1038
A N A L Y T I C A LC H E M I S T R Y
1%. Mixing WRE done by a magnetic stirrer. Polarograms and current m ~ a u r e m e n t swere made within 2 minutes after mixing.
Carbon disulfide yields two cathodic waves with half-wave potentials, independent of both p H and concentration, a t - 1.3 and - 1.7 volts (os. saturated calomel electrode). The presence of 0.006% gelatin completely suppresses a maximum on the first wave, The waves are of good form for analytical work (Figure 1) and in the concentration range 2 X 10-4 M to 2 X 10-8 M I a constant value of the ratio id/Crn2/3t*'6 of 2.52 X lo3 'a'liter'sec*'/z mol e.mg .'/s was found a t 2' C. for the first wave (Figure 2). The height of the second wave was 0.95 that of the first. A plot of log i/id - z for the first wave mas linear and had a slope of 0.076, indicating irreversibility. -4ssuming a two-electron sq. em. reduction, a value of the diffusion coefficient of 5.3 X per second is calculated at 2' C., while if the number is one, a value of 2.1 x 10- sq. cm. per second is found. Taking a temperature coefficient of 2% per degree centigrade] a two-electron reduction would correspond a t 25' C. to D = 0.84 X sq. cm. per second and a one-electron reduction to D = 3.3 X
sq. cni. per second. The latter value is close to that of small uncharged molecules (of the order of 3 X 10%sq. cm.per second). Hence, both waves correspond to a one-electron reduction. Probably a free radical is formed in the first reduction. The products formed in both reductions react with cupric copper, for the presence of an equimolar amount of cupric chloride in 0.1 M potassium chloride causes the disappearance of both carbon disulfide waves (Figure l), probably by a process similar to that involved when oxygen is reduced in slightly acid solution ( 1 ) . ACKNOWLEDGMENT
Acknowledgment is made to the Graduate School of the University of Minnesota for a grant n hich enabled the authors to carry out this work. LITERATURE CITED
(1) Kolthoff, I. bI., and Lingane, J. J., "Polarography," p . 309, New York, Interscience Publishers, 1941. RECEIVED for review October 26, 1961. Accepted December 13, 1951.
Improved Stirrer for Special Freezing Point Determinations SAMUEL KAYE Sational.4dcisory Committee f o r Aeronautics, Clecelund, Ohio
HE determination of freezing and melting points of many hydrocarbons and synthesis intermediates is required in the preparation of high purity fuels. Reliable freezing or melting data for some compounds are difficult to obtain because of lack of thermodynamic equilibrium ( 2 ) , supercooling, and abnormal crystallization behavior. The standard procedure for deterY
The reciprocating stirrer,n-hich, after a portion of the liquid had crystallized, frequently generated enough heat to remelt the crystals, was replaced by a rotary, vane-type stirrer. The vanes consisted of broad, thin, metallic surfaces which permeated the entire freezing tube and thus aided the establishment of thermodynamic equilibrium. Use of a freezing tube lined with powdered glass, and a seeding wire, reduced supercooling and the formation of glasses. AI'PARATU S
Figure 1 shows a .side xiew of the apparatus. A worm gear on the shaft of motor Jf drives an asle, on one side of which is at,tached an eccentric, E, for driving the standard reciprocating stirrer ( 1 ) . A set of bevel gears, B , on the oppositeside transmits power t o a vertical shaft on which a drive pulle D, is mounted. The drive pulley id fitted with a spiral spring b e p t o a second pulley, P , mounted a t the top of the stirrer shaft,, 8. This arrangement prevents breakage of the platinum resistance thermometer)
I
w'
2570r
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1
a
$
RECIPROCATING STIRRER
2570/
ROTARY STIRRER
',
2550-
0 W
2530-
T I M E , MINUTES
Figure 2.
