard substance. The comparison may be made in the same solution as an internal standard, or in a second solution aa an external standard (8). The fundamental principle involved, aa expressed by Raoult's law, states that the partial pressure of the solvent in an ideal solution is proportional to
the mole fraction of solvent. Assuming that dilute real solutions obey this law, it may be combined with the ClausiusClapeyron equation to obtain the familiar expression for the ebullioscopic molal boiling point constant: where K b is the ebullioscopic constant,
M 2is the gram molecular weight of the solute, T B the elevation of the boiling point, and tol and 202 the weight in grams of the solvent and solute, respectively. Two requirements of this equation as stated are that K sbe completely independent of the nature and concentration of the solute and that the boiling point elevation of the solvent be dependent only upon the concentration of the solute, regardless of its nature. For many real systems, Raoult's law is only approximated a t higher concentrations. Therefore, to correct any deviations that may exist, it will be necessary to extrapolate some function of AT to zero concentration to obtain data in dilute solutions which are valid for the relationship. The comparative standard technique is based upon a simple ratio of the ebullioscopic constant relationship for two solutes, assuming an equal amount of
solvent is used in each case. It is expressed by :
where M , and M , are the molecular weights of the known and unknown respectively, Kb(,) and Kb(.) are the ebullioscopic constants, AT, and AT, are the boiling point elevations, and w. and w. are the solute weights. At a finite concentration, systems which show unequal deviations from Raoult’s law yield different values for the ebullioscopic constant of the solvent. For these systems, a correction factor, which is the ratio of the ebullioscopic constants a t finite concentration, must be applied to the experimental data. Where the systems show the same or no deviation from Fhoult’s law, the ratio of the ebullioscopic constants will be equal to one and no correction is required. EXPERIMENTAL
Apparatus. The details of t h e ebullioscopic apparatus used in this study and a procedure for its use in t h e determination of molecular weights by one of t h e comparative standard methods have been published (6). The temperature difference between the two ebulliometers was determined by a differential thermoelement whose e.m.f. was suitably amplified and recorded. Table 1.
3
4 5 6 7
8
9 10
11
12 13 14 15 16 17 18
19 20 21 a
The solutes listed in Tables I, 11, and
I11 cover a molecular weight range of 100 to 900 grams. They include 21
PSU No.
All of the hydrocarbons identified by
PHU numbers were obtained from thc American Petroleum Institute Research Project 42 at the Pennsylvania State University (Joseph A. Dison, director), and were used as received. n-Octacosane was an Eastman chemical and also used as received. The m-terphenyl (Monsanto) and triphenylmethane (Eastman) hydrocarbons were of high purity, but were recrystallized prior to use from their respective solvents. The four n o n h y d r o c a r b o n s , r e s o r c i n o 1 (Fisher’s certified grade), benzil (Fisher’s reagent grade), Coumarin, and tristearin (Eastman’s high purity) were used without additional purification. Determination of t h e Ebullioscopic Constant. The pure dry solvent for this work was stored in a hurct (Fisher Scientific Catalog No. 20-1 10) having a t o p reservoir suitably protected by drying tubes. The buret was filled by filtering solvent into it through a n appropriate desiccant (active silica gel for benzene and cyclohexane, anhydrous calcium sulfate for acetone). The ebulliometers were thoroughly cleansed, and, for the ebullioscopic constant method, dried by vacuum before they were filled. A known amount of the dry solvent, 5 ml. accurate to 0.01 ml. a t 25‘ C. was used. I n these measurements, the amount of solvent in the vapor phase must be constant, and this was accomplished by controlling
7-n-Hexyleicosane n-Oc tacosane 11-n-Decyldocosane n-Hexatricontane n-Tetratetracontane Nonfused aromatics 1,sDiphenylethane m-Terphenyl Triphenylmethane 7-Phenyltridecane 1,5-Diphenyl-3( 3-cyclopentylpropy1)pentane 1,7-Diphenyl-4( 3-phenylpropyl) heptane 15-Phen ylnonacosane
Fused aromatic 1,10-Di(a-naphthy1)decane Nonfused naphthenes 1,ZDicyclohexylethane 1,5-Dicyclohexyl-3(3-cyclopentylpropy1)pentane 1,7-Dic clohexyl-4(3-cyclohexyGropy1)heptane 15-Cyclohexylnonacosane Fused naphthenes Perhydropyrene 6-n-Octylperhydrobenz(de)-
Formula Weight, G./Mole
Rate of Change of Kb with Concn. (G./1000
Kb at 0.05 Molal Concn.
