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ANALYTICAL LETTERS, 17(A20), 2359-2368 (1984) 

EXTRACTION OF COPPER(I1) FROM AQUEOUS THIOCYANATE SOLUTIONS 

INTO CHLOROFORM AND SUBSSQUENT SPECTROPHOTOMETRIC DETERMINA- 

TION 

KEY WORDS: Extraction, spectrophotometric method, Copper(II), 

aqueous thiocyanate, 2-benzoylpyridine-2-pyridyl- 

hydrazone, 2,2Ldipyridil-Z-pyridylhydrazone. 

J.A. Stratis, I.N. Papadoyannis and A.N. Anthemidis. 

Laboratory of Analytical Chemistry, University of Thessaloniki, 

54006 Thessaloniki Greece. 

ABSTRACT 

Traces of copper(I1) can be quantitatively extracted from 

aqueous thiocyanate solutions into 2-benzoylpyridine-2-pyridyl- 

hydrazone (BPPH) or 2,2'-dipyridyl-2-pyridylhydrazone (DPPH) 

chloroform solution. Optimal conditions given for the extrac- 

tion are based on a critical study of the relevant factors such 

as the effect of the pH, thiocyanate and reagent concentration. 

Very small amounts of copper can be recovered from large sample 

volumes and determined directly. 

INTRODUCTION 

The solvent extraction of metal chelates have been used 

extensively in preconcentration and separation procedures for 

many metals. The theory of extraction process was developed 

Copyright 0 1984 by Marcel Dekker, Inc 

2359 

0003-27 19/84/ 1720-2359$3 .SO/O


2360 STRATIS, PAPADOYANNIS, AND ANTHEMIDIS 

by Kolthoff and Sandel ' 

by Oosting 

3,4,5 

, Irving and Williams and then 

The majority of chelating ligands used in extraction are 

weak acids, HL, which react in the form of their conjugate 

bases, L-, with metal ions, Mn+, to give uncharged extract- 

able chelates, ML,. In the last two decades some new orga- 

nic reagents have become available which can be considered as 

slightly modified ferroin chromagens. These include certain 

substituted hydrazones (-N=C-NH-N=C-c=N-) that were intro- 

duced by Lions et a1 6'7 Many of these reagents are very 

sensitive for spectrophotometric determination of metals and 

some of them have been examined for extraction of cations and 

their subsequent spectrophotometric rnicr~determination~'~ 

It was observed, that some ions such as cyanide, cyanate, tllio- 

cyanate etc., increase the extraction of copper(I1) complex with 

pyridine-aldehyde-2-pyridylhydrazone (PAPHY), 2-benzoylpyridine- 

2-pyridylhydrazone (BPPH) and 2,2'-dipyridyl-2-pyridylhydrazone 

(DPPH) into organic solvents. An indirect determination of cya- 

nide, cyanate, thiocyanate and selenocyanate by atomic absorption 

spectroscopy and spectrophotometry was developed 

10,ll . These 

considerations prompted a study for possible copper preconcen- 

tration from aqueous solutions and spectrophotometric micro- 

determination of copper. A comparison between BPPH and DPPH 

for extraction of copper(I1) and subsequent spectrophotome- 

tric microdetermination of copper is reported. 

EXPERIMENTAL 

Reagents 

The synthesis of 2-benzoylpy+idine-2-pyridylhydrazone 

(BPPH) and 2,2'-dipyridyl-2-pyridylhydrazone (DPPH) has been 

I 1


EXTRACTION OF COPPER(I1) 2361 

previously reported (1 2,13). Solutions of BPPH and DPPH were 

prepared by dissolving the required weight in chloroform 

and ethanol. These solutions are stable and can be kept 

for several weeks in amber-glass bottles. 

Standard copper(I1) solutions were prepared daily from 

a stock solution 1000ppm Cu(I1) ("Titrisol" Merck) . A standard 

solution of thiocyanate was prepared by dissolving the ap- 

propriate amount of potassium thiocyanate in distilled water. 

The solution obtained was standardized by titration with sil- 

ver nitrate solution conductometrically. 

All other solutions of cations and anions were prepared 

by dissolving analytical grade reagents in distilled water. 

Chloroform was also analytical grade reagent (Merck) and it 

was used without any further purification after being sa- 

turated with distilled water and conversely all distilled wa- 

ter used in extraction procedures was saturated with chloroform. 

Apparatus 

Spectrophotometric measurements were done on Zeiss 

model PMQ 3 and Unicam SP 700A spectrophotometers with 

a 10 nun quartz cells. The pH values were measured by a 

Radiometer model PHM64 Research pH meter calibrated by NBS 

pH standards at 25t0,5 OC. 

