determination of arsenic by cathodic stripping try with a hanging mercury drop electrode

8/2/2019 Determination of Arsenic by Cathodic Stripping try With a Hanging Mercury Drop Electrode

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2189nal. Chem. 1980, 52 , 2189-2192good flatness of response. Rh6G is spectrally somewhat flatterin response than Rhl lO, but it is appreciably (64 times) lesssensitive.

Surprisingly, RhB, one of the best quantum counters wehave observed in methanol, yielded one of the poorest polymersystems; its spectral response is highly depen dent on con-centratio n. Th e only reason for using i t in film counters isits wide availability. Ghiggino has, however, reported RhBin PVA (17 wt %) to be flat to h670 over the range 250-530nm which makes it currently the only film counter calibratedinto the further UV.For t he short wavelength region (


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2190 ANALYTICAL CHEMISTRY, VOL. 52, NO. 13, NOVEMBER 1980cathodic stripping technique for arsenic with a hangingmercury dr op electrode.

E X P E R I M E N T A L S E C T I O NAppara tus. Voltammograms were obtained with a PrincetonApplied Research Model 174polarographic analyzer with a Model303 static mercury drop electrode. The electrochemical cell withthis unit was equipped with a silver-silver chloride referenceelectrode, platinum wire counterelectrode, and the workingelectrode which can be used either in the hanging mercury dropmode or as a controlled-drop dropping mercury electrode. There

is a provision for purging the cell with nitrogen and then directingthe nitrogen flow above the solution. The voltammograms wererecorded on a Houston Instrument Omnigraphic 2000 X-Y re-corder.Reagents. All chemicals were reagent grade unless otherwisespecified. Standard arsenic stock solution containinglo00pg/mLAs(II1) was prepared by dissolving 1.320g of arsenic trioxide ina minimum amount of 20% (w/v) potassium hydroxide solution.The solution was acidified to pH 2 with 20% (v/v) sulfuric acidand diluted to 1 L. Working solutions of the desired arsenicconcentrations were prepared from the stock solution daily.Selenium solution containing lo00 pg/mL Se(1V) was preparedby dissolving 1.000g of selenium powder in a minimum volumeof nitric acid, evaporating to dryness, adding 3 mL of water, andevaporating to dryness again. The addition and evaporation ofwater was repeated twice. The residue was dissolved in 10% (v/v)hydrochloric acid and diluted to 1L with 10%hydrochloric acid.Further dilutions were made as required.Cath odic S tr ip pi ng Procedur e. Voltammograms were ob-tained in 0.36N sulfuric acid containing 50 pg/mL selenium(1V).Additions of arsenic working standards or selenium solutions tothe cell were made with an Eppendorf pipet . Nitrogen wasbubbled into th e solution at such a rate as not to distu rb thehanging mercury drop electrode. After 4min the potential of -0.50V was applied for 1.5 min, and then nitrogen was directed to sweepabove the solution. Bubbling of nitrogen into he solution removedoxygen and provided the required stirring. The potential wasswitched to -0.60 V for 15 s, and the linear scan in the cathodicdirection was commenced at a rate of 50 mV/s.Determination of Arsenic in Orch ard Leaves by CSV.Sample (10.3g) was accurately weighed into a Teflon decom-position vessel (6), mL of nitric acid was added, and the vesselwas closed and placed into a 150 "C oven for 2 h. After allowingthe vessel and contents to come to room temperature, the samplewas transferred to a 10-mL volumetric flask with the aid of water.An aliquot (1 mL) was added to a 50-mL borosilicate beaker and1mL of magnesium nitrate (75 mg of MgO/mL in dilute "OB)was added. The solution was heated to dryness on a hot plateand was then ashed in a muffle furnace at 450 "C for 30min. Theresidue, consisting mainly of magnesium oxide, was moistenedwith 2 mL of water and dissolved by adding 0.1-mL incrementsof 18N sulfuric acid. The solution was then made weakly alkaline(pH 8) by dropwise addition of ammonia water and was trans-ferred to a small separatory funnel. The beaker was washed witha minimum amount of water, and the wash water was added tothe separatory funnel. The solution was shaken with 2 mL ofdithizone in carbon tetrachloride (10pg/mL) for 1or 2 min. Afterthe separation of the aqueous layer the carbon tetrachloride layerwill be green or colorless in the absence of trace metals. If thecolor is pink, red, orange, etc., in the carbon tetrachloride layer,there are trace metals present. The organic layer was drainedoff, and the extraction was repeated with an additional 2 mL ofdithizone solution until the carbon tetrachloride layer was greenor colorless. The aqueous solution containing arsenic(V) wastransferred to a 5-mL beaker; the funnel was rinsed with aminimum amount of water , and the rinsings were added to thebeaker. Then 1 mL of nitric acid was added, and the solutionwas evaporated to dryness on a hot plate at low heating settingsso as o avoid bumping. Finally, the temperature was raised untilwhite fumes appeared. The residue, now consisting mainly ofmagnesium sulfate, was dissolved in 2 mL of 5 N hydrochloricacid. Arsenic(V) was reduced to arsenic(II1) by adding 0.5 mLof 85% (w/v) sodium bromide solution and 0.1 g of hydrazinesulfate, covering the beaker with a watch glass, and heat ing ona steam bath for 30min. The solution was transferred to a 10-mL

