CHAPTER G

USE OF DITHIOCARBAMATES IN INORGANIC ANALYSIS

The N-substituted dithiocarbamates have found considerable use in analytical methods for heavy metals. Since the researches of Uel6pine24 these compounds have been used for the detection and quantitative determination of traces of copper in various material~17*~~. In recent years it has been demonstrated that the usefulness of the dithiocarbamates is by no means limited to copper analysis.

The substituted dithiocarbamic acids are not specific reagents, but, as has been pointcd out (see pagesg, 46), form with many heavy metals stable complexes very sparingly soluble in watcr. Eckert2’ in his comprehensive review shows that, in spite of the lack of specificity, separations are possible on the basis of one of three schemes: (a) by means of what Eckert calls ‘positive selection’, involving pscipitation or solvent extraction of the required mctal as il dithiocarbamate salt, while interfering metals arc masked by other complcxing agents; (b) by ‘negative selec- tion’ or in other words, by precipitation o7extraction of inter- fering metals as dithiocarbamate complexes, the required metal being masked i f necessary; (c) by means of the differing stabilities and distribution ratios ot-the clithiocarbamate complcses. Separations based on these principles have been presented by EcIccrtz’, Imt somr expansion hrrr on tIw abovc summary is warran trd.

-A I i \ \ J

set ot conditions~Thc reader is refcrrecl to tlic works of 1 3 0 d c ‘ ~ - ~ ~ and Bode and Neuman13 for a discussion of the stabilities of the

158 DITHIOCARBAMATES

salts of diethyldithiowbamic acid and their extractabilities in relation to the pH of the solution. In the latter workslanu Bode has investigated systematic'dy the precipitation by diethyl- tlit1iioc:~rb;un:itc of individual metals of the groups of the I'eriodic Table, the estractability of thcse salts into carbon tetrachloride, and the ultraviolet absorption spectra of these

of a dithiocarbamate complex can be ctal forming a stronger complex of the

hus we have the 'Verdrhgungsreihe' of E ~ l i e r t ~ ~ , which lists the metals in order of increasing strength of coniplrs formation with diethyldithiocarbamic acid mid at p1-I 5 .G: Mn, Asm, Zn, Snu, Fern, Cd, Pb, Bi, Con, Ni; Cu, Ag, Hg (see page 46). and Wickbolda have published series of metals for the solubilities of the dithiocarbamate salts. \Vii.kbold lias detcrniincd that sodium dietliyldithiocarbamntc * at 1'1-1 9 (in thc prescnce of 'ammonia and tartrate) prccipitatcs the following metals, in order of d solubility: FcIII , Mn", Zn, SlF,TluI, Cd, Bi, Pb, Co, g, I-Tg. This follows very closely the series presented by Malatesta (givcn earlier on page 46). Wickboldm has shown that in the presence of other compl(:xing agents (ie., the 'masking' agents in the scheme of Eckert, above), only certain of the metals form complexes with dicthylditl iocarbamate. In the presence of the disodium salt of ethplcnediaminetetracetic acid, only Sb", Tlm, Bi, l'b (only partly), Cu, Ag and Hg are precipitated. In the presencc of cy;inidc, only Sbm, TlIIJ, Cd, Bi and Pb are precipitated. Wickbold found further that, if the precipitations were at- tempted at pH 12 (instead of at pH 9.0 as abovc) and again in thc presencc of tartrate, the order of solubility is the same except that FeIn, Mn, Zn and S P do not precipitate. If cyanide is d d c d to thc solution at pH 12, only Tim, Cd, Bi and 1% are prccipitat c d . If both cyanide and ethylenediaminetetracetate are added at 1'1-I 12, only TP, Bi and Pb (partially) form insoluble dicthyldit hiocarbamates.

Nazareiiko and BiryukGa have shown that Bi, Pb and Cd can be es tracted as the diethyldithiocarbamates into chloroform

USE IN INORGANIC ANALYSIS I59 from aqueous solutions of ~ H I I - I Z containing tartrate and cyanide. Several recently reported analytical procedures for various metals have used selective extraction of the metal dithiocarbamates into an organic solvent from aqueous solutions of definite pH v a l ~ e s ~ ~ ~ ~ ~ ~ 1 . Malissa and GomiSE~k~~ have found that at pH I the diethyldithiocarbamates of Fe, Co, Ni, V, Cu, As, Sb, Sn and Pb could be quantitatively extracted into chloroform; Cu, Sb and Sn could be extracted out of 6 N HC1 solution as the pyrrolidineditliiocarbamates. Usatenko and Tulyupas6 have shown that from an ethyl acetate solution of the cliethylditliiocarbamate complexes of heavy metals, zinc, cadmium and lead are extracted with 1-2 N HCl, iron and bismuth with 5-6 N HCl, nickel and copper are removed only by using concentrated hydrochloric acid, and cobalt is not cxtractablc with this acid.

Thus, by controlling pH conditions and by adding other V D r i a t e complexing agents, the precipitation or lieavy metal -bamic 'acids can be controlled.

