Science Fair Idea - I can't choose!

[irregular]

Hi Donna!

1. Thanks! Good news - it worked! At first I was getting really hopeless. I realized that the instruction was asking for the resistance of the copper wire and carbon, while I was testing the aluminum and the carbon! I then measured the resistance of the copper wire and electrode - nothing. My dad explained that one end of the copper wire must touch the electrode, so there is a flow. We played around with that, and finally... success! Yay!

2. Wow, that's a marvelous explanation! I will make sure that I comprehend it and include it in my hypothesis! I will continue to do a little more research on acetate as well for supporting facts.Using another substrate like you recommend would be amazing!

3. Okay, so we have guests over tonight,again! They're are coming in about half hour though, so I have time to start right now. I am getting starting in about 2 minutes. Wish me luck!

Thanks!

[irregular]

Hi Donna!

(read my post above before this one).

Ahh, I was sounding so positive in my previous post. My outcome wasn't that positive, however. I quickly had to prepare the conductive solution for the cathode, connect the alligator clips and the copper wires, turn on the air pump, add the methylene blue, add the bacteria, all in a rush. The bacteria was used nearly 24 hours after collection. The mud didn't become blue, as some projects showed the anode looking blue. Well, the container is an opaque red, I can see through but with some opaqueness. It's been about 10 minutes from startup time. In separate periods, teh multimeter is showing 0.1-0.7 millivolts of power. For the majority of the time, it stays as 0 millivolts.

I'm not sure what to do - the MFC is producing power (though little), although I'm scared that there may be something going wrong, as the bacteria was exposed to a good amount of oxygen when my Dad and I were setting up.

A few notes:
1. The anode is not full to the time, so there is oxygen, although I think that the mud is on the bottom of the water, so it may not be exposed to the oxygen.
2. In the cathode, the lid is not on very tight as the air pump tubing needs space to enter.
3. The salt bridge is not touching the mud solution.
4. I will read, right now, the results of other students who've made the MFC, and am going to check if my MFC is producing significantly low voltage than others. If something is wrong, I might consider re-doing my MFC...
5. In this MFC I have not added anything. So, I guess this would be the organic compounds.
6. When I move the multimeter around or apply pressure to the leads of the multimeter, the voltage on the multimeter starts varying up to 3 millivolts.
7. The resistance shows infinity.

Thanks!

[donnahardy2]

Hi Irregular,

Your set up sounds OK, except the infinite resistance is a problem that you need to solve as soon as possible. Your MFC may not start producing power right away because the bacteria will have to equilibrate in their new environment and start growing. However, you will never be able to measure power produced because the resistance is infinite and this means there is a break in you circuit. Maybe the electrodes are not in contact with the copper wire. Maybe the problem is the salt bridge. Use your multimeter to isolate the problem, and see if you can make the connection.

Donna Hardy

[deleted-71588]

Try putting the DVM on a DC microAmp scale to measure the current flow. What do you get?

For the MFC to supply power, it needs to have a way for current to flow from the annode wire to the cathode wire external to the apparatus. Note: because electons carry a negative charge, the electrons are flowing from the cathode to the annode externally.

With the meter on a DC voltage scale there won't be any significant current flow so the bio-chemistry won't be operating to supply power. Not to say that you don't want to occasionally measure the voltage (aka open circuit voltage), you just don't want to leave it that way for long. With the meter on a DC current scale, the meter will be a low resistance path between the annode and cathode so that the MFC will operate as intended and you can leave it that way most of the time.

Another interesting measurement would be to put the DVM on the highest Ohms (resistance) scale and measure the resistance between the two electrodes. Measure it and then reverse the leads and measure it in the opposite polarity. Don't leave it connected this way long, just long enough to take the readings.

Knowing these resistance measurements will help me understand better what may or may not be happening.

[irregular]

Hi Donna and Craig!

Yes, you're right about the infinite resistance. I thought that the problem was with the copper wire, as I just wrapped it around the electrodes. My thinking changed when I tested the resistance between the copper wire and electrode itself - both electrodes individually- and there was some resistance! When I tested the resistance between both copper wires, there was still infinite resistance. This was because there was oxygen exposure involved and there wasn't a path for the electrons to travel. So, I had to secure the connection between the salt bridge and anode/cathode using duct tape, electrical tape and teflon tape.

