Journal of Nuclear and Radiochemical Sciences, Vol. 11, No. 2, pp. 13-18, 2010

ArticlesRadiotracer Study of Synergistic Effects of Organophosphorus Compounds on the
Extraction of Silver by N-(thio acetyl) benzamide into n-butanol
P. Dey and S. Basu*
The Department of Chemistry, the University of Burdwan, Burdwan 713104, India
Received: October 12, 2010; In Final Form: December 9, 2010
The extraction behaviour of Ag(I) from an aqueous nitrate medium employing (N-thio acetyl) benzamide(HA) in
n-butanol has been investigated in presence of several organophosphorus donors like, tri-butyl phosphine oxide,
tri-phenyl phosphine oxide and tributyl phosphate at pH 4.0 adopting radio tracer technique. The synergism wasfound in presence of neutral donors due to the formation of the adduct [Ag (A) (H2O) (B)] in n-butanol (B denotes
neutral donor). The overall equilibrium constant for each of the three ternary complexes and also for binary system were calculated from extraction data. It was found that the extent of extraction of Ag (I) in organic phase bothas the binary complex as well as ternary complex increased with increase in temperature. The temperature dependence of the equilibrium constants were investigated in order to evaluate the thermodynamic parameters likeenthalpy (∆H°), entropy (∆S°) and free energy (∆G°). Extraction mechanism was finally supported by I.R spectraof ternary adduct.

1. Introduction

2. Experimental

Silver occurs in small quantities in nature along with coinage metals and is a metal of commercial impor tance.
Inhalation exposure to silver leads to adverse cardiovascular or
musculoskeletal effects and respiratory irritation. Acute inhalation of aerosol containing colloidal silver leads to ultra-structural damage and disruption of cell of tracheal epithelium. It
also leads to abdominal pain as well as gastrointestinal pain.1-2
Because of the above mentioned hazards; there is always a needfor a good separation method of Ag (I) from industrial andphotographic wastes. This precious metal is used largely in
medicine3, the disinfection of water4, photography5, electronic
devices, and cloud seeding.6 Silver also enters the environment
from industrial waters, because it often occurs as an impurity
in copper, zinc, arsenic, and antimony ores.7 Bowen8 also hadsuggested that silver might pose a potential risk as a water pollutant because of the lack of recycling of mined silver. Liquid–
liquid extraction methods are frequently used for the separationand preconcentration of silver.9-12 A considerable amount ofwork has been carried out on solvent extraction of silver withsulfur-containing extractants from nitrate or sulphate media13-21,
or calixarene derivatives22-24 and calixarene analogs25, quaternary amine with chelating ligand HTTA,26 as mixed extraction
or extraction of Ag (I) from aqueous nitrate medium with host–
guest assemblies involving macrocycles and different carboxylic acids.27-28 But only a few attention has been paid on thesynergistic effect of phosphine oxides on the extraction of Ag(I) from aqueous nitrate medium.29
In the present investigation, systematic study of synergistic
effects of phosphine oxides and tributyl phosphate on theextraction of Ag (I) from aqueous nitrate medium with N-(thioacetyl) bezamide into n-butanol is being reported. The ligand
was synthesized and cha racter ized in our laborator y.
Temperature dependence of extraction equilibrium have beenstudied in order to evaluate the thermodynamic parameters
responsible for the extraction. Finally, IR data were used tosupport the extraction mechanism.

