Analytical Distillation Frederick
T
E. Williams,
Hercules Research Center, Wilmington, Del.
review covers the period September 1963 to September 1965. I t may be the last biennial review for &JALYTICAL CHEMISTRY Of distillation as an analytical technique, for it has suffered from the blight of its own defects and the vast virtues of gasliquid chromatography. Perhaps, like the American chestnut tree, analytical distillation, after years of apparent dormancy, may put forth a few tentative shoots of new life from old stumps, but they will offer only a fleeting glance at past grandeur, and after a season will be no more. Janiei and Martin ( 9 ) , when they experimentally confirmed the earlier speculations of Martin and Synge (11), uncovered an area of research that, through the efforts of hundreds of workers, ha5 undergone in one decade one of the most fantastic developments in the history of science. Not only has gas-liquid chromatography (GLC) supplanted analytical distillation for all practical purposes as a means of analysis, but its principles (ar a rehult of the low concentrations in its system of operation) have let it become effective in areas from which time and temperature effects have forever excluded analytical distillation; moreover, while analytical sample requirements changed from (at best) milliliters for analytical distillations to microliters for GLC, the factor of time dropped from hours for distillation to minutes for GLC. The supreme irony arose when instrumental developments for GLC necessitated some considerable intellectual effort in devising ways and means of measuring, accurately, small enough samples for quantitative results. During the last five years (1960-65) further encroachment into the exclusive domain of laboratory distillation has taken place through the development of preparative GLC ( 3 ) . This natural evtension of the analytical technique to large-scale operation has run into the expected difficulties attending scaleup (these have long been familiar to workers in analytical distillation), HETP’s did not remain almost vankhingly small as in analytical GLC, column capacity did not increase in the hoped-for ratio R 2 / r 2 ,and the effects of heat of adsorption and desorption, to say nothing of the severe problem of cooling vast quantities of dilute gas solutions to recover desired solutes, leave something to be desired in largescale or preparative GLC. Perhaps the profoundest effect of HIS
GLC vis-%-vis distillation is the great reduction in the necessary size of experimental operations in chemical research. When only a few microliters of sample are needed for a complete analysis, only a few hundredths of a mole of reactants need be involved in studying a reaction instead of the molar or multimolar quantities needed if analytical distillation were one of the required analytical procedures. This use of very small samples not only saves time and materials, but should, in the long run, engender much more careful techniques required in handling small quantities of materials. What, then, is the position of distillation in the scheme of things? As a strictly analytical technique, it is completely outclassed and outdated by GLC; for all practical purposes, it is dead. In process development, fractional distillation still functions uniquely to assess distillation for ultimate use in a commercial process, particularly operating conditions, expected temperature levels, azeotrope formations, etc., and determination of in-the-barrel yields. The developments in small-scale distillation during the quarter of a century between 1930 and 1955 were such as to provide quite satisfactory equipment and operating techniques for the preferred uses of distillation outlined above. Additional developments might have been forthcoming had not the glamour of the digital computer turned workers’ heads away from the more pedestrain pursuits of developing distillation equipment to the elegance of distillation theory (with simplifying assumptions), Fair ( 6 )has reviewed developments in distillation technology as disclosed in the literature for the period June 1963 to September 1964. A survey for an additional year reveals that the general trend of interests is in the same direction noted by Fair. The integration of computer techniques and distillation technology has tended toward perfection, but frequently lack of interest in correlating computer results and practical results in actual operations has thrown a shadow of the ivory tower across the computer results. For those who retain interest in laboratory distillation, there have been no outstanding developments during the last two years. Tiong and Waterman (15) describe a spinning band column having an efficiency of 50 theoretical plates and usable to 250’ C. a t pressures
