Patent Application: US-201213354396-A

Abstract:
a carbon paste electrode is modified with a chemical agent that is selective for a plurality of lanthanides and actinides elements . the modified carbon paste electrode selectively has different voltages applied thereto where a first voltage is used to cause the deposition of one or more lanthanides or actinides from an industrial or environmental sample onto the electrode , and , subsequent to removal of the electrode from the sample and insertion into a second sample where concentration of lanthanides or actinides is preferred , a second voltage is used to cause the deposited lanthanides and / or actinides to be discharged from the electrode for concentration into the second sample .

Description:
the description below shows the fabrication and testing of certain exemplary electrodes and methods according to the invention . it will be recognized by those skilled in the art that the electrodes , the materials used for their fabrication , and methods of use can be varied within the spirit and scope of the appended claims . methods and systems for the electrochemical transfer of f - series element constituents present in a first medium to a second medium are described herein . a modified paste electrode can be utilized to accumulate f - series elements within the paste when the voltage applied to the electrode is held at a first voltage and subsequently released from the paste when the voltage applied to the electrode is held at a second voltage . the ability to accumulate f - series elements enables a range of applications wherein said elements can be transferred between media . in particular embodiments the transfer of said elements can serve to concentrate the elements ( e . g . when the volume of the second medium is less than the volume of the first medium or where the elements from plurality of first media are deposited in a common second medium ). an exemplary modified carbon paste electrode is schematically depicted in fig7 ac . in reference to fig7 a a typical implementation for a modified carbon paste electrode comprises a paste 703 on the surface ( portion of the component in diffusive communication with the media ) of a conductive element 702 encased within an insulating housing 701 , and a connector 704 operable to connect the conductive element 702 to an external voltage source . the implementation depicted in fig7 a provides an example wherein the surface or tip of the conductive element 702 is coplanar with the tip of the insulating housing 701 . other embodiments may comprise configurations where the tip of conductive element 702 and paste 703 are not coplanar . in particular embodiments the paste 703 may also be present in a void in the insulating housing 701 as depicted in fig7 b . in such implementations the surface of the conductive element may not extend to the tip of the insulating housing . fig7 b may also comprise an embodiment wherein the conductive element 702 is a porous material and the paste has been distributed into the pores of the conductive element 702 . in yet further embodiments , the tip of the conductive element 702 may extend further from the tip of the insulating housing 701 as depicted in fig7 c . common to all configurations is that the paste 703 is applied to the portion of the conductive element 702 that is in diffusive communication with the external medium ( not shown ). the insulating housing 701 may comprise any material that does not conduct electrical current . examples include , but are not limited to glasses , ceramics , polyethylene , crosslinked polyethylene ( either through electron beam processing or chemical crosslinking ), polyvinyl chloride ( pvc ), kapton , rubber - like polymers , oil impregnated paper , teflon , silicone , and / or modified ethylene tetrafluoroethylene ( etfe ). the conductive element 702 may comprise a material that can conduct electrical current . examples include , but are not limited to platinum , gold , silver , glassy carbon , brass , copper , graphite , porous graphite , and / or molybdenum , and combinations thereof . the paste 703 applied to the portion of the conductive element 702 that is in diffusive communication with the external medium is composed of three general components : a binder , a conductive component , and a modifier . the binder serves to adhere the paste 703 to the surface of the conductive element 702 and provide a fluid like medium to uniformly disperse the conductive component and modifier within the binder . traditional binders comprise organic liquids which link mechanically the conductive component and modifier . however , besides this main function , the binder as the second main moiety of carbon paste co - determines its properties . typical parameters required for binders are : i ) chemical inertness and electroinactivity , ii ) high viscosity and low volatility , iii ) minimal solubility in aqueous solutions , and iv ) immiscibility with organic solvents . example binding agents ( binders ) used for preparation of carbon pastes include , but are not limited to ; mineral ( paraffin ) oils ; namely , i ) nujol or a similar trade - mark product and solvent for spectroscopy ii ) uvasol iii ) aliphatic and aromatic hydrocarbons , including their iv ) halogenated derivatives , as well as v ) silicone oils and greases , or nearly solid silicone rubbers . the conductive component within the paste typically comprises a carbonaceous material . in particular embodiments powdered carbon ( graphite ) as the conductive component within the paste provides for proper function of an electrode or a sensor in electrochemical measurements . suitable carbonaceous materials should obey the following criteria : i ) particle size in micrometers , ii ) uniform distribution of the particles , iii ) high chemical purity , and iv ) low adsorption capabilities . naturally , the type and quality of graphite used , as well as its overall amount in the carbon paste mixture , are reflected in all typical properties of the respective mixture . a typical carbon powder comprises spectroscopic graphite with particles in the low micrometric scale ( typically , 5 - 20 mm ). alternatives to graphite include but are not limited to i ) soot and charcoal , ii ) acetylene black , iii ) glassy carbon powders with globular particles , iv ) pulverized diamond of both natural and synthetic origin , v ) template carbon , vi ) porous carbon foam , and vii ) carbon microspheres viii ) fullerenes , ix ) carbon nanofibers or various types of x ) carbon nanotubes . in general the conductive component is present in a concentration of between 5 g / ml binder and 0 . 2 g / ml binder within the paste . the modifier in the paste generally comprises an organic compound that contains a ( or a plurality of ) functional group that demonstrates a preference for ligating to f - series elements . in the preferred embodiment the organic compound has an affinity for a range of f - series elements . in yet further embodiments a plurality of organic compounds may be incorporated as modifiers wherein each organic compound provides a preference for binding a distinct subset of elements within the to f - series elements . examples of organic compounds include but are not limited to , 1 , 2 - dihydroxybenzene - 3 , 5 - disulfonic acid , 2 - hydroxyisobutyric acid , trimetaphosphoric acid , trans - 1 , 2 - cyclohexylenedinitrilotetraacetic acid , 2 - hydroxy - 2 - methylpropanoic acid , iminodiacetic acid , nitrilotriacetic acid , ethylenedinitrilotetraacetic acid , diethylenetriamine - pentacetic acid , 2 , 2 ′, 2 ″, 2 ′″-( 1 , 4 , 7 , 10tetraazacyclododecane - 1 , 4 , 7 , 10 - tetrayl ) tetraacetic acid , n -( 2 - carboxyphenyl ) iminodiacetic acid , dihydroxycyclobutenedione , 3 , 6 , 9 , 12 - tetrakis ( carboxymethyl )- 3 , 6 , 9 , 12 - tetraazatetradecane - 1 , 14 - dioic acid and derivatives of these compounds . derivatives comprise compounds with identical f - element ligation sites and a similar structural motif ( e . g . 2 - hydroxyisobutyric acid and 2 - hydroxybuteric acid ). in general the modifier ( or modifiers ) is present in a concentration of between about 0 . 1 mm and 20 mm within the binder . the concentration of modifier can be tailored to the f - element content within the intended operational media . fig8 provides an operational process for accumulating f - elements present in a medium and transferring the f - elements to a second medium . a modified paste electrode can be utilized to accumulate f - series elements within the paste when the voltage applied to the electrode is held at a first voltage within a first medium . implementations wherein the first medium is an aqueous medium the applied voltage required is about − 0 . 05 to − 0 . 05 v ( vs . ag / agcl ). the duration of time necessary for accumulation will vary with the specific implementation . once the f - elements are accumulated , the electrode can be transferred from the first medium to a second medium . an optional cleaning step may be added during this transfer ; here the cleaning may involve a physical or chemical cleansing of the body of the electrode . once the electrode is placed in the second medium , a second voltage is applied to release the f - elements accumulated in the paste . implementations wherein the second medium is an aqueous medium the applied voltage required is about 0 . 1 to 0 . 09 v ( vs . ag / agcl ). the specific voltage applied should be dependent upon both the operational media and the organic compounds present within the paste . more specifically , the accumulation voltage should be a voltage wherein the organic group is reduced and the release or stripping voltage should be at a voltage wherein the organic group is in it &# 39 ; s electrochemical ground state ( e . g . not oxidized or reduced ). the process of accumulation and stripping may be repeated or cycled between media . in a specific particular embodiment , lanthanide cations in solution can be rapidly sequestered onto and subsequently removed from a modified carbon paste electrode by cycling the voltage of the electrode . the electrode comprises a paste produced by mechanically mixing 5 grams of acheson 38 carbon with 3 milliliters of paraffin oil . prior to forming the paste with the carbon , the paraffin oil is modified by mixing approximately 5 millimoles ( 300 milligrams ) of alpha - hydroxyisobutyric acid ( hiba ) into the 3 milliliters of paraffin oil . the required range of concentration of hiba in paraffin oil is 0 . 