Use of Physical Constants in Analysis Extension to Liquid Halogen and Oxygen Organic Compounds E. H. GILMORE, MARIE MENAUL, A N D V. SCHNEIDER Texas Technological College, Lubbock, Tex. Charts showing the relation between structure and refractive index, density, and boiling point of representativeliquid halogen compounds and compounds of carbon, hydrogen, and oxygen are presented. How these charts can be used to shorten and simplify the analysis needed in connection with research work is described and illustrated.
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HE main feature of the procedure described here is the use of physical constants, usually used to help in final identification, to serve also in the classification of an unknown. The classification requires the use of charts showing the relation between molecular structure and refractive index, density, and boiling point of pure compounds. These three physical constants, when compared with the plotted data of known compounds, limit the class possibilities very sharply and show quickly which tests and chemical derivatives, if any, are needed for final identification. A search of the literature shows that Smith in 1930 plotted refractive index against density to aid in the identification of individual compounds ( 1 7 ) . He recognized the value of the chart, but did not plot enough data a t that time to show the segregation of classes in limited fields. Samygin in 1936 showed that several classes of aliphatic compounds fell in separate and distinct fields when so plotted (16). Similar graphs have been published in connection with hydrocarbon analysis (10,16). That the general principles involved have received wide attention
Figure 2. Density and Refractive Index of Some Classes of Saturated Aliphatic Compounds
in the petroleum industry is shown very strikingly by many other outstanding discussions correlating physical constants and structure of hydrocarbons (1-6, 9, 11-1 4,18-20). The present paper, based on work with halogen- and/or oxygen-containing compounds, extends the ideas developed in connection with hydrocarbon research to the analysis of other liquid organic compounds (16).
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CONSTRUCTION OF GRAPHS
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Figure 1. Density and Refractive Index of Oxygenated Saturated Compounds Compared to Parent Hydrocarbons 892
An ideal method for plotting the data on any one class of compounds would be to plot a r'eference series in the class completely, and then to plot not more than two or three compounds in each isomeric series (1). For example, on Figure 6, the straight-chain primary alcohols in the aliphatic monoalcohol class are plotted completely enough to serve as a reference series. Data for the members omitted can be easily estimated by interpolation. On this same figure, 2pentanol can be used to illustrate how one compound serves to estimate data on an isomeric series. Its boiling point is 18.2" C. lower than that of 1-pentanol, its density is 0.0056
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Figure 3. Density and Refractive Index of Several ClassSaturated Aliphatic Compounds
data on chlorine compounds, the book by Huntress is available ( 7 ) ,and for additional data Chemical Abstracts seems to be the best source. The chemical handbooks and reference books which contain tabulated data are also good supplementary sources. The halogen derivatives, which have been studied in some detail, present a more difficult problem in analysis than do the carbon, hydrogen, and oxygen compounds. The possible number of halogen compounds is probably in the millions. At present, fewer than 5000 can be found even partially described in chemical literature. If a halogen compound be chosen at random, the chance of its being mentioned in chemical literature is very slight; hence, some method of interpolating and extrapolating data is necessary in any scheme of analysis. Charts of the kind presented here simplify the interpolation appreciably. Figures 7 and 8 give data for a very few of the simpler halogen compounds. The dash-dot lines on these two figures show direction for finding other members of a particular series of compounds whose successive members differ by a -CH*group. By interpolation in
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lower, and its refractive index is 0.0039 lower. _--Other members of the 2-01 series should differ similarly from their isomers in the reference .c: * -. D I0 s--'-.-.KETOA L DEHY DES---; 4series. The differences in constants will be I A8 .'e. I larger for isomers of smaller molecular weight *.* !ALcOmD\EsTtrf+ and a little smaller for the isomers of larger molecular weight. +.-ALCOHOLS--.#.-.ETHER-ALCOHOLS*.-E THE R-ACIDS-----* -. A C I D S - - - - + I It is self-evident that the physical constants I .46 I for the compounds of any one series of a given +--I. . I 1H E R Jclass should vary in some regular pattern as the JI.KU!ONt.5----L.PA?&&,K E T O - E T H E R I molecular weight increases, and that the differnr AND A L D E H Y D E S c.-.-.-.-.-.-~IESTER~-.--. +.E T H E R - E S T E RS.* ences in constants between isomers of two series I .44 *.14J should have some uniformity. These are the 0.0" basic ideas to use with the charts. K.