I was first introduced to vegetative electric energy during my freshman year in high school, when our chemistry teacher showed a lemon battery at work as we were studying Electrochemistry. I got very intrigued with the idea of generating electricity with lemons, and I later learnt that many organic substances exist which can also produce electricity.
The next year of my high school, I heard about the global consumption of power, and how the earth's natural resources were getting depleted, and got an idea that if natural organic batteries were developed, the resources of the earth would remain stable. Thus I took up this study to help me explore the possibility of organic fuel cells and its capability of generating electromotive force.
For this study I will take potatoes into consideration because of their high acid content and the relative accessibility of it.
The acid responsible for the generation of EMF within the potato is Phosphoric acid, but my experiment will deal with what causes the change in the EMF when the potatoes are boiled. Initially I thought the cause lied in the varying concentration of the electrolyte, but upon further study and research found the reason to lie within the cell membranes of the potatoes that get ruptured during the boiling process of the potatoes, thereby varying the EMF generated.
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After maize, wheat, and rice, potato is the world's fourth most important food crop with an annual production of more than 323 x 106 tons with more than one-third coming from developing countries. Thus if potatoes do prove to be beneficial asset, it can able easily adopted by those who are lacking electrical infrastructure as part of the daily routine since it is cheap and requires no special skills for assembly.
How does the induced electromotive force generated from the potatoes depend on the state of the potato (i.e. Fresh potato vs. Boiled Potato)?
The first batteries were researched and invented by Volta when he made "a device capable of producing electricity by the mere contact of conducting substances of different species." The invention of "Voltaic battery" had marked the birth of a new era in the development of modern physics and made a significant change in our lifestyle. Battery technology has without a doubt seen progress, starting from it being dependent on organic/biological matters to it becoming more efficient using inorganic-reaction-based technology. However from the end of the 20th century, biological batteries were just a mere science experiment performed in highs school, however with the growing concern of depleting the earth's resources, there has been a new found interest in the development of organic fuel cells.
In order to highlight this growing interest, I have performed a study regarding the basic school experiment of a potato battery. For the first part of my study, I will perform the normal experiment by making a potato cell, using Zinc and Copper electrodes and recording the electromotive force (EMF) generated. Now, for the second part, I will boil the potatoes and record the readings of the EMF generated. I will compare the two results, and comment about my observations, and make possible conclusions about why there is a change in EMF generated or why there is no change in EMF generated.
BACKGROUND INFORMATION - ELECTROCHEMISTRY
Electrochemistry deals with the inter-conversion of electrical energy and chemical energy. This study will deal with the conversion of chemical energy into electrical energy (Electrochemical Cells).
An electrochemical cell mainly consists of two major components: left hand electrode (LHE) and the right hand electrode (RHE). In LHE, oxidation (loss of electron) takes place and is called the anode. In RHE, reduction (gain of electron) takes place and is called cathode. Anode is generally of that metal (or substance) which readily loses electrons (i.e. Oxidized easily). Cathode is a metal which readily accepts electrons (i.e. Reduced easily).
There are two specific ways in order to create an electrochemical, voltaic or galvanic cell.
Put the LHE (anode) into the solution of the electrolyte of the Cathode (containing the ions of the cathode). This allows the anode to loose electrons per atom and the ions present in the electrolyte accept the electrons. Thus, the cathode ions from the solution in this manner get deposited in order to form the metals of the respective cathode and the metal anode goes into the solution as ions. The reaction can be understood with two half-cell reactions:
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Oxidation M Anode (S) Mn+(aq) Anode+ ne-
Reduction: Mn+(aq) Cathode + ne- M Cathode(S) _ ___________________________________________________
Overall Reaction: M Anode(S) + Mn+(aq) Cathode M Cathode (S) + Mn+(aq)Anode
M Anode(S) is the element that gets oxidized at the anode,
M Cathode(S) is the element that gets reduced at the cathode,
ne- is the number of electrons lost/gained during the reaction
A rod of that metal is prepared and placed into one of its own solution in LHE to get anode. In RHE, a rod of metal that loses electrons less easily as compared to the metal of LHE is prepared and put into one of the solution to get the cathode. LHE and RHE are also known as two-half cells. Now the electrons move from anode (LHE) to cathode (RHE) and hence a current flow is maintained in the external circuit. This current flow is due to the fact that a potential difference is created this and this is called the E.M.F, electromotive force of a cell.