1 - 20 mol / l . this paste is then applied to the end of a teflon electrode body . as a group of metal cations , the lanthanides accumulate from a solution of 0 . 1 m licl onto the carbon paste surface within 30 second when a voltage of − 0 . 4 v ( vs . ag / agcl ) is applied . the sorbed lanthanide cations can then be quantitatively stripped off the electrode surface into a different solution by applying an oxidative step of + 0 . 8 v ( vs . ag / agcl ). materials : reagent grade graphite , licl , paraffin oil and 2 - hydroxyisobutyric acid ( hiba ) were used as received , from fisher scientific , ( waltham mass . usa , www . fishersci . com ). for the multi - element analysis , a stock solution containing 10 ppm of analytes ( sc , y , la , ce , pr , nd , sm , eu , gd , tb , dy , ho , er , tm , yb , lu and th ) in 2 % hno 3 was purchased from high - purity standards , ( charleston s . c . usa , www . highpuritystandards . com ). this solution was used as received . electrode fabrication : procedures for preparation of the cp electrode were modified from adams . 26 in brief , 5 grams of acheson 38 grade graphite was mechanically mixed with 3 ml of paraffin oil using a glass mortar and pestle to produce a thick , uniform paste . to prepare the hiba - cp electrode , 5 mmoles of solid hiba were added to 3 ml of paraffin oil and mixed until the slurry was homogenous . then five grams of carbon were added and mixed to form the paste . procedures for smoothing and renewing the electrode followed those of adams , which is herein incorporated by reference . 26 electrochemical procedures : electrochemical measurements were made using a model 100b potentiostat , 15 ml teflon ™ electrochemical cell , and 3 mm cp teflon ™ electrode body purchased from bioanalytical systems inc . ( west lafayette indiana usa , www . basinc . com ). the 15 ml teflon ™ electrochemical cell was constructed and setup similarly to schumacher et al . 3 the reference and auxiliary electrodes were ag / agcl and pt wire , respectively . solutions within the electrochemical cell were purged with purified ar prior to conducting an experiment . cyclic voltammetric ( cv ) experiments were typically scanned at a rate of 100 mv / s starting at + 0 . 8 v to − 0 . 4 v . double potential step chronoamperometric ( dpsc ) and chronocoulometric ( cc ) experiments were stepped from + 0 . 8 v to − 0 . 4 v and back to + 0 . 8 v vs . ag / agcl . the background electrolyte was 0 . 1 m licl that was ph adjusted using 2 % hno 3 . pre - concentration and stripping experiments followed the procedures outlined in wang 27 , 28 with modifications . after a deposition step of 30 sec , the cp or hiba - cp electrode was removed from the cell , wiped with a kim - wipe on the insulating shroud and transferred to a separate vial containing 2 ml of 2 % hno 3 . a stripping step from − 0 . 4v to + 0 . 8v vs . ag / agcl for 30 sec , was performed and the solution analyzed by icpms . a typical experiment involving conditioning the cp or hiba - cp electrode , pre - concentration of the trivalent f - element and stripping into 2 % hno 3 required approximately five minutes . the cell was cleaned between experiments following the procedures of schumacher et al . 3 icpms procedures : inductively coupled plasma mass spectrometry ( icpms ) measurements were performed on an agilent 7500 icpms utilizing an internal indium and rare earth standard and scanned in the positive mode . prior to analyzing any samples , the instrument was calibrated with a set of prepared standards in 2 % hno 3 and plain 2 % hno 3 was used as the blank to correct for background . analyses of electroanalytical and icpms data were performed using graphpad prism version 5 . 02 for windows , ( graphpad software , san diego calif . usa , www . graphpad . com ). fig1 a shows the electrochemical response for a cp electrode to 0 . 1m licl at ph 3 . 5 . while this voltammogram is only the response to background electrolyte , a series of scans were performed in various solutions containing hiba and / or f - elements to determine if cp exhibited any electrochemical response . in these cases , the cp electrode showed no electrochemical response to dissolved f - elements . fig1 b shows the electrochemical response for a hiba - cp electrode to 0 . 1m licl at ph 3 . 5 ; the pka for hiba is 3 . 7 . 29 for this voltammogram , the scan rate was 100 mv / s and started at + 0 . 