1095
V O L U M E 27, NO. 7, J U L Y 1955 the results: of moving boundary analyses are preferred by many; however, analyses of mixtures of albumin and gamma globulin or of sera of a group of healthy individuals also enable evaluations to be made and in some respects might be better justified. Periodic analyses of stored samples of the same serum or serum pool have proved useful. Martin and Franglen (IO) have described independently t h e i18e of paper rihboris for applying serum. They remove the strip after 20 minutes. Interruption of the experimeiit in this way .disturbs the equilibrium established in the chamber and is less likely to enable transfer of all protein from the applicator st’rip. Quantitative measurements of the intensity of staining can he utiliaed with much more confidence than mere inspection of the patterns, as some have advocated. Estimates of the amounts present in a given zone, as judged bJ- inspection of patterns, and also as to the configurat.ion of the peaks can be grossly erroneous. Subcomponenbs, however, can at times be detected bet’t’erhy inspection than by densitometry when the concentration of protein in such components is low. Subcomponents in the alpha?, beta, and gamma zones are frequently visible in the zones separated on paper, whereas their existence is less clearly demonstrated by the moving boundary method. This is one of many phases of serum protein behavior newly opened for study by the method of zone electrophoresis. ACKUOWLEDGRIENT
These studies were carried out under sponsorship of the Office of the Surgeon General, E. S. Department of the Army. The
authors are iiidebted to Georg F. Springer for loan of a Grassinann apparat.us during the earlier part of this study. LITERATURE CITED (1) Block,
R. J., Durrum, E. L., and Zweig, G., “Manual of Paper
Chromatography and Paper Electrophoresis,” Academic Press, New York, 1955.
(2) Fijher, B., Am. J . Clin. Pathol.. 23, 246 (1953). (3) Gilman, L.. Gould, E. F., and Reinhold, J . G., Pepper Laboratory of Clinical Medicine, University of Pennsylvania, Philadelphia, Pa., unpublished data. (4) Grassniann, W., and Hannig, K., Hoppe-Seyler’s 2. phusiol. Chem.. 290. 1 (1952). ( 5 ) Grassmann, W., Hann’ig, K., and Knedel. AI., Deut. med. Kochschr.. 76, 333 (1951). ( 6 ) Hnedel, W., M e d . Monatsscht., 5 , 707 (1951). ( T ) McDonald, H. J., Lappe, R. J., llarbach, E. P., Spitser, R. H., and Urbin, 11. C., Cjin. C h e m . , 5, 35, 51 (1953): ( 5 ) McDonald, H. J.. Alarbach. E. P., and Urbin, M.C., Ibid., 5, 17 (1953). (9) Alackay. I. R., S’olwiler. K . , Goldsn-orthy, P. D., Eriksen, N., and Wood. P. A , J . Clin. Invest., 33, 853 (1954). (IO) llartin. S . H., and Franglen, G . T., J . C l i n . Pathol., 7 , 87 (1954). (11) Bommerfelt. S.C., Scand. J . (’lin.& Lab. Inz,est., 5, 299 (1953). (12) Tiselius, .I.,and Flodin. P., :ldmuces in Protein Chem., 8 , 461 (1953j. (13) Wunderly, C . , Chimia (Swift.), 7 , 145 (1953). RECEIYEIIfor review September i . 1954. .iccepted March 7, 195.5. Presented before the Division of Biological Chemistry, Symposium on Electromigration in Stabilized Electrolytes, a t the 124th Jleeting of the AMERICAN CHEMIC&L SOCIETY. Chicago, 111.. September 1953, and in part a t the hleeting-in-hliniaturc. . ~ I E R I C A S C“E\IIC.AL ~ O C I E T Y .Pliiladelphia, P a . ( J a n u a r y 29, 1953.)
Voltammetry at Constant Current Application to lead Ion in Nitric Acid Solution M. M. NICHOLSON and J. H. KARCHMER H u m b l e O i l and Refining Co., Baytown, Tex.
