Determination of heavy metals in milk by potentiometric

stripping analysis using a home-made flow cell

Emma Mu~

noz

*

, Susana Palmero

Departamento de Quımica (

Area de Quımica Analıtica), Facultad de Ciencias, Universidad de Burgos, P/ Misael Ba~

nuelos s/n, 09001 Burgos, Spain

Received 18 June 2003; received in revised form 15 October 2003; accepted 16 October 2003

Abstract

A method is described for sensitive and reliable determination of trace concentrations of cadmium, lead and copper in real

samples by stripping potentiometry with a home-made flow cell. The selection of the experimental conditions was made using
experimental design methodology. The optimum conditions for the method include an acetic acid–acetate buffer mixture (pH 3.4) as
supporting electrolyte, an electrolysis potential of

)1.1 V and a flow rate of 3 ml min

1

. The detection limits were 6.51

· 10

10

mol l

1

for cadmium, 4.60

· 10

10

mol l

1

for lead and 8.59

· 10

10

mol l

1

for copper, with an electrolysis time of 900 s. The relative standard

deviations at this concentration level were 0.038, 0.086 and 0.095, respectively. In order to check the analytical accuracy, standard
solutions have been used and recoveries close to 100% were obtained. The method was successfully applied to the determination of
cadmium, lead and copper in powdered milk, using the method of standard additions.
 2003 Elsevier Ltd. All rights reserved.

Keywords: Potentiometric stripping analysis; Flow-cell; Milk

1. Introduction

Although heavy metals have industrial uses, their

potential toxicity for man and animals is the object of
several studies. For some elements the effects are accu-
mulative and it is necessary to control his level in con-
sumed food. Atomic absorption spectroscopy has
mostly been used for carrying out these determinations
(Bag et al., 1999; Conti, 1997).

Interest in flow systems related to electroanalysis has

increased considerably during the last years. Estela,
Tomas, Caldera, and Cerda (1995) have shown that flow
potentiometric stripping analysis in continuous mode is
a very useful technique for the separation of elements
that are electroactive in potential regions close to each
other. With the growing concern regarding heavy metal
contamination in food samples, the importance of this
method has increased considerably in the last years, not
only for quality control but also for purposes of envi-
ronmental monitoring. The determination of trace ele-
ments using the potentiometric stripping technique has

been described and reviewed in numerous works (Chow,
Davey, & Mulcahy, 1995; Estela et al., 1995; Frenzel &
Br€

atter, 1986; Huiliang, Jagner, & Renman, 1988; Lo

Coco, Ceccon, Ciraolo, & Novelli, 2003; Luque de
Castro & Izquierdo, 1991; Ostapczuk, 1993; Riso, Le
Corre, & Chaumery, 1997). The technique has been
successfully applied to trace element analysis in natural
waters (Adeloju, Sahara, & Jagner, 1996), in wastewater
and in biological samples (Adeloju et al., 1996; Estela
et al., 1995; Jagner, Renman, & Wang, 1994; Luque de
Castro & Izquierdo, 1991; Ostapczuk, 1993). With vol-
tammetric techniques, and in particular with pulsed
voltammetric techniques, the dissolved reversible cou-
ples and high concentrations of inorganic salts produce
interfering signals. This is not the case with potentio-
metric stripping analysis (Jagner & Westerlund, 1980;
Kryger, 1980).

Potentiometric stripping analysis (PSA) is a two-step

technique consisting of an electrolysis step and a strip-
ping step. The electrolysis step, commonly performed
using a mercury film-coated glassy carbon electrode, is a
pre-concentration step in which metal ions are reduced
to free metal and electrodeposited as amalgam on the
working electrode. The measurements are made in
the stripping step during which the metal is reoxidized.
The reoxidation can be a chemical or electrochemical

*

Corresponding author. Tel.: [phone]; fax: +34-947-

258831.

E-mail address:

[e-mail]

(E. Mu~

noz).

0956-7135/$ - see front matter

 2003 Elsevier Ltd. All rights reserved.

doi:10.1016/j.foodcont.2003.10.006

Food Control 15 (2004) 635–641

www.elsevier.com/locate/foodcont

process. During the stripping phase in the chemical
oxidation, the electric circuit is interrupted and a
chemical oxidant, such as dissolved oxygen or mercury
ions, establishes the reoxidation. In electrochemical
oxidation, an anodic current is imposed on the electrode
to reoxidate the electrodeposited metals. The change in
the electrode potential (E) over time (t) during the
reoxidation process is monitored. A stripping potentio-
gram, E vs. t, is obtained.

