Chemically modified mesoporous silica material (SBA-15) was used for the construction of Tl(I) selective carbon paste electrode. The best response was found with the electrode containing 10% modifier as electrode material. The electrode has a lower detection limit of 6.0 × 10−9 M in a working concentration range of 1.0 × 10−8–1.0 × 10−1 M. The selectivity coefficient calculated by match potential method (MPM) shows the high selectivity of electrode towards Tl(I) over other tested ions. The electrode was successfully applied as an indicator electrode for the titration of 0.01 M TlNO3 solution with standards EDTA solution and for sequential titration of mixture of different anions.
Thallium is a soft and pliable metal. The metal is obtained as a byproduct of refining zinc or lead. The Tl(I) salts are soluble in water and easily absorbed in human body. Thallium compounds are quite toxic and some have been used as rat poisons [
Silica is widely used for the synthesis of various mesoporous materials because it is chemically inert, inexpensive, and easily available and has high thermal stability. The order mesoporous silica was first reported in 1992; since then various mesoporous silica materials have been synthesized. The SBA (Santa Barbara Amorphous) type silica is a high order material possessing a regular two-dimensional hexagonal arrangement of pores. The SBA-15 exhibits large surface area, large pore size of the same dimensions, and thick framework walls, which helps to capture the target species [
The analytical grade reagents, 2-[5-(chloromethyl)-1,2,4-oxadiazol-3-yl]-3-methylpyridine (COMP), 3-(triethoxysilyl)propyl amine (TESPA), and SBA-15, were purchased from Sigma-Aldrich (India). The graphite powder with 1-2
All the potential was measured with the help of digital potentiometer (Equiptronics EQ-602, Mumbai, India) using modified carbon paste electrode in conjunction with an ECIL, India double junction Ag/AgCl reference electrode containing 10% (w/w) potassium nitrate solution in the outer compartment. The pH of test solutions was measured with a conventional glass pH electrode.
A solution of 2-[5-(chloromethyl)-1,2,4-oxadiazol-3-yl]-3-methylpyridine (COMP) (0.001 mole) in 50 mL THF was added to a solution of 0.01 mole of 3-(triethoxysilyl)propyl amine (TESPA) in 10 mL THF at 0°C. The solution was refluxed for 1 h at room temperature producing an oily product. The excess of solvent and unused reactants were removed at reduced pressure. The residue was washed with ethanol producing a white solid (Scheme
1H NMR (CDCl3):
13C NMR (CDCl3):
The organofunctionalized TMPA-SBA-15 hybrid was obtained by copolymerization of TMPA and SBA-15 under controlled conditions at 30°C. A mixture of SBA-15 (3 g in 100 mL of dry toluene) and TMPA (0.001 mol) was refluxed for 10 h. The resultant precipitate was air dried. The final product was then washed with ethanol and acetone for 20 h and then dried for 24 h at room temperature and for 1 h at 100°C (Scheme
The modified silica based carbon paste electrode was prepared by mixing fixed amount of graphite powder, paraffin oil, and TMPA-SBA-15. All the components were added in weight percentage. The components stir till the homogenous paste was obtained. The paste was than filled into a glass tube of 2 mm diameter. A copper wire was inserted from the opposite end to the glass tube for electrical contact. The electrode was removed from the tube and smoothed with the help of bond paper. For batter complexation kinetics at electrode surface a new electrode surface was generated by removing extra carbon paste and scratching the old electrode surface. The electrode surface was than conditioned by 0.001 M solution of TlCl at pH 4.8. The pH of solution was adjusted by phosphate buffer solution [
The following cell assembly was used for potentiometric investigations: Ag, AgCl(s) ∣10%, KCl (l)∥test solution∣ modified carbon paste electrode.
The waste water samples were collected from the different local areas. Recovery experiments were conducted by spiking the water samples with appropriate amount of Tl2SO4. A test solution of hairs of cattle crazing around river was prepared as follows.
The hairs (0.05 gm) were washed with THF, ethanol, and distilled water and burned in a muffle furnace at 460°C. The residue was dissolved in concentrated nitric acid and evaporated till the solid residue was obtained. The obtained residue was than dissolved in water to get clear solution. The test solutions of sample were obtained by taking fixed volume of clear solution of the residue.
