Determination of Nitrites in Water by In-electrode Coulometric Titration in Reticulated Vitreous Carbon Electrode

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152 _Acta Chim. Slov._ 2015, _62,_ 152–158 DOI: 10.17344/acsi.2014.838

_Scientific paper_

# Determination of Nitrites in Water by In-electrode Coulometric Titration in Reticulated Vitreous Carbon Electrode
## Katarina Lenghartova, 1 Lukas Lauko, 2 Frantisek Cachob 2 and Ernest Beinrohr 1,2, *

1 _Department of Chemistry, Faculty of Natural Sciences, University of SS. Cyril and Methodius in Trnava, J. Herdu 2,_
_917 01 Trnava, Slovakia_

2 _Institute of Analytical Chemistry, Faculty of Chemical and Food Technology, Slovak University_
_of Technology in Bratislava, Radlinskeho 9, 812 37 Bratislava, Slovakia_

_* Corresponding author: E-mail: ernest.beinrohr@stuba.sk_

_Received: 22-07-2014_

## Abstract

Nitrite in water samples was determined by in-electrode coulometric titration in a reticulated vitreous carbon (RVC)
electrode of 100 ppi porosity. The sample was mixed with dilute sulphuric acid and sodium sulphate, aspirated into a
flow cell with the porous electrode and nitrite was oxidised to nitrate by constant current of 5 μA at which the potential
of the electrode was monitored. The limits of detection and quantification were found to be 0.4 and 1.2 μg/L, respectively. The repeatability and reproducibility were 2.2% and 2.6%, respectively. The bias at 100 μg/L were found to be
0.3%. The duration of the measurement is 2–3 min depending on the nitrite concentration. There were a few interferents
only, neutral and cationic surfactants decreased and increased slightly the signal, respectively. Humic acids above 30
mg/L increased the signal by 10%. Drinking and surface water samples were analysed and the results matched well those from the photometric method.

Keywords: Nitrite determination, drinking water, surface water, in-electrode coulometric titration, chronopotentiometry

## 1. Introduction

Nitrites in small amounts are natural components of
the environment as a part of the nitrogen cycle. Higher
amounts in soil and water are the result of enhanced application of nitrogen containing fertilisers. Sodium nitrite is
used for curing of meat because it prevents bacterial growth
and gives the product a desirable colour. Because of the relatively high toxicity of nitrite, the maximum allowed nitrite concentration in meat products is 200 mg dm –3 . Nitrite in
blood is highly reactive with haemoglobin and causes methaemoglobinaemia. Under certain conditions nitrites in
meat can react with degradation products of amino acids,
forming nitrosamines, which are known carcinogens. 1

For the determination of nitrites various analytical
methods are used such as volumetric analysis for higher
levels, spectroscopic, electrochemical and separation methods for lower concentrations. 2

Nitrite is mostly detected and analysed photometrically by making use of the Griess reaction, involving the
formation of a deep red-coloured azo dye as the reaction
product of NO2– ions with sulfanilic acid and naphthyl-1
amine in acidic media. 3 These methods including also some other modified reagents enable the determination of
nitrites at concentrations down to the low μg dm –3 range.

Of electrochemical methods amperometry on glassy
carbon electrode coated with gold 4 or modified with complexes of Fe(III) and Co(II) 5 was successfully used for nitrite determination water and orange juice samples. The
detection limit was near 10 μg dm –3 . Carbon black-modified electrodes were used for amperometric detection of
nitrite and nitrate through oxidation of nitrite 6 . Nitrates
were reduced first to nitrite and then measured.

