Table I summarizes the pK, values determined from the preceding ElI1us. p H curves. The values in parentheses are in the literature data for reduced forms taken from Northey ( I d ) or data of Bell and Roblin (3). Oxidation-Reduction
Potentials.
From t h e individual E,,, us. p H diagrams i t is evident t h a t t h e majority of t h e sulfas studied have approximately the same redox potentials over the entire p H range. I n the physiological p H range, sulfadiazine, sulfapyridine, sulfanilamide, sulfamethazine, and sulfisomidine all have formal potentials around f0.85 volt us. S.C.E., which become more positive with higher acidities. Sulfathiazole has a redox potential somewhat lower than the others at p H 7 , about +0.75 volt. Determination of Sulfa Drugs from Tt‘hile Anodic Limiting Current. there is no particular need for new analytical methods for t h e sulfa drugs, t h e limiting current us. concentration behavior of the compounds is of interest. Table I1 shows typical calibration data for low levels of sulfapyridine. The constancy of i,/C falls off markedly a t the lower concentrations; this is not completely explainable. Part of the discrepancy is naturally associated with the increased difficulty of accurate limiting current measurements at lo^ concentrations. Attempts to determine sulfa drugs in blood and urine samples r e r e unsuccess-
Table II. Limiting Current vs. Concentration for Sulfapyridine C (M x 105) i ~pa., iL/C
2.0 2.8 3.6 4.4 5.2
1.8
4.5 7.4 9.4 11.1 13.0
6.0
0.91 1.61 2.05 2.13 2.14 2.16
ful, because of very high residual current levels in these fluids. The presence of the drug could be definitely established, but no quantitative work appeared possible. The anodic polarography of Gantricin, a n N1 substituted sulfa containing the heterocycle
-C
/O\
I/ CHa-C-C-CHs
N
I1
1
gave
interesting results. At certain p H values, the normal oxidation wave split into two waves of approximately equal height. The compound Nl-(n-butylcarbamyl) sulfanilimide gave a straightforward El vs. p H curve n-ith the following indicated p K values: ~ K R ( N=~ ) 2.6, PKOCN~)= 3.5, and ~ K R ( N=~ ) 5.4. The a t pH 7 is about f0.9 volt us. S.C.E. ACKNOWLEDGMENT
Samples of the Pure sulfa drugs used in this study were generously donated
by Ciba Pharmaceuticals, Inc., Merck 8: Go., Inc., and Eli Lilly and Go. LITERATURE CITED
Adams, R. N., Reilley, C. N., Furman, K.H., ANAL.CHEM.2 5 , 1160 (1953). Adams, R. N., Voorhies, J. D., Zbid., 29, 1690 (1957). Bell, P. H.. Roblin, R. A., J. Am. Chem. SOC.64,2905 (1942). Bergman, I., James, J. E., Trans. Faraday Soe. 50, 60 (1954). Clark, W. &I “Studies ., on Oxidation-Reduction,” U. S. Public Health Service, Hygienic Lab. Bull. IX (1928). Elving, P. J., Komyathy, J. C., Van Atta, R. E., Tang, C., Rosenthal, I., ANAL.CHEM.23, 1218 (1951). Julian, D. R., Ruby, W. R., J. Am. Chem. Soc. 72, 4719 (1950). Levitan, K’.I., Kolthoff, I. M., Clark, W. G., Tenenberg, D. J., Zbid., 65, 2265 (1943). Lord, S. S., Rogers, L. B., ANAL. CHEM.26, 284 (1954). Michaelis, L., “Oxidation-Reduction Potentials,” J. B. Lippincott, Philadelphia, 1930. (11) Morris, J., Department of Chemistry, Howard University, private communication, JL,I! 1956. Northey, E., 8ulfonamides and Allied Compounds,” Reinhold, New York, 1905. Parker, R. E., Adams, R. N., ANAL. CHEW28, 828 (1956). RECEIVED for review June 19, 1957. ACcepted October 21, 1957. ‘Division of Analytical Chemistry, 130th Meeting, ACS, Atlantic City, N. J., September 1956.
