Separation of Gold (III) as Its Ion-pair Complex with 18-Crown-6 from Hydrochloric Acid Media by Means of Solvent Extraction

ANALYTICAL SCIENCES 2001, VOL.17 SUPPLEMENT i725

2001 © The Japan Society for Analytical Chemistry

†To whom cor r es pondence s houl d be addr es s ed E-mai l : honj o@kenr oku. kanazawa-u. ac. j p

Separation of Gold (III) as Its Ion-pair Complex with 18-Crown-6 from Hydrochloric Acid Media by Means of Solvent Extraction

Cristina T. C

AMAGONG

and Takaharu H

ONJO^†

Department of Chemistry, Faculty of Science, Kanazawa University, Kakuma-machi, Kanazawa-shi, Ishikawa, 920-1192, Japan

A new solvent extraction technique for the separation of gold(III) at trace amounts from hydrochloric acid media was developed based on the formation of an ion-association complex of tetrachloro gold anion, AuCl₄^- with 18-crown-6 (18C6) oxonium cation in 1,2-dichloroethane. The effect of various factors (crown ether, acid concentration, solvent, shaking time, stripping reagent etc.) on the extraction and separation of gold(III) was investigated. Quantitative extraction was achieved using 0.05 mol dm^-3 18C6 in 1,2-dichloroethane at 4 mol dm^-3 HCl within 5 min. Back-extraction was carried out by 5-min shaking with 0.1 mol dm^-3 Na₂S₂O₃ in 0.2 mol dm^-3 NH₃, followed by FAAS determination. Interference studies revealed excellent tolerance of the method to foreign ions. The method’s high selectivity was demonstrated by a spike recovery of >96% for gold(III) and by the negligible extraction of silver(I). Using first-class silver reagents, the quantitative recovery of traces of gold(III) was achieved up to a 20-fold preconcentration factor.

(Received on August 9, 2001; Accepted on September 13, 2001)

Gol d and si l ver are comm onl y f ound associ at ed wi t h each ot her. Sever al met hods for gol d anal ys i s , howe ver, possessed l i mit ed t ol er ance for t he co-exi st ence of si l ver and vi ce versa. In an anal ys i s of gol d i n or es by fl ow i nj ect i on on -l i ne sor bent ext r act i on pr econcent r at i on t echni que,¹ t he pr esence of >2. 5 μ

g cm

^-3 sil ver i nter fer ed si gni fi cant l y wi t h t he det er mi nat i on. In anot her st udy,² t he use of met h yl et h yl ket one/ chl or of or m ( M EK/ CHCl₃) syst em was f ound l i mi t ed by t he i ncor p or at i on of si l ver i n t he gol d ext r act. On t he ot her hand, t he pr esence of gol d at mor e t han 10 g dm^{- 3} cons i der abl y s uppr es s ed si l ver ’s si gnal i n El ect r ot her mal -AAS³ and no i mpr ovement was made even aft er gol d mat r i x reduct i on t hr ough addi ti on of var i ous or gani c r educt ant s . Hence, a met hod devel opment for separ ati ng gol d and sil ver pr i or t o anal ys i s i s cons i der ed neces s ar y.

Sever al met hods have poor sel ect i vi t y as shown i n t he li mi ted separ ati on of gol d and si l ver. One st udy⁴ uses t hi osul fat e and ammoni a aqueous medi a ext r act ed by a pri mar y ami ne-ami ne oxi de mi xt ur e, gi vi n g onl y a separ ati on fact or (α_{A u / A g}) of 15. In addi ti on, t he use of an expensi ve ami ne oxi de was f ound i mpr act i cal . Sast r e et al .⁵ on t he ot her hand, ext r act ed gol d fr om aqueous cyani de medi a by LIX 179 in t he pr es ence of s i l ver. The maxi mum separ at i on fact or obt ai ned i s 9, whi ch i s much l ower t han t he pri mar y ami ne-ami ne oxi de p r oce d u r e a n d m or e ove r, t he eq ui l i b r i u m p H ( 9 . 8 )

i s bas i c whi ch can pr om ot e met al hydr ol ys i s . Bur maa et al .⁶ st udi ed t he pr efer ence of di but yl sul fi de and sel eni de for gol d i n con cent r at ed HCl s ol ut i ons and of di but yl s el eni de and t el l uri de for si l ver i n concent r at ed HNO3 s ol ut i ons . A r evi ew⁷ ci t ed t he pot ent ial of t hes e extr act ant s, but no i nvest i gat i on was r epor t ed on t he act ual separ at i on of gol d and si l ver fr om a com m on aci d sol ut i on.

