Polarographie Determination of Dialky Idit hiophosphates R. F. MAKENS, H. H. VAUGHAN', Department
and
R. R. CHELBERGl
o f Chemistry and Chemical Engineering, M i c h i g a n College o f M i n i n g and Technology, Houghton, M i c h .
Six sodium dialkyl dithiophosphates salts, ethyl through butyl, are determined polarographically. Curves of current versus concentration are linear up to 10-3M. The anodic half-w-ave potentials become increasingly negative with respect to the saturated calomel electrode as the size of the alkyl group increases. Information concerning the electrode reaction was obtained from the relation between the limiting current and the half-wave potentials at various concentrations of the same salt and by separate experiments, wherein products could be recovered, using a mercury macro electrode at the same half-wave potentials found in the polarographic work. The probable electrode reactions are the oxidation of the mercury to the mercurous ion and a reaction of this ion with the dialkyl dithiophosphate ion to produce free mercury and the mercuric salt. A polarographic determination of a dialkyl dithiophosphate was made at a micro platinum anode and again at a platinum macroelectrode, using the same half-wave potential. Bis-(dialkyldithiophosphoryl) disulfides are formed at this anode.
A
METHOD for the polarographic determination of dialkyl dithiophosphates b y Oxidation a t the dropping mercury anode is described and possible electrode reactions are considered. These dithiophosphates are used in dilute solutions for ore flotation ( 2 , '7)) insecticides (6),and oil additives (1). T h e polarographic behavior of the dialkyl dithiophosphates has not previously been described. EXPERIMENTAL
Apparatus. A Sargent Model XI1 photographic recording polarograph was used with a n H-type cell (11) immersed in a constant temperature bath a t 25.0' C. The capillary for the dropping mercury electrode had an m2'3t1'6value of 2.061 mg.2'3 sec.-1'2 a t -0.15 volt us. saturated calomel electrode (S.C.E.)in the supporting electrolyte used for the analysis. A Leeds and Northrup Type K potentiometer with a Type R mirror galvanometer served for instrument calibration. For a supplementary experiment designed to give additional information on the nature of the electrode reactions, a stationary platinum microelectrode as described by Muller (13) replaced the dropping mercury electrode. The cell I R drop was measured m-ith a n impedance bridge from the mercury well through the saturated calomel electrode. Equipment for the controlled potential synthesis of macro quantities of product, a t the half-wave potentials observed in the polarographic work, I\ as similar to that described by Lingane ( I O ) , except t h a t a porous cup was used t o separate the t u o electrode compartments. A similar arrangement was employed, with the indicated change of electrode, for the electrolysis a t a macro platinum anode. Reagents. The free diethyl, di-n-propyl, diisopropyl, diisobutyl, di-n-butyl, and di-sec-butyl dithiophosphoric acids were all freshly prepared starting with the desired alcohol and phosphorus pentasulfide (3,4, 1 6 ) , all of which distilled a t 3-mm. pressure (8). The di-n-butyl and di-sec-butyl compounds, however, decomposed excessively; hence their sodium salts were prepared from crude acids. T h e free acids were converted to their respective sodium salts by the addition of a n equal volume of anhydrous diethyl ether and subsequent treatment with a n excess of anhvdrous sodium carbonate. T h e sodium carbonate was added slowly with stirring to avoid excessive frothing. After reaction ceased, the volume was tripled by the further addition of anhydrous acetone. The product was filtered through a large sintered-glass funnel and the Present address, Jones and Laughlin Steel Gorp., Segaunee, Blich. Present address, Michigan College of Mining and Technology, So0 Branch, Sault Ste. Marie, Jlich. 1
2
filtrate was evaporated a t 50-mm. pressure a t 45' to 50" C. Large crystallizing dishes were used t o provide shallow layers. T h e solid products were washed with petro!eum ether and dried overnight in a vacuum desiccator. These salts should be protected from moisture during preparation or a gummy product will be obtained. Analyses for sulfur and phosphorus are shoiT-n in Table I.
Table I.
Analyses of Sodium Dialkyl Dithiophosphates and Disulfides for Phosphorus and Sulfur S %70 p, % Compound
Analyzed Found Theory 30.89 30.79 Diethyl salt 26.52 27.11 Di-n-propyl salt 26.95 27.11 Diisopropyl salt 24.08 24.06 Di-n-butyl salt 23.72 24.06 Diisobutvl salt 24.06 Di-see-b;tyl salt 23.78 Disulfide from diisopropyl salta 29.68 30.08 34.69 Disulfide from diethyl saltb 31.73 a Bis-(O,O-diisopropyl dithiophosphoryl) disulfide. b Bis-(0,O-diethyl dithiophosphoryl) disulfide.
