378
ANALYTICAL CHEMISTRY
collected from two cows fed antioxidant equivalent to 10 times the amount expected to be consumed in a cow’s daily ration of alfalfa. The data show that the method is sufficiently sensitive to detect the antioxidant in the cow’s milk within 2 hours after ingestion,
Table 111. Recovery of Antioxidant from a Series of Eggs Antioxidant ildded t o Egg Yolk, P.P.M. 3.4
Total Apparent Antioxidant, P. P. hl.
Corrected - . ~ ~ Antioxidant Value (after KMnOb Treatment), P.P.M.
9.7
5.2 3.7 3.3 2.8 3.9 0.2 0.2 0.2
8.4
6.9 7.5
Table 11.
a
8.4
Amount of Antioxidant Found in Milk
c o w 880, c o w 1120, Time Sample Taken P. P.*M. P.P.hl. 7 December, A . M . 0 0 8 December, P . M . @ 0.05 0.02 0.04 0.03 9 December, A . M . 9 December, P . M . ~ 0.19 0.14 Antioxidant given a s drench 2 hours before this milking.
0
5.5 5,2
Table IV. Antioxidant Found in Rat Tissues after Feeding for 200 Days Antioxidant in Diet,
%
Control
Known amounts of antioxidant were added’ to samples of egg yolk prior to assay and replicate determinations made (Table 111). Despite all precautions, occasionally some compounds still interfered with the determination-that is, they were extracted from the tissues along with the antioxidant and were quenched by the permanganate. Thus, chicken liver shoiyed as much as 1.6 p.p.m. in some controls. Accordingly, the values obtained by this method should be regarded as approximate maximum valuks only. Such errors can be partially resolved by use of a sufficient number of replicated samples. Thus, as in Table IV, the general picture is evident, although obvious inconsistencies are present in the controls. Preliminary experiments with urine from dogs and rats showed the presence of some component which reacted like the antioxidant when treated with permanganate. Further experiments are necessary to eliminate this interfering material.
4.9
0.0125
Rat No. 1
2
Liver 0 0
3
0
4 5
0
6
0.20
0.40
0 0
7
0
8 9
0
10 11
0
12 13
Tissue Assay, P.P.M. Fat Kidney 0 2 0 0 0 3
0 0 0 11 11
0 0
22 21 74 50
0
55
0
Muscle 0 0 0
0
0
0 0
0 0
6 3
1 0
3.5 3 8 5 4
1 1.5 1 1 1
the aid of F. X. Gassner, Colorado A & M College, who supplied egg samples, and S. W. Mead, University of California, who supplied milk samples, both from animals fed Santoquin. LITERATURE CITED
(1) Ferrebee, J. W., J. Clin. Invest. 19, 251 (1940). (2) Thompson, C. R., Znd. Eng. Chem. 42, 922 ( 1 9 5 0 ) .
ACKNOWLEDGMENT
The authors wish to acknowledge the assistance of E. D. Walter and I. V. Ford with some of the analyse8 required in the development of this method. They also wish t o acknowledge
Automatic
iR
RECEIVEDfor review September 26, 1955. Accepted November 26, 1955. Mention of a n y product6 does not imply t h a t they are endorsed or recommended b y t h e Department of Agriculture over others of a similar nature not mentioned.
Drop Compensator for Polarographic Use
WARREN JACKSON, JR., Chemical and Physical Research Division, The Standard O i l Co. ( O h i o ) , Cleveland, O h i o , and PHILIP J. ELVING, Department o f Chemistry, University of Michigan, A n n Arbor, M i c h .