The two separate containers are connected by a inverted tube "U" shaped tube called as salt bridge. The salt bridge contains solution of strong ionic salts like NaCl, NaNO3 and KCl etc. (salts of most reactive alkali metals) soaked in colloidal solution of agar-gel which only allows movements of ions, not water. The role of the slat bridge is very important as it allows the continuous discharge of the cell. The salt bridge keeps the two solutions electrically neutral to one another. In the Fe-CuSO4 cell, in the left cell as Fe loses electrons, excess of positive charge in the form of Fe2+ is collected near the LHE and as Cu2+ ions gets discharged accepting electrons form Fe in the right hand cell, excess of negative charge in the form of SO42- is accumulated near the RHE. Now the salt bridge provides positive charge to RHE (in form of K+ ions) and negative charge to the LHE (in the form Cl-) and thus bringing about the neutrality of two solutions. If this does not take place, a reverse potential difference is created in the two compartments and thus breaking the continuous supple of voltage (current), which is the purpose of the cell.
The efficiency of a cell is determined by the tendency of LHE to loose electrons and the tendency of RHE to accept electrons. A measure of cell efficiency is called as electromotive force (EMF) or the voltage or the difference in potentials of two electrodes. EMF is defined as the difference in the potential across LHE and RHE to which electrons from anode travel to cathode.
My experiment consists of the above explanation with regard to a Secondary Battery or also called Galvanic Cell, which uses the main principles of the method mentioned above, but lacks a salt bridge but the cell membranes within the potato act as a salt bridge. The electrolyte in the potatoes is the phosphoric acid which does not actively participate in the reaction, since its main purpose is to make Zn loose electrons by oxidizing it, the potato provides the protons and the Cu plate remains unaffected by the acid bath.
My storage battery is the potato, with the anode plate is made up of Zinc (Zn), while the cathode plate is Copper (Cu). The electrolyte which initiates the reaction or makes the reaction possible in potatoes is phosphoric acid (H3PO4).
My experiment will involve the use of iron nails (Zn+2/Zn) acting as anode, and copper plates (Cu+2/Cu) as cathode.
These are placed in an electrically conductive solution that allowsÂ ionsÂ to travel freely between the two metals in this case potato. The acid steadily eats away at the Zinc, a chemical reaction that releases spare zincÂ electrons. These electrons then join with spareÂ hydrogen ionsÂ in the acid to create hydrogen gas.
Meanwhile, the copper remains unaffected even when submerged in acid but as soon as a conducting wire is connected between it and iron electrons flow from copper to Iron. The spare iron electrons are still intent on forming hydrogen gas, but they have an easier time doing it with the hydrogen surrounding the iron anode. So the electrons from the copper cathode travel through the wire to get to the iron. Batteries exploit this flow of electrons, therefore producing induced EMF.
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In most of the batteries, there is internal resistance which makes it impossible for the battery to produce 100% of its maximum potential difference. The same is applicable for the potato battery in the form of GAII (Galvanic apparent internal impedance, a trait related to both the salt bridge function of a given tissue delineated between electrodes and to the "battery internal resistance" properties). This electrical impedance can be a classified into further categories which is out of scope of this study. But the concept of GAII is useful as it can explain the relation between the EMF generated from a boiled potato as compared to a fresh potato.
