Laboratory Experiment pubs.acs.org/jchemeduc
Prussian Blue Coated Electrode as a Sensor for Electroinactive Cations in Aqueous Solutions Houston Byrd,* Blake E. Chapman, and Christopher L. Talley Department of Biology, Chemistry & Mathematics, University of Montevallo, Montevallo, Alabama 35115, United States S Supporting Information *
ABSTRACT: Prussian Blue (PB) is an excellent material as a sensor for electroinactive cations because of its electrochemical behavior and its zeolytic character. A simple 3-h laboratory designed for a quantitative analysis or an instrumental methods course is reported. This laboratory studies the transport of various cations into a PBmodified electrode upon reduction. Cyclic voltammograms are used to monitor the transport of ions such as K+ and Rb+ and observe the exclusion of Na+ and Mg2+ ions. The ability for cations to enter the PB film is a function of the hydrated radius of the cations being analyzed. KEYWORDS: Upper-Division Undergraduate, Analytical Chemistry, Laboratory Instruction, Hands-On Learning/Manipulatives, Electrochemistry, Ion Selective Electrode, Instrumental Methods alternating Fe(II) and Fe(III) ions bridged by CN− ligands and charge balancing metal cations.12 However, not all cations have the ability to transport into the PB film because the open cavities can only accommodate ions up to 182 pm.10 Itaya demonstrated that K+, Rb+, NH4+, and Cs+ have the ability to penetrate the film due to their small hydrated radius, whereas Li+ and Na+ cannot (see Table 1). Itaya’s work was later confirmed by undergraduate students utilizing a quartz crystal microbalance to monitor the incorporation of cations into PB films.8 The electrodeposition of the PB films and monitoring of transport of ions into the film via CV are skills that can be easily taught to undergraduate students. Therefore, this laboratory is designed for an analytical undergraduate course to be completed in one 3-h laboratory session. The aims of this laboratory are as follows: (i) demonstrate simple electrochemistry experiments by electrodepositing PB onto standard electrodes, (ii) demonstrate the transportation of ions into the film through cyclic voltammetry, and (iii) open discussions on sensors via ion selective electrodes.
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on selective electrodes are one of the most important subgroups of electrochemical sensors1−3 with more than 7000 papers published on this topic by 1990.4 However, few laboratories for undergraduates have been developed to investigate this area.5 Prussian Blue (PB)6−10 is an excellent material as a sensor for electroinactive cations because of its electrochemical behavior6−9 and its zeolytic character.10 The crystal structure of insoluble PB, solved in 1977, indicates the iron ions define a simple cubic lattice.11 The alternating Fe(II) and Fe(III) ions are linked by linear cyanide groups (Figure 1). The carbon atoms are coordinated to the Fe(II) ions, whereas the nitrogen atoms are coordinated to the Fe(III) ions. The ideal crystal structure assumes an ordered face-centered unit cell that has Fe(II) vacancies, which are located in the center of one-fourth of the unit cells.12 This open structure leaves large cavities and tunnels in three directions, which can be occupied by cations when the PB is reduced. In 1982, Itaya et al. published a paper6 on the electrochemistry of PB-modified electrodes and the transport of various cations into the film upon reduction. The equation for only the electrochemical process is
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Fe4 III[Fe II(CN)6 ]3 + 4M+(aq) + 4e−
Salt Solutions
(PB) II
II
⇌ M4Fe4 [Fe (CN)6 ]3
All reagents were purchased from Aldrich and used without further purification. The experiments were carried out with both chloride and nitrate salts of Li+, Na+, K+, Rb+, and Mg2+. All monocation solutions (distilled water) were prepared at a concentration of 0.1 M and the dication solutions (distilled water) were prepared at a concentration of 0.05 M. The pH was not adjusted although several papers6,7 indicate the best
(1)
After several cyclic voltammetry (CV) cycles with cations in solution, a structural change takes place where approximately one-fourth of the Fe(III) ions are replaced with the metal cations.7 The inclusion of these ions is necessary to maintain the electroneutrality of the film. The ideal structure (Figure 1) of reduced film has no Fe(II) vacancies or coordinated water molecules, but instead possesses an ordered cubic structure of © XXXX American Chemical Society and Division of Chemical Education, Inc.