Fogg and Alonso 7 determined nitrites by differential
pulse stripping voltammetry by making use of the adsorption of an azo dye formed by nitrate on hanging mercury

_Acta Chim. Slov._ 2015, _62,_ 152–158 153

drop electrode. The detection limit of the procedure for 5
min accumulation time was 7 ng dm –3 . Chamsi and Fogg 8

used an electrochemically pre-treated glassy carbon electrode in a flow system for nitrite detection through its direct electrochemical oxidation. Concentrations above 3 μg
dm –3 could be measured. The use of bare glassy carbon
electrode was also demonstrated. 9 Differential pulse voltammetric with gold ultramicroelectrode was used for nitrite sensing in natural waters. 10 The calculated detection
limit was reported to be 30 μg dm –3 . A cobalt film electrode on copper disk was reported for nitrite determination in
water samples through their reduction in non-deaerated
solutions 11 with a detection limit about 10 μg dm –3 .

Potentiostatic coulometry is a selective and accurate
method for the determination of nitrite content in water
samples through direct electrochemical oxidation to nitrate at a platinium electrode 12 . However, the method is more
suitable for higher nitrite concentrations only. Contents
down to 100 μg dm –3 can be determined by coulometric
microdetermination via diazotization of 1-amino-4-naphthalenesulfonic acid 13 . He _et al_ 14 determined nitrite by coulometric backtitration under flow conditions. Nitrite was
reduced by hydrazine and the excess of hydrazine was
coulometrically titrated by electrogenerated bromine. The
detection limit was about 5 μg dm –3 .

Nitrites can be oxidised in porous electrodes as well
which enables their determination by in-electrode coulometric titration (IECT). The method is in fact constant
current chronopotentiometry in thin-layer arrangement
which makes use of a constant current for direct electrochemical conversion of analyte species in the small pores
inside the electrode bulk. The void volume of the electrode determines the volume of the sample being “titrated”
by current. Unlike coulometric titrations in classical coulometric cells, the electrode here serves as a generator as
well as the indicator electrode. Its potential follows the
changes of the analyte concentration at the electrode surface. If the pore diameters are near or less than the diffusion layer thickness, the analyte concentration is virtually
uniform in the pores. Hence, the potential is controlled by
the analyte bulk concentration. In such a case, the sudden
change of the potential in the instance when the analyte
concentration at the electrode surface drops to zero (corresponding to the chronopotentiometric transition time)
corresponds also to a virtually zero concentration in the
bulk – it is actually the titration end point.

The paper presents a simple analytical procedure for
nitrites by making use of their direct electrochemical oxidation in the RVC electrode.

## 2. Materials and Methods

2. 1. Instruments
Flow-through chronopotentiometric measurements
were carried out on a flow-through electrochemical analy

ser EcaFlow model GLP 150 (Istran, Ltd., Bratislava, Slovakia; internet address: www.istran.sk) equipped with two
solenoid inert valves, a peristaltic pump and a microprocessor controlled potentiostat/galvanostat. The measured values of potential were treated by a 16-bit 78 kHz
analog to digital converter. The block diagram and the
operation mode of the system were given earlier. 15 The
signals were recorded and evaluated by the memory-mapping technique. 16,17 In the resulting chronopotentiograms
the content of the cannels (counts) in the channel counter
were plotted against the potential values attributed to the
particular channels. Hence, the chronopotentiometric signals were not obtained and displayed as a wave-like dependence of potential _vs_ time, but as a peak in the relation
of counts _vs_ potential. Formally, the counts are expressed
in s/V units, but actually they are dimensionless numbers.
Obviously, the integrated value of a peak ( _A_ ) is a dimensionless number as well for it is a sum of the content of
channels between two potential values. The peak area is
directly proportional to the chronopotentiometric transition time (ι) of the corresponding electrode process. The
value of ι expressed in seconds can be obtained from the
peak area by dividing it with the sampling frequency _f_ of
the analog to digital converter of the instrument: ι = A/f.

The measurement consisted of two main steps: i) the
background signal was measured first by means of the
blank sample, ii) then, the sample or standard solution
was measured. The background signal was subtracted
from this signal yielding a true background corrected net
signal. The operation parameters are listed in Table 1 .

