Polarographic Method for Copper, lead, and Iron Using a Pyrophosphate Background Solution C. A. REYNOLDS' AND L. B. ROGERSO, Stanford University, Calif.
A linear relationship between diffusion current and concentration has been found for the reduction of copper (II), iron (IIL), and lead (11) in a 0.1 M sodium pyrophosphate solution. I t is possible to utilize these complexes for analysis of several types of alloys. Because iron and lead form a single wave, one of them must be determined by another procedure if significant amounts of both are present.
C
OPPER (11),lead (11))and iron (111) (IO) as well as antimony (111),uranium (VI), and other ions (11) appear to have analytically useful waves in a 0.1 M sodium pyrophosphate solution, whereas aluminum, cadmium, cobalt (11),magnesium, and nickel have waves which are not suitable for polarographic analysis either because the ion is not reduced before hydrogen is evolved or because a flat nave is formed (IO). Although Sartori (12) reported that cadmium could be analyzed in a pyrophosphate background solution, his results are not clear, and the long flat wave found for cadmium by Rogers and Reynolds (IO) would be very difficult to employ in an analytical procedure. During the course of studies involving solutions of sodium pyrophosphate, it became desirable to establish for several ions the relationship between polarographic diffusion current and concentration. The relationships were first determined by using carefully prepared fitandard solutions and then checked, after devising a simple procedure, by analyzing alloys of kiiown composition. In a 0.1 n/f pyrophosphate solution, copper (11) is reduced in two steps having half-wave potentials of -0.40 and -1.33 volts versus the saturated calome1,electrode (S C.E.). The reductions of lead (11)to the metal and iron (111)to iron (11)have half-wave potentials of -0.69 and -0.82 volt, respectively. Therefore, in a solution of 0.1 M pyrophosphate containing a mixture of copper, lead, and iron, the copper waves can be distiqguished easily, whereas lead and iron, which have half-wave potentials less than 0.2 volt apart, produce a single wave reprewnting a sum of the two. Although the procedure described below does not separate lead and iron, it can nevertheless be applied to many commercially available alloys that contain a negligible quantity of either of these elements. If appreciable amounts of both lead and iron are present, another method (5, 6) must be combined with the present one in order to determine the amount of each element rather than the sum. Until 1941, polarographic procedures for alloys had not been examined extensively ( 4 ) . hIore recently, a number of articles have described the application of various electrolytes, such as citrate, tartrate, or cyanide, to the analysis of aluminum- (5, 13, 1 4 ) , copper- (6, Y, 8, 15, 16), and zinc-base ( 1 , 3, 9, 14) alloys. Usually the same or a slightly modified procedure can be used to analyze beryllium- and magnesium-base alloys because these elements have very negative half-wave potentials.
Standard solutions of the cations and of the sodium pyrophosphate were prepared as before (IO). In addition, a solution of 0.1 M beryllium nitrate was standardized by precipitating the oxide with ammonia followed by an ignition a t 1000" C. ( 2 ) . A solution of gum ghatti was prepared by allowing 20 grams of soluble gum ghatti from Eimer and Amend to stand in 1 liter of water for 24 hours. The resulting solution was filtered and 10 ml. of chloroform were added to prevent bacterial decomposition. Tests of the linearity between concentration of reducible ion and diffusion current were carried out in the usual way for concentrations in the range of lo-* to Jf (Figure 1). PROCEDURES
Brass Samples. -4weighed sample of about 0.1 gram is dissolved in 1 ml. of concentrated nitric acid and, after the oxides of nitrogen have been expelled by boiling, the solution is made up to 250 ml. in a volumetric flask using a 0.1 M solution of pyrophosphate. To a 50-ml. aliquot of this solution, 1 ml. of a 2% solution of gum ghatti and 1.0 gram of solid sodium sulfite are added, following which the solution is polarographed. Because the amount of copper in the alloy is much greater than the amount of lead, it is advantageous to use different sensitivities for determining the two elements. The amount of iron present in most commercial brasses is too small to interfere with the determination of lead. In addition, the diffusion current for iron is smaller than that of lead of the same molar concentration. As a result, the percentage of iron varied from 0.05 to about 0.2 in the five brass samples reported in Table I without interfering noticeably with the lead determination.
Table I.
