Oxidation of ferricyanide

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1KB010 Bachelor project 15 credits June 2019

Oxidation of ferricyanide

An electrochemical study of HCF(III) redox reactions

Department of Chemistry, Angstrom Laboratory

Written by: Johan Rye-Danjelsen Supervisor: Mun Hon Cheah, Molecular Biomimetics, Department of Chemistry, Angstrom Laboratory

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Author

Johan Rye-Danjelsen Swedish titel

Oxidation av ferricyanide: En elektrokemisk studie av HCF(III) redox reaktioner English Title

Oxidation of ferricyanide: An electrochemical study of HCF(III) redox reactions Supervisor

Mun Hon Chea, Molecular Biomimetics, Department of Chemistry, Angstrom Laboratory Abstract

The reversible reduction of hexacyanoferrate(III) to the Fe(II) form is a well-known standard in electrochemistry. It is however reported to be highly dependent on the solvent environment.¹ This electrochemical project has aimed to examine the behaviour of ferricyanide in aprotic environment by changing the solvent to acetonitrile, a medium polar solvent with a high dielectric constant. The method used for research is cyclic voltammetry. Ferricyanide has been examined in aqueous and aprotic solvent and has been studied in comparison to ferrocene, which is a well-known chemical substance.

When studying ferricyanide in acetonitrile solvent under inert conditions, a completely new voltammetric picture was drawn where the standard reduction potential in aqueous solvent of 43 mV (SCE) was dramatically lowered and a new redox reaction with a higher reduction potential seemed to be present at -97mV (Ag/Ag⁺) when ferricyanide went through redox process from 0 V to +1.3 V to -0.5 V and back to 0V again. When examining the redox process an EC reaction was found to be coupled to the new peak. This process was studied within the time limit for the project. Some properties of the possible chemical has been presented and the conclusions are discussed in the report. Suggestions are made for how to move forward with the research.

Swedish abstract

Den reversibla reaktionen av hexacyanoferrate(III) till Fe(II) formen är en välkänd och välstuderad process inom fältet elektrokemi. De etablerade egenskaperna är rapporterade att vara starkt beroende av lösningens egenskaper.² Detta elektrokemiska projekt har som mål att undersöka ferricyanide och dess egenskaper i aprotisk lösning genom att byta ut lösningen mot acetonitril, ett medium som är polärt med en hög dielektrisk konstant. Metoden som används är cyklisk voltammetri. Ferricyanide har undersökts i akvatisk och aprotisk lösning och har studerats i jämförelse med ferrocene som också är ett väl dokumenterat kemiskt ämne.

När ferricyanide studerades i acetonitril lösning under inert tillstånd ritades en helt ny voltammetrisk bild av ämnet upp från akvatiska jämförelsepunkter, då standard reduktions potentialen på 43mV (SCE) blivit dramatiskt sänkt och en ny redox potential med en högre reduktions potential uppstår vid -97mV (Ag/Ag⁺) när ferricyanid går genom redoxprocess från 0V till +1.3V till -0.5V och åter till 0V. När redoxprocessen studerades kunde en EC reaktion kopplas till den nya redox potentialen vid -97mV (Ag/Ag⁺). Denna process studerades inom den givna tidsramen för projektet. Vissa egenskaper hos det kemiska ämnet presenteras och några slutsatser dras och diskuteras i rapporten. Förslag på vägar att gå vidare med forskning föreslås.

Ämnesord

Ferricyanid, Hexacyanoferrat, HCF(III), Elektrokemi, Oorganisk kemi, Redox reaktioner Key words

Ferricyanide, Hexacyanoferrate, HCF(III), Electrochemistry, Inorganic chemistry, Redox reactions

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Objective and Goals

The aim of the project is to examine the redox reactions of ferricyanide (HCF) in acetonitrile solution. Reduction of HCF(III) to HCF(II) in aqueous solvent is a well-known one-electron reduction process with a reduction potential of 43 mV (SCE).³ It has been reported that the reduction potential of HCF(III) is highly dependent on solvent composition and the solvents electron donor’s qualities.⁴ In acetonitrile, the reduction potential is reported to shift to -106 mV (SCE).⁵ In addition to the reported shift in reduction potential of HCF(III), a new oxidation process is observed that is not previously reported in literature. The aim of this project is to compare the reduction chemistry of HCF(III) in acetonitrile to those recorded in aqueous solvent and to examine the previously unreported oxidation of HCF(III) by using cyclic voltammetry.

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Methodology

The best way to understand cyclic voltammetry is to study a voltammogram. And a good way to do that is to use a well-known reversible one-electron reduction process like ferrocene/ferrocenium in acetonitrile.

Figure 1: CV of 2 mM ferrocene in acetonitrile solvent at 100 mV/s scan rate with 0.2M tetrabutylammonium hexafluorophosphate salt as supporting electrolyte salt.