Table 1: Operation parameters of the flow-through analyser

Parameter Dimension Value
Aspirating potential mV 0
Quiescence potential mV 600
Quiescence time s 5
Terminal potential mV 1100
Standby potential mV 0
Oxidation current μA 5
Sample volume mL 4
Flow rate mL min –1 3

Compact flow-through electrochemical cells of type
104 with Pt auxiliary and built-in Ag/AgCl reference electrodes was employed. As working electrodes the macroporous E-104 electrode was used (all from Istran, Ltd., Bratislava, Slovakia). The electrode E-104 was fabricated from
a reticulated vitreous carbon plug of 100 ppi (pores per
inch) porosity (Electrosynthesis Co. Inc., Lancaster, New
York, USA) with 10 mm and 4 mm in diameter and length,
respectively (Fig. 1 ). Its effective volume and active surface area were 300 μL and 17 cm 2, respectively. The pores
were almost regular with diameters of about 500 μm.

For the determination of the electrode reaction stoichiometry the E-56 electrode was used (Istran, Ltd., Brati

154 _Acta Chim. Slov._ 2015, _62,_ 152–158

slava, Slovakia). It was made of a carbon cylinder (height
6 mm, inner and outer diameter 5 mm and 10 mm, resp.).
The cylinder was filled with crushed reticulated vitreous
carbon material and closed from both sides with porous
frits. The effective volume and active surface area of the
electrode were approximately 35 μL and 45 cm 2, respectively. The pores in the electrode were irregular with diameters of 5 to 20 μm.

Figure 1: RVC electrode

The potentials throughout this paper are expressed
against the Ag/AgCl reference electrode.

The accuracy of the results was checked by spectrophotometry on a Varian Cary 50 Bio UV-visible spectrophotometer.

2. 2. Reagents and Solutions

CRM for nitrite: 1.000 ± 0,002 g dm –3, CRM CZ
9075(1H), Czech Metrological Institute, Prague, Czech
Republic.

Carrier electrolyte for IECT: 0.1 mol dm –3 Na2SO4;
0.01 mol dm –3 CH3COONa; 0.01 mol dm –3 CH3COOH.
Reagent for sample adjustment in IECT: 1.0 mol
dm –3 Na2SO4, 0.05 mol dm –3 H2SO4

Colour reagent for nitrite 18 : 4 g of 4-aminobenzene1-sulfonamid is dissolved in a mixture of 10 mL 15 mol
dm –3 H3PO4 and 50 mL water. On dissolving 0.2 g of
dihydrochlorid N-(1-naftyl)-1,2-diaminoethane the volume of the solution is adjusted to 100 mL with water. At
2–5 °C the reagent is stable over a month.

2. 3. Procedures

_Sample treatment_ : The samples were stored at 5 C°.
Prior to analysis the samples were filtered and in the case
of high hydrocarbonate contents were boiled.

_IECT_ : The sulphuric acid and sodium sulphate solution was added to the sample to a final concentration of
0.1 mol dm –3 and 0.005 mol dm –3 in Na2SO4 and H2SO4,
respectively. Alkaline samples were neutralised with diluted sulphuric acid first.

_Photometry_ : To 40 mL sample 1 mL of the colour
reagent was added, the volume was adjusted to 50 mL
with water and the solution was stirred up. On 20 min the
absorbance was measured at 540 nm. The blank sample
was prepared and measured in the same way just water
was taken instead of the sample.

## 3. Results and Discussion

Porous flow-through electrodes exhibit several special features, the main important ones being the large electrode surface to volume ratio and the thin layer character.
The former property ensures high electrochemical conversions the latter facilitates direct coulometric titrations inside the electrode bulk.

The surface to volume ratio is decisive for applications where high recoveries of the electrolysis are requested, e.g. for metal removal in waste water treatment
19–21 or electrochemical pre-concentration for atomic
spectroscopy. 22 Obviously, the electrochemical properties such as electrochemical signal to background ratio
are of no significance here. On the contrary, by making
use of the thin-layer character of porous electrodes, e.g.
in in-electrode coulometric titrations (IECT), the signal
to background ratio is of high importance for it influences the detection limit of the method which is decisive
when trace concentrations are to be determined. The
signal to background ratio is inversely proportional to
the surface to volume ratio and this fact limits the geometrical arrangement of porous electrodes, especially
the pore size.

