DISCUSSION
Table 111.
Correction of Anode Potentials (About 0.001M CdSOl in 1N HC1. 2.667 mv./sec., 7 = 4.0 sec., t = 23.5' zk 1' C. Eilz, Volts)
Anode DePt" vice Without With -1.705 -0.648 -1.758 -0.645 -1.688 -0,646 Av. -1.717 -0.646
Without -1.681 -1.670 -1.679 -1.677
9.7 9.7
9.8 9.7
Pt* With -0.648 -0.641 -0.645 -0.645 id,
9.5 9.6
9.6 9.7 9.8
N ic
Without -0.269 -0.272 -0.268 -0.270
7 ) ~=
2.32 mg./sec.,
With -0.638 -0.638 -0.638 -0,638
Hg Without -0.635 -0.637 -0.632 -0.635
9.6 9.6 9.6 9.6
9.6 9.6 9.5 9.6
POOP With -0.643 -0.641 -0.638 -0.641
pa. 9.7 9.7 9.6 9.7
9.7 9.7 9.7 9.6 9.7 9.7 a Platinum wire, 0.75 X 1.6 mm. Platinum wire, 0.75 X 6.0 mm. c Nickel wire, 0.50 X 8.2 mm. d A stationary layer of mercury on the bottom of the cell was employed as anode.
9.7
Av.
EXPERIMENTAL RESULTS
The performance of this corrector was checked under the conditions used in ordinary polarography. I n Figure 5, results from the automatic corrector are compared t o those obtained conventionally b y using calculated corrections for iR1 and anode potential. Measurement of the resistance of the sample solution is difficult, and the errors in it are noticeable. Therefore, performance of this corrector was also checked under the condition that the resistance of the cell solution was negligibly small compared with other resistance; the resistance medium was installed intentionally in series with the cell instead of the resistance of the sample solution. The experimental results are shown in Figure 6 and Tables I and I1 where i d is the mean value of diffusion current, and i is the mean value of the current at any given point on the polarogram. I n Table I when the inserted resistance (R3)is zero, Elis(calcd.) (calculated from the e.m.f. value of the mercury pool electrode, E,, against a n external reference electrode) and the half-wave value
9.5 9.6 9.6 9.6
of the total applied e m f . , Vl,~( observed), did not differ significantly from those obtained by the automatic corrector. The wave forms of both polarograms also showed little difference. The polarogram obtained with the H-type cell was useful in checking the performance of the corrector, but it had a little distortion which might have resulted from the potential drop across the reference electrode. When the resistance mas inserted intentionally in series with the cell, the distortions were also compensated for completely b y the automatic corrector. The experimental results indicate that the corrector performs satisfactorily (Figure 6). To polarize the anode intentionally, the two platinum microelectrodes and a nickel microelectrode were used as the anode instead of the mercury pool electrode. The half-wave potentials and the diffusion currents obtained with and without the 'automatic corrector are compared in Table 111. The over-all results are mithin the limits of normal polarographic accuracy.
With this instrument, the auxiliary reference electrode must be inserted in the cell. However, there is no obstacle even though the auxiliary reference electrode is very small, because no current flows through the regulating circuit. The sliding contact, C, in Figure 1 can follow the current oscillations resulting from the mercury drop, as shown by the results in Figures 4 and 6. The auxiliary potential applier of the all-electronic system should simplify this instrument and give better results than the servomechanical system employed. However, the results obtained were identical. Details of the all-electronic instrument will be reported. There are several advantages with this automatic corrector. S o preliminary measurement for the potential drops is required. K h e n the conventional polarograph is required, the lead to the reference electrode should be connected to the anode: then the iR of the external resistance alone is eliminated. Therefore. a potentiometric recorder of relatively high internal resistance could be used for the current balancing recorder. ACKNOWLEDGMENT
The author n-ishes to espress his sincere thanks t o Nasayoshi Ishibashi, Taichiro Fujinaga, Xagahiko Aikawa, and Kotaro Konishi for their cncouragement and many valuable suggestions and to X s t u o Kashima and Yaichiro Shibasaki for their assistance in ohtaining the experimental data. LITERATURE CITED
(1) Ilkovii., D., A x . 4 ~ .CHEM. 24, 917
(1952). (2) Jackson, W.,Elvjng, P. J.. Ibid., 28, 378 (1956). (3) Pecsok, R. L., Farmer, R . K., Ibid., 28, 985 (1956). RECEIVEDfor review July 1, 1957. Accepted March 21, 1958.
