**3.5 Application of the polyphenols as a pH sensor and dye removal. 3.5.1 The pH sensors**

Hydrogen ion is ubiquitous species encountered in most chemical reactions. It quantified in terms of pH –the negative logarithm of its activity:

$$\text{pH} = -\log\left[\text{H}\right] \tag{5}$$

The pH sensors are widely used in chemical and biological applications such as environmental monitoring (water quality), blood pH measurements and laboratory pH measurements amongst others. The earliest method of pH measurement was by means of chemical indicators, e.g. litmus paper that changes its colour in accordance to a solution's pH. For example, when litmus is added to a basic solution it turns blue, while when added to an acidic solution the produced colour is red. Since many chemical processes are based on pH, almost all aqua samples have their pH tested at some point. The most common systems for pH sensing are based upon either amperometric or potentiometric devices. The most popular potentiometric approach utilises a glass electrode because of its high selectivity for hydrogen ions in a solution, reliability and straight forward operation. Ion selective membranes, ion selective field effect transistors, two terminal microsensors, fibre optic and fluorescent sensor, metal oxide and conductometric pH-sensing devices have also been developed . However, these types of devices can often suffer from instability or drift and, therefore, require constant re-calibration. Although litmus indicators and other abovementioned pH sensors are still widely used in numerous areas, considerable research interest is now focused on the development of chemical or biological sensors using functional polymers. Thus, electrosynthesized polymers are considered to be good candidates as pH sensors due to the fact that they are strongly bonded to the electrode surfaces during the Electropolymerization step.

### **3.5.1.1 POCP modified Pt- electrode as pH sensor**

#### *3.5.1.1.1 Potentiometric study of POCP*

In recent years, there has been a growing interest in electropolymerized film chemically modified electrodes and their application as potentiometric sensor particularly as PH sensor. To investigate the effect of the thickness on the potentiometric response. POCP modified electrode of different thickness were prepared. Fig. (15) shows the potentiometric response of POCP film electrode in a wide range of pH (2-11) as a function of thickness ,we observed that this electrode gave a linear response over pH range with potentiometric slope values ranging from 26.6 to 40.72 mV/pH with the difference of the thickness as summarized in Table (7).


Table 7. the potentiometric response of POCP modified electrode with different thickness at different range of pH.

Hydrogen ion is ubiquitous species encountered in most chemical reactions. It quantified in

The pH sensors are widely used in chemical and biological applications such as environmental monitoring (water quality), blood pH measurements and laboratory pH measurements amongst others. The earliest method of pH measurement was by means of chemical indicators, e.g. litmus paper that changes its colour in accordance to a solution's pH. For example, when litmus is added to a basic solution it turns blue, while when added to an acidic solution the produced colour is red. Since many chemical processes are based on pH, almost all aqua samples have their pH tested at some point. The most common systems for pH sensing are based upon either amperometric or potentiometric devices. The most popular potentiometric approach utilises a glass electrode because of its high selectivity for hydrogen ions in a solution, reliability and straight forward operation. Ion selective membranes, ion selective field effect transistors, two terminal microsensors, fibre optic and fluorescent sensor, metal oxide and conductometric pH-sensing devices have also been developed . However, these types of devices can often suffer from instability or drift and, therefore, require constant re-calibration. Although litmus indicators and other abovementioned pH sensors are still widely used in numerous areas, considerable research interest is now focused on the development of chemical or biological sensors using functional polymers. Thus, electrosynthesized polymers are considered to be good candidates as pH sensors due to the fact that they are strongly bonded to the electrode

In recent years, there has been a growing interest in electropolymerized film chemically modified electrodes and their application as potentiometric sensor particularly as PH sensor. To investigate the effect of the thickness on the potentiometric response. POCP modified electrode of different thickness were prepared. Fig. (15) shows the potentiometric response of POCP film electrode in a wide range of pH (2-11) as a function of thickness ,we observed that this electrode gave a linear response over pH range with potentiometric slope values ranging from 26.6 to 40.72 mV/pH with the difference of the thickness as summarized in