\aQ.o.c The relative positions of the fields for paraffins, *-------cone-ring cycloparaffns, one-ring aromatic hydrocarbons, and the saturated oxygenated deI A2 \*\. / rivatives of these three types of hydrocarbons L. I ,/' are shown on Figure 1. Figures 2 and 3 show \r ,/' approximately which class of saturated aliphatic 1 4 0 1 . -4 " oxygenated derivatives can be found a t any given density and refractive index. Figure 5 _----_-_--serves similarly for aromatic compounds. Fig.as -9 0 .95 Dzo 1.00 1.05 I .IO ure 4 shows the position of different classes of Figure 4. Density and Refractive Index of Cyclohexane Derivatives cyclohexane derivatives; how a change in ring and of Several Ether Series structure affects the position on the chart is also shown by the five different ether series. The smaller the ring, the lower the compounds will be on the chart, such a series, the structure of an unknown may be deduced. Figand an oxygen atom in the ring lowers the position on the chart. ure 9 illustrates other kinds of series which may be similarly used. Figures 2, 3, 4, and 5 can be used for preliminary classification ANALYTICAL PROCEDURE of an unknown and in the case of mixtures, where the point representing the unknown will be in between those of the comIn an analysis of a liquid sample, many variations in the use of ponents, for forecasting classes present so that the purification charts such as the ones presented here can be made. In any case, will not be entirely blind trial and error. the first step is to determine density a t 20' C. and refractive index Figure 6 illustrates the kind of detailed graph needed for maxifor the sodium D line a t 20' C. Corrections of 0.001 per ' C. on mum help in an analysis. The dash-dot lines are for the referdensity and 0.00045 per ' C. on refractive index will be found ence series plotted most completely. Boiling points are given in accurate enough for most cases, if it is inconvenient to make the degrees centigrade a t atmospheric pressure, or a t the pressures tests a t exactly 20' C. Data on the unknown are compared with indicated by superscripts. For carbon, hydrogen, and oxygen those plotted on Figures 1 to 8 and on other similar graphs concompounds, the textbook by Huntress and Mulliken has considerstructed by the analyst, and used with history of the sample to able data easily available for plotting other such charts (8). For judge what class or classes of compounds are probably present.
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894
ANALYTICAL CHEMISTRY
appreciably by their use. Another feature which should be emphasized is the amount and accuracy of detail which can be predicted about the structure of an unknown from the three physical constants only. For example, an unknown with dso = 0.785, n g = 1.404, and boiling point = 98-100' C. cannot only be classified immediately as an aliphatic monoether, but the boiling point in relation to its position on Figure 6 and the averages given below for effect of unsaturation show that it is unsaturated. Assuming that this particular compound is not yet described in the literature, its probable structure can be quickly deduced by interpolation and extrapolation of data known for other unsaturated ethers, when such data are plotted on a graph similar to Figure 6. Another example was a cut obtained by frsctionating the products from the oxidation of ethylene in the presence of sodium acetate. The data on this cut were: boiling point = 160-'170" C. (uncorrected), di0 = 0.836, n 2 = L 1 1 I I I I J 1.4042. These data, plotted on Figure 2, indi.9b 1.00 1.04 D20 1.08 1.12 1.16 4 cated that the compound waa probably a diether. Figure 5. Refractive Index and Density of Saturated Aromatic The data plotted with known diethers on a Compounds detailed chart not given here lined up very well with straight-chain diethen having the oxygen atoms separated by two carbon atoms. Interpolation of boiling points indicated a molecule containing approximately eight carbon atoms. Because boiling point was obtained incidental t o purification, less than 5 minutes was needed to obtain the remainder of the experimental data used in classification. The information on structure might be complete enough in some research work to allor omitting the further testing by breaking up into alcohols or alkyl halides, as would probably be done if older classification methods were used. When two or more functional groups are in the same molecule, such will usually be indicated when its data are compared with data plotted on an auxiliary chart similar to Figure 6. Consider a compound such as ethoxyacetaldehyde, with dso = 0.942, n g = 1.3956, and boiling point 105" C. Its refractive index and density place it in the aliphatic acid field, but its boiling point does not match the acids at that particular position in the field. A class test might be needed to show the presence of the aldehyde group, and with that confirmed, it would be obvious that Figure 6. Saturated Aliphatic Ethers, Alcohols, and Oxides there is another functional group in the molecule, an ether group, This emphasizes a point already brought out, that the position of the point representing the unTo obtain maximum help, a detailed chart such as Figure 6 should known and its boiling point must be completely accounted for be constructed. If, during separation and purification, refractive by the structure and functional groups of the molecule. index and density are determined occasionally and plotted on In the case of mixtures, the point representing the unknown will such a chart, successive points will line up toward the point reprebe in between those of the main components on the charts, and senting the compound being purified. Properties of individual closer to the one present in largest amount. Hence, some idea of compounds, or of classes indicated to be present by their position the components present is obtained, which helps in choosing on the graph, determine each step in procedure until final identimethods of separation to use. It is not likely that a main comfication is made. Boiling point often serves to distinguish beponent will be missed, as has happened in using older methods of tween saturated and unsaturated compounds or between comanalysis. pounds in two different classes whose fields overlap on the graph. Several general rules can be given about the position of the difIt can also be a distinguishing property in choosing between comferent classes on the charts. If the oxygenated compounds are pounds of two isomeric series of the same class. considered as being derived from the corresponding hydrocarbons, DISCUSSION introducing an oxygen atom to make an ether group has the least effect on the constants. The next least effect is for the oxygen Even where the usual class tests are not entirely eliminated by atom to be in an alcohol, aldehyde, or ketone group. Still farther use of the charta, the time for an analysis usually will be shortened