Kb a t Zero Concn.
Deviation,
3.80 2.76 2.82 2.80 2.85 2.80
-0.36 -1.78 +O. 36 -0.36 +1.42 -0.36
No change No change
2.80 2.76 2.82 2.80 2.85 2.80
2.82 2.77 2.82 2.84
+0.36 -1.42 $0.36 4-1.07
No change No change
502
182.3 230.2 244.3 260.5
No change No change
2.82 2.77 2.82 2.84
126
334.5
2.84
+1.07
No change
2.84
171 135
370.6 484.9
2.79 2.82
-0.71 +O. 36
131
394.6
2.79
-0.71
520
194.4
2.81
127
346.7
2.79
-0.71
- 7.30 - 1.60
172 136
388.7 490.9
2.84 2.84
+1.07 +1.07
No change
- 1.60
2.89 2.84
578
218.4
2.83
+ O . 71
-10.08
2.81
196 132
343.6 414.7 Average
2.80 2.83 2.81
-0.36 +0.71 *O. 75
- 3.52
2.77 2.80
Par ~1iiiI 1d 6,ll-Di-n-amylhexadecane
anthracene l,lO-Di( a-decaly1)decane
pure hydrocarbons having different structural types and four nonhydrocarbons.
Experimental Data on the Ebullioscopic Constant for Benzene
Solute 1 2
Calibration of Thermometric System, A precision potentiometer was used as a standard millivolt source and its e.m.f. fed into the amplifier-recorder system. The relationship between this input and t h e output of the system was linear over t h e range 0 t o 10 mv. within t h e ability t o read t h e recorder chart (0.002 chart reading). The copper-constantan thermocouples, amplifier, and recorder were calibrated against platinum thermohms in the vicinity of the boiling temperature of the solvent over a span of 0.0 to 0.08 mv., which was the expected range of the elevation of the boiling point with successive additions of various solutes. A linear relationship between the temperature rise and the e.m.f. produced was found. Materials. This study includes three solvents, benzene, cyclohexane, and acetone; each one was chosen because of its availability in high purity, and structural variety. They were distilled before use and dried over appropriate desiccants. Benzene was an especially pure grade of 99.92 mole %, as determined cryoscopically. Cyclohexane was Phillips pure grade having a purity of 99 mole %, and acetone was a standard commercial sample having a determined density of 0.7847 gram per milliliter.
22 51 a
7 190 205 519
Compared against n-octacosane PSU No. 176.
366.7 366.7 394.7 450.8 507.0 619.2
.,
70
0.00
c.1 x
106
No change No change No change No change
++ 3.27 3.20 + 6.94
- 2.96
VOL. 31, NO. 11, NOVEMBER 1959
2.82 2.86 2.84 2.80 2.78
0
1885
the boiling rate. If the condensation ring is claudy, indicating a second phase is present, the solvent should be discarded, the ebulliometers cleaned and dried again, and a fresh charge of dry solvent used. Before the apparatus was considered ready for solute addition, the system was allowed to equilibrate until a driftrfree temperature difference was maintained between the ebulliometers for a t least 15 minutes.
Table II.