An objective of this work was to use experimental

design methodology to optimize the experimental con-
ditions required for the most ideal determination of
cadmium, lead and copper using the minimum number
of trials. Experimental designs are more efficient than
the ‘‘one-a-time’’ experiments since they allow one to
detect interactions between factors that could lead to
false conclusions. Several authors have successfully
employed experimental designs in the optimization of
experimental variables in electroanalytical (San Martın,
Sanllorente, & Palmero, 1998) and spectroscopic tech-
niques (Saurina & Hern

andez-Cassou, 1995).

During analysis, the working electrode must be cov-

ered with a mercury film, which is thick enough to dis-
solve all the mercury soluble metals that are reduced
during plating. These processes were carried out in one
step where the working electrode is pre-coated with
mercury film and the metals are simultaneously electro-
deposited. Since transition time, s, is a measure of the
metal concentration, this parameter was chosen as the
response variable. Central composite designs with rep-
lications at the central point have been used in order to
analyze the response surface obtained. The method has
been tested by using standard solutions (Merck). In this
work we have chosen powdered milk as a real sample to
calculate heavy metal concentrations.

2. Experimental

2.1. Flow system

Electrochemical experiments were run using a wall-jet

cell laboratory-made (Palmero, Mu~

noz, & Garcıa-

Garcıa, 2000), a Miniplus 3 peristaltic pump (Gilson
S.A., France) equipped with polyvinyl chloride tubing
of appropriate diameter (0.2–4 mm) and a computer-
controlled potentiostat (PSTAT 10, Eco Chemie/Auto-
lab, Utrecht, The Netherlands).

A diagram of the transparent flow cell laboratory-

made is shown in Fig. 1. This cell has an epoxy resin
(‘‘Epofix’’) body (0.7 cm diameter, 4.5 cm length)
defining a 700 ll enclosed volume and a ‘‘wall-jet’’
configuration with a three-electrode system. The glassy
carbon disk working electrode is a commercial electrode
(3 mm diameter); the reference electrode of Ag/AgCl
and the counter electrode were made in the laboratory

(Mu~

noz, Palmero, & Garcıa-Garcıa, 2002). As can be

seen in Fig. 1, electrodes can be removed from the cell
for cleaning without stopping the flow.

The experimental parameters were optimized using

the experimental design methodology (Montgomery,
1991). Analysis of the data was done with Statgraphics



Plus for Windows 4.0 (Statistical Graphics Corp.,
Rockville, MD 20852-4999, USA) on a personal com-
puter equipped with a Pentium microprocessor.

2.2. Reagents

High purity nitrogen was bubbled through the solu-

tions for 20 min before each experiment in order to re-
move dissolved oxygen and then kept under a nitrogen
atmosphere.

All the solutions were prepared used high purity de-

ionized water (Milli Q, Millipore instrument). The metal
solutions to find the optimum conditions were prepared
using their nitrates. The used reagents are ‘‘Suprapur’’
and P.A. Merck quality.

The mercury film was formed under continuous flow

conditions together with the metals, from a solution
containing 1

· 10

4

mol l

1

mercury ions in an acetic–

acetate buffer at pH 3.4.

3. Results and discussion

3.1. Optimization of experimental variables

Prior to each analysis, the used mercury film was

wiped off with a tissue and the electrode cleaned with
deionized water. The study was conducted in solutions
containing 1

· 10

7

mol l

1

of cadmium, lead and copper

ions, and 1

· 10

4

mol l

1

of mercury ions as oxidising

Fig. 1. Flow cell: (1) cell block; (2) sample inlet; (3) glassy car-
bon working electrode; (4) reference electrode; (5) counter electrode;
(6) sample outlets.

636

E. Mu~

noz, S. Palmero / Food Control 15 (2004) 635–641

agent. The pH is adjusted to 3.4 with an acetic acid/
acetate buffer that does not present complexation
properties for the ions studied or for the oxidising agent.
After deaeration, an electrodeposition time and a
stripping time are performed at the appropriate poten-
tial and the peaks for cadmium, lead and copper in the
sample recorded. Electrodeposition and the chemical
stripping steps took place at the same flow rate.