(Eq1a) Nernst equation for redox systems: 𝐸 = 𝐸⁰+ (𝑅𝑇/𝑛𝐹) 𝑙𝑛(𝑂𝑥𝑖𝑑𝑖𝑧𝑒𝑑 𝑎𝑛𝑎𝑙𝑦𝑡𝑒) (𝑅𝑒𝑑𝑢𝑑𝑒𝑐 𝑎𝑛𝑎𝑙𝑦𝑡𝑒)

In the equation, F is Faraday’s constant, R is the universal gas constant, n is the number of electrons and T is the temperature in degrees Kelvin.

For ferrocene, this means that:𝐸 = 𝐸⁰′ + 2.3026 (𝑅𝑇/𝐹) 𝑙𝑜𝑔₁₀^[𝐹𝑐⁺^]

[𝐹𝑐]

Nernst equation is a powerful tool when it comes to predict how a system will respond to a change in concentration of species or electrode potential and can be used in reference to reversible systems.⁶The equation relates to the potential of an electrochemical cell (E), the standard potential of the species in the cell (E⁰’) and the relative activities of the oxidized and reduced analytes in the system at equilibrium.

When following the curves of ferrocene in the CV seen in Figure 1, we start at point A and stop at point H. At point A, 100% ferrocene exists in the unoxidized form and the potential is at 0V.

At point B, called the E1/2, the Nernst equation predicts the following:

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𝐸 = 𝐸⁰′ + (𝑅𝑇/𝐹) 𝑙𝑛 (1)

Since ln(1) equals to 0, the following approximation is made for reversible systems:

(Eq1b) 𝐸 = 𝐸^0′ = 𝐸_1/2

Point B can give the value for the standard potential, which means that if the settings for the experiments were to be repeated the following would happen:

If 2mM ferrocene concentration in acetonitrile solvent with 0.2M tetrabutylammonium electrolyte salt were to be put in a cell under inert conditions and approximately 39mV potential would be applied (as indicated by Figure 1), the Ferrocene and Ferrocenium would reach equilibrium.

Point C is called the Anodic peak potential. It’s where the diffusion works at it’s fastest rate and where in this example the analyte ferrocene is instantly turned into it´s oxidized form ferrocenium. This is where the Peak anodic current is observed. The current is dependent upon the constant stream of oxidized analyte from the surface of the W.E. At the same time, a layer of reduced analyte has been built at the W.E. surface, called the diffusion layer. This layer continues to grow throughout the scan. This makes scanning slower at points D, E, F, G, etc.

At Point D, the Switching potential or E2, the potential is reversed and the current subsequently turns negative.

At Point E, The Eq1b applies again and the same (negative) potential for E1/2 = E⁰’ satisfies the Nernstian equilibrium.

Point F is called the Cathodic peak potential and the difference between Point C and Point F (expressed in mV) is called the EPP, or the peak-to-peak separation. Chemical reversibility is a term used to describe whether or not an analyte is re-oxidizable or not after having been reduced. Is the product reusable or not. This term is not only appropriate in terms of chemical reversibility, but also in terms of electrochemical reversibility as a process that is

electrochemically induced by raising the electron’s activity in an electric field must be reversible in kinetic terms. If the process is sluggish (a relative expression for an electron) then the process is referred to as irreversible.

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The practical part: Che Cell

The Cell in this project was made of glass with a plastic top with a tight fit. The container was 20mL in size and was filled with approximately 5mL acetonitrile solvent in each experiment.

Figure 2a. Electrode A is a homemade (Ag/Ag⁺) silver/silver chloride electrode, see Figure 2b. This type was used as Reference Electrode (R.E.), through the whole project. In aqueous solvent, an (Ag/Ag⁺) R.E. with an internal solvent of 0.3M potassium chloride was used and in acetonitrile an (Ag/Ag⁺) R.E. with an internal solvent of 0.3M

tetrabutylammonium

hexafluorophosphate (TBAPF6) was used. Three small holes and two very small ones are present in the top on the Cell for the three electrodes and the two small plastic tubes that barely can be made to fit through, see Figure 2.

Figure 2a: Whiteboard drawing of the Cell (I) From the project, complete with a pre-bubbler (II).

Electrode B is a glassy carbon Working Electrode (W.E.) with a working area of 3 diameter.

The actual chemistry of the whole project has been centred on those 7.068*10^-2 cm² of area, since the electron transfer process has it’s centre there.

When the Cell undergoes an oxidation process, the analyte is oxidized on the surface on the W.E.:

i) A ⇌ A⁺ + e^-

When the Cell undergoes a reduction process, the analyte is reduced on the surface on the W.E.:

ii) A⁺ + e^-⇌ A

The analyte in Figure 1 was Ferrocene, which was oxidized into ferrocenium and reduced back again. The analyte in each experiment is defined as the chemical of interest for the experiment.

Electrode C is a Platinum wire coil that is used as Counter Electrode (C.E.). The end points of electrodes A and B should be as close together as possible, due to the Ohmic drop (IR drop), which means the overpotential that results from the electrons flowing through the solution of the Cell. The IR drop is dependent on the distance between the R.E. and W.E. The last

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experiments were performed with the W.E. and the R.E. so close together that a paper couldn’t fit in between.

