A new and sensitive reaction rate method for spectrophotometric determination of trace amounts of thiourea in different water samples based on an induction period
© Arab Chamjangali et al.; licensee Springer. 2015
Received: 30 November 2014
Accepted: 10 February 2015
Published: 4 March 2015
Thiourea (TU) has various industrial, agricultural and analytical applications. TU has been labeled as having carcinogenic activity. Hypothyroidism was induced in animals by using TU. A simple and sensitive spectrophotometric reaction rate method was proposed for the determination of trace amounts of TU.
The method is based on the inhibitory effect of TU on the rate of meta cresol purple (MCP) with bromate in the presence of bromide. The reaction progress was followed by monitoring the absorbance of MCP at 525 nm.
The effects of different variables on the sensitivity of the proposed method were studied and optimized. Under optimum conditions a linear relationship between induction period time and TU concentration was found in the concentration range of 0.10 – 6.0 μg mL−1 of TU. The detection limit (3σ) of 0.020 μg mL−1 was found. The relative standard deviations for six replicate determinations of 0.10, 2.0 and 5.0 μg mL−1 of TU were 2.3%, 1.8% and 1.1%, respectively.
In this study a new reaction system was proposed for the kinetic spectrophotometric determination of TU in water samples. The new method not only benefit from high selectivity and sensitivity, but also it has the advantage of fast and simple operation.
Thiourea (TU) has various industrial, agricultural and analytical applications. This material is widely used in photography as a fixing agent and also removes stains from negative. In agriculture, it is used as fungicides, herbicides and rodenticides (Pérez-Ruiz et al. 1995) and also to decrease the content of nitrifying bacteria in soil (Smyth and Osteryoung 1977). TU is also used for induction of early ripening in several fruits (de Oliveira et al. 2004). In analytical chemistry TU is used as a spectrophotometric reagent for determination of several metals (HE et al. 1999). TU is also used as a reagent for copper electrolytes refinery (Akeneev et al. 2005). Compounds of TU are often added to citrus fruits as a fungicide during cold storage. TU has been labeled as having carcinogenic activity. Hypothyroidism was induced in animals by using TU (Sokkar et al. 2000; Bhide et al. 2001). Therefore, determination of TU at trace levels is of interest. There are some problems with determination of TU in waste water. These problems are due to the existence of a large number of organic components with relatively high concentrations, non-extractable nature of TU with traditional organic solvents and poor volatility of TU, which is not easily analyzed by gas chromatography (Toyoda et al. 1979). However, various methods have been proposed for the determination of TU such as titrimetry with iodine (Amin 1985; Pillai and Indrasenan 1980) or N-bromosuccinimdie (Sarwar and Thibert 1968), Raman spectroscopy (Bowley et al. 1986), spectrophotometry (Abd El-Kader et al. 1984; Hutchinson and Boltz 1958), polarography (Trojánek and Kopanica 1985), voltammetry (Stará and Kopanica 1984), liquid chromatography-mass spectrometry (Xiao-Lan et al. 2009), high performance liquid chromatography (Rethmeier et al. 2001), ion selective electrode potentiometry (Radić and Komljenović 1991), FTIR (Kargosha et al. 2001), and tandem mass spectrometry (Raffaelli et al. 1997).
One of the useful methods which have been widely applied to trace determination of analytes is kinetic spectrophotometric method in which the main required equipment is a spectrophotometer. Based on the literature survey only a few numbers of indicator reactions for the kinetic determination of TU by spectrophotometric method have been published (Abbasi et al. 2009; Abbasi et al. 2010). All of these methods are based on the catalytic effect of the TU on the certain indicators reactions. To the best of our knowledge, there is no report on the use of induction period effect of TU for its kinetic determination and so this is the first report on the kinetic determination of TU based on induction period on the MCP-bromate-sulfuric acid as a novel reaction system.
In the present report a new sensitive and selective kinetic spectrophotometric method is proposed for determination of TU based on the induction period associated with TU on the catalytic oxidation of MCP by bromate. TU acts as an inhibitor on the catalytic effect of bromide ion. The reaction induction period at 525 nm is proportional to the TU concentration.
Reagents and solutions
A 1000 μg mL−1 stock standard solution of TU was prepared by dissolving 0.1020 g TU (Merck) in distilled water and diluting it to 100 mL. In order to prepare the working solutions, appropriate dilution of the stock standard solution was carried out. For preparing 0.013 M potassium bromide solution in 100 mL volumetric flask, 0.1544 g of KBr (Merck) was dissolved in distilled water and diluted to the mark. A 100 mL 5.2 × 10−4 M MCP solution was prepared by dissolving 0.0200 g of MCP (Merck) in 25 mL ethanol and diluted with distilled water in a 100 mL calibrated flask. In order to prepare a 100 mL bromate ion solution (0.030 M), 0.5010 g of KBrO3 (Merck) was dissolved in distielld water. Sulfuric acid solution (0.30 M) was prepared by diluting a known volume of concentrated solution (Merck) and standardized against sodium carbonate. All reagents used in this study were of analytical grade and double distilled water was used to prepare sample solutions.
