V O L U M E 2 4 , N O . 6, J U N E 1 9 5 2
973
For a particular molecule, if a plot of the diffusion currents against the reciprocals of the square root of the viscosity coefficients of the solvents shows a linear relationship, then, as the slope, nK, is a canstant, the electron change would be the same in all the solvents. This condition was met by all the nitroalkanes and n-butyl nitrate. I
00
Figure 2.
I
20
I
4 0
I
60
1
yfi,
8 0
I
100
1
I
120
POlSES-"2
Effect of Viscosity of Solvents upon Diffusion Current of 0.001 M Nitroethane
The diffusion coefficient of nitroniethane in methanol is given as 2.53 X 10-6 cm.2 sec.-l a t 16" C. ( 3 ) . From polarogram obtained for 0.001 M solutions of nitroniethane in 0.5 f i f lithium chloride in methanol a t 16", the calculated value of the electron change, using the Ilkoviit equation, was 3.9. As the wave heights and the half-wave potentials of the nitroalkanes are of the same magnitude, the electron change would be expected to be the same for nitromethane, nitroethane, 1-nitropropane, 2-nitropropane, 1-nitrobutane, and 2-nitrobutane. The final reduction products would probably be the alkylhydroxylaniines. There is general agreement that the nitroalkanes in aqueous solution are redured polarographically xith an electron change of 4 to the hydroxylamines ( 1 , 6, 8, 9). On the assumption that IZ is equal to 4)the diffusion coefficients of the nitroalkanes in methanol were calculated froni the IlkoviE equation and were compared with the diffusion coefficients calculated with the follo\~-ingrelation :
derived from the StokesEinstein equation, Khere ,T' is the molal volume of t h e molecule. These data are given in Table 1- and a comparison of the square root of the diffusion coefficient cal-
Table
Y. Diffusion Coefficients of 0.001 M Nitroalkalies in Methanol as Solvent (0.5 J1 in lithium chloride)
( D g )
c
Diu DSE~ Cm.2 sec.-l X 10s Sitraniethane 26.4 2.69 1.43 Nitroethane 24.4 2.30 1.30 1-Nitropropane 22.7 2.00 1.21 2-Nitropropane 23.3 2.10 1.20 1.41 1-Nitrobutane 19.1 1.14 1.55 2-Nitrobutane 20.1 1.14 D I calculated from Ilkovia e uation, n = 4. b 0.~8, calculated from Stokes-%instein equation. 16
111
0.73 0,75 0.78 0.75
0.90 0.86
culated from the Stokes-Einstein equation; that calculated from the IlkoviE equation shows that this ratio approaches 1 as the nitroalkane molecular size increases. This indicated that in methanol for molecules as large as nitrobutane, or larger, the Stokes-Einstein equation can be used to calculate diffusion coefficients for polarographic use. For n-butyl nitrate a 2-electron change was calculated for t,he reduction in all the base solutions studied. A 2-electron change has been reported for the reduction of ethyl nitrate and cyclohcsyl nitmtcl in a q u v o u ~vthmol (4). LITERATURE CITED
(1) De Vries, T., and Ivett, R. TV,, IND.ENG.CHEM.,AN.AL.ED., 13,339 (1941). (2) Hale, c. H., ANAL. CHEM.,23, 572 (1951). (3) "International Critical Tables." Vol. V, p. 72, S e w Tork, McGrsw-Hill Book Co., 1929. (4) Kaufman, F., Cook, H. J . , and Davis, S. >' "Electrolytic I., Reduction of Organic Sitrate Esters," Abstracts, 117th lfeetine. -4MERICAN C H E X I C I L SOCIETY. Detroit. Mich.,,~1950. (5) Lewis, WY'R., Quackenbush, F. IF'., and De Vries, T., ASAL. C H E Y . , 21, 762 (1949). ( 6 ) Miller, E. TV., Arnold, A. P., a n d ;Istle, 11.J., J . A m . C'hevr. Soc., 70, 3971 (1948). (7) Parks, T. D., and Hansen, K. A., ASAL. CHEM.,22, 126s (1950). (8) Petru, F., ColEection Ctechosloc. Chem. Cornmum., 12,620 (1947). (9) Stewart, P. E., and Bonnei, JV. A,, AXAL.CHEM.,22,793 (1950). (10) Stone, H. W.,and Skavinski, E. R., ISD. ENG.CHEM.,.Isa~. ED., 17,495 (1945). I ~ L C E I ~for E Dreview March 10, 1951. Accepted April 7, 1952. Presented before the Pittsburgh Conference on Analytical Chemistry and Applied Spectroscopy, March 5 , 1951. Abstracted from t h e Ph.D. thesis submitted by S a r h a n Radin t o t h e Graduate School of Purdue University, February 1951.
