An Optical Chemical Sensor for Determination of Nickel in Water and Hydrogen Peroxide Samples

Application of nickel in different industries has been developed and so contamination of natural water is a great concern due to its potentially toxic effects on living beings. Therefore, fast monitoring of Ni in aqueous samples is important. In this work, we fabricated a sensitive optical sensor for determination of nickel in mineral water samples and hydrogen peroxide solutions. The optode was prepared by incorporation of 1-(2-pyridylazo)-2-naphthol and sodium tetraphenylborate in a plasticized poly (vinyl chloride) membranes containing dioctyladipate as a plasticizer. The influence of several parameters such as pH, base matrix, solvent mediator and ligand concentration were optimized. Comparison the obtained results with previously reported sensors revealed that the proposed method, in addition to fast and simplicity, provided good linear range (1.70–85.20 μmol L) and low detection limit (0.17 μmol L). The precision (relative standard deviation) was better than 1.55% for 7 replicate determinations of 17.10 μmol L of Ni in various membranes. Corresponding Author: Saeed Babaee , Faculty of Chemistry and Chemical Engineering, Malek Ashtar University of Technology, Tel.: +98 2122970168, Fax: +98 2122962257, E-mail address: Safnba@Gmail.com


Introduction
Nowadays, the pollution of natural water by heavy metals is a great concern due to their potentially toxic effects on living beings, therefore the detection and monitoring of toxic metals in water samples is necessary and very important [1].
Due to wide applications of nickel in different industries [2][3][4][5][6], its contamination of natural water leads to serious environmental hazards.Nickel and some its compounds can cause an allergic reaction, asthma, lung cancer and leukemia in human body originates [7][8][9][10].Amount of nickel should not be more than 1.70 µmol L -1 in drinking water resources [11].
Hydrogen peroxide is used as a disinfectant, oxidizer and so forth in usual applications [12].High concentration of hydrogen peroxide can be used either as a monopropellant or as an oxidizer for military applications [13].If cationic impurities exist in commercial grade of hydrogen peroxide solutions, its purification to military grade (over 85%) can cause to explosion.So nickel concentration in commercial H 2 O 2 solutions must be lower than 0.68 µmol L -1 [14].
Because of oxidation medium or presence of oxygen bubbles in H 2 O 2 solutions, it is difficult to direct determining any ions before initial pretreatments.Therefore, it is important to develop a safe, selective and sensitive technique for the rapid measurement of Ni 2+ in different media.
Determination of nickel is performed by several techniques such as x-ray absorption spectroscopy [15], flame and electro thermal atomic absorption spectrometry [16][17][18][19][20], atomic emission spectrometry [21], spectrophotometry [22,23] and fluorescence spectroscopy [24].Among the methods, spectrophotometric methods offer many appealing characteristics including: simple instrumentation, rapid response times and easy operation.These properties are desirable to the future design and development of portables analytical devices for nickel analysis.
Recently, an interest has been increased on the development of optical sensors compared to electrochemical sensors [25].These sensors have better analytical characteristics [26] and they do not require internal and external reference devices, their time preconditioning are short and are not subjected to electrical noise [27].The choice of the optode matrix is governed by the parameters such as permeability for analyte, cost, good mechanical properties and immobilization suitability for chromophore along with uptaking [28][29][30].
The most widely used polymers in optical sensors are poly (vinyl chloride) groups.They have many desirable features and compare well with sol-gel matrices for most applications [31].Several optodes have been reported in trace analysis of different analytes such as metal ions, anions and organic compounds [32][33][34][35][36][37].
which is readily soluble in common organic solvents such as methanol, ethanol etc.It forms coloured complexes with a large number of metal ions [38].
In this research, we introduced a selective,

Membrane preparation
The membrane consisted appropriate amounts of active components.30.0 mg of PVC, 75.0 mg of DOA, 8.0 mg of PAN and 5.0 mg of NaTPB were transferred in a glass vial and dissolved into 1 ml THF.The solution was immediately shaken vigorously to achieve complete homogeneity.A glass plate (1×9×50 mm 3 ) was cleaned with pure THF and then placed in the spin-on device.
Ninety micro liters of the above solution was injected to the glass plate.After 30 s spinning, at rotation frequency of 600 rpm, the membrane was located in ambient air and allowed to dry in air for few minutes.

Procedure
The Hydrogen peroxide (10-20%) samples were prepared freshly from commercial hydrogen peroxide (30%) solution after filtration.Then, each of samples after Ni (II) spiking was subjected to the membrane methodology.

Results and Discussion
Preliminary investigations

Membrane composition
The Due to the complete mass transfer of Ni 2+ ions into the membrane and decreasing of response time, the presence of an anionic additive such as NaTPB facilitates the ion-exchange equilibrium [42].The effect of NaTPB was investigated in the range 3.0-7.0mg (membrane no.11-15 of Table 1).It is shown that the highest absorbance is recorded by using 5.0 mg of NaTPB.

