An increasing number of fields in everyday life require the development and application of ever more modern and efficient chemical sensors and biosensors, namely to be integrated in intelligent control system networks. The applications of sensor electrode extend to all fields, from drug detection to clinical diagnosis, from the control of industrial quality and safety, from combating bioterrorism to health care and from environmental monitoring to food quality control and analysis of ions in water. The general way to create sensing units is by using a hierarchical assembly of building blocks as thin film sensing structures which are able to adsorb or interact in some way with the molecules to be detected [1]. The sensor’s classification is accomplished in different ways and it can range from very simple to complex. Sensors are commonly divided, according to the principles of signal transduction, into groups: optical sensors, chemical sensors, electrochemical sensors, electrical sensors, mass sensitive sensors, magnetic sensors, thermometric sensors and other sensors (based on emission or absorption of radiation) [2] Potentiometric complexometric titrationis one of the classical titrimetric methods developed for the rapid and quantitative chemical analysis of metal ions. The ions of interest are titrated with the chelator of choice through a coordination complexation reaction and rapidly form stable monodentate or multidentate complexes. The chelator is sometimes called the complexing reagent or more simply, titrant. The end point can be identified by a metallochromic indicating .dye, which shows a color change, or by other instrumental indicators, such as ion selective electrodes. [3]. The thin layer can be applied by using different techniques such as screen printing [4], chemical and electrochemical deposition [5], ultrasonic spraying [6] and laser technics [7].

Recently, some studies interested in studying the composites of sensor electrodes. Due to the properties of the composite materials of sensor electrode such as higher signal-to-noise ratio, time and speed of response mechanical stability enabling their application in flowing systems, and resistance toward passivation. The last requirement is especially important because electrode fouling is probably the biggest obstacle to more frequent applications of electroanalytical methods in environmental analysis [8-10].

Abu Ghalwa et al , investigated of the preparation of conductive glass / thymol blue TB and bromothymol Blue (BRB ) sensors electrodes by spin coating of the (TB) and (BRB ) indicators on conductive glass formed from F-SnO2 for used as indicator electrodes to potentiometric acid-base titration in aqueous solution [11-12].

Eriochrome Black T is a complexometric indicator that is used in complexometric titrations, e.g. in the water hardness determination process. EBT is blue in a buffered solution at pH 10. It turns red when Ca²⁺ ions are added show Figure 1 [13].

Figure 1: structure of Eirochrome Black T.

This study focuses in thin film-based sensor devices to applications in detection of total hardness by synthesis Glass/ Eriochrome Black T indicator (G/EBT) (as thin film sensor based on conductive glass) electrode used in complexemtric titration in aqueous solution .

Chemicals

The chemicals used in potentiometric titrations and preparation the electrode were tetraethylorthosilicate (TEOS), Eriochrome Black T, hydrochloric acid, ammonia, acetic acid, phosphoric acid, sodium hydroxide, sulfuric acid, citric acid and disodium phosphate. The chemicals are of analytical pure grade.

Synthesis of Materials

Preparation of Hydrolyzed TEOS

 A mixture of 2.5 ml of absolute ethanol, 0.86 ml of 0.1M NH₃ were added to 2.5 ml of TEOS under stirring. The obtained solution was kept under stirring at room temperature until a homogeneous clear solution was obtained. The solution was aged at least for 24 hours before used in the coating process. The hydrolyzed TEOS solution was used as a host matrix for the indicators.

Preparation of Indicators

Indicators solution Erochrom black T (EBT) (1x10^{-2 M) were prepared using absolute ethanol as solvent.}

Stock Solution of Indicators

The sample solution was prepared by mixing 1 ml of blank hydrolyzed TEOS solution and 1 ml the indicator.

Preparation of Silica-immobilized Thin Films

Substrate Cleaning

Glass were activated by concentrated H₂SO₄ for 24 hours, then washed with distilled water and ethanol. The surface was finally rubbed with cleaning paper.

Preparation of glass/TB electrodes using Spin coating method

All thin films layers prepared in this work were made by spinning three drops of the solutions onto a clean glass slide. The coating process was performed using the spin coater machine at 900 rpm spinning speed for 1 min. period time. To obtain multilayer's of thin films a subsequent spin coating method was performed after gradually drying of the previous layer at room temperature for 24 hours, then dried at 80 ^oC for another 48 hours. And repeat the spin coating two or three time.

Where the conducting substrate is usually conducting glass, consisting of glass coated with a thin layer of F-doped SnO₂

Sensor design of potentiometric cell

The potential of the indicator electrode relative to that of the reference electrode was measured on a digital multimeter model YDM 302C (China). Potentials were measured to ±5 mv. The potential of Eriochrome Black T, sensor indicators electrodes was measured vs. a saturated calomel electrode (SCE). The error in the measurement of the potential due to liquid- junction potentials in these electrolytes is estimated to be about 0.001 V.