In order to optimize the experimental variables:

electrodeposition potential, E

d

, electrodeposition time,

t

d

, and flow rate, v

flow

, we used the experimental design

methodology to find the best conditions with the mini-
mum trial.

Firstly, a 2

3

central composite design for each metal

was applied. This design has replications in the central
point in order to estimate the residual error. Two levels,
high and low, for each factors to be optimized were
chosen to carry out the experimental designs. Next,
experiments were done with all possible combinations.
The response to be optimized was the transition time, s,
obtained of the stripping potentiogram. The values that
correspond to the high (+) and low (

)) levels and to the

central point (0) for each factor are the following:

t

d

ðþÞ ¼ 300 s E

d

ðþÞ ¼
0:9 V

v

flow

ðþÞ ¼ 12:0 ml min

1

t

d

ð0Þ ¼ 200 s

E

d

ð0Þ ¼
1:1 V

v

flow

ð0Þ ¼ 7:5 ml min

1

t

d

ð
Þ ¼ 100 s E

d

ð
Þ ¼
1:3 V

v

flow

ð
Þ ¼ 3:0 ml min

1

From analysis of the variance (ANOVA) in Table 1,

it can be deduced the effects which have P -values less
than 0.05, indicating that they are significantly different
from 0 at the 95.0% confidence level. In order to com-
pare the results for the three metals, Fig. 2 shows the
mean effects for each factor and each metal.

The electrodeposition potential can influence signifi-

cantly the response of the cadmium but E

d

and none of

the interactions in which it is involved are significant
factors in lead and copper determination. The negative
value was limited by the undesirable zinc electrodepos-
ition which can be present in the sample and form
intermetallic compounds with copper. So, the chosen
electrodeposition potential was

)1.1 V.

The flow rate and electrodeposition time can influ-

ence significantly the response of the three metals.
Observation of the Fig. 2 indicated that the signals were
negatively affected by the increase of flow rate, and
positively affected by the increase of electrodeposition
time. So, the best response for the three metals should be
obtained for low flow rate and long electrodeposition
time.

The next step was to optimize electrodeposition time

and flow rate by fixing the potential at

)1.1 V. In this

case, a 2

2

central composite design was chosen and the

corresponding high and low levels and the central point
for each factor are the following:

t

d

ðþÞ ¼ 700 s

v

flow

ðþÞ ¼ 5:0 ml min

1

t

d

ð0Þ ¼ 450 s

v

flow

ð0Þ ¼ 3:0 ml min

1

t

d

ð
Þ ¼ 200 s

v

flow

ð
Þ ¼ 1:0 ml min

1

Variance analysis (Table 2) shows that the flow rate only
was influence in the determination of cadmium, so it was
fixed again according to the cadmium results. The re-
sponse for cadmium shows a maximum response at
3 ml min

1

(Fig. 3), which was taken as the optimum

flow rate. The electrodeposition time has P -values less
than 0.05 with a clear tendency to larger values for the
three metals.

The relation between the transition time, s, and the

electrodeposition time, t

d

, was studied in a solution

containing 1

· 10

7

mol l

1

of cadmium, lead and copper

ions, and 1

· 10

4

mol l

1

of mercury ions in acetic acid/

acetate buffer (pH 3.4). The electrodeposition time was
varied from 100 to 1800 s, the electrodeposition poten-
tial was

)1.1 V and v

flow

was 3 ml min

1

. The correlation

coefficient (r

2

) for s vs. t

d

dependence was 0.990 and the

slope 3

· 10

4

for cadmium, 0.991 and 6

· 10

4

for lead,

and 0.992 and 8

· 10

4

for copper. The linear was

influenced by the chosen electrolysis time and the lowest
detectable

amount of

the

metals

decreases

with

increasing electrolysis time. So, finally, we have chosen
the following conditions for calculating the detection
limits:

E

d

¼
1:1 V;

v

flow

¼ 3 ml min

1

and

t

d

¼ 900 s

3.2. Detection limits

The determination of detection limits is one the most

significant aspects of any trace analytical method
(Massart, Vandegiste, Deming, Michotte, & Kaufman,
1988). Once the optimum parameters for the analysis
were chosen, the next stage was to estimate the detection
limit of the three elements. A solution containing the
acetic acid/acetate buffer and 1

· 10

4

mol l

1

of mercury

ions was pre-electrolised at

)1.1 V and a flow rate of

3 ml min

1

. Stripping was performed in the same solu-

tion and the flow rate was kept constant at 3 ml min

1

.