1
2
3
EXPERIMENTAL
4
The equipment used in this study has been descrihed (IO). Polarograms were recorded using a Leeds & Xorthrup Electrochemograph in conjunction with the calibrated dropping mercury electrode and an outside saturated calomel electrode. A the:m+ stat maintained the temperature of the solution at 25.0 * 0.1 O
Analyses of Brass Samples for Copper and Lead
Sample
5
c.
-1 Present address, Department of Chemistry, University of Kansas, Lawrence, Kansas. 2 Present address, Massachusetts Institute of Technology, Cambridge, Mass.
Sample samples.
176
Weighing Gram Stated value 0.1103 0.0714 0.0482 0 0856 Stated value 0.0653 0,1809 0.1242 0.1759 Stated value
Copper, %
Lead, %
75.52 6.12 75.5 6.2 75.4 6.3 75.4 6.3 75.6 6.1 66.63 3.73 66 6 3.7 66.2 3.9 65.7 3.8 66.6 3.8 84.17 3.76 0.1008 84.3 3.8 0.0642 84.4 3.6 0.0961 84.2 3.8 0.1162 84.1 3.7 Stated value 71.23 4.48 0.1169 71.4 4.6 71.4 4 4 O.lOQ7 71.3 4.6 0.0658 0.0897 71.2 4.4 Stated value 70.29 0.964 0.0782 70.4 1.0 1 .o 70.2 0,1088 0 1656 0.9 69.2 1.1 69.4 0 .11 l'l 5 was Bureau of Standards Brass 37; others were Thorne Smith
V O L U M E 21, NO. 1, J A N U A R Y 1 9 4 9 Aluminum- and Zinc-Base Alloys. The same general procedure was used in preparing solutions of these alloys for analysis. However, it was found best to dissolve the alloy by adding very slowly 2 ml. of 1 to 1 hydrochloric acid followed by 0.5 ml. of concentrated nitric acid. Table I1 gives the results of analyses on alloys from the Xational Bureau of Standards. Beryllium- and Magnesium-Base Alloys. No suitable Bureau of Standard samples were on hand with which to test the method, but presumably the procedure described for an aluminum alloy could be used without modification. The absence of interferences from large amounts of magnesium and beryllium was substantiated by preparing synthetic alloys from standard solutions. A representative series of analyses of synthetic beryllium and synthetic magnesium alloys are shown in Table 111. DISCUSSION
I t is very important not to add too much acid in dissolving the sample. Large amounts of acid can introduce errors by affecting the pH of the resulting solution and by causing a decrease in the amount of pyrophosphate available for complexing by converting it to phosphate. If a larger amount of acid is used to dissolve a sample, it is desirable to neutralize the excess acid by adding a solution of sodium hydroxide until a permanent precipitate is just formed. Analyses carried out using this modified procedure gave results agreeing with those listed in the tables, whereas erratic results often appeared when the bulk of the excess acid was not eliminated before adding pyrophosphate. Every trace of oxides of nitrogen must be removed before adding the solution of pyrophosphate. In the analysis of zinc-base alloys the presence of a small amount of these oxides appeared to result in very large diffusion currents for both copper and lead. The waves had their normal half-wave potentials but the currents were sometimes 50 times higher than the expected value. The presence of a suspension of stannic oxide did not interfere with the determination of copper and lead in brass. Furthermore, the precipitate did not appear to dissolve appreciably in pyrophosphate during the time required to make an analysis. Had it dissolved, it would not have interfered because the tin (IV) complex does not reduce before hydrogen is evolved (IO). The maximum in the first wave for copper (El/*= -0.40 volt) spread over a wide range of potential, but it caused no difficulty
177 Table 11. Analyses of Aluminum- and Zinc-Base Alloys for Copper and Iron Weighing Copper, % Iron, % Uram 1 Stated value 7.87 1.53 7.77 1.50 0.0817 1.49 7.7f 0.1062 1.5O. 7.9 0.0432 0.1172 1.56 7.88 2 Stated value 4.11 0.395 0.1165 4.12 0.39 0,1348 4.20 0.38 0.1077 4.0a 0.40 0.0924 4.l o 0.39 3 Stated value 2.82 0.048 0,0762 2.Bn 0.06” 2.72 0.062 0.1165 2.72 0,050 0.1342 0,0425 2.9 0.045 a Kumber of significant figures in results was limited by sensitivity selected for specific polarogram. Sample 1. Bureau of Standards aluminum-base casting alloy 86B. Sample 2. Bureau of Standards aluminum alloy 85. Sample 3. Bureau of Standards zinc-base die casting alloy 94. Sample
Table 111.