Procedure 

Into a 50 ml calibrated flask were pipeted in the fol- 

lowing order: 0.5-10 ml of a copper standard solution (0.50 

ppm, 1.00 ppm, 1.25 ppm 10.00 ppm or 25.00 ppm according to 

the experiment), the appropriate amount of the thiocyanate 

solution, buffered to the required pH by phosphate buffer 

solution and ionic strength 0.1 by Na2S04 and diluted to



the volume with distilled water. 10 ml of this solution was 

transferred into a 50 ml separatory funnel, an equal volume 

4 

of BPPH or DPPH in chloroform (1.2X10- M or 7.5~10-~~) was 

added and the two layers were shaken for 30 min. An aliquot 

of 2.5-3 ml of chloroform extract was transfered to a lorn 

cell and the absorbance at maximum wavelength was measured 

against the reagent blank. 

RESULTS AND DISCUSSION 

Absorption spectra 

The visible absorption spectra of BPPH-Cu, BPPH-Cu-SCN, 

DPPH-Cu and BPPH-Cu-SCN in the aqueous and in the chloroform 

phase are shown in Fig. 1. It can be seen that the copper(I1) 

complexes of BPPH and DPPH give absorption maxima at 480 

and 483 nm respectively, whereas their chloroform extracts 

in the presence of thiocyanate give the corresponding absorp- 

tion maxima at 535 and 525 nm. There is no effect on the spec- 

tra of BPPH-Cu and DPPH-Cu from thiocyanate in the aqeous 

phase, on the other hand thiocyanates increase the extraction 

of these complexes and the degree of extraction of BPPH-Cu 

and DPPH-Cu into the chloroform layer depends on the thiocya- 

nate concentration. 

Effects of experimental conditions 

Measurements of absorbance were repeated at least twice 

and the average of the three absorbance values was taken 

in calculating R, degree of extraction. Since the volumes of 

the two phases were equal, R=A1/Ao, where Ao=absorbance for 

100% extraction, Al=the absorbance for the first extract. A, 

was taken as the sum of the absorbance measured after succes- 

sive extraction of the same aqueous phase containing a known


EXTRACTION OF COPPER(I.1) 2363 

100 500 

A ( riin) 

Fig 1. Absorption spectra of: BPPH-Cu in aqueous phase(1) 

BPPH-Cu in extraxt (2) , extract Of BPPH-Cu-SCN (3), DPPH-Cu 

in aqueous phase (4), DPPH-Cu in extract (5), extract Of DPPH- 

Cu-SCN (6). ICu"] 3.95X10-5M. [BPPH] = 1 .2X10-4M, 

[DPPH] = 1 .2Xlr4M, [SCN-] = ~xIO-~M, pH = 7 .O 20.2. 

quantity of metal with several aliquots of ligand (BPPH or 

DPPH) in chloroform. In presence of thiocvanate ions, the ab- 

sorbance of the second extract was generally very low and 

near to zero. No independent measurement of the concentration 

of metal left in the aqueous phase was made, hence very high 

and very low values of R may be in error. 

60 0 

The effect of pH on the extraction of copper as BPPH-Cu 

and DPPH-Cu from aqueous solutions into chloroform layer is 

shown in Figure 2. 

The optimum pH range for the determination of copper(I1) 

after extraction as BPPH-Cu is 3.8-9.0 and as DPPH-Cu is 5.6 

-10.5 in presence of thiocyanate (curve 2 and 4, Fig 2) and 

the extraction of copper(I1) with BPPH or DPPH can be done



2 L 6 8 1 0 

PH 

Fig. 2. Effect of pll on the extraction of copper. Curve 1: 

BPPII-Cu , curve 2 : RPPII-Cu with thiocyana tc, curve 3 : 

DPPH-Cu, curve 4: DPPH-Cu with thiocyanatc. [Cu 1 

2 .5 X 

5 

10- M, [BPPII] Z7.5 X lr5M, [DPPll] z7.5 X 10-5M, [SCN-] 

4 

2.5 X 10- M. 

better and over a wider pH range when thiocyanate is in ex- 

cess in the aqueous solutions. The pH of aqueous solutions 

was used to be 7.0k0.2. The effect of ligands concentration 

and the effect of thiocyanates concentrationon the extraction 

of copper (11) were studied. 

Figure 3 shows the effect of ligands concentration. The 

concentration of BPPH or DPPH must be 2.2 times greater than 

copper(I1) concentration for quantitative extraction. The li- 

gand concentration was used to be threefold molar excess over 

copper(I1) Concentration. 

Figure 4 Shows the effect of thiocyanate concentration. 

