more soluble in water and salt solutions than the corresponding basic salt. This information leads t o the following hypothesis: During the electroanalysis, cobalt is deposited as the metal until the cobalt concentration in the plating bath drops t o about 0.0051V. ilt this point COSOS3CO(OH)2 H,O and/or related compounds are deposited. .It the end of the deposition, part of the cobalt surface compounds are washed off or dissolved by the wash water, but enough remains on the deposit to give positive errors. The absolute errors due to such a process n ould be essentially independent of the total amount of cobalt if the initial concentration is above 0.00LV. With a sufficiently large sample the error ~oouldbe negligible. This seems to be true n-hen heavy cobalt deposits are made (6). I s the reaction and the washing of the deposits xvould not be expected to be reproducible, it is not surprising that there is a large variation in the positive errors. The formation of surface compounds is to be distinguished from the occIusioii of foreign material to which reference is generally made. Experiments mere made in which the depositions were purposely stopped before electrolysis was complete. The amount of metal that remained undeposited was determined with the liquid counter. Coincidence corrections were applied and suitable counting periods were used to provide the necessary accuracy. Deposits were removed, washed, dried, and neighed as before. The results are shown in Table V. The deposits tlint n-eigh 35 mg. or
Table V. Test for Formation of Surface Compounds at Low Concentrations
(49.60 mg. of Co Remaining Deposit, in Soln., hfg. Mg. 50 40 0 09 50 79 0 08 35 75 14 35 41 00 9 64 38 88 10 95 40 45 10 28 30 36 20 65 46 37 4 88
cobalt taken) Sum, hfg.
50 50 50 50 49 50 51 51
49
87
10 64 83 73 01 25
Error, Mg. $0 89 $1 27 +O 50 +1 04
+O 23 + I 13 + I 41 +1 65
less, after correction for the cobalt in the liquid, all show positive errors and are not significantly smaller than those obtained when nearly complete deposition is made. Hence, it appears either that the surface compound explanation as given above is incorrect, a t least insofar as the assumption that surface compounds form only n-hen the concentrations of the cobalt are very low; or that the formation of surface compounds a t low concentration causes so small a part of the positive error that its effect was not observed in this test. Another possibility is that as a result of concentration polarization near the cathode, surface compounds still form a t the cathode, even though the cobalt concentration is greater than 0.005N in the bulk of the solution. LITERATURE CITED
(1) Brophy, D . H., IND.ENG. CHEM., ASAL.
ED.3, 363 (1931).
(2) Csokan, P., Z. anal. Chem. 119, 418 (1940). (3) Fine, M. M., U.S. Bur. Mines, Rept. Invest. 3370,59 (1938). (4) Fischer, A., Sleicher, A,, “Electroanalytische Schnellmethoden,” pp. 235, 237, Ferdinand Enke, Stuttgart, 1926. (5) Guzman, J., Rial, M., Anales SOC. espaii. f i s . y quim. 34, 636 (1936). ( 6 ) Hague, J. L., Maczkowske, E. E., Bright, H. A., J. Research Natl. Bur. Standards 53, 353 (1954). (7) Jilek, A,, VieSfal, J., Collection Czechoslov. Chem. Communs. 7, 512 (1935). (8) Kolthoff, I. M., Sandell, E. B,., “Textbook of Quantitative Inorganic Analysis,” 3rd ed., p. 410, Macmillan, New York, 1952. (9) Kpvalenko, P. N., Zhur. Priklad. Khzm. 24,951 (1951). (10) Lingane, J. J., “Electroanalytiral Chemistry,” p. 289, Interscience, New York, 1953. (11) Lundell, G. E. F., Hoffman, J. I., J.Znd. Eng. Chem. 13, 540 (1921). (12) Nicol, A., Ann. chim. 2,670 (1947). (13) Perkin, F. M., Hughes, W. E., Chem. News 101. 52 (1910). (14) Perkin, F. i f . , Prebble, W. C., Trans. Faraday SOC.1, 103 (1905). (15) Smith, E. F., “Electroanalysis,” 6th ed., pp. 137, 146, Blakiston’s, Philadelphia, 1918. (16) Taggirt, W. T., J . Am. Chem. SOC. 25, 1039 (1903). (17) Wagenmann, K., Metall. u. Erz 18, 447 (1921). (18) Watts, 0. P., Trans. Am. Electrochem. SOC.23,99 (1913). (19) Youpq: R. S., “Industrial Inorganic Analysis, p. 73, Wiley, New York. 1954. ~
RECEIVEDfor review March 4, 1958. Accepted May 12, 1958. Taken in part from a dissertation presented to thr Graduate School of The Ohio State University by Darnel1 Salyer in partial fulfillment of the requirement for the doctor of philosophy degree.