Reproducible Platinized Platinum Electrode for Anodic Polarography WILLIAXI 31.~IAcNEVINAND RIICK-IEL LEVITSKY' Ohio S t a t e Unicersity, Coluntblts, Ohio
K
ECEKTLY ( 2 ) it mas observed that the current obtained with the microelectrode in anodic polarograplly could be appreciably increased if the surface of the electrode were coated with a layer of electrolytically deposited platinum. Difficulty was experienced in reproducing the current, using succee sively prepared electrodes, and it is with this problem that the present paper is concerned. The electrode support used in this work was a rotating microelectrode similar t o that described by Kolthoff and Lingnne ( 1 ) .
A 3-mm. length of 20-gage platinum wire was attached to the rotating shaft. In the origina1,design the shaft was coated with 1 Present address, Grasselli Division, E. 1. d u pant de N~~~~~~ and Wilmington, Del.
c0,,
wax and the wax was removed from the end Of the platinum wire. This process usually loosened the wax surrounding the wire, and electrode surface of unknown area was exposed to the solution. early in the prasentwork by coating This problem lvr&s the platinum wire with glass. Glass tubing was drawn to the diameter of the wire. A short section was placed OVW the wire and heated with a small flame until it collapsed and attached itself to the wire. The glass over the end of the wire waa then ground off with emery paper. The esposed end of the platinum wire was polished with fine silicon carbide powder suspended on cloth. Finally the shaft was coated with ceresin was, which overlapped on the glass-covered shank of the platinum electrode. When prepared in this way, the electrode area is constant. The exposed end Of the electrode may be polished repeatedb- without changing the area of the electrode surface. The glass surrounding
974
Dissolution of mercury at low positive potentials limits its use in anodic polarography. A platinized platinum electrode had been found to be very sensitive in the estimation of methyl and ethyl alcohols from anodic currents, but reproducibility from one platinum deposit to another was difficult. The present work was undertaken to study the factors involved in obtaining reproducible platinum deposits. Reproducibilitywas possible if the platinum wire base was sealed in glass, polished, and plated at a rotating electrode from a 3% chloroplatinic acid solution, 0.06% in lead acetate, mercuric chloride, or copper sulfate. It was possible to obtain anodic currents for to molar dextrose reproducible to 52% between platinum deposits, chloroplatinic acid solutions, or electrodes. The reproducible platinized platinum electrode will extend the use of polarography in anodic reactions. Dextrose and galactose can be determined using this electrode.
the electrode adheres firmly enough to withstand mechanical damage from the polishing operations. Electrolysis of a solution of chloroplatinic acid was used to produce a coating of platinum black over the end of the platinum electrode. It was soon observed that the kind and amount of metallic impurity in the chloroplatinic acid had a marked effect upon the physical quality of the electrolytic deposit of platinum black and upon the current sensitivity of the electrode in subsequent polarographic measurements. For example, when reagent grade chloroplatinic acid was used, the deposit was black. When chloroplatinic acid was made from spectroscopically pure platinum the deposit was bright. The difference was traced to the effects of copper and mercury in the reagent grade chloroplatinic acid. According to the label on the chloroplatinic acid bottle, 0.2670 of heavy metals may have been present. Spectrographic examination indicated that these were mainly copper and mercury with a lesser amount of antimony. Because electrodes coated with platinum black have a much greater current response, the effect of the kind and amount of other metallic impuritieg on the production of platinum black was investigated. It was found that salts of lead, copper, and mercury were especially effective in producing the black deposit, while salts of zinc, nickel, cadmium, and iron did not decrease or destroy the brightness of the platinum deposit. The addition of h7 drochloric acid to the chloroplatinic acid had a marked effect in decreasing the current response of the electrode in subsequent polarographic measurements. This effect is presumed to be due in part to the compleuing of the lead and mercury ions with chloride. The