Anodic Stripping Voltammetric Determination of Nitrite Using Carbon Paste Electrode Modified with Chitosan

doi:10.4236/ajac.2011.22035

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American Journal of Anal yt ical Chemistry, 2011, 2, 284-288

doi:10.4236/ajac.2011.22035 Published Online May 2011 (http://www.SciRP.org/journal/ajac)

Anodic Stripping Voltammetric Determination of Nitrite

Using Carbon Paste Electrode Modified with Chitosan

Ibrahim H. I. Habib

Microanalytical Chemistry Lab, Applied Organic Chemistry Department, National Research Centre, Cairo , Egypt

E-mail: [email protected]

Received December 2, 2010; revised March 7, 2011; accepted May 14, 2011

Abstract

A simple method for anodic stripping voltammetric determination of nitrite using carbon paste electrode

modified with biomolecular chitosan, is described. In this method, the electrode is activated electrochemi-

cally by scanning 5 replicates over the potential range from +500 to +1400 mV immersing in 0.5 M HCl so-

lution. Following this step, the nitrite sample containing 0.1 M KCl is pre-concentrated on the activated elec-

trode at +500 mV for 30 s. The deposited anions are then oxidized by different modes of sweep in the oxida-

tion direction. Chemical and electrical parameters affecting the voltammetric measurements are optimized.

The peak current is linear proportional to the 2



concentration within the range 0.41 - 4.1 µg/ml, with

detection limit 0.187 µg/m using differential pulse mode. The relative standard deviation is 0.285 % for 2.46

µg/ml (five replicates). No interference is observed due to oxygen dissolved in the sample so that nitrogen

purging is not needed in this case. The result obtained by the modified electrode is more accurate and selec-

tive than the unmodified electrode.

Keywords: Chitosan, Carbon Paste Electrode, Nitrite, Anodic Stripping Voltammetry

1. Introduction

Nitrite is one of the pollutants widely found in natural

waters, wastes from fertilizers or intentional additions of

nitrites for corrosion control or protection of meat prod-

ucts. Nitrite ions can interact with amines to form ni-

trosamines which are known as carcinogenic substances.

They also may react with hemoglobin and reduce the

oxygen carrying capacity of it [1,2]. Thus, its concentra-

tion in various types of samples has to be controlled by

sensitive methods.

The most commonly used method for nitrite determi-

nation ranging from trace to ultra trace levels is the elec-

trochemical techniques. This includes polarography [3-7],

amperimetry [8,9] and voltammetry [10-14]. The sensi-

tivity can be further enhanced if the surface of bare elec-

trodes is modified chemically with active thin film of-

fering significant advantageous for design and develop-

ment of electrochemical sensors. According to the litera-

ture survey, all electrodes modified by complexes of iron

[15,16], palladium [17], iridium [18], cobalt [19], copper

[20], mercury [21], cetyltrimethylammonium bromide

[10], phosphomolybdic [22,23] or silicotungestic acids

[24,25] have been used successfully for electrochemical

oxidation of nitrite. Boron-doped diamond electrode [26]

has also been applied for monitoring nitrite, but it needs

special instruments not ever available elsewhere.

Chitosan, on the other hand, is natural product extracted

from the shell of shrimps, crabs and insects [27]. Because

of its ability to bind strongly with many metals [28], chi-

tosan and its derivatives have been applied for electro-

chemical determination of silver [29,30], platinum [30],

palladium [30], gold [30,31], lead [32,33] and iron [34].

In the present work, carbon paste electrode modified

with natural chitosan represents the simplest and repro-

ducible analytical tool to determine nitrite in samples.

2. Experimental

2.1. Apparatus and Reagents

A Metrohm model 693 VA processor and 694 VA stand

equipped with a Ag/AgCl-3 M KCl and a platinum

counter electrode are employed. The modified carbon

paste electrode is used as the working electrode and pre-

pared as described below.

I. H. I. HABIB285

2.2. Preparation of the Modified Carbon Paste

Electrode

The carbon paste is prepared by mixing 175 mg paraffin

oil (Fluka), 250 mg synthetic carbon powder 1 - 2 micron

(Aldrich) and 48 mg low-viscous Chitosan (Fluka) in

agate mortar. The electrode is consisted of stainless steel

tube with i.d. 2.5 mm and 30 mm deep, moving through

it an inner screwed stainless steel connector. The tube is

coated externally with Teflon. The tube is packed with

the modified carbon paste, compressed with inner screw

and smoothed on a wetted Whatman filter paper.

2.3. Procedure

The fresh surface electrode is first immersed in 0.5 M

HCl solution and electrically activated by direct current

sweep five replicates from +500 to +1400 mV with scan

rate 50 mV/s. The solution is then exchanged by 10 ml of

0.1 M KCl solution containing 0.41 - 4.1 µg of nitrite

and pre-concentrated for 30 s at +500 mV with stirring at

2000 rpm. After resting for 10 s, one of electrical modes

(direct current tast DC, differential pulse DP, square

wave SW or first-harmonic alternating current AC1) is

ramped from +500 to +1400 mV with scan rate 50 mV/s,

pulse amplitude 50 mV, pulse duration 20 ms, measure-

ment time 10 ms, frequency 30 Hz for SW and AC1. The

experiment is triplicated without electrode re- generation

and two standard solutions of nitrite are added sequen-

tially. Average of current peaks due to the sample and

standard solutions are taken. For new electrode, the ex-

periment is repeated with new activation of electrode as

described before.