Effects of pH
The influence of media pH on the sensor response was studied in the range 4-8.As it is shown in Figure 3, optode absorbance increased at pH 6.0 and then decreased.At pH<6, protonation of the ligand prevents its reaction with Ni 2+ ions and at pH>6.0, the response decreasing could be due to the hydrolysis of Ni 2+ ions that is caused to incomplete diffusion of Ni 2+ cations into the membrane.Therefore, a buffer with pH 6.0 was chosen in all experiments.In comparison, the optimum pH for Ni-PAN complex in aqueous solution has been reported as 6.5 [43].

Response time of optode
Response time of optodes is defined as the diffusion time of the metal ions from solution into the membrane (slowest step in complexation process) [44].
The effect of this parameter on the optode response was studied ( for more than 2 hours.

5. Membrane properties
The properties of the optode membrane were measured by recording absorbance changed at 570 nm from individual solutions of 8.52, 17.04 and 51.12 µmol L -1 of Ni.As it is seen in Figure 4, in all of the three cases, the optodes reached to 98% absorbances after 10 minutes.
The stability of membranes was tested for 2 hours and during this period a mean difference of absorbances for the mentioned solutions was ±0.007.
Also the membrane responses were stable for one month in air.
The salting-out phenomenon on the optode response was investigated by adding different amounts of sodium nitrate.The results indicated that this parameter had no effect on the membrane response, up to 0.04 mol L -1 of NaNO 3 and above this concentration it was reduced slightly.This is due to a decrease in the activity of Ni 2+ ions at higher concentration of electrolyte which reduces the interaction of nickel (II) cation with PAN in the membrane.
The sensor regeneration was studied by using of different compounds such as hydrochloric acid, nitric acid, sulfuric acid, sodium fluoride and oxalic acid in different concentrations.It was found that all of the reagents could not regenerate the optode membrane thoroughly and thus the membrane could be used as a probe (single test).

6. Analytical characteristics
Table 2 summarizes the analytical characteristics of the optimized membrane.In this manner dynamic linear range was resulted within 1.70-85.20 µmol L -1 of nickel and detection limit was 0.17 in various membranes was 1.55% and so the method is reproducible during the experiments.
Table 3 presents a comparison between the proposed optode and the other sensors for determination of nickel previously [43,[45][46][47][48].It is obvious that the obtained results of this work are comparable with these existing sensors.In some cases, it provides better linearity range and detection limit.

Study of interferences
The selectivity of this sensor for determination of 8.52 µmol L -1 of Ni +2 was summarized in Table 4.The      respectively.Therefore, the proposed sensor can be successfully applied for the determination of nickel in the mentioned samples.

Conclusion
The proposed optode is a precise, low cost and sensitive device for determination of nickel, based on PVC membrane.Also the proposed method, in addition to fast and simple, provides a wide dynamic range, reliable reproducibility and a good limit of detection.
EDTA was used as masking agent and the method could be made selective in this way.A comparison of the proposed optode with the previously reported sensors indicates that the proposed method in some cases provides wider linear range and lower detection limit.
Finally, the fabricated sensor can be successfully applied

Apparatus
Absorption measurements were carried out on a Hitachi-U 3310 model Lambada-25 double beam UV-Vis spectrophotometer.The pH of the solutions was measured by a Metrohm model 691 pH/Ion Meter using a combined glass electrode.
sensing membrane (optode) was placed in a beaker filled with 20 ml of the test solutions containing EDTA (1.0×10 -2 mol L -1 ) and different concentration of Ni 2+ (1.70 -85.20 µmol L -1 ) at pH 6.0.After 10 min the optode was mounted into the spectrophotometer directly and its net absorbance was recorded at 570 nm against a blank membrane.Water samples (Mineral and river) were collected in 1 L amber glass bottles from Haraz (Anahita, Polur) and Jajrud (Lavasan) rivers in Iran.After sample pretreatments[34], 3 ml of each sample was spiked with appropriate amount of Ni (II) and was subjected to the above procedure.

Figure 2 .
Figure 2. Effect of plasticizer nature on the response of the membrane after 10 min (a) and on the membrane leakage% after 30 min (b).Conditions: 34.08 µmol L -1 Ni 2+ ; T=25 • C; membrane layer containing 30.0 mg of PVC, 75.0 mg of each plasticizers, 8.0 mg PAN.

Table 2 .
Analytical characteristics of the proposed sensor.

Table 1 .
Effects of membrane composition on the absorbance of the proposed optode

Table 3 .
Comparison of optical sensor papers for Ni (II) determination a Linear Dynamic Range; b Limit of Detection tolerance limit was defined as the concentration of added ion causing less than ±5 ٪relative error.From Table4, presence of alkaline metals and anions such as sulfate, chloride and others did not have adverse effects on nickel uptake.As seen, bivalent and some trivalent cations can interfere at different ratios.

Table 4 .
Tolerance limits of diverse ions on the Ni +2 (8.52 µmol L -1 ) determination a a ≤ 5.0% Deviation in the absence of masking agent.
to nickel monitoring in water and hydrogen peroxide samples.According to the best of our knowledge no manufactured optode has been reported in the literature for the determination of Ni in H 2 O 2 solutions.

Table 5 .
Determination of Ni 2+ in different spiked samples a Sample sources as described in the text.b Mean value of three replicate determination ± Standard deviation.c Not detected.