3. 1. Stoichiometry of the Electrode Reaction

The stoichiometry of the electrode reaction was determined by making use of the E-56 electrode which
enables a complete and fast electrolysis of the analyte in
the pores of the electrode. 23 The pore diameters (5–20 μm)
are within the diffusion layer thickness and the electrode
reaction is no more diffusion controlled. Hence, differences in diffusion constants do not affect the rate of electrolysis.

The chronopotentiometric transition time ι can be
expressed by means of the Faraday laws of electrolysis as
follows:

ι = z F c Veff /I (1)

where _z_ denotes the charge number of the electrode process, _F_ the Faraday constant in C mol –1, _c_ analyte concen

_Acta Chim. Slov._ 2015, _62,_ 152–158 155

tration in mol dm –3, _Veff_ the effective void volume of the
porous electrode in dm 3, and _I_ the applied current in A.

The effective void volume of the electrode can be
obtained by analysing solutions of known concentrations
of species and with known stoichiometry of the electrode
process. Potassium ferrocyanide in KCl solutions was
used to find out the electrode volume by making use of Eq
(1). The volume _Veff_ was found to be 36.1 ± 0.6 μL. By
analysing solutions with known nitrite concentrations and
taking this volume, Eq (1) resulted a charge number of
2.01 ±0.05 which implies the following equation for the
electrode reaction:

NO2– + H2O – 2 e– → NO3– + 2 H+ (2)

3. 2. Optimisation of Experimental
Parameters

As Eq (2) implies, the oxidation potential of nitrites
is pH dependent, by decreasing the pH value the potential
is shifted to more positive values and _vice versa_ . At pH values below 1 the nitrite ions decompose rapidly and the results are less reproducible. Moreover, in such solutions
the oxidation peak is shifted to potentials where the background signal may obscure the signal of nitrite, especially
in the presence of chloride ions which are oxidised to
chlorine.

In less acidic and neutral solutions the oxidation
peak is shifted to more negative values and is well separated from the background signal up to pH values of 8–9
(Fig. 2 ). In more alkaline solutions there is a competitive
electrode reaction, namely the oxidation of hydroxide
ions:

(3)
4 OH – - 4 e – → O2 + 2 H2O

Like the nitrite oxidation this reaction is pH dependent, the oxidation peak is shifted to more negative values
when increasing the pH value. Above pH 8 the background caused by this oxidation is enhancing considerably and the signal evaluation is no more reliable or even
possible (Fig. 2 )

Owing to the low currents used for the oxidation, the
ionic strength of the solution had a small influence on the
peak position and width only. Acidic solutions yielded
more reproducible signals than neutral and slightly alkaline ones, so the testing and sample solutions were adjusted
to contain 0.1 mol dm –3 and 0.005 mol dm –3 in Na2SO4
and H2SO4, respectively.

In porous electrodes with pore sizes significantly
larger than the diffusion layer thickness the response
(chronopotentiometric transition time) obeys more the
Sand equation than Eq (1), i.e. the concentration dependence is no more linear, especially at higher current
densities. To retain linearity, the current densities
should be as low as possible. However, very low current
densities may induce undesirable effects, namely i) a
significant portion of the applied current may be consumed by non-faradayic processes such as double layer
charging and electrode material reactions, ii) the duration of the electrolysis may become unacceptably long.
Hence, compromised conditions should be found at
which the linearity and reasonable measurement times
can be assured.

Currents below 3 μA prolonged the measurement up
to 10 min and more. Over 20 μA the measurement was
significantly faster but there was a considerable non-linearity of the response, especially at the lower concentration
range (below 10 μg dm –3 ). Oxidation current of 5 μA was
found to generate sufficiently linear response and measurement times of 3–10 min.