Rapid Polarographic Method for Hydroperoxides in GasoIines MARVIN 1. WHISMAN and BARTON H. ECCLESTON Petroleum Experiment Station, Bureau of Mines, U. S. Department o f the Interior, Bartlesville, Oklu.
b A modification of the polarographic method of analysis for hydroperoxides in gasoline requires relatively simple and inexpensive equipment and substantially reduces the time for analysis. The differences between the modification and the polarographic method are not significant. The modification
1638
ANALYTICAL CHEMISTRY
has proved valuable in motor gasoline stability studies, where large numbers of analyses must be performed and sample is limited to a few milliliters.
A
N EXTENSIVE STUDY of motor gasoline stability by the Bureau of Mines has created the need for a
rapid method of estimating hydroperoxides in small quantities of gasoline. Chemical methods (1, 3, 4,7, 8) are widely accepted. Although they give good precision and accuracy, in general, they are time consuming and require large samples for analysis. Polarographic study of organic peroxides
32 0
- I 3 VOLTS 1
2801 24.0
i
-
!
$ 2 0 0 w CL u
a
160-
L
0
v)
2c
12or 8.0-
4 0-
0
-0- .3 - V.O- I-T. S-
I
- ----' 0
Figure 1. gasoline
I
-0 2
-0.4
-0.6 -0.8 -1.0 APPLIED E.M.F., VOLTS
-1.2
-1.4
-1.6
Typical hydroperoxide reduction wave of thermally cracked Gasoline depolarized with silica gel and ultraviolet irradiated
(5, 6, 9 ) has received some attention in recent years. The polarographic method for hydroperoxides in gasoline is more rapid than chemical methods but sacrifices something in precision, particularly for peroxide numbers below 5. This paper describes a modification of a polarographic method for determining hydroperoxides in motor gasoline and statistically compares data obtained by this modification with those obtained by the arsenite chemical method ( S ) and the polarographic method ( 5 ) . The niodification is more rapid than the polarographic method, requiring about 6 minutes for a complete analysis and 1 t o 2 nil. of sample. The equipnieiit involved is simple to us? and maintain and is less expensive than the polarograph. APPARATUS A N D MATERIALS
Polarogram3 reported were recorded with n Sargent Model XXI polarograph. A Sargent Ampot was used for measuring diffusion currents in the modification of the polarographic method. An H-type calomel cell, similar to that described by Eccleston et al. (W), was used for both polarographic and Ampot measurements. The two-piece design of this cell simplifies cleaning operations between runs. The cell was not thermostated, but room temperature \vas held a t 24.5" 0.5" C . The dropping mercury electrode had a dropping time of 2.73 seconds and a capillary constant, n z 2 W 6 , of 2.669 mg.2'3 sec.-l/z A solution of 0.3X lithium chloride (c.P.) in equal volumes of absolute methanol (Union Carbide Chemicals Co.) and benzene (Phillips Pure Grade, 99 mole yo minimum) was used as the electrolyte-solvent for all analyses reported. Commercial tert-butyl hydroperoxide (Union Bay State Chemical Corp.) and cuniene hydroperoxide (McKesson R. Robbins'r were assayed by making a
*
*
weighed dilution in a 50/50 mixture of iso-octane and benzene and running a series of analyses by the arsenite chemical method to establish purity. Standard hydroperoxide solutions were made for calibration purposes b y volumetric dilution of these assayed hydroperoxide stock solutions.
