974
A N A L Y TI C A L CHEMISTRY:
9
graphs, and the directors of Beecham Research Lahoratories, Ltd., for permission to publish this paper. LITERATURE CITED
(1) Consden, R., Gordon, A. E.,and Martin. A. J. P., Bioebem. J . (London),38, 224 (1944). (2) Curson, G., and Giltrow, J.,N a t w e , 173, 314 (1954). (3) Datta, p., ~ e n t E. , C., and ~ ~H,, science, ~ ~ 112, 621 i
s.
11 qmi. ~ ~...,
(4)
Hardy, T. L., end Holland, D. 0.. Chemistry & Zndustru, 1952, 855.
( 5 ) Harris, G.. J . Inst. Brewing, 58,417 (1952). (6) Hsrrir, G., and Pollock, J. R. 1.Zbid.. . 59, 20 (1053). (7) Jepson, J. B., and Smith, I., .Valure. 172,I100 (1953). (8) Levy, A. L., and Chung, D., .~NAL.Cnsu., 25, 396 (1053). (0) Redfield. R., Biochem. et Biophys. Ada. 10, 344 (1 953). (lo) ~ ~ ~ k L,l B,,~ and ~ dunderwood, , J, c,, Cnmr.. 26,.
1557 (1054). A.,and Oreskes, I., Science, 119, 124 (1054). ~(11) Scrifer. , (12) Smith. I., Natuve, 171,43 (1053). RECEIVED
for review July 10, 1954.
loeepted February 1. l%X.
Twenty-Stage Molecular Distillation Unit F. W. MELPOLDER, T, A. WASHALL, and J. A. ALEXANDER The Atlantic Relining Co., Philadelphia, Pa. A 20-stage countercurrent molecular still has been developed for the distillation of high boiling samples. The use of the still is expected to permit a more complete analysis of heavy petroleum products. The still, with a capaoity of 1500 ml., consists of one large and 19 small still pots connected in series. The unit can he operated as an equilibrium-type still for samples less than 260 ml. and as a hatch distillation unit for larger volumes. Using a test mixture of Oetoil Ihis(2-ethylhexy1)phthalatel and Octoil S Ibis(2-ethylhexyl)sebacate], an efficiency of 0.8 theoretical plate per stage was found. Automatic safety controls allow the still to operate unattended.
distillate and reflux wa8 provided for efficient fraotionittion. This design is similar in principle to the method used by F a a e e t t and McCowen (S), except that pumps between stages were eliminated.
I
N T H E past few years several new itnalytical tools have been
developed for the separation and analysis of high boiling petroleum stocks. As the first step in a separations program it is often advantageous to prepare narrow boiling fractions in order t o enhance the degree of separation in subsequent operations. Because of the thermal instability of hydrooarbons at the higher temperatures, it is necessary t o carry out the distillation at the lowest practicable pressure. Scveral types of high vacuum rectifying stills have becn described recently. Byron, Bowman, and Coull ($) have developed a rotary rectifying column consisting of two concentric tubes. The inner tube is cooled and rotated while the outer tube is heated and stationary. Brewer and Madorsky ( 1 ) obtained a pnrtid separation of the meroury isotopes in a 10-cell countercurrent reflux still. Fawoett and McCowen (S) developed a countercurrent redistillation unit for continuous distillation. A multicolumn, countercurrent falling-film still was developed by Mndorsky ( 6 ) . The individual stills consist of a cancentric.evaporata- surrounded by a water-cooled condensing are3. Each column is provided with a combination magnetic pump and reservoir. A multistage still developed by Mair, Pignocca, and Rassini (6) consists of 8 large pot and a reotifying Eection contsining 50 stages. I n the following work, an attempt was made to design a simple multistage molecular still possessing a relabively high separation effieienoy.
-
1.
.
contents of t h a t pbt before %assing downward as reflux to the next lourer pot. T h e result is that the lighter components are progressively distilled to the higher stages, while the heavier camnonents are carried down to the loiver stages. Chamber e is merely B reflux unit, The d$tillate from pot 20 is returned to that pot as reflux through tube f. Tubes f also
DESIGN O F MULTISTAGE STILL
. .