-
V O L U M E 2 2 , NO. 7, J U L Y 1 9 5 0
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away from the hydrocarbons are acetals and diethers, then esters, a side branch. Switching the position of two halogen atoms and finally the acids. The distance of difunctional compounds has very little effect on the constants. For example, l-chlorofrom their parent hydrocarbon is roughly the sumof the distances 2-fluoropentane will have practically identical constants with of the monofunctional compounds from the parent hydrocarbon. l-fluoro-2-chloropentane, but both will be noticeably different The effect of any group in a big molecule is less than that of the from an isomer, such as 1-chloro-5-fluoropentane. Data can same group in a small molecule. Therefore, as molecular size be found to contradict any of the above rules. However, it is increases, the field for any class of compounds moves closer and believed that most of these contradictions are due to inaccuracies closer to the field for the parent hydrocarbon, and the difference in published data. between isomers is less for large molecules than for small ones. The amount of variation from averages made to show the effect From data now available on the oxygenated and halogenated of unsaturation and also to show the effect of isomerization indicompounds, the average effects of creating a double bond by cates that the published data on an individual compound may be eliminating hydrogen from molecules of molecular weight between very inaccurate. Hence, some method of checking accuracies is 100 and 160 are approximately: an increase of 0.035 in density, desirable. If all data available on two isomeric series are plotted an increase of 0.030 in refractive index, and very little change in on a graph such as Figure 6, it will be easy to see the approximate boiling point. The effects are largest in aromatic compounds and amount of error in individual data, inasmuch as some regular patr next largest in other cyclic compounds. The effects also vary tern is to be expected between successive members of each series, widely with position of the double bond in relation to other groups as well as between isomers in the two series. which possibly activate the double bond. Compounds which might be expected to polymerize readily seem to have the higher constants. For example, a comparison of data on the two 1phenylpropenes shows that the one having the double bond conjugated with the benzene ring has approximately 17" C. higher boiling point, 0.017 higher density, and 0.034 higher refractive index than the other isomer. The average difference in density of saturated and unsaturated bromine and iodine compounds is appreciably higher than the average for other halogen compounds. In an analysis, use of the above averages will help in judging whether the unknown is unsaturated when using graphs containing only data on saturated compounds. Once the unknown is established as an unsaturate, enough indiv dual data on unsaturated compounds should be plotted to place the unknown in one particular series if possible. Correlation of unsaturation and change of structure with change in physical constants has been by far more extensively studied in connection with hydrocarbons than with any other class of compounds. The discussions referred to above are a few of the better known papers on this subject. Qualitatively, the same conclusions reached for hydrocarbons apply to oxygenated and halogenated compounds. A methyl group in 2position results in the compound's having lower refractive index and density than the corresponding n-compound, but branching farther away from the end of the molecule raises both refractive index and density. An ethyl or larger side branch has about the same effect on boiling point as a methyl side branch, and about double the effect on refractive index and density. The effect of several changes in structure is roughly additive, except that adjacence of groups may cause an additional effect. For example, two adjacent tertr or neocarbon atoms result in appreciably higher constants than those for isomers where they are not sdjacent. Usually, cis compounds have higher constants than the trans isomers. An oxygen atom or a halogen atom can be considered qualitatively as being like another carbon atom, but the effects of their position are greater. Thus, secondary straight-chain alcohols hake, on an average, about 18" C. lower boiling point than the corresponding primary alcohols, compared to about 8" C. lower boiling point for isomers where s methyl group is shifted from the end to make Figure 8. Fieldr for Simpler Cyclio Halogen Compoundr
ANALYTICAL CHEMISTRY
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Figure 9. Illustration of Different Ways to Interpolate
Huntress, E. H., “Organic Chlorine Compounds,” New York, John Wiley & Sons, 1948. Huntress, E. H.,and Mulliken, S.P., “Identification of Pure Organic Compounds,” Order I. New York. John U‘ilev & Sons. 1941. (9) Kinney, C. R.,and Spliet”hoff,W.’L.,-J. Org. Chem., 14, 71 (1949).