Thus the EMF generated from one potato is because of the potential difference created by the electrodes as in the above mentioned cases. But since the number of potatoes remains constant, the reacting species also is constant, i.e. when two potatoes are used, each potato will have an zinc and copper plate, and thus when the zinc gets oxidized by the potatoes, same electrons will enter the iron electrode from the copper, thus EMF generated should be same. But this is where my experiment differs.
Experiments have already been conducted on fresh potatoes and the induced EMF but, I planned to boil my potatoes and observe the readings of the EMF generated and compare the results obtained from performing the experiment with raw potatoes. The potatoes by default will be similar and will be microwaved in KCl solution for scientific vigor, and then after certain attainment of room temperature, the EMF generated will be recorded. The readings and the graph will make clear weather the boiling of potatoes changes the EMF and what makes the EMF generated to change.
The induced EMF generated from the experiment being performed with boiled potatoes compared to raw potatoes will decrease since the concentration of phosphoric acid will decrease, since the potatoes are boiled in aqueous solution, thus diluting the already present phosphoric acid, and thus since the concentration of the electrolyte decreases so will the rate of oxidization and reduction, eventually leading to the decrease in the EMF generated. The GAII may also play a part since when the potatoes are boiled the inner temperature of the potato increases causing denaturation and this might affect the flow of electrons thereby affecting the EMF generated.
MATERIALS AND METHOD:
LIST OF ALL MATERIALS/APPARATUS
Name of Apparatus
Zinc plates of similar dimensions
Copper plate of similar dimensions
Vernier LAB QUEST
4.9 g of KCl
Commercially available potatoes were used throughout, due to ease of accessibility and for economic factors. The mineral composition of the potatoes has been given in Table 1 of the appendix. I compared the EMF generated from cells made of potatoes treats as follows
For the preparation of the Galvanic cell, the potatoes in both cases were cut into 5x2x2cm and were sandwiched between the Iron and Copper plates.
Potato Denaturation by Boiling
I compared the electrical energy generated from untreated potatoes compared to that of treated potatoes. For scientific vigour, I immersed the sliced potatoes in 1 mol dm-3 KCl solution and microwaved at 800W for 5 minutes.
Measurement of EMF
The amount of EMF (V) generated was evaluated using a Vernier Lab Quest connected to the cell. The measurement was also taken for Current (I) and Power (P). These measurements were taken over a period of 2 hours over a constant load of equal resistance. In order to prevent the potato coming in contact with air it was covered with Parafilm in order to reduce drying and oxidation.
The independent variable in this experiment is the potatoes, or the state of the potatoes i.e. boiled or fresh. Thus the experiment will be carried out with fresh potatoes, and then further into boiled potatoes.,
For similar concentration, and volume of acid in potatoes, similar sized potatoes were taken so that the result will not deviate
Set diagram for potatoes is
As you can see, the potatoes act as independent batteries, providing induced EMF as they are connected in series. The reason they act as a battery is because the copper and zinc electrode undergo redox reactions in the presence of the acid which acts as an electrolyte, which creates a potential difference and this is calculated to be EMF
The dependant variable is the EMF generated by the potatoes when arranged in series.
It will be measured with a Vernier Lab Quest which is connected to the computer
The potential difference will be calculated, between the two extremes of the electrodes (anode and cathode => Zinc and copper plate). This given criteria is same for both the set up.
The unit of measure is the Volt. The readings will be taken for two hours for each.
The apparatus used was same throughout the experiment, since this will reduce mean deviation and the calculations will be done with respect to the other readings therefore, error is less
The temperature in the room was controlled and was kept at 300K and this is with respect to the room temperature and not the temperature of the potato.
The arrangement of the potatoes and the beakers was done in series since that would accurately judge between the EMF discrepancies between boiled and unoiled potatoes.
Similar sized potatoes were taken in the hopes that the concentration of phosphoric acid would be similar; therefore the readings will not have much discrepancy relative to each other.
When the potatoes were boiled, all were boiled to the same temperature, for the same amount of time, and were removed from the water bath at approximately the same time
The apparatus was cleaned thoroughly before performing each experiment so as to reduce discrepancies in the readings, with respect to other readings.
