Separation and Spectrophotometric Determination of Microgram Quantities of Mercury Using Diethyldithiocarbamate E. A. HAKKILA and G. R. WATERBURY Universify o f California, 10s Alamos Scientific laboratory, 10s Alarnos,
b Microgram amounts of mercury are separated from most metallic ions b y extraction of the mercury-diethyldithiocarbamate complex into carbon tetrachloride from a carbonate buffered solution containing (ethylenedinitril0)tetraacetate and cyanide to mask interfering ions. The mercury is determined b y measuring the absorbance of the extract a t 278 mp. O f 32 ions investigated only copper, thallium (Ill), and bismuth(ll1) interfere seriously. If more than 200 pg. of ruthenium are present, it must b e removed b y hypochlorite oxidation. Based upon data for 14 determinations at each concentration, coefficients of variation of 5 and 2 for 10 and 50 pg. of mercury, respectively, were obtained. One analyst can perform up to 30 determinations per day.
70
B
an analytical method was needed for detrwnining sinall concentrations of niercury in solutions containing cerium, iron, lanthanum, molybdenum, plutonium, ruthenium, and zirconium, various published procedures for determining mercury were considered. Volumetric procedures using stilboxine (8) or phenylthioscmiicarbazide (7') as indicators have heen described for this determination, but both methods are suscept,ible to interferences by the other metallic ions expected t o be present. The spectrophotometric determination of mercury with dithizone (11) suffers from tlie instability of the ditliizone reagent. Kevertheless, dithizone is perhaps the most widely used reagent for determining small concentrations of mercury. A turbidimetric method (5) involving the reaction between bin (11) and arsenic(II1) is highly susceptible to errors caused by light intensity, reaction time, and Icaction tc'mperature nietliylditliiorarl,arnate has been extensively invastigatcd (1-d> 9 , IO) as an estractant for various ions. Bode ( 3 ) found that mercury tlit:tliyltlithiocarbamate is quantitatively extracted into carbon tetrachloridtL from aqueous solutions in thc p H raiigc of 4 t o 11 and suggclsted its use in prcliminaiy yeparations of nioi'cury from numerous eleECAUSE
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ANALYTICAL CHEMISTRY
N. M .
mcnts. However, this carhori t,etrachloride extract was reported to have no absorption maximum suitable for the quantitative determination of mercury. The present' investigation showed that a maximum a1)soriiance occurs a t a ware length of 278 nip and that tlie molar absorptivity is approximately 33,000. A sensitive method, based upon this large absorbance a t 278 mp3 was developed in which the mercurydiethyldithiocarbamate complex is extracted into carbon tetrachloride from a potassium cyanide- (ethylenedinitrilo) tetraacetate (EDTA) solution adjusted to a p H 9.3 to 10.0, and the absorbance of the carboii tetracliloridc extract is measured. Although this met'hod was developed specifically for t'he determination of mercury in solutions containing cerium, iron, lanthanum, niolybrlenum, plutonium! ruthenium, and zirconium, i t should be applicable to a wide variet'y of samples, providing proper dissolut'ion procedures arc used. Methods are published for prcparing organic arid biological samples (11) and n i i n r 4 samples (6) pi,ioi, to t'lie cletri,ininatioii of mercury. REAGENTS
Analytical reageut grade chemicals and low-conductivity ivater were used in all reagents. The preparation of special reagents is described below. Diethyldithiocarbamate
Solution,
0.27,. Dissolvt. 0.20 grain of Eastman K h i t e Label o r rmgcnt of similar purity in tlistill(d jvater, add 2 drops of 207, sodium hydroxide, and dilute to 100 ml. E x t r w t for 5 minutes each n.ith two 25-1111, portions of carbon tetrnchloritle, centrifuge the aqueous portion for 5 minutes t o remove dispersed carbon tetrachloride, and carc,fullp transfcr the aqueous solution t o a stoppered flask. (Ethylenedinitri1o)tetraacetic Acid Disodium Salt (EDTA) Solution,
57,.
