Spectrochimica Acta Part A 75 (2010) 1152–1158
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Spectrochimica Acta Part A: Molecular and
Biomolecular Spectroscopy
journal homepage: www.elsevier.com/locate/saa
Kinetic spectrophotometric method for trace determination of thiocyanate based
on its inhibitory effect
Radhey M. Naik
∗
, Basant Kumar, Abhas Asthana
Department of Chemistry, University of Lucknow, Lucknow 226007, India
article info
Article history:
Received 13 July 2009
Received in revised form
30 December 2009
Accepted 31 December 2009
Keywords:
Determination of thiocyanate
Potassium hexacyanoferrate(II)
Kinetic spectrophotometry
Inhibitory effect
Spiked water
Saliva samples
abstract
A kinetic spectrophotometric method for the determination of thiocyanate, based on its inhibitory effect
on silver(I) catalyzed substitution of cyanide ion, by phenylhydrazine in hexacyanoferrate(II) is described.
Thiocyanate ions form strong complexes with silver(I) catalyst which is used as the basis for its determi-
nation at trace level. The progress of reaction was monitored, spectrophotometrically, at 488 nm (
max
of [Fe(CN)
5
PhNHNH
2
]
3-
, complex) under the optimum reaction conditions at: 2.5 × 10
-3
M [Fe(CN)
6
]
4-
,
1.0 × 10
-3
M [PhNHNH
2
], 8.0 × 10
-7
M [Ag
+
], pH 2.8 ± 0.02, ionic strength () 0.02 M (KNO
3
) and temper-
ature 30 ± 0.1
◦
C. A linear relationship obtained between absorbance (measured at 488 nm at different
times) and inhibitor concentration, under specified conditions, has been used for the determination of
[thiocyanate] in the range of 0.8–8.0 × 10
-8
M with a detection limit of 2 × 10
-9
M. The standard deviation
and percentage error have been calculated and reported with each datum. A most plausible mechanistic
scheme has been proposed for the reaction. The values of equilibrium constants for complex formation
between catalyst–inhibitor (K
CI
), catalyst–substrate (K
s
) and Michaelis–Menten constant (K
m
) have been
computed from the kinetic data. The influence of possible interference by major cations and anions on
the determination of thiocyanate and their limits has been investigated.
© 2010 Elsevier B.V. All rights reserved.
1. Introduction
Thiocyanate has extensive usage as ammonium thiocyanate
is used as important starting material in the manufacture of
thiourea [1], while sodium thiocyanate solution is used as dis-
persive medium for copolymers, obtained in the manufacture of
acrylic fiber [2]. The ammonium and alkali metal thiocyanates are
used in agriculture chemicals as weed killers, dying and print-
ing of textiles, paints for inhibiting rust, photography [3] and
petroleum tracer in oil fields to investigate the distribution of oil
deposits and the stratigraphic structure as well as effectiveness of
water blocking [4]. In respect to above applications, thiocyanate
ion (SCN
-
) is significantly less toxic than that of CN
-
(cyanide) but
its exorbitantly high level can inhibit normal uptake of iodine in
thyroid which reduces the formation of thyroxine [5]. The renal
elimination of low levels of cyanide ion takes place due to its even-
tual conversion to thiocyanate (SCN
-
) ions which is catalyzed by
mitochondrial enzyme rodhanese produced in lever and kidney
[6].
Thiocyanate is also present in low concentrations in human
blood serum, saliva and urine. The high concentration of thio-
∗
Corresponding author. Tel.: +91 9450466126; fax: +91 522 2740916.
E-mail address: naik rm@rediffmail.com (R.M. Naik).
cyanate in body arises from tobacco smoke and clinical studies
have shown that the saliva thiocyanate concentration is higher for
smokers than for non-smokers [7]. The level of thiocyanate is con-
sidered to be a biomarker for distinguishing between smokers and
non-smokers. The saliva thiocyanate may also have an antibacte-
rial role in the mouth which subsequently decreases the corrosion
potentials of amalgams [8]. If the content of this ion is little higher
in the body than normal, the protein dialysis will be affected and it
may even result in coma [6].
The determination of thiocyanate is important in view of its
contaminations in environment which generates the highly toxic
chemical species, such as CN
-
(cyanide), CNCl (cyanogenchlo-
ride) by irradiation and chlorination [1]. The determination of
thiocyanate at trace level has received the attention of analytical
chemists and environmentalists for its hazardous effect on human
health. In recent years, a number of analytical procedures have been
used for the determination of thiocyanate such as ion selective
electrodes, based on a variety of carriers [9–21]. The separation
techniques, such as gas chromatography [22,23], electrophore-
sis [24,25] potentiometric [26], and spectrophotometric methods
(SPMs) [27–35] were also applied to thiocyanate determination in
biological samples.
However, many of these methods are tedious to perform and
involve expensive chemicals and harmful reagents. The use of
kinetic catalytic methods (KCMs), based on the phenomenon of
1386-1425/$ – see front matter © 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.saa.2009.12.078