The functionalized mesoporous material is an effective material for the determination of metal ions from solution. The modifier loaded on the surface of the mesoporous silica (SBA-15) material increases the conductivity of the material. The silica material provides the extra flexibility to the organic modifier to bind with different metal ions. The use of modified silica material could also avoid the background potential caused by direct use of polar binding material. The response mechanism of modified silica carbon paste electrode is based on the rapid complexation kinetics at electrode solution interface. The appropriate amount of electrode material can give the best possible response mechanism; therefore the electrodes of varying amount of TMPA-SBA-15, graphite powder, and paraffin oil were constructed and their response characters were recorded (Table
Effect of electrode components on response characters of modified carbon paste electrode.
Composition, % (w/w) | Slope (mV/decay) | Linear range (M) |
LOD (M) | Response time (s) | |||
---|---|---|---|---|---|---|---|
TMPA-SBA-15 | Graphite powder | Paraffin oil | |||||
CPE1 | 0 | 75 | 25 | 10.6 ± 2.5 | — | — | — |
CPE2 | 2 | 75 | 23 | 21.6 ± 1.4 |
|
|
25 |
CPE3 | 3 | 75 | 22 | 24.4 ± 1.2 |
|
|
22 |
CPE4 | 5 | 75 | 20 | 26.3 ± 1.2 |
|
|
18 |
CPE5 | 7 | 75 | 18 | 28.4 ± 1.1 |
|
|
15 |
CPE6 | 8 | 75 | 17 | 31.4 ± 1.1 |
|
|
12 |
CPE7 | 9 | 75 | 26 | 35.6 ± 1.1 |
|
|
10 |
CPE8 | 10 | 75 | 15 | 42.4 ± 1.0 |
|
|
8 |
CPE9 | 10 | 74 | 16 | 42.4 ± 1.0 |
|
|
8 |
CPE10 | 11 | 73 | 16 | 40.8 ± 1.0 |
|
|
8 |
CPE11 | 12 | 69 | 19 | 40.2 ± 1.0 |
|
|
8 |
CPE12 | 15 | 65 | 20 | 38.6 ± 1.0 |
|
|
8 |
The potential generated in absence of electroactive material due to some impurities is called background potential which may affect the potential response of the electroactive species. Thus the electrode without TMPA-SBA-15 (CPE1) was prepared and its potential response was recorded. The electrode CPE1 shows a nonlinear potentiometric response with the slope of 10.6 ± 2.5 (mV/decay of activity). To investigate the effect of electrode components on potential response the electrodes of different compositions were prepared. The data presented in Table
Potential response of CPE8 for different cations.
The response time of the electrode assembly (CPE8) was investigated in terms of change in potential with time for 0.01 and 0.001 M solution of Tl(I) ion. All the measurements were performed at constant pH (5.2) of test solution. It was observed that the electrode CPE8 reached to a stable potential in a very short time of about 6 s. To evaluate the reversibility of the electrode the effect of concentration on potential response of the electrode was investigated for whole concentration range. The results were obtained by changing the concentration from higher to lower (1.0 × 10−2–10−8 M) and then from lower to higher concentration; the variation of potential with time for CPE8 was studded for whole concentration range from lower (1.0 × 10−8–1.0 × 10−2 M) for CPE8. The average response time of the electrode assembly was almost the same for both sequences. Thus the electrode CPE8 is sufficiently reversible. The pH of test solution was maintained with the help of phosphate buffer solution (Figure
Variation of potential with time for CPE8.
The large surface area and covalent interaction of mesoporous silica (SBA-15) with modifier TMPA avoids the leaching of electrode components that may occur from physical encapsulation. Thus electrode based on modified silica (SBA-15 or MCM41) can show stable response towards target species for a very long time. In the present study the life time of the electrode CPE8 was investigated with the help of change in LOD and slope of calibration curve. It was observed that the LOD and slope remain the same for a period of 10 months. However after 10 months small change in slope and detection limit was observed. Thus the CPE8 can be used for the selective determination of Tl(I) for more than 10 months. The electrode CPE8 used all the measurements at least three times in a week for more than one hour (Table
Change in slope and detection limit with time for CPE8.
Time (months) | Slope (mV/dec. of activity) | LOD |
---|---|---|
1 | 42.4 ± 1.0 | 6.0 × 10−9 M |
2 | 42.4 ± 1.0 | 6.0 × 10−9 M |
3 | 42.4 ± 1.0 | 6.0 × 10−9 M |
4 | 42.4 ± 1.0 | 6.0 × 10−9 M |
5 | 42.4 ± 1.0 | 6.0 × 10−9 M |
6 | 42.4 ± 1.0 | 6.0 × 10−9 M |
7 | 42.4 ± 1.0 | 6.0 × 10−9 M |
8 | 41.3 ± 1.0 | 6.0 × 10−9 M |
9 | 40.1 ± 1.0 | 8.0 × 10−9 M |
10 | 39.3 ± 1.0 | 8.6 × 10−9 M |
11 | 36.2 ± 1.0 | 1.2 × 10−8 M |
12 | 33.8 ± 1.0 | 6.7 × 10−8 M |
13 | 32.1 ± 1.0 | 1.5 × 10−7 M |
The effect of pH on the response of the carbon paste electrode modified with organofunctionalized silica was studded in the range of 0–10.6. As shown in Figure
Effect of pH on potential response of CPE8.