Dissolve 50 grams of reagent grade E D T A ant1 25 grains of sodium carbonate in 800 nil. of distilled Rater, adjust t h e pII of the solution t o 9 t'o 9.5 with 20% sotliuni hvdroxide, and dilute t o 1 1itk.r. M e r c u r y Standard Solution. Dissolve 0.500 gram of reagent grade mercury in nitric acid and dilute to ,500 nil. v i t h distilled n t e r t o prcspaie
a stock solution. Dilute 5.00 ml. of this solut'ion t o 500 ml. with 2 nil. of c o n c t ~ ~ t r a t enitric d acid and distilled water. The diluted solution contuins 0.01 nig. per nil. of merciiiy. RECOMMENDED PROCEDURE
Determination of Mercury. Traiisfer a n aliquot of t h e sample solution containing 10 to 60 pg. of mercury to a 50-ml. beaker. Add 1 ml. of nitric acid (specific gravity 1.42) and warm on n steam bath for 10 minutes to ensure oxidation of the mercury to the (11) oxidation state. Add 5 ml. of the buffered I.;DTA solution and adjust the p H to 9.3 to 10.0 with 20% sodium hydroritlc, using a p H meter. (-4Beckman hfodel G p H meter e q u i p p d with glass and calomel microelectrotlcs is suitable. Because of the pota2s'111111 chloride salt bridge separating the caloinel and the sample solution, no mercury contamination from the elwtrode was espected or found in an!- of the determinations.) Quantitat.i\-rly transfer the solution to a 50-ml. test tube, and add 1 ml. of 1.0% potassium c y n i d e solution, 1 ml. of 0.2% diethyltiithiocarbaniate solution, and 10.0 nil. of carbon tetrachloride. Stir the niisture for 5 minutes a t a rate suficic,iit to mis the two layers complrtc~ly. K i t h the stirrw running, lower the t& tube away from the stirrer and rinse t'lie stirrer into the test tube with distilled water. .-\lion the layers to separnte for 10 minutes. With n 4-iiil. s-rinrre Diuet transicy a. portion of the " caibo; tetrachloride layer to :I c l t w i , dry, quartz absorption i d , taking care not to transfer any of the aqueous layer. To prevent i n t e r from entering the pipet, gently prcss the plunger of the syringe as the tip is lowered mid raised through the aqueous layer, and then wipe the tip with :in absorbant tissue. To clean the pipet rinse l\ith acetone and dry b y suction. All traces of acetone must be removrtl before pipetting the next sample or rmults will be high. hleasure the absorbance of the carbon tetrachloride solution a t a wave length of 278 nip wing reagent grade carbon tetrachloride as a reference liquid. From a calibration curve plotted from tlata obtained by extracting a rcagciit blank and kn0R-n amounts of mercurydiethyldithiocarbamate from a p H 9 . 3 to 10 buffered EDTA-potassium cj-aA
nide solution, calculate the ainount of mercury in the sample. A reagent blank and one or two known concentrations of mercury should be determined to verify the calibration curve cach bime a set of samples is analyzed. Removal of Large Amounts of Ruthenium. If more t h a n 200 pg. of ruthenium are present, adjust t h e volume of tlic: sample aliquot t o approsiniatcJly 5 ml. with distilled m t e r , and adjust the pH t o 2 t o 5 using 38% hydrochloric acid and 207, sodium hydroxide. Warm the solution on a steam bath, add 3 to 4 drops of 6% sodium hypochlorite, and heat for 5 to 10 minutes to volatilize the ruthenium. If the solution is brown, indicating the presence of ruthcniuni, add a few more drops of hypocahlorite solution and continue warming, but do not allow to evaporate to dryness. If ruthenium oxide deposits around the rim of the beaker, carefully wipe off with an absorbant, tissue. Acidify t'he solution ivith 1 to 2 drops of 387, hydrochloric acid and licst for 2 minutes. Add 2 to 3 drops of 6% sulfurous acid and continue heating for 5 to 10 minutes, but do not allow the dohitioil to evaporate to dryness. Reinove from the steam bath, :illow to cool, and continue as described iinder Deterniination of hIercury. DISCUSSION AND RESULTS
Extraction of dithiocarbamate.