current response of the electrode in subsequent polarographic use was found to be affected by the concentrations of lead, copper, and mercury salts in the chloroplatinic acid. When lead, copper, and mercuric nitrates and acetates were added to reagent grade chloroplatinic acid, a maximum of current responEe resulted when the concentrations of the salts were 0.06 to 0.08%. At concentrations greater than 0.08%, the current response decreased and the deposit became loosely adherent. It was decided therefore to use a concentration of 0.06% of impurity for the preparation of the electrode. As a rule a 3% solution of chloroplatinic acid was used for the electrolytic deposition of the platinum. Although the maximum concentration of metals in the original solid chloroplatinic acid was 0.2676, this corresponded to approximately 0.01% of metallic impurity in the prepared 3% solution. The amount of impurity added is therefore a t least six times that present in the chloroplatinic acid itself, so that variations in impurities in the reagent grade chloroplatinic acid are unlikely to have much effect upon the
ANALYTICAL CHEMISTRY total impurity present and upon the current response of the electrode. I t was found early in the work that a more adherent deposit and a more reproducible polarographic current were obtained if the electrode were rotated during the deposition of the platinum black. The current response of the platinized electrode also varies with the rate of rotation of the electrode during the deposition of the platinum black. I t increases as the rate of rotation of the electrode increases. It was observed that the size of the bubbles of hydrogen evolved during the electrolysis was smaller the higher the rate of rotation. The greater current response is assumed due to the greater state of subdivision of the platinum black when smaller bubbles of hydrogen are being formed. A rotation rate of 1300 r.p.m. was finally selected as satisfactory. PREPARATlOh- O F ELECTRODE
-43% solution of reagent grade chloroplatinic acid was prepared and 0.06% of lead acetate added. The electrode was rotated a t 1300 r.p.m. during the deposition. A voltage of 2.1 volts JTas applied and was found not to vary noticeably from this value a t any time. Under these conditions the current was about 1 ma. A plating time of 100 seconds was used. Experiments in which the current was intentionally made more or less than 1 ma. indicated that a constant number of coulombs must be used in preparing the electrode in order to get maximum reproducibility. As the deposition was normally performed, however, the current did not stray from 1.0 ma. from one deposition to another and a plating time of 100 seconds could always be used.
Table I.
Reproducibility of Currents with Replatinized Electrode Preparation Current, w.5 1 2 3 4 5 6
a
39.6 39.6 38.7 40.3 39.6 38.7 7 39.6 hl ea n 39.4 B v . dev., % 1.1 Electrode 1, platinizing solution A, used for these measurements.
Although a platinized electrode could be used repeatedly with only occasional indication of aging, it was found more practical to replatinize the electrode each time it was used. The electrode surface was cleaned and polished by rubbing with fine silicon carbide powder, suspended on cloth, for 20 to 30 seconds. The electrode was washed, blown dry to remove particles of the abrasive, and replatinized. The platinum black deposit was finally washed briefly with distilled water. The entire replatinixing operation requires not more than 3 minutes. The conclusion is therefore reached that a reproducible platinized platinum electrode of high current response can be prepared if the following points are observed: The platinum wire base should he surrounded with glass. The end of the electrode should be thoroughly polished with emery and silicon carbide powder. Black deposits of platinum give appreciably higher currents than bright ones and are therefore preferred. The electrode should be rotated a t 1300 r.p.m. during the electrolytic deposition of the platinum black. An impurity of 0.06y0of the acetates or nitrates of lead, copper, or mercury should be added to the chloroplatinic acid. The plating is carried out in a 3% chloroplatinic acid solution a t 2.1 volts and about 1 ma. As an example of reproduclbi!ity, the data of Table I show the currents obtained for electrode 1 after successive replatinizations. iM dextrose These are the anodic currents for the oxidation of
975
V O L U M E 2 4 , NO. 6, J U N E 1 9 5 2 Table 11.