Square-Wave Polarograph RANDALL E. HAMM Department o f Chernisfry, Universify o f Ufah, Salt lake City, Ufah
A square-wave polarograph was designed and constructed with a circuit such that the fundamentals of the method could b e studied. Its characteristics were determined for several inorganic ions. Performance i s reported as a function of the variable elements in the circuit. It is less sensitive than the Barker square-wave polarograph, but its resolution is approximately the same.
T
limiting factor in t h e application of conventional direct-current or sine-wave polarography to lower concentrations is the capacitative current associated with the double layer at the mercury drop. Barker and Jenkins ( I ) developed an apparatus which HE
350
ANALYTICAL CHEMISTRY
superimposcd a small square-wave signal on the conventional direct current voltage span of the polarograph and then with appropriate circuitry measured the current that flo\ved just before each change in the applied square-wave potential. If the time constant of the cell circuit was small enough, this allowed the capacitative current to fall essentially to zero, and the faradic current to be measured separately from the capacitative current. Ferrett and Milner (2-6) have described several applications of the Barker square-wave polarograph. Alternating current polarography has the advantages that derivative-type waves are obtained, in which the reduction of each reducible species is displayed independently of any species previously reduced, and materials which are re-
duced completely irreversibly-i.e.,oxygen-are of negligible magnitude in the final measurement of normal concentrations of reversibly reduced materials. Square-wave polarography has all the advantages of sine-wave polarography and the elimination of the capacitative current permits measurement of much lower concentrations. The purpose of this investigation was to develop and test a square-wave polarograph, starting from a completely different circuit design than that used by Barker and Jenkins. An instrument was wanted which would be simpler in electronic circuitry, t o reduce the cost, and more versatile in the frequency used and in the fraction of the time during which measurement was t o be allowed. The instrument was of considerable use as a n analytical instrument and
is still useful €or the fundamental study of square-wave polarography. INSTRUMENT DESIGN
Figure 1 is a circuit diagram of the general arrangement of the components. The direct current scan circuit, shon-n within the dashed lines in Figure 1, was the conventional circuit of a Model XXI Sargent Polarograph, which was altered only by the insertion of switches 8 1 and S2, so that the instrument could be operated either as a conventional polarograph or as a square-wave polarograph. The square-wave generator was a Tektronix Type 105 square-n-ave generator. The amplifier was a Tektronix Type 112 direct-coupled amplifier. The filter was the quadruple parallel-T circuit constructed according to the outlines given by Kelley and Fisher (8). The attenuating resistances, R40 and R41, were such that the (2.50 mv.) Brown recorder normally on the Model XXI polarograph nould record 20-mv. full scale. It was necessary to place the sensitivity switch of the Model X X I a t 1.500 pa. per mm.
s2
I
A . C. Amplifier
-
Gating Circuit
-
- Cathode Filter
Follower
Genera tor I
Figure 1.
Square-wave polarograph
R1. '/2, 1 , 2, 5, 10 ohms wire-wound, 1 % R2. 5,10,25,50, 100,200 ohms wire-wound, 1 % R3. 5 kohms wire-wound 1 % R40. 36 K wire-wound, 1 % R41. 4 K wire-wound, 1 % R42. 170-ohm potentiometer R43. 5.95 ohms when operating as square-wave polarogroph S 1, S2. Switches
t3oov
345 Figure 2. c1, C l l . c2. c3. c4. C5, C6, C7. C8, C9. c10. c12. c13. C14. c15. R4, R9, R14, R33, R26. R5. R 1 5 , R39. R6. R 7 , R10, R 1 1. RE. R12. R13. R16, R34. R 1 7 , R18, R21, R22. R19, R20. R23, R24.
0.04 pf. 0.005 pf. 12 ppf. 0.003 pf. 100 ppf. 56 ppf. 1000 ppf. 0.01 p f . 0.10 pf. 1 pf. 0 . 5 pf. 10 kohms 15 kohms 2-Mohm potentiometer 470 kohms 18 kohms 12 kohms 1 kohms 100 kohms 5 . 6 kohms 330 kohmr 180 kohms
Gating and cathode follower circuits R25. R27. R28. R29. R30. R31, R37. R32. R35. R36. R38. v1, v2, v3, v5. v4.
