Journal of Modern Education Review, ISSN 2155-7993, USA May 2013, Volume 3, No. 5, pp. 407–415 Academic Star Publishing Company, 2013 http://www.academicstar.us

407

Microscale Analytical Potentiometry: Experimental Teaching with Locally

Produced Low-Cost Instrumentation

Lilia Vierna1, Arturo Garcia-Mendoza2, Alejandro Baeza-Reyes2

(1. Biology Department, Autonomous National University of México;

2. Analytical Chemistry Department, Faculty of Chemistry, Autonomous National University of México, Mexico)

Abstract:MicroISE, microbiosensors and microreference electrodes built with low cost locally produced

materials are presented, to perform experiments to determine several analytes in instrumental analysis teaching.

Creativity in experimental learning emphasizing ecological and green aspects are achieved.

Keywords: low cost, microISE, microbiosensors, ionic liquids

1. Introduction

Microscale laboratory has been widely used in General Chemistry mainly in Synthetic Chemistry (inorganic

and organic chemistry). Analytical Chemistry approaches just concern to titrimetric determinations with acid —

base indicators using 5 mL pipets as burets to teach semi quantitative analysis aspects. Chemical Instrumentation

has been developed mainly for basic or applied research so it is difficult to access to such devices in teaching

laboratories. In our laboratory, we have developed low cost equipment with locally materials to perform

micropotentiometric measurements to teach Instrumental Analytical Chemistry: pH microsensors based on electro

generated conductive polymers or W0 rods, AgCl or Ag2 Smicrocrystal with Ag0 transduction to measure chloride

and sulfide anions and also microbiosensors based on fresh vegetable tissue to detect hydrogen peroxide, urea and

ascorbic acid in several samples including foods, isotonic solutions and microbial systems.

2. Experimental Design.

2.1 Solid StateMicrosensors

The pH determination is achieved with a combined micropotenciometric system: A drawing carbon rod

covered with polyaniline, PANI, obtained applying a direct potential, 9V, pulse according to Figure 1.

Microreference electrode is made by introducing a Cu0 rod in a plastic tip that contains water or a Cu(II) 10

mM solution. A piece of cotton is used as liquid junction separation. The PANI electrode can be substituted by a

W0 micro rod fixed in a plastic tip as well. Both pH solid-state sensor and microreference electrode are fixed in a

25 plastic vessel to performed pH measurements connected to a simple voltmeter as shown in Figure 2.

Alejandro Baeza-Reyes, Ph.D., Professor, Analytical Chemistry Department, Faculty of Chemistry, Autonomous National

University of México; research areas: electro analytical chemistry. E-mail: baeza@unam.mx.

Microscale Analytical Potentiometry: Experimental Teaching with Locally Produced Low-Cost Intrumentation

408

Figure1 AnodicPolyaniline Recovering of a Micro Carbon Rod in A 0.1 M Aniline Bisulphate Dissolved in A 0.5 M

Sulfuricacid. A 30s 9 V Potential Pulse is Applied.

Silver — silver chloride microreference electrodes have been used successfully in potenciometric

experiments as well. This kind of electrode is constructed by means of an Ag0 wire with electrochemical covering

of AgCl. This procedure is easily reproducible with a 9V battery using the Ag wire as anode and a carbon rod as

cathode in a hydrochloric acid 0.1 M solution, by 10 seconds. A single plastic tip with KCl saturated solution can

be used as first chamber, however a second chamber has shown better results whenever the double junction in the

electrode restricts diffusion of analytes towards the reference electrode.

MicroISE based on Ag0|AgCl(s) to measure chloride ions or Ag0|Ag2S(s) to measure HS-, are prepared in a

similar way by oxidizing a microsilver rod in HCl 0.1M for the former and Na2S 0.1 M for the latter microsensor.

In both cases the same microreference electrode can be used.

Figure 2 Typical Plastic Microcell Shown with Two Solid State Microsensors: W0and PANI as pH Sensors. Combined

PANI-Microreferenceor W0-Microreference Electrodes Can be Used Alternatively Connected to a Multimeter to Determine pH.