I knew that the salt bridge had to be in contact with the mud/water solution, or else the protons won't be able to flow (pictures of MFCs show this as well). So, that meant that I turn the MFC and lay it down, horizontal instead of vertical.

I added another 1 1/2 cup of distilled water, put the mud back in, and started up the MFC. I am not looking for the voltage/current right now, just for a secure MFC and to make sure that there is not infinite resistance.

As for the current in my previous set up yesterday, the current was low. I tried again today, and it ranged from 001-006. Oh, so I think I understand better now. I need to have the multimeter on the current scale.

I will get back to you on the highest resistance scale test.

Thanks guys for your help, thoughtfulness and patience!

An additional question:
- Will I be able to reuse my electrodes, salt bridge, and distilled water for my next experiment?

[donnahardy2]

Hi Irregular,

It sounds like you have a working unit, so this is good, even though the current is low. It might increase as the bacteria start growing in their new warmer environment. Please let us know what the resistance and voltage measurements are, although do follow Craig’s precaution and don’t leave the multimeter set on voltage longer than necessary to take the measurement. If you have current and either resistance or voltage measurements, you can calculate the electric power (watts) that your MFC is producing.

http://en.wikipedia.org/wiki/Electric_power

You will have to look at your electrodes and salt bridge and decide if they are reusable. Salt bridges fail on a regular basis, so look for any breaks in the agar after you have used it. The anode electrode will have a biofilm (slimy layer of bacteria) on it at the end of the experiment and this will allow the bacteria to start growing again quicker when you add new mud and an extra carbon source for the next experiment. I don’t think it would be possible to clean the biofilm from carbon paper.

Let us know about your results.

Donna Hardy

[irregular]

Hi Donna!

Okay, so after 24 hours of no leakage, I figured out that this MFC design is effective. Since I was out of town 12 hours after the setup, my first noting was this morning, so 24 hours. Measurements:
Voltage - 92 (on 200mV)
Resistance - 1272 (on 2000 ohms)
Current - 0

As you see, there is no current, and there is very high resistance (like you warned me). My main problem is the current + resistance. How do you recommend that I reduce the resistance, and gain a current to produce electricity?Thanks for including the Wikipedia page, I will take a look at it right now.

So I will check my salt bridge after use. As for my electrodes, if I reuse them, it sounds as if it will not be a controlled variable. I will make new electrodes.

Here is my plan: This MFC right now does NOT have an aquarium air pump attached, and is still working.
1. no added substrate (organic compounds), no air pump
2. no added substrate (organic compounds), air pump, solution for high resistance
3. substrate (acetate), no air pump
4. substrate (acetate), air pump, solution for high resistance

If this doesn't work out, then:
1. no added substrate (organic compounds), no air pump
2. added substrate (acetate), no air pump
3. added substrate (formic acid, propionic acid or butyric acid), no air pump

If I have extra time, I will add...
5. added substrate (formic acid, propionic acid or butyric acid), no air pump
6. added substrate (formic acid, propionic acid or butyric acid), air pump, solution for high resistance
... to my original plan

Thanks, wish me luck!

[donnahardy2]

Hi Irregular,

Your measurements are interesting; your MFC is working because you have voltage, but the resistance is so high that no current can flow. You either need to increase the metabolism of the bacteria so you will have some current to measure, or decrease the resistance.

If you don’t have the air pump in the cathode chamber, you might be limiting the amount of oxygen necessary to convert the electrons and protons to water. Since your background reading has indicated that oxygen is needed in the cathode chamber, I would recommend going ahead and adding the air pump to the cathode chamber. This might increase the flow of electrons (current).

Do you have another glass tube available? You could also try switching to a difference salt bridge to see if that will help, or just add another salt bridge to double the area of agar available.

What temperature is it in the furnace room? Is the MFC located at the warmest spot in the room (remember that warm air rises)?

When is your project due? You need to have a few days for each experiment, and unless you are planning to set up multiple MFC’s (which is a lot of work), I recommend not doing the no pump experiments. You want to plan to stop your experiment a few days before the project is due so you will have time for complete data analysis.

