Submission: January 16, 2020; Published: March 06, 2020
*Corresponding author: Nasser M Abu Ghalwa, Chemistry Department, Al-Azhar University, Gaza, Palestine
How to cite this article: Nasser M Abu Ghalwa*, Fawzi kodih, Nader B Farhat and Rewaa J Abu Saelik. Synthesis of a Novel Sensor Electrode Based on
Bromothymol Blue as an Indicator Electrode in Potentiometric Acid-Base Titration in Aqueous Solution. Organic & Medicinal Chem IJ. 2020; 9(3): 555764. DOI: 10.19080/OMCIJ.2019.09.555764
A modified bromothymol blue BTB electrode was prepared by spin coating of the BTB indicator on conducting glass substrate, and its use as indicator electrode to potentiometric acid-base titration in aqueous solution at 298 K was developed. The change of the open circuit potential with pH (E-pH) curve is linear with slope of 0.052 V/Dec at 298 K. The standard potential of this electrode, E0, was determined with respect to the SCE as reference electrode. The recovery percentage for potentiometric acid-base titration using G/BTB as indicator electrode was calculated
Keywords: Potentiometry; Sensor; Conducting Glass; Titration; Indicator Electrode; Bromothymol Blue
At the recent years, there has been an expanding interest for chemical analyzers, which are suited for a quick, accurate and in field ion analysis. There is a great interest for easy to use, sensitive, low-cost and maintenance free chemical analyzers for economical real time analysis of environmentally important ions . Titration is a very useful and reliable technique, which is widely used in different fields such as the food industry, scientific research, and chemical, clinical and pharmaceutical laboratories. Potentiometric titration belongs to chemical methods of analysis in which the endpoint of the titration is monitored with an indicator electrode that records the change of the potential as a function of the amount usually the volume of the added titrant of exactly known concentration and the potentiometric titration have a more selective character when acid-base, complexometric or precipitation reactions are explored [2-5].
The Potentiometric sensors based on the potential gap between a working electrode (WE) and a reference electrode (RE), at zero current . This potential is related to the activity of the analyte in the solution, which is considered an effective concentration, in contact with the electrode surface. In potentiometric sensors, a local equilibrium is established at the sensor interface,
where either the electrode or membrane potential is measured,
and information about the composition of a sample is obtained from the potential .
In acid base titration, the unknown concentration of an acid or base is determined by neutralizing the analyte with an acid or base of known concentration. pH indicators are frequently weak acids or bases which is used in titrations in analytical experiments to determine the extent of a chemical reaction. Because of the subjective determination of color, pH indicators are susceptible to imprecise readings [7,8]. Different types of electrode have been used as acid-base indicator electrodes. Of those, the most frequently used are the titanium, hydrogen–palladium and other electrodes [9-12].
Recently, researchers have been interested in studying the composites of conducting electrode film. Composite materials of sensor electrode have superior properties from where broader potential window, 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 [13,14].
Bromothymol blue (also known as bromothymol sulfone
phthalein and BTB) is a pH indicator for reactions between strong
acids and bases. It is mostly used in applications that require
measuring substances that would have a relatively neutral pH
(near 7). A solution of bromothymol exhibits two forms acidic
forms (yellow color) and basic form (blue color)  (Figure 1).
This study focuses on the bromothymol blue indicator BTB to
prepare a new modified electrode coated on conducting glass film
as sensor (glass / indicator electrode), for used in potentiometric
acid base titration.
The chemicals used in potentiometric titrations and
preparation the electrode was tetraethyl orthosilicate (TEOS),
bromothymol blue (BTB), hydrochloric acid, ammonia, Acetic acid,
phosphoric acid, sodium hydroxide, sulfuric acid, citric acid and
disodium phosphate. The chemicals are of analytical pure grade.
A mixture of 2.5 ml of absolute ethanol, 0.86 ml of 0.1 M of HCl
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 (0.001 M) of bromothymol blue, 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 for each indicator.
Preparation of Silica-immobilized Thin Films
Glass were activated by concentrated H2SO4 for 24 hours, then
washed with distilled water and ethanol. The surface was finally
rubbed with cleaning paper.
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
multilayers 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 80oC 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
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 Bromothymol blue, Thymol blue, sensor indicators
electrodes was measured vs. a saturated calomel electrode (SCE).
The error in the measurement of the potential due to liquidjunction
potentials in these electrolytes is estimated to be about
The solution in a beaker is stirred by means of a magnetic
stirrer. The electrodes (indicator and reference) were dipped
slowly into aqueous solution (acid or reductant). After the steady
state potential was attained, the titration of the acid was carried
out by addition of 1 ml of the base to the acidic solution, waiting
until the steady potential is established and then measured. The
potential variation depends on the type of the base, the progress of
neutralization process and on the initial concentration of the acid
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 reached.