2.2 Microbiosensors

A microchamber to put fresh tissues of vegetal containing a certain active enzyme is built by placing a C0 or a

W0 rod transductor into a plastic tip cut properly to form a small chamber as shown in Figure 3.

A piece of fresh potato is used to profit the peroxidase activity to determine H2O2 in samples dissolved in a

pH = 7 phosphate buffer solution, PBS. A paste formed with soybean flour obtained directly by grinding soybean

grains in distilled water is used to determine urea since this tissue presents a high urease activity. A skin cucumber

extract with PBS present ascorbic acid oxidase activity used to measure ascorbic acid content.

PANI

Micro

Figure 3 M

3. Exp

Microp

Nikolsky po

determined b

since the co

Figures 4 an

Figure 4 Ty

Figure 5 Plo

oscale Analytic

Microchamber

perimental

pH sensor bas

otentiometric

by a linear an

orresponding

nd 5.

ypical Calibrat

ot

mV

E

H

Naf

[

[10 60

Solution p

10

A

al Potentiomet

r (A) to Built M

l Results

sed on PANI

behaviors. T

nalysis of the

slope yields

tion Plot E

Linear Fitti

BSH

a ] , Where E2

pH = 4.0. Linea

E[mV]

(10ΔE/60)

try: Experimen

Microbiosensor

was calibrat

he selectivity

type:

10 60

the selectivit

f(pH) for PA

ing Analysis Yi

2 in Presence o

ar Fitting Anal

ntal Teaching w

r Based on FreElectrode (B).

ted with seve

y of the PANI

mV

E

H

Nf

[

[0

ty coefficient

ANI Solid State

ields .E= 7336

of 0.04 M < [Na

lysis Yields b =

B

with Locally P

esh Vegetable T.

eral pH buffe

I sensor to H

BSH

Na ]

t HNak

, d

e Microsensor

pH.mV- 651 ,

a+] < 018M, E1

= 0.9951 1; m

Produced Low-

Tissues Combin

er solutions in

H+ in presence

directly. Resu

Calibrated wi

R2 = 0.9998.

without Na+, i

m = 0.0001 and

[Na+]/[H+

-Cost Intrumen

ned with A Mic

n order to de

e of Na+, as e

lts obtained a

ith Several Buf

in 5 mL of Bip

r2 = 0.9899.

+]BS

ntation

409

croreference

etermined the

example, was

are shown in

ffer Solutions,

phtalate Buffer

9

e

s

n

r

Microscale Analytical Potentiometry: Experimental Teaching with Locally Produced Low-Cost Intrumentation

410

From the results presented above it is conclude that the propose PANI solid state microsensor is suitable to

determine pH in aqueous solutions.

Figure 6 shows a typical titration plot obtained with a W0 microsolid state pH sensor.

Figure 6 Typical Titrationplot, pH = f(v), of 0.5 mL HCl 0.1 M with NaOH 0.0998 M, monitored with aW0 Microsolid State pH Sensor and aCu0|H2O|| MicroferenceElectrode, A linear Fitting Analysis Yields E = 84mV-28pH, r2 = 0.9964.

PANI micro pH sensor has been used successfully in non-aqueous solvents. Indeed in pure acetonitrile this

sensor has shown a linear behavior, E = 244.89-20.39pH, r2 = 0.9923, calibrated spectrophotometrically with

bromochresol blue in this solvent (results not shown).

On the other hand, it is very well known Liquid Ionics are a special family of solvents with amazing

properties; nevertheless the high cost of them has complicated and discouraged the searching for new

physicochemical parameters. We have been able to evaluate the electrochemical behavior of ferrocene and

p-benzoquinone in several ionic liquids media, in order to compare electrochemical reactivity in these media. A

suitable microcell was designed for potentiometric, cyclic voltammetric, chronoamperometric and

chronopotentiometric experiments with small solvent volumes, V 500 L. Also, a set of microelectrodes was

designed for this task, included a micro-reference electrode based in each ionic liquid. As a matter of fact,

p-benzoquinone reduction was studied in two ionic liquids: tetrafluoroborate of 1-butyl-3-methylpyridinium,

[bmpy][BF4] and tetrafluoroborate of 1-ethyl-3-methylimidazolium, [emim][BF4]. In both media, two

experimental conditions were used: pure ionic liquids and buffered acidity levels. A buffer of methanesulfonic

acid and sodium methanesulfonate prepared in these media was used. A W0microelectrode was designed and used

as an acidity levels sensor together with aAg0|AgCl(s)micro reference made in those media. A Nernstianpotential

— concentration linear behavior was found using perchloric acid as leveling agent acidity levels, as shown in

Figure 7.