Figure 2: Chronopotentiograms of the blank and 1.0 mg dm –3 nitrite at pH 2.6 (curves A and B) and pH 9.1 (curves C and D), respectively.

156 _Acta Chim. Slov._ 2015, _62,_ 152–158

3. 3. Interferences

The influence of various species common in water
samples was investigated in concentration ranges which
are typical for underground, surface, drinking and mineral
waters (Table 2 ).

Hydrocarbonate over 400 mg dm –3 interferes owing
to changes of pH value and release of carbon dioxide
which forms bubbles in the solution and electrode. Such
samples should be boiled prior to the analysis to remove
hydrocarbonate. This step does not affect the nitrite content and the subsequent measurement.

The anionic surfactant does not influence the nitrite
signal. There is a slight increase of the signal at concentrations above 10 mg dm –3 of the cationic surfactant, at 20
mg dm –3 the signal is 10% higher than without the surfactant. The neutral surfactant decreases the signal, at 20 and
100 mg dm –3 there is a 4% and 20% decrease, respectively. Humic acids at concentrations above 20–30 mg dm –3

slightly increase the signal. Surprisingly, Fe(II) ions in

Table 2: Tested species for interference study

creased the signal at concentrations above 0.5 mg dm –3

only, i.e. they were actually not oxidised to Fe(III) under
the applied parameter setting and pH. The remaining species did not exert any influence on the results.

3. 4. Analytical Figures of Merit

The overall performance of the elaborated procedure was tested by analyses of model samples. The results
are summarised in Table 3 . The values of limit of detection (LOD) and limit of quantitation (LOQ) were calculated by making use of the upper limit approach. 24 The repeatability of the results _sr_ was determined by performing
10 analyses of the same solution in a fast sequence and is
expressed as the relative standard deviation of the results.
The reproducibility _sr,_ was determined by analysing fresh

solutions with the same nitrite content but during a 10
days period of time. The bias _e_ gives the absolute value of
the difference between the obtained results and the nominal or reference concentration relative to the reference va

Tested range, Interfering
Tested species Added as
mg dm –3 concentration a , mg dm –3

Fe 2+ FeSO4.7H2O 0.1–1 0.8 (+)
Ca 2+ CaSO4.2H2O 100–400 none
Mg 2+ MgSO4.7H2O 100–400 none
NH4+ (NH4)2SO4 20–200 none

NH4+ (NH4)2SO4 20–200 none

Cl – NaCl 20–2000 none
HCO3– NaHCO3 100–1000 400 (–)

HCO3– NaHCO3 100–1000 400 (–)

NO3– NaNO3 50–300 none

Anionic surfactant SDS 10–500 none
Neutral surfactant Tween 80 10–100 60 (–)
Cationic surfactant TBAB 8–20 10 (+)
Humic substance Sodium humate 5–50 30 (+)

SDS: sodium dodecylsulphate, Tween 80: polysorbate 80, TBAB: tetrabutyl ammonium bromide; Nitrite
concentration: 100 μg dm –3

a Concentration of the tested species causing a 10% increase (+) or decrease (–) of the signal

Table 3: Analytical figures of merit

Obtained at
Parameter Symbol Dimension Value
concentration, μg dm –3

Limit of detection LOD - μg dm –3 0.4
Limit of quantitation LOQ - μg dm –3 1.3
Repeatability sr 100 % 2.2
Reproducibility sr, 100 % 2.6

Bias e 100 % 0.3
Accuracy Re 100 % 100.3
Range - - μg dm –3 1.3–1000
Linearity R * - 1 0.9997
Measurement duration – min 2–3

- R: regression coefficient of the concentration dependence ι = 0.1530119 ρ, where ι is the signal (transition time) in seconds and ρ is the nitrite concentration in μg dm –3 . Regression data valid for the 0 – 20 μg
dm –3 concentration range.