Donna Hardy

[irregular]

Hi Donna!

To increase the metabolism, I will add leftover blended kitchen scraps (banana peels,etc), which will act as a substrate. I am doing some reading on decreasing the resistance of the MFC. I will also find a way to include the air pump into the cathode. I will look at applying these three changes after 36 hours of running the MFC. To keep this a controlled variable, I will have to do the same thing with the acetate MFC.

Unfortunately, I do not have an additional glass tube - I checked with my school as well, they just have plastic tubing.

The MFC is placed at the warmest area of the furnace room, close towards the furnace. The temperature is approximately 22 degrees Celsius.

The fair is on April 8+9. The abstract is due on April 2. I am looking at finishing around the 27 of March. You're right, I will need some extra time. Here is my revised plan:th

1. added substrate (organic compounds), resistance decrease, air pump input after 36 hours
2. added substrate (acetate), resistance decrease, air pump input after 36 hours

With the extra time, I will preferably..
3. added substrate (formic acid, propionic acid or butyric acid), resistance decrease, air pump input after 36 hours

If not possible, I will repeat my experiments to ensure proper results.

Thank you!

[donnahardy2]

Hi Irregular,

I recommend that you don’t add the kitchen scraps for this is experiment. The current run is your control experiment with no added nutrients, but you should try to optimize the MFC for maximum power conditions. You could do a kitchen scrap experiment for the next run, or do the acetate, but you are not going to have time to do very many experiments. You do have measurable results with this set up, so you will be able to measure the effect of the added nutrients with your next set up. Go ahead and give this MFC a few more days to produce power. Your temperature is close to optimum, so you can’t do anything with the temperature. Let us know what tomorrow’s readings are after you add air to the cathode.

Setting up the MFC and having it actually work is a major accomplishment, so congratulations on your results so far!

Donna Hardy

[irregular]

Hi Donna!

I took your advice and decided not to add the kitchen scraps. Eager, throughout the day I took measurements, just for the sake of enjoyment. I noted that the mV went down, and came into the negative numbers!

I knew that this must've been since there is no apparent substrate, and also because there was a small leak. To fix the leak, my dad and I tried un-taping and re-taping duct tape several time. Then, I thought of using a candle to create heat between the glass and the plastic. The wax didn't really help. Finally, my dad mentioned that we have something called a silicon caulking gun, kind of like rubber soldering. We applied that and we will let it "cure" and solidify for a while. Hopefully it works.

Also, I have completed my "How a Mediator Microbial Fuel Cell works" section. I may make additional edits later on. Thanks for previously offering to read it and provide some advice. Here it is, in the quoted section:

How a Mediator Microbial Fuel Cell Works
The first and foremost step in the production of electricity from a Microbial Fuel Cell is the feeding of bacteria, using an electron donor (substrate). This substrate is oxidized by the bacteria (without the involvement of oxygen), since the bacteria liberate electrons in the energy-rich substrate (Bennetto, 1990). The electron leaves the substrate as hydrogen, which reduces into a proton and two electrons (Schwartz, 2007). “…This process is of course a complex one involving many enzyme catalyzed reactions. It progresses through a series of intermediates involving successive oxidation-reduction changes, and in this respect resembles an electrochemical process” (Bennetto, 1990).

An electron acceptor (mediator) invades the membrane of the bacteria, and “steals” the electrons and protons in the bacteria. Thus, the mediator undergoes reduction, since it carries electrons. The mediator, along with the electron(s) and proton(s), departs the bacterium. It shuttles back to the electrode in the anode chamber and releases the electrons and protons. Hence, the mediator undergoes oxidation, although the electrode becomes negatively charged with the electrons (Bennetto, 1990). Anaerobic conditions are needed or else the oxygen will collect all the electrons, since it has a greater electronegativity than the mediator (“Microbial Fuel Cell”).

The electrons pass through the electrode, into an external circuit, and arrive at the electrode in the cathode chamber (creating a positive current). Usually, there is a load in the external circuit which the electrons may power (Logan et al., 2006). This electrode in the cathode chamber becomes the positive electrode. The protons travel through an agar salt bridge or a proton exchange membrane, and then arrive in the cathode chamber as well.