The concentration of the three metals was then deter-
mined by the standard additions method.

The detection limits can be estimated from linear

calibration plots obtained in the range of 2.15

· 10

10

–

4.20

· 10

9

M of cadmium, lead and copper ions with an

electrodeposition time of 900 s. The equations of the
calibration curves are s

¼ 3  10

7

c

þ 0:0213 for cad-

mium, s

¼ 5  10

7

c

þ 0:0494 for lead and s ¼ 9  10

7

c

þ

0:0036 for copper. The determination coefficients of

E. Mu~

noz, S. Palmero / Food Control 15 (2004) 635–641

637

cadmium, lead and copper are 0.9950 (n

¼ 9), 0.9956

(n

¼ 9) and 0.9933 (n ¼ 9), respectively.

The detection plots constructed by the Clayton

method using the DETARCHI calculation program
(Ortiz & Sarabia, 1994) have been used. The detection
limits for cadmium, lead and copper, after four deter-
minations and 900 s of pre-electrolysis was found to be
6.51

· 10

10

,

4.60

· 10

10

and

8.59

· 10

10

mol l

1

,

respectively. The relative standard deviations at this
concentration level are 0.038 for cadmium, 0.086 for
lead and 0.095 for copper.

3.3. System testing

In order to evaluate the analytical performance of the

method, model samples were analyzed using the recov-
ery determination. The samples were acidified with
acetic acid/acetate buffer (pH 3.4) and then were spiked
with 9.00

· 10

10

M of cadmium, lead and copper ions

(standard solutions Merck). Five replicates for each
sample were analyzed by PSA. By the standard addition
method and an electrodeposition time of 900 s, the
concentrations of cadmium, lead and copper found were

Table 1
Analysis of variance for 2

3

star composite design to optimize flow rate, v

flow

, electrodeposition potential, E

d

, and electrodeposition time, t

d

, in

cadmium (Panel A), lead (Panel B) and copper (Panel C) determination by PSA

Source

Sum of squares

Degree freedom

Mean square

F

-ratio

P

-value

Panel A

a ;b

A: v

flow

14943.9

· 10

7

1

14943.9

· 10

7

172.56

0.0057

B: E

d

214385

· 10

7

1

214385

· 10

7

2475.51

0.0004

C: t

d

122092

· 10

7

1

122092

· 10

7

1409.80

0.0007

AA

1742.62

· 10

7

1

1742.62

· 10

7

20.12

0.0463

AB

2.50600

· 10

7

1

2.50600

· 10

7

0.03

0.8806

AC

579.744

· 10

7

1

579.744

· 10

7

6.69

0.1225

BB

31035.6

· 10

7

1

31035.6

· 10

7

358.37

0.0028

BC

281190

· 10

7

1

281190

· 10

7

324.69

0.0031

CC

7617.25

· 10

7

1

7617.25

· 10

7

87.96

0.0112

Lack of fit

42850.6

· 10

7

5

857.012

· 10

7

98.96

0.0100

Pure error

173.205

· 10

7

2

86.6023

· 10

7

Total (corr.)

467524

· 10

7

16

Panel B

c ;d

A: v

flow

8357.9

· 10

5

1

8357.9

· 10

5

276.28

0.0036

B: E

d

7.7013

· 10

5

1

7.7013

· 10

5

0.25

0.6640

C: t

d

14143

· 10

5

1

14143

· 10

5

467.50

0.0021

AA

1743.0

· 10

5

1

1743.0

· 10

5

57.63

0.0169

AB

245.00

· 10

5

1

245.00

· 10

5

8.12

0.1042

AC

93.310

· 10

5

1

93.310

· 10

5

3.08

0.2211

BB

498.35

· 10

5

1

498.35

· 10

5

16.47

0.0557

BC

1.8600

· 10

5

1

1.8600

· 10

5

0.06

0.8273

CC

158.74

· 10

5

1

158.74

· 10

5

5.25

0.1491

Lack of fit

2089.5

· 10

5

5

417.91

· 10

5

13.81

0.0689

Pure error

60.500

· 10

5

2

30.252

· 10

5

Total (corr.)