Sol vent ext r act i on t echni ques ar e cons i der ed i nval uabl e t ool s for t he separ at i on pur poses of pr eci ous met al s.⁷ At pr esent , our r esear ch gr oup has done s ever al s t udi es on t he us e of cr own et her s f or t he separ ati on of di ffer ent met al s found adj acent t o each ot her i n t he per i odi c t abl e by sol vent ext r acti on t echni ques. The syner gi st i c act i on of t henoyl t r i fl uor oacet one and di benzo-18- cr own-6 ( DB18C6) was found s ucces s ful i n t he separ at i on of cadmi um and zi nc⁸ i n s ol ut i on, as well as for l anthanum and bar i um.⁹ Separ ati on of t he pr eci ous met als such as pl ati num,^{1 0} pal l adi um,¹¹ r hodi um^{1 2} and i r i di um^{1 3} fr om each ot her was also made possi bl e t hr ough t he f or mat i on of i on-associ at i on compl exes wit h cat i oni c di cycl o-18- cr own-6 ( DC18C6) .

Pr evi ous s t udies1 4 , 1 5 on t he us e of cr own et hers f or gol d ext r act i on wer e also found, howe ver, none of whi ch r epor t ed any pr act i cal appl i cat i on. In t hi s wor k, t he pot ent i al of 18C6 cr own et her for t he ext r act i on and s eparat i on of gol d fr om s i l ver was i nves t i gat ed. The appl i cabi li t y of t he pr op os ed met hod f or t he i s ol ati on, pr econcent r at i on and det er mi nat i on of t r aces of gol d was exami ned by s pi ke r ecover y s t udi es usi ng fi r st -cl as s si l ver r eagent s .

i726 ANALYTICAL SCIENCES 2001, VOL.17 SUPPLEMENT

Experi mental

Appar at us

A Hi t achi Pol ar i zed Zeeman Type At omi c Abs or pt i on/ Fl ame Emi ss i on Spect r ophot omet er, model Z-6100, a Tai yo Re ci pr o Shaker, model SR- II and a Tom y Sei ko Swi n g Type Cent r i fuge, model CD-50R, wer e us ed.

Reag ent s

St andar d s ol ut i ons wer e pr epar ed by di l ut i on of a 1, 000μg cm^-3 gol d( III) s t ock s ol ut i on (f = 1. 003) i n 1. 0 mol dm^{- 3} h ydr ochl or i c aci d ( C IC A-M er ck, J apan) t o t he wor ki ng r ange. 18C6 ( 99% pur it y) was pur chas ed fr om Acr os Organi cs , US A. Al l aqueous s ol ut i ons wer e pr epared us i ng di s ti ll ed-dei oni zed wat er ( Nanopur e syst em, Bar nst ead Lt d. USA) . Al l ot her chemi cal s wer e of guar ant eed gr ade and wer e us ed wit hout fur t her pur i fi cat i on.

Procedure

Sol vent Extr acti on. A s ampl e ali quot ( 5 cm³) of 4 mol dm^{- 3} h ydr ochl or i c a ci d s ol ut i on cont ai ni ng gol d ( 0. 5 – 10 μg cm^{- 3}) was pl aced i nt o a 30-cm³ gl as s -s t opper ed cent ri fuge t ube. A 5 cm³ of 1, 2- di chl or oet hane s ol ut i on cont ai ni ng 0. 05 mol dm^{- 3} 18C6 was added t o t he cent ri fuge t ube, and t he mi xt ur e was shaken on a shaki ng machi ne for 5 mi n.

Phas e s epar ati on was car r i ed out by cent r i fugat i on at 2, 000 r pm for 10 mi n. The organi c phas e was col l ect ed by pi pet t i ng i nt o anot her centr i fuge t ube f or back-ext r act i on.

Back- ext r acti on. The gol d-l oaded or gani c phas e ( 5 cm³) was shaken wit h 5 cm³ of 0.10 mol dm^-3 Na₂S₂O₃ in 0.10 mol dm^-3 NH₃. The amount of gol d s t r ipped fr om t he organi c phas e was det er mi ned by Fl ame-AAS at 242. 8 nm under i nst r ument `s opt i mum s et ti ngs .