EXPERIMENTAL PROCEDURE
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dx.doi.org/10.1021/ed3008318 | J. Chem. Educ. XXXX, XXX, XXX−XXX
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Laboratory Experiment
Figure 1. Idealized structure of the insoluble form Prussian Blue showing the cubic structure (left). The inclusion of metal cations, upon reduction of the PB film (right), changes the original crystal structure. Structures are adapted with permission from ref 12. Copyright 2010 Loretta Lawton. The cyanide ligands are only shown to coordinate on one Fe3+ cation and are left out of the reduced structure (right) for clarity.
Figure 2. Typical cyclic voltammograms of a PB-modified electrode. (A) The solid line is with 0.1 M KCl, the dash line is with 0.1 M NaCl. (B) The solid line is with 0.1 M RbCl, the dash line is with 0.05 M MgCl2. In all cases, the sweep rate was 20 mV/s and all potentials are referenced to Ag/ AgCl/sat’d KCl electrode.
results are obtain when salt solutions are adjusted to a pH of 4.0.
equipment. Therefore, the data shown are from films freshly prepared without cycling them in a K+ solution.
Working Electrodes
Cyclic Voltammetry (CV)
Stationary voltammetry electrodes were purchased from BASi. The electrodes are coated in a solvent-resistant CTFE plastic body. The electrode disk diameter was 1.6 mm and the disks used in this experiment were gold, platinum, and glassy carbon. There was no real advantage for one electrode over another; therefore, all data shown uses a platinum working electrode. However, the PB film does not strongly adhere to the disk using these standard electrodes. Itaya also noticed this phenomenon and mounted his electrodes on glass.6 However, for the laboratory purposes at the undergraduate level, these electrodes work fine up to approximately 20 cycles. The PB film can be easily removed by wiping the surface with a Kimwipe.
The potentiostat−galvanostat used in all experiments was a Princeton Applied Research Verastat II. All monocation salt solutions (distilled water) were 0.1 M in concentration, whereas the dication solutions (distilled water) were 0.05 M. The cell contained an Ag/AgCl/sat’d KCl reference electrode and a Pt wire as the counter electrode. A sweep rate of 20 mV/s was used between 0.6 V and −0.2 V and the CV was cycled 3 times.
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HAZARDS The use of HCl should be contained to a fume hood when preparing the 0.05 M HCl solution. Concentrated HCl is very hazardous in case of skin contact (corrosive, irritant, permeator), eye contact (irritant, corrosive), and ingestion. Caution should be used when weighing out K3[Fe(CN)6] and FeCl3·6H2O because both are hazardous in case of skin contact (irritant), eye contact (irritant), and ingestion. Eye protection should be worn at all times during this laboratory.
Deposition of Prussian Blue
The procedure published in this Journal was followed.7 Aqueous solutions (distilled water) of 0.05 M HCl, 0.05 M K3[Fe(CN)6], and 0.05 M FeCl3·6H2O were mixed in this order in a volume ratio of 1:2:2. The working electrode and a Pt counter electrode were immersed in this solution. While vigorously stirring the solution, the film is galvanostatically deposited by passing a current density of 40 μA/cm2 for 5 min (300 s). Once the film was obtained, it was rinsed with distilled water and then placed in a holder for further use. It is important to note that Vincente et al.7 cycled their films in 0.1 M KCl 15 times to stabilize the films. This works very well; however, time may be a factor depending on size of the class and availability of
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RESULTS Several authors have demonstrated that the CV’s are strongly affected by the cations in solution.6−9 A current generated on the reduction of the film is indicative of cation transport into the open channels of the PB film. The inclusion of these cations is necessary to maintain electroneutrality. The inclusion or exclusion of ions into the PB film is generally a function of the B
dx.doi.org/10.1021/ed3008318 | J. Chem. Educ. XXXX, XXX, XXX−XXX
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Laboratory Experiment
hydrated radius of the ions.6 Ions that possess a hydrated radius less than 180 pm are expected to enter the channels, whereas larger ions are excluded.10 Figure 2 shows four cyclic voltammograms produced by undergraduate students. It is clear that K+ ions are transported into the film, whereas Na+ ions are excluded (Figure 2A). This is observed by the reduction wave of the PB film in the K+ solution followed by corresponding reoxidation wave. The appearance of these waves indicates that the K+ is transporting into the films to balance the charge within the film. When the film is placed in a Na+ solution, both the reduction and oxidation waves are nonexistent, which indicates that Na+ ions do not transport into the film. This is in excellent agreement with previous published results.6,8 Experiments were also performed with Rb+ ions and Mg2+ ions and again the same dependence of transport on the solvated radius was observed (Figure 2B). Finally, experiments were attempted with Li+ ions, however the PB films were completely stripped from the surface of the electrode. A summary of these results and previously published results are listed in Table 1 and it shows the variety of cations that can be used in the laboratory experiment.
Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We would like to thank Stephen O’Donnell for helpful discussions throughout this process. (1) Bobacka, J.; Ivaska, A.; Lewenstam, A. Potentiometric Ion Sensors. Chem. Rev. 2008, 108, 329−351. (2) Bühlmann, P.; Pretsch, E.; Bakker, E. Carrier-Based Ion-Selective Electrodes and Bulk Optodes. 2. Ionophores for Potentiometric and Optical Sensors. Chem. Rev. 1998, 98, 1593−1687. (3) Zen, J.-M.; Kumar, A. S. A Mimicking Enzyme Analogue for Chemical Sensors. Acc. Chem. Res. 2001, 34, 772−780. (4) Frant, M. S. Historical Perspective. History of the Early Commercialization of Ion-Selective Electrodes. Analyst 1994, 119, 2293−2301. (5) A search of abstracts in this Journal containing “ion selective electrodes” only produced 20 articles and none after 2005. (6) Itaya, K.; Ataka, T.; Toshima, S. Spectroelectrochemistry and Electrochemical Preparation Method of Prussian Blue Modified Electrodes. J. Am. Chem. Soc. 1982, 104, 4767−4772. (7) Garcia-Jareño, J. J.; Benito, D.; Navarro-Laboulais, J.; Vincente, F. Electrochemical Behavior of Electrodeposited Prussian Blue Films on ITO Electrode: An Attractive Laboratory Experience. J. Chem. Educ. 1998, 75, 881−884. (8) Deakin, M. R.; Byrd, H. Prussian Blue Coated Quartz Crystal Microbalance as a Detector for Electroinactive Cations in Aqueous Solution. Anal. Chem. 1989, 61, 290−295. (9) Karyakin, A. A. Prussian Blue and Its Analogues: Electrochemistry and Analytical Applications. Electroanalysis 2001, 13, 813− 819. (10) Ware, M. Prussian Blue: Artists’ Pigment and Chemists’ Sponge. J. Chem. Educ. 2008, 85, 612−620. (11) Buser, H. J.; Schwarzenbach, D.; Petter, W.; Ludi, A. The Crystal Structure of Prussian Blue: Fe4[Fe(CN)6]3·xH2O. Inorg. Chem. 1977, 16, 2704−2710. (12) Lawton, L. Structure Property Relationships in Prussian Blue Analogues and Hydrogen Bond Mediated Metal Complexes [Online]. Ph.D. Thesis, The University of Glasgow, Scotland, U.K., September 2010. http://theses.gla.ac.uk (accessed Apr 2013). (13) Robinson, R. A.; Storkes, R. H. Electrolyte Solutions; Butterworths: London, 1955.
Table 1. Relationship of Hydrated Radius and Transport of Ions into the Prussian Blue Film Ions
Hydrate Radius/pm
+
Li Na+ K+ Rb+ Cs+ NH4+ TEA+ Mg2+ Ba2+
a
237 183a 125a 118a 119b 125b 281b,c 346a 288b
Transport no no yes yes yes yes no no no
a
Data obtained from ref.13. bData obtained from refs 6 and.8. cTEA+ is the tetraethylammonium ion.
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CONCLUSION This experiment accomplishes the goals of teaching electrochemistry and introducing students to ion selective electrodes. One advantage is that it can be easily accomplished in a single 3-h laboratory session. While one group of students is preparing their own PB modified electrode, the other groups are preparing their individual salt solutions to investigate. The deposition and CV (3-cycle) for one salt solution takes approximately 10−15 min. This experiment has been introduced as the last lab in the quantitative analysis course as a complement to the lectures on electrochemistry.
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REFERENCES
ASSOCIATED CONTENT
S Supporting Information *
The material provides a complete detailed description of solution preparation and experimental details. A description of the materials used and any hazards are also provided. This material is available via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
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dx.doi.org/10.1021/ed3008318 | J. Chem. Educ. XXXX, XXX, XXX−XXX