Polarograph with Automatic Corrector for the Electrode Potentia I SYOTARO OKA’ instruments Division, Shimadzu Seisakusho, Ltd., Kyoto, Japan
,A new polarographic instrument automatically corrects the potential of the indicator electrode in accordance with the applied voltage. The difference between the electrode potential and the applied potential is set on the auxiliary potential applier, which is controlled servomechanically. This instrument continuously compensates for any potential drops arising from internal and external resistances in the cell circuit. Electrolysis can b e carried out using the indicator electrode against the mercury pool anode or
other electrodes. As the voltage axis of the resulting polarogram is always represented against the reference electrode, the polarizable micrometallic electrode can also b e used as the anode. Reproducibility and accuracy are within the limits of normal polarographic analysis.
T
HE accurate determination of halfwive potentials is essential for many applications of polarographic analysis, but the values determined with the auto-
matic recording polarograph in common use are sometimes in error. Much work in organic polarography has been conducted in solutions containing little or no water. The electrical resistances of these cell solutions are very high, resulting in distortion of the current-voltage curves. This problem mas investigated by IlkoviE ( 1 ) and Jackson and Elving ( g ) , and automatic compensators for 1 Present address, Instrument Division, Shimadzu Manufacturing Co., Kyoto, Japan.
VOL. 30, NO. 10, OCTOBER 1958
1635
internal resistance drops lvere presented. Pecsok and Farmer (3)have shown that the resistances of electric circuits also distort the current-voltage curves, and they proposed a new type of cell voltage control circuit. I n the conventional polarograph, the mercury pool electrode must retain a constant potential, providing an area of a t least 1 sq. cm. Otherwise, accurate determination of half-wave potential is almost impossible. A new polarograph, composed of an electronic servoamplifier and an auxiliary potential applier, automatically corrects the potential of the indicator electrode. The operating characteristics and the results obtained are reported.
E,
=
E , - iXR
- E,
,,/’
where ZR is the total resistance of the cell circuit. T o make E, equal to E,, a reference calomel half-cell is installed very near the indicator electrode in the polarographic cell. The basic circuit of this polarograph is shoirn in Figure 1, in which Et can be stated in terms of the other e.ni.f. values in the circuit A-B-C-D-E: VI
/
,
D
/’
/
I
Lt-1
IW I
I I1
1 I””f””
1,
I
El
I
1
(1)
ii
/
PRINCIPLES AND APPARATUS
I n polarography, the half-wave potential refers to the saturated calomel half-cell. Therefore, correction for the potential of the mercury pool electrode is necessary to determine the half-wave potentials from the recorded polarogram. The internal resistance, R1, of the cell and the resistance medium, Rz, are contained in the cell circuit, and undesirable potential drops occur when current, i , flows through the resistances. The actual potential of the indicator electrode, E,, differs from the applied voltage, E,, by the sum of these undesirable potential drops, which is indicated by izR, and the potential of the mercury pool anode, E,:
/’
/’
,,
.
I L-
___
€1.
6-volt storage battery 1.5-volt dry cell RI. Resistance of the sample solution RP. External resistance: sum of r6, r7, resistances of lead wire, etc. n. 200-ohm variable r?. 40-ohm potentiometer r3. 40-ohm variable
Et = Ea - zXR - E ,
+ E,,,.
(2)
E,,, is the auxiliary voltage for the compensation of i z R plus E,. E,,, must be equal in magnitude, but opposite, to the sum of i z R and E,. E,,, is set automaticallj- on the cell circuit. For complete and continuous correction, the potential of the indicator elrctrode must be equal to the applied voltagp : n here
E, = E, (3) Subtracting Equation 3 from Equation 2, - 1x12 - E ,
E,,, 7,
/’
=
E,
=
(-1)
0
-+ ax12
/
SM
Basic circuit of polarograph
€2, €3.
E,,,
A
/’
Figure 1.
*
t - - - - J
(5)
2-ohm potentiometer 200,000-ohm variable r6. 200-ohm potentiometer r7. 1 00-ohm potentiometer Amp. 1: Amp. 2. Electronic amplifier BM1, BMz- Balancing motor SM. Synchronous motor rl.
rj.
ll~lieii the magnitude of E,,,. i b controlled automatically to fulfill the condition of Equation 3, undesirable potential drops are compensated for by E,,, as shonn in Equation 5 . I n Figure 1, nhen E i is equal t o E,, the potential difference, e, between the refercnce electrode and point A in the potential applier of the polarograph reduces to zero. With the electronic servosystem, the potciitial difference becomes the error signal. e, which is amplified by Amp. 1; the amplified voltage drives the bnlwiiciiig motor, BM1, in the proper directioii t o correct the Yoltage error by sliding the contact, C, on the auxiliary potential applier. The automatic corrector K B S designed
T,
X
Figure 2. VI. Mechanical vibrating chopper 71, Tz. 6SL7 vacuum tube T3. 6V6 vacuum tube X. 6.3-volt alternating current CI,C7. 0.0005 mfd. CZ, Ca. 0.01 mfd. C3, CIO. 5 0 mfd.