Analyses of Synthetic Beryllium- and Magnesium-Base Alloys
Sample 1 stated value
2 stated value
Copper, % 6.75 6.70 6.92 6.80 6.71 3.26 3.33 3.26 3.30 3.31
Iron, % 4.21 4.31 4.16 4.26 4.19 1.96 1.88 1.89 1.92 1.97
52.1%
Be
87.7% M g
in the determination of copper because the second wave ( E I / ~ = -1.33 volt) was usually used for this purpose. However, such an extended maximum did interfere with the determination of lead whose half-wave fell at -0.69 volt. A solution of gelatin suppressed the copper maximum but, if sufficient gelatin were added to supress the maximum completely, it also depressed the diffusion current of the lead wave to a small fraction of its true value. Gum ghatti, on the other hand, suppressed the copper maximum sufficiently well to enable a complete lead wave to be observed without at the same time noticeably affecting the diffusion current for lead. The use of gum ghatti is also recommended for mixtures of copper with iron, although in this case the half-wave potential for iron (-0.82 volt) is sufficiently more negative than that for lead (-0.69 volt) to decrease automatically the interference from the maximum of the copper wave at -0.40 volt. Therefore] the amounts of gelatin required to eliminate interference from the copper maximum are too small to affect adversely the diffusion current for iron. LITERATURE CITED
0
950
500
750
1000
PER LITER Figure 1. Linear Relation between Concentration and Diffusion Current for Copper, Iron, and Lead MILLIGRAMS
(1) Cozzi, D., Mikrochemie uer. Mikrochim. Acta, 31, 37 (1943). (2) F u r m a n , N . H., “Scott’s S t a n d a r d M e t h o d s of Chemical Analysis,” 5th e d . , p. 139, New Y o r k , D. Van N o s t r a n d Co., 1939. (3) Hawkings, R. C . , and T h o d e , H. G., ISD. ESG. CHEM.,ANAL. ED., 16, 71 (1944). (4) Kolthoff, I. M.,a n d Lingane, J. J., “Polarography,” p. 328, Kew Y o r k , Interscience Publishers, 1941. (5) Kolthoff, I. M., a n d iMatsuyama, G., ISD. EXG.CHEM.,ANAL. ED., 17, 615 (1945). (6) Lingane, J. J., I b i d . , 18, 429 (1946). (7) Milner, G. 11‘. C., A n a l y s t , 70, 250 (1945). (8) Milner, G. W. C., MetaIZurgia, 36, 257 (1947). (9) Publication S u b - c o m m i t t e e , British S t a n d a r d s Institution Panel, “Polarographic a n d Spectrographic Analysis of High P u r i t y Zinc a n d Zinc A4110ys for Die Casting,” pp, 1-32 (chapter b y Nickelson, A . S.,a n d Randles, J. E. B.), London, H . M .Stationery Office, 1945. (10) Rogers, L. B . , a n d Reynolds, C. A., J . A m . Chem. SOC.,in press. (11) Rogers, L. B . , a n d Reynolds, C . A,, unpublished work.
ANALYTICAL CHEMISTRY
178 (12)
(13)
Sartori, G., Gart. chim. itaZ., 64, 3 (1934), through Kolthoff, I. M., and Lingane, J. J., “Polarography,” p. 270, New York, Interscience Publishers,1941. Semerano, G., and Capitano, V., Mikrochemie ver. Mikrochim.
Acta, 30, 71 (1942). (14) Spalenka, hl., Z . anal. Chem., 128, 42 (1947). (16) Toropova, V. F., J . Applied Chem. (U.S.S.R.),18, 177 (1945).
(16) Tyler, W. P., and Brown, W. E., 15,520 (1943).
IND. ENG.CHEM.,ANAL.ED.,
RECEIVED May 14, 1948. Taken from a thesis submitted by C. A. Reynolds to the Department of Chemistry and the Committee on Graduate Study of Stanford University in partial fulfillment of the requirement for the degree of doctor of philosophy, July 1947
Apparatus for Electrolysis at Controlled Potential C. J. PENTHER
AND D.