A small addition of solute was made. Solids a t room temperature were added as weighed crystals or pellets: liquid samples are added from a fine needle of a hypodermic syringe firmly supported above the ebulliometer by a clamp. From the loss in weight of the syringe, the weight of the solute was obtained. After each sample addition, the solvent was refluxed into the condenser to wash down any portion of the solute that
Experimental Data on the Ebullioscopic Constant Ka in Cyclohexane
Formula Weight, G./Mole
Zero Concn.
366.7 366.7 394.7 450.8 507.0
3.22 3.23 3.15 3.17 3.14
+O .63
$0.94 0.00 + O . 63 -0.32
+9 76 $5.92 $7.02 +5 60 +7.73
502
182.3 230.2 244.3 260.5
3.19 3.12 3.12 3.18
-0.31 -0.63 -0.95 +O .95
-9 20 -12.80 -7.84 0.00
126
334.5
3.20
0.00
-3.04
171 135
370.9 484.9
3.22 3.21
+O. 63
t0.31
-4.00 -6.40
131
394.6
3.19
-0.31
-9,44
520
194 4
3.20
127
346.6
3.15
hexvluroDv1)heDtane 172 1,5-C~cioh&ylno~acosane 136 Fused naphthenes lga Perhydropyrene 578
388.7 490 9
3.18
3 14
218 4 343 6 343 6 414 7 Average
Solute 1
2
30 40 54
7 8" 9" 105 11 12 13
14 15 16 17
Paraffins 6,l I-Di-n-amylhexadecane 7-n-Hexyleicosane n-Octacosane 11-n-Decyldocosane n-Hexatriacontane Nonfused aromatics 1,ZDiphenylethane m-Terphenyl Triphenylmethane 7-Phen yltridecane
1,5-I~!phenyl-3( 3-cyclopentylpropy1)pentane 1,7-Diphenyl-4(3-phenylpropy1)heptane 15-Phenylnonacosane Fused aromatic 1,IO-Di(a-naphthy1)decane Nonfused naphthenes 1,2-Dicyclohexylethane 1,5-Dicyclohexyl-3(3-cyclopentylpropy1)pentane
PSU No.
22 51 7 190 519
1,7-Dicyclohexyl-4(3-cyclo-
18
20a
6-n-octylperhydrobenz(de)-
21
1,lO-Di(adecaly1)decane
4
anthracene
196 196 132
Kb a t
Rate of Change of Kb with Concn., Deviation, G./1000 % G. X 106
0.00
-2.48
-1.56
+3.60
-1 88
-0.63
+3.33 $6 56
3 14
-0 32
-5 60
3 3 3 3 3
17 23 18
+O 63 +O 94
-1 53
15" 20
+067
-0 63
3 ~ 51a 0
-1 06 0
Lot A ; rest are lot B. Experimental Data on the Ebullioscopic Constant Rate of Change Kb at Formula Kb at O f Kb with 0.05 Weight, Zero Deviation, Concn. G./1000 Molal Solute G./lfole Concn. 76 G. x' 104 Concn.
Table 111.
2'
23 24 19
9 10
22
23 24 8 9
35 0
*
I n Acetone + 16 1 84 0 00 110 1 - 7 2 1 84 0 00 146 1 0 0 + l 60 210 2 1 87 - 21 40 -1 09 218 4 1 82 - 6.0 244 3 1 84 000 - 12 24 260 5 1 83 -0 54 .\verage 1 84 5 ~ 054 In Benzene Csing hlixed Type Solutes -159 6 2 82 +O. 71 110.1 Resorcinol - 26.3 2 80 -0.00 146.1 Coumarin 2 79 -0.36 - 6.91 210.2 Beneil 2 77 -1.07 No change 230 2 m-Terp henyl 2 82 No change + O . 71 244 3 Triphen lmethane 2 82 + O . 71 388 691 5 y cery Ttristearate +O. 59 .\verage 2 80 PSU No. 578. PSU No. 502.