Development of the theory of constant-current electrolysis was begun many years ago, but its application to analytical chemistry has been fully recognized only recently. In order to explore the potentialities of these techniques, a versatile pen-recording instrument and a convenient electrolysis cell were designed and fabricated. In a study of the constant-current voltammetry of lead ion in nitric acid solution, an over-all precision equivalent to an average relative deviation of =tl% was obtained over the range of 0.0002 to 0.02 mole per liter. These results confirm the analytical usefulness of the technique. Although about 40% of the lead was present as the mononitrate complex in the cell solutions, no kinetic effect on the electrollsis process was observed. In general, this method, like conventional polarography, should he applicable to the analysis of electroreducible or -oxidizable materials. One possible advantage of constant-current procedures is that the electrodes can be stationary rather than dropping or rotating.
A
LTHO‘CGH diffusion-dcpeiident c.lectroly& a t constant current has been Ftudied for many years, recent papers ( 1 - 5 , 9 ) have emphasized its value in the investigation of electrode processes and its analytical significance. Delahay has applied the terms “chronopotentiometry” and “voltammetr\- at constant current” to these measurements, in which the course of polarization of a n electrode under forced constant current is follon-ed potentiometrically as a function of time. The electrode is in
contact n i t h an unstirred solution, R hich contains a supporting electrolyte in addition to the depolarizer being studied. An ingenious experimental arrangement u ith oscillographic recording is described by Gierst and Juliard (6). Delahay and coworkers (1- 4 ) have made mathematical analyses of the diffusion problems corresponding t o a number of specific types of electrode processes, and the resulting equations provide a sound theoretical basis for interpretation of many observed effects. The potential-time curve recorded in the presence of a depolarizer is characterized by a transition time, during which the rate of change of potential is relatively small. This time interval is of primary analytical importance because of its dependence on concentration. I n the absence of certain kinetic complications ( 3 ) and preceding electrochemical reactions (1), this relatiomhip takes the form of the Sand equation ( I O ) for zero surface concentration:
In Equation 1, C is the bulk concentration of the reacting ion or molecule (moles per milliliter), D is its diffusion coefficient (square centimeters per second), i is current (amperes), T is the transition time (seconds), A is the area of the polarized electrode (square centimeters), and the other symbols have their usual meanings. Thus, if io represents current density, the quantity i6 r1/2/Cis a constant characteristic of the reacting material and might be termed the “transition time constant” by analogy t o the “d8usion current constant” of conventional polarography.
ANALYTICAL CHEMISTRY
1096 I n the case of a reversible electrode reaction, the potential a t r/4 is equal to the polarographic half-wave potential (3). This paper describes a versatile pen-recording instrument for making constant-current electrolysis measurements over a range of experimental conditions. The design of a convenient electrolysis cell is described, and the utility of the technique is demonstrated by its application to the determination of lead ion in nitric acid solution. EXPERIMENTAL
centrations from 0.0001 to 0.02M in lead ion were prepared in approximately 0.2M nitric acid. The mercury electrodes were triple-distilled C.P. mercury (W. H. Curtin & (20.). ,Matheson prepurified nitrogen was passed over copper turnings a t 450' C. and through water before entering the cell. Procedure. The cell was mounted through rubber connections in a water bath controlled a t 25.00' =k 0.01' C. The temperature control system was turned off immediately before each polarization run t o prevent mechanical disturbance of t h e cell. After some preliminary variations in details, the following procedure was found t o give reproducible results in the solutions of lead nitrate in nitric acid.
Instrument. A battery-operated constant current source and current and voltage measuring circuits were built into a standard instrument cabinet, which also housed a rapid response Brown Electronik strip chart recorder. Details of the direct current circuit are shown in Figure 1. The current and voltage measuring sections were constructed from precision resistors, those of 20 ohms and higher having j=O.l% tolerance. The electrolysis current, ranging from about 10 t o 10,000 pa., is reset through a 500-ohm resistor before application t o the eel! The 200-volt source was a single series of Burgess No. 5308 45-volt B batteries, mounted on Teflon and glass insulation. The shunt may be used with the panel microammeter, Weston 643, or with an external precision meter to which the proper series resistance is added.
S.C. E.
Figure 2.