1
.
..
. . ~
I
~~
~~
vapo;was
to reduce seriously the fractionation efficiency of the column. A new type of still was desianed, therefore, to eliminate the flow of vapor- from any particular stage. Twenty glass molecular stills of the type described by Mulliken and Harkins 171 were connected in series as a sinele unit with snitabk tnhes io; transfer of distillate and reflug. Countercurrent BOW of ~~
~~~~~~
Figure 1.
Still assembly
975
' V O L U M E 2 7 , NO. 6, J U N E 1 9 5 5 serve as constant level devices to maintain a constant liquid level in each of the small still pots and the reflux unit. Vertical tube3 9. extending above the still pots, connect to the vacuum system through a manifold line. These tubes are also used to -charge sample t,o the pots, to remove fractions, and to hold short thermometers Tyhich indicate liquid temperatures in the pots. Ground.glass caps at the top of the tubes :~llon-easy access to the pots. . Electrical heat is supplied to the m a l l rtill pots by means of radiant heaters clad nit11 The large still pot is ced direct1)- beneath it. heated by a 375-watt infr Asbestos cylinders lined with aluminum foil enclose the heaters A total of five Variac and bottom portions of t er currents of the $mall transformers are used to :i still pots. Each tranefoi,nier controls four heaters xhich are connected in parallel. A constnnt heat iiiput to each of four still pots can be attained by carefully adjustiiig the sl+icing hetn-een the heater and still pot. A separate tr:in4'ornier is used to conitrol the heat input for the large $till pot.
are placed in the closed position while stopcoclis j and k are opened. The small still pots and the reflux unit are charged ivith sample through the vertical tubes, and the ground-glass caps are lubricated and set in place. The vacuum umps are turned on, and heat is applied to the pots. I n tKe startup operation, a manual switch is turned to bypass the safety controls. ris soon as a pressure of 5 microns of mercury or less is reached, the switch is then turned back to the automatic control pqsition. A final pressure of about 1 micron of mercury is obtamed. The heat is gradually increased until distillate is seen to drip from the distillate delivery tubes. Further adjustment is made until a constant rate of 1 drop every 3 seconds is obtained. I n the first 4 hours of operation. additional adjustments in the heat are required because of the accumulation of the less volatile coniponents in the lon-er pots and more volatile components in the upper pots. Thereafter. no further adjustments need to be made and the still is allowed to operate unattended overnight. I-suall>-16 hours of operation suffice for the contents of the still to approach equilibrium. At the end of this interval, the heaters are turncd off and the system vented to the atmosphere. The caps are removed, and the contents of the pots are withdrawn n-ith a 25-nil. h!-poderniic syringe attached to a capillary tube. .I small amount of sample caught in the reflus transfer tubes cannot be retrieved easily. This method of operation can be extended to the distillation of even smaller volumes of sample. I n this case. a sufficient number of the uppermost still pots are wed and heat is applied onl>-to those still pots containing sample. 111
I
\
L Figure 2.
REFLUX
Low-er stages of 20-stage molecular distillation unit
Tlie vacuum s>-stem consists of a combination of the conventional mechanical pump and a n oil diffusion pump (Distillation Products, Inc., Model 1IBlOO). A cold trap is placed between the pumps and the still. However, this is normally not used except for the distillation of the more volatile samples.. A large manifold and distribution lines connect the vacuum svstem with each 4 1 Dot A Pirani aane monitors the Dressure of the system. Electrical Safetv Controls. Two electrical safety devices a ~ c used to allox t h e still to operate unattended. These control3 are incorporated in the electrical heating circuit3 to protect the system in the event of air leakage or water failure. 2. central power relay is installed to cut off the current to all heater elements in response to two sensitive relays. The first relay is in the Pirani gage circuit t o monitor the pressure of the s!-steni. High pressure in the system generates an unbalance current in the Wheatstone bridge circuit of the Pirani gage. This current operates a Weston sensitive rela>- which in turn actuates the power relay through an electronic switch. The U-eston relay ii set to operate a t 8 microns of mercur- pressure. A second electronic switch is used to monitor the !rater flonthrough the condenser of the oil diffusion pump. The sensitive grid connection from the electronic w i t c h is placed in the effluent water stream, causing the tube to remain conducting until the n.ater flon. is cut off. The relay in the elrctronic sn-itch is then de-energized, \rhich in turn caiises the main rcl:~!- to turn o f f the pon.er to the 2ic:ittv.G. OPERATIOS O F STILL
Tlie 20-stage unit c a n be used cit!irr :is n i l rqidihriuni type of still or as a hatch tli~tillationstill. In the former method of operation. only the 19 sniil1,l sti!l pots and the reflux unit are used. The det:d$ of operation :ire a s follon-s: Stopcocks h and
~l
i
li
19
Figure 3.