1886
Resorcinol Coumarin Bend Perhydropyrene' '_rriphenylmethane ,-Phenyltrldecane*
+
0
ANALWCAL CHEMISTRY
2 2 2 2 2
72 77
77 77
82 2 98
might have adhered to the walls. Care was taken to avoid the loss of solvent vapor from the condenser, thereby preventing changes in concentration. When the temperature reading was constant for 10 minutes, this value was used as the base for the next addition of solute. The Amv., read to the nearest 0.2 of a small chart division before and after the addition, was equivalent to the boiling point rise of the solvent. It may be converted to AT to resemble values for Ka as done in the following tables, but this is not necessary in evaluating the ebullioscopic constant or in the determination of molecular weight. Additions of solute were repeated until sufficient data were obtained. The value of the ebullioscopic constant & is calculated by: MWlhV. (3) K c = .-iimGr where Amv. has been substituted for AT in Equation 1. For succeasive individual additions, the accumulated value of Kb was plotted with respect to total concentration and extrapolated to zero to obtain the ebullioscopic constant a t infinite dilution. Experimental Kb values will differ somewhat from the theoretical, because they reflect both instrumental and solvent characteristics. Such K , values should be used with the ebulliometer in which they are determined, then checked for agreement with each new batch of solvent. To determine a molecular weight, the data for several additions of solute were obtained. The accumulated term wlA mv. ~ O O Owl
was plotted with respect to total concentration and extrapolated to zero. The molecular weight was then calculated by dividing the intercept into the ebullioscopic constant. Determination of Molecular Weight by the Comparative Methods. For the determination of molecular weight by the comparative methods, t h e procedure for the external standard technique required d a t a to be determined in the same manner as for the ebullioscopic constant method. The elevation of the boiling point for the addition of a known amount of solute in a known amount of solvent must be determined, and the data compared with those for a standard b!. Equation 2 where w. and tus are now concentrations of grams per thousand grams of solvent. This was equivalent to the determination of the ebullioscopic constant a t a finite concentration. As the internal standard method was a relative measurement, the restrictions governing the procedure were not necessarily as rigid aa those regarding the ebullioscopic constant or external standerd techniques and, therefore, it had a certain timesaving advantage.
32
0
1000,
700 600 -
7
310t
I I
800 -
3 00
u
2
,IC I
I
O
I
Jooc
I
l
o
.. UI
I
400-
I
f 300-
t
0
I
2.m
1
-:200-
~
s
I ,7-diphenyl- 4(3-phenyl propyl l'ncplone
~
0 GIovoI-HIII Solufer 0 A P I Hydrocarbons
I
0 n-octocorane
A
~
6-n_-oclyIperhydrobenz (&lanlhrocenr 271
.--J
0
10
20
30
40
M €0 70 80 90 Concentration g / I M x ) o Solvent
110
Variation of K p with solute concentration
Figure 1. benzene
Weighed additions of the material whose molecular weight was to be determined were not made until successive additions of the internal standard gave Pmv./w values that agreed within 3%. The procedures for addition of solutes, temperature equilibration, and determination of boiling point rise from Amv. were the same as previously described. In practice, each addition of unknown solute was preceded and followed by additions of an internal standard. At least two such groups of data were required for a molecular weight determination. The molecular weight of the unknown was calculated for each addition of unknown from Equation 2, neglecting the K I ratios. AI,
100
=
(Amv./w.M.) ____ (Amv;/wu).
(4)
The term (Amv./w). is an average value for the reference standard before and after the addition of the unknown sample. RESULTS
Typical examples of the rate of change of the ebullioscopic constant K b with concentration are illustrated for benzene In Figure 1. These curves represent examples of positive, zero, and negative changes of K D with increasing concentration. Curve a, having a positive dope, is for a nonfused aromatic type, t,7 - diphenyl - 4(3 - phenylpropy1)hepLane; curve b, with no change in slope, is fur a paraffin, n-octacosane; curve c represents a fused-ring naphthene, 6 Q - octylperhydrobenz(de)anthracene. Table I presents a summary of experimental data for the ebullioscopic constant of benzene as obtained for 21 samples of pure hydrocarbon solutes of various structural types. The paraffins And several of the nonfused aromatics have a negligible rate of change of Kb with concentration, while the remaining Aromatics, Nos. 12, 13, aqd 14, show
I
275
K,
20
Figure 2.