> At pH range (2-12) At pH range (5-9) -slope, (mV/pH) r2 -slope, (mV/pH) r2

Table 7. the potentiometric response of POCP modified electrode with different thickness at

 40.72 0.96 56.19 0.98 37.70 0.99 42.12 0.98 31.70 0.96 35.09 0.99 26.60 0.93 28.11 0.85

pH = - log [H] (5)

**3.5 Application of the polyphenols as a pH sensor and dye removal.** 

terms of pH –the negative logarithm of its activity:

surfaces during the Electropolymerization step. **3.5.1.1 POCP modified Pt- electrode as pH sensor** 

*3.5.1.1.1 Potentiometric study of POCP* 

Table (7).

No of Cycles

different range of pH.

**3.5.1 The pH sensors** 

From Figure 15 and Table 7, it is clear that the calculated potentiometric slope decreased as the polymer film thickness increased. Where the thin film of POCP electrode shows potentiometric slope equal to 40.7mV/pH and the thickest POCP shows decrease in potentiometric slope equal to 26.60 mV/pH, this decrease may be attributed to the diffusion rate of hydrogen ion across this modified electrode.

But when we use more limited pH range (4-9) as shown in Fig. 16 we noticed that the potentiometric response improved where the potentiometric slope was found to be 56.19 mV/pH for the thin POCP and decreased to 28.11 mV/pH for the thickest POCP film, from this result we noticed that this electrode might not an effective pH sensor for more basic or more acidic solutions but it is a good sensor in moderate pH range.

Fig. 15. POCP response at different pH values (2-12), the prepared film after different no of cycles.

Electropolymerization of Some Ortho-Substituted Phenol Derivatives on Pt-Electrode from

Aqueous Acidic Solution; Kinetics, Mechanism, Electrochemical Studies and Characterization of… 45

Fig. 17. P OCP response at different days, the prepared film from the first day to the ninth day

We have shown that the potentiometric responses to pH changes of the different modified electrodes are linear in the range 4–9. These responses must be mainly attributed to the polymer films rather than the platinum substrate. Possible explanation is the affinity of the numerous hydroxyl groups and Cl atoms to the protons in solutions. The reaction of H+ onto polymer creates local charge density excess at the electrode surface. Surface reactions seem to take place on the polymer film, essentially protonation and deprotonation of

When the equilibrium is reached at the polymer/solution interface, we can write the equilibrium expression K of the surface reaction (1) and the equilibrium potential E as:

According to this mechanism of reaction, we are waiting for a potentiometric response slope of 59 mV/pH unit at 25 °C at all pH values. But our electrodes showed lower response slope. The presence of anionic and cationic responses of the polymer film electrodes, due to

P (Polymer) +H+ PH+ (6)

(7)

(8)

superficial OH groups of the polymers as symbolically described as follow;

*3.5.1.1.3 Response mechanism* 

and

Fig. 16. POCP response at pH range (4-9) with different thicknesses

#### *3.5.1.1.2 Electrode stability*

The Potentiometric response of the chemically modified POCP electrode was examined over a period of 9 days in order to test the stability of the electrode. We observed that the chemically modified electrode shows linear behavior from pH (4-10) during 9 days as shown from Table 8 and Fig. 17 and the sensitivity of this coating decrease considerably with time since the slope varies from 40.7 at the first day to25.5mV/pH unit at the ninth day. Consequently polymer coating is not stable over a period of 9 days and the sensor cannot be used for several measurements. However, this electrode can be sensitive for one use sensor due to its sensitivity during the first measurement. It is important to note that the POCP film was found to peel off the supported electrode and it is a possible reason for the change of response slope.


Table 8. The slope and r2 values of POCP at different time intervals.