3. Results and Discussion

Preliminary experiments are carried out to optimize the

sensor composition. The height of peak current obtained

for nitrite is used to compare the performance of the dif-

ferent chemical and physical parameters. Each parameter

is analyzed triplicate with the same sensor and the cor-

responding average value of peak height is plotted. The

working electrode containing 40% (w/w) water-immis-

cible organic binders (pasting liquids), viz. paraffin or

silicon oils, with/without 4% chitosan are prepared and

examined under identical conditions, i.e. immersing in

0.5 M HCl prior accumulation and differential pulse

stripping voltammetric measurements of nitrite in 0.1 M

KCl solution. It is obvious from Figure 1, the 40% par-

affin oil with 4% chitosan accumulates afforded highest

stripping peak at potential of about +900 mV. Thus, the

modifier enhances the sensitivity of the electrode twice

more than using paraffinic bare CPE. This enhancement

Figure 1. Differential pulse DP cathodic stripping voltam-

mograms for determining nitrite in absence and presence of

chitosan with scan rate 60 mV/s, pulse amplitude 50 mV in

0.3 M KCl.

could be explained by the fact that the – NH2 group is

converted into 3



 making it easy to increase the

electrostatic attraction of negatively charged nitrite ions

on its surface in a similar manner for determining nega-

tively charged palladium, platinum30 and lead32 chloride

complexes

The sensitivity and linear responses towards the nitrite

concentrations can also be enhanced by increasing the

proportion of chitosan up to 10% (w/w) chitosan. Be-

yond this proportion a depression in the sensitivity is

observed as shown in Figure 2.

On the other hand, the influence of HCl and KCl, sep-

arately or in mixture solutions, revealed that in acidic

solutions (pH < 3.3) nitrite is escaped as NO gas and lost

Figure 2. Effect of chitosan (%, w/w) on the stripping cur-

rent at different concentrations of nitrite (0.41 - 2.46 µg/mol

NO2) using DP mode with scan rate 60 mV/s, pulse ampli-

tude 50 mV in 0.3 M KCl.

I. H. I. HABIB

286

more when nitrogen gas is bubbled giving up weak peak

current. In a neutral medium, pH > 3.3, nitrite anion is

predominated promoting to strong peak current. Thus, in

order to avoid NO gas formation, two separating solu-

tions on two sequential steps are achieved, 0.5 M HCl

solution for the protonation of chitosan followed by 0.1

M KCl solution for applying as a neutral supporting

electrolyte medium. The latter effect is as depicted in

Figure 3 where the concentration of 0.1 M KCl gives the

highest peak current and best linear relation.

The variation of scan rate from 10 to 120 mV/s re-

vealed that the diffusion current is related linearly with

square root of scan rate, but the oxidation reaction pro-

ceeded irreversibly as the peak potential shifted linearly

to more positive potential with increasing the scan rate.

No reduction peak is observed on the reverse scan within

the studied potential range indicating that the overall

process is chemically irreversible in agreement with the

previous reports for nitrite oxidation at different types of

electrodes [35,36].

Modulating the direct current tast ramp DCT with

various wave forms by using superimposed differential

pulse DP, square-wave SW or first harmonic a.c. modu-

lation AC also studied to explore which one of modes

will enhance the sensitivity.

As shown in Figures 4 and 5, variation of current with

concentration of nitrite ions showed a linear behavior

between 0.41 and 4.1 µg/mol of nitrite with correlation

coefficients 0.994, 0.999, 0.998 and 0.998 for different

modes of sweep DC, DP, SW and AC1, respectively.

The detection limit based on “three standard deviations

of the blank 3sB method [37] is found to be in turn, 0.281,

0.187, 0.207 and 0.241 µg/ml nitrite. The analytical pa-

rameters for nitrite determination are reported in Table 1.

Figure 3. Effect of KCl on the stripping current at different

concentrations of nitrite (0.41 - 2.46 µg/ml NO2) using 10%

chitosan carbon paste electrode and DP mode with scan rate

60 mV/s and pulse amplitude 50 mV.

Figure 4. Different modes of sweeps, DCT, DP, SW and

AC1, at different concentrations of nitrite (0.41 - 2.46 ug/ml

NO2) using 10% chitosan carbon paste electrode with scan

rate 60 mV/s, pulse amplitude 50 mV in 0.1 M KCl.

Figure 5. Regression lines for determining nitrite using

different modes of sweep.

Table 1. Analytical parameters for nitrite determination at

chitosan modified CP electrode.

ModeR Slope,

nA/µg

Intercepte

nA/µg LOD µg Repeatability

RSD, %

AC10.998204.78– 74.76 0.241 0.668

DP 0.999208.35– 66.17 0.187 0.285

DC 0.994360.86– 173.00 0.281 1.018

SW 0.998404.86– 169.084 0.206 0.437

An extremely attractive feature of the chitosan modi-

fied carbon paste electrode is its highly stable response

toward nitrite oxidation. Repeatability “RSD” using dif-

ferent mode is found to be 1.018%, 0.285%, 0.437% and

0.668%, respectively, for five successive determination

of 2.46 µg/ml nitrite. For the same modified chitosan

I. H. I. HABIB287

electrode, reproducibility, without and with successive

cut off/polishing five times, is found to be 0.285% and

0.347%, respectively, for 2.46 µg/ml nitrite and using DP

mode.

It is concluded that the new and simple carbon paste

electrode modified with chitosan has been shown to offer

comparable performance to the more expensive glassy

carbon or diamond electrodes for the practical use to

analysis 2ions at trace concentrations in real samples

by anodic striping voltammetry with high sensitivity and

selectivity.

NO

4. Acknowledgements

Thanks are due to Volkswagen Stiftung, Kastanienallee

35, 30519 Hannover, Germany, for the financial support

of purchasing the Metrohm model 693 VA processor and

694 VA stand.

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