Determination of Thiocyanate by Kinetic Spectrophotometric Flow Injection Analysis

A simple and rapid �ow injection spectrophotometric method has been developed for the determination of amount of thiocyanate in wastewaters and well water samples. It is based on the reaction of thiocyanate in hydrochloric acid with janus green and bromate. Reaction wasmonitored spectrophotometrically bymeasuring janus green absorbance at λλmax = 554 nm.e calibration graph was linear over the range 0.02–1.0 μμgmL and the detection limit was 0.016 μμgmL (sssss = s). e throughput was 25 samples per hour.


Introduction
SCN − is a metabolite of cyanide detoxi�cation with similar toxicity that has received attention from researchers in different �elds such as medicine, food chemistry, and environmental sciences [1].iocyanate usually exists in industrial waste waters, pesticides and organism metabolites.It is also present in low concentrations in human serum, saliva, and urine [2].
Different methods to determine thiocyanate in wastewaters, synthetic samples, biological samples, and food have been reported in the literature including spectrophotometric [3][4][5][6], �uorometric [7,8], chromatographic [9][10][11], and electroanalytic approaches [12].Some of these methods have a short linear range time consuming and expensive.Flowinjection analysis (FI) is characterised by its simplicity and speed, the inexpensive equipment needed and the accuracy of its results.It is an important alternative to other analytical methods, with clear advantages in terms of the short time required for each assay.In the bibliography, some FI methods [13][14][15] have been proposed for the determination of SCN − in real samples.
In this paper an FI method, using spectrophotometric detection at 554 nm, is described for the determination of thiocyanate.e method is based on inhibitory effect of thiocyanate on the oxidation of janus green by bromate in acidic medium.e proposed method has been successfully applied to the determination of thiocyanate in wastewaters and well water samples.

Experimental
2.1.Reagents.Analytical-reagent grade and doubly distilled water were used.A 2 × 10 − M janus green solution was directly prepared by dissolving 0.1022 g of janus green (Merck, M = 511.06g/mol) in distilled water and diluting it with distilled water in a 100 mL volumetric �ask.A 1 × 10  g/mL stock standard solution of thiocyanate was prepared by dissolving 0.1673 g of potassium thiocyanate (Merck, M = 97.181g/mol) in distilled water and diluting it to 100 mL.Working solutions were prepared by appropriately diluting the stock standard solution.A 100 mL 0.5 M potassium bromate solution was prepared by dissolving 8.35 g of KBrO  (Merck, M = 167 g/mol) in distilled water and diluting it to mark in a 100 mL volumetric �ask.Hydrochloric acid solution (2.0 M) was prepared by diluting a known volume of its concentrated solution (Merck).

Flow Injection System.
Figure 1 shows the schematic diagram of the �ow system.e 8-channel peristaltic pump (Ismatec, MCP process, IP 65) was �tted for pumping solutions.Silicon rubber tube with 0.8 mm i.d. was used for delivery of the solutions.A mixed solution of janus green, bromated, and hydrochloric acid as a carrier stream was delivered through silicon rubber tubing (at 40 ∘ C). e thermostatic water bath (Gallen Kamp Griffin, BGL 240 V) was used at a given temperature of 40 ± 0.1 ∘ C. e standard solution of thiocyanate was injected into a carrier stream with a sample injector (Rhedyne, model 9125).An UV-visible spectrophotometer (2501 CECIL) equipped with a �owthrough cell with 10 mm path length connected a recorder was used for monitoring the variation in the absorbance spectrum.
2.3.Recommended FI Procedure.e �ow-injection system is shown in Figure 1.Aliquots (200 L) of SCN − solutions prepared at different concentrations, 0.02-1.0g mL −1 , were injected into a carrier stream containing of 0.5 M hydrochloric acid reagent stream, which was then merged with 0.4 M bromate stream and 1.5 × 10 −5 M janus green stream; �ow rate, 18 mL h −1 ; reaction coil, 50 cm; detection wavelength, 554 nm.Calibration graph was prepared by plotting the absorbance of the peak maximum versus SCN − concentration.

Results and Discussion
Janus green (Figure 2) undergoes an oxidation reaction with bromate in acidic medium at very fast rate.We found that ultratrace amounts of SCN − have an inhibitory effect on this reaction.Mechanism of Reaction.Bromate is reduced by the chloride ion in acidic media: Janus green reacts with the products of the reaction and is decolorized: In presence of thiocyanate, we have following reaction in acidic medium [16]: Indeed, in presence of thiocyanate, reaction rate of 2 is decreased and thiocyanate has a inhibitory effect in this reaction and this reaction rate is sharply decreased by addition of trace amounts of thiocyanate.
is reaction is followed spectrophotometrically by controlling absorbance change of the dye at 554 nm by a �ow injection method.To have best change absorbance between sample and blank, the effect of reagent concentrations and manifold variables on the analytical signal was studied.As a result, for determination of 0.4 g mL −1 , thiocyanate analytical signal was obtained 14.25.

Effect of Chemical
Variables.e in�uence of bromate concentration, janus green concentration and hydrochloric acid concentration, temperature, and ionic strength on the analytical signal was studied to �nd the optimum conditions.In �gures drawn, peak height is change in absorbance (maximum peak height with the best baseline).e in�uence of hydrochloric acid concentration on the analytical signal was studied in the presence of 0.1 g mL −1 SCN − and the optimum bromate concentration (Figure 3(a)).e results indicate that the best concentration for hydrochloric acid is 0.5 M. erefore, the optimum hydrochloric acid concentration was selected to be 0.5 M.
e effect of janus green concentration on the analytical signal was studied at 0.1 g mL −1 SCN − and the optimum bromate concentration.e results in Figure 3(b) show that by increasing janus green concentration up to 2.0 × 10 −5 M, the analytical signal increases, but because of high concentration of janus green in channels and possibility of color stick to the wall channel, janus green concentration of 1.5 × 10 −5 M was selected for next study.
Experiment was done at different bromate concentration.Figure 3(c) shows the maximum peak height with the best baseline is at 0.4 M. While higher bromate concentration decrease the signal.erefore, a bromate concentration of 0.4 M was selected as the optimum bromate concentration.
Figure 4 show the in�uence of temperature on the maximum signal (ΔA) was studied for the range of 15-50 ∘ C, under above conditions, as previously described.e results show that by increasing temperature up to 40 ∘ C, analytical signal increases.So temperature was �xed at 40 ∘ C.