Fig. 17. P OCP response at different days, the prepared film from the first day to the ninth day

#### *3.5.1.1.3 Response mechanism*

We have shown that the potentiometric responses to pH changes of the different modified electrodes are linear in the range 4–9. These responses must be mainly attributed to the polymer films rather than the platinum substrate. Possible explanation is the affinity of the numerous hydroxyl groups and Cl atoms to the protons in solutions. The reaction of H+ onto polymer creates local charge density excess at the electrode surface. Surface reactions seem to take place on the polymer film, essentially protonation and deprotonation of superficial OH groups of the polymers as symbolically described as follow;

$$\text{P (Polymer)} \nrightarrow{\text{H}^\*} \xrightarrow{\text{H}^\*} \text{PH}^\* \tag{6}$$

When the equilibrium is reached at the polymer/solution interface, we can write the equilibrium expression K of the surface reaction (1) and the equilibrium potential E as:

$$\text{K} = \frac{[\text{PH}^+]}{([\text{P}][\text{H}^+])} \tag{7}$$

and

44 Electropolymerization

Fig. 16. POCP response at pH range (4-9) with different thicknesses

Time, (day) -slope, (mV/pH) r2 1 42.66 0.97 2 34.71 0.94 3 29.2 0.94 4 27.46 0.87 5 24.46 0.89 6 24.51 0.93 7 29.23 0.92 8 31.60 0.90 9 24.51 0.93

Table 8. The slope and r2 values of POCP at different time intervals.

The Potentiometric response of the chemically modified POCP electrode was examined over a period of 9 days in order to test the stability of the electrode. We observed that the chemically modified electrode shows linear behavior from pH (4-10) during 9 days as shown from Table 8 and Fig. 17 and the sensitivity of this coating decrease considerably with time since the slope varies from 40.7 at the first day to25.5mV/pH unit at the ninth day. Consequently polymer coating is not stable over a period of 9 days and the sensor cannot be used for several measurements. However, this electrode can be sensitive for one use sensor due to its sensitivity during the first measurement. It is important to note that the POCP film was found to peel off the supported electrode and it is a possible reason for the

*3.5.1.1.2 Electrode stability* 

change of response slope.

$$E = E\_0 + \left(\frac{RT}{F}\right) \ln\left(\frac{[\text{PH}^+]}{[\text{P}]}\right) = E\_0' + \left(\frac{RT}{F}\right) \ln[\text{H}^+] \tag{8}$$

According to this mechanism of reaction, we are waiting for a potentiometric response slope of 59 mV/pH unit at 25 °C at all pH values. But our electrodes showed lower response slope. The presence of anionic and cationic responses of the polymer film electrodes, due to

Electropolymerization of Some Ortho-Substituted Phenol Derivatives on Pt-Electrode from

Elovich models were employed to interpret the experimental data, as shown below:

Pseudo-first-order equation:

Pseudo-second-order equation:

Intraparticle diffusion equation:

Elovich equation:

diffusion models.

Pseudo-firstorder

Pseudo-second-

Intraparticle diffusion

order

Aqueous Acidic Solution; Kinetics, Mechanism, Electrochemical Studies and Characterization of… 47

The adsorption capacity increases rapidly during the initial adsorption stage and then continues to increase at a relatively slow speed with contact time. The obtained result reveals that, at the beginning, the adsorption mainly occurs on the surface of POHP so the adsorption rate is fast. After the surface adsorption is saturated, the adsorption gradually proceeds into the inner part of POHP via the diffusion of MB dye into the polymer matrix, leading to a lower adsorption rate In order to evaluate the adsorption kinetics of MB dye onto POHP, the pseudo-first-order, pseudo-second-order, intraparticle diffusion and

t/qt =1/k2q2e+t/ (10)

qt = kit0.5 + (11)

 qt = b + a ln (12) Where qe and qt are the amounts of dye adsorbed (mg mg−1) at equilibrium and at time t (min), and t is the adsorption time (min). The other parameters are different kinetics constants, which can be determined by regression of the experimental data. The validities of these four kinetic models are checked and graphically represented in Fig. 19. The corresponding kinetic parameters and the correlation coefficients are summarized in Table 9. Based on linear regression (r2) values, the adsorption kinetics of MB dye have the following order. pseudo-second order > Elovich > pseudo-first-order > intraparticle