Conformance with Beer's law 

The extraction of copper(I1) from aqueous thiocyanate 

solutions is nearly complete, 97% extraction as BPPH-Cu-SCN, 

++



Pig. 3. Effect of the liqands concentration on thr extraction 

of copper (IT), Curve 1: effect of FPPIl concentration, cur- 

ve 2:effect of DPPFI conccntration, [cu 1~3.95 

[SCN-] 14 X I O-4M, 

100- 

Fj 

L50- 

Q 

rr 

i- 

X 

w 

I 

d 

pH = 7. OkO. 2 

++ 

I 

1 2 3 4 5 I 0 2 0 3 0 

15CN-I :IC ut I 

x 10-5M, 

Fig. 4 Effect of the thiocyanate concentration on the ex- 

traction of copper(I1). Curve 1 in presence of BPPH, 

++ 5 

curve 2 in presence of DPPH. [Cu ] = 3.95X10- MI [BPPH] = 

1 .ZX10-4M, [DPPH] = 1 .2X10-4M, pH=7 .OfO.2.


2366 STRATIS, PAF'ADOYANNIS, AND ANTHEMIDIS 

95% extraction as DPPH-Cu-SCN under optimum conditions. To 

increase accuracy and sensitivity of copper(I1) determination 

a single extraction was carried out9 . The lowest 

effective volume ratio of aqueous solution to organic sol- 

vents for a single extraction was determined. This ratio 

10.14 

was found to be 10: 1 . Figure 5 shows the results 

for single extraction. 

Color stability and precision 

The color intensity of chloroform extracts remains cons- 

tant at least for 3 hours. The reproductibility of the measu- 

rements expressed as relative standard deviation is 1.02% for 

BPPH-Cu-SCN system (X=535nm) and 1.51% for DPPH-Cu-SCN sy- 

stem (A=525nm). The detection limit for both ectraction 

systems is 5 ppb (V :Vo= 50:5). 

W 

Effects of diverse ions 

Diverse ions studies carried out on BPPH-Cu-SCN and 

DPPH-Cu-SCN for copper(I1) determination showed that prac- 

tical analytical procedures were feasible. 0.20 parts per 

million concentration of copper(I1) can be measured without 

interference in the presence of 100 ppm of A13+, As3+, As5+, 

+ + + 2+ 2+ 

, C1-, Br-, I-, CN-, SCN-, NO;, PO:-, 

Na , X , NH4 , Ca , Ba 

2+ 2+ 2+ 2+ 

SO:-. Zn , Co , Ni , Hg , interfere completely at the 

level of 2 ppm. Mn2+ interferes at the level of 5 ppm the de- 

termination of copper(I1) through the BPPH-Cu-SCN extraction 

system, but it does not interfere when copper(I1) is determin- 

ed through the DPPH-Cu-SCN system. 

Comparison between BPPH and DPPH for copper extraction 

Both of the ligands have approximately the same analytical 

properties. The synthesis of BPPH is more difficult than that



100 20 0 

ICU+'l 

300ppb 

Fig. 5. Calibration graphs for Qu(I1) determination. Curve 1 

for BPPH-Cu-SCN system, curve 2 for DPPH-Cu-SCN system. 

4 

[SCN-] -2X10- M, 

Vw=50ml, Vo=5ml. 

[BPPH] =I .2Xl O-4M, [DPPH] =I .2Xl CI-~M 

of DPPH. On the other hand BPPH-Cu-SCN degree of extraction is 

slightly higher than that of BPPH-Cu-SCN and the system BPPH- 

Cu-SCN gives better reproducibility, lower standard deviation, 

for determination of copper(I1). 

REFERENCES 

1. I.M. Kolthoff and E.B. Sandell, J. Amer. Chem. SOC. 1939, 

63 1906. 

- 

2. H. Irving and R.J.P. Williams, J. Chem. SOC. 1949,1841. 

3. M. Oosting, Anal. Chim. Acta 1959 2 301. 

4. M. Oostinq, Anal. Chim. Acta 1959 21 397. 

5. M. Oosting, Amal. Chim. Acta 1959 1 505. 

6. J.F. Geldard and F. Lions Inorg. Cheni. 1963 - 2 270. 

I. R.W. Green, P.S. Hallwan and F. Lions Inorg. Chem. 1964 

- 3, 376. 

8. M. Otomo, Anal. Chim. Acta 1980 116, 161.



9. J.A. Stratis, A.N. Anthemidis and G.S. Vasilikiotis, 

Analyst, 1984 109 373. 

10. J.A. Stratis, Doctoral Thesis, University of Thessaloniki, 

1979. 

11. G.S. Vasilikiotis and J.A. Strat 

29, 209. 

- 

12. J.E. Going and R.T. Pflaum. Anal 

s. Microchemical J. 1984 

Chemistry 1970 42, 1098. 

13. G.S. Vasilikiotis, Th. Kouimtzis, C. Apostolopoulou and 

A. Voulgaropoulos, Anal. Chim. Acta 1974 2, 319. 

14. I.N. Papadoyannis, Th. A. Kouimtzis and G .S. Vasilikiotis 

Microchemical J. 1983, 28, 347. 

- 

Received July 19, 1984 

Accepted September 17, 1984

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