I
W DETAIL OF 5'
26 08-
I
Freezing Point Determination of Water REClPROCIilNG S T R R E R
Figure 1. Rotary Stirrer
mining these data ( 1 ) has been modified by the use of a stirrer differing from the reciprocating stirrer of illair et al. a t the National Bureau of Standards. This has reduced or eliminated some of the difficulties
TIYE. LIINUTES
Figure 3.
Freezing Point of Benzene
1039
V O L U M E 24, N O . 6, J U N E 1 9 5 2
2760r
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3
Vanes of brass shim stock are soldered onto the lower 6 inches of the stirrer, 8’. These vanes are cut in the form of a screw; so as to give both a churning and a propelling action. The edges of the vanes are serrated in order to produce a high unit pressure where they touch the powdered glass of the freezing tube. The pitch of the vanes is interrupted a t about 360’ intervals, so that the vanes may slice through the crystals when the liquid has frozen. This should permit a larger portion of the liquid to be solidified before the stirrer stops. The stirring shaft itself is perforated with 3/~l-inchholes t o allow the liquid propelled upward to recirculate downward and in an outward direction.
RECIPROCATING STIRRER ROTARY STIRRER
sToppY
27.401
0
20
40
100
80
60
I20
TIME, MINUTES
Figure 4. Comparison of Heat of Stirring of Rotary and Reciprocating Stirrers
Several stirrers of varying vane pitch were constructed to meet the special behavior of different compounds. If the crystals formed remained coarse and granular, a steep pitch rvas used. If the crystals were fine and inclined to cake, a shallow pitch was used, and the vanes then cut a path through the solid! thereby permitting the stirrer to operate over a greater portion of the freezing curve.
A seeding wire was made by drilling 0.008-inch holes radially in No. 18 Nichrome wire. The wire is wetted by the liquid under observation and then dipped into a slurry of crystals of the same liquid. The crystals adhering t o the y e t wire are then cooled in a liquid nitrogen bath. When the temperature in t.he freezing tube is at the melting point of the crystals, the wire is quickly inserted into the freezing tube and scratched against the sides. 23 4 0 -
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Lo o - - - - R ECIPROCATING STIRRER
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STIRRER
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Freezing Point Determination of Allylbenzene
2240-
Tj when freezing of the liquid causes the stirrer to slow down or stop; the spring belt acts as a safety clutch and slips on the pulleys, thus preventing shearing of the thermometer. The stirrer consists of a shaft, S, of 0.5-inch brass tubing about 8.5 inches long which turns in two Oilite bearings fitted into the upper and lower ends of yoke Y . It is supported by a hmss thrust collar, C, which rests on the top of the lower bearing. The lower end of the shaft is set into a 2-inch sleeve of Bakelite, K , lrhich provides an insulating space between the stirring section ;tiid the exposed section. .k flat disk at the top of the stirrer vanes prevents crystals from being conveyed out the top of the rube by wren- action. The thernlonleter is set in the annular + ] ) w e n-ithin the tube.
19.90
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1940 L
20
40
60
80
T I M E . MINUTES
Figure 6.
5 05 “C
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8
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22200 2 0 4 0 60 80 100 TIME, MINUTES
Figure 5 . Melting Point Curve of 1,l-Diphenylbutane Rotary stirrer
The ireezing tube was identical with the one described i n ( 2 ) , escept that finely powdered glass was fused onto the sides duiing its construction, The scratching of the stirrer vanes against the rough lining contributed to crystallization in some cases, as attested by visual observation through an unsilvered freezing tube and Dewar flask. RESULTS AND DISCUSSION
20.00
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xx x
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TIME, MINUTES
Figure 3,
M
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120
I40
1
Freezing Point of Allylbenzene Rotary stirrer
Figures 2 and 3 are comparisons of the freezing points of Lvater and benzene with the two stirrers. Figure 4 is a comparison of t,he heat of stirring of the two stirrers when the rate of reciprocating or rotary motion is 120 cycles per minute. Water belonroom temperature was placed in the freezing tube and the teniperature rise with the stirrer moving was compared with the temperature rise when it was stopped. The area between the line showing the rise in temperature and a horizontal line measures the increase in heat content of the system. Calculations showed that. the rotary stirrer evolved about 25 to 50% less heat than the reciprocating stirrer. Figure 5 is an illustration of a borderline determination with allylbenzene. This particular sample, a synthesis intermediate, afforded no satisfactory curve with the reciprocating stirrer, but with the rotary stirrer a curve was obtained from which the melting point was evaluated. Also illustrated is a run (Figure 6) in which two crystalline modifications appeared; the low melting form was composed of transparent needlelike crystals and the higher melting form consisted of granular opaque crystals.