2.1. Reagents and instrument. The chelating ligand (Nthio acetyl) benzamide was synthesised by the reaction of thioacetamide and benzoylchloride, derived form benzoic acid,
which were obtained from Aldrich, USA. The donor reagentsused in this work e.g tri-butyl phosphine oxide (TBPO), triphenyl phosphine oxide (TPPO), tri-butyl phosphate (TBP),
were also collected from Aldrich; USA. The solvents were
purified by standard procedures. All other reagents used wereof A.R. grade. A stock solution of silver was prepared by dissolving A.R. AgNO3 in deionized water and a working stock
was prepared by appropriate dilution. In order to spike thesolution, an appropriate volume of stock solution of Ag(I) was
mixed, with few drops of tracer solution containing, 110m Ag as
AgNO3 supplied by BRIT, India and finally made up to themark. Microanalysis of carbon, hydrogen and nitrogen content
of the synthetic ligand were achieved in a Perkin-Elmer 2400
CHNS-O elemental analyzer. Infrared spectra of ligands andadduct complexes were recorded in a Perkin-Elmer FTIR
Model RX1 Spectrometer. A systronics model 335 digital pHmeter equipped with a single electrode was used for pH measurement. A single channel γ -ray spectrometer coupled with awell-type NaI (T1) detector of Nucleonix, India make was used
for radio-activity measurement. The radioisotope 110m Ag was
assayed by its 667 kev photopeak. Thermal analyses ofextracted species were carried out in a PerkinElmer Diamond
TG/DTA analyzer heated from 30 °C – 750 °C under N2.
2.2. Extraction procedure. In the general extraction procedure, 5 ml aqueous solution of AgNO3 containing~ 52 µg
spiked Ag(I), and adjusted to a pH 4.00 was extracted withequal volume of n-butanol solution of chelating ligand for aperiod of 30 minutes in a mechanical shaker. Ionic strength of
the solution was kept fixed by using 0.50 M KNO3. For synergistic study, organic phase was mixed with appropriate donor
reagent of desired concentration. After the equilibration, twophases were separated and radio-activities of equal volumes ofboth of the two phases were measured. Distribution coefficient(D) were computed from,*Corresponding author. E-mail: basu_sukalyan@yahoo.co.in© 2010 The Japan Society of Nuclear and Radiochemical Sciences
Published on Web 01/14/2011

14

Dey

J. Nucl. Radiochem. Sci., Vol. 11, No. 2, 2010
D=

Radioactivity in organic phase
Radioactivity in aqueous phase

Material-balance in the extraction was checked from the countsof two phases separately.
2.3. Synthesis and characterisation of the chelating agent.
The chelating ligand was synthesized by the general condensation reaction between benzoyl chloride and thioacetamide (1:1).
The starting material, A.R. benzoic acid (2.00 g) was refluxedfor 2 h with successive addition of thionyl chloride. The excess
of chlorinating agent was removed by slow evaporation. In abeaker, a saturated solution of thioacetamide (1.24 g) in double
distilled nitrobenzene solvent, free from any water, was prepared. The solvent was purified by drying over calcium oxide
followed by distillation before use. This thioacetamide solution was then condensed with benzoyl chloride for 1 h over aboiling water bath. The brown color precipitate so appeared
was separated by filtration process, recrystallized from chloroform solvent and dried. The dried compound melts at (94 ±2°C). The proposed formula of the ligand (Scheme 1) is ingood agreement with the stoichiometries obtained from their
CHN elemental analysis data as well as IR and 1H NMR data.
From CHN analysis, Found, % ; C 60.26, H 4.98, N 7.65
Calculated, % ; C 60.33, H 5.02, N 7.82. In the IR spectrum of
the ligand absence of band at 3760 cm-1 due to νOH (present inoriginal acid ) and clear presence of sharp peaks at 1689 cm-1
(due to carbonyl group of acid)30 corresponding to support onlyhydroxyl group of acid take part in condensation reaction.
Further observation of νC=S at 1175 cm-1 supported the view thatthe thio group in thioacetamide is not affected in the course ofsynthesis. 1H NMR data also agrees well, δ, at 7.6 ppm (5H,
Aromatic), methyl proton occur at 2.3 ppm δ (s 3H, methylgroup), Amide proton –HN- arise at 8.1 ppm δ (s, 1H, proton)31
COOH