Plot of
I
I
2 80 2 85 2 90 at 0 0 5 Molal Concentrollon
K, with
I 2 95
respect to molecular weight
in
positive changes of K b . The naphthenes, nonfused and fused, produce negative slopes with the exception of No. 18. Using these pure hydrocarbons, the range of solute molecular weights covered in benzene was 182 to 619 grams. The values for K b at infinite dilution in benzene varied from 2.77 to 2.85, yielding a mean of 2.81. Individual values differed from the mean by at most -1.7% to +1.42%, with an average deviation of O.75Yob. This deviation is mainly experimental and could be substantially reduced with further refinements in the apparatus and operating procedure, and with increased sensitivity and stability of the thermometric system. The data for the K b determinations of cyclohexane are presented in Table TI, representing lots A and B. The lots were taken from the same original batch of solvent, but were treated and purified separately. At the time of the study of lot A, eight pure hydrocarbons were available, ranging in molecular weight from 218 to 507 grams. At zero concentration, they produced a mean Ka of 3.15 with individual deviations varying *0.95."j,, and an average difference of 0.51%. Using 12 additional hydrocarbons spanning the molecular weight range from 182 to 491 grams, the K b a t infinite dilution of lot B was 3.20 with an average deviation of 12.677~and individual variations falling between -1.88% and +0.94%. Solute No. 20 was studied in each of the two lots of solvents, yielding a K b of 3.17 in lot A with a +0.63'% deviation from the mean, and a value of 3.23 with a +0.94% deviation in lot B. The mean K , values for the two lots differ by 1.5%, but if the data should be combined, the resulting value for the mean K b would be 3.18 with an average deviztion of 0.85% and a maximum variation of 1.9%. Solute behavior in this solvent is generally less ideal than that observed in benzene and the slopes are more pronounced. The
K b values for paraffins are concentrationdependent at finite concentrations showing positive changes, while those for the aromatics and naphthenes show both positive and negative variations in slope with the exception of solutes 10 and 21 which have no slope. Because many of the hydrocarbons were only partially soluble in acetone or displayed erratic behavior due to instability in the system caused by foaming, the data obtained in this medium are supplemented with determinations using nonhydrocarbon solutes Nos. 22, 23, 24 (Table 111). The molecular weight span was rather limited, from 110 to 260 grams, and at infinite dilution an average K b of 1.84 was obtained with individual maximum variations being - 1.09% and 1.60%; the average deviation was o.54~0. The rate of change of K bwas generally negative with the exception of solutes 24 and 22, which showed no change and a slightly positive slope, respectively. The hydrocarbon solutes demonstrated comparatively larger concentration effects over the polar nonhydrocarbon materials. I n Table 111, data determined in this laboratory are tabulated for several of the solutes previously studied by Glover and Hill (3) in benzene, spanning molecular ranges from 100 + ? 900 grams. Resorcinol, coumarin, and benzil show negative changes in K s with respect to concentration changes, while tristearin has a positive slope. m-Terphenyl, No. 8, and triphenylniethane, KO. 9, were previously shown in Table I and are fairly ideal in their behavior. With these materials, the mean expermental K b obtained a t infinite dilution was 2.80 which is within the limits set by the previous determinations in benzene using the pure hydrocarbons. The maximum deviations were - I .07% and +o.71YOwith a mean value of 0.59%. Figure 2 shows a plot of Ka us. molecular weight for all of the soiutes studied in benzene, regardless of slope, at 0.05 molal concentration; line a is a plot of the data for some of the compounds also
+
YOL. 31, NO. 1 1 , NOVEMBER 1959
1887
studied by Glover and Hill (3) and showing a similar relationship; line b represents the average value of 2.81 if the data of Table I are included, with a deviation of *1.4Yo bounded by the dotted lines e. I n Table IV, data obtained by two operators are listed for several determinations of K b for benzene using two solutes. The average value of the K b constant using n-octacosane is 2.82 with a n average deviation of o.9i'70. TWO operators obtained the value 2.84 using 7-phenyltridecane as the solute. One of the four runs in octacosane had a 1.77% deviation; however, if it is neglected, the average values are in excellent agreement with the one in Table
I. Some selected results of molecular weights determined by the comparative atandard methods using benzene and cyclohexane as solvents and some of the pure hydrocarbons from Table I as solutes are given in Table V. Runs 1 through 6 in benzene and 8 through 11 in cyclohexane are examples of the
Table IV.