1636
ANALYTICAL CHEMISTRY
Details of amplifier circuit CE, Cs. 10 mfd. CB,Cg. 0.05 mfd. C11. 1 mfd. Clz. 4 mfd. C13, Cl4. 8 mfd. R1. 500,000 ohms 1 megohm RP, R7, Re, RII.
Ria.
10,000 ohms 100,000 ohms Rg. 1 -megohm potentiometer RIP. 2 megohms Rl3. 500 ohms R14. 150,000 ohms Rib, R16. 50,000 ohms R3, R6,
Ra, Rj.
D.M.E..2.667""hc.. 7=4.Osec.. iii=2.40"~/r.s..t.22.0~~l~C. 1 0
1
1
1
1
1
1
1
-0.5
1
1 -1.0
1
1
VOLTS Figure 4. Polarograms obtained with ond without outornotic corrector Oxygen In isopropyl olcohol containing 0.1M lithium chlorids and roturatsd with air
Figure 3. Ty,pic01 setup of outomotic corrector with convention01 ,olorogroph
Figure 5. Comporison of manually ond automoticolly corrected polorograms Oxygen In isopropyl alcohol containing 0.1 M lithium chloride ond mturctted with air
Dropping mercury electrode, 0.001M cadmium d a t e in I N ammonium chloride and 1N ammonia, 2 . 6 6 7 mv./sec., T = 4.0 sec., E = 2.22 mg./iec.. t = 23.0' & 1 "C.
to occupy a relatively small space beside thr Shimadzu polarograph Model RP-2. The Amp. 1 used in Figure 1 is the Shimadzu servoamplifier Model H, whosr diagram is given in Figure 2. The sensitivity of the amplifier is within 1 mv. Other characteristics of t h r amplifier and the slide rmistance arr given. An identical amplifier is used in t h r current recorder in Figure 1. T h r typical setting of the polarograph and accessory unit is shown in Figure 3.
This accessory unit will be suitable for use with any conventional polarograph. such as the photorecording polarograph. A corrccted polarogram which was recorded automatically by this instrument is shown in Figure 4. A currenevoltage curve, which was accurately corrected and easily measured, was obtained in many cases without preliminary measurement of the internal and external resistanccs and the POtential of the mercury pool anode.
Table I. Half-Wove Potentiols b y Various Methods (0.001M CdSO, in 1N NH,Cl and 1 N NHo. 2.667 mv./sec., I = 4.0 sec., E = 2.22 me./sec.. 1 = 23.0O l o C. Acconted value of E ,.. , , for Cd++is -0.81 volt us. S.C.E.) Em, Volt "8. S.C.E.
+
VI,* (Otmd.), Volt us. Hg
t , ,), -0.5
-0.6
, ,
-0.7 -0.8 -09 -1.0
-1.1
Table 11.
Volt"
Calcd.
Ohsd.*
present mrthod
Comparison of Holf-Wove Potentiols, Diffusion Currents, ond Slope of Waves
(0.001M CdSO, in 1N NH,Cl and NH.. 2.667 mv./see., I = 4.0 8 m , rn = 2.22mg./sec., 1 = 23.0' 1' C. Polarogrsms recorded with and without automatin corrector) InsertPd Slope of Log Resistance, V,n or&p Volt i d , pa. i / i d - i, Mv.
*
VOLTS
Ohm 0
Figure 6. Change of half-wove potentials b y undesired potentiol drops, and outomotic correction Dropping mercury electrode, 0.001M cadmium ivlfote in 1 N ammonium chloride end IN ammonia, 2.667 mv./sec., .j = 4.0 iec.. i= 2 . 2 2 mg./rec.. f = 23.0' l o C.
a
Without-
Withb
Without
-0. --0.806* 9.0 5,000 -0.831" - 0 . 8 ~ 9.1 i n , nnn -0.88G0 - 0 . 8 ~ 9.0 H-cell -n .8z:v ... 8.9 Against mercury pool electrode, iR not corrected
With
Without
9.0
30 89 138
9.1 8.9
...
Against saturated ralomel electrode.
VOL 30, NO. IO, OCTC
35
With 3n 30 31 ..
~
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