J. POMPEO, Shell Development Company, Emeryuille, Calif.
A unitized controlled potential apparatus is described which contains a source of accuraie reference potential with n range of 0 to 4.41 volts, a sensitive unbalance detector and electrode potential correcting unit, and a source of line-supplied direct current which is independent of line voltage variations. Using a mercury cathode, and with initial current as much as 100 times final current, the seositivitp to changes in electrode potential is about 1 mv. and the instantaneous deviation from the control point not more than 1 1 0 mv.
T
H E apparatus described was designed t o be fully automatic and as nearly foolproof as possible, for use a8 a routine tool in an analytical laboratory. I t can be used for the preparation of samples in the systematic polarographic analysis of inorganic or organic mixtures, as xell as in organic synthesis ( d ) , and has been proposed as a means of direct analysis of alkali metals by separation using a rotating silver anode and mercury cathode by the method of Smith ( 5 ) . Earlier apparatus designed to accomplish work of this nature has been described by Hickling ( d ) , Caldwell, Parker, and Diehl (I), and more recently by Lingane (3). The requirements of a routine laboratory apparatus were met by combining the best features of the above-mentioned equipment-namely, complete line operation (except for the battery in the reference potential potentiometer), wide range of voltage and current, high sensitivity, and freedom from need of operator attention and maintenance.
Mounted directly above the potentiometer are two meters with corresponding range switches used to measure electrode potential and current. The one on the left measures current on four ranges of 0.1, 0.5, 1.0, and 5.0 amperes. The one on the right (ranges changed after photographing) measures cathode-anode potential in three ranges of 3.0, 15, and 30 volts. The upper large knob to the right of the potentiometer is the Helipot voltage control. A friction clutch between the Brown motor and Helipot shaft permits setting the electrode voltage manually when starting an electrolysis. The motor is rated a t 27.5 r.p.m. and a 5 to 1 gear reduction is used to drive the potentiometer. As the Helipot has ten turns, the effective maximum 27 5 speed of the slider is -= 0.55 r.p.m. 5 x 10 The lower knob is the amplifier gain control, which enables the operator to adjust the amplification to the optimum value of maximum sensitivity without hunting. The potentiometer gain control mounted in the Electronik amplifier is disconnected and shielded leads are extended to the panel control. The change in cell voltage is indicated by two pilot lamps which are mounted above the upper knob and are identified as increasing
METHOD
A block diagram of the apparatus is shown in Figure 1. A b e d s & Northrup potentiometer, Type 7665-5 with a special
f7?
range of 4.44 volts, is used as a reference potential. The difference voltage between this reference voltage and that developed between the mercury or platinum cathode and a saturated c a b me1 half-cell is detected by a Brown Electronik amplifier (Brown Instrument Company, 4482 Wayne Ave., Philadelphia 44,Pa.). The output of this amplifier controls the direction and speed, corresponding to the sign and magnitude of input unbalance, of a two-phase motor which drives a Helipot potentiometer (Helipot Corporation, 1011 Mission St., South Pasadena, Calif.). A voltage picked off the sliding arm of this potentiometer varies the grid voltage of a pair of vacuum-type rectifier tubes, the plate currents of which serve to vary the output of a saturable reactor. The reactor maintains the primary voltage of a step-down transformer supplying the low voltage rectifier at a value that provides a constant cathode potential independent of electrode current and line voltage.
SENSITIVITY
APPARATUS
OYETER
The entire a paratus is housed in a steel case occupying a table space 50 cm. 8 0 inches) long by 45 cm. (18 inches) deep. The rear 7.5 inches are occupied by the alternating current power supply and covered with a cane-pattern sheet metal cover which provides adequate ventilation as well as protection. The front section of the cabinet is made of 18-gag6 steel sheet and supports the Duralumin control panel on a slope of 30” running from a height of 6 inches a t the front edge to 12 inches a t the rear. The Brown amplifier is secured as a unit to the inside rear wall at the bottom. As shown in Figure 2, a photograph of the complete apparatus, the Leeds & Northrup potentiometer has been removed from its case and mounted on the panel.
I
I
Figure 1. Block Diagram of Controlled Potential Electrolysis Unit
