65th annual ise meeting lausanne, switzerland 2014depa.fquim.unam.mx/amyd/archivero/paper_ise... ·...
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s15-026
65th Annual ISE Meeting
Universidad Nacional Autónoma de México
Lausanne, Switzerland 2014
Conductometric
Microscale electroanalytical chemistry experimental teaching with locally produced low-cost instrumentation
Arturo García-Mendoza, Adrián de-Santiago, Alejandro Marín-Medina, Alejandro Baeza* Departamento de Química Analítica, Facultad de Química, UNAM
Ciudad Universitaria, 04510, México. D.F. [email protected], * [email protected]
References
(1) Vierna L, García-Mendoza A, Baeza A J, Mod. Edu. Rev. 2 (2012) 243-251 (2) 3thd International symposium of chemistry in microescale, Universidad Iberoamericana, May 18-20, 2005. México. (3) El Microlaboratorio. Available on line: https://www.youtube.com/user/elmicrolaboratorio
Introduction Microscale Chemistry is a set of techniques that attempt to reduce to the minimum the scale of the components used in a conventional manner for a successful experiment in the area of chemistry. In recent years microscale chemistry has proved very useful in several areas such as shown in Figure 1.
Potentiometric
Online Resources
(1) Instrumental Analytical Chemistry. Supporting documents, class notes, tests and experimental protocols. Available at: http://depa.fquim.unam.mx/amyd/curso.php?curso=178
(2) Analytical Chemistry. Chemical Equilibria in Solution. Supporting documents, class notes, tests and experimental protocols. Available at: http://depa.fquim.unam.mx/amyd/curso.php?curso=243
Aim of the Work Our aim is to show the achievements we have made in our laboratory, where we have developed low-cost equipment with locally materials, as listed in Figure 2, to perform potentiometric, conductometric, electrochemical and photocolorimetric measurements to teach Instrumental Analytical Chemistry.
Microscale Chemistry
Lower cost of operation.
Less storage space.
Shorter operation.
Equipment and fast
operation at low cost. Fieldwork
and classroom.
Less waste.
Reduced risk of operation.
Low water consumption
and reagents.
Figure 1. Positive aspects of Microscale Chemistry.
Common materials • Plastic, acrylic, glue, small
bottles, coal mines, short and thin copper wires, cotton, epoxy.
Low cost materials • Disposable syringes and
plastic tips, stainless steel, silver, tungsten welding, computer fans, connections and circuits used in electricity and electronics
Specific materials • Low-cost multimeters to
measure voltage, current and electrical resistance
Figure 2. Left: Common materials used to construct the equipment. Right: Localities where this proposal has been carried out successfully.
Conclusions ① Good analytical results have been achieved in under and graduate courses in most of Chemistry Colleges in Mexico, Central and South America in a period of 15 years. ② In all latter colleges the methodology presented has given them their first opportunity to achieve electrochemical experimental approach. ③ It is possible to construct doser equipments and measuring instruments to get chemical information through operations of calibration proper with a level of precision associated acceptable, if operated in accordance with good laboratory practices.
Electrochemical
Photocolorimetric
Figure 3. Potentiometric low-cost microequipment
6.0
8.0
10.0
12.0
14.0
16.0
0 200 400 600 800 1000 1200
pH
VAdded MetH [µL]
Non-aqueous solvent titrations in acetonitrile.
NH2
CH3
+ CH3
O
O
S OH
H H
H
N+
CH3 CH3
O
O
S O-
Figure 4. Typical potentiometric plots.
ΔΕ = -logCascorbic acid - 59 r² = 0.9998
-59
-58
-57
-56
-55
-54
-53
-52 -8 -6 -4 -2 0
∆E/
RE
[mV
]
log Cascorbic acid
Calibration plot ∆E = f(logC)
Ascorbate oxidase (extracted from the peel of fresh cucumber)
as recognition element in the construction of a biosensor.