O

O

SOCl2
Reflux 2h

C Cl

NH2CSCH3
Double distrilled
Nitrobenzene solvent 1hCHNminimum for Carbontetrachloride. The trend of extraction fororganic solvents used follows the order: Carbontetrachloride <Chloroform < Xylene < Toluene < Ethylacetate < iso-amyl alcohol < n-butanol (Table 1.). Butanol being polar solvent allows
water molecules in metal coordination sphere and thus, extracts
the hydration metal in most.
3.3. Effect of pH in aqueous phase. Experiments were carried out in different pH by using 0.014 mol.dm-3 N- (thio acetyl)
benzamide into n- butanol. At first, it was observed that thepercentage of extraction increased with increasing pH but
above pH 4.0, the extent of the extraction decreases. The plotof logDo against pH resulted in a straight line with slope ∼ 1(Figure 1.) indicating that only one H + ion was liberated from
HA during the extraction processes. An optimum pH of 4.0
was selected for the experiments. [Do = distribution coefficientin presence of only ligand]3.4. Extraction of Ag (I) with N-(thio acetyl) benzamide(HA). Extraction of Ag (I) from nitrate medium was investigated using n-butanol as a diluent. The concentration of ligand
was varied from 0.35 × 10 -2 mol.dm-3 to 1.45 × 10 -2 mol.dm-3 inTable 2. The percentage of extraction was found to increase
with increase in ligand concentration at fixed pH 4.0. At higher
concentration of ligand, effective stripping of the water molecules take place from the hydrated silver ion sphere by theligand, rendering the species much more organophilic innature. The linear plot of logDo against concentration of ligand
with a slope ∼ 1 Figure 2, indicating one molecule of ligand(HA) is involved in the extraction of silver according to equation (1). As the ligand is bidentate in nature, only one moleculeof ligand is sufficient to balance the charge of monovalent
metal ion.
The overall binary extraction is described as,k+
Ag+aq + [HA] org + 2H2O →← [Ag(A)(H2O) 2] org + [H]

CH3
S

-0.4

Scheme 1. Synthesis of chelating ligand N-(thio acetyl) benzamide.

3.1. Effect of equilibration time. Preliminary studies on
the extraction of Ag (I) by N-(thio acetyl) benzamide(HA) in
n-butanol show that the two phase reaction is fairly rapid andthe equilibrium is reached within 30 minutes. The increase of
the time of equilibration does not affect the extraction equilibrium.
3.2. Effect of organic diluents. The effect of organic diluent on the extraction was studied and it was found that maximum extraction was obtained in the case of n- butanol and
TABLE 1: The extraction of Ag (I) with 0.0018 mol.dm-3
of N-(thio acetyl) benzamide in different diluent at pH 4
D0
0.135
0.241
0.341
0.356
0.132
0.456
0.689

% Extraction
11.89
19.41
25.42
26.25
11.61
31.31
40.79

logDo

-0.6

3. Results & Discussion

Diluents
Carbontetrachloride
Chloroform
XyleneTolueneEthylacetate
Iso-amyl alcohol
n-butanol

(1)

-0.8

[ligand] = 0.0014 mol.dm -3

-1.0
-1.2

2

3

4
pH

5

6

Figure 1. Plot of logD0 vs pH for Ag(I) –ligand system in butanol.

TABLE 2: Exaction of Ag(I) with N-(thio acetyl) benzamide ligand only in butanol at pH = 4.0
LigandConcentration
mol.dm-3×10 -2

Do% Extraction

N-(thio acetyl)
benzamide

0.35
0.75
1.05
1.4

0.073
0.163
0.276
0.491

4.21
14.05
21.38
29.07

Radiotracer Study of Synergistic Eeffects of Organophosphorus

J. Nucl. Radiochem. Sci., Vol. 11, No. 2, 2010
A, [TBPO]
B, [TPPO]
C, [TBP]

-0.05

-0.4

15

-0.10

logDmix

-0.6

logDo

-0.8

-0.15
-0.20
-0.25

-1.0

-0.30
-1.2

[Ligand] = 0.0075 mol.dm -3

-2.7
-2.40

-2.25

-2.10

-1.95

-2.6

-1.80

-0.15

-2.3

-2.2

Figure 4. Plot of logD mix vs log[donor] for extraction of Ag(I) –[ligand]- donor ternary system at pH 4.0 in n- butanol keeping [ligand]= fixed.

[TPPO] = 0.0021 mol.dm -3-0.30

logD mix-2.4

log[Donor]

log[ligand]Figure 2. Plot logD0 vs. log[ligand] for Ag(I) – ligand system.

