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ANALYTICAL CHEMISTRY, VOL 50, NO. 6, MAY 1978
Analytical and Mechanistic Studies of the Electrochemical Reduction of Biologically Active Organoarsenic Acids Richard K. Elton and William E. Geiger, Jr." Department of Chemistry, Unwersity of Vermont, Burlington, Vermont 0430 1
The electrochemical reductions of dimethylarsinic acid (DMAA) and methane-arsonic acid (MAA) have been investigated by dc polarography, differential pulse polarography, controlled potential coulometry, and by analysis of bulk electrolysis products. The arsenicals are electroactive in aqueous buffers and in nonaqueous media in which the acidic supporting electrolyte, guanidinium perchlorate, is employed. A direct method of analysis, based on differential pulse polarography, is reported. Detection limits of roughly 0.1 pg/mL (for MAA) and 0.3 pg/mL (for DMAA) are achieved with nonaqueous electrolytes, and working curves are linear over at least 3 orders of magnitude change in concentration. A procedure for separately analyzing MAA and DMAA in solutions containing both acids is given, based on prior separation by ion-exchange chromatography. The mechanism of the reduction of DMAA in pH 4 buffer was studied, and dimethylarsine was the major product identified.
Redox processes involving organoarsenic compounds are known to be important in natural surroundings and in in-vivo processes. Perhaps t h e most important organoarsenicals in this connection are t h e methyl-substituted arsines and t h e oxides and acids derived from these arsines. h'ood ( 2 ) and Hrarnan (2) have recently pointed ut that these species are interrelated in nature by a series of oxidation-reduction reactions and biological methylation reactions in an apparently cyclic manner. Methanearsonic acid (MAA, 1) and dimethylarsinic acid (DMAA, 2 ) commonly known as cacodylic acid. play a significant role in this cycle as they are formed both by microbial action on inorganic arsenic oxides. and by atmospheric oxidation of' methylated arsines ( 3--6i. CH,AS(O)(OH)~ ( C H ,) I A s ( 0 ) ( O H ) 1 2 llnder aerobic conditions, 1 and 2 may also undergo oxidation and demethylation to form inorganic arsenate (As04"1 again (7). Very large amounts of 1 and 2 enter the environment each year because of t h e wide application of these compounds, or their simple salts, as herbicides and, less often. as pesticides
(8). Methods for analysis of trace amounts of arsenic are well established (9,. Only recently, however have there been a t t e m p t s to develop methods of analysis specific for certain orgnnoarseriicals. For reasons already cited, these efforts have been focused on t h e determination of 1 and 2, and have involved reactirig t h e acids to produce volatile arsine derivatives, which are then detrcted by methods such as gas chromatography ( 1 0 1 5 i , discharge o r microwat e emission spectroscopy ( I S , Id), or other methods ( Z f , 1 5 ) . These methods have generally relied on temperature-dependent volatilization procedures f'or separation of'the various arsine derivatives formed in t h e reactions. LVe report data in this paper showing that 1)IMand DMAA can be electrochemically reduced in both aqueous arid nonaqueous media, and a direct niethod of' analysis, based on 0003-2700/78/0350-07 12501.OO/O
differential pulse polarography, is reported. Aside from any analytical utility, t h e electroreduction of organoarsenic acids is also of interest from a mechanistic point of view, for pentavalent organoarsenicals are known to undergo reduction to lower valent arsenic species both in vivo (16)and in the environment(1, 2, 7). It was hoped t h a t t h e present study would shed light on t h e reduction pathways of pentavalent organoarsenicals under controlled conditions. A preliminary account of some of our findings has already appeared (17).
EXPERIMENTAL Reagents. (Caution: Volatile arsines like ASH,, As (CH,,,,
and the partially methylated homologues are extremely toxic materials. All experiments involving these species, including bulk reductions of 1 and 2, must be carried out in a well-ventilated hood with proper attention to safety procedures). Dimethylarsinic acid (Fisher), was recrystallized twice from absolute ethanol by slow evaporation at -15 "C, washed several times with cold methanol, and dried in vacuo at 100 "C, giving crystals with a melting point 199-199.5 "C (lit. 200 "C). Disodium , recrystallized from 4:l methanearsonate (DSMA, Alfa, 9 5 7 ~ )was methanol:water, washed with cold ethanol, and vacuum dried, melting point 134-140 "C (lit. 13C140 "C). Methanearsonic acid (MAA) was prepared by twice passing a 1.570 solution of recrystallized DSMA through a column packed with 500 mL of Fisher Rexyn l 0 l H strong acid cation-exchange resin at the rate of 1 mL/min. The solution was evaporated to 3 7 ~of the original volume, cooled, and the platelike crystals collected were recrystallized from ethanol, and dried at 100 "C in vacuo, and checked for purity by infrared spectrometry ( 1 8 ) . Dimethylarsine, (CH3)*ASH, and methylarsine, CH3AsH2,were produced by reduction of DMAA and MAA, respectively. The reductant was either sodium borohydride (14) or zinc amalgam (19). In each case the reduction was carried out under nitrogen, and the volatile arsines were swept out of the reaction vessel under nitrogen into a toluene trap kept at -5 "C for (CH3),AsH or -15 "C in the case of CH3AsH,. Tightly stoppered toluene solutions of the arsines were stored under N2 at --15 "C and used as gas chromatographic standards as needed. Tetramethyldiarsine, (CHJ4As2,was produced by the reduction of IIMAA with sodium hypophosphite in HCI (20). Typically, about 30 mg of dry DhIA.4 was placed inside a 5 m L , 3-necked flask. one neck of which was covered with a serum cap, with the other two were used to flush nitrogen through the cell. Ten milliliters of 6 M HCI containing 8 g N a H P 0 2 was added by sl-ringe, while stirring the