Found 14 75 13.23 13.40 11.77 11.80 11.79 15.01 16.73
Theory 14.90 13.11 13.11 11.68 11.68 11.68 14.46 16.75
Supporting Electrolyte. Perchloric acid, 0.10005, was used as a supporting electrolyte. All solutions were made t o contain the same concentration of this reagent. Compounds Prepared for Comparative Purposes. Certain compounds were prepared for identification of their properties with those of compounds isolated a t the macro mercury and platinum anodes. Bis-( 0,O-diisopropyl dithiophosphoryl) disulfide was prepared by oxidizing the sodium salt with iodine in potassium iodide solution. Upon recrystallization of the resulting solid from ethyl alcohol the product melted a t 93.8" to 94.6" C. Determination by the Cottrell method gave a molecular weight of 418 as compared with 426.58 for the theoretical value. The corresponding ethyl compound was prepared by reaction of sodium diethyl dithiophosphate with copper sulfate, whence the disulfide was extracted from ether and dried. The ethyl bis compound is a nearly nonvolatile oil. T h e molecular weight \vas determined as above to be 382 while the theoretical value is 370.46. The analysis calculated for (CZHjO),P(S,SS(S)P(OC2H5h: carbon, 25.92; hydrogen. 5.44; found carbon, 26.03; hydrogen, 5.54. Analyses for sulfur and phosphorus on the t p o compounds prepared are shown in Table I. Xercuric di-n-butyl dithiophosphate was prepared (12) by reaction of the mercuric ion with the sodium salt and was found t o have a melting point of 60.0" t o 61.5" C. The n-propyl compound was prepared in a similar manner. The melting point was 6 i . 3 " to 69.6" C. x i t h indications of decompoqition. T h e same t n o mercuric dialkyldithiophosphates n ere made by reacting mercurous nitrate in 0.1N perchloric acid with the sodium dialkyl dithiophosphate, the latter in equivalent amounts or in slight excess. I n the latter case, the products are gray solids, easilr separable by acetone extraction into equivalent amounts of free mercury and the corresponding mercuric dialkyl dithio-SP(S)(OR)2 -+ H g phosphate. T h e reaction is: H e + + Hg [SP(S ) ( O R121 2. Polarography of Dialkyl Dithiophosphates. -4n H-cell (11) holding 30 ml. of a solution and containing the desired weight of the sodium dialkyl dithiophosphate in O.lOOOLVperchloric arid was deoxygenated by a stream of purified nitrogen and a polarogram made in accordance with the procedure recommended for the unit used (17). Voltage settings obtained from the camera ring scale were checked with the Type K potentiometer. The half-wave potentials were determined also for two dialkyl dithiophosphates a t 1 X lO-3M using the stationary platinum electrode as described by Muller (13)in place of the dropping mercury electrode. Other equipment and conditions were the same as for previous runs.
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Electrolysis with Macro-Size Anodes. The electrolysis of dialkyl dithiophosphates using relatively large electrodes (10) was
1062
1063
V O L U M E 27, NO. 7, J U L Y 1 9 5 5 made to obtain anodic reaction products in quantities large enough for examination. The cathode was, in each instanre, smooth platinum, separated from the anode by a porous cup. T h e anodes were either mercury or smooth platinum. Voltage measurements of both cathode and anode potentials versus the saturated calomel electrode were made potentiometrically, with a gradual increase of the applied voltage until the anode was a t the half-wave potential determined polarographically for the concentration of dialkyl dithiophosphate and elect,rode used. A411 determinations were made in 0.1X perchloric acid. Osygen was removed with nitrogen as was done in the polarographic determinations. Stirring was performed mechanically. Products xvere recovered by filtration through fritted-glass crucibles, or, in the case of the bis-(O,O-diethyl dithiophosphoryl) disulfide, formed a t the platinum anode by extraction of the aqueous solution with ether. From Figure 1 and Table 11, a linear relationship is seen to esist up to 1 millimole per liter for the polarographic determinations ( 1 4 ) . Only the normal salts are plotted in Figure 1 since the isopropyl salt behaves Pimilarly t'o tmhen-propyl salt and the see-butyl and the isobutyl salts have nearly the same values as shown for the n-butyl salt. I n Figure 2 it is shown that the log i d z's. E12 is linear for the polarographic determination of sodium diethyl dithiophosphate. Edslierg ( 6 ) has shown that El!* = E' - 0.03 X 2 log id for the oxidation of 2 Hg+Hg?+" or a slope of approximately 0.06. T h e value of the slope determined in Figure 2 is 0.068. I n consideration of the products formed a t a mercury macro-anode, of the reaction observed n-hen mercurous ion is added t o a solution of a soluble dialkyl dithiophosphate, and of the information obtained from Figure 2, it would appear that, the following sequence of reactions would best account for the behavior observed: 2Hg HgZ"+
+
Hg?++
+ -SP(S)(OR),
-+
Hg
+ 2e-
+ Hg[SP(Sj(ORj?]2
It does not appear possible t h a t the dropping mercury electrode is behaving like an inert electrode, as a very different
0
Table 11. Polarographic Data on Sodium Diethyl Dithiophosphate 1Iillimoles per Liter 0.106 0.333 0.666 0.833
id, r a .