Upper stages of 20-stage molecular distillation unit
I n operating the still as a batch distillation unit, the still is charged as indicated previously up to maximum volume of I500 nil. The first 260-ml. portion of sample is used to fill the 19 m a l l pots and reflu.: chamber while the remaining portion is placed in the large still pot. The startup operation is the same a i that described for the equilibrium-type operation, with the addition that heat is also applied to the large still pot. When the contents of the still approach equilibrium. after 16 hours of operation, distillate withdranal is begun. This is accomplished in one of two wavs. I n the first method, stopcock j is turned to the open popition. Fractions can then be collected periodically by opening stopcock h and draining the contents of the reflux chamber. The system can be fitted with either a large single ieceiver or a receiver capable of holding four separate smaller fractions. I n order to change the receiver, stopcocks h and k are closed, and the receiver is vented to the atmosphere through stopcock m. which is closed immediately. The receiver is then iemoved. I n resuming operation. the receiver is replaced and
976
ANALYTICAL CHEMISTRY
stopcock k is opened, thereby permitting evacuation of the receiver. The second method of product withdrawal involves continuous removal of distillate After the contents of the still pots approach equilibrium, stopcock h is opened and the desired rate of distillate withdrawal is achieved by adjusting the flow through stopcock j. When removing the receiver during this method of operation, stopcocks h and k are closed, the receiver is vented and then removed. After the receiver is replaced, stopcock k is opened. Stopcock h is opened only after the pressure in the receiver has been sufficiently reduced. The distillation is continued until the large still pot is depleted. At this time, all heat is turned off, and the contents of the still pots are allowed to cool hefore venting the system to the atmosphere. The last fractions are obtained bv emptying the 19 small still pots and the reflu\ unit as described previously. V0L.Z DISTILLED
EFFICIENCY TEST
Figure 5.
I n order to test the efficiency of the still, an attempt w a q made to find a suitable test mixture. Octoil bis(2-ethylhexyl) phthalate and Octoil S bis(’2-ethylhexyl) sebacate have been thoroughly tested by Trevoy (8) but the relative volatility of this mixture is too large for stills having more than four or five theoretical plates. Another miuture, Octoil-n-octyl phthalate, was also studied by Trevoy. However, he found that the relative volatility changed markedlj with composition and therefore was unsuitable ss a test mixture.
100
80 v)
2 60 0
*
40
0 J
20
0
20
40
60
80
100
V O L %DISTILLED
Figure 4.
Tlolecular distillation of 50 mole Octoil S
Octoil and
0 One-stage molecular still
A 20-stage molecular still
I n the absence of a n ideal test mixture, the Octoil-Octoil S mixture was distilled in order t o observe the degree of qeparation obtainable. It \vas possible to determine the efficiency of a limited numher of still pots in which the composition varied from 10 to 90% of the lighter component. This test was made prior to modification of the still. The 20 small still pots were used and an equilibrium type of distillation was carried out. A 50 to 50 mole % mixture of Octoil and Octoil S was charged to the still pots. The mixture was allowed to reflux for 40 hours and then the contents of the still pots were collected. The refractive index of each fraction was measured and its composition was found from a calibration curve. The results plotted in Figure 4 show that the first and last 30y0 portions distilled were nearly pure Octoil and Octoil S, respectively, while the middle 40% was a mixture. A calculation was made of the plate efficiency of the center five pots. Using the relative volatility of 2.6 reported by Trevoy and the Fenske equation ( 4 ) ,a total of 4 theoretical plates for five pots was found, or a ratio of 0.8 theoretical plate per pot. For comparison, the separation of the same Octoil -0ctoil S mixture in a one-plate still under similar conditions is also given in Figure 4. I n this distillation it is interesting to note that the initial fraction was only i 7 . 7 mole % Octoil whereas the final fraction was almost pure Octoil S.