to free it of oxygen. One or 2 ml. of sample (2 ml. for peroxide numbers below 5 ) was then pipetted into a 10ml. volumetric flask and filled to the mark with oxygen-free electrolyte-solvent. This dilution was transferred to the polarographic cell and bubbled with helium for 4 minutes. (Nitrogen may be used for degassing but may require longer bubbling periods for complete oxygen removal.) The helium was first passed through a bubbling tower containing equal parts of methanol and benzene to minimize evaporation losses in the analysis cell. At the end of the 4-minute bubbling period, the dropping mercury electrode was placed in operating position and the bubbling continued for 1 minute. The solutions were blanketed with helium during the run to prevent oxygen absorption. Polarograms were recorded using a 2.0-volt span (0.148 volt per minute), damping off, dropping mercury electrode negative. Ampot current measurements were made a t -0.3 volt and -1.30 volts. These measurements vere made by reading the microammeter a t the maximum inflection of the needle. Corrections for I R droD were made to -polarographic half-wa;e potentials, although exact voltages are not essential to obtaining quantitative measurements of diffusion current. The cell resistance was about 1000 ohms.
CALIBRATION OF POLAROGRAPH A N D A M P O T
A calibration curve for the polarograph was made b y running standard terl-butyl hydroperoxide solutions in several concentrations ranging from 0.1 to 50 peroxide number and plotting diffusion current at the half-wave potential us. peroxide number (milliequivalents of active oxygen per liter). The solvent used for these dilutions was a 50/50 mixture of iso-octane and benzene. The Ampot method was calibrated by measuring the difference in diffusion current a t -0.30 volt and - 1.30 volts and plotting the difference us. peroxide number as determined chemically. This is a straight-line relationship up to a peroxide number of 100, and Lewis (6) reports linearity u p to a peroxide number of 250. PROCEDURE
A stock solution of electrolyte-solvent was prebubbled 15 minutes with helium Table II.
EXPERIMENTAL RESULTS
Table I shows the half-wave potentials Table
1.
Half-Wave vs. Standard Calomel Electrode
Sample Cumene hydroperoxide Catalytically cracked gasoline Catalytically cracked gasoline (depolarized with silica gel) Thermally cracked gasoline (depolarized with silica gel) Thermally cracked gasoline (UV irradiated) Catalytically cracked gasoline (UV irradiated) Hydrogenated catalytically cracked gasoline hlethyl ethyl ketone hydroperoxide Platformate gasoline tert-Butyl hydroperoxide
El/%
Volts
-0 879 -0 881 -0 886 -0 887 -0 900
-0 901 -0 909
-0 949 -0 974 - 1 035
Typical Peroxide Number Analyses
Sample Chemical Hydrogenated catalytically cracked gasoline 8.3 9.2 Catalytically cracked gasoline 13.7 Hydrogenated catalytically cracked gasoline 24.3 tert-Butyl hydroperoxide Platformate gasoline pluil TEL 27.3 29.8 Catalytically cracked gasoline Thermally cracked gasoline 30.2 39.9 Platformate gasoline Platformate gasoline plus TEL 51.7 Catalytically cracked gasoline 52.6 Thermally cracked gasoline 61.5 Catalytically cracked gasoline 63.4
Peroxide Number Polarograph
Ampot
7.9 11.3 13.6 23.8 25.4 33.7 31.6 39.1 50.7 58.6 65.6 64.9
8.5 12.8 13.4 23.2 26.4 31.2 31.4 36.9 42.8 54.3 61.3 59.1
VOL. 30, NO. 10, OCTOBER 1958
1639
Table 111. Standard Deviation of Method by Differences between Duplicate Determinations
Peroxide Standard Deviation Number Chem- PolaroRange ical graph Ampot Ot05 0.018 0.102 0.079 5 to 10 0 075 0 to 25 0 136 0 203 10 to 25 0 109 0 581 25 to 100 0.270 0 603 for a variety of materials. The agreement of these values justifies dispensing with the polarogram and relying entirely upon two diffusion current measurements as shown in Figure 1. With this simplification of the polarographic procedure the need for the polarograph itself is obviated, and any simple device that is capable of polarimetric measurements can be substituted. The data reported herein were obtained using a Sargent Ampot, u-hich is simple to use and relatively inexpensive. DISCUSSION
A statistical analysis of duplicate results on 41 samples covering a range from 0 to 100 peroxide number indicates that the standard deviation for the Ampot method is not substantially
different from that for the polarographic method. The F ratio found for each group of data is below the critical value, which may be exceeded once in 10 times under the hypothesis that there is no difference in precision. A portion of the data used to calculate the above standard deviations is tabulated in Table TI. There is, hoFvever, evidence for a substantial difference between precision of the chemical arsenite method ( 8 ) and the precision of the other two methods in the 0 to 5 peroxide number range. This can be seen in Table I11 by examining the standard deviation (IO) of each method, which was determined by the differencp between duplicate runs. A comparison of accuracy was not possible for the three methods because of the multitude of interferences possible in the gasoline boiling range. These interferences are not known for either the arsenite method or the polarographic method. However, wide acceptance of chemical methods for hydroperoxides and the good agreement between results by the arsenite chemical method and the polarographic and Ampot methods indicates that the proposed modified procedure will prove valuable where time cf analysis is a factor and high precision of results in the range of 0 to 5 peroxide number is not essential.