In short, the trick is to keep everything extremely clean, to scrub the W.E. really well and to make sure to use every trick in the book you can find - quite literary. No one invents every trick on their own.

The acetonitrile experiments were performed inside a fume hood, all experiments were

performed under inert conditions, the solutions had 0.2M electrolyte salt and were prebubbled with N2 gas.

The R.E. works under ideal conditions, under which it exhibits constant potential. It also has little change with temperature cycling. As it is not in direct contact with the chemical process of the Cell, it stays obedient to Nernst equation. Figure 2b shows a whiteboard drawing of a homemade reference electrode. A is the platinum wire, which has been electroplated with silver, after which the part of the wire that is on the inside of the R.E. is converted to silver chloride by anodic treatment with hydrochloric acid.

In Figure 2b, B is the top sealing. C is the internal solvent and D is the frit. It is important that the frit is not leaking, thereby influencing the integrity of the R.E. The electron transfer through the frit is important in order to use the R.E., but all data acquisition from the R.E. is depending on the R.E. as a chemical reference system operating without interference of the actual experiments.

Figure 2b: Whiteboard drawing of a R.E.

When the Cell undergoes an oxidation process, the following happens inside the R.E.:

i) Ag(s) ⇌ Ag⁺ + e^-

When the Cell undergoes a reduction process, this process happens inside the R.E.:

ii) Ag⁺ + e^-⇌ Ag(s)

Prebubbler and N2-gas

For aqueos solutions, the prebubbling was made simply by bubbling N2 gas through the solutions for 5 minutes and letting the solutions settle before each scan. For acetonitrile experiments, a prebubbler, marked as II in Figure 2, was used to prevent the volatile acetonitrile liquid from escaping from the Cell together with the gas and thereby make the readings increasingly inaccurate as the gassing continued. The pre-bubbler (II) saturates the N2 gas before bubbling through the solution of acetonitrile in the actual Cell (I). A closed 20mL container was used, containing 5mL acetonitrile with N2 gas bubbling through it to

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become saturated. The N2 gas was further led into the electrolyte cell, where the scans was performed.

The saturated N2 gas was bubbled through the solutions for 5 minutes before each cycle of experiments and the solution was allowed to settle for a few seconds before each scan. The plastic tubing was drawn out of the solution but the gas was kept on so that a steady low stream of saturated N2 gas kept the integrity of the Cell from O2 interference. The solution was bubbled with gas every few minutes and stirred between each experiment.

The technical part: monitoring the CV:s

A potentiostat regulates the flow of potential between the Start point, Switching point and End point and the resulting current is monitored and plotted on a voltammogram. Another name for A is E1, meaning the Starting potential, as the potential is 0 at the start of the scan (in the example Figure 1). D is called E2, the Switching potential. This is also illustrated by Figure 3, that illustrates a (generic) CV experiment in which the starting point (A) is the start of the scan, the Switching point is D and the End point is H. If E1 and E2 results in a chemical reaction and the formation of a new chemical species, the reverse scan may be extended beyond E1 to learn more about the system under study and it´s electrochemical reactivity.

There are two commons ways of storing CV data: the US

convention and the IUPAC convention. For the purpose of this project, the IUPAC

convention is used. This means that the oxidation peaks will appear on the top half of the CV:s, while the reduction peaks will appear on the bottom half’s.

Figure 3: Applied potential as a function of time for a generic CV experiment. A is the starting point, D is the switching point and H is the End point for the experiment.

Scan rate

The scan rate controls how fast applied potential is scanned. This has a direct effect on the size on the diffusion layer and can be used in addition with the Randles-Sevcik equation to determine the Diffusion constant or whether an analyte is freely diffusing in solution or not.

An experiment often collects CV:s with several scan rates for comparison.

(Eq 2a) ip = (0.446) * nFAC⁰* (nFvDo/RT)^1/2

For the settings of this project and with a linear graph plotted with the X-axis plotted as the (Scan rate)^{^1/2}, which in Eq 2a stands for the v term, the following equation is applicable to use when determining the Diffusional constant, D0.

Time

Potential A

D H

End

Start

E1 E2

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From 2a follows that:

ip = (0.446) * FAC⁰* (F/RT)^1/2* (D0)^1/2* (v)^1/2

Further modified to accommodate the “X slope” from the equation from the Excel graphs, where y = kx + m gets very small ‘m’ values, the m will be set to m = 0 as the intercept is the target.

(Eq 2b) D0 = ( "X slope"

(0.4463) ∗ FA𝐶^𝑜 ∗ (F/RT)^1/2)²

The Randles-Sevcik equation was modified to calculate the diffusional constant D0, for Ferricyanide in aqueous and acetonitrile solutions and for Ferrocene in acetonitrile solution.

The chemical that is of interest for the analysis is referred to as the analyte in the reaction.

According to the Randles-Sevcik equation, the peak current (ip) is dependent upon several factors, including the active area of the W.E. (A, expressed in cm²), the bulk concentration of the analyte of the experiment (C⁰, expressed in mol*cm^-3) and finally how the analyte diffuses and reacts in relation to the chemical environment (D0, expressed in cm²*s^-1).