For recording the potential difference between the polarized and reference electrodes, full-scale sensitivities of 1, 2, and 4 volts are available with calibration made directly against the standard cell. By means of two Helipots in series with the recorder, the voltage scale may be shifted in either direction as much as the full scale value. The recorder, Model N o . 153X18VX-66X2, with 2.5-mv. range, is similar t o that employed by Delahay and Rfattax (4). The original chart drive mechanism provides chart speeds of 1 to 4 inches per second. Rapid pen response, about 1 second full scale, was retained with the high input impedances by means of circuit modifications made according to Minneapolis-Honeywell specifications (8). Cell. The electrolysis cell, Figure 2, v a s designed to provide essentially uniform current density over the surface of the polarized electrode. The inside diameter of the loir.er section is 1.76 em., and the area of mercury exposed to the solution was estimated to be 2.57 sq. em., assuming a spherical zone determined by the uppermost point of the meniscus, observed in a cathetometer, and the circle of contact with the glass. Other features include a built-in calomel electrode which dips nithin 2 mm. of the mercury surface, a removable auxiliary electrode assembly with fritted-glass disk, and side arms to permit addition and withdrawal of materials xhile the cell is mounted in the viater bath. T o remove oxygen, nitrogen may be introduced through either side arm. The auxiliary anode compartment contained an agar plug prepared from 0.5.M potassium nitrate, a pool of mercury covering the plug, and a potassium chloride solution. This anode composition was chosen to minimize contamination of the sample by diffusion of oxidation products. A cell of this design should be equally suitable for mercury pool polarography. Materials. The lead nitrate and nitric acid 15-ere Baker analyzed reagent products. Solutions a t exact rounded con-
Electrolysis cell
For the cathode, 3.0 ml. of mercury were measured into the cell with each sample of solution. Oxygen was removed before the first run by passage of nitrogen for 15 minutes through the mercury and solution, by way of the lower capillary tube. Five minutes' bubbling, usually through the solution only, preceded each additional run. The cell was tapped, when necessary, to disperse the lead deposited a t the mercury surface. The overall precision of the measurements indicates that variations in electrode area were not greater than 1 or 2'%. The glass surface was not coated with a water repellant (3). Figure 3 is a useful aid in selection of a suitable electrolysis current. Plots of current us. transition time for several values of nC and electrode area were made from Equation 1, with D = 1.00 X 10-5 sq. em. per second. With the present equipment, transition times in the range of 3 t o 20 seconds are desirable. After setting the compensating voltage t o place the curve in a convenient position on the chart, the preselected current is applied t o the cell by the reversing switch or the adjacent battery switch. Recordings were made a t a chart speed of 1inch per second. RESULTS ASD DISCUSSION
A typical recorded potential-time curve is shown in Figure 4. Distortion by capacity effects, recorder lag, and additional electrode reactions necessitates some arbitrariness in any procedure for evaluation of transition time. The results in Table I are based on a point-tangent method introduced by Delahay and Berzins (3),in which r is taken as the vertical distance from point P to the initial tangent, A B . P is selected visually by the criterion d * E j d t 2 = 0. E , 4 corresponds to a vertical distance r / 4 from the curve to the tangent. With one exception, each T&>.. value reported in Table I is the average of three or more recordings on a single filling of the cell, agreeing usually within 1 or 2%. Initial runs were disregarded in compiling the table because they often included some extraneous M aves which disappeared in later recordings. T h e quantity ir'l*jG is found to be a constant, 1.44 X l o 3 amperes sec.l'* cm.3 per mole, with an average deviation of i l % in the concentration range 0.0002 to 0.0251. Below 0.0001M the paves were too ill defined to menwie.
1097
V O L U M E 27, NO. 7, J U L Y 1 9 5 5 Table I.
Chronopotentiometric Data on Lead Nitrate in 0.2M Nitric -4cid at 25" C.
C,
hfoles per Liter 1 x 10-4 2 5 1 x 10-3 2 5
El/& Volt
9
it Ma. 0.100 0.150 0.250 0.480 1.200 2.50 3.00 5.00 5.00 10.00
a
Omitting 1 . 8 X 103.
a 1 1
x
10-2
TW.,
us.
Seconds 3.1 4.0 8.4 10.4 5.48 8.33 5.67 8.04 8.26 8.21
S.C.E. -0.47 -0.43 -0.42 -0.40 -0.40 -0.41 -0.39 -0.41 -0.40 -0.40
ir112/C,
Amp. Sec.1'2 X Cm.3 Moles-' ( x 10-3) 1.8 1.81 1.45 1 .4,5 1.40 1.44 1.43 1 42 1.4-1 1.43 Av. 1 . 4 4 "
value based on equivalent conductance of lead ion a t infinite If all of the data in Table I happened to dilution is 0.98 X fall in a sufficiently high io/C range to give only the lower, kinetically determined, limit of i o P / C , then the corresponding diffusion coefficient would be 2.99 X 10-j sq. cm. per second. This high value appears unlikely, and one concludes that if dissociation precedes the electrochemical reaction, it is too rapid to be observed easily on the equipment used.