50 kohms 680 kohmr 25-kohm potentiometer 150 kohms 270 kohms 22 kohms 1 . 8 kohms 500-ohm potentiometer 7 . 5 kohmr, 10 watts
50-, loo-, 300-, 500-, 1 000-, 3000ohm wire-wound, 1 % 12AU7 6AS6 1 N38
Diodes. Leads. 1 . Square-wave input 2. Input from amplifler 3. Ground to fllter 4. Output to fllter 5. Input from filter 6,7. Output to recorder
VOL. 30, NO. 3, MARCH 1958
35)
when operating as a square-wave polarograph, to keep the resistance in t h e cell circuit lo^. The slide-wire resistance on the Model XXI polarograph is a 170-ohm potentiometer, so that the resistance seen by the cell was of the order of 10 to 100 ohms for the spans usually used. The power supplies used were 0.1% regulated (IO), Well regulated power supplies are essential, if this type of instrument is to obtain reproducible results. An H-type cell with a sintered-glass junction between the saturated calomel reference electrode and the dropping mercury electrode compartments was used, Best results were obtained when a connection was made from the calomel electrode through a 2000-pf. electrolytic capacitor to the mercury pool ( 6 ) , as shown by the dashed connection in Figure 1. With this bypass capacitor in the circuit. the calomel electrode established the direct current potential at the dropping-mercury electrode, but the resistance of the cell for square-wave purposes could be maintained at a few ohms, if the inert electrolyte concentration was of the order of liM. The gating and cathode follower circuits are shown in Figure 2. The method of functioning of the gating circuit is that the square-wave inp u t (normally 25 volts peak to peak) goes through the capacitance, C2, which gives a trigger from the positive rise of the square-wave to the cathode of tube V2a. The circuit associated with tube V2 is a monostable multivibrator (IO), which is triggered into its unstable state, where it stays for a period of time depending upon the magnitude of C4, R6, and R7. The output from the plate of V2a is fed through capacitor C6 to point A . The incoming square rvave is also fed through capacitor C1 to the grounded diode, resistance, R14, circuit, which gives a trigger from the negative edge of the square wave. This trigger is amplified by tube V l a and passed through capacitor C5 to point A . Figure 3 is a diagram of the relationships of the various pulse shapes to the input square wave. a is the original square wave, c is the output at the plate of tube V2a, where the position of the negative edge is variable by control of potentiometer R6, and d is the pulse shape a t point A. The triggers pass from 14through capacitor C7 to the grid of tube V3a. The circuit associated with tube V3 is a bistable multivibrator ( I O ) , which will respond to triggers by flipping from one conducting state t o the other. The pilot at Figure 3,e, shon p the output from the plate of tube T'3a. This pulse passes through capacitor C15 to grid 1 of tube V4. This is equivalent to placing a negative 75 volts on this grid during all times except when the positive pulse is placed on the grid. This tube will be nonconducting while the negative 75 volts is on grid 1, but become conducting during the time of the pulse, and the current flowing will be controlled by the potential placed on grid 3 from the amplifier through the cathode follower (tube V l b ) . The signal of the current
352
ANALYTICAL CHEMISTRY
U
U
Figure 3. a.
b. C.
d. e.
f.
Pulse shapes
Square-wave Current flowing in cell when no reduction occurs Output from monostable multivibrator, V 2 Triggers to bistable multivibrator, point A G a t e potential to gate tube Output from gate tube
from the amplifier is shown in Figure 3,b. The potentiometer a t R28 is used to adjust a proper grid bias on grid 3 of tube V4. For the work reported here it was found that -8.0 volts on this grid gave a good linear output. The curve of Figure 3,f, is the output a t the plate of tube V4, where the height of the negative pulse is controlled by the signal placed on grid 3 of tube V4. As the positive and the negative parts of the capacitative and faradic currents (see Figure 3,b) from applying a square wave to a dropping mercury electrode were symmetrical, i t was decided in the design of the circuit to ignore the negative pulses and get the information from only the positive pulses. As the repetition rate was of the order of 100 or 200 times per second, this gave a series of pulses occurring a t this repetition rate and permitted adequate nieasurement of the faradic currents. Loss of gain in this neglect of the negative pulses was only a factor of 2, and made the resulting circuitry simpler than would be required to include the information from both positive and negative pulses. The series of negative pulses pass through capacitor C13, are rectified by the pair of diodes, and pass to capacitor C14. The voltage on this capacitor is applied to the grid of tube Ti5 either directly or through the filter by operation of switch 83. Tube 1'5 is a cathode follon-er introduced to enable use of the high impedance Oak Ridgetype filter and a low impedance recorder of the Brown type. The potentiometer a t R35 was added to act as a grid potential adjustment on tube V s t h a t is, a zero adjustment on the recorder. I n the construction of this circuit considerable care should be used to shield all 10%- level lines and to avoid as much as possible 60-cycle pickup. INSTRUMENT CHARACTERISTICS
Figure 4. Square-wave polarogram of cadmium solution 1 .OO X lo-' mole per liter in 1 .OM KCI with and without fllter
t
50
L o a d R e s i s t a n c e (ohms) I
I
!