Kinetic models Parameters r2

k1(min−1) 0.02303

k2 (mg mg−1 min−1) 40.32

Elovich a 6.46×10-4

Table 9. Kinetic parameters for adsorption of MB onto POHP.

Ki(mg mg−1min−0.5) 2.68×10-4

b 6.59×10-4

qe, (mg mg−1) 1.81×10-3 0.934

qe, cal (mg mg−1) 0.0036 0.999

C(mg mg−1) 1.31×10-3 0.891

0.972

Log (qe − qt ) = log qe − k1t/2. (9)

the presence of ions (K+, Na+, Cl- ,….etc of the buffer solutions) in the different solutions probably caused this difference of response slope.

### **3.5.2 Dye removal**

Water resources are of critical importance to both natural ecosystem and human developments. Increasing environmental pollution from industrial wastewater particularly in developing countries is of major concern. Many industries like dye industries, textile, paper and plastics, use dyes in order to color their products. As a result they generate a considerable amount of colored wastewater. The presence of small amount of dyes (less than 1 ppm) is highly visible and undesirable. Many of these dyes are also toxic and even carcinogenic and pose a serious threat to living organisms. Hence, there is a need to treat wastewaters containing toxic dyes and metals before they are discharged into the water bodies.

Many different treatment methods, including biological treatment, coagulation/flocculation, ozone treatment, chemical oxidation, membrane filtration, ion exchange, photocatalytic degradation and adsorption have been developed for the removal of dyes from wastewaters to decrease their impact in environment. Among these methods, adsorption is a well known separation process and is widely used to remove certain classes of chemical pollutants from waters, especially those that are practically unaffected by conventional biological treatments.

#### **3.5.2.1 POHP as MB dye adsorbent**

#### **3.5.2.1 Adsorption kinetics**

The adsorption kinetics was conducted to determine the optimum adsorption time for the adsorption of MB dye by POHP. The effect of the contact time on adsorption of MB onto POHP is represented in Fig. 18.

Fig. 18. The effect of contact time on adsorption of MB onto POHP.

The adsorption capacity increases rapidly during the initial adsorption stage and then continues to increase at a relatively slow speed with contact time. The obtained result reveals that, at the beginning, the adsorption mainly occurs on the surface of POHP so the adsorption rate is fast. After the surface adsorption is saturated, the adsorption gradually proceeds into the inner part of POHP via the diffusion of MB dye into the polymer matrix, leading to a lower adsorption rate In order to evaluate the adsorption kinetics of MB dye onto POHP, the pseudo-first-order, pseudo-second-order, intraparticle diffusion and Elovich models were employed to interpret the experimental data, as shown below: Pseudo-first-order equation:

$$\log\left(\mathbf{q}\_{\rm e}-\mathbf{q}\_{\rm t}\right) = \log\mathbf{q}\_{\rm e} - \mathbf{k}\_{\rm t}/2.\tag{9}$$

Pseudo-second-order equation:

$$\mathbf{t}/\mathbf{q}\_t \equiv \mathbf{1}/\mathbf{k}\varepsilon \mathbf{q}^2 \mathbf{e} + \mathbf{t}/\tag{10}$$

Intraparticle diffusion equation:

$$\mathbf{q}\_t = \mathbf{k} \mathbf{i} t \mathbf{e} \,\mathrm{s} + \tag{11}$$

Elovich equation:

46 Electropolymerization

Water resources are of critical importance to both natural ecosystem and human developments. Increasing environmental pollution from industrial wastewater particularly in developing countries is of major concern. Many industries like dye industries, textile, paper and plastics, use dyes in order to color their products. As a result they generate a considerable amount of colored wastewater. The presence of small amount of dyes (less than 1 ppm) is highly visible and undesirable. Many of these dyes are also toxic and even carcinogenic and pose a serious threat to living organisms. Hence, there is a need to treat wastewaters containing toxic dyes and metals before they are discharged into the water

Many different treatment methods, including biological treatment, coagulation/flocculation, ozone treatment, chemical oxidation, membrane filtration, ion exchange, photocatalytic degradation and adsorption have been developed for the removal of dyes from wastewaters to decrease their impact in environment. Among these methods, adsorption is a well known separation process and is widely used to remove certain classes of chemical pollutants from waters, especially those that are practically unaffected by conventional biological

The adsorption kinetics was conducted to determine the optimum adsorption time for the adsorption of MB dye by POHP. The effect of the contact time on adsorption of MB onto

Fig. 18. The effect of contact time on adsorption of MB onto POHP.

,….etc of the buffer solutions) in the different solutions

the presence of ions (K+, Na+, Cl-

**3.5.2.1 POHP as MB dye adsorbent** 

**3.5.2.1 Adsorption kinetics** 

POHP is represented in Fig. 18.

**3.5.2 Dye removal** 

bodies.

treatments.

probably caused this difference of response slope.

$$\mathbf{q}\_l = \mathbf{b} + \mathbf{a} \ln \tag{12}$$

Where qe and qt are the amounts of dye adsorbed (mg mg−1) at equilibrium and at time t (min), and t is the adsorption time (min). The other parameters are different kinetics constants, which can be determined by regression of the experimental data. The validities of these four kinetic models are checked and graphically represented in Fig. 19. The corresponding kinetic parameters and the correlation coefficients are summarized in Table 9. Based on linear regression (r2) values, the adsorption kinetics of MB dye have the following order. pseudo-second order > Elovich > pseudo-first-order > intraparticle diffusion models.


Table 9. Kinetic parameters for adsorption of MB onto POHP.

Electropolymerization of Some Ortho-Substituted Phenol Derivatives on Pt-Electrode from

Fig. 20. Adsorption isotherms of MB onto POHP

0.000

0.005

0.010

0.015

0.020

0.025

**q**

**e**

adsorption is favorable.

adsorption intensity.

POHP

POHP.

Aqueous Acidic Solution; Kinetics, Mechanism, Electrochemical Studies and Characterization of… 49

The essential characteristics of the Langmuir isotherm can be expressed in terms of a

**Ce**

0 5 10 15 20 25 30

 RL = 1/(1 + bCo) (14) where Co (mg L−1) is the initial dye concentration. If the value of RL lies between 0 and 1, the

The Freundlich isotherm is an empirical equation assuming that the adsorption process takes place on heterogeneous surfaces and adsorption capacity is related to the concentration of dye at equilibrium is described by Fig. (21) and the following equation:

where Kf (mg mg−1) is roughly an indicator of the adsorption capacity and 1/n is the

qm b RL r2 n Ln Kf r2

0.023 0.34 0.42 0.99 1.74 5.16 0.972

Sample Langmiur Freundlich

Table 10. Langmiur and freundlich isotherm constants of the adsorption of MB dye on

log qe = log Kf +1/n log Ce (15)

dimensionless constant separation factor RL represented by the equation .

Fig. 19. Adsorption kinetics of MB dye onto POHP.

### **3.5.2.2 Adsorption isotherm**

The adsorption isotherm of MB dye onto POHP is represented in Fig 20. The adsorption capacity of MB dye increases with the increase of dye concentration. This may be attributed to the extent of a driving force of concentration gradients with the increase of dye concentration. Then tends to level off, this is due to the saturation of the sorption site on the adsorbent.