Both the electrons and protons meet in the cathode (Schwartz, 2007). If oxygen is present in the cathode chamber, as an oxidizing agent, water will be produced (Schwartz, 2007). This occurs since a hydrogen ion (a proton) and an oxygen molecule combination result in a water molecule. This process continues until all substrate is consumed.

A Mediator Microbial Fuel Cell simply differentiates from a Mediator Microbial Fuel Cell since it does not have a mediator to aid in electron transfer. There are various ways how electrons may be transferred. The Microbial Fuel Cell may contain electrochemically active bacteria that can transfer the electrons directly from the bacterial respiratory enzyme to the electrode (“Microbial Fuel Cell”). Some developing research identifies nanowires to be one of possibly more electron transfer methods.

* According to the internet, propionic acid is in milk (http://www.answers.com/topic/propionic-acid) and swiss cheese (http://www.webexhibits.org/butter/glossary-pr.html), butyric acid is inbutter, and the following link showed various definitions for formic acid (http://www.answers.com/topic/formic-acid). These would be interesting to use!

Thanks so much for your effort, encouragement and help!

[deleted-71588]

Using Ohms Law: 92 mV / 1272 Ohms means you have about 76 uA of current. I suspect that your DVM either doesn't have a sensitive enough DC current scale or you weren't on the most sensitive current scale.

What model DVM are you using? That will help me know what your DVM is capable of.
Did you get the same reistance reading in both directions? Knowing this will determine what modifications to your setup are appropriate.

[irregular]

Hi Craig!

The most sensitive DC current scale I have is 200 uA, and that is at which I have been measuring from.
Here is a link which shows some information on my multimeter:
http://www.made-in-china.com/showroom/v ... -838-.html

What do you mean the same resistance in "both directions"? Switching the leads to opposite copper wires?

Thanks Craig!

DONNA: My previous message to you is above. Thanks!

[deleted-71588]

The most sensitive DC current scale I have is 200 uA, and that is at which I have been measuring from.

Following the link to the DT-838 specifications page: DCA: 2m-20m-200m-10A ± 1. 0%
The picture does show a 200uA scale which should have been sufficient to get a current reading based on the other data. I'm guessing that the 200uA scale is not +/- 1% accurate and they just left off the specification for how accurate it is. In any case, you should be seeing a current reading based on Ohms law unless the resistance of the MFC is dependent on current direction and the resistance in the expected direction of current flow is at least an order of magnitude larger than the value you gave.

What do you mean the same resistance in "both directions"? Switching the leads to opposite copper wires?

Yes. Assuming you use a red test probe in the red jack and a black test probe in the black jack, On the Ohms scale, the black lead will be the common to the negative terminal of the internal 9 volt battery and the red test probe will be connected via an internal resistance based on the scale that is in series with the external circuit you are measuring the resistance of. The meter acutally measures the voltage drop across the internal resistance to determine the value displayed. If the entire battery voltage is dropped across the internal resistor, the external resistance must be 0 Ohms. If negligible voltage is dropped across the internal resistance, the external resistance is significantly larger than the internal resistance and an "open circuit" indication is given.

By switching leads, you determine if there is anything in the external circuit (outside the meter) whose resistance is sensitive to the direction of current flow. In the case of your MFC, there are several components that can be sensitive to the current direction. That is why I asked to know the resistance measured in both current flow directions.

[donnahardy2]

Hi Irregular,

You have done a really good job in putting all of the information together, but I think you need to simplify it to make it clearer for your readers. Most of your audience will be reading about microbial fuel cells for the first time, so I would avoid adding too much detail. Here are my suggestions:

How a Mediator Microbial Fuel Cell Works

First paragraph: Just describe what an MFC is; try to put the following ideas into your own words. I have fused the basic facts from your first and third paragraph here as a suggestion:

The purpose of a microbial fuel cell (MFC) is to capture the electrons produced by metabolism of bacteria to produce electricity. Bacteria grow under anaerobic conditions in the anode chamber of the MFC and oxidize organic compounds to obtain energy for cell metabolism and growth. The end products of this complex enzymatic process are electrons and protons, and this process continues until all of the organic fuel in the anode chamber is consumed. The electrons are moved through an external circuit and the protons travel through a salt bridge and combine with oxygen in the cathode chamber to produce water. The movement of electrons through the circuit produces an electrical current, which can be measured and used like a standard battery to power a load (an electrical device).