I
!
10 2 Square W a v e (mv p - p ?
Figure 5.
Linearity with signal input
Figure 4 s h o w a typical square-wave polarogram of a 1.00 X M solution of cadmium chloride in 1.OM potassium chloride, both with and without the filter. All instrument settings were left undisturbed for those two recordings, except switch 83, which was operated to take the filter out of the circuit. The attenuation of the desirable signal by the filter is not serious in any way and the more readily measured filtered recording is highly desirable. There is a slight lag in the filtered recording, but this would be a disadvantage only if measurement of the half-wave potential were the object, and even then it could be eliminated by determining the half-wave potential on separate scans taken forward and backward and averaging the results. The half-width of the peak without filter was 0.045 volt. Linearity with Signal Input. A series of runs on 1.00 X IO-$ -11 cadmium chloride in 1 . O M potassium chloride was made as a function of squarewave potential applied t o the cell.
niagiiitude of resistance R2 (subsequently called the load resistance), and the position of the output resistance, R38, used. Figure 5 shows that the output is linear with both the squarewave input and the load resistance used. The peak heights n-ere found to be directly proportional t o the output resistance used. Gate Width. T h e effect of t h e ~ i d t hof t h e gate used on t h e peak M cadmium heights. when a 1.00 X chloridc solution in 1.0M potassium chloride \\as run, is plotted for three frequencies in Figure 6. The gate n idth causes a nonlinear variation in the pcnk height obtained. The leveling off of these curves with the wider gate \\ idths indicatrs less variation in peak hright with t h e widrr gate nidths. This result has t o be used with sonir degree of compromise in both gate n idth and frequency, because the gate must bc sufficiently narron to remove the capacitance current, and yet wide cnough t o avoid rapid change of output 11 ith gate nidtli. For most of the 11-ork reported in this paper a gate n i d t h of ’ 2o cycle and a frequency of 230 cycles prr sccond n cre used. Linearity with Concentration. A seiies of solutions of lead, indium, and c.atlminni ions was examined, using this instrument in t h e concentration mole per range 5.0 X lov6to 1.0 X liter. For each ion the peak height rras a linear function of concentration belon1.0 x 10-4 mole per liter, but above this concentration the peak height was slightly lower than would be expected from the results for the solutions of Ion er concentration (Table I). The pcak output, reported in colunin 3 of Table I, \vas calculated by dividing thc~peak height in millimeters by the product of the load resistance, the q u a r e - n ave amplitude (in millivolts), and the output resistance. The constant. sho\\n in column 5 , was calculated 1))- dividing the peak output by the concentration of the solution. To compare these results, independcntly of the capillary characteristics uscd, the 1.00 X 10-3Ll!f solutions were run by classical polarographic tcchniques, using the same solutions and eypc’rimental conditions, exccyt that dissolved oxygen m s removed by bubbling pure hydrogen through the solutions (T:iblr I). The values correspond to d3usion current constants (id/Cm*13 of 3.79, 4.87, and 3.68 for lead, indium, and cadmium. These values igree very w l l r i t h the values reported by Koltlioff and Lingane ( 8 ) . The 1 d u e s of k determined for these three lone, \\hen compared with the classical polarographic wave heights, agree well for the three ions, when it is assumed that the peak height is related to the square of the number of electrons, as predicted by the equation derived by t l / b )
Table I.