Effect of Manifold Variables.
In�uence of variables such as reaction coils, injected volume, and �ow rate on the analytical signal was studied.e peak height depends on the residence time of the sample in the system that is affected by �ow rate and reaction coil lengths.

Species assayed
Tolerated ratio  Species / thiocyanate Ca 2+ , Na e results during the investigation of the effect of three reaction coil lengths show that by increasing the reaction coil length up to (RC 1 ) 30 cm, (RC 2 ) 40 cm, and (RC 3 ) 150 cm, the sensitivity increases.At shorter distances, there is not enough time for reagents to be mixed and above this reactor length, increased dispersion will decrease the peak height (Figure 5).e in�uence of sample volume was tested under optimum conditions.e results show that the peak height rose by increasing the volume of sample loop volume, but the injection of a large amount of sample results in peak  F 6: Effect of sample loop volume on the sensitivity.Conditions: 0.5 M hydrochloric acid, 2 × 10 − M janus green, 0.4 M bromated, 0.1 g mL −1 thiocyanate, temperature of 40 ∘ C, �ow rate of 18 mL/h for each channel, three reaction coil lengths from RC 1 to RC 3 , 30 cm, 40 cm, and 150 cm, respectively.T 2: Determination of thiocyanate in real samples.

Sample
Added Expected Founded Recovery (g mL −1 ) (g mL −1 ) (g mL broadening and tailing.us, a sample volume of 200 L was selected (Figure 6).e effect of �ow rate was tested under optimum chemical conditions.e results show that the best pump �ow rate (the maximum peak height and minimum dispersion) will be obtained in 18 mL h −1 for each channel.At lower �ow rates the dispersion will be high whereas at greater �ow rates the reaction may be incomplete (Figure 7).T 3: A comparison of detection limit and linear dynamic range of several methods with the proposed method.

Method
3 detection limit Linear dynamic range Reference (g mL −1 ) (g mL

Analytical Characteristics of the FI Spectrophotometric
Method.e calibration graph was obtained by the procedure described in Section 2, whereby a series of standard solutions was analysed in triplicate to test the linearity.e calibration graph obtained was linear in the range 0.02-1.0g mL −1 .e equation of the calibration graph is (ΔA = 1.8469C thiocyanate + 13.5212, where ΔA is change in absorbance (maximum peak height with the best baseline) for the sample and C thiocyanate is thiocyanate concentration expressed in g mL −1 , with a correlation coefficient  = .9954.e limit of detection calculated according to the recommendations of IUPAC [17], was 0.016 g mL −1 ( = 1).e between-run precision of the method was tested by analysing �ve replicate samples of 0.3, 0.5, and 0.8 g mL −1 of thiocyanate.e coefficients of variation were ±.68%, ±.56%, and ±.45%.
3.4.Selectivity.e interference of possible coexisting substances was studied.e sample containing 0.1 g mL −1 of thiocyanate, and various concentrations of the foreign substance was injected into the �ow system.e tolerance limit was taken as the concentration causing an error of not more than ±3% in the determination of thiocyanate.e results obtained are shown in Table 1.As can be seen, the proposed method is very selective.
3.5.Analytical Application.e proposed method has been successfully applied to determine thiocyanate in the real and synthetic water and wastewater samples.e results in Table 2 represent that good recoveries in all samples were obtained.

Conclusion
A simple, rapid, and selective �ow injection inhibition procedure is developed for the determination of thiocyanate with spectrophotometric detection.is method can be used for the determination of g mL −1 amounts of thiocyanate with a sample rate of 25 ± 5 samples/h.e main advantages of the method are its simplicity and its large dynamic range which make it possible to determine thiocyanate in the real samples with satisfactory results.e �ow injection system developed for the determining thiocyanate is inexpensive; it employs available reagents, allows rapid determination at low operating cost, and provides simplicity, adequate selectivity, and a low limit of detection compared to some of referenced methods.A comparison of detection limit and linear dynamic range of several methods with the proposed method is shown in Table 3.

3 −F 3 :F 4 :
], [janus green] ×10 5 (mol L −1 ) Effect of (a) hydrochloric acid (b) janus green, and (c) bromate concentrations on the mean of peak height (  ) of 0.1 g mL −1 thiocyanate standard solution.Effect of temperature on the sensitivity.Conditions: 0.5 M hydrochloric acid, 2 × 10 − M janus green, 0.4 M bromate, 0.1 g mL −1 thiocyanate, sample loop volume of 200 L, �ow rate of 18 mL/h for each channel, three reaction coil lengths from RC 1 to RC 3 , 40 cm, 40 cm, and 150 cm, respectively.T 1: Effect of common excipients and other possible interferents on the determination of 0.1 g mL −1 of thiocyanate.

3 F 5 :
Effect of reaction coil lengths on the sensitivity.Conditions: 0.5 M hydrochloric acid, 2 × 10 − M janus green, 0.4 M bromated, 0.1 g mL −1 thiocyanate, temperature of 40 ∘ C, sample loop volume of 200 L, and �ow rate of 18 mL/h for each channel.