reaction. The mixture turned white, then cleared after 10 min, leaving at the bottom tiny droplets of cacodyl which were cautiously removed by syringe and identified by NMR (2Z).Trimethylarsine (Orgmet Inc.) was used as received. Preparation of guanidinium perchlorate, (GP). C(NH,),C104, f'ollou-edthe recommendations of Breslow and Drury (22).The GP was crystallized from acetonitrile, filtered under nitrogen, and dried in vacuo at 36 "C. Tetrabutylammoniurn hexaf'luorophosphate (Bu,NPF,) was prepared by the reaction of ammonium hexafluorophosphate and tetrabutylammoniurn iodide in acetone. Addition of water precipitated the product, which was filtered and recrystallized three times from ethanol. It was dried in vacuo at 100 "C. Spectrograde acetonitrile (Eastman) and methanol (Fisher) were used without further purification. Absolute ethanol was prepared by refluxing ethanol for 24 h over magnesium turnings C 1978 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 50, NO. 6, MAY 1978
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1 D Figure 1. Cell used for the bulk electrolysis of dimethylarsinic acid and methanearsonic acid. The cell consists of an inert gas bubbler, A; a compartment, B, for the reference electrode; a dropping mercury electrode, C; a ground glass joint, D, to accommodate the bubbler; a vent, E: a platinum wire auxiliary electrode, F; ball joints, G, to connect to a constant temperature bath; an (optional)evacuated compartment, I; fine sintered glass frit, J; a mercury pool, K, connected to the potentiostat by a tungsten wire, L; a magnetic stirring bar, M; a cooling jacket, N; a 102175 ball joint, 0; and a two-way stopcock, P, to direct the inert gas stream either through the solution or over the solution. The cell cap contains 5 portals. They are a ground glass joint, Q, for the reference electrode compartment; a ground glass joint, R, for the bubbling device; a ground glass joint, S, for the auxiliary electrode: a portal, T, to vent the cell, and an opening, U, to accommodate the dropping mercury electrode
with a crystal of iodine added. It was then slowly distilled through a 12-inch column, and the middle fraction collected and stored over 4A molecular sieves. Distilled water was treated with several milliliters of 607~phosphoric acid per liter, to prevent codistillation of traces of amines, and distilled directly. the first 10% being discarded. Technical grade toluene was washed twice with concentrated H2SO4,once with 2070 Na2C03in water, and twice with water. After drying over MgSOI, the decantate was refluxed for 20 min over sodium with rapid stirring, then cooled, decanted, and slowly distilled directly using a 14-inch Vigreux column. Technical grade pentane (Fisher) was distilled through a 14-inch column. Practical grade m-xylene (Eastman) was purified according to the method of Matthews (23). It was then refluxed for 24 h over PZOj, and distilled at 137.5 "C. Britton-Robinson buffers (24)were made from boric acid, acetic acid, and phosphoric acid, all 0.04 hl, and titrated to the d value with 0.2 hl NaOH. More concentrated buffers con of the three acids at 0.25 M each, titrated with 1.6 M NaOH. A McIvaine buffer ( 2 5 ) was prepared using 0.1 M citric acid (Eastman) adjusted with 0.2 M S a 2 H P 0 4 . Ion-exchange experiments utilized Fisher Rexyn 101 cation-exchange resin. Instrumentation. The Princeton Applied Research Corp. Models 173 and 174 potentiostats were used for all electrochemical experiments. Coulometry w'as performed by manually integrating the recorded current-time curves. Recorders and other equipment were as previously noted (26). For gas chromatographic analysis of arsines produced in the electrolysis, an Aerograph Hi-Fy model 600D gas chromatograph equipped with a flame ionization detector was used. CH3AsH2 and (CH3),AsH were analyzed on either 10% silicone rubber SE-30 on Chromasorb 101 or 20% silicone Dow 11 on Firebrick (6-ft X '/s-inch, col. temp. 40-70 "C). (CH314As2was analyzed on 2 7 ~
Figure 2. Trapping system used in the identification of volatile electrolysis products, consisting of a water vapor trap. A; a cold toluene trap, 8 ; and a scrubbing tower, C. The system contains two -15 O C ice-acetone baths, E and G; toluene, F; a side arm for the removal of chromatographic samples, H; a chromic acid oxidizing solution, I; and minimal amounts of tygon tubing, J. to connect glassware. In mass spectral analysis, the trap, D, is used in place of 8 . This trap is kept at - 129 OC in a frozen pentane slush. K; and a 10130 ground glass joint, L, fits the inlet chamber of the mass spectometer. The water vapor trap was not employed in chromatographic experiments
Carbowax 20M on Firebrick at 75-100 "C. Helium flow rates of 20--30 mL/min were employed. and sample sizes were about 5 p L , For mass spectrometric analysis of effluents from bulk electrolysis solutions, a Hitachi Perkin-Elmer KMU-BD mass spectrometer was employed. The arsines were trapped as described and kept at -15 "C until opening the sample to the mass spectrometer chamber. Electrolytic Generation of Arsines. Bulk electrolysis experiments were conducted in the cell depicted in Figure 1. The design follows the recommendations of Harrar and Shain (27) for ideal potential control and low potential gradient across the working electrode surface. These precautions were necessary because of the close proximity of the arsenical wave to that of the background discharge. The solution. stirred by a magnetic stirring bar, M. is electrolyzed at a mercury pool working electrode. K. The platinum wire auxiliary electrode. F. is housed in a separate compartment. 1. contained hy a large fine frit that is located directly over the working electriide. A reference electrode is inserted into a separate chamber. F3, which contains a small fine frit, .J, that is placed in t h e region between the auxiliary and working electrodes. Polarography may he prrformed h y the insertion of a dropping mercury el