Res ults and Discuss ion

Eff ect of crown et her

The s el ect i vi t y of t he cr own et her for t he s epar at i on of gol d and s i l ver was hi ghl y cons i der ed i n t he opt i mi zat i on. The cr own et her s i nves t i gat ed wer e DC18C6, 18C6 and DB18C6. The extr act i on behavi or of gol d and s il ver was exami ned at 1-12 mol dm^{- 3} h ydr ochl or i c aci d s ol ut i ons us i ng 0. 05 mol dm^{- 3} con cent r at i on of t he cr own et her i n 1, 2 - di chl or oet hane us i ng a 5 - mi n s haki ng t i me. A 1:1 phas e vol ume r at i o was adopt ed. Am on g t he i nves t i gat ed cr own et hers , 18C6 gave excel l ent s el ect i vi t y ( Fi g. 1) for t he t et r achl or o gol d ani on, AuCl4-, whi ch i s r eadi l y ext ract ed i n t he organi c phase whil e sil ver remai ns i n sol uti on. The f or mat i on of unext r act able s il ver chl or o compl exes , AgCln( n - 1 ) -, account s for s i l ver `s negl i gi bl e ext r act i on at hi gh hydr ochl or i c aci d con cent r at i ons . Moreover, Fi g. 1 al so shows t hat at ≥ 2 mol dm<sup>- 3</sup> aci d s ol ut i ons , t he hi gh s el ect i vi t y of 18C6 al s o r es ult ed i n gol d`s quant i tat i ve extr act i on. For s ubs equent exper i ment s , a 4 mol dm^-3 concent r at i on of h ydr ochl or i c aci d was us ed.

Fi g. 1 Effe ct of h ydr ochl or i c aci d on t he ext r act i on of gol d ( IV) (▅) and sil ver ( I)( 〇 ) by 0. 05 mol dm-³ 18C6 i n 1, 2-di chl or oet hane.

Sel ect i on of back- ext racti ng agent

Vari ous s t ri ppi ng r eagent s wer e st udi ed us i ng an or gani c phas e s ol ut i on of 0. 05 mol dm^{- 3} 18C6 i n 1, 2- di chl or oet hane l oaded wit h 10μg cm^{- 3} gol d. A 1:1 phas e vol ume was mai ntai ned. The s tr i ppi ng s ol ut i ons i nves t i gat ed wer e: 11.0 mol dm^-3 HCl + 0.05 mol dm^-3 KSCN, 1.0 mol dm^{- 3} N H3-NH4Cl ,^{1 1} 0.10 mol dm^{- 3} NaOH,^{1 6} 0.30 mol dm^{- 3} t hi our ea,⁵ 0.10 mol dm^-3 Na₂S₂O₃⁴ and 0.10 mol dm^-3 Na₂S₂O₃ in 0.10 mol dm^-3 NH₃.⁴ Back-ext r act i on us i ng 0.10 mol dm^-3 Na2S2O3 in 0.10 mol dm^-3 NH3 was abl e t o r ecover gol d quanti t at i vel y fr om t he l oaded organi c phase aft er a 30-mi n s haki ng t i me.

Eff ect of s olvent

Di ffer ent organi c s ol vent s wer e i nves t i gat ed for t he opt i mum ext r act i on of gol d wi t h 18C6 fr om 4 mol dm^{- 3} h ydr ochl or i c aci d s ol ut i on. The exami ned s ol vent s wer e as fol l ows : chl or of or m, o-di chl or obenzene, ni t r obenzene, 1, 2- di chl or oet hane, cycl ohexane, t ol uene and benzene.

It was f ound t hat t he extr act abi li t y of gol d was quant it ati ve as t he i on pai r compl ex wi t h pr ot onat ed 18C6 i n chl or of or m, 1, 2- di chl or oet hane and o- di chl or oben zene. 1, 2- di chl or oet hane was sel ect ed, because i t i s cheaper t han o-di chl or obenzene and does not dri p eas il y fr om t he pi pett e t i ps i n cont r as t t o chl or of or m.

Eff ect of s haki ng t i me and phas e vol ume r ati o The opt i mum s haki ng ti me was as cer t ai ned by meas ur i ng t he r ecover y of gol d aft er 15 sec – 45 mi n s haki ng per i ods , as s hown i n Tabl e 1. It was f ound t hat t he met hod gave r api d at t ai nment of ext r act i on equi li bri um for gol d i n ei t her for war d or backwar d extr act i on wi t hi n 3 mi n. A 5-mi n s haki ng t i me was t aken as opt i mum for bot h pr ocedur es t o ens ur e compl et e r ecover y.