Second paragraph: An electron acceptor or mediator, such as methylene blue, is used in an MFC to facilitate the transfer of electrons from the bacterial cells to the anode electrode. The mediator molecule acts as a shuttle and is continuously recycled as it is reduced when accepting the electrons from the bacteria, by direct membrane contact, and is then is oxidized when it transfers the electrons to the anode electrode.

Third pargraph: (great paragraph!) A Mediator-less Microbial Fuel Cell simply differentiates from a Mediator Microbial Fuel Cell since it does not have a mediator to aid in electron transfer. There are various ways how electrons may be transferred. The Microbial Fuel Cell may contain electrochemically active bacteria that can transfer the electrons directly from the bacterial respiratory enzyme to the electrode (“Microbial Fuel Cell”). Some developing research identifies nanowires to be one of possibly more electron transfer methods.

4th paragraph: The bacteria used in an MFC are usually freshly collected samples from the benthic layer of fresh water lakes. This is a good source for mixed cultures of anaerobic bacteria that use a variety of organic and inorganic compounds as terminal electron acceptors for respiration. The use of mixed cultures results in higher power production compared to using pure cultures.

Insert a drawing of a bacterium showing the steps involved in metabolism to produce protons and electrons.

Insert a drawing of an MFC with all components labelled: This will help your readers understand what you are talking about. You can add a description of the function of each part of the MFC.

5thparagraph: A brief history of MFC’s.

6th paragraph: Applications of MFC’s.

Donna Hardy

[irregular]

Hi Donna and Craig!

CRAIG:

My dad and I looked at the caulking compound we applied and it solidified. However, when we laid the MFC down, it leaked. We applied duct tape for security and laid it down again. My multimeter thankfully showed some measurements (Voltage: 100mV, Resistance:1030, Current: 0).

Unfortunately, the resistance is so big that no current is there. Also, the MFC started leaking again. My dad and I made a PVC pipe salt brige to test and see if it might provide better results than the glass salt bridge. At this stage, my dad and I will ultimately try anything and everything possible.

As for the lead test, I switched leads. With the black lead on the anode side, the measurement with the lead on the red side was obtained, although in a negative number. For example, with the red lead on the anode side I obtained 130mV. With the black lead on the anode side I obtain about -130mV. Current and resistance stay the same. Does this seem right to you? What can you conclude? Okay, does the red lead signify the internal resistance, and black lead signify external resistance? So this will be an open circuit, right?

DONNA:

Thanks so much! When I was writing this section, I was aiming for a chronological format of the section. It seems as if you're looking at more of a brief explanatory type, then the mediator details below. I made my edits according to chronology one I had in mind. Is this fine to you, or do you want me to change the format of the section into your layout?

I removed all direct quotes although included in-text parenthesis references to keep my referencing in order. I edited based on your suggestions. As for the anaerobic detailed paragraph, I decided to merge this and some previous information I had into one section in the glossary for the bacteria. Here it is:

Bacteria are one of the earliest origins of life on Earth. They are microscopic unicellular organisms which we come in contact with daily (“Bacteria”). However, they are yet incredibly interesting. They can produce protons and electrons through reactions with the consumption of a variety of substrates.

Since the Microbial Fuel Cell uses the protons and electrons to produce electricity, bacteria are a good source. There are many sources where bacteria is obtained from, including industrial wastewater, benthic layer anaerobic samples in lakes or streams, grown cultures or mixed cultures.

In this Microbial Fuel Cell, freshly collected samples from the benthic layer were obtained, since a water mass was readily available and doable. This source conatins mixed cultures of anaerobic bacteria, which is proven to be an excellent source of energy-producing bacteria compared to pure cultures.. Mixed cultures may use a variety of organic and inorganic compounds as electron acceptors for respiration.
Anaerobic bacteria and anaerobic conditions in the anode chamber of the Microbial Fuel Cell are needed or else the oxygen will collect all the electrons, since it has a greater electronegativity than the mediator (“Microbial Fuel Cell”). Temperature is dependant on bacterial growth as well. Bacteria usually have an optimum temperature which they grow with the best.