Absorption-time graphs at a fixed wavelength and absorption spectra were recorded on a Shimadzu UV-160 Spectrophotometer with a pair of 1.0 cm quartz cell. In order to control the temperature of the reaction a water bath thermostat (n-BIOTEK, INC, model NB-301) was used in this study. A stopwatch was also applied to record the time of the reactions.
General procedure for determination of TU
The reagent solutions and water were kept at 25°C in the thermostatic water bath for 30 min. An appropriate volume of sample or standard solutions were transferred to a 10.0 mL standard flask, then 1.0 mL of 0.30 M sulfuric acid, 1.0 mL of 5.2 × 10−4 M MCP and 1.0 mL of 0.013 M potassium bromide solution were added sequentially and the mixture was then diluted to ca. 8 mL. After mixing, 1.0 mL of 0.030 M KBrO3 was added and diluted to the mark with doubly distilled water. The stopped clock was started and after transferring ca 2 mL of this reaction mixture to spectrophotometer cell. The change in the absorbance at 525 nm was recorded against water for the first 15–450 s reaction time interval. The same procedure was applied for the measurement of the blank solution (without TU). In order to construct a calibration graph reaction induction period (tip) was plotted against TU concentration in a series of standard working solutions.
Results and discussion
Optimization of variables
In order to find the optimum conditions, the effects of different variables on the absorbance changes of the catalyzed (ΔAc) and inhibited (ΔAi) reaction were studied during a fixed time of 15–115 s. One-at-a time optimization procedure was applied in this study and the difference between absorbance changes of catalyzed and inhibited reaction (ΔA = ΔAc - ΔAi) was determined and used as an analytical signal.
Based on primary studies, acidic media is the best media for observation of reaction induction period caused by TU. Thus, different acids such as sulfuric, hydrochloric and nitric acid with the same concentration were tested to find the best type of reaction medium. According to the results, sulfuric acid showed higher sensitivity. So, sulfuric acid was chosen as the best reaction medium.
The sensitivity increased slightly as the concentration of MCP increased from 1.0 × 10−5 to 5.7 × 10−5 M and then it deceased. For this reason, 5.2 × 10−5 M of MCP was selected for the recommended procedure.
The other parameter which has a severe effect on the rate of both catalyzed and inhibited reactions in this study is the temperature. So, by applying the optimized concentration of reagents, the effect of temperature was studied in the range of 5 – 40°C. The results showed that the analytical signal was increased with increasing temperature. However 25°C (about room temperature) was used during this study.
In order to study the impact of ionic strength on the reaction induction period (analytical signal used in construction calibration curve) potassium nitrate (2.0 M) was used under the aforementioned optimum values. Based on the obtained results, the induction period was not affected by the ionic strength up to 0.32 M (maximum value tested) of sodium nitrate.
Interferences for the determination of TU (0.50 μ g mL −1 )
Tolerance limit (W Species /W TU )
Co+2, Na+, Ca+2, Al+3, K+, NO3 −, S2O8 −2, Urea , Glucose
Cd+2, Zn+2, Mn+2, Mg+2, Fe+3
Citrate, Acetate, Ba+2
EDTA, Pb+2, Oxalate
Formate, Salicylate, PO4 3−
Cl−, Tartaric acid
Cr2O7 2−, Ag+
Calibration and analytical parameters
The analytical parameters of reported methods for determination of TU
Linear range (μg mL −1 )
Detection limit (μg mL −1 )
Toyoda et al. 1979
Kinetic catalytic spectrophotometry
Abbasi et al. 2009
Kinetic catalytic spectrophotometry
Abbasi et al. 2010
Tandem Mass Spectrometry
Raffaelli et al. 1997
Kinetic induction base spectrophotometry
0.10 – 6.0
Results for determination of TU in different spiked water samples
Add (μg mL −1 )
Found (μg mL −1 )
RSD% (n = 3)
Real sample analysis
The proposed method was effectively used for analysis of different spiked water samples (mineral and springer water). In analysis of water samples the procedure described in the experimental section was applied and the concentration of TU was calculated using constructed calibration graph. The results are shown in Table 3. The validity of the proposed method in the analysis of real samples is evident from the calculated recoveries. The student’s t- test at 95% confidence level did not show any systematic error in the proposed method and thus confirms its reliability.
In this study a new reaction system was proposed for the kinetic spectrophotometric determination of TU in water samples. The new method not only benefit from high selectivity and sensitivity, but also it has the advantage of fast and simple operation. Besides, using cheap reagents and simple instrumentations in which minimum maintenance is required are the other reasons for favorably of the proposed method.
The authors are thankful to the Shahrood University Research Council for the support of this work.
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