-2.5kAg+aq + [HA] org + [B] org + H2O →
←

-0.45

[Ag(A)(H2O)(B)] org + [H] +aq-0.60

Hence, ternary adduct formation may be described as,
Obviously, Equilibrium constant (K) is shown by[TBP] = 0.0084 mol.dm -3

-0.75

(4)

[TBPO] = 0.0032 mol.dm -3

-0.90

K=
-2.5

-2.4

-2.3

-2.2

-2.1

-2.0

-1.9

[Ag(A)(H2O)(B)] org [H] +aq[Ag+] aq.[HA] org.[B] org(5)log[Ligand]Figure 3. Plot logDmix vs log[ligand] for extraction of Ag(I) –[ligand]- donor ternary system at pH 4.0 in butanol keeping [donor] = fixed.

Dmix =[Ag(A)(H2O)(B)] org + [Ag(A)(H2O) 2] org[Ag+] aq

(6)

But, distribution coefficient (D mix) for mixed extraction is,Combining Eqs. (2), (5) and Eq (6), we get
The distribution coefficient is defined by,
D0 =[Ag(A)(H2O) 2] org[Ag+] aq

K=
(2)

Thus equilibrium constant (k) for binary extraction is related tothe distribution coefficient by,logk = logDO – log[HA] – pH

(3)

3.5. Synergistic extraction of Ag (I) complex in presenceof phosphine oxide andtributyl phosphate. The distribution
ratio of Ag (I) extracted with HA increases by the addition ofdifferent organophosphorous compounds. This is attributed to
the formation of a more lipophilic adduct with these donors. In
the ternary extraction system, the log Dmix vs. log [HA] plots atfixed concentration of donor and fixed pH of aqueous phaseyielded slope of ∼1 Figure 3, while the log Dmix vs. log [donor]
plots at fixed concentration of ligand and fixed pH of aqueous
phase showed straight line behavior with a slope also ∼1 in
Figure 4. suggesting the composition of the extracted species of
the type Ag(A)(B). The overall reaction of the generally
accepted synergistic extraction of silver ions is given by Eq. (4)where one molecule of ligand and one molecule donor (B) arepresent in the extracted adduct.

(Dmix – Do)[H] +aq[HA] org [B] org(7)

Taking logarithm and rearranging,
logK = log (Dmix – Do) – log[HA] org – log[B] org – pH

(8)

Extraction of Ag (I) with pure donor is negligible under thepresent condition.
The adduct formation reaction in the organic phase may be
represented as
KS
Ag(A)(H2O) 2 + [B] org →[Ag(A)(H2O)(B)] org + H2O(9)3.6. Degree of synergism in extraction and equilibrium
constant. The degree of synergism can be quantified by synergistic coefficient (SC). It is used to describe enhancement in
the equilibrium distribution ratio when two or more extractants
are used as a mixture for extraction. SC = log [Dmix/(Do +DS)]
Where D mix , D o and DS denote the distribution ratios of ametal ion with mixture of ligand and donor, pure ligand (O),and only donor (B) respectively. In the present case, DS = 0,Dey

J. Nucl. Radiochem. Sci., Vol. 11, No. 2, 2010

Donor
TBPO

TPPO

TBP

Conc of donor
mol.dm-3
0.00210.00320.00420.00210.00310.00410.00380.00480.0059

Dmix
0.573
0.782
0.912
0.550
0.761
0.881
0.487
0.592
0.686

%
Extraction
27.16
32.56
47.69
25.92
31.57
46.84
32.75
37.18
40.68

SC

logβ

0.590
0.678
0.750
0.525
0.666
0.730
0.472
0.557
0.621

3.075
3.071
3.035
3.049
3.069
3.027
2.715
2.735
2.731

Where, SC = Synergistic coefficient = log D mix /(D 0) and β(mol-1) = (D mix – Do)/Do [B], apparent formation constant B =Donor
TABLE 4: Equilibrium constants for binary and ternary
systemBinary system
Systemlogk
Ag(I) –ligand-2.62

