V O L U M E 2 0 , NO. 11, N O V E M B E R 1 9 4 8
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communication-there is a linear relation between the quantity (1 - R )/ R and volume fraction of adsorbent. The quantity S was practically constant until a high fraction of Celite was present in the mixture; it therefore showed no consistent variation with composition and v-as not plotted.
LITERATURE CITED (1) LeRosen. 4 . L., J . Am. Chem. Soc., 67,1683 (1946).
RECEIVED March 3, 1948. Part of a thesis submitted b y Walter A. Roth t o t h e Graduate sohoo1 of Louisiana State University in partial fulfillment of the requirements for the master's degree.
Rate of Movement of a Chromatographic Zone as a Function of Temperature ARTHUR L. LEKOSEN AND CHARLES -4.RIVET, JR., Louisiana State C'nicersity, Baton Rouge, La. In order to use the term R (ratio of movement of zone on column to movement of solvent in column) for study of the behavior of substances in the chromatographic column, i t is necessary to know how this quantity depends on the variables in the system. Some data are available for all important variables except temperature. For this variable only a qualitative statement is on record that changes i n zone sequence will take place with changing temperature. In the present work three systems have been studied
T
HE factors affecting the rate of movement of a single
chromatographic zone in a system consisting of only one adsorbent and one solvent have been investigated quantitatively by several workers. LeRosen ( 4 ) has developed an empirical equation expressing R for the leading edge of a zone as a function of initial concentration and initial volume. For the trailing edge of a zone R was found to be independent of both initial concentration and volume. Weil-Malherbe (6) has developed similar equations for Vt (the volume of filtrate collected before the solute first appears in the filtrate) as a function of quantity
A
z 0
0.30
k t 0
%".20t3 6
and the variation of R for the leading edge of the zone is recorded graphically as a function of temperature. All the R-temperature curves show a flat region a t about 20" to 35" C. and two out of the three show rapidly increasing values of R with temperature outside this region. No special control of temperature is necessary for these systems in the region 20' to 35" C., but since other systems maldiffer from these, i t is desirable to report the temperature a t which a given R determination is made.
of adsorbent, initial volume, and quantity of solute. Austin and Shipton (1) have shown that R is independent of the rate of filtration. Strain ( 5 ) has indicated that the adsorption sequence of certain organic compounds is dependent to some extent on temperature. This adsorption sequence is directly proportional to the rates of movement of the individual compounds. This study was undertaken primarily to determine the part temperature plays as an uncertainty factor in ordinary chromatographic work, particularly in standardization of adsorbents and studies of rates of movement of chromatographic zones. The system, o-nitroaniline-silicic acid-benzene, was chosen for study. The results are given in Figure 1. Fortunately for this system, temperature had relatively little effect on R over the range 20" to 35" C. It could be concluded therefore that any measurement of R a t ordinary room temperature would be fairly reliable (all other variables being held constant) and no special precautions need be taken for controlling the temperature. The question as to whether or not the behaviar shown in Figure 1 was characteristic of all systems led to the selection of two other systems having R values ranging from 0.1 to 0.3 a t room temperature. D a t a for the systems o-nitroaniline-silicic acidchloroform and lycopene-calcium hydroxide-benzene are given in Figure 1. All three systems show the least change in R with temperature over the range 20 to 35 O C. and consequently any room temperature measurement of R for these systems would be fairly reliable. The temperature, however, should be stated when R values are given for other systems. DISCUSSION
" I
Figure 1. Relation between Ri (Leading Edge of Zone) and Temperature
I t has been shown (3) that R is identical with the fraction of the total time spent in solution by a solute molecule as it moves down the column. If T , is defined as the average time a solute particle spends in solution between successive adsorptions and To as the average time the particle spends on the adsorbent, then R is given by T 8 / (T , T,). Although it is not now possible to describe exactly the behavior of T , and Ta as functions of temperature, the following can be said with reasonable certainty.
o-Nitroaniline-ailicic acid-benzene. o-Nitroanilingsilicic %d-chloroform. 8. Lycopene-calcium hydroxide-benzene. Each point represents average of 5 to 10 determinations of RI on a single column
For a region of relatively small change in R with temperature, Tal. one of the following must be true for the fraction, T a / (T. Case 1. The relative ratesof change of the numerator and the
IO'
20'
I
I
40' 50' T E M P E R A T UR E
30'
.
I 60"
+
+
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ANALYTICAL CHEMISTRY
denoniiiiatoi must be equal. I n any event, both f', and Ta must be changing in the same direction. Case 2. Both the numerator and the denominator ( T , and total time) must change relatively little with temperature. The slopes dT8/dt and cll',/dt must be relatively flat if d R / d t is to approach zero. S o w it is reasonable to assame that rlT,'dt cau never be positive, since with increasing temperature the probability that any adporbed molecule will have sufficient energy to leave the adsorbent becomes greater. The average time a particle spends on the adsorbent would therefore tend t o decreasp. Case 2 srems to he the more likely of the tn-o possibilities.