As for pictures of the bacterium, here is what I have found. A few of these look relatively complicated which I could explain simply using a caption:
http://www.microbialfuelcell.org/www/in ... nisms.html
http://1.bp.blogspot.com/_DZH2cmCoois/R ... cteria.jpg
http://1.bp.blogspot.com/_DZH2cmCoois/R ... cteria.jpg

There are many MFC drawings which are available on the Internet which I will research on. Alternatively, I could draw an MFC myself using the simple Paint program on Windows. I will be sure to briefly explain the components.

QUESTION: Where should I incorporate the scientific/mathematical process such as conversion of a substrate into electrons and protons? (e.g. C12H22O11 + 13H2O ---> 12CO2 + 48H+ + 48e-..... from wikipedia)

EDIT:

* According to the internet, propionic acid is in milk (http://www.answers.com/topic/propionic-acid) and swiss cheese (http://www.webexhibits.org/butter/glossary-pr.html), butyric acid is inbutter, and the following link showed various definitions for formic acid (http://www.answers.com/topic/formic-acid). These would be interesting to use!

Do you think that these would be appropriate for each acid, which would be usable in an MFC?

Thanks both of you very much!

[donnahardy2]

Hi Irregular,

Your results are great; your current has increase to 97 uA! And you have decreased the resistance in your MFC. I think maybe your bacteria are starting to grow and metabolize a little faster. Maybe you will get into a positive current range soon. Trying a new salt bridge sounds like a good idea. Be sure to say thanks to your Dad.

I really like the new paragraphs you have written for you background paper. I have added comments in parentheses below:

Bacteria are one of the earliest origins of life on Earth. They are microscopic unicellular organisms which we come in contact with daily (“Bacteria”). However, they are yet incredibly interesting. They can produce protons and electrons through reactions with the consumption of a variety of substrates. (If you want to, you could add more information about bacteria from http://en.wikipedia.org/wiki/Bacteria that are related to your project. They are prokaryotic (lack cell organelles) .3 to 5 microns in size, and typical concentrations in soil are 40 million in a gram of soil)

Since the Microbial Fuel Cell uses the protons and electrons to produce electricity, bacteria are a good source. There are many sources where bacteria is obtained from, including industrial wastewater, benthic layer anaerobic samples in lakes or streams, grown cultures or mixed cultures. (The bacteria produce electrons and protons as a result of energy production, and they require an electron acceptor to eliminate the electrons. In an MFC, the anode electrode is used as the electron acceptor). (The formula C12H22O11 + 13H2O ---> 12CO2 + 48H+ + 48e is the general equation for respiration of a sugar by plants and animals. Your anaerobic bacteria are not using this pathway, but you can include this to show where the electrons from an MFC come from. This information will fit into this paragraph.)


In this Microbial Fuel Cell, freshly collected samples from the benthic layer were obtained, since a water mass was readily available and doable. (this sounds like you are starting your procedure section; just say that the benthic mud from a fresh water lake is a good source of bacteria for MFC’s. ) This source contains mixed cultures of anaerobic bacteria, which is proven to be an excellent source of energy-producing bacteria compared to pure cultures... Mixed cultures may use a variety of organic and inorganic compounds as electron acceptors for respiration.
Anaerobic bacteria and anaerobic conditions in the anode chamber of the Microbial Fuel Cell are needed or else the oxygen will collect all the electrons, since it has a greater electronegativity than the mediator (“Microbial Fuel Cell”). Temperature is dependant on bacterial growth as well. Bacteria usually have an optimum temperature which they grow with the best. (Great!)

I like the diagram in http://www.microbialfuelcell.org/www/in ... nisms.html, as it shows how the electrons and protons are produced at the cell membrane of the bacteria. You can include this picture with a citation because it will help your reader understand how an MFC grows.

You should draw your MFC, or take a picture and add diagrams. Labeling the function of each part of the MFC will help readers understand the MFC.