-0.0040
-0.0030
-0.0020
-0.0010
0.0000
0.0010
0.0020
0.0030
0.0040
-300
-200
-100
0
100
200
300
400
500
0 100 200 300 400 500 600 700 800 900
∆E/
RE
[mV
]
VAdded AgNO3 [µL]
Coupled plots (a) titration of a mixture of halides with AgNO3 ∆E/RE= f (VAdded AgNO3)
and (b) second derivative (∆∆2E/∆V2)= f (Vaverage2).
No salt-bridge needed.
(a)
(b)
Figure 7. Upper: MIMP: Minimal Instrumentation MicroPolarograph. Lower left: Microcoulometer. Lower right: Electrochemical cell and electrodes.
Figure 8. Typical electrochemical analysis. Figure 9. MIMC: Minimal
Instrumentation MicroColorimeter. Figure 10. Typical Lambert-Beer-Bouger Law calibration plots.
Figure 5. Conductometric low-cost microequipment
Figure 6. Typical conductometric titrations plots.
0.00
0.20
0.40
0.60
0.80
1.00
0 0.2 0.4 0.6 0.8 1
Ψ
Vadded of NaOH [mL]
Conductometric titration of a strong acid with a strong base. Both C= 0.01 mol*L-1
0.002
0.004
0.006
0.008
0.010
0.012
0.014
0.016
0.00 0.20 0.40 0.60 0.80 1.00
k[µS
*cm
-1]
Vadded of AgNO3 [mL]
INN�H+Cl- + Ag+ ⇌ AgCl↓+ INN�H+
Conductometric titration of Ranitidine hydrochloride,
C= 0.01 mol*L-1, with AgNO3, C= 0.01 mol*L-1.
0
5
10
15
20
25
0.00 0.10 0.20 0.30 0.40 0.50 0.60 0.70 0.80 0.90 1.00
Ψ
VAdded NaOH [mL]
Conductometric titration of Naproxen with NaOH C= 0.1 mol*L-1 taken from one tablet dispersed in a mixture of 10 mL H2O-Ethanol. Sample V= 0.5 mL
0.00
0.20
0.40
0.60
0.80
1.00
-6 -5 -4 -3 -2
Abs
orpt
ance
Log [C0]
Ringbom Plot
A= 0.3961 CCu[II] + 4x10-5 r² = 0.9966
0.0000
0.0002
0.0004
0.0006
0.0008
0.0010
0.0012
0 0.0005 0.001 0.0015 0.002 0.0025 0.003
A
[Cu(NH3)4]2+ [mol*L-1]
Calibration plot for Cu(II) as [Cu(NH3)4]2+
A = 0.3419C CSA - 0.0474 r² = 0.9994
0.07
0.10
0.13
0.16
0.19
0.22
0.25
0.40 0.45 0.50 0.55 0.60 0.65 0.70 0.75 0.80 0.85
A
CSalicilic acid [mg*mL-1]
Calibration plot for Salicilic acid as AS-Fe(III)
-5
15
35
55
75
95
115
135
0 500 1000 1500 2000
i [µA
]
ΔE/RE [mV]
Electrochemical window
ABS/Ascorbic acid
Oxidation of Ascorbic acid, C= 0.01 mol*L-1, in Acetate buffered media (pH= 5)
0
10
20
30
40
50
60
70
80
90
t turn
[s]
nH+
Coulometric calibration plot tturn= f (nH
+ ) r2 = 0.9975
Electrochemical Reaction WE, cathode: H2O + 2e- → H2(g)↑+ 2OH-
Tritation Chemical Reaction: OH- + HA ⇄ A- + H2O
-200
-150
-100
-50
0
50
100
150
200
250
300
-600 -400 -200 0 200 400 600 800
∆E/RE [mV]
i [µA]
ASV. Solution of Cu(II),
C= 6 mmol*L-1 in Acetate buffered media
C= 1.0 mol*L-1, pH= 5. WE: Pt
RE: Ag | AgCl(s) | | AE: Graphite
-150
-100
-50
0
50
100
150
200
250
300
-1000 -500 0 500 1000 1500 2000
i (µA)
∆E/RE (mV)
Microchronoamperometry of polyaniline (PANI)
Electrodes modified