$1) /-02 -0.4 -0.6 -an -1.0

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Flgure 1. Effect of selenium(1V)on the electrochemical behavarsenic(II1)at HMDE: (1 ) = 0.1 N H,SO,; (2) = (1) + 2. 0 pg/m(3) = (2 ) + 0.2 pg/mL Sew; (4 ) = (3) + 2.0 pg/mL As'"; Scan20 mV/s.volumetric flask with the aid of water and diluted to volCathodic stripping voltammetry was performed on an aliquoutlined above except tha t deposition and equilibration carried out at -0.4 and -0.5 V vs. Ag-AgC1, respectively. centration of arsenic in the sample was determined frombration curves prepared by adding arsenic(II1) standards tblank th at had undergone the same treatment as the sam

R E S U L T S A N D D I S C U S S I O NAttempts to deposit arsenic on a hanging mercury electrode from 0.1 to 1.0 N sulfuric or hydrochloric ac

th e absence of selenium(IV) were only part ly successful,no consistent signals could be obtained. Thi s could beto t he close proximity of th e two reduction waves of arso tha t some reduction to arsine would occur during thposition step. These difficulties were overcome by the addto th e solution of a trace of selenium(1V). In an acid medselenium(n7) is reduced to selenium(-11), accompanied bformat ion of mercuric selenide at about -0.1 V, followethe reduction to mercury and hydrogen selenide a t aboutv (7).Figure 1 illustrates t he effect of selenium(1V) on th e trochemical behavior of arsenic(II1). Th e voltammogshown in Figure 1 are d c voltage scans of 0.1N sulfuricto which arsenic(II1) or selenium(1V) was added. Scanthat of 0.1 N sulfuric acid. Scan 2 is that of 2 pg/mLsenic(II1). Th e reduction t o arsenic(0) begins at aboutV, followed by the reduction to arsine. Th e peak at -0.is due to th e reduction of arsenic(O), which deposited ohanging mercury drop electrode. Scan 3 is that of a solcontaining 2 pg/mL arsenic(II1) and 0.2 pg/mL seleniumDue to the presence of selenium(1V) th e apparent reduof arsenic(II1) to arsenic(0) is shifted toward a more pospotential, beginning at about -0.25 V, and the peak a t -V is greatly increased. Th e peak at -0.58 V is due treduction of deposited mercuric selenide to mercury(0hydrogen selenide. Scan 4 is tha t of 4 pg/m L arsenic(II10.2 pg/m L selenium(1V). Th e same behavior was obseafter th e solution containing selenium(1V) was electrolto deposit mercuric selenide on the HMD E an d th e solwas replaced with the one containing arsenic(II1) buselenium(1V) and th e experiment repeated with t ha tHMDE. Furthermo re, th e same general behavior wasobserved in 0.1-1.0 N hydrochloric acid and perchloric a

It is evident from Figure 1 hat the deposition potentiarsenic is critical an d should be between -0.4 an d -0.50.1 N sulfuric acid. At a potential below abou t -0.25

hy muit vt kimbngzon trong


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ANALYTICAL CHEMISTRY, VOL. 52, NO. 13, NOVEMBER 1980 2191

80 10 20 30 40 50 60 70 80 90

Sdm*lrnjlV)Cmcmntntlm, q m LFigure 2. Effect of selenium(1V) concentration on arsenic(II1) cathodicstripping peak height: (1) 25 ng/mL and ( 2 ) 250 ng/mL As"', in 0.36N H,SOd.reaction occurs, and at a potential greater than -0.58 Vmercuric selenide is reduced to mercury(0) and hydrogenselenide. For selenium to have the described effect on theelectrochemical behavior of arsenic, i t is necessary thatmercuric selenide be present on th e working electrode. Therequired deposition potential in a given supporting electrolytecan best be found by experiment.

