1
VOLTAMMETRIC MONITORING OF BIVALENT IRON IN WATERS
AND EFFLUENTS, USING A DENTAL AMALGAM SENSOR
ELECTRODE. SOME PRELIMINARY RESULTS.
Øyvind Mikkelsen and Knut H. Schrøder
oyvind.mikkelsen@chem.ntnu.no, knut.schroder@chem.ntnu.no
Norwegian University of Science and technology
Department of Chemistry
N - 7491 TRONDHEIM, NORWAY
ABSTRACT
A very sensitive method for detection of iron (II) in the ng/L rage with a deposition
time of 180 s by use of differential pulse anodic stripping voltammetry on the novel
dental amalgam electrode is introduced and tested in different supporting media. A
well-defined peak for the oxidation of iron metal to ferrous ion was observed around
– 750 mV in citrate, oxalate and nitrate solutions. Good linearity has been found (r 2avg
= 0,998) for different concentration ranges. The deposition had to be performed at a
potential lower than – 1050 mV. At deposition potentials more positive than this, the
iron peak was absent in the stripping step, making it possible to measure iron
independent of lead and cadmium by use of subtractive stripping. Measurements of
real samples from river and seawater without any significant change in the pH are
demonstrated. The use of this stable and non-toxic electrode together with the
unique sensitivity makes the reported system suitable for implementation in
continuous monitoring systems for online detection of iron in rivers and in seawater.
KEYWORDS: Voltammetry, Dental Amalgam, Electrode, Iron, Seawater, Heavy metals.
2
1. INTRODUCTION
The importance of monitoring the content of heavy metals in waters and effluents is
obvious and several analytical methods are available [1-5]. Being in situ methods,
voltammetric and potentiometric stripping methods are suitable [6-8], however the
toxicity of mercury as electrode material makes limitations in particular for use in the
field.
About 4,7 percent of the earth's crust is consisting of iron, that makes it to one of the
most common metals. In water iron forms numerous complexes and plays an
essential role for the biological activity [9]. Trivalent iron has a very low solubility and
for that reason the concentration is very low except when extremely stable
complexes are formed, like in seawater where 99% of the iron is found in organic
complex ligands, mainly as oxy-hydroxides and colloidal matter [10].
Continuous monitoring of the content of iron in waters is of great interest both from a
chemical and from a biological point of view. Of particular interest is determination of
iron (II) in seawater. Iron (II) is assumed to be present as a transient state in oxygen
rich surface water through photo-reduction of iron (III) in organic complexes [11]. Also
deposits from air are possible explanations for the presences of iron (II). Increased
amounts of iron (II) may cause an increase in the bioavailable iron amount, because
colloidal or particulate iron uptake cannot occur without thermochemical or
photochemical dissolution [12,13].
3
At a mercury drop electrode it is possible to study the reduction of complexed ferric
ion to ferrous ion, by use of tartrate or citrate media. A voltammetric signal for the
ferric-ferrous complex couple will appear at a potential near the point where the
mercury electrode starts to collapse due to oxidation. However, a reversible welldefined signal may be obtained at pH values less then 7.
More complicated are measurements of the reduction of ferrous ion to the metal. At
the mercury electrode this reaction occurs at a potential between – 1.3 to – 1.6 V vs.
SCE, depending on the used complexing media and pH. These values are about 0.7
V more negative than the standard half-wave potential for the ferrous-ion to iron
metal reduction. This indicates that there is a large overvoltage involved in this
reaction, a phenomenon that is known for metals that are insoluble or have a low
solubility in mercury [14]. Because of this, the ferrous-ion to iron metal reduction
coincides with the hydrogen evolution reaction and results in difficulties and low
sensitivity for detection of the ferrous ion. In pyrophosphate buffer, iron (II) forms a
complex and a peak potential can be observed at – 500 mV with a detection limit of
about 0.5 g/L [15]. However, this analysis is restricted to a solution with pH 9.
More common on the mercury drop electrode is therefore to measure iron by use of
differential pulse adsorptive cathodic stripping voltammetry as suggested by van den
Berg et. al. [16], which have reported a detection limit of 0,12 nM with 60 s adsorption
time.
We have introduced a non-toxic electrode material similar to dental amalgam [17-19]
for use in voltammetry, especially for use in field. In analyses for this purpose, it is
4
preferred not to use mercury film electrodes or any other mercury surface-treated
electrodes [20], but solid electrode materials that are stable over a long time period.