_Acta Chim. Slov._ 2015, _62,_ 152–158 157

lue. The accuracy _Re_ refers to the ratio of the obtained value to the reference one (method recovery). The analytical
range refers to the concentration range limited from the
lower and upper end by the LOQ and the highest linear
response, respectively. Measure of linearity is the regression coefficient _R_ of the concentration dependence.

These data imply that all criteria of a sound analytical performance are fulfilled and the method has the capacity to deliver reliable results in a broad concentration of
nitrites.

The performance of the elaborated procedure was
tested with analyses of real samples.

3. 5. Real Samples

Various water samples were collected and analysed
by the elaborated method (Table 4 ). The accuracy was
checked by analysing spiked samples and by means of
spectrophotometric measurements.

The spike recoveries were found to be in the range
of 81–116%. Except for two samples a satisfactory match
was found between the electrochemical and photometric
methods as well. For the flavoured soft drink sample
(Table 4 ) the chronopotentiometric method delivered an
overestimated value, though the spike recovery was normal. The probable reason was the colour additive to the
sample which interfered through its electrochemical oxidation and enhanced the chronopotentiometric signal.

## 4. Conclusions

The determination of nitrites through their direct
electrochemical oxidation in porous electrodes (in-electrode coulometric titration) was found to be a simple and
fast way of analysis of water samples for nitrites. In most
cases, the sample requires the addition of a solution containing dilute sulphuric acid and sodium sulphate only
which is a significant advantage over the commonly used
photometric methods which make use of toxic organic
reagents. There are few interferents in samples commonly
analysed. The interfering effect of high carbonate content

Table 4: Nitrite in underground, drinking and surface water samples

can be eliminated though boiling the sample prior to
analysis. The porous electrode is cheap and its lifetime is
over several hundreds of analyses which make the running
costs extremely low. Owing to the satisfactory performance, simplicity, speed and low running costs is the method
suitable for routine use and in applications in water monitoring devices.

## 5. Acknowledgements

This publication was supported by the Competence
Center for SMART Technologies for Electronics and Informatics Systems and Services, ITMS 26240220072, and
by the Slovak Grant Agency VEGA (project No.
1/0419/12). F. C. and E. B. appreciate the financial support of the grant agency APVV (project No. APVV-079711).

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Sample IECT Photometry Spike Accuracy Range of found
μg dm –3 μg dm –3 recovery % % concentrations μg dm –3

1 21.6 ± 0.9 21.7 ± 0.1 112 99 3.0–21.6
2 0.50 ± 0.05 0.52 ± 0.04 85 96 0.4–1.4
3 1.2 ± 0.2 1.37 ± 0.04 109 88 1.2–3.3
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5 46.1 ± 2.9 46.3 ± 0.5 105 98 46–146
6 131 ± 1 130.5 ± 1.0 105 101 17–131

1: underground water; 2: drinking water; 3: natural mineral water carbonated; 4: soft drink orange flavoured, slightly carbonated; 5: creek and river
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Nitrit smo dolo~ali v vodnih vzorcih z znotraj-elektrodno kulometri~no titracijo v elektrodi iz mre`astega steklastega
ogljika (RVC) s poroznostjo 100 ppi. Vzorcu smo dodali razred~eno `veplovo kislino in natrijev sulfat ter ga posesali v
preto~no celico s porozno elektrodo. Nitrit se je oksidiral do nitrata pri konstantnem toku 5 μA, pri ~emer smo spremljali potencial elektrode. Meja zaznave je bila 0,4 μg/L, meja dolo~itve pa 1,2 μg/L. Ponovljivost je bila 2,2%, obnovljivost
pa 2,6%. Odstopanje pri 100 μg/L je bilo 0,3%. Meritev je trajala 2–3 min, odvisno od koncentracije nitrita. Ugotovili
smo le nekaj interferentov: nevtralni surfaktanti so rahlo zni`ali signal, kationski pa so ga rahlo zvi{ali. Huminske kisline nad 30 mg/L so zvi{ale signal za 10%. Analizirali smo vzorce pitne in povr{inske vode, rezultati so se dobro ujemali z rezultati fotometri~ne metode.

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