For your substrate choice, it would be better to see if you could find a sample of pure propionic acid or pure butyric acid, but I don’t know if that would be possible. If you add butter and Swiss cheese, the whole foods will contain other molecules. White vinegar is composed of acetic acid only, so you will be adding pure acetate if you add vinegar.

Donna Hardy

[deleted-71588]

As for the lead test, I switched leads. With the black lead on the anode side, the measurement with the lead on the red side was obtained, although in a negative number. For example, with the red lead on the anode side I obtained 130mV. With the black lead on the anode side I obtain about -130mV. Current and resistance stay the same. Does this seem right to you? What can you conclude? Okay, does the red lead signify the internal resistance, and black lead signify external resistance? So this will be an open circuit, right?

Switching leads on voltage or current scales should simply change the polarity + to - or - to + so this is as expected, but more than I asked for. Note: +0 and -0 are the same so current reading of 0 is what it is. Does not make sense to me why it isn't non-zero.

Resistance being the same for both polarities is what I expected; however, that further confounds me on why you aren't seeing any current reading on the 200 uA scale.

This is telling me that your high resistance value isn't caused by something that is affected by the directional flow of current (or electrons in the opposite direction). Since the ion exchange in the cathode and annode chambers should be slightly sensitive to polarity, it is telling me that most likely your high resistance is coming from the salt/agar bridge.

The ways to reduce resistance (increase conductivity) are:
1) Use a larger diameter salt/augar bridge - conductivity increases proportional with the cross section area
2) reduce the length of the salt/augar bridge while making sure both ends are completely in contact with solution - resistance increases proportional to length
3) Add more salt to the augar in its preparation to increase the conductivity of the material itself

With an existing salt bridge, the only way to decrease the resistance is to remove augar/salt from one or both ends to decrease the length. The danger in doing this is you might loose the seal the augar makes with the inside of the tube and you might have to re-melt it to form a new seal.

[irregular]

Hi Donna and Craig!

DONNA:

Thanks so much for reviewing my bacteria explanation! It really helps a lot! My only concern is regarding the respiration section. I am planning on creating a separate term for "anaerobic respiration" in the glossary, underneath "Bacteria". I will merge the section relating to anaerobic respiration in bacteria and put it in a new term.

I will write my entire research report and then possibly present it to you, as it is taking a lot of my time to write, edit. It would be faster if I could write everything roughly, and then edit as a whole. I would like to finish writing by Sunday, and have editing done MAXIMUM by the next weekend, alongside my experiment.

How could I use this picture (http://www.studentsguide.in/microbiolog ... thways.jpg) to explain anaerobic respiration? What do you think of the reference itself (http://www.studentsguide.in/microbiolog ... ation.html)?

I will look into drawing/taking a picture of my MFC and the substrates

CRAIG:

Thanks so much! The reason why I wasn't obtaining a current is because my lowest Amperage measurement is 2000uA, not 200uA. I will have to calculate current with Ohms Law.

DONNA AND CRAIG:

My dad and I will restart and make a new MFC with a salt bridge with a larger diameter, shorter length and more salt, involving the PVC pipe. Another student has used a similar pipe and was successful. We will start up the new one tomorrow with acetate - the first variable. How much vinegar do you recommend I put in?

I will be trying to do more work these days, please don't mind if I reply a little later than usual. Thanks so much, you guys have been helping immensely.

[deleted-71588]

The reason why I wasn't obtaining a current is because my lowest Amperage measurement is 2000uA, not 200uA.

Even if you aren't, I'm still puzzled. The ohms law calculation for 130 mV / 1272 Ohms = 102 uA. (102 uA / 2000uA) * 100 = 5% of full scale so something yet to be explained is happening.

The lowest DCV scale is 200 mV full scale. If we assume that the DVM internal circuitry is such that the shunt resistance used to measure current is such that 2000 uA accross the shunt resistor yields 200 mV, then the DCM's internal resistance on the 2000 uA scale would be 100 ohms. 100 ohms load on an MFC with an internal resistance of 1272 ohms means the DVM shunt resistance is a little over 7% of the total resistance in the current loop of the whole circuit (test equipment and MFC and test leads) which is a bit high and will cause and explain measurement inaccuracies, but it is not enough to explain the lack of any current reading. I'm still puzzled.