Materials and Method

The chemicals used in this project were reagent grade, 98% pure or higher.

All experiments that included aprotic solvents, i.e. acetonitrile, was conducted under the protection of a fume hood.

Tetrabuthylammonium Ferricyanide, TBA HCF(III), is used as ferricyanide with 1mM concentration in all experiments with ferricyanide in acetonitrile solvent, with 0.2M TBAPF6

as supporting electrolyte salt.

For ferricyanide in aqueous solvent, potassium ferricyanide, K3Fe(CN)6 was used to prepare a 1mM (aq) solution with KCl as 0.2M electrolyte salt.

For ferrocene in acetonitrile solvent, tetrabutylammonium tetrafluoroborate (TBA PF6) was used to prepare a 0.2M electrolyte acetonitrile solution. Ferrocene was added to 2mM analyte concentration.

The electrolyte salt used, namely potassium chloride for aqueous solvents and

tetrabutylammonium tetrafluoroborate for acetonitrile solutions, were selected for several qualities for their respective solvents including: high solubility, electrical conductivity and supposed lack of interference with the experiment. The last part is a bit tricky, since an

experiment which results you already know everything about is of use for training, but not for scientific pursuits. Having access to good research material can give insights into most

pitfalls, however. As can a veteran colleague or a good supervisor.

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Synthesis of Tetrabuthylammonium Ferricyanide

30mL (d) water was added to 2.8706g Tetrabuthylammonium chloride (TBA Cl).

0.876 g Potassium ferricyanide (K3HCF(III)) was added to the mixture. The colour turned distinctly yellow. TBA HCF(III) was extracted with dichloromethane, DCM, 8 times, after which the yellow yield was dried with magnesium oxide. After drying, TBAHCF(III) was filtrated and brought to evaporation for approximately 20 minutes.

When evaporation was finished, no crystal formations could be seen in the round bottom flask. The yield had formed into a residue that was sticking hard to the side of the flask. The residue was scraped down to the middle of the flask and was rinsed with ether 8 times. When rinsing with ether the first few times, the residue gained an increasingly brighter yellow colour. The residue was dissolved into approximately 20 mL of acetonitrile and the flask was covered and left for the night to see if crystals would begin to form spontaneously. The next morning, however, there were no crystals visible in the solvent.

Ether was added dropwise into the solvent to force a crystallization process. The dropwise process took a long time, as the amount of ether that was added became approximately 40 mL until a cloudiness formed atop of the solution and something akin to a snowing process started. The process went on for a quarter hour, under which a powder settled on the bottom of the solution. After approximately a quarter hour the process subsided, but a few drops of ether made the process continue. The process was repeated once again with a lesser amount of ether after having filtered the solution with vacuum. After the second vacuum filtering, the product weight was 1.1173g which amounted to a yield of 44.69%. The product, now solidified and purified, was Tetrabuthylammonium Ferricyanide, TBA HCF(III).

The reaction can be written in two steps:

i. 3[TBA]Cl (aq) + K3[HCF(III)] (aq) ⇌ 3KCl (aq) + [TBA]3 [HCF(III)] (aq)

The stochiometric amounts of tetrabuthylammonium chloride and potassium ferricyanide was mixed together in a solution of 30 mL distilled water.

DCM

ii. [TBA]3 [HCF(III)] (aq) → [TBA HCF(III)] (s)

TBAHCF(III) (aq) was extracted with dichloromethane, purified and synthesized into TBAHCF(III) (s), referred to hereafter as TBA HCF(III) or tetrabuthylammonium ferricyanide.

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Results

Ferricyanide in aqueous solution

The oxidation of ferricyanide is a well-studied reaction, making it an excellent place to start for a new researcher in the field, in need of training. It also makes an excellent reference point for ferricyanide in acetonitrile. The IR drop proved hard to manage in the experiments. In order to get around this problem, the results are compared to the reaction of a well-known reversible reaction, ferrocene in acetonitrile, which is highly reversible. To prove that the IR drop is the reason for the high readings, a “trumpet-plot” is presented (see Figure 10.

Figure 4: Ferricyanide in aqueous solvent at different scan rates.

For details regarding concentration, see Materials and Methods.

To determine the reversibility of the reaction, a linear graph was plotted using the Excel software. The results are presented in Figure 5. A trendline was added to assert the linear nature of the graph and to get an equation for the trendline. The equation for the trendline gave the intercept for the Modified Randles-Sevcik equation (the “X-term”) which is needed to calculate the diffusional coefficients for ferricyanide in aqueous solvent.

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Figure 5: Peak current vs. square root of the scan rate for the

ferri(III)/ferro(II)cyanide redox reaction in aqueous solution under inert conditions.

Eq 2b was used to calculate two D0 values for ferricyanide in aqueous solvent. Because the intercepts are the same, the D0 values for the oxidation and reduction peaks will be the same:

1,09E-4 cm² s^-1.