internal standard technique. In runs 1 and 2, solutes n-hexatriacontane and n-octacosane, both having no variation of Ka with concentration, are compared, with the latter as the standard. The two determinations of the molecular weight of n-hexatriacontane indicate individual variations for calculated molecular weights; however, the deviations from the average value are small and within 1.6% of the formula weight. I n runs 3 and 4 the molecular weights of perhydropyrene a n d 6-n-octylperhydrobenz(de)anthracene,each of which shows negative variations of Kb in benzene, were determined against the same internal standard. Values fall within 1.5 and 2.5% of the respective formula fveights. I n run 5 in benzene, two materials with positive slopes, 1,7diphenyl4(3-phenylpropyl)heptane and l&phenylnonacosane, were compared, while in run 6 two with negative slopes, 6 - m - octylperhydrobenz(de)anthracene standard and l,lOdi(adecalyl)decane, were studied. The average values obtained were within 1.6% for the two
Repeatability of Ebullioscopic Constant Determination in Benzene by Two Operators Ka at Deviation,
Solute n-Octacosane
Trial IO
IIa 1116 IVb
7-Phenyltridecane
Av.
I5
IIb
%
Zero Concn. 2.79 2.80 2.87 2.83
Av.
-1.06
-0.71 +1.77 +O. 35 f0.97
2.82 2.84 2.84
0.00
0.00
2.84
0.00
DISCUSSION
Operator 1. b Operator 2. 0
The data as tabulated in Tables I through IV for the determination of
Table V.
Run No. 1 2
3 4
5 6 7"
with positive changes of Kb, and 1.4% for the pair having negative changes. I n runs 8 and 9 in cyclohexane, n-octacosane, which has a positive slope, was deliberately used as a n internal standard against perhydropyrene and m-terphenyl, each showing negative variations of Kb in this solvent. Deviations from the formula weight resulted from as much as 8 to 16%. The next two runs, 10 and 11, matched solutes with similar slopes against each other; for materials with negative slopes in cyclohexane, triphenylmethane waa used as the standard and m-terphenyl as the unknown, which yielded an agreement with the true formula weight of the latter within 0.4%. With ll-ndecyldocosane as the standard and n-octacosane as the unknown, each showing positive slopes, the agreement waa 1.5%. Run 7 in benzene and 12 in cyclohexane illustrate the second comparative method, the external standard technique. The data in these two examples have been taken from the appropriate K s curves at several different concentrations. The first line of data, in each run indicates what would be the observed values for the molecular weight of the unknown. Materials having opposing K bchanges with respect to concentration have been deliberately chosen again and values agreeing to within only 5 and 870, respectively, were obtained. The second line includes a correction for the differences in the rate of change of K s and drastically reduces the error to &0.2% in each w e .