Effect of Source of Chloroplatinic Acid on Anodic Current Current, pa." Lot A
39.6 Mean 0.9 Av. dev., % Electrode 2 used for these measurements.
Lot B
39.8 2.3
in a buffered solution prepared from equal volumes of 0.2 AT potassium dihydrogen phosphate and 0.2 S sodium hydroxide solutions. .4 second test of reproducibility involved the use of different preparations of chloroplatinic acid. Solutions were prepared from each of two lots of solid reagent grade chloroplatinic acid. Electrode 2 was replatinized several times from each solution and the anodic current measured with the same dextrose solution referred
to in Table 1. The currents obtained are shown in Table 11. Although the precision of the currents obtained with platinizing solution B is not so good as was usually obtained, the average current agrees well with the average obtained for platinizing solution A. In the work reported in Tables I and 11, two different electrodes were used. They were prepared in the same way, starting with 20gage wire. I t is apparent on comparing the average currents for electrode 1 shown in Table I, with those for electrode 2 shown in Table 11, that the current response of the platinized platinum electrode can be duplicated with the usual precision of polarographic current measurement. LITERATURE CITED
I. M . , and Lingane, J. J., "Polarography," p. 440, Xew York, Interscience Publishers, 141. (2) MaoNevin, TV. M., and Sweet, T. R., Quart. J. Studies on Alcohol, 12, 46-51 (1951). (1) Kolthoff,
RECEIVED for review J a n u a r y 11, 1952. Accepted March 15, 1952. From a thesis submitted t o t h e Graduate School of T h e Ohio S t a t e University in partial fulfillment of t h e requirements for t h e degree of daetor of philosophy, March 1952.
Determination of Dihydrostreptomycin and Mannosidostreptomycin with Periodic Acid W. AUBREY \'AIL1 AND CLARK E. BRICKER Department of Chemistry, Princeton University, Princeton, N. J .
D
This work was started in order to improve the chemical method for the determination of dihydrostreptomycin. During this preliminary investigation, an observation was made which has led to a differential spectrophotometric method for the simultaneous determination of mannosidostreptomycin and streptomycin in mixtures. Dihydrostreptomycin when treated for a specified time with periodic acid yields 1.02 moles of formaldehyde, whereas streptomycin produces only 0.20 mole. Prior to the formaldehyde determination with chromotropic acid which forms the basis for the determination of the dihydrostreptomycin, the iodic acid and the excess periodic acid are precipitated with lead acetate. The differential spectrophotometric method for mannosidostreptomycin and streptomycin utilizes the fact that when any of the streptomycins are treated with periodic acid and lead acetate, a characteristic color is formed. Because these colors for the streptomycins are very time-sensitive, optimum conditions for maximum sensitivity and differentiation must be carefully followed. These methods should find application in the manufacture, control, and physiological investigation of the streptomycins. The method for dihydrostreptomycin is believed to be the most reproducible chemical method so far reported. The reaction of lead acetate with the streptomycins which have been oxidized with periodic acid should provide a useful check on other chemical methods for deter.mination of mannosidostreptomycin.
IHYDROSTREPTONYCIS, n hich is made by the catalytic hydrogenation of streptomycin, has been determined by two chemical methods which differ in principle from those reported for streptomycin. Garlock and Grove (8) and Colon, Herpich, Johl, Seuss, and Frediani ( 4 ) reported methods based on the oxidation of dihydrostreptomycin with sodium metaperiodate and periodic acid, respectively. In both cases, the fornialdehyde that was formed during the oxidation was removed from the reaction mixture by distillation and then determined by t h e chromotropic acid procedure ( 1 , 3 ) . More recently, Hiscox ( 9 ) suggested a spectrophotometric method for dihydrostreptomycin based on its characteristic absorption a t 265 mp after acid hydrolysis. Although periodates are fairly specific reagents for oxidizing organic compounds containing adjacent hydroxyl or amino groups, this specificity is lost when the reaction mixture is heated ( 7 ) . Colon et al. ( 4 ) report that streptomycin, which theoretically should yield little, if any formaldehyde with periodic acid oxidation, gives with their specified distillation conditions 0.6 mole of formaldehyde per mole of streptomycin. Dih-drostreptomycin, furthermore, yields 1.6 moles of formaldehyde per mole of this compound instead of the expected 1.0 mole. The accuracy of their method depends therefore on a very careful control of the time of distillation. Garlock and Grove ( 8 ) allow the dihydrostreptomycin to stand overnight n-ith sodium metaperiodate, destroy the excess periodate with sodium thiosulfate. and then distill. As the chromotropic acid method is one of the most rapid and reliable methods for determining formaldehyde, and periodic and iodic acids interfere positively with this procedure, a more convenient method for removing this interference from a reaction mixture T V ~ Ssought. Roberts and Bricker, in extending their method ( 2 ) for the determination of end unsaturation in organice 1
Present address, American Cyanamld Co., Stamford, Conn.