Eckert's third scheme for the utilization of dithiocarbamic

. .

acids in metal analysis utilizes the above differential stabilities of the heavy metal salts as well as the distribution ratios of these complexes between organic and aqueous phases. If a solution of the dithiocarbamate of the metal Me, in an organic water- immiscible solvent (such as CHCI,) is shaken with an aqueous solution of a salt of the metal Me,, there will be set up an equili- brium according to the following equation:

m(Mezn+) aq. + n (Me, (DEDC) m) or6 . + n ( Me,m+) aq. + tll (Me, (DEDC) n) org .

If the ratio of the stability constant

wcz(DEDC)n] aq. [Mer*+] [DEDC-In aq. KB =

and the distribution ratio

References p . 164

-.

. . .

- -

160 DITHIOCARBAMATES

of thc dithiocarbamate of metal Me, is much greater than the corresponding ratio for the metal Me,, ;.e..

thcn tllcrc is obtained a displacement of the metal Me, from its dit1iior:arhamatc complex by Mc,. Thc: mctal Mc, appears in tlic organif: ph;isc: (as thc c1itliioc:vbamatc salt), thc mctal Mc, ap- pc:tring in the aqueous phase.

rlbslqkiomctric analysis of the metal dithiocarbamate in a noli-polar solvent is the most convenient method of determina- tioil of tlic metal, although other methods such as turbidime- tric”#, I~eterometric~ and gravimetric46 methocls have been sug- gested ;md arc quite useful. SedivecSs and Fritz and Sutton30 haye developed a titrimetric method for mercuryn, using cupric ions as an internal indicator, based on the observation by !%div-e~nd Va9QIP’ that shaking a cupric dithiocarbamate in an orginic solvent with aqueous Hg2f causes the copper complex lo I I C I~oken, the inorc stable mercuric .dithiocarbamate being iortned. & d i v c ~ ~ ~ used the more conventional sodium diethyl- clithiocnrbamate, whereas Fritz and Sutton30 used bis(z- 1~ydrox~ethyl)dithiocarbamate. The latter reagcnt had been sog~estcd by Geiger and MUlle+, ant1 again by Dclaney” for c:oppcr analysis in that the copper complex is water-soluble and thc neal for extraction with a non-polar solvent is eliminated.

T I I ntltlition to tire esamplcs of the ‘positive selection’ method of Eckel-t cited above, a few recent applications arc noteworthy. Kreimel- and Butylkinm have used lead diethyldithiocarbamate to estract copper into chloroform from aqueous copper sulfate at pl-14 or lower. Cobalt and nickel are not extracted. Wood and X i ~ l i o l s ~ ~ have employed a preferential extraction of the dictliyldithiocarbamate of lead to determine trace amounts of this metal in zirconium and its alloys. According to Busev cl O I . . ‘ ~ , indium can be separated from gallium by precipitating tlic former wit.h dicthyldit1iiocarbnm;tte a t pl l 3-5 in thc pl-escmcc of oxalic acid. Under these conditions, gallium is not

USE IN INORGANIC ANALYSIS IGI precipitatcd. M a l i ~ s a ~ ~ has shown that sodium diethyldithio- carbamate and sodium pyrrolidinedithiocarboxylate both react with niobium but not with tantalum in the range of pH 2-6. Berger and Elvers4 have found that aqueous sodium diethyl- dithiocarbamate rcmoves only cadmium from a chloroform soliltion of thc 1-(2-pyridylazo)-z-nnphthol complcsrs of cad- mium and zinc. Tlic photomctric dctcrmination of coppcr in aluminum and stccl using diethyldithiocarbamate was shown feasible by Andreev and I<aW if iron is masked with citric acid at pH 9-9.5 and nickel with ethylenediaminetetracetate.

Martin48 has uscd the ‘negative selection’ approach to the determination of zinc in the presence of cobalt and copper. He uscd two complexing agents, diethyldithiocarbamate and di-2- naphthylthiocarbazone, in an application of the following considerations. The complescs of coppcr and cobalt with the dithiocarbamate are formed in prcfcrcnce to thosc with di-2- naphthylthiocarbazone, and arc stable towards dilute acid. The di-z-niiphtliylthiocarbazonc complcx of zinc is formcd in preference to thc diethylditliiocarbaatc complex, and is unstable to acid, so that the zinc appears completely in the water layer upon extraction of the chloroform solution of the complcxcs with 0.5 N hydrochloric acid.

I n their mcthod for determining aluminum (in thorium oside) as the quinolate, Goldstein ct aLM recommend the removal of the intcrlcring mnngancsc and cobalt by a prior extraction with diethyldithiocarbamate. Fritz and Johnson-Richard2g used a preliminary extraction with diethyldithiocarbamate if thorium were present, in thcir colorimetric determination of uranium with arsenazo. Chernikhov el al.’B, in their complexomctric determination of thorium, extracted the interfering mangnnesc into chloroform as its diethyldithiocarbamnte at pH 5.