Ternary System
Donor
logK
logKS
TBPO
TPPO
TBP

0.403
0.378
0.051

3.023
2.998
2.671

The apparent stability constant (β) is given by: β = (Dmix – Do)/Do [B]
Synergism was observed at all the compositions studied.
Keeping the concentration of ligand constant it was observedthat the synergistic coefficient increased with an increase in
the concentration of the synergist, organophosphorus extractant
Table 3, at fixed pH of the aqueous phase. The trend in SC andβ-value shown in Table 3, for organophosphors donors taken up
here follow the order: TBPO >TPPO> TBP keeping parity with
their electron donor abilities. Thus, TBP having much poor
donor ability with respect to two phosphine oxides results only
low values of stability constant in compare to TBPO and TPPO
Again If we compare the logK (ternary equilibrium constant)and logKS (adduct formation equilibrium constant) values Table
4, it is observed that the above two parameters are found to behighest in case of tri-butyl phosphine oxide (TBPO). The
logKS values clearly show that TBPO is the most effective synergistic agent among the three organophosphorus donors usedin present investigation. This trend can be correlated with therelative basic natures of the oxophosphorus donors. Somewhat
similar trend was observed in the synergistic extraction of othertransition metals and actinides. 32-34 Overall enhancement
effects of organophosphorus compounds are found to be veryhigh. The liquid-liquid extraction of a metal ion with a chelating agent is essentially determined by the stability and extractability of uncharged metal complex e.g. [Ag (A) (H2O) (B)] inpresent case. Stability of ternary adduct reported in present
work is found to be much higher than corresponding organophosphorus adduct of EHPNA complexes of lanthanides35 or
salicyldyl complexes of Zr(IV).36 Presence of soft ligand for asoft metal like silver maybe responsible for higher stability ofadducts and addition of phosphine oxide donors on such system
accelerates the degree of extraction into nonpolar solvent.
3.7. Extraction mechanism and thermodynamic parameters. Aqueous solution of silver ion exist as Ag(H 2 O) 4 + i.e.
tetra coordinated silver ion with two molecule of water held
firmly and two loosely bound molecules of water37, due to the

0.5
logK

TABLE 3: Synergistic extraction of Ag (I) by mixture ofc h e l at i n g l i g a n d a n d or g a n op h o s p h or u s do n or.
Concentration of ligand = 0.0075 mol.dm-3, pH = 4.0, solvent = n-butanol, D0 = 0.163,

0.0A, [Ligand] = 0.0073
B, [TBPO] = 0.0021
C, [TPPO] = 0.0021
D, [TBP] = 0.0038

-0.5
-1.0

logk

16

-2.0
-2.5
3.15

3.20

3.25

3.30

3.35

-1

3

1/T K × 10

3.40

3.45

Figure 5. Temperature variation plot of equilibrium constant ofbinary and ternary systems. The line ‘A’ only presence of ligand, and‘B’, ‘C’, ‘D’ lines are the synergistic mixture of different donor presence of fixed ligand concentration. Exact concentration of donors
and ligand in mol.dm-3 are shown in the figure.

TABLE 5: Thermodynamic parameters related to binary
and ternary extraction system
System∆H°(kJ.mol-1)

Ag(I) –Ligand
23.62 ± 0.2
Ag(I) –Ligand-TBPO 10.13 ± 0.1
Ag(I) –Ligand-TPPO 7.62 ± 0.2
Ag(I) –Ligand-TBP
5.32 ± 0.3∆S°(JK .mol-1)-1∆G°(kJ.mol-1)
at 27°C

29.56 ± 0.5 14.81 ± 0.1
42.44 ± 0.1 -2.61 ± 0.4
33.27 ± 0.3 -2.36 ± 0.2
21.47 ± 0.2 -1.12 ± 0.2

fact that silver ion is a soft metal and water molecules are hardin nature.38 HSAB theory also confirms that, the selective
ligand containing N or S donor atoms interact strongly with
soft acids like Ag(I) even in the presence of high concentrationof protons and loosely bound water molecule are released fromco-ordinated sphere.
The equilibrium extraction constants logk and logK for thestudied complexes were obtained from the equilibrium metal
concentration in the organic and aqueous phase using Eqs. (3)and (8). From the values of equilibrium extraction constant at
the temperature range investigated, enthalpy changes were calculated using the Vant’s Hoff equation.39
logK = –∆H°+
2.303RT∆S°
2.303R(10)