Existence of the ion P b S 0 3 + has been demonstrated recently by Herahenson, Smith, and Hume ( 6 ) , who reported a formation constant of 3.3 a t an ionic strength of 2. Delahay and Berzins (3) have shown theoretically that when a slow dissociation step precedes the electrochemical reaction, its influence appears as a variation of i0d/* with io a t constant concentration. An equivalent and somewhat more general function is i 0 d ' 2 / C os. io/C. For a reaction of the type RIX s PIT X, the per cent decrease in i o P 2 / C from the upper to the lower limit is 100 KiormCz/(1 Kform Cz), xhich is also the percentage of M present as L I S in the bulk of the solution a t equilibrium. Kfoimrepresents the formation constant of LIS, and X is present in large excess. I n the present case, with 0.251 nitrate concentration, kinetic control should cause a 40% decrease in i0+*/C a t high ioC ratios. A trend of this nature was not observed in Table I, which includes i o / C values from 175 to 389 ampere centimeters per mole. From C the geometrically measured electrode the average i ~ l ' ~ /and area, a diffusion coefficient of 1.07 x 10-5 sq. cm. per second is calculated from Equation 1, assuming no kinetic control. T h e
+
01
I-, 1
01
Figure 3.
+
Figure 4. Recorded potential5ime curve for 0.0111 lead nitrate at a ma.
It is of interest to compare the quarter-lvave potentials with polarographic half-wave potentials of lead (6): -0.386 volt 2s. S.C.E. in 0.2X sodium nitrate, -0.383 volt in acidified 0.251 sodium perchlorate. With the accumulation of lead from repeated TRANSITION T I M E , SECONDS 05 I 5 10 50 100 polarizations, a shift of the wave toward 10000 more cathodic potentials would be expected. This shift vas apparent in the 5000 static potential measured before each passage of current but v a s negligible a t the quarter u-ave, where El.r often varied lees than 0.01 volt in six recordings. 1000 The procedure of recording repeated polarizat,ions on the same sample has 500 the advantage removing some surface m W contaminants during the first run hut W must be applied wit,h caution, since I a traces of other interfering materials may be deposited simultaneously. I t 100 g I is estimated that reduction of an ad+* sorbed monolayer of oxygen to hydro50 5 gen peroxide on the mercury pool used LL 2 xould require about 0.7 millicoulomh, a quantity easily measured on this instrument. I n preliminary work with 10 lead in nitric acid a peculiar overvoltage effect was observed. The first 5 potential-time curves frequently split into tn-0 viaves, one near the normal -0.4 volt, the other a t -0.6 volt. I n , p * C . , ; y T i ' ' ! subsequent recordings on the same \I ' ~1 sample a single wave appearkd in the 05 I 5 IO 50 100 500 1000 TRANSlTlOY T I M E , SECONDS -0.6-volt region, m-hile the value of ir''";C was not affected by the change Transition time us. electrolysis current for various concentrations in potentiul. The wave returned to and electrode areas ,
,
, - ,
..
~
~
'\-,r:-.
1098
ANALYTICAL CHEMISTRY
its normal potential range after nitrogen vas passed several minutes through the mercury in contact with the solution, and the shift did not reappear. Various attempts to establish the source of this interference have been unsuccessful. It was eliminated in the measurements reported in Table I by empirical use of the bubbling procedure. In conclusion, the results on lead ion confirm the usefulness of constant current voltammetry as an analytical method. With ordinary precautions, an over-all precision equivalent to rt 1% average relative deviation in lead concentration was obtained over most of the concentration range investigated. Similar experimental technique is applicable to study of the composition and thickness of films on metal surfaces ( 7 . 11). ACKNOW LEDGMENT
The writers are indebted to Paul Delahay for helpful discussions, t o members of the Baytown Refinery Instrument Department for construction of the instrument, and to Carl P. Tyler for aid with the evperimental work.