Recently we have also reported that silver and other noble metals alloyed with a few
percent of mercury, bismuth or other metals with high overvoltage towards hydrogen,
act in a similar way [21,22]. Such solid electrodes have a high overvoltage towards
hydrogen and are found to give stable results over weeks and even longer, thus
being useful for online and remote monitoring of waters, effluents [23], and food and
beverages [24].
In the present work the deposition and the subsequent anodic stripping of ferrous ion
from the dental amalgam electrode have been studied. From the preliminary results it
is found that the deposition occurs at a potential far more positive than at the mercury
drop electrode. This can be exploited analytically, and it is found that there is a
unique sensitivity for iron on the dental amalgam electrode, and that iron can be
measured together with zinc, lead and copper. Interestingly, it has been found that
the iron must be deposited at a potential more negative than –1050 mV, otherwise no
signal appears.
Iron (II) concentrations in the ng/L range were detected in real seawater samples.
Also freshwater and tap water samples have been analyzed with only minor sample
pretreatment by use of anodic stripping voltammetry. With further optimization and
enhanced predeposition time it is reasonable to assume a detection limit for iron (II)
of down to 50 ng/L or lower.
2. EXPERIMENTAL
5
All the voltammetric scans were performed as differential pulse anodic stripping
voltammetry (DPASV). When real samples where used, the analyses were performed
directly in the sample with the addition of given salts only. The sample (60 ml) was
purged with nitrogen for about 5 min, then after a deposition time at a given potential a
scan was performed. The indicator electrode was made of dental amalgam (6 mm in
diameter) and prepared as described elsewhere [18,19]. A platinum wire was used as
the counter electrode, and the reference electrode was an Ag/AgCl/KCl (sat') electrode.
Analyses of iron (II) in different electrolytes and two different Norwegian real samples
and tap water were performed. The real samples were a) seawater from Trondheim
harbour, b) lake freshwater from Lillesand south in Norway. The seawater was analysed
within an hour after it was collected, and the freshwater within two days after sampling.
The seawater was analysed by two different voltammetric procedures. In the first
procedure, ammonium oxalate was added to seawater, and in the second tri-sodium
citrate was added.
The freshwater sample was analysed by adding ammonium oxalate (0,2 g to 75 mL
sample). The freshwater sample was sent to an external laboratory (Norwegian
Institute of Nature Research, NINA, Trondheim, Norway) for complimentary analyses
by ICP-MS. The used apparatus was a Thermo Finnigan Element (double
focusing). Further instrumental details may be given if inquired.
Standard solutions were prepared by dissolving iron (II)-sulphate in water and dilute
to solutions of 200 mg/L. The water was rinsed by Millipore Elix and then with Millipore
6
Milli-Q Gradient system. All reagents were of analytical grade only. The voltammetric
analyses were performed in 60 ml test cells. The solutions were purged with nitrogen,
supplied from Norsk Hydro (5.0), under stirred conditions for about 5 minutes before
each scan and iron standard was added for quantification. The electrodes were
polished to a shiny surface, with ¼ mm diamond paste on Struers polishing
equipment. The electrodes were stored in distilled water when not in use.
All the voltammetric analyses were performed by commercial available equipment
constructed by Oceanor AS, Trondheim, Norway in collaboration with the present
authors. This voltammetric apparatus is in particular applicable for implementation in
industrial process and aquatic water systems for online monitoring.
3. RESULTS
Both real samples and samples prepared in the laboratory were analyzed. Initially
different electrolyte solutions were tested out. From these results it was found that
oxalate and citrate were the most promising ones. Also sodium nitrate was found to
be usable, however the iron peak was wider and not so well defined as in citrate and
oxalate solutions. In ammonium buffer (pH 8,6) the iron peak was totally absent, even
in a 2 mg/L iron (II) solution the peak was not observed. This can be explained by
complexion with ammonium with a shift towards a more negative value.
By using DPASV, Fig. 1 shows the successively addition of iron (II) in oxalate and
citrate media. Fig. 1 a) and b) show the detection of iron (II) in oxalate (0.01 M) media
in a high (166,7 to 500 g/L) and a low (16,7 to 50 g/L) concentration range. In both
7
cases linear relationships were found. Fig. 1 c) shows detection of iron (II) in the
concentration range 1.67 to 50 g/L in citrate solution. The difference between 20
and 60 s pre-deposition time was also tested out for a specific iron (II) concentration,
and the result given in Fig. 1 d shows that an iron deposit is formed.