The vol ume r at i o of t he aqueous phase t o t he or gani c phas e was var i ed dur i ng gol d ext r act i on, and t he pr econ cent r ati on fact or was det er mi ned.

For a s i ngl e extr act i on, i t was found t hat t he quant it ati ve r ecover y of gol d was at t ai nabl e up t o 20- f ol d ( Vaq/ Vorg = 10 :0. 5) .

0 2 4 6 8 1 0 1 2

0

2 0 4 0 6 0 8 0 1 0 0

A g ( I ) A u ( I I I )

% E

HCl (mol dm

^-3

)

ANALYTICAL SCIENCES 2001, VOL.17 SUPPLEMENT i727

Tabl e 1 Effect of shaki ng t i me on gol d extr acti on Recover y, %

Ti me

For war d ext r act i on

Backwar d ext r act i on

15 s ec 90 61

3 mi n 100 98

5 mi n 98 101

10 mi n 97 100

15 mi n 98 99

30 mi n 100 97

45 mi n 99 100

HCl : 4 mol dm^-3; 18C6: 0.05 mol dm^-3; Solvent: 1,2- Dichloroethane

Compos i t i on of ext r act ed s peci es

Us i ng t he s l ope anal ys i s met hod, t he comp os i t i on of t he ext ract ed s peci es on t he ext r act i on of gol d wi t h 18C6 i n 1, 2- di chl or oet hane was i nves t i gat ed at a fi xed hydr ochl or i c aci d concent r at i on of 4 mol dm^{- 3}. The pl ot of l og D vs l og [18C6 ]o r g ga ve a s t r ai ght l ine wit h a sl ope of 1. 01 as s hown i n Fi g. 2, and t he r es ul t i s cons i s t ent wit h a pr evi ous res ear ch made by Kos hi ma et al.^{1 5} Thi s suggest s t hat under hi ghl y aci di c condi t i ons , t he tet rachl or o gol d ani on, AuCl₄^-, i s ext ract ed as an i on- as s oci at i on compl ex wi t h t he s t abl e 18C6 oxoni um cat i on. Thi s r es ul t expl ai ns t he i next ract abi li t y of gol d under basi c condi t i ons , wher e t he for mat i on of 18C6 ox oni um cat i on i s i mpos s i ble. The r es ult i ng i on-as s oci at i on pr oduct , [H3O∙18C6]⁺[AuCl4]^-, i ndi cat es a met al t o cr own et her r at i o of 1 :1.

Fi g. 2 Effect of var yi ng t he concent r at i on of 18C6 on t he di st r i but i on r at i o of gol d i n 1, 2- di chl or oet hane at 4mol dm^-3 HCl .

Eff ect of f orei gn ions

The i nt er fer ence due t o t he pr es ence of for ei gn i ons i n t he extr act i on and det er mi nati on of gol d was i nves t i gat ed, and t he met hod`s t ol er ance l i mi t f or each for ei gn i on was i denti fi ed. Under opt i mum condi t i ons , var i ous concent r at i ons of t he for ei gn i on wer e s pi ked i ndi vi duall y i n 1μg cm<sup>- 3</sup> gol d s ol ut i on befor e extr act i on. The r ecover y dat a ar e s ummar i zed i n Tabl e 2. It was obs e r ve d t ha t t he met hod gave excel l ent t ol er ance for for ei gn i ons . Al t hou gh i r i di um<sup>1 3</sup> at 4 mol dm<sup>- 3</sup> h ydr ochl or i c a ci d s ol ut i on i nit i al l y r educed gol d `s r ecover y t o ab out 50% , t he us e of back-was hi ng t e chni que was abl e t o r el ieve t hi s i nt er fer ence.