For potentiometric assays, tungsten sensor presented a differential response to different levels of acidity in

[bmpy][BF4]. Its performance was tested in various evaluations volumetric Lutidine with HClO4, which was

obtained in the typical evolution potential during the analytical operation as can be seen in Figure 8. This

methodology has allowed us to reduce costs and waste materials.

Micro

Figure7

Figure 8 T

The ch

(Mohr Meth

It has b

shown that i

the potentiom

is needed.

oscale Analytic

Typical PotenExperiment

Titration Plot, p

hloride micro

hod). Figure 9

been possible

it is feasible

metric titratio

al Potentiomet

ntiometric Calts Were Perfor

pH = f(v), of 0.5

and a A

ISE can be u

9 shows a typ

e to work w

to find their

ons of a mixtu

try: Experimen

librationplot Ormedusing A M

5 mL Lut 0.1 M

Ag0|AgCl(s) Micr

used to mon

ical calibratio

ith halides m

respective c

ure of halide

ntal Teaching w

Obtained for a Multimeter to D

M with HClO4

roference Elec

nitoring titrati

on plot to det

mixtures with

oncentrations

s with no mo

with Locally P

W0 Micro SensDetermine the P

0.1 M, Monito

ctrode in [bmpy

ion plots of

termine chlor

h these micro

s without pri

ore than 1 mL

Produced Low-

sor for AcidityPotential in Ea

red with a W0 M

y][BF4].

AgNO3 solut

ride in 1 mL s

oelectrodes. S

or sample tre

L of silver nitr

-Cost Intrumen

y Levels in [bmach Solution.

Microsolid Sta

tions with N

standard NaC

Several exper

eatment. Figu

rate 0.1 M. N

ntation

411

mpy][BF4],

ate pH Sensor

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No salt bridge

d

e

s

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Micro

412

Figure 9

Figure 10

Ag

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Figure

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oscale Analytic

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g|AgCl(s), Micr

bove calibrati

tions and chlo

11 shows a t

on is perform

sing method:

10

200 L samp

lphide conten

al Potentiomet

Plot E = f(log[Cce of Cu0|H2O|

ot of 500 L of

roreference of

on plots has

oride content

typical calibr

med in a bora

∆

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a Mixture of N

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been succes

in commerci

ration plot to

ate buffer solu

10, 20 30 an

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ntal Teaching w

with Several 1 ing Analysis Y

NaCl, KCl and

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ssfully used t

al french-frie

determine su

ution pH = 9

10

nd 40 L of

is obtained ac

with Locally P

mL NaCl StanYields: E = -7.9m

d KI Aprox. 0.1

ystem, Δ(Δpe/Δ

to control chl

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ulfides, HS-,

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a standard so

ccording to:

Produced Low-

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from asulpho

ndard additio

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olution of

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ns. MicroISE: ACl-], r2 = 0.9950.

O3 0.1 M Using

lot is Also Show

nt in commer

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g MicroISE:

wn.

rcial Isotonic

ulture media.

nd Gran Plot

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where vabsc i

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413

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the generated

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.

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de HalogenurChil. Ed. Cient., oro en desinfect

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rmine Urea in

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ponding anal

cucumber ext

roperly to sev

well. The use

s creativity

ven in comple

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ros a MicroescVol. 3, No. 1, p

tanteshospitalar

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ntation

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tracts in PBS

veral analytes

e of low cost

in analytical

ex media.

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cala Total conpp. 22–25. rios”, Rev. Cub.

and the use of

garette smoke”,

d

S

s

t

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,

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