1040
A N A L Y T I C A L CHEMISTRY
Figure 7 illustrates again the phenomenon which the rotary stirrer seems to detect far more rea'dily than the reciprocating stirrer-the appearance of two crystalline modifications. From the shape of the curve evidence for two modifications of crystals for 1,l-diphenylbutane is shown, one melting a t -25.05' C., and another a t -28.35' C. I n other runs only the higher or the lower melting form appeared. The apparatus described is used when the dompounds cool to a viscous liquid or to a glass, and when the conventional techniques will not yield satisfactory freezing curves. I n such cases the crystallization is so slow that the heat evolved by virtue of fusion and stirring exceeds the rate of heat transfer through the walls of the freezing tube. The rotary stirrer produces a lower heat of stirring than the reciprocating stirrer, which is shown by calibration curves. At the same time, the larger area of conducting metallic surfaces im-
mersed in the liquid provides a uniform temperature throughout the fluid in addition to adequate agitation for thermodynamic equilibrium. ACKNOWLEDGMENT
The author is grateful to John H. Lamneck for the sample of allylbenzene and to Kasper T. Serijan for the sample of 1,l-diphenylbu tane. LITERATURE CITED
(1) Glasgow, -4.R., Streiff, A. J., and Rossini, F. D., J. Research Natl. Bur. Standards, 35, 355 (1946). (2) Mair, B. J., Glasgow, A. R., and Rossini, F. D., Ibid., 26, 621 (1941). RECEIVED for review J u n e 6, 1951. Accepted January 4, 1952. Presented before t h e Division of Analytical Chemistry a t t h e 119th Meeting of t h e A M E R I C ~CHEVICAL U SOCIETY, Cleveland, Ohio.
lodometric Codetermination of Copper and Iron ROBERT C. BRASTED School of C h e m i s t r y , University of Minnesota, Minneapolis, M i n n .
HE iodometric titration for the estimation of copper is a well Testablished procedure. The corresponding titration for the iron(II1) ion, though known (a), has not received the attention it deserves. The usual procedure in the determination of copper in either an alloy (free of arsenic and antimony) or an ore is to buffer the iron interference with fluoride ion or to remove the copper by reduction to the metallic etatr by aluminum (6, 6). S o instances have yet been found in the literature in 1% hich the copper and iron have been codetermined iodometrically using aliquot portions of a single sample. There is a wide variety of proceduIes for the pretreatment of a nitric acid solution containing dissolved copper to eliminate nitrous acid prior to the iodometric titration. The nitric acid solution may be evaporated to fumeswith sulfuric acidand hydrochloric acid or evaporated to the appearance of copper oxide ( 4 , 6 , 7 , I O ) . The titration of a freshly boiled and cooled solution is also recommended. In any of these procedures errors are likely through the loss of solution by spattering. Urea is effective in removing nitrous acid from dilute warmed solutions. A precipitate of urea nitrate is formed, however, when urea is added to nitric acid solutions of about 1to 1 concentration. The waiting period involved in allowing a solution to assume room temperature after boiling is tedious. It is obvious that the iodometric titration of a solution containing even small amounts of nitrous acid will be in error. The presence of excessive quantities of mineral acids, especially nitric acid, has been reported by Khitehead and Miller ( I O ) as a source of m o r in the iodometric titration of copper. The purpose of this investigation is threefold:
prior to the preparation of a 1 to 1 nitric acid solution used in the experiments. EXPERIMENTAL
Copper Determination in Presence of Nitric Acid. Stock solutions of a brass sample were prepared by dissolving the alloy in 1 to 1 nitric acid. The solutions were not evaporated nor were any other mineral acids added. After complete reaction the solutions were boiled, cooled, transferred Tyith any precipitated metastannic acid to a volumetric flask, and diluted to volume. The concentration of nitric acid in the solutions was about 0.14 111. Aliquot portions of these stock solutions n-ere used for both the copper and iron determination. The amount of copper added (see Table I ) was computed from the thepretical copper content of the brass, 59.9070 by electrolytic analysis. The iodometric determinations of copper by the several procedures later described are in good agreement a-ith the electrolytic
Table I. Detn. So.