The plot of logK against 1/T Figure 5 is a straight line where
the slope gives the enthalpy of reaction (∆H°) and intercept
corresponds to entropy (∆S°) value and these data were used toobtain the corresponding free energy changes. The results are
summarized in Table 5. The thermodynamic data can be
explained on the basis of two factors associated with the process of extraction e.g, 1) The dehydration of the metal cationand the hydration of the released protons, 2) The combinationof metal cation and ligand anion in the organic phase and thesimultaneous deprotonation of the for med chelates. 4 0
Concerning the enhanced entropy contribution, the literature
data41 in similar cases show that the degree of order which islost during the dehydration of the cation is not compensated by
the order caused by the hydration of the proton. Again, it iswell-known that for inner-sphere complexes the entropy term isfavored and the enthalpy term is disfavored i.e.,.42 In such com-

Radiotracer Study of Synergistic Eeffects of Organophosphorus

J. Nucl. Radiochem. Sci., Vol. 11, No. 2, 201017

Figure 6A. Thermal plot of binary extracted complex.

Figure 6B. Thermal plot of ternary extracted complex.

plexes the hydrated sphere is disrupted more extensively andthe net entropy and enthalpy changes are positive. On the other
hand the lower values of (∆H°) of adduct than single extractionof metal, is indicative of relatively smaller heat effects involvedin the formation of the extracted complex. It is interesting tofind that the enthalpy change of the synergistic extraction process is relatively small in the experimental temperature range.
From these values it is clear that the adduct complex formation
reaction proceeds spontaneously in the forward direction, withrelatively high entropy gain than binary complex at the standard state conditions. This maximum entropy gain is explainedby the extensive disruption of the hydration sphere when adductis formed and hence net disorder of the system which contributes to increase positive ∆S° change as a feature of the innersphere complexes. Therefore synergistic effect of neutral donor
is explained by the replacement the co-ordinated water molecule from primary co-ordination sphere of the metal.
3.8. Infrared spectroscopy. The infra-red spectrum of theextracted compound was recorded after evaporating the solvent. From spectral data, it was observed that the νC= S band
was shifted from 1175 cm-1 in free ligand to 1125 cm-1 in theextracted complex.30 This indicated coordination took placethrough ‘S’. Silver being soft metal prefers to bond soft centre‘sulfur’ atom in ligand. Hence νC=S frequency was shifted todown field. Instead, new absorption bands appear at 1556 cm-1,
which was assigned to the C=N stretching vibrations. Thisimplies that the complexes incorporate deprotonated ligands.43
Similarly the νP=O band has been shifted from 1144 cm-1 in freephosphine oxide donor to 1115 cm -1 which characterize the
P=O-Ag bond in adduct complex. Presence of water molecules
in both binary and ternary extracted species ware indicated bya band at 3455 cm-1 and at 1384 cm-1.
3.9. Thermal analysis of extracted complexes. Ag (I) is
generally know to form 2-coordinates and 4-coordinates complexes but with ligands having π - bonding ability tetra-coordinate complex predominate.44 Hence, in binary extract
complexation with HA having π - acid character, two cites areoccupied by bidenate HA while two other sites are filled bywater molecules in order to keep electroneutrality. In presenceof donor (B), one water molecule is replace by donor in ternarycomplex. Presence of water molecules in binary and ternarycomplexes were indicated by IR data. Number of water molecules was confirmed by thermal analysis of extracted complexes which were evaporated to remove the solvent. Both TG
and DTA data were recorded between 30 °C to 750 °C in astatic atmosphere of N2 (Figure 6A & 6B). Results clearlyshow a loss in weight which nearly corresponds to two molecule of water in binary system and one molecule of water by
ternary complex. Loss of water was 8.36% against a theoretical value 11.07% in binary complex while it was 5.77% against
theoretical value 3.5% in mixed adduct in a temperature range
above 100 °C. If there would not be any water molecule present in batter case, there would not be any loss in weight in thistemperature range.

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J. Nucl. Radiochem. Sci., Vol. 11, No. 2, 2010

4. Conclusion.
Bidentate chelating agent used in this work has fair degreeof extractability for silver from aqueous nitrate medium.
Extraction is further enhanced by the addition of organophosphorus donors which enter into the inner-sphere of metal complex by the dehydration. Applications of such ligand may be ofresearch interest for the solvent extraction of other soft metalions.
5. Acknowledgement.
One of the authors (P. Dey ) acknowledges CSIR for providing him a fellowship. Authors also thank Dr.(Mrs) S.
Bhattacharyya (R.R.R. College, W.B, India) for her interest in
the present work.
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