20-stage molecular distillation of 50 mole Octoil and Octoil S 3 A
+
4fter 2 hours’ distillation .4fter 6 hours’ distillation After 16 hours’ distillation After 32 hours’ distillation
Another test run was made to determine the length of time required for the composition of the liquid in the 20 small pots to approach equilibrium. -450 to 50 mole yomixture of Octoil and Octoil S was charged t o the still and the distillation was started up. Several drops of liquid were removed from the still pots at intervals of 2, 6, 16, and 32 hours for refractive index nieasure‘ment. The results of this test (Figure 5 ) show that equilibriuni was nearly attained a t 16 hours. An additional 16 hours of oprration did not increase the degree of separation. After the still was modified, a t,echnical grade of eicosme wax was distilled. A total of 1500 ml. of sample !vas charged t o t,he still and distillation was begun in the usual manner. h distillate throughput of about 40 nil. per hour was established. At this throughput, it was calculated that t,he vapor pressure of the eicosane wax was about 0,020 mm. of mercury. After refluxing for 24 hours, the contents of the reflux unit were emptied. Thereafter, the reflux unit v a s emptied only at the beginning and end of the eight-hour work day. Between distillate withdrawals, the still was operated a t total reflux. Approximately 28 nil. mere collected every 24 hours with a n effective reflux ratio of approximately 30 to 1. Minor adjustments of the heater voltages w r e required at intervals of 2 or 3 days to compensate for the increase in distillation temperature. Composition of the fractions was determined by melting point determinations. Figure 6 h x v s
1
1
V O L U M E 27, NO. 6, J U N E 1 9 5 5
977
that concentrates of normil CIS,CIS, C?o, C", and CZZwere obtaintid. %. slight decrease in melting point was noted near the end of FeveraI plateaus. This is attributed to isoparaffin and cyclop;traffin hydrorarhons present, in the xas. The still has heen used t o prepare narrow boiling fractions from lubricating oil stocks as the first step in a separations program. The opcration of the unit has been satisfactory for the heaviest fraction distilled so far. In this rase a maximum temperat,ure of 600" F.~ w olxerved. s LITERATURE CITED (1) 131,ewer.
4.K., and Madorsky, S. L.. J . Renearch S a t l . Bur.
Staridads, 38, 129 (19.17).
( 2 ) Bpron. E. 8.. Bowman, J. R., and Coull, J., Ind. Eno. Chem., 43, 1002 (1951).
Fawvcett, E . W.. and McCoxen, J . L., U. 9. Patent 2,073,202 (llarch 9, 193i). (4) Fenske, 31. R . , I n d . Eng. Chern.. 2 4 , 4 8 2 (1932). (5) liadorsky. S.L., J . Research S a t l . Bur. Standards, 44, 136 (1960). (6) lIair, B. J.,Pignocco, A. J . , and Rossini, F. D., ASAL.CHEY.,2 7 , (3)
190 (1955).
(7) Llulliken. R. S.. and Harkins, W. D . , J . A m . C h e m . SOC.,44, 37 (1922). (8) T r e r o y . D. J.,Ind. Eng. C h e m . , 44, 1888 (1952). RECEIVED for review Noremher 26, 1934. Accepted February 5 , 1955. Presented hefore tlie Division of Petrolellin Chemistry at the 125th Meeting of the A M E R I C ~ CHEMICAL S SOCIETY, Kansas City, N o . , 19.54,
Determination of Traces of Nickel and Zinc in Copper and Its Salts LOUIS MEITES Sterling Chemistry Laboratory, Y a l e University, N e w Haven, Conn.
A solution of the sample in an ammoniacal ammonium chloride medium containing excess hydrazine is electrol>zedwith a mercury cathode, the potential of which is automatically maintained constant at -0.85 volt Z'S. S.C.E. for the removal of the copper. Nickel and zinc are determined polarographically in the residual solution. With certain modifications in rare instances, results accurate and precise to about &t;% or better can easily be attained over the range of nickel and zinc contents from lo-' to 10-6%. An entire analjsis can be completed within 7.5 minutes.