A n automatic device has been developed which continuously compensates for the iR drop i n a polarographic cell. The device is particularly useful in the polarographic examination of solutions having very high electrical resistance, and can be used readily in conjunction with any conventional type of polarograph. The compensator completely eliminates time-consuming manual calculations for iR drop. It continuously senses the cell current and produces a potential equal to the product of the cell current and the previously measured cell resistance. This compensating potential is then introduced into the cell circuit in such a way as to cancel the effect of the iR drop. The compensator has been tested under a variety of conditions with both dropping mercury and solid electrodes, and for both reduction and oxidation processes at the indicating electrode, Reproducibility and accuracy are within the limits of normal polarographic analysis.
recent development work in organic polarography ]MIUCH has been conducted in solutions containing little or no water. I n many such cases, the electrical resistances of the cell solutions are very high, resulting in distortion of the current-voltage curves. Interpretation of such curves frequently requires time-consuming, manual correction for the internal resistance of the cell solution. Polarographic analysis, under such conditions, would be greatly facilitated by automatic correction of the entire curve. I n 1952, IlkoviE ( 4 ) presented a paper in Europe describing an automatic compensator; no details of its construction or operation are available in this country. More recently, Arthur, Lewis, and Lloyd (1) described a device for the automatic correction of recorded polarogams, which employed a strip-chart function plotter and two reference electrodes; effective voltage rather than applied voltage was then recorded directly. An automatic iR compensator constructed from standard electrical equipment and suitable for use with any conventional
V O L U M E 28, NO. 3, M A R C H 1 9 5 6
379
polamgraph is described in the present study. The simplicity of its operation is discussed as well as the accuracy of the results obtained. PRINCIPLES AND APPARATUS
&e,
R, is la&, a n indesired ootential drop oecur~in the cell
usually d&e by manuil, point-by-&& or transcribed polarograms.
corGection of reoorded
CELL
POLllRO'llh**
i L__.
-
C(
.cI_-
_.-=sic
The device described eliminrrws.s ~ u i l u u n ~ G U L ~ ~ . C L I I U I Iuy U ULserting into the circuit a continuously compensating source of potential, whose potential equals in magnitude hut opposes the iR drop in the circuit. Thus, the cell potential is always equal t o the applied potentkl and no corrections of reoorded polsragrams are required. A circuit for accomplishing this oompensation is shown in Figure 1. A voltage proportional t o the undesired iR drop in the cell is developed &erossa small resistance, R,, and is amplified in amplifier of @in K to a value which compensates for the iR drop. This amplified voltage is then inserted back into the circuit in opposition t o the iR drop. The relationship between the various parameters required for accurate compensation is readily derived. When the output resistance of amplifier K is made negligibly small, the cell potential can be stated in terms of the other circuit e.m.f. values:
Figure 2. Typical setup of compensator, using commercially available amplifier
+
E..u = E...M - iR,.u - iRt KiRl (1) The requirement for complete compensation is that the cell potential always be equal t o the applied potential: E,eu =
Esppiied
(2)' 3)
Figure 3.
Polarograms o b t a i n e d with a n d without a u t o m a t i c iR correction
2.5 ml. ofdibutylph~nylenedisrninein 100 m l . of 1 to 1 is-octaneisopropyl alcohol, U.1M in LiCl Opal wax-impregnated graphite electrode US. Ag-AgCl c a t h o d e 1.24 m i . m e r second; $tirred test aolutioo: 19.000 o h m s oell
4,
(Gataloe No. XYU14YU-Li~I. and a Brown Dotentlometer recorder. kodifiea to operate as a servo-amplifier,-were all tested experil
t o select"m arbitrary value of 100 or lb00 for ( K - 1). T o obtain
Typical Compensatbr. One particular adparatus for i R cam-
The resir linear pa tentidmeter. The procidure o u t l i n d requires that the gain, K , Iof the amplifier be constant and equal t o the arbitrarily chosen VItlue within B tolerance which is better than the desired accuracy of compensation. Amplif ier Characteristics. The performance of the compensation c i mtit (Fimre 1) is largely determined by the characteristics . of the m
self-balancing potentibmeter, having a gain of 101. A 1000-ohm 10-turn potentiometer served as the resistance, Ri. Thus, the range of the instrument extended to 100,000 ohms cell resistance. The output of the amplifier w m connected in series with the cell, and polarized so that it acted in opposition to the iR drop and added to the applied potential. A voltmeter connected across
fie7 RlP!
fre