Figure 2 show the change of the open circuit potential (E) of
the glass/ Bromo thymol blue (G/ BTB) indicator electrodes with
PH. The relation between potential and pH (E-pH) plot of the G/
BTB indicator electrode fits straight line with slope of 52.66 mV
and 53.11 mV at 298 K. This value is close to the magnitude of the
term 2.303 RT/F (where: R gas constant, T absolute temperature
and F Faraday constant) at the corresponding temperature (59.1
mV at 288 K). From Figure 2 the E0 value of the sensor electrode,
i.e. the potential at [H+] =1, is computed as 279.1 mV relative to the
saturated calomel electrode and can determination by:
This equation is applicable for the reversible behavior of
working electrode. From the developed Nernst equation, we
indicate that working electrodes can be used as pH-indicator. At
high or low pH, the electrode indicates pH less than true value
as pH glass electrode, it may be due to damage in electrode or
existence of alkali metal ions in solution too.
Effect of Concentration of Acid on Potentiometric Titration
Figures 3a,b,c represent the relation between the volume
of 0.1 M NaOH with potential shift in the titrations of different
concentrations of acetic acids, phosphoric acid and hydrochloric
acid for G/BTB electrodes, where the relation between the volume
of 0.1 M HCl with titrations of different concentrations of ammonia
represent in figure 3d. The variation of G/BTB electrode potential
at 288 K with the different volumes of standard NaOH and HCl
followed typical potentiometric titration curves. These curves
show slight decrease in potential (to more negative values) with
the addition of the titrant.
Location of Endpoints
The locating endpoints shown in Figure 4. Figure 4 represents
ΔE/ΔV against V for the potentiometric titrations of hydrochloric
acid, Acetic acid and phosphoric acid, against 0.1 M NaOH where
the ammonia against 0.1 HCl. From the plots the values of
endpoints are determined. The molar amounts A of CH3COOH acid,
HCl acid, CH3COOH acid, experimental and theoretical amounts of
standard NaOH, Be, Bt and recovery percentage (R%) for acid-base
titrations using BTB indicator electrode are listed in Table 1.
Where table 2 represent the molar amounts of Phosphoric
acid experimental and theoretical amounts of standard NaOH, Be,
Bt and recovery percentage (R%) for acid-base titrations using
BTB indicator electrode.
The values of the recovery percentage, R%, for all above
titrations are calculated from equation (8). From the plots the
values of endpoints and the values of the recovery percentage, R%,
are determined as
where Be is the experimental amount of base and but is the
theoretical amount of base calculated from the stoichiometric
equations of neutralization reactions. It is clear from these data
that the working electrode can be used as indicator electrode
with the satisfactory percentage recovery not less than 88%
in potentiometric titrations. These differences in the recovery
percentage may be attributed to the impurities in the reagents.
The values of pKa (acid equilibrium constant) and pKb (base
equilibrium constant) for different acids can be determined
using the method of half neutralization . They are close to the
previously reported values listed in Table 3 for the tested acids.
It is well known that the response time of the sensor is one of
the most important factors in its evaluation and is defined as the
time between the addition of analyte to the solution and the time
when a limiting potential has been reached [17,18]. Figure 5 show
the response time of the G/BTB sensor at different concentration
of phosphoric acid, acetic acid, Hydrochloric acid, ammonia and
NaOH respectively. Response time, in the range of (100-450)
seconds was achieved, which rendered the sensor highly practical.
The pH of the electrolyte solution depends on the degree of
dissociation of the acids and bases present, which is temperature
dependent. And this effects on the behavior of the sensor . To
study the thermal stability of the sensor, calibration graphs were
constructed at different test solution temperatures 15, 25, 35 and
45 ̊C. According to Figure 6. The sensor would be used for pH
measurements in the range from (2-11). At lower temperatures,
like 283 K, the slope of the sensor was about 34.12 mV/decade.
However, when the temperature of the test solutions was adjusted
to 298 K, the slope significantly increased to 52.66 mV/decade. By
raising the temperature to 308 K and 318 K the slope increased to
64.64 mV/decade and 66.54 mV/decade respectively.
Figure 6 shows the square of the correlation coefficient (r2) for
pH measurements using the sensor, at different temperatures, as
compared to pH values obtained by a conventional pH electrode
(Hanna Instruments HI 1131 pH combination electrode) was
found to change as the temperature increases where as r2 values
for measurements at 283 K, 298 K, 308 K, and 318 K were 0.9655,
0.9383, 0.9482, 0.9876, respectively. This indicates that better
results could be obtained at 298 K. Where figure 7 represent the
Correlation between the conventional glass electrode (pH meter)
and G/BTB indicator electrode.
a. In the present study the trials were made for the
preparation of the modified electrodes of type glass/
Bromothymol blue G/BTB and their use as sensor
indicator electrodes in the potentiometric acid-base
titrations in aqueous solution at 298 K
b. The recovery percentage for potentiometric acidbase
titration using G/BTB as indicator electrode was
c. The E-pH curve is linear with slope of 0.052 V/decade
for the G/BTB electrode at 298 K. This value is close to
the theoretical value 2.303 RT/F (0.059 V at 298 K).
d. The standard potential of the tested electrode, E0, is
computed as 279.11mV with respect to SCE as reference
electrode. Acetic acid, phosphoric acid, hydrochloric acid
and ammonia were successfully potentiometric titration
with NaOH as titrant in aqueous medium at 298 K.
e. In this study applied the different temperature like 283 K,
298 K, 308 K, and 318 K were the correlation coefficient
(r2) 0.9655, 0.9383, 0.9482, 0.9876, respectively.