The solution in a beaker is stirred by means of a magnetic stirrer. The electrodes (indicator and reference) were dipped slowly into aqueous solution (hardness or reductant). After the steady state potential was attained, the titration of the hardness was carried out by addition of 1 ml of the EDTA to the complex solution, waiting until the steady potential is established and then measured. The potential variation depends on the concentration of the EDTA, the progress of titration process and on the initial concentration of the hardness to be titrated. The results were reproducible to satisfactory value of ± 5 mV for potential measurements. The process of addition of the titrant was repeated until the equivalence point was

The E-[Hardness Ions or Ca²⁺ and Mg²⁺] Relation of EBT Indicator Electrode

Figure 2 displays the change of the open circuit potential (E) of the G/ EBT electrode with [hardness ions or Ca²⁺ and Mg²⁺]. The E- hardness plot of the G /EBT electrode fits straight line with slope of 53.85 mV at 298 K. This value is close to the magnitude of the term 2.303 RT/F at the corresponding temperature (0.059 V at 298 K). From Fig.2 the E⁰ value of the EBT electrode, i.e. the potential at [hardness] = 0, is computed as 21.52 mV relative to the saturated calomel electrode.

Figure 2: The E-[hardness ion] relation G/EBT Sensor at 298.

The Response Time of the Sensor

Figure 3 show the response time of the G/EBT sensor at different concentration of hardness ions and EDTA response time, in the range of (100-450) seconds was achieved, which rendered the sensor highly practical

Figure 3: Response time of G/EBT Sensor at different concentration of hardness ions.

Effect of Temperature of the G/ EBT Sensor on the Response Characteristics

The EBT sensor response was evaluated at different temperatures, Figure 4. At lower temperatures, like 288 K, the slope of the sensor was about 45.22 mV/decade and the sensor would be used for different concentration measurements of hardness (Ca⁺² and Mg⁺²). However, when the temperature of the test solutions was adjusted to 298 K, the slope significantly increased to 53.85 mV/ decade. By raising the temperature to 308 K and 318 K the slope increased to 54.28 mV/ decade and 59.54 mV/decade respectively. Figure 4 shows the square of the correlation coefficient (r²) for [hardness ions or ca²⁺ and Mg²⁺ ] measurements using the solid-state sensor, at different temperatures .It was found that as the temperature increases the r² values for measurements at 288K, 298 K, 308 K, and 318 K were 0.971, 0.953, 0.944, 0.966, respectively. This indicates that better results could be obtained at 298 K.

Figure 4: Effect of temperature at different [Hardness ions] values on the slope of G/EBT Sensor.

The Relation between Correlation Theoretical Analysis of Hardness Ions and G/EBT Indicator Electrode

Figure 5 displays the correlation analysis of different solutions of hardness ions and G/EBT indicator electrode, it can be easily recognized that excellent correlation between the results. The slope of this relation was 0.951 and the r² was 0.9985. This indicates that G/EBT indicator electrode potential values are closed to the values of standard analysis of hardness ions.

Figure 5: Correlation between theoretical analysis of hardness ions and G/EBT Sensor.

Potentiometric Complexometric Titration

Hardness Ions

Figure 6 represents the relation between the volume of 0.1M standard ethylenediaminetetraacetic acid (EDTA), with each potential shift in the titrations [hardness ions or Ca²⁺ and Mg²⁺]. The variation of the EBT electrode potential at 298 K with the different volumes of standard EDTA followed typical potentiometric titration curves. These curves show slight decrease in potential (to more negative values) with the addition of the titrant.

Figure 6: The effect of different cocentration of hardness ions on Potentiometric titration against 0.1 M EDTA at 298 K.

Where Figure 7 represents ΔE/ΔV against V for the potentiometric titrations of calcium chloride CaCl₂, and magnesium sulphate (MgSO₄) with 0.1M standard EDTA, respectively. From the plots the values of end points are determined. The obtained results are listed in Table (10) for calcium chloride CaCl₂, and magnesium sulphate MgSO₄ .The calculated values of (R%) are listed in Table (10). [162].

Figure 7: The effect of different concentration of hardness ions on Potentiometric titration against 0.1 M EDTA at 298 K (for locating end points).

Potentiometric of Tap Water

Figure 8 explains the relation between the volume of 0.1M standard ethylenediaminetetraacetic acid (EDTA), with each potential shift in the titrations [hardness ions or Ca²⁺ and Mg²⁺ ] For tap water . The variation of the EBT electrode potential at 298 K with the different volumes of standard EDTA followed typical potentiometric titration curves. These curves show slight decrease in potential (to more negative values) with the addition of the titrant.

Where Figure 9 shows ΔE/ΔV against V for the potentiometric titrations of calcium chloride CaCl₂, and magnesium sulphate MgSO₄ with 0.1M standard EDTA, respectively. From the plots the values of end points are determined. The obtained results are listed in Table 1 for calcium chloride CaCl₂, and magnesium sulphate MgSO₄ .The calculated values of (R%) are listed in Table 1

Figure 8: Potentiometric titration of tap water using G/EBT Sensor against 0.1M EDTA at 298 k.

Figure 9: Potentiometric titration of tap water using G/EBT Sensor against 0.1M EDTA at 298 k (for locating end point).

Table 1: The molar amounts H of Hardness ion concentration, experimental and theoretical amounts of standard EDTA, (B_e and B_t ) and recovery percentage (R%) for complexometric titrations using EBT indicator electrode.

According to this study, The dependent spin coating a thin films of Eriochrome Black T indicator on conductive glass formed from F-SnO2 as glass/indicator was studied in complexometric titration in aqueous solution .The E- hardness plot of the G /EBT electrode fits straight line with slope of 53.85 mV at 298 K. The standard potential of the above electrode E0, was determined with respect to the SCE as reference electrode. The recovery percentage for complexometric titration using G/EBT as indicator electrode was calculated. The result show that the electrode very sensitivity for detection of total hardness.

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