from 1 to 760 torr. The spinning band was of Teflon. Another spinning band column suitable for the analysis of hydrocarbon mixtures was described by Chanda and Ghosh (2). This unit was 45 cm. long. The authors claim the presence of high boiling components, either naturally present, or deliberately added, improve the efficiency of separation. Ziolkowski, Filip, and Kawaha (20) studied a column with rotary cones alternating with fixed cones [probably similar to a design of Mair and Willingham ( I O ) ] . Increasing speed of rotation of the cones was found to improve efficiency, particularly a t higher vapor loads in contrast to the findings of Mair and Willingham in their work. One of the oldest types of laboratory rectifying columns (Warren, 1864), the helical coil dist,illation column, has been the subject of efficiency studies by Morton, King, and LCIcLaughlin (12). Several tube diameters from 0.42 cm. to 1.95 cm. were investigated. Details of operation a t 20 torr of a spiral made of 1.10-cm. diameter tubing is given in detail. These columns appear to be useful for low-pressure fractionation of heat-sensitive materials. For columns of pilot plant size, Ellis et al. (6) have used a packing made up of vertical layers of multistrand knitted wire packing four inches long, and six inches in diameter in a 6-inch diameter column 5 feet long. This packing had an H E T P of 2.45 inches and a notably low pressure drop. Sperando et al. (16) developed a multichannel packing made from woven wire fabric; it is recommended for vacuum rectifications. Some attention has been given to test mixtures for determining the efficiency of rectifying columns. Romani (14) has suggested three mixtures suitable for columns of low efficiency: cyclohexane-toluene (nonpolar type), acetonebenzene (polar - nonpolar), and acetic acid-water (polar type). These mixtures are easily produced, purified, and analyzed. Inclusion of polar-type mixtures is a departure from traditional types of test mixtures. New vapor-liquid equilibrium data have been determined for the benzenecarbon tetrachloride text mixture by Haughton ( 8 ) . Although the new data agree fairly well with some other published data on this system, the general discrepancies among the data for the system benzene-carbon tetrachloride rule it out as a reliable test mixture. Auxiliaries for use with analytical VOL. 38, NO. 5 , APRIL 1966
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fractionating columns are few. Van Sway (17) has developed a fraction collector for use in vacuum fractionation. A piston pump ejects the collected fractions through a relief valve into suitable storage vessels a t atmospheric pressure. Williams (19) has described an all-glass fraction cutter with a selflubricated valve. This unit is suitable for vacuum operation a t temperatures as high as 300’ C. Little has been added during the past two years to the fund of knowledge dealing with analytical distillation; not only has distillation equipment been of minor interest, but operating procedures and improved techniques have been scarcely considered. Pichler and Fetterer (13) have confirmed earlier findings on the consonance between continuous and intermittent withdrawal of distillate from a fractionating column. Barker, Jenson, and Rustin ( 1 ) developed equations for predicting the rate of approach to equilibrium of a batch distillation column. The equations were solved by an analog computer. .4ccuracy of the equations was tested with a methylcyclohexane-toluene test mixture in a 4.5-inch diameter bubble tray column. For the column studied, condenser holdup was the most important variable affecting the rate of
approach to equilibrium. In most packed analytical columns, condenser holdup is generally very slight, and should therefore have only a slight effect on rate of equilibration. Haring and Knol (7) discuss the influence of reflux ratio on the separating effect in a fractionating column. They distinguish between separating performance as functioning at partial reflux and separating power as functioning under total reflux. Elliev et al. (4) developed equations to establish a similar relationship. Finally, a somewhat optimistic role is predicted for distillation by Wilcox (18). He devised special equations to establish conditions for preparing ultrapure materials. These equations are simpler than those for the usual batch distillation, since the quantity of the second component is assumed to be very small. This approach may be helpful also for the ultimate recovery in purest form of compounds separated by preparative GLC where some contamination by eluted fixed phase from the GLC column has occurred. LITERATURE CITED
(1) Barker, P. E., Jenson, V. G., Rustin, A., J . Znst. Petrol. 49 (478), 316-27 (1963). (2) Chanda, SI., Ghosh, K. K., Chem. Age (India) 14, 517 (1963).
