A comparative study of four 20-membered N2S4-crown ethers as ionophores for polymeric membrane silver selective electrodes

Article abstract of DOI:10.1016/j.cclet.2009.11.028

Four 20-membered N₂S₄-monoazathiacrown ethers have been synthesized and explored as neutral ionophores for Ag⁺-selective electrodes. Potentiometric responses reveal that the flexibility of the ligands has great effect on the selectivity and sensitivity to Ag⁺ ions. The electrode based on ionophore 9,10,20,25-tetrahydro-5H,12H-tribenzo[b,n,r][1,7,10,16,4,13]tetrathiadiaza cycloicosine 6,13-(7H,14H)-dione (C) with 2-nitrophenyl octyl ether (o-NPOE) as solvent in a poly(vinyl chloride) (PVC) membrane matrix shows a measuring range of 1.0 × 10^-6 to 1.0 × 10^-3 mol/L with a Nernstian slope of 54.9 ± 0.3 mV/decade. This electrode has high selectivity for Ag⁺ with negligible interference from many other cations and can be used in a wide pH range of 3.6-9.2.

Full text of DOI:10.1016/j.cclet.2009.11.028

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Chinese Chemical Letters 21 (2010) 464–467
www.elsevier.com/locate/cclet
A comparative study of four 20-membered N2S4-crown ethers as
ionophores for polymeric membrane silver selective electrodes
Jun Zhang a,b,c, Shun Yang Yu a, Tan Ji Yin a, Xue Feng Hu a, Wei Qin a,
*
a Key Laboratory of Coastal Environmental Processes, Yantai Institute of Coastal Zone Research, Chinese Academy of Sciences,
Yantai 264003, China
b Institute of Oceanology, Chinese Academy of Sciences, Qingdao 266071, China
c Graduate University of Chinese Academy of Sciences, Beijing 100049, China
Received 17 August 2009
Abstract
Four 20-membered N2S4-monoazathiacrown ethers have been synthesized and explored as neutral ionophores for Ag+-selective
electrodes. Potentiometric responses reveal that the flexibility of the ligands has great effect on the selectivity and sensitivity to Ag+
ions. The electrode based on ionophore 9,10,20,25-tetrahydro-5H,12H-tribenzo[b,n,r][1,7,10,16,4,13]tetrathiadiaza cycloicosine
6,13-(7H,14H)-dione (C) with 2-nitrophenyl octyl ether (o-NPOE) as solvent in a poly(vinyl chloride) (PVC) membrane matrix
shows a measuring range of 1.0 ꢀ 10ꢁ6 to 1.0 ꢀ 10ꢁ3 mol/L with a Nernstian slope of 54.9 ꢂ 0.3 mV/decade. This electrode has
high selectivity for Ag+ with negligible interference from many other cations and can be used in a wide pH range of 3.6–9.2.
# 2009 Wei Qin. Published by Elsevier B.V. on behalf of Chinese Chemical Society. All rights reserved.
Keywords: Ion-selective electrodes; Silver; Ionophore; Azathiacrown ether
Crown ethers containing S and/or N as donor atoms are of interest as they exhibit high affinities towards transition
metal ions [1]. Most of transition metal ionophores for use in ion-selective electrodes (ISEs), such as Ag+-ionophores,
possess S and/or N atoms. The selectivity of crown ethers towards Ag+ is tunable by altering the composition of donor
atoms, ring size and ligand geometry [2]. Many crown ethers have been utilized as Ag+-ionophores [3,4]. However,
most of them suffer from problems of complicated synthesis procedures and poor selectivities. In this work, four 20-
membered N2S4-crown ethers have been synthesized as Ag+-ionophores by a simple route with high yields, and
electrodes based on these ionophores show excellent affinities to Ag+. The effects of flexibilities of these ligands on the
response of polymeric membrane Ag+-ISEs have been studied.
1. Experimental
The ionophores A–D were synthesized by a three-step reaction (Scheme 1). A solution of 4 or 5 [5] (0.5 mmol) in
DMF (50 mL) and a solution of ethane-1,2-dithiol or benzene-1,2-dithiol in DMF (50 mL) were added simultaneously
to 50 mL DMF containing 2 mmol anhydrous Na2CO3 over 1.5 h at room temperature. At the end of this period, the
* Corresponding author.
E-mail address: wqin@yic.ac.cn (W. Qin).
1001-8417/$ – see front matter # 2009 Wei Qin. Published by Elsevier B.V. on behalf of Chinese Chemical Society. All rights reserved.
doi:10.1016/j.cclet.2009.11.028