Tabl e 2 Effect of f or ei gn i ons on gol d ext r act i on

For ei gn i on Tol er ance l i mi t ( t i mes t hat of gol d)

Recover y of Gol d, % Na⁺ 300 103

K⁺ 300 100

Li⁺ > 500 96 Rb⁺ > 500 97 Mg²⁺ 400 98 Ca²⁺ > 500 96 Cu²⁺ 300 100 Fe³⁺ > 400 96 (ppt) Zn²⁺ 300 96 Cd²⁺ > 400 100 Mn²⁺ > 400 101 Pb²⁺ > 500 96 Ni²⁺ 400 98

Co²⁺ > 500 95 NO3- > 500 101 SO42- > 500 97 SCN^- 400 98

Pd²⁺ > 400 102 Rh³⁺ > 400 95 Pt⁴⁺ 200 98 Ru³⁺ 200 102 Ir⁴⁺ > 200 99^a HCl : 4 mol dm^-3; 18C6: 0.05 mol dm^-3; Solvent: 1,2- Dichloroethane; Shaking time: 5 min

a:Aft er back-was hi ng

Anal yti cal per f or mance

A l inear cal ibr ati on cur ve (r²=0. 9999) was obt ai ned i n t he r ange of 0–10μg cm^-3 Au i n 0. 5 mol dm^{- 3} h ydr ochl or i c a ci d aft er meas ur ement s of peak ar ea of absor pt i on us i ng FAAS at 242. 8 nm. The det ect i on l i mit (aver age of bl ank val ue pl us t hr ee t i mes i t s s t andar d devi at i on) was 0. 25 ng cm^{- 3}. The pr opos ed met hod ( Fi g. 3) was appl i ed t o t he s epar at i on of gol d and si l ver us ing s pi ke r ecover y st udi es and t he r esul t s ar e summar i zed i n Tabl e 3.

The preci s i on was good ( s t andar d devi ati ons < 6% ).

A ba ck-was hi ng s t ep was neces s ar y f or t he compl et e s eparati on of gol d and s i l ver, ot her wi s e, 2-6% s il ver can be found i n t he gol d ext r act . In al l cas es , gol d was quant it ati vel y ext ract ed by t he pr opos ed met hod.

-5.4 -5.2 -5.0 -4.8 -4.6 -4.4 -4.2 -1.35

-1.20 -1.05 -0.90 -0.75 -0.60 -0.45 -0.30 -0.15

s l o p e = 1 . 0 1

l o g D

l o g [ 1 8 C 6 ]

o r g

i728 ANALYTICAL SCIENCES 2001, VOL.17 SUPPLEMENT

Fi g. 3 Anal yt i cal s cheme for t he det er mi nat i on of gol d t r aces i n hydr ochl or i c a ci d medi a

Tabl e 3 Separ at i on of gol d and s il ver Metal added,

μg cm^-3

Metal found, μg cm^-3a

Recovery,

%

Au Ag Au Ag Au Ag

1 1 1.01 ±0.01 0.04 ±0.01 101 4 10 1 10.03 ±0.15 0.06 ±0.01 100 6 100 1 95.67 ±1.53 0.02 ±0.01 96 2

1 1 0.99 ±0.04 0^b 99 0

1 10 1.00 ±0.01 0^b 100 0

1 100 1.01 ±0.02 0.029 ±0.005 ^b 101 0.03 HCl : 4 mol dm^-3; 18C6: 0.05 mol dm^-3; Solvent: 1,2- Dichloroethane; Shaking time: 5 min

a: Mean value of 3 determinations b: Aft er back-was hi ng

Tabl e 4 Trace enr i chment of gold in si l ver samples Sample,

100 μg cm^-3

Precon- centration

factor

Gold Added, μg cm^-3

Gold Found μg cm^-3a

Recovery,

%

AgNO3 1 1 0. 96 ±0.02 96

10 0. 1 1. 02 ±0.04 102

Ag₂SO₄ 20 0. 05 1. 04 ±0.06 104 HCl : 4 mol dm^-3; 18C6: 0.05 mol dm^-3; Solvent: 1,2- Dichloroethane; Shaking time: 5 min

a: Mean value of 3 determinations

It i s al s o i mpor t ant t o ment i on t hat t he 4 mol

dm^{- 3} con cent r ati on of h ydr ochl or i c a ci d adopt ed

l i mi ts t he pres ence of s il ver i n s ol ut i on t o a maxi mum of 100μg cm^{- 3} due t o t he obs er ved pr eci pi t at i on ( AgCl ) at hi gher l evel s . On t he basi s of gol d `s ext r act i on behavi or i n Fi g. 1, t hi s l i mi tat i on can be r el ieved by us i ng mor e concent r at ed hydr ochl or i c aci d s ol ut i ons .