(3) Chem. Eng. Xews 43 (26), June 28, 1965. (4) Elliev, Y. E., Devyatykh, G. G., Dozorov, 5‘.A., Zh. Fiz. Khim. 37,2179 (1963). ( 5 ) Ellis, S. R. AI., Porter, XI. C., Jones, K. E., Trans. Znst. Chem. Engrs. (London) 41, 212 (1963). (6) Fair, J. R., Ind. Eng. Chem. 56 (lo), October 1964. (7) . . Haring, H. G., Knol, H. W., Rec. Trav. Chim. 83; 645 (1964). (8) Haughton, C. O., Brit. Chem. Eng. 10, 237 (1965). (9) -James, A. T., Martin, A. J. P., Analyst 77, 915 (1952). (10) &fair,B. J., Willingham, C. B., J . Res Xatl. Bur. Std. 22, 519 (1939). (11) Martin, A. J. P., Synge, L. M., Baochem. J . (London)35, 1358 (1941). (12) Morton, F., King, P. J., McLaughlin, A., Trans. Inst. Chem. Engrs. (London) 42 (8) T-285-T-295, T-296-T-304, NO. 182 (1964). (13) Pichler, H., Fetterer, E., Erdoel Kohle 17,97 (1964). (14) Romani, J. M., Genie Chim. 90, 29 (1963’l \ _ _ _ _
(15) Tiong, Sie Sevan, Waterman, H. I., Znsenieur (Utrecht) 72. Ch. 71-82 (1960). (16) Sperando, A., Richard, M., Huber, M., Chem. Zngr.-Tech.37, 322 (1965). (17) Van Sway, &I., Rev. Sci. Znst?. 35, 164 (1964). (18) Wilcox, W. R., Ind. Eng. Chem. Fundamentals 3, 81 (1964). (19) Williams, F. E., “Techniques of Organic Chemistry,” Vol. IV, 2nd ed., Wiley, New York (1965). (20) Ziolkowski, Z., Filip, S., Kawaha, Z., Przemysl Chem. 42, 512 (1963).
Electroanalysis and Coulometric Analysis Allen 1. Bard, Department
of Chemistry, The University of Texas, Austin, Texas 7871 2
T
HIS PAPER surveys the literature and developments during 1964 and through December 1965, although papers published before 1964 which have not appeared in previous reviews in this series have also been included. BOOKS AND REVIEW ARTICLES
A number of books dealing with electroanalytical chemistry and electrochemistry have appeared since the last review. Volume 2a of “Comprehensive Analytical Chemistry” (296) deals with electrical methods and contains one chapter on an introduction to electrochemical analysis and another on electrodeposition by A. J. Lindsey. Purdy’s book, “Electroanalytical Methods in Biochemistry” (218), contains an elementary discussion of controlled potential coulometry and coulometric titrations, as well as other electroanalytical methods; Table 6 in this book lists a number of coulometric titrations that have been performed, the limits of concentration for the determinations, and their accuracy; and 88 R
ANALYTICAL CHEMISTRY
Table 7 gives the supporting electrolytes for many coulometric titrations. The new edition of “Polarographic Techniques” by Meites (179) contains sections on controlled potential electrolysis and coulometry and coulometric titrations. The description of the techniques involved in controlled potential electrolysis measurements and the discussion of perturbing effects in coulometry are of special interest. An introductory book on electroanalytical methods has also appeared (161). Other books dealing with electrochemistry which may be of interest to workers in the field include “The Encyclopedia of Electrochemistry” (105), Conway’s “Electrode Processes” (53), Delahay’s “Double Layer and Electrode Kinetics” (62), and Zuman’s “Organic Polarographic Analysis” (SIO), which discuss the fundamental principles and electrode reactions which form the basis of electroanalytical techniques. The recent book, “ h p e r o m e t r i c Titrations,” by Stock (259) contains numerous examples of this end point
detection technique, so valuable in coulometric titrations. An English translation of the book of Abresch and Claassen on ‘[CoulometricAnalysis” (1) has appeared, as well as a new book on this subject by Patriarche (207). Several review articles on electroanalysis have been published. Szabadvary’s (264) paper gives brief biographies for many of the pioneers and research workers in the field of electroanalysis. Foreign language reviews of electroanalysis (191, 198) (Japanese), ($1) electrogravimetric analysis (Dutch), coulometric analysis (5, 182) (Russian), and controlled potential coulometry (257) (French) have also appeared. NEW TECHNIQUES
Flow Electrolytic Methods. Interest has been revived in techniques based on electrolysis of flowing streams of solutions for analysis or separations. Sporadic reports in the past have been concerned with attempts a t carrying out the electrolysis of a flowing