LITERATURE CITED
(1) Berzins, T., and Delahay, Paul, J . -4m.Chem. Soc., 75, 4305 (1953). ( 2 ) Delahay, Paul, “Xew Instrumental Methods in Electrochemistry,” Chap. 8, Interscience, S e w York, 1954. (3) Delahay, Paul, and Berzins, T., J . -4m.Chem. SOC.,75, 2486 (1953). (4) Delahay. Paul, and Mattax, C. C., Ibid., 76,874 (1954). (5) Gierst, L., and Juliard, A. L., J . Phys. Chem., 57, 701 (1953). (6) Hershenson, H. hl., Smith, AI. E., and Hume, D. N., J . -477~. Chem. SOC.,75, 507 (1953). (7) Hickling, A., and Taylor, D., Trans. Faraday Soc., 44, 262 (1948). (8) Ninneapolis-Honeywell Regulator Co., Minneapolis, LIinn., Bull. B15-10 (1948). (9) Reilley, C. N., Everett, G. W., and Johns, Richard, I s . 4 ~ . CHEM.,27, 4S3 (1955;. (10) Sand, H. J. S..Phil.Mag., 1,45 (1901). (11) Wakkad, S. E. S. El, and Ernara, S . H.. J . Cheni. Soc., 1953, 3504.
RECEIVED for review December 29. 1034. .Iccepted February 28, 1955.
Determination of Oxygen in Metals without High Yacuum by Capillary Trap Method WILLIAM G. SMILEY University o f California, Los Alamos Scientific Laboratory, Los Alamos,
The determination of oxygen in metals by conventional vacuum fusion methods is rather slow and complicated. This paper describes a simple and rapid method which does not use high vacuum. The sample is dropped into molten platinum in a graphite crucible. Any oxygen in the sample reacts to form carbon monoxide, which is swept out by a stream of argon at atmospheric pressure. A modified form of Schiitze’s reagent oxidizes the carbon monoxide to carbon dioxide, which is condensed in a capillary trap and measured with a capillary manometer. The apparatus is sensitive to 0.3 y of oxygen. Cuprous oxide samples gave 99% of the theoretical value. Results are given for samples of iron, steel, aluminum, and thorium. A determination usually takes 12 minutes.
0
S Y G E S in metals is usually determined by so-called vac-
uum fusion methods (6). Essentially, the sample is fused in vacuum in an induction-heated graphite crucible, often with a +lux of molten metal, and the gaseous products are collected by a fast diffusion pump and analyzed, oxygen from the sample appearing chiefly as carbon monoxide. A McLeod gage is used for the final measurement. The outstanding advantage of the method is high sensitivity, fractions of a microgram being easily measured with a McLeod gage. Objections t o the method, which have limited its use somewhat. are the complicated apparatus, the necessity for a specially trained operator, and the tedious procedure. I t is difficult to adapt the method t o routine 15ork; each sample commonly requires an hour or more, and much time is spent in such preliminary work as changing crucibles, loading Lamples, and reducing the blank to a constant value. .1 method not involving vacuum was used by Singer ( 4 ) , who i n e p t a stream of nitrogen over the crucible, and after oxidizing carbon monoxide to carbon dioxide, determined it gravimetrically, ns in combustion methods for carbon. Because of its relatively poor sensitivity, this method is limited to large samples. The present paper describes a method which eliminates high
N. M .
vacuum and most of its associated troubles, yet is sensitive to less than 1 y of oxygen. The sample is dropped into a bath of moIten platinum in a graphite crucible. -4stream of argon a t atrnospheric pressure removes the carbon monoxide and carries it through a modified form ( 6 ) of Schiitze’s reagent (S), which converts it to carbon dioxide. The carbon dioxide is condensed out in a capillary trap and measured with a capillary manometer. The desired sensitivity is achieved by using a small volume rather than by measuring low pressures. Only a mechanical pump is used, and only the capillary trap is evacuated. A 10-minute period is sufficient for subRtantially complete r e covery of oxygen from many samples, and the whole determination takes only 12 minutes. The crucible can be changed quickly, and the blank is restored to its operating level in an hour or tMo. Samples are introduced one at a time, through7a
I
F
A
F
Figure 1. A.
Schematic diagram of apparatus
Ascarite and magnesium perchlorate
D. Sample holder
F . Induction furnace .If. hlanostat R. hfodified Schutze reagent T . Capillary t r a p and manometer 1.. Uranium furnace I,!, V2, Va, Vd, Vb. VG. Brass bellonw type valves Ti’. Glass wool and magnesium perchlorate