Comparison of Square-Wave and Normal Polarograph
Reducible Ion
Supporting Electrolyte
Mole/Liter
Phi+-
1M HSOl
1.0 X
Concn.,
5 . 0 x 10-4 2.5 X
1.0 X 5.0 X 2 . 5 X 10-5 1 , O X low5 5.0 X i 1 o x 10-3 5 0 X lo-‘
1.u ~ c 0 1M HC1
In+++
2 5 x 10-4 1 0 X lo-‘ 5 0X
2 5 x 10-j 1 0 X 10-6 1 o x 10-3
ix 1x1
CdT+
5 0X 2 5X
1.0x
5.0 x 2.5 X 1 .0X 5.0 x
10-5 10-5 10-5 10-6
Kambara ( 7 ) . If the indium solution was not slightly acidified with hydrochloric acid, the peak height as considerably loiTer than if acidified. The classical polarograms taken under these two conditions were not distinguishable. The difference in the square-wave polarograms probably results from a slight degree of irreversibility introduced by the hydrolysis of the indium ion in the solution which is not acidified. Further investigation is in progress. The shape of the peaks for these three ions was found to be related t o the number of electrons in the process-Le., the width of peaks was inversely proportional t o t h e number of electrons. T h e half TT idths determined m r e about 32 niv. for indium and 48 niv. for cadmium and lead, v-hen the filter n a s used a t the normal scan rate, or 30 mv. for indium and 45 niv. for cadmium and lead without t h r filter or a t extremely slow scan rates. TheRe results indicate only a slight amount of lag induced by use of the filter. Resolution of Mixtures. A mixture of copper, lead. a n d cadmium ions
Figure 7. Square-wave polarogram of mixture of copper, lead, and cadmium ions in 1.OM potassium chloride Each 1 . O O per liter
X
I
Cd
Peak Output 0.0270
k
o.oii~
0.00775 0.00321 0.00163 0.00082 0.00@327 0.000161 0 0558 0 0295
o
0152
o
00157 00063 0248
0 0 0 0
Classical Polarogram, Amp. 8.15
00627 00315
27.0 29.2 31.0 32.1
32.6 32.8 32.7 32.2 55 8
62 63 24 28 31
0 0144 0 00784 0.00318
10.50
59 0 62 8 62 7 63 0
8
0 8 8
i.92
3
31.8
0.00157 O.OOOi92
31,4 31.7 32.0 31.0
0.000320 0.000155
(each a t 1.00 X mol per liter) in l . O M potassium chloride was run. The curve resulting (Figure 7 ) has separate peaks shown independently of ions reduced at the more positive potentials. To s h o n how sharp these resolutions
i
l
l
l
l
l
/d
50t
I
I
I
Ga’e
v‘idtil
I
0
:
I
32
I
0 3
Figure 6. Effect of gate width on peak height a t three frequencies
Pb r\
I
cu
male
Volts v s S.C.E VOL. 30, NO. 3, MARCH 1958
353
could become, a solution 0.10M in lead ion and 5.00 X in cadmium ion was run to determine the height of the cadmium peak. The resolution gave a separate cadmium peak which permitted the calculation of the cadmium ion concentration n-ith as great a n accuracy as if the lead had not been present. This is the determination of the more difficultly reducible species when the two ions are in the ratio of 1 to 20,000. Figure 8 s h o w the reduction of a mixture of cadmium (1.00 x mole per liter) and indium (1.0 X loe4 mole per liter) from 1.OM potassium chloride, with 0.1 mole per liter of hydrochloric acid added. Under these conditions the half-wave potentials differ by about 40 mv. and the conventional polarograph would not permit accurate calculations of the amounts of the two ions. The two peaks shown n-ere sufficiently well resolved to make possible calculation of the amounts of each ion present. Reproducibility. In t h e investigations outlined a t concentrations mole per liter the near 1.0 X square-wave polarograph gave measurements of about the same reliability as a conventional polarograph for the reduction of a reversibly reduced species. Because the capacitance current did not become a factor in the measurements as the concentration was reduced, the results became less reliable only when the noise in the circuit began to become a n important factor. As the circuit was constructed, concentrations as low mole per liter could be deas 1.0 x termined and further care in shielding should permit the determination of somewhat lower concentrations. CONCLUSIONS
When the performance of this instrument is compared with the report given by Ferrett and others (6) on three recently developed polarographs, it is
in the presence of a completely irreversibly reducible species, even if they are normally reduced a t the same potential; and analysis by normal polarograph techniques where the square-wave tech-. nique is not necessary. ACKNOWLEDGMENT
Figure 8. Square-wave polarogram of mixture of indium and cadmium ions
The author wishes to acknowledge the assistance of the Chemical Research Operation of the Hanford Atomic Products operation, ryhere this research was first started, and particularly indebtedness to Robert Connally of this laboratory for the preliminary design work on the circuit. He is also greatly indebted to the Research Corp. for a research grant which has permitted t h e continuation of this research a t tha University of Utah.