Kurta, S. S., Jr., and Headington, C. E., IND. ENG.CHEW,ANAL.ED.,9,21 (1937). (11)Kurta, S. S.,Jr.. hlills. I. W.. Martin, C. C.. Harvey, W. T., and Lipkin, M. R., Ibid., 19, (10)
LITERATURE CITED
(1) Boord, C. E., Petroleum Refiner, 21, 372 (1942). (2) Driesbach, R. R., I d . Eng. Chem., 40,2269 (1948). (3) Edgar, G., and Calingaert, G., J. Am. Chem. S O L , 51, 1540 (1929). (4) Francis, A. W., Chem. Revs., 42, 107 (1948). (5) Francis, A. W., I n d . Eng. Chem., 36,256 (1944). ( 6 ) Gooding, R. M., Adams, N. G., and Rall, H. T., ISD. ENG. CHEM., ANAL.E~.,.18, 2 (1946).
175 (1947). (12)Lipkin,’M. R.,and Kurta, S. S., Jr., Ibid., 13. 291 (1941). (13) Lipkin, M. R., and Martin, C. C., Ibid., 19, 183 (1947). (14) Love, R. M., Padgett, A. R., Seyfried, W. D., and Singleton, H. M., Ibid., 19,37 (1947). (15)Samsain. h1. M..J . Phus. Chem. (U.S.S.R.). 8,-839 (1936). (16) Schneider, V., Stanton, G. W., and Watkins, E., Refiner Natl. Gasoline Mfr., 18,112 (1939). (17) Smith, P. E.,Chemist-Analyst, 19,KO.3,p. 4 (1930). (18) Thorne, H.M., Murphy, W., and Ball, J. S.,IND. ENC.CHEM.. ANAL. ED.,17,481(1945). (19)Ward, A. L., and Kurta, S.S.,Jr.. Ibid., 10,559 (1938). (20)Ward, A. L.,Kurtz, S. S., Jr., and Fulweiler, W. H., “Science of Petroleum,” Vol. 11, p. 1172, London, Oxford University Press, 1939. RECEIVED February 21, 1949.
Coulometric Titrations with Electrically Generated Ferrous Ion W. DONALD COOKE
AND
N. HOWELL FURMAN, Princeton University, Princeton, N. J .
Ceric sulfate and potassium dichromate solutions were titrated with ferrous ion generated at a platinum cathode. The end point was obtained potentiometrically using a platinum-tungsten electrode system. The reductions were carried out at constant current, and the time of the electrolysis was measured.
C
OULOMETRIC methods of analysis are made possible by Faraday’s fundamental researches upon electrochemical equivalence. Quantitative experiments using the coulometric principle have long been included in courses in physical chemistry. Quinn and Hulett (16) utilized the method with silver and cadmium coulometers in series for estimating the equivalent weight of cadmium. The first extensive analytical studies of the coulometric method were those of Szebelledy and Somogyi (19). The method depends upon the measurement of the amount of current, working at 100% efficiency, necessary to complete a chemical oxidation or reduction, These procedures fall into two general classes: those that are carried out a t constant controlled electrode potential, and those at constant current. In the former method, originally devised by Hickling (8), the potential of the anode or cathode is controlled so that only the desired reaction is able to take place. The process is allowed to continue until only a small residual current flows which in-
dicates complete reaction. The amount of current involved is measured by means of a coulometer which could be chemical (1I), electronic (I), or mechanical (18). This method, though possibly more versatile than the constant current procedures, is at present limited by the lack of a simple indicating coulometer having an accuracy of at least 0.1%. Lingane applied this method, using chemical coulometers, to the determination of copper, bismuth, and lead (II), as well as the halides (10). Constant current procedures have been applied to electrolytio stripping of plated deposits. Because the degree of polarization is less in electrolytic solution of an electrode, such processes can often occur with theoretical efficiency a t constant current. Xakhar’evskiI (22) determined copper and lead in this fashion. Metallic copper and lead dioxide were first deposited and then stripped off an electrode with a measured amount of current. Campbell and Thomas (9, 4 ) , as well as Francis ( 6 ) ,used thia principle to measure the thickness of metal films.