Effect of Nitric Acid on Iodometric Titration of Copper
Copper Coppera Error, Added, G. Found, G. XIg.
1
0.0761
2
0.0761
0.0762
3
0.0761
0.0767
4
0.0761
0.0894
5
0.0844
0.0846
6
0.0844
0.0844
7
0 0761
0.0817
8
0.0761
0.0763
9 10
0.0844 0.0761
0,0847 0.0771
11
0.0761
0.0767
12
0.0844
0.0847
REAGENTS
13
0.0761
0.0771
Sodium thiosulfate, sodium hydroxide, acetic acid, potassium iodide,. potassium acid phthalate, and ammonium acid fluoride of analytical reagent grade were used without further purification. The thiosulfate solution was standardized against copper. Sulfamic acid \%-ascommercial grade, twice crystallized, sulfatefree. Nitric acid was Du Pont C.P. reagent, 70% by weight, specific gravity 1.42. N o attempt was made to remove oxides of nitrogen
14
0.0844
0.0850
1. To develop a simplified rocedure for the preparation of a n alloy sample (as brass free or antimony and arsenic) whereby neither evaporation, fuming, boiling, and cooling, nor elimination of nitric acid is necessary prior to an iodometric titration. 2. To develop a procedure whereby copper and iron may be COdetermined volumetrically on aliquot portions of a single sample using a single standard solution. 3. To determine the best buffer conditions using fluoride ion to eliminate iron interference, in the determination of copper.
0 0761
Remarksb
0.0 Freshly boiled solution, 0.14 M H s 0 3 , no sulfamic acid added +O.l 0.3 gram of sulfamic acid added hefore addition of K I + 0 . 6 Solution aged 24 hours, no sulfamic acid added +13.3 Solution aged 3 days, no sulfamic acid added + O 2 Solution aged 24 hours, 0.3 gram of sulfamic acid added 0 0 Solution buffered with NaOH and HOAc before titrationc, 0.3 gram of sulfamic acid added 4-5.6 6 ml. of 1 : l HSOa, no sulfamic acid added 0.3 gram of sul+ 0 . 2 5 ml. of 1 : 1 "03, famic acid added +0.3 Same detn. 8 +1.0 10 ml. of 1:l HPiOa, 0.3 gram of siilfamic acid added of "08, 0.3 gram of sulfamic +0.6 1 added , of 1:l "03, 2 grams of sul+0.3 I - --id added +1.0 : 1 HKoa, 0.3 gram of sulid added : 1 "03, 2 grams of sul+0.6 famic acid added ~
0 Calculated from total thiosulfate less t h a t attributed t o iron present in original brass sample. b Unless otherwise stated, no sodium hydroxide-acetic, acid or acid phthalate buffers used. Titrations made on nitric acid solut!ons. c NaOH. 4 A' added t o appearance of Cu(0H)z. Acetic acid added until Cu(OH)x just disappears.