J. Zhang et al. / Chinese Chemical Letters 21 (2010) 464–467
465
Scheme 1. The synthesis route of crown ethers A–D.
solvent was removed under reduced pressure. The solid was washed with water and dried in vacuum. The products so
obtained were recrystallized from DMF to give white crystals of A–D [6–9].
The membranes contained (totaling 134.6 mg) ionophore, 2-nitrophenyl octyl ether (o-NPOE), poly(vinyl
chloride) (PVC) and sodium tetrakis[3,5-bis(trifluoromethyl)phenyl]borate (NaTFPB) with a thickness of 180 mm.
All the electrodes were conditioned with 0.01 mol/L AgNO3 overnight. Membrane potentials were measured in the
galvanic cell: SCE//1 M LiOAc/sample solution/ISE membrane/inner solution/AgCl/Ag.
2. Results and discussion
The optimization of ISE membrane was performed by varying different plasticizers and amounts of ionophore and
ion-exchanger. The membrane containing (in wt%) ionophore (1.0), o-NPOE (65.7), PVC (32.9) and NaTFPB (0.4)
shows the best response to Ag+. The selectivity coefficients of Ag+-ISEs were measured according to Bakker’s method
pot
Ag;Et4N
[10] (Table 1). Using log K
¼ 4:84 for the ionophore-free membrane [11], formal complex formation constants,
log b = 6.47, 7.32, 9.56 and 4.63, were obtained for ionophores A–D, respectively. The data show that electrode based
on ionophore C has better selectivity and sensitivity to Ag+ compared to other three ionophores (Fig. 1). Since the
number of donor atoms and the ring-member are the same, the hardness of the benzene group plays an important role in
the selectivity of A–D to Ag+ by affecting the flexibility of the crown ethers. The ISE using ionophore C shows
Nernstian response over a concentration range of 10ꢁ6 to 10ꢁ3 mol/L with a response time of <15 s. The standard
deviation of the Nernstain response slope for one electrode is 0.3 mV/decade (n = 3). The ISE can be used in a pH
range of 3.6–9.2 with a lifetime of 2 months.
The fabricated ISE based on ionophore C was successfully used as an indicator electrode for potentiometric
titration of chloride ions. Fig. 2 shows a typical titration curve of KCl solution with AgNO3 as titration reagent, which
indicates that the end-point can be accurately determined by the proposed ISE.
Table 1
Potentiometric selectivity coefficients, log KApogtJ, and response slopes obtained with membranes based on ionophores A–D.
Ion J
Iono. A
Iono. B
Iono. C
Iono. D
Li+
ꢁ5.68 (47.83)
ꢁ5.35 (60.82)
ꢁ5.12 (57.93)
ꢁ3.63 (53.64)
ꢁ6.64 (29.60)
ꢁ10.83 (35.87)
ꢁ6.27 (32.81)
ꢁ6.47 (32.32)
ꢁ5.85 (34.35)
0.54 (56.94)
ꢁ7.00 (48.75)
ꢁ6.74 (63.56)
ꢁ6.28 (52.40)
ꢁ5.06 (53.63)
ꢁ7.76 (24.83)
ꢁ7.39 (29.32)
ꢁ7.47 (25.12)
ꢁ8.04 (27.35)
ꢁ6.13 (27.39)
ꢁ0.58 (63.13)
ꢁ0.64 (55.02)
ꢁ9.39 (55.35)
ꢁ8.96 (53.85)
ꢁ8.64 (50.78)
ꢁ6.94 (52.68)
ꢁ10.38 (26.94)
ꢁ9.95 (29.81)
ꢁ9.36 (21.34)
ꢁ9.74 (22.55)
ꢁ8.55 (29.05)
ꢁ0.99 (60.30)
ꢁ3.08 (58.75)
ꢁ6.66 (56.0)
ꢁ6.21 (63.3)
ꢁ5.99 (53.90)
ꢁ4.84 (53.70)
ꢁ7.57 (25.9)
ꢁ7.27 (33.9)
ꢁ7.24 (34.8)
ꢁ7.23 (26.8)
ꢁ6.62 (35.0)
ꢁ1.55 (81.4)
1.41 (58.03)
H+
Na+
K+
Mg2+
Ca2+
Cu2+
Cd2+
Pb2+
Hg2+
Et4N+
0.65 (58.31)

466
J. Zhang et al. / Chinese Chemical Letters 21 (2010) 464–467
Fig. 1. EMF responses of the ionophores (A–D)-based Ag+-ISEs. Inset shows the time-dependent EMF response of the C-based Ag+-ISE.
Fig. 2. Titration curve of 50 mL of 10ꢁ3 mol/L KCl with 0.1 mol/L AgNO3 obtained by using Ag+-ISE based on ionophore C.
Acknowledgments
This work was financially supported by the Chinese Academy of Sciences (No. KZCX2-YW-410), the National
Natural Science Foundation of China (No. 40776058), the National 863 High Technology Project of the Ministry of
Science and Technology of China (No. 2007AA09Z103), the Outstanding Youth Natural Science Foundation of
Shandong Province (No. JQ200814) and the Taishan Scholar Program of Shandong Province.
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