Determination of Molecular Weight by Comparison Techniques
Formula Weight, Solute G./Mole Standard In Benzene n-Hexatriacontane 507 n-Octacosane n-Hexatriacontane 507 n-Octacosane Perhydropyrene 218 n-Octacosane 6-n-Octylperhydrobenz344 n-Octacosane (de)anthracene 485 1,7-Diphenyl-4(3-phenyl- 15-Phenylnonacosane propy1)heptane 415 1,10-Di(a-decaly1)6-n-Octylperhydrobenzdecane (de)anthracene 15-Phenylnonacosane 485 6-n-Octylperhydrobenz(de)anthracene 485
In Cyclohexane 218
Experimental Mole Weights, 1 2 3 4
G./Mole Average
Deviation from Formula Weight, yo
505 517 209
509 501 226 340
515
$1.6
221
325
533 513 227 352
336
+1.4 -2.3
479
482
467
479
477
-1.6
430
402
402
402
409
+1.4
473
467
457
451
462
-4.7
485
486
484
485
485
f0.2
513
521 225 318
513
+1.2
+7.8 246 248 235 +15.6 274 281 266 -0.4 225 238 227 229 +1.5 366 418 394 434 401 11 +7.8 238 245 252 258 248 120 10.2 231 230 229 230 230 Second line includes correction for differences in rate of change a First line indicates observed values for molecular weight of unknown, 8 9 10
Of
n-Octacosane n-Oc tacosane Triphenylmethane 11-n-Decyldocosane n-Octacosane
Ks.
1888 s
ANALMlCAL CHEMISTRY
Perhydropyrene m-Terphenyl m-Terphenyl n-Octacosane m-Terphenyl
230 230 395 230 230
231
249
239 260 226
molecular weight by the ebullioscopic constant K b technique indicate that this method is valid a t least over the molecular weight range of 100 to 900 grams for nonvolatile solutes. I n this work, the ebullioscopic constant a t infinite dilution is independent of molecular weight and solute type. For petroleum hydrocarbons benzene is more ideal than cyclohexane or acetone. Many materials show no variation of Kb with concentration, while the solutes that influence changes in K b do so to a comparatively lesser degree in benzene than in the other two solvents. The data for cyclohexane in Table I1 support the contention that Kb values reflect solvent characteristics, therefore necessitating the calibration of each new batch of solvent. Because the difference in the mean values for K bfrom each of the two lots falls within the limits set by the K , method, the data conceivably could be incorporated into one value; but the data appear to be more consistent if they are isolated according to the solvent lot in which they were studied. This solvent effect haa also been veritied many times in the routine application of ebulliomztry in this laboratory. Comparison oi the results obtained in acetone with those obtained in cyclohexane, for the same compounds, show that the slope of the K b curve is about the same for triphenylmethane, while in the cases of 7-phenyltridecane and perhydropyrene, there is a marked increase in the negative character of the slope in acetone, indicating that cyclohexane is to be preferred. I n the acetone series, hydrocarbon and nonhydrocarbon materials yield the same value of liba t infinite dilution, but the relatively nonpolar pure hydrocarbons have the most significant slopes. I n benzene, which is less polar than acetone, the reverse is ture. Here the polar nonhydrocarbon solutes exhibit the greatest rate of change of Kb with respect to concentration, while the values for hydrocarbons are much less sensitive to concentration changes. In benzene, as in acetone, all of the solutes extrapolated to the same K b value. The maximum deviation for any solute in any solvent w a s 1.9% and molecular weights calculated from the data were within 2y0. With this apparatus, while agreement between values obtained for the same solute is quite good, they are little more reliable than those obtained using a variety of solutes (Table IV). Glover and Hill (3) report that a t 0.05 molal concentration for a number of solutes, the ebullioscopic constant vaned with the molecular weight of the