Mercury-Diethyl-
Mercury-diethyldithiocarbamate is quantitatively extracted b y carbon tetrachloride from a q u r o u ~solutions adjusted t o pH -1 t o 11. Honnver, because t h e diethylt1ithioc:iri)nrnatc reagent', which absorbs strongly in t h e ultraviolet, also extra(+ froin solutions more acitl t h a n p H X.5, a n d hct*auae mercury coprecipitatc~s if iron is present, a t n pH ai)oro 10, thc optimum p€I range for the, clxtmction is 8.5 to 10.0. Stability of Reagent. One main :idvantage of the diethyldithiocnrbanintc p r o c d u r e for dctcrmining merc ~ i r y:is cAoinpared t o the widely used tlithizoni~mrthotl is t h e stability of tlie forincr rcagent. \\'heroas dithizone solutions iiiust. lie standardized tlaily :rnd stored in :i rcfiigoxtor, clic,thyldithiocarl,Rlnate solutions are st:il)lv for :it least 6 n.rc>kswhen stored in the dark. A s slion.11 in Table I, txlsults olitainrd using reagent solutions that ai'r freshly prrpared or 6 m c k s old ai'e iiot significantly diffrreiit. To detemine t'he homogeneity of the solid diethyltlithiocar~~amate, 0, 10, 30, antl 50 pg. of mercury were determined iising reagent solutions prepared from fiw t liffcrc~nt lots of dict'hyldithiocarIianiatc. i.c:agent,. The averages of the absoi~l)anws a t each mercury conctmtration o k i i n e d using solutions pi+ pared from cach lot, of reagent, were r o m p a r d with thc average of 14 drtt~i~minations at the sarnr mcrviii'y cun-
centration using one reagent solution. As indicated in Table 11, no detectable difference in results was observed. T o determine the stability of t.he mercur?.-diethyldithiocarbamate extract, absorbances were measured as a function of time following extraction, at the 10- and 30-pg. concentration levels of mercury. The absorbance increased slowly with time of standing. The cause for this increase was not determined but $vas presumed to he decomposition of the diethyldithiocarbamatc. As shown in Table 111, the absorbance of the carbon t'etrachloricle extract should he measured h e t w e n 10 and 30 minutes after cstract'ion for reliahle results. Interferences. Using aqueous solutions iiiore alkaline than pH 8.5, several metals (3, 9) , including copper, iron, plutonium, and ruthenium, coextract with mercury as the rcspcctive diethpldit1iiocarl)amate complexes, and metals sucli as cerium, lanthanum, and zirconium precipitate as the hydroxides which carry some mercury. Coppcr, which is a trace inipu1,ity in most of the reagents used. intcrferee seriously hecause 1 pg. of copprr causes a n error equivalent to 6 pg. of mei~cury. Cpanidt' antl E D T A w r e investigated as complexing agcnts to hold the interfering iiictals in the aqueous phase. Five millilitc~rsof 5% E D T A eliminated intcrfcrence caused by a tot'al of 2 mg. of cerium, iron, lanthanum, plutonium, and zirconium. Boclc (3) reported that cyanitlr hintlc:,s the estraction of copper to a greatci, clc~greclthan that, of mercury, hut doc.s not completely mask either meta,l. dn invcstigation of the (Jffect of n r i o u s concentrations of c,yanidc on tlic txstractioii of nici~rurytlio t h yltli t h iocarlmnatt' iiit o carhon trtrachloride indicated that 10 ma. or less of cyanidc causcd no loss in seiisitivity 01' precision, but larger amounts reduce t,hc rst,ractability of mei~ciiry-. Using 10 mg. of potassium cyanitlc. up t o 2 pg. of coppcr do not apprcciahly interfere \Tit11 the determination of 10 to 60 pg. of nirrcury. In addition. u p to 200 pg, of ruthenium can be tolcratc~ci in the samplc with no separation. 'fhe effect of cyanide on the extraction of mercury and copper is shown in l'ahle TV. If 5 ml. of 5% E D T A and 10 nig. of potassium cyanide are added to the aqueous phnsc. and the pH is cai,c,fully atljust,rtl. a n ci't'oi' of l i w than 0.A pg. of mcrcury in t h r di~tcmninationof I O pg. is caused 1)y 1 mg. of cach of the folloiring c.lmitnts: Gmup IA, IIA, antl rare rarth mrtals, aluiiiinum, aiwnic (111). cadmium, chromium(1IT) antl (VI), ironiII) and (111). manganese iII), molyl)di~iiiirn(T.rI)niobium, plutonium (LV), thorium, tin( 11) antl (W ) titanium. tungsten, uraniuni(IV), vanIn adium, zinc, and zii.coniurn. ~
~
Table I. Stability of Diethyldithiocarbamate Solution with Time Absorbance of C01, Extract0 Hg -4tIded, Solution Solution PK. 1 day old 6 weeks old
u
0.045 0.044 0 214 0.214 0 555 0.5ST 50 0 0 913 0 913 Averages of 14 determinations. 10.0 :30 0
Table II. Effect of Using Different Lots of Diethyldithiocarbamate on the Determination of Mercury Hg One Lot Five Lots A\iided, -4bsorb- U, pg. Absorb- U, pg. pg. ance Hg ance Hy 0 0 044 0 40 0.045 0 49 10 0 0.214 0 56 0 216 0 45 30 0 0 557 0 80 0 555 0 73 50 0 0 913 1 03 0 911 1 19
Table 111. Effect of Time on Absorbance of Carbon Tetrachloride Extract Time aft,er Ahsorban ce Extr:tct,ion 10 pg. Hg 30 pp. fi