In 1.OM KCI, 0.1M HCI, each 1 .OO X mole p e r liter
seen that the sensitivity of this instrument is not so great as the Barker square-wave polarograph. and that its resolution is approximately the same, as shown on resolution of the indiumcadmium peaks (Figure S). This instrument is, hon-erer, considerably simpler electronically and therefore much cheaper to build. It can be simplified further by replacing the expensive square-wave generator with a single tube astable multivibrator without appreciably altering the results obtained. The instrument was constructed for use either as a square-wave polarograph or as a conventional polarograph and has proved remarkably versatile. This versatility arises because the instrument permits determination of lower concent,rations than the conventional polarograph; determination of small quantities of more difficultly reducible species in the presence of large quantities of a more easily reducible species; determination of a reversibly reducible species
LITERATURE CITED
Barker, G. C., Jenkins, I. L., Analysi. ~
77, 685 (1952).
Ferrett, D. J., Milner, G. Zbid., 80, 132 (1955). Ibid.. 81. 192 la^' Ferrett, ’D. J., Milner, G. J . Chem. Soc., 1956, 1186. Ferrett, D. J . , Milner, G. Shalgosky, H. I., Slee, I”
IT. C.,
\-Y”Y,.
W. C..
IT. C.,
L. J.,
Analvst 81, 506. (1956).
Gutmann, F., Universlty of Sydney, Sydney, Australia, private communi-- t : n m Kamba ra, T., Bull. Chem. SOC.Japan \rrn”I”II.
27,523, 527, 529 (1954).
Kelley, M. T., F isher, D. J., A x . 4 ~ . CHEM.28, 1130 (1956). Kolthoff, I. M., Lingane, J. J., “Polarography,” 2nd ed., Interscience, New York, 1952. Natl. Bur. Standards, “Handbook of Preferred Circuits,” Kavy Aeronautical Electronic Equipment,” Savaer 16-1-519 (1955). RECEIVEDfor revien. July --22, 1957. Accepted Sovember, 23, 1931. Northwest Regional Meetlng, ACS, Spokane, Wash., June 13-14, 1957. A considerable amount of work performed at Hanford Laboratories, Hanford Atomic Products Operation, Richland, Wash.
Automatic Determination of Uranium in Process Streams H. W. BERTRAM, M. W. LERNER, G. J. PETRETIC, E. S. ROSZKOWSKI, and C. J. RODDEN U. S. Atomic Energy Commission, New Brunswick, N. J.
b An automatic instrument composed of a derivative polarograph and a sampling and proportioning system has been developed to analyze high concentrations (100 to 200 grams per liter) of uranium in process streams. The derivative polarograph, based upon the resistance-capacitance circuit, scans the applied voltage in a reverse direction to decrease peak oscillations. Concentrations of ura-
354
*
ANALYTICAL CHEMISTRY
nium are recorded a t 5-minute intervals. Factors affecting the precision and accuracy are briefly discussed.
T
numerous chemical analyses required in the process control of uranium purification involve high costs. Automatic instrumentation of many of these control tests would reduce both laboratory costs and material holdup time in the plant. HE
I n the current uranium refining processes there are many points at which an aqueous process stream must, be analyzed for control purposes, Rodden (IS) has summarized all these control points, giving the number and type of analyses necessary at each, for representative diethyl ether and tributyl phosphate extraction plants. The ore digestion section of either type of plant is especially suitable for auto-
’