Both ferri(III) and ferro(II)cyanide are expected to behave reversibly in aqueous solvent and to have similar D0 values. This is not expected to be the case in aprotic solvents, however. As long as there are water molecules around the ferro(II) complex, they should interfere with any structural changes. The ferri(III) complex are not expected to behave similarly, as the ion-pair formation is weaker⁷.

The literature values for ferricyanide in aqueous solvent is: D0(ox): 7.3*10^-4 cm² s^-1 and D0(red) 7.1*10^-4 cm² s^-1.⁸

Scan rate [mV/s]

Ipa [mA]

Ipc [mA]

Epp [mV]

E_1/2 [mV]

50 mV/S ^0.043 ^-0.044 ⁷³ ^36.5

100 mV/S ^0.0645 ^-0.0635 ⁷⁵ ^37.5

200 mV/S ^0.088 ^-0.089 ⁷⁸ ³⁹

500 mV/S ^0.114 ^-0.114 ⁸⁹ ^44.5

1000 mV/S ^0.198 ^-0.195 ^92.8 ^46.4

1200 mV/S ^0.22 ^-0.217 ^100.8 ^50.4

Table 1: Results chart from experiments on ferricyanide in aqueous solvent.

The E1/2 is roughly equal to E’⁰ according to Nernst equation (Eq1b).

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Ferrocene in acetonitrile solution

Ferrocene is a highly reversible one-electron reaction that has been studied extensively.

Establishing a reference point for ferrocene was important for two reasons: first for the reason that it’s a well-established one-electron reaction in acetonitrile with the same concentration of electrolyte salt. This leads to the second reason, that the point of reference leads to the

establishing of a so-called ‘Trumpet plot’ which is a direct comparison between the Ipa and Ipc values of ferrocene and ferricyanide in acetonitrile solution plotted against the Log10(scan rate). It establishes that reversibility can exist for ferricyanide as well, if it stays inside the lines of ferrocene and if the intercept of Figure 11 is also linear and low. That would mean a low rise of peak-to-peak level rate.

Figure 6: Ferrocene in acetonitrile solvent at different scan rates.

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Figure 7: Peak current vs. square root of the scan rate for the ferrocenium⁺/ferrocene redox reaction in acetonitrile solution under inert conditions.

Eq 2b was used to calculate two D0 values for ferrocene in acetonitrile solvent. Because the intercepts are the same, the D0 values for the oxidation and reduction peaks are the same:

2.73*10^-5 cm² s^-1.

The literature value for ferrocene in acetonitrile solvent is only one: D0(ox/red): 2,60*10^-5 cm² s^-1.⁹

Ipa [mA]

Ipc [mA]

Epp [mV]

E1/2 [mV]

10 mV/S ^0.0203 ^-0.0210 ^79.3 ^39.7

20 mV/S ^0.029 ^-0.030 ^81.3 ^40.7

50 mV/S ^0.0460 ^-0.0475 ^79.3 ^39.7

100 mV/S ^0.064 ^-0.064 ^89.3 ^44.7

200 mV/S ^0.094 ^-0.093 ^89.3 ^44.7

500 mV/S ^0.121 ^-0.120 ^102.5 ^51.3

1000 mV/S ^0.178 ^-0.179 ^112.3 ^56.2

1200 mV/S ^0.192 ^-0.192 ^115.8 ^57.9

Table 2: Results chart from experiments on ferrocene in acetonitrile solvent.

The E1/2 is roughly equal to E^0’ according to Nernst equation (Eq1b).

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Ferricyanide in acetonitrile solution

Two types of systems have been studied previously and reference points have been made to both ferricyanide in aqueous solvent and ferrocene in acetonitrile solvent. Now it’s time to move on to basic measurements on ferricyanide in acetonitrile solvent. As previously stated, it is a well-known system and it is highly reversible, but it has also been stated that ferricyanide is highly dependent upon the donor properties of the solvent environment. Noftle and Pletcher goas as far as to say that ‘water is the unusual solvent’.¹⁰ The research is now moving into the oxidation of ferricyanide and the properties of the ferri (III)/ferro(II)cyanide redox couple in acetonitrile solvent. To help with this, two sets of reference points are already included in this paper.

Figure 8: Ferricyanide in acetonitrile solvent at different scan rates.

Figure 9: Peak current vs. square root of the scan rate for the

ferri(III)/ferro(II)cyanide redox reaction in acetonitrile solution under inert

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Eq 2b was used to calculate the two D0 values for ferricyanide in acetonitrile solvent. Because the intercepts are quite different, the D0 values for the oxidation and reduction peaks are quite different as well: D0(ox) for ferricyanide is 2.45*10^-6 cm² s^-1 and D0(red) for ferrocyanide is 4.36*10^-6 cm² s^-1. The different D0 values for the oxidation and reduction peaks means that when the reaction starts with 100% ferri(III)cyanide and the electrical current is positive, the electron transfer stage is kinetically slower than the ferro(II)cyanide reduction stage (when the electrical current is negative). The ratio of speed inequality is 2.45/4.36 for the

oxidation/reduction peak and 4.36/2.45 for vice versa.¹¹ The reference used for the D0 found has a different method: it starts the redox reaction by reducing ferrocyanide into ferricyanide.