0.73 1.39 2.83 3.36 4.17 6.37 12.38 14.60
1.000
a
2.000 4.000 5,000 Corrected for cell I R drop.
E l / * u s . S.C.E., volt5 -0.010 -0.028 -0.052 -0.056 -0,062 -0.075 -0.097 -0.102
on-BUTYL SALTS
id/ em.?1st'
'8
2.12
2.01 2.03 1.95 1.98 1.52 1.51 1.45
Table 111. Polarographic Data on Dibutjl Dithiophosphates 1\Iillimoles per Liter 0.167 0.333 0,500 0.667 0,833 1,000
Isobutyl,
sec-Butyl, ~ dr a , .
2dr
0 18 0 GO
5.000
7 , r~ a .
...
0.20 0,63
0.99 1.54 1.89 2.24 4.47 10 32
0.90
0.60
0.98 1.48 1.93 2.48 4.88 13 56
10.000
n-Butyl,
ra.
1.50 1.91 2 25 3.52 8 57
At the platinum macroelectrode an oil vas produced v, hen a 10-3M sodium diethyl dithiophosphate n as electrolyzed a t a potential of 0 70 with respect to the saturated calomel electrode. The sodium diisopropyl dithiophosphate 1% as ovidized similarly. A white solid formed hich n as filtered, n ashed M ith mater, and
:i
ETHYL SALT
9 n-PROPYL SALT
2ob
result was obtained with the substitution of a flowing junction type of micro platinum electrode made in accordance with the directions of Muller ( I S ) . The 23112 us. the saturated calomel electrode shifted from -0.062 volt for the dropping mercury electrode to +0.696 volt for the micro platinum electrode when a l O - 3 M solution of sodium diethyl dithiophosphate was electrolyzej. Similar behavior was observed by Kolthoff and Barnum ( 9 ) for cystein oxidation a t a dropping mercury and a t a platinum electrode.
0
IO
8.0
0.2 0.15
0.5
I
Figure 1.
' '
0
I I I
I
I1111111 5.0 iao
.01
0.5 10 MILLIMOLES PER LITER
Relation of current to concentration of sodium dialkyl dithiophosphates
.05
.03
. .07
.09
.I I
EI,~ IN VOLTS Figure 2.
id cs. El/? of sodium diethyl dithiophosphate
Log
6
.I3
~
’1064
ANALYTICAL CHEMISTRY LITERATURE CITED
dried over fresh barium oxide. The melting point was 89.0” to 92.0” C. Upon recrystallization from ethyl alcohol the melting point was redetermined as 92.5’ to 93.8” C. A mixed melting point with the bis-(O,O-diisopropyl dithiophosphoryl) disulfide prepared previously by direct oxidation with iodine produced no change in melting point. It) would appear t,hat the reaction a t the micro platinum electrode is: 2-SP(S)(OR)* -+ (RO),P(S)SS[S]P(OR)2 2eThe El;?us. saturated calomel electrode values for the dialkyl dithiophosphates become increasingly negative as the molecular weight increases. At 0.5 X 10-3111 t’he Eli2 values determined were for isopropyl, -0.104 volt; n-propyl, -0.131 volt; secbutyl, -0.203 volt; isobutyl, -0.200 volt; and n-butyl -0.204 volt. This increase would be expected if insoluble precipitat,es of increasingly lower solubilities were being formed (15). At’ concentrations up to 1 X 10-3M the i d values for all the butyl salts studies are nearly identical, with deviation becoming apResults are parent only at concentrstions above 1 X shown in Table 111.