Although it is perhaps preferable to eliminate interfering ions by the application of the above principles, i t is not always necessary to do SO, as shown by Cl~i l ton~~”. It w;1s dcmonstrittcd that copper, nickel and cobalt can be determined simultaneously by solution of simultaneous equations involving the absorption Relerenccs p . rG4

I Gz DITHIOCARBAMATES

spectra of the metal dithiocarbamates in carbon tetrachloride solution.

Table VI lists some of the analyses of heavy metals involving clithiocnilnmntrs which have appeared in the chemical literature,

TABLE VI REPRESENTATIVE EXAMPLES OF THE USE OF DITHIOCARBAMATES

IN ANALYSIS OF METALS

AI

Sb As Bi

Cd

Cr

co

c u

Ga In

dicthyl

dicthyl

dicthyl silver diethyl di rncthyl

h S t raiiiclliylciic dit*lliyl dictliyl tcl.raincthylcne

+:mi nophcnyl

dicthyl dic thy1 tcl raniethylene dil:thyl dicthyl dic.thyl diethyl ziiic clibcnzyl tcl ramethylene bir (2-h ydroxycthyl) tetramethylenc tetrametliylene

divthyl

d i l ~ l l l p l

NH3, tartrate, PH 5-7

PH 5.2

EDTA, CN-. NH, -

CN-, EDTA NH,. citrate

CN-

tartrate, p11 6

-

-

citrate tartrate, pH 6

NH,, citrate citrate CN- tartrate, pH 6

-

- - pyrophosphate

titrimetric

colorimetric colorimetric colorimetric colorimctric

coloriincl ric colorinictric ha tcromctric coloriinctric t i trimctric ccilorinici ric grnvinictric colorimetric gravimetric colorimetric hcterometric colorimetric colorimetric grnvimctric colorimctric colorimctric colorimetric colorimetric colorimetric

50

26

27 66

18

G2

54*67 9 62 71 3'*3* 40

46 62

8 2 0 , 5 5 42 46

62 23 62

62

2 0

2

USE IN INORGANIC ANALYSIS 163

PH 5 - tartrate, pH 6

CN-

CN- tartratc CN-, tartrate

and citrate

- -

-

gravimetric colorimctric gravimetric gravimctric gravimetric colorimetric titrimetric titrimetric colorimetric

ampcromctric

- colorimetric - colorimetric CN-, tar tra t c color i nictric ICCNS,cystcine coloriiiiclric N1i3, citmtc. colorimctric

EDTA titriinctric mmi licteronictric pyridinc, EDTA titrinictric tart ra tc titriiiirtric

' pl-I 8.5

colori mc t r ic colorimetric colorimctric colorimetric

tartrate, p H 6 gravimetric citrate colorimetric

colorimetric

- colorimetric

- titrimetric

tartrate, pH 6 gravimetric - colorimetric

Fe

Pb

rtig Mn

Hh'

Mo

Ni

Pd

Rh

Ag

diethyl tetramethylem diethyl

pcntamcth ylene tctramcthylene diethyl dicthyl diethyl

cyclollcxylc thy1

dicthyl dicthyl tctraincthylcnc cliciliyl tl i I N t yl tlicthyl

bis( 2-hydroxycthyl) tlicthyl dicthyl tlictliyl

plrcnyl tetrainethylene dimethyl 5-phenyl-I -pyrazolyl

diethyl diethyl . tctramethylcne

tctraincthylcnc

3azapenta- methylene

diethyl tetramethylene

28

62 46 36 14 62

72 70 63

64

53 62 2 s

58 21

30 6, 7 59 cis,

40 62

40 15

46 20,22

62

6 2

56

46 62

(Continued)

I %

164 DITHIOCARBAMATES

TABLE VI (costinucd)

‘I‘c ~~entalllcthplcnc tlictliyl

Sn 1.ctramctliylcnc ’ V i rlictliyl I I ~lictliyl V tc?tramctliylcne %n diethyl

t c traine th ylcne tlictliyl Iiis(r-hydroxyctliyI)

CN- tartrate, c u p

ferron, CN- - PH 2 -

- - - di thizonc

gravimctric 14 colorimetric I I, 35

colorimctric 6 2

colorimctric 57 colorimctric 74

colorimctric 62

colorimctric 39 coloriinctric 6 2 potcntiomctric Gn colorinictric 47

:rnd thcb particular dithiocarbamate used. The reader is cau- 1 iont2d that this list is not intcnded to be cshaustive. Thc table (10~s. . IIO\VCVCT, indicate the widc application 01 ditliiocnrbamic ,i citl dt ~rivativcs to nictnl analysis. Sodium diethyldithio- carbamxte L- - is hm- , cn inTable TrI, a‘iirimbcr of other dithiocarbamates liave been employTd. ];or example, Musil and H a a P have introduced recently the ammonium salt of ‘phenylhydrazineditliiocarbamic acid’ (from phenylhydrazine, carbon disulfide and ammonia) as a reagent for >ilvcl-, coppcr and 1c;icI.

-

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I GH DITHIOCARBAMATES

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