The amount of insertion of the Iron and copper into the potato was same throughout all the experiments at 3Â±0.1cm.
The potatoes were all sliced up into the following dimension 5 x 2 x 2 cm and were sandwiched between the electrodes.
The part of the potato exposed to the air was covered with Parafilm in order to prevent the potato from drying and reduction.
The copper plate and the iron nails used were the same throughout the experiment, so was the location where the experiment took place so as to keep all errors due to pressure and temperature constant.
The same water bath was used to boil the potatoes, in order to keep the potatoes at constant temperature with regard to each other.
The time taken for recording the EMF generated from the potatoes in both cases was taken as 2 hours.
DATA AND GRAPHS
ACTUAL REACTIONS TAKING PLACE
Zn: Zn Zn++ + 2e- , E0 = 0.76V,
Cu: 2H+ + 2e- H2 , E0 = 0.00V
Zn + 2H+ Zn++ + H2, âˆ† E0 = 0.76V
REASON FOR THE REACTIONS AND ANALYSIS OF DATA
My results conclude that Zn electrode and the reduction of hydrogen at the Cu electrode are the dominating reactions which give rise to EMF, Current Density and the potential difference.
Maximum power delivered by boiled potato cells with ruptured membranes may reach values an order of magnitude higher than that generated by untreated potato. When the data was compared, a direct relationship between the ability of the potato battery to deliver power and GAII (Galvanic apparent internal impedance, a trait related to both the salt bridge function of a given tissue delineated between electrodes and to the "battery internal resistance" properties) becomes evident. The significant increase in electric energy generation with membrane destruction shows that the ionic diffusivity through the tissue bridge between electrodes is the reason behind this phenomenon, as effective diffusivity of protons increases with membrane rupture. In contrast, the rate of proton flux is reduced when cell membranes are intact probably due to the tortuosity of the extracellular space as well as the equivalent reduction in the concentration of the electrolytes per unit volume when the intracellular fluids do not actively participate in the ionic transport.
From the data and the graphs it is clearly visible that my hypothesis was inaccurate, since the EMF generated did not decrease with the boiling of potatoes, but increased and also lasted longer under the same external load compared to the fresh potato. The potato serves only as a medium for the movements of electrons from the zinc electrode. The potato supplies the protons thus generating electricity. Fresh potatoes do it, but the strong internal resistance makes it very inefficient. Boiling the potato destroys membranes and possibly some part of the cell walls, thus reducing significantly the internal resistance and increase 10 folds the generation of power. The bio electrolytic low power electrical energy source introduced in this study brings an dimension to the utilization of the globally fourth most abundant crop accessible essentially all over the world, made of solid components and requires low financial investment compared with solar or conventional batteries.
The experiment was conducted in a non-ideal conditions which could lead to errors:~
The Parafilm had foreign bodies or had an unwanted flaw which could have not given me an accurate reading
The reading of the electronic balance may also have a manufacturing defect, thereby leading to a difference in the times taken.
The lab quest may be defective or may have been inaccurate which may have given inaccurate results.
The microwave may not have operated throughout the five minutes at 800W, thus leading to a variation in the temperature achieved by the potato in order to break the cell membrane.
There might have been a gap or hole in the Parafilm leading to increased drying of the potato thereby affecting the EMF generated.
Human parallax error when adjusting the volume of the solutions by taking only the lower meniscus.
The apparatus used may contain remnants of other chemicals leading to an impure solution.
The temperature of the room was taken to be constant, but there might have been fluctuation in the actual temperature thus leading to heat loss, and null results.
The electronic balance might not have been zeroed out to take the new reading or might have had impurities which could have given inaccurate readings
The microwave may not have run for exactly 5 minutes, thus leading to different boiling degrees
EMF of the potato was taken every 3 seconds from the start of the reaction and thus the increase/decrease would not be exactly accurate, leading to a discrepancy in data.