Gol d preconcent r at i on

The pot ent ial of t he met hod for gol d pr econ cent r at i on was al so i nvest i gat ed. For t hi s pur pos e, AgNO3 ( 99. 8% pur i t y) and Ag2SO4 ( 90%

pur i t y) fi r s t -cl as s r eagent s wer e s pi ked wi t h 0. 05 – 1. 0μ

g cm

^-3gol d and t he r es ult s ar e gi ven i n Tabl e 4.

Thei r gol d cont ent was under detect i on l i mi t ( 0. 25 ng cm^{- 3}) aft er 20 t i mes pr econcent r at i on. The obs er ved quant i t at i ve r ecover y ( >96% ) on t he ot her hand, s hows t he capabi l it y of t he met hod t o pr econ cent r at e t r ace level s of gol d i n s i l ver mat r i ces up t o 20-fol d.

Bas ed on t he r es ult s by i on-as s oci at i on compl ex f or mat i on, t he pr opos ed met hod was found t o gi ve excel l ent sel ect i vi t y f or gol d ext ract i on over s i l ver and per for m 20- f ol d pr econ cent r at i on. The us e of a mor e s ens it i ve AAS i ns t r ument t her efor e, combi ned wi t h t he met hod`s hi gh t ol er ance for co-exi s t i ng f or ei gn i ons , can l ead t o bett er per for mance whi ch can be appl ied i n t he det er mi nati on of ver y l ow l evel s of gol d s uch as t hat found i n nat ural wat er s.^{1 7}

References

1. S. Xu, L. Sun, and Z. Fang, Anal. Chim. Acta, 1991, 245, 7.

2. E. Ivanova, N. Jordanov, I. Havezov, H. Stoimenova, and S.

Kadieva, Fresenius J. Anal. Chem., 1990, 336, 501.

3. I. Karadjova, S. Arpadjan, and L. Jordanova, Fresenius J.

Anal. Chem., 2000, 367, 146.

4. J. Zhao, Z. Wu and J. Chen., Sep. Sci. Technol., 1999, 34, 2061.

5. A.M. Sastre, A. Madi, J.L. Cortina, F. J. Alguacil, J. Chem.

Technol. Biotechnol., 1999, 74, 310.

6. G. Burmaa, V.A. Potapov, S.V. Anosova, and A.A.

Khlebnikova, Zh. Anal. Khim., 1992, 47, 930.

7. Y.B. Qu, Analyst, 1996, 121, 139.

8. M. Billah, and T. Honjo, Fresenius J. Anal. Chem., 1997, 357, 61.

9. T. Tsuchiya and T. Honjo, Bunseki Kagaku, 2000, 49, 895.

10. K. Z. Hos s ai n, and T. Honj o, I nt er nat i onal Sol vent Ext r act ion Congres s ’99, Book of Abs t r act s, Bar cel ona, Spai n, 1999, 104.

11. K. Z. Hos s ai n, and T. Honj o, Fres eni us J. Anal.

Chem., 2000, 367, 141.

12. K. Z. Hos s ai n, and T. Honj o, Fres eni us J. Anal.

Chem., 2000, 368, 480.

13. K. Z. Hos s ai n, C. T. Camagong and T. Honj o, Fres eni us J. Anal. Chem., 2001, 369, 543.

14. S. Fang and L. Fu, I ndi an J. Chem., 1994, 33A, 885.

15. H. Kos hi ma and H. Oni s hi , Anal yst, 1986, 111, 1261.

16. A. M. Sastre, A. Madi and F. J. Al guaci l , Hydromet al l urgy, 2000, 54, 171

17. R. Ci du, L. Fanfani, P. Shand, W.M . Edmunds, L. Van`t dack, R. Gij bels , Anal. Chi m. Act a, 1994, 296, 295.

Au (10μg cm^-3), Ag (10μg cm^-3)

←4 mol dm^-3 Hydrochloric acid, 5 cm³

←0.05 mol dm^-3 18C6 in 1,2- Dichloroethane, 5 cm³ Shake for 5 min

Centrifuge (2000 rpm for 10 min)

Aqueous phase Organic phase Sample solution

Organic phase 18C6

Aqueous phase Gold

Determine gold by AAS

AgCl

n(n-1)- [H3O∙18C6]⁺[AuCl4]^-

18C6, 5 cm³

←0.1 mol dm^-3 Na2S2O3 in 0.2 mol dm^-3 NH₃, 5 cm³ Shake for 5 min

Centrifuge (2000 rpm for 10 min)