Fig 1 a) and b) Detection of iron (II) with anodic differential pulse stripping voltammetry (DPASV) in
ammonium oxalate (0.01 M) solution. Freshly made iron (II) solution added in sequences to solutions
of a) 166,7 g/L, 333,3 g/L and 500 g/L and b) 16,7 g/L, 33,3 g/L and 50 g/L. Scan parameters
as follows: pre-deposition at - 1500 mV for respectively 20 and 180 s, equilibrate time 10 s, scan rate
15 mV/s, modulation pulse 50 mV. c) Detection of iron (II) with DPASV in tri-sodium citrate (0.02M)
solution. Addition of iron (II) standard to solutions of 1,67 g/L, 3,34 g/L, 5 g/L, 15 g/L, 25 g/L, 50
g/L, pre-deposition time 180 s. d) Comparison between 20 s and 60 s pre-deposition time for a
specific solution. All samples (60 mL) purged with Nitrogen for 4 min.
The possibilities for analyzing iron (II) in real samples using DPASV were also
performed. In Fig. 2 detection of iron (II) in tap water is shown. The water sample was
8
taken directly from the tap and purged with nitrogen for about 4 minutes, and than
immediately after this oxalate was added and the sample analyzed.
55
50
Zn
45
I (  A)
40
35
Cu
30
25
20
15
Fe
10
5
-1500
-1000
E (V)
Pb
-500
0
Fig. 2 Tap water (80mL) added ammonium oxalate solution (20 mL, 0,1 M). Zinc observed at - 1150
mV, iron at -700 mV, (lead at - 580 mV as a tail on the iron peak), and finally copper at - 240 mV.
Scan parameters (DPASV) as follows: pre-deposition at - 1500 mV in 180 s, equilibrate time 10 s,
scan rate 15 mV/s, modulation pulse 50 mV. Sample (60 mL) purged with Nitrogen for 4 min.
Iron (II) was then detected in seawater using DPASV. In the first procedure, oxalate
(0.1M) was used. In the second procedure tri-sodium citrate (0.02 M) was used under
the same conditions.
In oxalate solution the iron peak was observed around – 800 mV and in citrate
solution the iron peak was observed at about – 650 mV. The oxalate solution had
somewhat better baseline, compared to the citrate solution. In both cases a
concentration of about 280 ng/L was detected, and a well-defined iron peak was
developed. Fig. 3 shows detection of iron (II) in a seawater sample.
9
17
I (A)
15
13
11
9
7
-1050
-850
-650
-450
-250
E (mV)
Fig. 3 Detection of iron in seawater from Trondheim harbour by standard addition method. Seawater
sample added ammonium oxalate solution (0,1 M), 285 ng/L found, pH = 8,1. Scan parameters was
as follows; dep. time was 180 s at - 1500 mV, equilibrate time 10 s, scan rate 15 mV/s, modulation
pulse 50 mV. Sample was purged with Nitrogen for 4 min before analyzing.
Fig. 4 Detection of iron in freshwater lake (pH 4.8) south in Norway (Lillesand). Sample (75 mL) was
added ammonium oxalate (0.2 g) and scanned with DPASV. Scan parameters as follows: predeposition at - 1350 mV (-1000 mV in first run) in 120 s, equilibrate time 10 s, scan rate 15 mV/s,
modulation pulse 50 mV. Sample purged with Nitrogen for 4 min. First run shows voltammogram of
sample after deposition and scan from -1000 mV, second run shows voltammogram of sample with
deposition and scan from - 1350 mV, and third run (dashed line) shows voltammogram after addition
of 100 g/L zinc, 300 g/L iron (II). Free metal concentration calculated to 15 g/L zinc and 340,1 g/L
iron (II).
10
Fig. 4 shows a sample from a fresh water lake taken south in Norway. The fresh
water lakes in this area is known to be quite acidic due to acid precipitation and a
bedrock low of lime, resulting in low pH and a high concentration of and aluminum.
Therefore a high concentration of iron (II) was expected in this sample, and 340,1
g/L iron (II) was found using DPASV (rsd. 3%). Complimentary ICP-MS
concentrations reported a total iron concentration of about 364,2 g/L (rsd. 2%).