8 v . a large reduction and oxidation signal was observed for the hiba - cp electrode . each new hiba - cp electrode typically required two to three conditioning scans to achieve a stable ( exceeding three hours ) electrochemical response . variability of peak intensity between electrodes was less than 10 %, which falls within the range expected for mcp electrodes . 27 additionally , the cathodic and anodic peaks varied less than 5 % between electrodes for a given scan rate . this demonstrates that once conditioned , the hiba - cp electrode yields a stable and reproducible response . watanabe et al . 30 reported that f - elements will adsorb to carbonaceous material in acidic environments . our result for the cp electrode is not in disagreement with watanabe et al . as the contact time for their study was much longer ( 3 - 4 hours ) 30 than the time period used in this study ( 1 - 2 min ). fig2 shows a series of cvs for a single hiba - cp electrode in 0 . 1 m licl where the scan rate was varied from 1 to 500 mv / s . the measured ratio of cathodic ( i c ) and anodic ( i a ) peak intensities was constant across scan rates at 1 . 12 ± 0 . 03 ( n = 3 ) suggesting the electrochemical response from the hiba - cp is chemically reversible . the plots of scan rate , u , versus i p and v 1 / 2 versus i p were inconclusive as to whether the observed results represent an adsorption or diffusion phenomenon . this result is not surprising given the range of v used . the shapes of the voltammograms did not follow the nernst equation , suggesting that electrochemical charge transfer is irreversibile . to further evaluate diffusion vs . adsorption , chronoamperometric analysis of the voltammetric wave shapes were conducted using the cottrell equation : where i = current ( amps ), n = number of electrons , f = faraday constant ( 96 , 485 c / mol ), a = area of the electrode ( cm 2 ), c = initial concentration of the analyte ( mol / cm 3 ), d = diffusion coefficient for the species ( cm 2 / s ), and t = time ( s ) was used to evaluate the waveforms . a plot of t − 1 / 2 vs . i deviated from linearity based on time of exposure to the analyte , suggesting other processes were either occurring at the surface of the electrode or impeding diffusion to the electrode surface . 31 , 32 a series of experiments were performed at different values of ph and no ph effects were observed , suggesting that the observed phenomenon is occurring on the surface of the electrode and not a direct result of solution conditions . to determine the number of electrons transferred per mole of hiba , a solution containing 0 . 1 mm k 3 [ fecn 6 ] and 0 . 1 m licl , which has a known 1 e − transfer , [ fe ( cn ) 6 ] 3 − + e − ⇄[ fe ( cn ) 6 ] 4 − , was analyzed by cv on a 3 mm pt electrode to determine the i p and integrated voltammetric wave area . a separate solution containing 1 mm hiba and 0 . 1 m licl was analyzed by cv on the same pt electrode . ( fig3 ) comparison of the ratios of magnitude of i p and integrated voltammetric wave areas for hiba to k 3 [ fecn 6 ] indicated an electron transfer of 1 . 2 ± 0 . 2 electrons per mole of hiba . kvaratskhelia and kvaratskhelia 33 examined the voltammetric responses of hydroxycarboxylic acids in aqueous solutions using various solid electrodes . their e 1 / 2 values of the observed waves on pt in 0 . 1 m naclo 4 occurred in the range of − 0 . 47 to − 0 . 49 v vs . a saturated calomel electrode . our voltammetric response for hiba is in agreement with their observed results . in a mcp electrode study involving complexes between rare earths and alizarin , li et al . 16 reported a 2 e − charge transfer irreversible process for alizarin that was not ph dependent . they point out that most electrode processes of organic compounds involve proton ion transfers thus a ph dependence is expected ; however , in the case of alizarin this was not observed . our characterization of mba , a 1 e − irreversible process with no ph dependence , agrees with the characterization reported by li et al . 16 for alizarin . fig4 shows the resultant anson plot 34 for a three second chronocoulometric experiment using both a hiba - cp and cp electrode in 0 . 1 m licl and 1 × 10 − 5 m la 3 + . the reduction lines are plotted as t 1 / 12 vs . q and the oxidation lines are plotted as θ vs . q , where θ =[ τ 1 / 2 +( t − τ ) 1 / 2 − t 1 / 2 ] as defined by anson . 34 for hiba - cp the reduction and oxidation lines exhibit different slopes with an intersection above the x - axis . the cp reduction and oxidation lines have nearly identical absolute values of their slopes and the lines intersect on the x - axis . one second and five second chronocoulometric experiments were also performed with nearly identical results to the three second experiment ( data not shown ). this range of timescales were chosen to minimize contributions from convective mass transport , which occurs at solid electrodes at time periods greater than 5 seconds . 26 according to anson , intersection above the x - axis represents the total coulombs of adsorbed reactant because this analysis of chronocoulometric data effectively removes any contribution due to double layer charging . 