American Cyanamid Co., Brit. Patent 588,090 (May 14, 1947). Buchanan, G. H., Mining a n d Met., 11, 567 (1930). Buchanan, G. H., U. S. Patent 1,868,192 (July 17, 1933). Carius, L., Ann., 112, 190 (1859). 26, 724-6 (1954). Edsberg, R. L., AXAL.CHEST., Gaines, J. C., J . Econ. Entomol., 46, 896-9 (1948). Gaudin, A. M., “Flotation,” IIcGraw-Hill, New York, 1932. Kabachuik, RI. I., and Rlastruykova, T. A , Bull. Acnd. Sci. C.S.S.R., NO. 4, 727-35 (1952). Kolthoff, I. h l . , and Barnum, C., J . Am.Chem. Soc., 62, 3061 (1940). Lingane, J. J., Ibid., 67, 1917 (1945). Lingane, J. J., and Laitinen, H. A.. IND. ExG., Cmnr., ASAL. ED., 11, 504 (1939). Mastin, T. W., Norman, G. R., and Weilmuenster, E. -L,,J. Am. Chem. Soc., 67, 1662 (1945). Muller, 0. H., Ibid., 69, 2992 (1947). Muller, 0. H., “Polarographic Method of Analysis,” 2nd ed., p. 69, Chemical Education Publishing Co., Easton, Pa., 1951. Revenda, J., Collection Czechosloa. Chem. C o m n ~ u n s . ,6, 453 (1934). Romieus, C. J., and Wohnsieddter, H. P., U. S. Patent 1,748,619 (Feb. 25, 1930). Sargent and Co., E. H., Chicago, Ill., “AIanual of Instructions for Sargent Model XI1 High Speed Photographic Polarograph.”
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ACKNOWLEDGMENT
T h e authors wish t,o express appreciation to Dale McCowen and Gary Babcock for repeztirig part of the experimental work described.
RECEIVED for review September 13, 1954.
Accepted hlarch 7, 1955
Polarographic Measurement of I-Noradrenaline and I-Adrenaline JOANNE HENDERSON’ and A. STONE FREEDBERG Department o f M e d i c i n e , Harvard M e d i c a l School, M e d i c a l Research Department, Yaminr Research Laboratories, and M e d i c a l Service, Beth Israel Hospital, Boston, Mass.
This work was initiated to meet the need for an accurate and simple method for the measurement of Z-adrenaline and Z-noradrenaline. The compounds were converted by iodate oxidation to their derivatives iodoadrenochrome or iodonoradrenochrome. Aliquots of the reaction solutions were measured polarographically. Half-wave potentials for iodoadrenochrome and iodonoradrenochrome were found to be Eli2 = +0.03 volt and El!?= $0.02 volt, respectively, in 0.1M acetic acidacetate buffer pH 4.52, 0.01% in gelatin. A linear relation between concentration and diffusion current was found over the investigated range of the equivalent of 1 to 50 y of adrenaline or noradrenaline. Hapid and accurate routine analysis of these amines can be carried out by the described method.
S
TLTDIES of adrenal medullary activity in man have been handicapped by the lack of a n accurate and reasonablv simple method for the measurement of adrenaline (epinephrine) and noradrenaline (Arterenol). The bioassay method developed b y von Euler ( 5 ) is carried out on two isolated or intact animal organs. The quantity of adrenaline and noradrenaline in an unknown mixture is calculated from the difference in response of the hen’s rectal cecum or rat uterus and the cat’s blood pressure, as compared with the response to known amounts of the two amines. The chemical methods now available have not been entirely satisfactory. Early emphasis on the colorimetric measurement of various derivatives of the amines ( 1 , 5 , 13) has been superseded by more sensitive fluorimetric techniques (9, 11). During the course of these studies, the publications of Keil-lfalherbe and Bone ( 1 4 ) and Manger and others (12) have come to the authors’ attention. They have not had the opportunity to examine this 1
Present address, Arthur D. Little Co., Inc., Cambridge, Mass.
method. Although the sensitivity has thus been increased, it has not been conclusively established t h a t the fluorescence measured is entirely t h a t of the adrenaline or noradrenaline derivatives.
M,
H
A
B Figure 1.
C
D
Structure of amines
A.
Adrenochrome E . Noradrenochrome C . Iodoadrenochrome D. Iodonoradrenochrome
The present study is concerned with the application of the polarographic method t o the measurement of adrenaline and noradrenaline. The catechol nucleus of adrenaline is oxidized at a potential so positive t h a t its measurement a t the dropping mercury electrode would not be practical. However, p - and oquinones are easily reduced and produce well defined, reversible polarographic waves which bear a linear relationship between concentration and diffusion current. Adrenochrome, the oquinone of adrenaline, although reversibly reduced over a wide range of pH, as shown by Wiesner (15),is unstable and eventually precipitates as melanin. Under suitable conditions, adrenaline is converted quantitatively to a stable derivative, iodoadrenochrome (Figure 1). Since this compound has the same o-keto structure as adrenochrome, it was expected that this derivative would produce a polarographic wave a t a similar potential. T h e present study presents a modification of the iodate oxidation described by Bouvrt (3) for the conversion of adrenaline to iodoadrenorhrome. The method has been extended to the prep-