The selectivity of electrode is an important parameter which dictates selectivity of the electrode toward a specific ion in presence of other interfering ions. In the present study the selectivity coefficient of CPE8 was calculated by match potential method (MPM) (Table
Selectivity coefficients of various interfering ions (B).
Interfering ion B | Selectivity coefficient ( |
---|---|
Al(III) | 4.3 × 10−5 |
Ga(III) | 1.3 × 10−5 |
Ho(III) | 1.3 × 10−5 |
La(III) | 1.6 × 10−5 |
Eu(III) | 1.7 × 10−5 |
Nb(III) | 1.6 × 10−5 |
Dy(III) | 1.8 × 10−5 |
Bi(III) | 1.3 × 10−5 |
Hg(II) | 1.3 × 10−5 |
Pb(II) | 1.3 × 10−4 |
Cd(II) | 1.2 × 10−4 |
Zn(II) | 1.1 × 10−4 |
Cu(II) | 1.3 × 10−5 |
Ag(I) | 1.1 × 10−5 |
Na(I) | 3.0 × 10−4 |
K(I) | 1.3 × 10−4 |
Li(I) | 1.7 × 10−5 |
The thermal stability of the electrode assembly was investigated in terms of thermal temperature coefficient (
The response characters of the electrode were compared with the previously reported best polymeric membrane and carbon paste electrodes (Table
Comparison study of CPE8 with previously reported electrodes.
Type of electrode | Concentration range (M) | Slope (mV/decay) | Reference |
---|---|---|---|
PVC membrane |
|
55 | [ |
PVC membrane |
|
57.27 ± 0.4 | [ |
PVC membrane |
|
56 ± 0.2 | [ |
Carbon paste |
|
[ | |
Carbon paste |
|
42.4 ± 1.0 | This work |
The electrode (CPE8) was successfully applied as indicator electrode for the titration of 20 mL of 0.01 M TlNO3 solution with standard EDTA (0.01 M) solution at pH 5.0. It was observed that the electrode potential sharply decreases at the end point of the titration. Thus the CPE8 can be used as an indicator electrode for the direct determination of total thallium content of the solution (Figure
Titration curve of Tl(I) with EDTA using CPE8 as indicator electrode.
Titration curve of 10 mL (0.01 M) TlNO3 with 0.003 M NaBr and 0.1 M NaCl solutions.
Titration curve of 10 mL (0.01 M) TlNO3 with 0.003 M NaBr, 0.1 M NaSCN, and 0.1 M NaCl solutions.
The practical utility of the electrode (CPE8) was investigated by recovery of Tl(I) ion from different synthetic water samples. The result, as the average of three separate determinations, is shown in Table
Recovery of Tl(I) ion by CPE8 from different water samples.
Sample | Added (M) | Recovery (%) | |
---|---|---|---|
CPE8 | AAS | ||
Tap water 1 | 0.0005 | 100.2 | 100.3 |
Tap water 2 | 0.0004 | 100.0 | 100.1 |
River water | 0.0003 | 102.3 | 102.4 |
Hair sample | 0.0003 | 101.4 | 101.5 |
Chemically modified silica (SBA-15) based electrode has been constructed and used for the selective determination of Tl(I) in various synthetic water samples. The mesoporous silica (SBA-15) was chemically modified by 3-(triethoxysilyl)propyl amine (TESPA) and 2-[5-(chloromethyl)-1,2,4-oxadiazol-3-yl]-3-methylpyridine as electroactive modifier. The electrode of the composition of 10% TMPA-SBA-15, 75% graphite powder, and 15% paraffin oil shows the best possible response in terms of slope (42.4 ± 1.0, mV/decay of activity), working concentration range (1.0 × 10−8–1.0 × 10−1 M) with LOD of 6.0 × 10−9 M. The CPE8 reached the equilibrium valve of potential in a very short time of about 8 s and could be used for a period of more than 10 months without any divergence in response characters. The electrode CPE8 was also used as an indicator electrode for the titration of 0.01 M TlNO3 solution with standard EDTA solution and sequential titration of mixture of different anions.
The authors declare that they have no competing interests.