As seen form Fig. 4 the response for lead is partly superimposed with the iron peak.
For that reason cadmium may interfere in the determination of iron. However, by
scanning from -1000 mV in addition to the scanning from -1350 mV will give a second
voltammogram were the iron peak is absent, thus subtracting the two
voltammograms will result in a subtractive stripping voltammogram containing only
the iron peak, as shown in figure 5. Using this procedure it will be possible to
measure iron independent of the cadmium or the lead concentration.
Fig. 5 Detection of iron in freshwater lake by subtractive stripping voltammetri. Solid curve shows
sample and dashed curve shows after addition of 300 g/L iron (II). Both scans subtracted a first scan
from - 1000 mV. All parameters given in Fig. 4.
11
Addition of trivalent iron did not give any effect on all the measurements given above.
This can be explained from the extremely low solubility of Fe(OH)3.
Conclusions
The preliminary results presented above show that the solid dental amalgam
electrode has a unique sensitivity for bivalent iron. The reduction from iron (II) to iron
has a very high over-potential at the traditional mercury drop electrode, a phenomena
well known for metals that are insoluble in mercury. This results in a reduction wave
that coincides with the hydrogen wave, and therefore low sensitivity. However, on the
solid dental amalgam electrode, which behaves more like a silver electrode with high
overpotential for hydrogen [25] this stripping peak for iron occurs at - 750 mV in the
specified solutions. Both freshwater and seawater samples have been analyzed
successfully with a minimum of sample treatment. Detection limits for 180 s predeposition time is estimated to be about 50 ng/L or lower. This method will be further
developed in our laboratories. With use of low frequency sound exposure [26] the
sensitivity can possibly be further enhanced. No iron peak is found when less
negative deposition potentials than –1050 mV is applied, this can be exploited to
avoid interferences with other species like cadmium and lead by using a subtractive
stripping technique, and to measure such species.
The solid dental amalgam electrode is suitable for implementation in continuous
online voltammetric apparatus for monitoring of iron as well as other metals. This
work that is in progress and will soon will be reported.
12
References
[1] J. M. Estela, C. Tomas, A. Cladera, V. Cerda, Critical reviews in analytical chemistry 1995, 25, 91.
[2] J. Wang, R. Sediadji, L. Chen, J. Lu, S. Morton, Electroanalysis 1992, 4, 161.
[3] Beyer M.E., Bond A.M., McLaughlin R.J., Anal. Chem., 47 (3), 479, 1995
[4] D. Jagner, E. Sahlin, B. Axelsson, R. Ratana-Ohpas, Anal. Chim. Acta 1993, 278, 237.
[5] J. Wang, B. Tian, Anal. Chem. 1992, 64, 1706.
[6] Hans-Joachim Diederich, Stefan Meyer, Fritz Scholz, Fresenius`s Journal of Analytical Chemistry,
1994, 349, 8/9, 670-675.
[7] Fernando Cordon, Silvana A. Ramírez and Gabriel J. Gordillo, Journal of Electroanalytical, 2002,
534, 2, 131-141.
[8] Alan M. Bond, Anal. Chim. Acta, 1999, 400, 1-3, 22, 333-379.
[9] [14] S. W. Wilhelm, D. P. Maxwell, C. G. Trick, Limnol. Oceanogr,, 1996, 41, 89.
[10] Turner, D.R., Whitfield, M. and Dickson, A. G., Geochim. Cosmochim. Acta, 1981, 45, 855.
[11] Colleine, R.J., Limnol. Oceanogr., 1983, 28, 83.
[12] Rich, H.W. and Morel, F.M.M., Limnol. Oceanogr., 1990, 35, 652.
[13] Van den Berg, C.M.G., Mar. Chem., 1995, 50, 139.
[14] Winkler, K.; Mojsa, R. Pol.J.Chem. 1995, 69: 5 731-741
[15] J.F. van Staden and M.C. Matoetoe, Anal. Chim. Acta 1998, 376:3:325-330
[16] H. Obata, C. M. G. van den Berg, Anal. Chem., 2001, 73, 11, 2522.
[17] Ø. Mikkelsen, K. H. Schrøder, Electroanalysis, 2001, 13, 8-9, 687.
[18] Ø. Mikkelsen, K. H. Schrøder, Analytical Letters, 2000, 33,15, 3253.