T""'
, ( cxterminat,ion of traces of nickel and zinc in metallic copper and its salts presents a rather difficult problem. Since copper in large amounts interferes in most procedures for the determinat,ion of nickel and zinc, it must be removed practirallj. complct~ely.without. contaminating the sample with significant ctnionnts of nickel and zinc which may he present, in the reagents. This pxper presents a method by n-hich as little as 10-q?O of eithci, nickel or zinc may be determined in cupriferous materials. The copper is removed by electrolyzing an ammoniacal solution of thr sample with a mercury cathode the potent'ial of x-ihich is licipt roi~.qt:inta t -0.85 volt Z'R.S.C.E. .is is shown by Figure 1. this potrntial suffice$ for the complete reduction of copper to the metal hut is without effect on nickel and zinc. The peculiar virtue of the electrolytic sixparation lies in the fact that, it practically eliminates the possihility of adventitious contamination of the s:tnipk. Indeed, this can be entirely eliminated by a simple prel)iir~ifir:ttionof the supporting electrolyte solution. PROCEDURE
T h e followiiig procedurc is recommended for the determination of nickel and zinc in sample? containing not more than ahout 20 times as much nickel as zinc, and not less than :$bout 5 X 10-477, of either. Dissolve a weighed sample containing about 1 gram of copper iri a minimal volume of water or dilute nitric arid and transfer to a 250-nil. volumetric flask. Add 60 ml. of a stock solution rontaining 1.V ammonia, 4M ammonium chloride, and 1M hydrazine hydrochloride. This solution should he made up fresh every week or two hecause of the slow air oxidation of the hydrazine, and should he stored in a thoroughly cleaned polyethylene bottle. Allow the mixture t o stand in the umtoppered flask for a minute or two. During this period the cupric ammonia complex is practically completely reduced t o the cuprous state according t o the NzHa = 4Cu(NHJ)2+ 4SH4+ equation 4Cu(NH3],++ N2. When vigorous evolution of nitrogen has ceased and the solution is nearly colorless, dilute it to volume and transfer a portion of it to a clean dry cell suitable for controlled potential electrolysis.
+
+
+
T h r cell used in this work was that, shown in Figure 3 of a previous paper (3), Jvhere its advantages over previously described cells were discussed in detail. I n this procedure the working anode is a helix of platinum wire wrapped around the stirrer shaft, and it is to depolarize this electrode that the hydrazine is included in the supporting electrolyte. As a referener electrode the author used a saturated calomel electrode of the type supplied for use with Beckman pH meters (Type B-I 1i0, Sational Techniral Laboratories, South Pasadena, Calif,). Unlike some instruments previously described, the .inalytical Inst,ruments, Inc., Bristol, Conn., potentiostat used in this work does not appear to object to the presence of tlie resistance of this type of electrode in its control circuit. Adjust the potentiostat t o maintain the potential of the mercury cathode constant a t -0.85 volt v s . S.C.E. and an efficient stirrer immersed in the mercury-solution interface t o keep the surface of the mercury in rapid motion, and alloiv the electrolysip to proreed unattended for 45 to 60 minutes. exclude air, from the solution during the electr oxidation of the cuprous ammonia complex at, the surface of the solution. ivill reduce the concentration of copper remainto about 0.01% of its initial value. Though the height of the copper wave in such a solution will be too small to interfere with the measurement of the nickel and zinc wave height. for most samples, it may very rarely be desirable t o continue the electrolysis for anot,her 15 minutes or FO to reduce the copper concentration still further. After the desired length of time, IT-ithdrair-about 50 ml. of the solution into a dry polarographic cell without interrupting t,he electrolysis current. Add one drop (0.05 nil.) of a 20/, aqueous solution of Triton S-100 (Rohm and Haas Co., Philadelphia, Pa.) to serve as a maximum suppressor, deaerate nitrogen or hydrogen, and record the polarogra from -0.8 to -1.5 volt,s vs. S.C.I