This has implications for solubility as the ferrocyanide is not soluble with all solvents. For this reason, the D0 value is not fully comparable neither in magnitude nor as D0(red)/D0(ox) speed reference. The D0 for ferri(III)/ferro(II)cyanide redox reaction is: 1.607*10^-5 cm² s^-1.¹²

Figure 10: The ‘Trumpet plot’ in which all three reference points are referenced to each other.

Peak current vs. the logarithm of the scan rate is plotted. The ferri(III)/ferro(II)cyanide redox reaction in acetonitrile solvent under inert conditions is marked as blue dots.

Figure 10 and 11 establishes that ferricyanide will remain reversible within 500 mV/s scans, despite the large peak-to-peak separation. Normally, the threshold for a reversible limit is 57mV or 2.22RT/nF.¹³ Scans for 1000 and 1200 mV/s are not within the reversible limit.

This, together with the (unequal) D0 values, are the first clues to the mechanism of the ferri(III)/ferro(II)cyanide complex and it’s functions.

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Figure 11: Peak separation vs. Log10 of the scan rate for the ferri(III)/ferro(II) redox reaction in acetonitrile solvent and the two points of reference.

Ipa [mA]

Ipc [mA]

Epp [mV]

E1/2 [mV]

10 mV/s ^0.0053 ^-0.0058 ⁷⁰ ³⁵

20 mV/s ^0.0073 ^-0.0076 ⁷⁰ ³⁵

50 mV/s ^0.0113 ^-0.0122 ⁸⁰ ⁴⁰

100 mV/s ^0.0147 ^-0.0156 ¹⁰⁰ ⁵⁰

500 mV/s ^0.025 ^-0.0280 ¹⁵⁰ ⁷⁵

Table 3: Results chart from experiments on ferricyanide in acetonitrile solvent.

The E1/2 is roughly equal to E^0’ according to Nernst equation (Eq1b).

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Ferricyanide experiments in acetonitrile

Two types of experiments on ferricyanide in acetonitrile are described in the contects of this project: TYPE I and II. The potential range is quite large and the scans are performed in two different directions, so there are a few words that needs explaining. First, there are two areas of interest: Area L is the area of low, 0 to -1.3 V (negative potential) and Area H is the area of High, 0 to +1.3V (positive potential). Second, when scans are first run in positive direction of the potential field, they are called forward scans. A run in negative direction is called a backward scan. When two sets of scans are made of the same area at the same scan rate after another area has been scanned inbetween, the first scan is called a Control scan and the last is called a Rescan. When they are laid atop one another to mark the differences, it’s called an Overlap scan. The scan that was made inbetween is called a Main scan. When a number of

‘runs’ is given, it basically means ‘laps’ of the CV. This nomenclature is the same for all experiments.

TYPE I

Figure 12: Full scan of ferricyanide in acetonitrile solution with 10 mV/s scan rate.

Control scan were performed Area L to determine reversibility of the peaks. As Figure 10-11 established, the reversibility is acceptable for scans up to 500mV/s. This experiment were run with two scan speeds: 10 mV/s and 500 mV/s. The experiment is run to compare what

repeated scans in area -0.5 to +1.3V are doing to the reversibility of the Area L and what the differences are between 10 mV/s and 500 mV/s scans.

The scans performed directly from 0V to

-1.3V and back to 0V shows similar reversible peaks recognisable from ferricyanide in aqueous solution, see Figure 4, but at a lower potential range. The experiments went on with

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scans into the ‘Higher potential area’, ‘Area H’, defined as the area with the potential range 0V to +1.3V, for 2.5 forward scans.

Figure 13a and b: Reversibility scans with scan rates 10 mV/s and 500 mV/s scans of Area L.

In the Type I experiments, the Forward scans included the following settings: Start at 0V, up to 1.3V, back to -0.5V and then back to 0V again. This was cycled 2.5 runs and stopped at 0V before the reduction peak. The area had previously been scanned and at sufficiently fast scan rates a new peak that seemed reversible appeared somewhere around the 100 mV line. After those two CV:s (each run for 2.5 forward runs), the working electrode (W.E.) was cleaned to avoid interference from residue on the W.E. surface and then a full scan of both areas was run in forward direction, 1 full run. These scans are referred to in the CV:s as ‘Rescan’ tracks.

When the scans went from Area L to H (times 2.5) back to L again, the CV:s of Area L are laid on top of the first CV and are presented as ‘Overlap scans’ of respective area, see Figure 15 a and b.

The results from Figure 12 seems small if A, the peak potential, is the main concern. B and C is of far more interest in this CV, however as they are completely new peaks. As can be expected from the ratio of speed inequality between the oxidation and reduction diffusion potentials, the reduction peak is far larger than the oxidation peak. The 2.45/4.36 ratio seems correct. The Peak positions are -0.65V (oxidation peak) and -1.05V (reduction peak).

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Figure 14: Full scan of ferricyanide in acetonitrile solution with 500 mV/s scan rate.