Figure 2 shows th e effect of selenium(1V) concentrat ion onthe cathodic stripping cu rrent of two different arsenic(II1)concentrations. Arsenic cathodic stripping peak heights in-crease to ab out 25 ng /mL selenium(1V); then higher con-centr atio ns of selenium(1V) have no effect until about 400ng/ mL selenium concentration when arsenic peaks becomedisto rted . On the basis of this finding, the selenium con-centration utilized to carry out arsenic cathodic stripping was50 ng/mL . It is worthwhile to note t ha t as arsenic(II1) con-centration is increased, the peak current due to t he reductionof mercuric selenide decreases.

In view of the above observations, a probable explanationof the role of selenium(1V) is as follows.At the potential between about 4 . 25 and -0.5 V, arsenic(II1)reacts with the deposited mercuric selenide, forming arsenicselenide and mercury.

2As3++ 3HgSe + 6e- - szSes+ 3Hg (1)Thi s would explain the ap paren t shift toward the positive

potent ial of the first arsenic wave. At the potential of about-0.72 V, arsenic selenide is reduced, resulting in arsine andhydrogen selenide.

As,Se, + 12e- + 12Hf - AsH3+ 3H,Se (2)I t is only necessary to perform cathodic scans in the usualmanne r, tha t is, deposition followed by equilibration a t the

same potential. Since in the present procedure, mercuricselenide strips ju st before arsenide, some overlapping of th etwo peaks occurs. To overcome this difficulty, the voltageduring the equilibration period was set at th e point a t whichmercuric selenide would be strip ped, thus eliminating theundesired current during the subsequent voltage scan.Figures 3 an d 4 how cathodic stripping voltammogramsof arsenic(II1) concentrations from 2 to 40 ng/mL obtainedin 0.1 N sulfuric acid in the presence of 50 ng/mL selenium-(IV) according to th e experimental procedure. The peakcurrents were measured from the base line established by thesuppor ting electrolyte. Th e detection limit depends on suchparameters as t he deposition time, stirring rate, an d the areaof the working electrode as well as the insolubility of arsenicselenide in the supporting electrolyte. The detection limit

I 1

I-0.6 -0s -1.0I . W n A#-AgCI

E)lgure 3. Arsenic(II1) cathodic stripping scans: (1) 0.36 N H2S04;2 ng/mL As"'; (3) 4 ng/mL Astn; (4)6 ng/mL As"'; (5) ng/mL As .

0.05uAI

r-0.6 -as -1.0E, Y V A I - A gC I

Figure 4. Arsenic(II1) cathodic stri ping scans: (1) 0.36 N H2S04; (2)10 ng/mL As"'; (3) 20 ng/mL As1! (4) 30 n glm L As"'; (5) 0 ng/mLAs"'.

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000=00io 200204 p"Arsmic(l ll) Conccniratinn, ng l a

Flgure 5. Some ty ical arsenic(II1) cathodic stripping calibration CUVBS:(1) 0-2 ng/mL As"; (2) -40 ng/mL As"'; (3) 0-300 ng/m L As"'.under the conditions used in this s tudy was 2 ng/mL arsenicand th e relative standar d deviation of the 10 ng/ mL peakswas 8.3%. Figure 5 shows some typical calibration curves from2 to 400 ng/ mL arsenic.

C chqutrnh incc khi cSe


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2192 Anal. Chern. 1980, 52, 2192-2195Among the commonly occurring elements expected to in-

terfere with this method would be copper, since it would alsodeposit at t he potential employed. Copper(I1) at a concen-tration 10 times th at of arsenic was found to reduce the arsenicpeak height by about half.For development of method for the analysis of samples suchas foods by the described cathodic stripping procedure, severalfactors had to be considered. These included the digestionof samples, reduction of arsenic(V) to arsenic(III), and theelimination of interferences. Digestion in a closed systemensured that no arsenic would be lost through volatilization,and heating with magnesium nitrate ensured the destructionof all traces of organic mat ter . Reduction with sodiumbromide in th e presence of hydrazine sulfate was effective anddid not introduce interfering ions into the solution. Inter -ference due to copper and other cations was easily eliminatedby removing them from solution by extraction with dithizonein carbon tetrachloride (8).The National Bureau of StandardsReference Material 1571 Orchard Leaves, certified to contain14 f Kg/g arsenic, was analyzed by the described procedureand yielded 13.2 bg/g arsenic (average of five determinations)with a standard deviation of 1.1pg/g.