28394

· 10

5

16

Panel C

e; f

A: v

flow

368.23

· 10

4

1

368.23

· 10

4

96.82

0.0102

B: E

d

65.929

· 10

4

1

65.929

· 10

4

17.33

0.0531

C: t

d

2099.8

· 10

4

1

2099.8

· 10

4

552.11

0.0018

AA

2.3842

· 10

4

1

2.3842

· 10

4

0.63

0.5115

AB

33.456

· 10

4

1

33.456

· 10

4

8.80

0.0974

AC

108.78

· 10

4

1

108.78

· 10

4

28.60

0.0332

BB

47.171

· 10

4

1

47.171

· 10

4

12.40

0.0720

BC

3.7812

· 10

4

1

3.7812

· 10

4

0.99

0.4238

CC

66.490

· 10

4

1

66.490

· 10

4

17.48

0.0527

Lack of fit

217.65

· 10

4

5

43.531

· 10

4

11.45

0.0823

Pure error

7.6066

· 10

4

2

3.8033

· 10

4

Total (corr.)

3069.9

· 10

4

16

The solution contained cadmium, lead and copper ions 1

· 10

7

M, mercury ions 1

· 10

4

M and acetic acid/acetate buffer (pH

¼ 3.4).

a

R

2

¼ 90:7975%.

b

R

2

(adjusted for d.f.)

¼ 78.9657%.

c

R

2

¼ 92:4278%.

d

R

2

(adjusted for d.f.)

¼ 82.6921%.

e

R

2

¼ 92:6624%.

f

R

2

(adjusted for d.f.)

¼ 83.2285%.

638

E. Mu~

noz, S. Palmero / Food Control 15 (2004) 635–641

9.14

· 10

10

M (with RSD of 0.017), 9.13

· 10

10

M (with

RSD of 0.014) and 8.89

· 10

10

M (with RSD of 0.046),

respectively. The mean analytical recovery was calcu-

lated as the ratio, expressed as percentage, between the
metal concentrations found to the concentration added
to the standard solution. The recoveries found were

v

flow

(mL min

-1

)

1

5

t

d

(s)

200

700

0.15

0.35

0.55

0.75

0.95

τ (s)

Fig. 3. Tendency of each experimental variable (flow rate, v

flow

; elec-

trodeposition time, t

d

) in the determination of cadmium

, lead

and copper

by PSA, according to the 2

2

star composite

design with E

d

¼
1:1 V. The solution contained cadmium, lead and

copper ions 1

· 10

7

M, mercury ions 1

· 10

4

M and acetic acid/ace-

tate buffer (pH

¼ 3.4).

v

flow

(mL min

-1

)

E

d

(V)

t

d

(s)

0

0.1

0.2

0.3

0.4

0.5

3 12 -1.3 -0.9 100 300

τ (s)

Fig. 2. Tendency of each experimental variable (flow rate, v

flow

; elec-

trodeposition potential, E

d

; electrodeposition time, t

d

) in the determi-

nation of cadmium

, lead

, and copper

by PSA,

according to the 2

3

star composite design. The solution contained

cadmium, lead and copper ions 1

· 10

7

M, mercury ions 1

· 10

4

M

and acetic acid/acetate buffer (pH

¼ 3.4).

Table 2
Analysis of variance for 2

2

star composite design to optimize flow rate, v

flow

, and electrodeposition time, t

d

, in cadmium (Panel A), lead (Panel B) and

copper (Panel C) determination by PSA with E

d

¼
1:1 V

Source

Sum of squares

Degree freedom

Mean square

F

-ratio

P

-value

Panel A

a; b

A: v

flow

2.5001

· 10

5

1

2.5001

· 10

5

0.24

0.6710

B: t

d

9845.7

· 10

5

1

9845.7

· 10

5

955.90

0.0010

AA

518.87

· 10

5

1

518.87

· 10

5

50.38

0.0193

AB

90.000

· 10

5

1

90.000

· 10

5

8.74

0.0979

BB

35.578

· 10

5

1

35.578

· 10

5

3.45

0.2042

Lack of fit

353.52

· 10

5

3

117.84

· 10

5

11.44

0.0814

Pure error

20.000

· 10

5

2

10.300

· 10

5

Total (corr.)