A: 1) The first run: the area has changed shape. This can partly be attributed to the

relative current change going from 10 to 500 mV/s. At 10 mV/s scans, the area had a different shape caused by the stream of electrons, but not anymore. Were these electrons flowing through slowly, which would explain this change or is this a matter of a changed perspective?

2) The whole area is consecutively lowered for each forward run, including the peak potential.

B: The Oxidation peak at -0.65V is increasing in size at expectant rate.

C: Something unexpectant has happened here. The peak that was previously at -1.05V has moved and is increasing in size with each forward scan. It’s now at -1.29V. The peak had the middle of it´s forward peak at -0.97 mV and was growing with each successive run as the +0.9V to +1.3V area was losing size on the CV. To understand this, it’s better to look at the overlap scans as well.

Figure 15 a and b: 10 mV/s and 500 mV/s overlap scans of Area L. The new peaks are more noticable at the faster scan rate of 500 mV/s, which corresponds to a greater change in the lower reversible peak. This also corresponds to a change in the shape of the “beak area” of the whole +1.0V to +1.3V

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“Beak” area, as can be seen in Figure 14: Area H. The scans of the area -0.5V to +1.3V were run for 2.5 forward scans, which can be seen on Figure 12 and 14.

The overlap scans give a clearer picture of the new peak area. The reduction peak is much larger at 500 mV/s scans (see C). At Figure 15b, the 500 mV/s scans clearly show a change in peak potential for the Rescan (see A, D). At A the peak was reduced from 2.75*10^-5A to 1.93*10^-5A and at D the the peak potential was reduced from -4.84*10^-5A to -3.73*10^-5A. At 10 mV/s point D was reduced from 6.23*10^-6 to 6.0*10^-6A.

The overall picture of the overlap scans is that the amount of analyte in the cell has changed, as well as the composition of compounds.

Discussion

Defining the area between +0.9V to +1.3V

- Homogenous Chemical step: better known as a ‘C-step’

- Possible analyses: simplest solutions EC or CE area

- Analyte (A) is transformed into A⁺ and e^- normally, which is referred to as the ‘E- step’

“E-STEP”: A ⇌ A⁺ + e^-

Reaction 1 (nondisclosed reversible/irreversible process At the EC area, it undergoes a reaction into a Byproduct (B⁺)

“C-STEP”: A ⇌ B⁺

Reaction 2 (irreversible process by means of homogenous chemical reaction).

Under reversible “nernstian” circumstances, this need not cause problems for the replenishing of analyte at the electrode surface, because the forces of diffusion works effectively enough to replenish the material. According to the effect of change concentration (Le Chateliéurs

principle), the diffusion itself strives for concentration equality and works to keep the materials at equilibrium (as defined by the stochiometric properties of the surroundings).

A is being replenished at the electrodes surface at a rate depending on the Scan rate, which sets the limit for how fast diffusion must work by setting the scan speed and therefore the time limit for the chemical reaction.

Figure 16a: Diffusion works fast and

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Observations

- A to B⁺ seems to have a very fast reaction rate, which would explain why no current is showing up flowing on the underside of the CV at the +1.3V to +0.9V area.

- If it is an EC mechanism, diffusion could explain why the reduction peak at C has

”moved”.

- Normally, a way to figure out if there is an EC mechanism is to raise the Scan rate.

That will theoretically restore the current on the underside of the +1.3V to +0.9V area. But that theory is based on the assumption that the rate constant of the reaction for the C-step is sufficiently slow for this method to be possible. In fact, the scan rates to try this are very fast, 75000 mV/s to 250000 mV/s and must be performed with microelectrodes. So how can we figure this out as far as we can under current conditions?

“Assuming an EC area, can we figure out something new, like why the peak at location C in Figure 12, 14 and 15 a-b moves between the 10 mV/s and 500 mV/s scans?“

Explanation

When the analyte goes into the EC area at 10mV/s, the diffusion works accordingly fast. The CV for a single run of forward scan of Area H, including the 0 to -0.5 elongation, takes about 2 minutes to run and the diffusion works along that time to bring equilibrium to the small 5mL amount of fluid inside the Cell. The total time for the 2.5 runs of forward scans of 10 mV/s took about 6 minutes of time for a single run.

This can explain why the peak at point C is seemingly changing location. As the electron transfer process is working on building up the material as regulated by the forces of the electric field, the diffusion is working it’s natural process at the reducing peak, making it smaller and making it appear at another place than where it would appear at a faster scan rate. The actual place for the reduction shows up at the 500 mV/s scans, at -1.29V which means that the E1/2 for the new peaks is in the middle of the area.

The 500 mV/s took a few seconds to run. During that time, the diffusion is no less powerful a force, but it has far less time to work through the 5mL solution inside the cell. The EC area is connected to the kinetics of electron transfer through an electric field, which means that the kinetic transfer process will happen through the C-step of Reaction 2, another process than the diffusional induced reversible process of Reaction 1.