ACK NO WLEDG M ENTThe author wishes to thank Mildred M. Cody, of FDA,York, and of the D epar tmen t of Home Economics andtrition of New York University, New York, and Th

Medwick, Professor of Pharmaceutical Chemistry, RuUniversity, New Brunswick, NJ, for their invaluable assisin th e preparation of this paper.

L I T E R A T U R E C I T E D(1) Arnold, J. P.; Johnson, R . N. Taknta. 1989, 76, 1191-1207.(2) Myers, D. J.; Osteryoung, J. Anal. Chem. 1973, 45 , 267-271(3) Foisberg,Y .; O'Laughlin, J. W.; Megargle, R . E.; Korlyohann, S. RChem. 1975, 47, 1586-1591.(4) Davis, P.H.; Dulude, G. R.; eiffin, R . M.;Matson, W. R.;Zink, E. WChem. 1978, 50, 137-143.(5) Cox, J. A.; Chang, K., Department of Chemistry and BiocheSouthern Illinois University, Carbondale, IL, 1975.(6) "Official Methods of Analysis of the Association of Official AnChemists", 12th ed.;George Banta Co. Inc.: Menasha. W I ,paragraph 25.106.(7) Christian. G. D.; Knoblock, E. C.; Purdy, W. C. Anal. Chem. W61128- 1132.(8 ) Lossen, G. L. Chemist-Analyst 1977, 67, 4.

RECEIVEDor review April 2,1980. Accepted August 18,

Absorbance Determination by Time Interval MeasurementsJ. Michael Ramsey" and William B. WhittenAnalytical Chemistry Division, Oak Ridge National Laboratory, P.O. Box X, Oak Ridge, Tennessee 37830

Absorbance measurements are accomplished by placlng thesample Inside the cavity of a laser which is impulsivelypumped. I f the laser contains an appropriate gain medium,there will exist a measurable time delay between pumplng andthe onset of lasing. It Is shown that this time delay Is relatedto the optical losses In the laser cavlty. Increased time delaysdue to an intracavity sample can therefore, be related to theabsorbance of the sample. The result of this process is thatabsorbance Is determined by measuring a differential timerather than optlcal power. Absorbances as small as aremeasured wlth an instrument utilizing this new approach. Inthe small absorbance reglme, the dlfferentlal delay time Islinearly related to the absorbance.

Optical absorbance measurements are among the oldest andmost utilized techniques employed by th e analyst. In recentyears, absorbance measurements have become more sophis-ticated, so th at , in many cases, detection limits are reducedsignificantly. Thes e new measurement techniques are pri-marily laser based such as laser photoacoustic, laser-inducedther mal gradient, an d laser intracavity absorption measure-ments. All of these methods are capable of measuring ab-sorbances of less than (1-3). We present in this papera new approach to the measurement of small optical absor-bance (N ~ O - , ) . Thi s novel method is also based upon the useof lasers and is compatible with all types of samples: gases,liquids, or solids. An imp ort ant feature of this approach isthat it accomplishes the absorbance determination bymeasuring a time difference rather tha n optical power. Sucha transformation is attractive because of the ease in whichaccurate an d precise time differences can be determined. In

addition, convenience is derived from the fact that ucertain conditions (e.g., low absorbance) this time diffeis linearly related to t he absorbance.

Th e new measurement scheme utilizes laser gain matewhich exhibit a relatively slow rise of optical gain afteinjection of pump energy. As a result of th e slowly risingthere is a significant time delay between the initiatiopumping and the onset of lasing. This delay corresponthe time required for th e optical gain to increase beyonoptical losses of the laser cavity. From th e dependendelay time on cavity losses, it follows th at one can deterthe absorption (an optical loss) caused by materials locin the laser cavity by measuring the resulting time delay.difference in delay time with and without the intracsample is indicative of th e absorption of that sample alaser wavelength.

In the following sections, the measurement scheme iplained in greater detail and t he expected response theically modeled. Th e experimenta l approach we utilimeasure the delayed lasing phenomena is described, andare presented which demonstrate the capabilities of sucinstrument.

T H E O R YThe gain media utilized in these experiments generallbe treat ed as four-level systems. An appropriate four-

energy diagram is shown in Figure 1. We will assumethe gain medium is instantaneously pumped (energy injeso as to populate level 3. The population density of leat this instant is defined asN3 These excited states now to populate the upper laser level, energy level 2. The rathi s relaxation is k,,, a characteristic of the active medThe nature of the upper laser level is that of a metastable(i.e., the population loss from level 2 is negligible over the

0003-2700/80/0352-2192$0OO/O 0 1980 American Chemical Society

determination of arsenic by cathodic stripping try with a hanging mercury drop electrode

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