[19] Ø. Mikkelsen, K. H. Schrøder, T. A. Aarhaug, Collection of Czechoslovak Chemical
Communications, 2001, 66, 3, 465.
[20] B. Yosypchuk, L. Novotný, Talanta 2002, 56, 971.
[21] Ø. Mikkelsen, K. H. Schrøder, The Analyst, 2000, 125, 12, 2163.
[22] Ø. Mikkelsen, K. H. Schrøder, S. M. Skogvold, L. T Findalen, 9th International Meeting on
Chemical Sensors, 2002, Boston USA.
[23] Ø. Mikkelsen, K. H. Schrøder, Environmental and Health Aspects of Mining, Refining and Related
Industries, 2001, Skukuza, Kruger National Park, South Africa.
[24] Ø. Mikkelsen, K. H. Schrøder, In Vino Analytica Scientia, 2001, Bordeaux, France.
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[25] Ø. Mikkelsen, K. H. Schrøder, Electroanalysis, 2003, in press.
[26] Ø. Mikkelsen, K. H. Schrøder, Electroanalysis, 1999, 11, 401.
14
Figures in text
Fig 1 a) and b) Detection of iron (II) with anodic differential pulse stripping voltammetry (DPASV)
in ammonium oxalate (0.01 M) solution. Freshly made iron (II) solution added in sequences to
solutions of a) 166,7 g/L, 333,3 g/L and 500 g/L and b) 16,7 g/L, 33,3 g/L and 50 g/L. Scan
parameters as follows: pre-deposition at - 1500 mV for respectively 20 and 180 s, equilibrate time
10 s, scan rate 15 mV/s, modulation pulse 50 mV. c) Detection of iron (II) with DPASV in trisodium citrate (0.02M) solution. Addition of iron (II) standard to solutions of 1,67 g/L, 3,34 g/L, 5
g/L, 15 g/L, 25 g/L, 50 g/L, pre-deposition time 180 s. d) Comparison between 20 s and 60 s
pre-deposition time for a specific solution. All samples (60 mL) purged with Nitrogen for 4 min.
15
55
Zn
50
45
40
I (A)
35
Cu
30
25
20
15
Fe Pb
10
5
-1500
-1300
-1100
-900
-700
E (V)
-500
-300
-100
100
Fig. 2 Tap water (80mL) added ammonium oxalate solution (20 mL, 0,1 M). Zinc observed at
- 1150 mV, iron at -700 mV, (lead at - 580 mV as a tail on the iron peak), and finally copper at 240 mV. Scan parameters (ADSV) as follows: pre-deposition at - 1500 mV in 180 s, equilibrate
time 10 s, scan rate 15 mV/s, modulation pulse 50 mV. Sample (60 mL) purged with Nitrogen for 4
min.
16
17
I (A)
15
13
11
9
7
-1050
-950
-850
-750
-650
-550
-450
-350
-250
E (mV)
Fig. 3 Detection of iron in seawater from Trondheim harbour by standard addition method.
Seawater sample added ammonium oxalate solution (0,1 M), 285 ng/L found, pH = 8,1. Scan
parameters was as follows; dep. time was 180 s at - 1500 mV, equilibrate time 10 s, scan rate 15
mV/s, modulation pulse 50 mV. Sample was purged with Nitrogen for 4 min before analyzing.
17
Fig. 4 Detection of iron in freshwater lake (pH 4.8) south in Norway (Lillesand). Sample (75 mL)
was added ammonium oxalate (0.2 g) and scanned with DPASV. Scan parameters as follows:
pre-deposition at - 1350 mV (-1000 mV in first run) in 120 s, equilibrate time 10 s, scan rate 15
mV/s, modulation pulse 50 mV. Sample purged with Nitrogen for 4 min. First run shows
voltammogram of sample after deposition and scan from -1000 mV, second run shows
voltammogram of sample with deposition and scan from - 1350 mV, and third run (dashed line)
shows voltammogram after addition of 100 g/L zinc, 300 g/L iron (II). Free metal concentration
calculated to 15 g/L zinc and 340,1 g/L iron (II).
18
Fig. 5 Detection of iron in freshwater lake by subtractive stripping voltammetri. Solid curve shows
sample and dashed curve shows after addition of 300 g/L iron (II). Both scans subtracted a first
scan from - 1000 mV. All parameters given in Fig. 4.