solute. This result as seen in Figure 2 and Table I11 has been confirmed for six of these solutes in benzene; however, as shown in Table I and Figure 2, the Kb for hydrocarbons is not dependent on molecular weight a t the same concentration but shows random variations. This scatter indicates a greater dependence on solute type rather than on molecular weight. The data for the systems indicate general agreement with the conclusions reported by Mair (6),who stated that the use of a pure standard hydrocarbon substance to determine the molal boiling point constant a t infinite dilution will allow this constant to be used atj a means of obtaining repeatable molecular weights on most unknown hydrocarbons. l n i s also held true for the nonhydrocarbon solutes studied, Table 111. The information presented in Table V would seem to indicate caution in the use of the comparative standard techniques. The average values for the observed molecular weights were good to within 2.5% in benzene; however, the accuracy may be improved by using materials of similar slopes which may be seen in runs 1, 2, 5, and 6 in benzene, where materials of zero, positive, and negative slopes were matched against solutes having corresponding slopes, producing values good to within 1.6% lor the internal standard method. I n the internal standard method, if the AT/w values for the additions of reference standard before and after the unknown did not agree within 3%, the boiling characteristics of the solution had changed or some other cause for deviation from ideal behavior had occurred, and the molecular %eight was not to be calculated from these data (6). This statement can now be reemphasized on the basis of results thet were deliberately run in cyclohexane using octacosane as the standard, in which the addition of standard quantities would not check because of this solute’s positive change of K b with concentration. When this standard was matched with materials exhibiting negative deviations, such as perhydropyrene and m-terphenyl, errors as high as lSy0 resulted; when studied with a material also having a positive slope, the error reduced to 1.5yo. A similar determination on materials with negative slopes fell well within this limit. The fact that the raw values obtained in the external standard determinations decreased in reliability with further additions of solute is due to the rate and direction of the change of K b for the two substances. When the appropriate corrections are applied there is excellent
agreement. This correction factor actually is the ratio of the & of the unknown material to that of the standard, evaluated at the concentration in question (Equation 2). If the two solutions should behave ideally or show the same deviations from Raoult’s law, the ratio of the Kb’s will be unity and the simple ratio of the elevation of the boiling point with respect to weight can be applied; however, this frequently i s not the case. CONCLUSIONS
The internal standard method is more rapid than the Kb technique, but unless suitable precautions are observed, it may involve considerable error. For nonpolar hydrocarbons, benzene is to be preferred oveq cyclohexane as the solvent because the behavior of the various solutes is more ideal in this medium. The observed data in benzene are fairly reliable and with the proper refinemenk can be quite accurate. For polar materials, such as the nonhydrocarbons studied, acetone is the better solvent because Kb is less sensitive to concentration effects in this polar medium. If, for solubility or other reasons, cyclohexane is to be used, careful consideration of the standard should be made with respect to solute type and its rate of change of K b with concentration. Information concerning the nature of the unknown would also be required, specifically, regarding the direction and rate of change of the slope. If this information has to be determined experimentally, the comparison techniques would lose their time-saving advantages and the ebullioscopic constant method would be preferred as the more reliable measurement. LITERATURE CITED
(1)
Boelhouwer, C., Waterman, H. I..
J. Inst. Petrol. 40, 116 (1954). (2) Deanesly, R. M., Carleton, L. T., IND. ENG.CHEM.,ANAL.ED. 14, 220 (19421 \ - - - - I .
(3) Glover, C. A . , Hill, C. P., ANAL.CHEM. 25, 1379 (1853). (4) Lipkin, AI. R., Martin, C. C., Kurtz, S. S., Jr., IND.ENG.CHEM.,ANAL.ED. 18,376 (i946). ( 5 ) Mair. B. J., d‘. Research Natl. Bur. . 8tanda;ds 14, 345 (1935). (6IrNes, K. van, Westen, H. A. van, AsDects of the Constitution of Mineral . .Oils;” Elsevier, New York, 1951. ( 7 ) Sullivan, I>. J., Fries, R. J., McClenahan, W. S., Willingham, C. B., ANAL.CHEM. 29, 1333 (1957). (8) Swietoslawski, W., “Ebulliometric Measurements,” p. 171, Reinhold, Piew York, 1945. RECEIVED for review February 19, 1959. Accepted July 20, 1959. Division of Petroleum Chemistry, 132nd Meeting, ACS, New York, N. Y., September 1957. Research supported by grant from Petroleum Research Fund administered by the
American Chemical Society.
VOL. 31, NO. 11, NOVEMBER 1959
1889