34 , 35 using the intersection value , 1 . 1 ± 0 . 3 × 10 − 6 c , with faraday &# 39 ; s law ; the total number of moles accumulated equals 3 . 6 ± 0 . 7 × 10 − 12 , n = 3 . fig5 is the results of chronoamperometric experiments with a hiba - cp electrode in varying solution concentrations of la 3 + ( 10 − 7 m to 10 − 3 m ) with 0 . 1 m licl as the background electrolyte at ph 3 . 5 . the data points were obtained by running 1 second chronoamperometric experiments ( an experimentally convenient time interval ) and measuring a background corrected i value at 500 ms . this time span was chosen because it excludes distortion due to charging current and minimizes contributions due to convective mass transport . 36 a new hiba - cp electrode was used for each change in the concentration of la 3 + and each point represents the average of triplicate runs . the shape of the graph shows a concentration dependent signal suggesting a sorption phenomenon . fig6 shows representative results from the pre - concentration experiments . the multi - element standard used contained all the lanthanides , minus promethium , plus scandium , yttrium , and thorium . the four graphs represent the range of responses of the lanthanides and shows that the hiba - cp electrode will pre - concentrate above the limit of detection ( lod ) for the icpms while the cp electrode does not pre - concentrate under the same conditions as the hiba - cp electrode . the counts for all the lanthanides were totaled and applied to a calibration curve to determine total moles accumulated , 3 . 0 ± 0 . 6 × 10 − 12 , n = 3 . comparing this number to the total moles adsorbed via the anson plot in fig5 , 3 . 6 ± 0 . 7 × 10 − 12 we find good agreement indicating that the hiba - cp electrode accumulates individual lanthanides or a mixture with equal efficiency . interestingly , for sc , ce and th the hiba - cp electrode did not pre - concentrate above lod . a possible explanation for the case of sc is that while in the same group as la , sc responds in solution more as a d - element while y , which does pre - concentrate , responds more like an f - element . to gain some insight into the mechanism of hiba - cp pre - concentration , a comparison of stepwise formation constants ( log k ) values for hiba in 0 . 1m ionic strength from martell and smith 29 and the total amount of f - element pre - concentrated by the hiba - cp electrode was conducted . since hiba exhibits a systematic increase in log k values across the series of lanthanides , 37 one would expect that if hiba - cp pre - concentration capability was solely a function of hiba in the electrode , then a similar trend would be observed . while in general , heavier lanthanides pre - concentrated more readily than lighter lanthanides , no direct comparison could be made indicating that more factors are involved in hiba - cp pre - concentration capability and further work is required to elucidate these factors . while this work has been performed in a neat solution , interferences are expected since hiba complexes to some extent with most metal cations present in solution . while many factors affect the strength of metal - ligand complexes , a good first approximation for determining potential interferences are thermodynamic stability constants . nash and jensen thoroughly discuss the solution chemistry aspects of metal - ligand complexes and provide an excellent justification for the use of stability constants for initial approximations . surprisingly , while hiba has been in use since its first reporting in 1956 , relatively little critically reviewed thermodynamic stability constant data are available for metal cations other than the f - elements . 29 while no stability constant data exists for a li + - hiba complex , taking considerations of ionic charge , radius , and strength of ion - dipole interactions , we estimate that li + interactions with hiba are minimal , resulting in little interference . in our case , li + was in 100 . 000 - fold excess of trivalent f - elements and did not serve as a major interference . the experiments above shows that an mcp electrode can be used to selectively bind f - elements . in the preferred embodiment , the modifying agent is a chemical agent with a selective binding affinity for f - elements . these modifying agents are included in the cp at a level of at least 0 . 1 mmol / l ( e . g . 10 milligrams of hiba per liter of paraffin oil ) but less than or equal to 20 mmol / l ( e . g . 2 , 000 milligrams of hiba per liter of paraffin oil ). these modifying agents are included in the cp at a level of at least 0 . 1 mol / l but less than or equal to 20 mol / l ( e . g . less than or equal to 10 mol / l ). combination of the f - element concentration methods described herein with the lanthanide separation methods described in clark et . al . 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