S o attempt is made to describe the behaviol of 1', and To over other temperature ranges in this paper. Unpublished theories relating T , and T , as functions of temperature have led to theoretical R-temperature curves similar to those in Figure 1. EXPERIMENTAL
Materials. Merck reagent silicic acid was ubeci its the adsorbent in systems I and 11. .In entire batch of adsorbent !vas prenashed in a large chromatographic tube (50 mni. in diameter) with one volume (the volume required to n e t the column completely) of distilled acetone follo~~-ed by one volume of anhydrous, peroxide-free ether. The adsorbent was dried over calcium chloride in a vacuum desiccator for 4 hours. Mild heat from an infrared lamp was used to hasten evaporation of the ether. The dried adsorbent was stored in a suitable air-tight container. Baker and Sdamson reagent calcium hydroxide (powder) was used in system I11 and received no prewashing. The benzene received no special treatment. The Mallinchrodt analytical reagent chloroform used was washed with one-tenth volume of water ( t o remove the alcohol), dried over calcium chloride, and distilled. Methods. The method used in determining R has been dewribed by LeRosen (2). A jacketed chromatographic tube, 250
mm. long, inside diameter 9 mm., was used. The temperature of the system (chromatographic tube plus a jacketed solvent reservoir) was controlled by a circulating water bath. I n all runs the packed column was brought t o the desired temperature and completely wetted with solvent before the solution was added (0.715 ml.). To facilitate even packing, all columns were packed before the tube \vas placed in the water jacket. The o-nitroaniline solutions were 0.001 M. The lycopene solution v a s 0.0001 M . I n most runs a second volume of solution nm added to the column after the first zone had moved donaward half the length of the column. Movement, of zones was folloived visually. Because of the diffuse nature of the shiv-moving o-nitroaniline zones n-hen adsorbed from chloroform, it m-as impractical to attempt determinations of R below room temperature. I n contrast, the sharply defined edges of lycopene zones on calcium hydroxide made it possible to make accurate determinations a t fairly low values of R. Khen chloroform was used as the solvent a t elevated temperatures small bubbles formed in the upper 10 mm. of the column and had to be removed iiianually before R \vas measured. -4t temperatures above 60" C. lycopene showed some tendency t o isomerize on the column. The amount of the faster moving isomer formed (neolycopene) never became great enough to change the initial concentration of the lycopene solution appreciably. LITERATURE CITED
(1) Austin, C. It., :tiid Shipton, J., J . CounciZ Sei. Ind. Research, 17, 115 (1944). (2) LeRosen, A. L., J . Am. Chem. Soc., 64, 1905 (1942). (3) Ibid., 67, 1653 (1935). (4) I b i d . , 69, 87 (1947). (5) Strain, H. H., ISD.EXG.CHEU.,ASIL. ED.,18, 605 (1946).
(6) Weil-Malherbe, H., J . Chem. SOC.,1943,303. KECEIVEDMap 1 7 , 1948. Presented before the Division of Analytical and Micro Chemistry a t t h e 113th Ifeeting of t h e . ~ > I E R I C A N CHEMICAL SOCIETY, Chicago, Ill.
Colorimetric Determination of Gold as Bromoaurate Separation of Small Amounts of Gold by Solvents Extraction F. A. E. RICBRYDF,' AND JOHN €I. YOE, L'ninersity of T'iryiniu, Charlotteszille, Vu.
.
The orange-colored bromoaurate ion provides a simple method for the colorimetric determination of gold. The color formation is immediate and permanent i n solutions w-ith pH less than 4, if a large excess of chloride is avoided. The sensitivity compares favorably with existing procedures. Small amounts of gold may be extracted from 2 M hydrobromic acid solutions into isopropyl ether, thereby effecting separation from other metals with colored ions.
S
EVERAL procedures for the colorimetric determination of gold have been proposed; the most successful are described in two recent monographs on colorimetric analysis (1, 19). The application of these methods is somen-hat limited-for instance, procedures that depend upon the formation of gold sols or of colored colloidal systems are affected by acidity and ionic stlength to a marked extent. iiumerous metallic ions, both colored and colorless, are reported t o interfere or give similar reactions in each of the recommended methods. I n some of the procedures the color formation is not permanent or is time-dependent. A detailed study of the application of the bromoaurate ion (AuBr4-) to the colorimetric determination of gold has revealed that a very simple practical procedure consists in the addition of bromide to an acidified solution of a gold (111) salt. I n so far as can be determined, this reaction has not been made the basis of a colorimetric method for gold. The converse procedure-namely, 1 Present address, Department of Chemistry, r n i v e r s i t y of Toronto, Toronto, Canada.
the use of gold salts for the determination of bromides in physiological fluids-has been described by several a orkers (9, 22, 23). With the cxception of chloride ion, the color reaction is not influenced by any colorless ion. Extraction of bromoauric acid into solvents such as ethers, esters, and ketones is virtually complete, and conditions have been worked out for a separation of gold from metals with colored ions by extraction into isopropyl ether from a 2 iM hydrobromic acid solution. The suggestion that gold might be separated from the platinum metals and mercury by extraction into diethyl ether was originally made by Nylius and Dietz (16), and Mvlius later showed the effect of acidity and total gold concentration upon the extraction coefficient (1.5, 1 7 ) . A decrease in the extraction coefficient, especially a t lower acidity, was noted a6 the total gold concentration was reduced. This same behavior has been found in the case of ferric chloride upon extraction into ethers, Lenher (12) showed that many organic compounds would extract gold chloride from acidified aqueous solutions, and