11006

· 10

5

10

Panel B

c; d

A: v

flow

8.5111

· 10

4

1

8.5111

· 10

4

0.23

0.6804

B: t

d

4870.3

· 10

4

1

4870.3

· 10

4

130.21

0.0076

AA

23.973

· 10

4

1

23.973

· 10

4

0.64

0.5074

AB

82.174

· 10

4

1

82.174

· 10

4

2.20

0.2765

BB

3.1518

· 10

4

1

3.1518

· 10

4

0.08

0.7989

Lack of fit

80.580

· 10

4

3

26.860

· 10

4

0.72

0.6266

Pure error

74.806

· 10

4

2

37.403

· 10

4

Total (corr.)

5151.7

· 10

4

10

Panel C

e ;f

A: v

flow

34.640

· 10

3

1

34.640

· 10

3

18.37

0.0504

B: t

d

380.70

· 10

3

1

380.70

· 10

3

201.89

0.0049

AA

6.4093

· 10

3

1

6.4093

· 10

3

3.40

0.2066

AB

1.5800

· 10

3

1

1.5800

· 10

3

0.84

0.4567

BB

1.5076

· 10

3

1

1.5076

· 10

3

0.80

0.4656

Lack of fit

27.511

· 10

3

3

9.1704

· 10

3

4.86

0.1753

Pure error

3.7723

· 10

3

2

1.8862

· 10

3

Total (corr.)

45495

· 10

3

10

The solution contained cadmium, lead and copper ions 1

· 10

7

M, mercury ions 1

· 10

4

M and acetic acid/acetate buffer (pH

¼ 3.4).

a

R

2

¼ 96:601%.

b

R

2

(adjusted for d.f.)

¼ 93.202%.

c

R

2

¼ 96:9838%.

d

R

2

(adjusted for d.f.)

¼ 93.9676%.

e

R

2

¼ 93:1239%.

f

R

2

(adjusted for d.f.)

¼ 86.2477%.

E. Mu~

noz, S. Palmero / Food Control 15 (2004) 635–641

639

101% for cadmium, 101% for lead and 99% for copper.
As show the results, a quantitative recovery of added
was obtained which proves the suitability of the method
in the simultaneous determination of cadmium, lead and
copper at current levels in food samples.

3.4. Determination of heavy metals in milk

Ten grams of powdered skimmed milk were dried in a

muffle furnace at the temperature of 100

C for an hour.

Then, the temperature was gradually increased and
maintained constant at 450

C for 4 h. The crucible was

removed and cooled in a diseccator. The dried sample
was weighed in the same crucible, and then dissolved
in the buffer (pH 3.4) at 20

C.

Fig. 4 shows the stripping potentiograms recorded in

the original sample by adding of increasing concentra-
tion of cadmium, lead and copper ions, with the acetic
acid/acetate buffer (pH 3.4) and mercury ions 1

· 10

4

mol l

1

. The electrodeposition time was fixed at 300 s

because metal concentrations are large enough to
determine the signal at this time. A standard additions
calibration method (with eight-point standard addition)
was used to determine cadmium, lead and copper con-
centrations directly in the milk sample. After four
determinations, we obtained the following results, which
are expressed as ppb (lg of metal for each kg of sampled
milk): 24.73 ppb of cadmium, 27.35 ppb of lead and
20.14 ppb of copper. The relative standard deviations at

this concentration level were 0.020 for cadmium, 0.033
for lead and 0.020 for copper.

4. Conclusions

The proposed method for the determination of cad-

mium, lead and copper in real samples is accurate,
simple and permits one to reach very low detection
limits. The procedure does not require a complicated
sample pre-treatment, thus keeping the contamination
risk to a minimum. The method may also be extended to
other food samples and include other metals.

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-0.2

0.0

0.2

0.0

0.5

1.0

1.5

2.0

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1:1 V, v

flow

¼ 3 ml min

1

and t

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640

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641