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The effect can be illustrated like this:

Figure 16a and b. Iillustrations of 10mV/s and 500 mV/s scans on time and diffusion rates and how it affects the concentration of Analyte on the Working Electrodes surface.

A comment on Figure 16b: it is a bit of a reach to claim that there is “no Analyte” (at all) left at the W.E. surface, but for illustrative purposes the picture has it’s uses.

If what we have are Fe(III) and Fe(II) complexes then we have a reasonable explanation for the difference in peak growth and reversibility:

The fact that the reducing peak is growing but that the oxidizing peak seems stable in size, might have something to do with the difference between the stronger π-e- donor qualities of the Fe(II) complexes than the Fe(III) complexes, suggesting that the ferrocomplex is not stable within it’s environment and that the irreversible product that comes from the C reaction is a result of ferricyanide turning into ferrocyanide, then undergoing the Reaction 2 (C-step).¹⁴

TYPE II

The same settings for Area L and H applies to this experiment, which means that the areas splits at 0V. In this experiment, the forward scans did not need to expand from 0V to -0.5V, because the area of focus was the Area L and the direct comarison between the Control and the Rerun scans. While Type I experimental settings focused on the changes on the peaks on both areas after several consecutive forward runs of Area H at 10 mV/s and 500 mV/s scan rates, Type II settings focuses on direct comparison of Area L after one Forward scan of Area H. First a Control scan of Area L is run, then a single Main scan of Area H is run and finally a Rescan of Area L is run. This experiment is made twice: one time for 10 mV/s and one time for 500 mV/s.

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Figure 17a and b: 10 mV/s and 500 mV/s overlap scans of ferricyanide in acetonitrile.

The 10 mV/s scan show more activity on the reduction peak than was expected from a single forward run by comparison with Figure 17b: A. 3.0*10^-6A peak potential change was certainly more from a single forward run than the Type I experiment showed from multiple scans. The difference between Figure 17a and b is small as well, amounting to differences that can be explained by the effect of the different amounts of current. These two scans shows no new oxidation or reduction peaks and neither do they show significant changes in peak potential.

The 500 mV/s scan shows a slight change between it’s Control and Rescan tracks, which is hard to interpret. The overall picture is that the amount of analyte has not changed to a great degree in the cell between the two scans.

Figure 17c and d: 10 mV/s and 500 mV/s scans of Area H taken in between the Control and Rescan tracks in Figure 17a and b.

Figure 17c and d shows the same approximate peak current and shape as in the beginning of the Type I scans, see Figure 12 and 14. This seems to suggest that:

1) The changes in peak current seen predominantly in Figure 14 are either depending on multiple consecutive forward scans or on the new oxidation/reduction peaks, as the changes are not apparent in Figure 17 a and b.

2) The conclusions that are to be drawn from Type II experiments seems to concurr with those from Type I.

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Conclusion

When examining the oxidation of ferricyanide in acetonitrile, an EC reaction was found to be at work coupled to the new peak with the middle of it’s forward peak at -97mV. The reducing peak undergoes buildup coupled to the chemical step.

Future analysis can reveal the identity of the oxidized form of ferricyanide.

Future work

Oxidation of ferricyanide in acetonitrile is an exiting subject that can be studied further. CV is a method that has scraped the surface under a short but constructive project.

Suggestions for moving forward with research:

- Simulations of the CV response

- Microelectrode CV using very fast scan rates

- Spectroscopy of the oxidized product with Bulk electrolysis including:

o FTIR spectroscopy, EPR spectroscopy and XRAY spectroscopy

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Reference list

1(a) Gutmann V., Gritzner G., and Danksagmuller K., Inorganica Chimica Acta, 17 1976 81; (b) Noftle R.E. and Pletcher D., Journal of Electroanalytical Chemistry, 1990 273, 293.

3L. Michaelis, Oxydations-Reductions-Potentiale Zweiter teil der

“wasserstoffionenkonzentration”, 100

5Noftle R.E. and Pletcher D., Journal of Electroanalytical Chemistry, 1990 274.

6N. Elgrishi K. Roundtree, B. McKarthy, E. Roundtree, T. Eisenhart, J. Dempsey, Journal of Chemical Education, 2017 199.

7Gutmann V., Gritzner G., and Danksagmuller K., Inorganica Chimica Acta, 17 1976 184

8Konopka S. J., McDuffie B., Analytical Chemistry 42 (14), 1970 1745

9A. M. Bond, K. B. Oldham, G. A. Snook, Analytical Chemistry 72 2000 3492-3496

10Noftle R.E. and Pletcher D., Journal of Electroanalytical Chemistry, 1990 276

11Compton R. G., Banks C. E., Understanding Voltammetry, 2nd Ed., 131, 133

12Gutmann V., Gritzner G., and Danksagmuller K., Inorganica Chimica Acta, 17 1976 83

13 N. Elgrishi K. Roundtree, B. McKarthy, E. Roundtree, T. Eisenhart, J. Dempsey, Journal of Chemical Education, 2017 199.

14Noftle R.E. and Pletcher D., Journal of Electroanalytical Chemistry, 1990 276

Oxidation of ferricyanide