ACKNOWLEDGMENT
The authors wish to thank Frank Schwartz for his guidance in preparing this paper, as well as Jack Hale and Joseph Thorstenberg for their assistance in determining the chemical analyses. LITERATURE CITED
(1) Dastur, 9. N., Lea, C. H., .tnwlyst 66, 90 (1941). ( 2 ) Eccleston, B. H., Morrison, M., Smith, H. AI., ASBL. CHEX. 24, 1745
(1952).
(3) Kokatnur, V. R., Jelling, >I., J . .4m. Chenz. SOC. 63, l U 2 (1941). 14) Lea. C. H.. J . SOC.Chenz. Znd. 65, 2886 (1946). (5) Lewis, W. R., &u:tckenhush, F. W., DeVries, T., -43.4~.CHEII. 21, 762
RECEIVEDfor revie\\. January 13, 1958 Accepted &lay 21, 1958. Kork EUPported in part by the Department of the Army, Ordnance Project TB5-0010C.
Determination of Beta-Chloropropionic Acid in Mixtures with Propionic and Alpha-Chloropropionic Acids Isotope Dilution Method JEROME G. BURTLE and JOHN P. RYAN Minnesota Mining and Manufacturing Co., St. Paul, Minn. Sr. MARIE JAMES
(GIBBONS)
College of St. Catherine, St. Paul, Minn.
b A simple and rapid method for analyzing acid mixtures obtained by the monochlorination of propionic acid is described. Propionic acid is estimated from the results of titration with standard alkali, or alternatively, from determination of the total chlorine content of the mixtures; j3-chloropropionic acid is determined by radiotracer isotope dilution analysis, and a-chloropropionic acid is determined by difference. A convenient method for the assay of radioactive j3-chloropropionic acid consists of plating the compound on a special planchet and counting it as a super-cooled Iiquid. 1640
ANALYTICAL CHEMISTRY
A
for production of acrylic acid from 8-chloropropionic acid, suggested by Staudinger (Ig), focused attention on the latter compound. Subsequent development of improved methods for performing this transformation ( I , 4, 23) increased interest in the chlorinated acid and stimulated development of numerous processes for its preparation (9). For reasons of low cost and over-all simplicity, direct chlorination of propionic arid in the presence of ultraviolet light or beta-orienting catalysts has been found attractive; several such processes have been described (3, 7 , 16). I n such procedures, optimum conditions PRACTICAL METHOD
for production of the beta-chlorinated isomer and for repression of polychlorin-
ated products invariably lead to formation of mixtures. These mixtures contain propionic, a-chloropropionic, and p-chloropropionic acids in varying amounts, usually in proportions of approximately 80 to 2 to lS% by weight (7) Mixtures of the above type are difficult to analyze completely, because there is no known quantitative method either for the estimation of one of the chlorinated acids in the presence of the other or for their separation. The two most attractive approaches to the development of an analyticnl method for '
-