If I were attempting to figure out this DC current measurement puzzle, I would purchase an inexpensive carbon film 100 ohm and 10 ohm resistor (1/8 watt or larger 10% or better) and use it as a shunt resistor placed externally between the annode and cathode wires and use your DVM on the DCV 200 mV scale to measure the voltage drop across this shunt resistance to be able to accurately measure current. The internal impedance (resistance) of the entire apparatus is not guaranteed to be constant as the biochemical state changes while the resistance of the external shunt resistance will stay constant to within several parts per million (minor variation with temperature which is far less than the accuracy of your meter so it won't affect the accuracy of your current readings).

[irregular]

Hi Craig!

When I meant that my lowest measurement was 2000uA, I didn't use the right words. The lowest scale on the DCV is 2000uA, not 200uA.

I will look into the resistor.

Thanks!

[donnahardy2]

Hi Irregular,

I think that writing the entire paper in draft form is an excellent idea; that will make it easier to see and then polish up. You are doing an excellent job on the background paper and I know how time consuming this is.

The reference you found is very good and would be perfect. It shows how electrons are transferred to different acceptors under anaerobic conditions. If you want, you could also add carbon to the list of electron acceptors because that is what the bacteria are using in the anode chamber of your MFC. Or, when you draw your diagram of your MFC, show the bacteria transferring electrons to the carbon electrode. Pictures and diagrams are always helpful in explaining complex processes.

If you are going to make a new salt bridge, and I think this is a good idea after reading Craig’s comments, to do a short trial with your current set up and the new salt bridge so you can compare results between experiments. If you switch to mud plus acetate and a new salt bridge, you won’t be able to attribute any change in power production to the acetate because you will be changing two parameters. In science projects, you want to change just one parameter at a time, even though this is difficult to do with MFC’s.

Your question about the amount of acetate to add is a good one. Here is one reference that used 5 mM acetate in an MFC:

http://www3.interscience.wiley.com/jour ... SRETRYTh=0
This reference compared the power production of MFC’s with different concentrations of acetate and found that there was good power production with a range of 400 to 800 mg/L acetate. Look at the graphs in the results sections to see what I am referring to. If you can’t open this reference, let me know and I’ll upload it; it’s really good:

http://pubs.acs.org/doi/pdf/10.1021/es048927c

Commercial vinegar has a concentration of 4 to 8%; this means there are 4 to 8 grams of acetic acid per 100 ml of volume. The Wikipedia site has lots of information about acetic acid.

http://en.wikipedia.org/wiki/Acetic_acid

Now, here are the calculations you need to determine how much vinegar to add to your MFC:

1. How much vinegar is 5mM?
You haven’t had chemistry yet, but for this project, you need to know that chemists commonly use concentrations in units of moles per liter. A mole of a compound is equal to 6.23 x 10 to the 23 molecules of the compound. If you look at the periodic table and look of the molecular weight of each element and add them up, you will get the molecular weight. From the Wikipedia website, you can see that acetic acid has 2 carbon, 2 oxygen, and 4 hydrogen atoms, so has a molecular weight of 2 x 12 +2 x 16 +4 x 1= 60 grams/mole, or 60 milligrams/millimole.

5 mM/L acetic acid x 60 mg/mM = 300 mg/L

So, you want to have a range between 300 to 800 mg acetic acid per liter of mud.

2. What volume of vinegar should be added per liter of mud?

Vinegar contains 4-8 grams or 4000-8000 mg of acetic acid per 100 ml, or 40-80 grams or 40,000 to 80,000 per 1000 ml or 1 liter. If you cannot find out exactly what the concentration of vinegar in your sample is, then assume it is 50,000 mg per liter (5%).
You want to dilute the vinegar from 50,000 mg/liter to 500 mg/liter (middle of optimum range), so use the following calculation:

500 mg x 1,000 ml/50,000mg = 10 ml vinegar per liter of mud.

So please measure the volume of your MFC and let me know how much vinegar you are planning to add to the anode chamber. If you don’t have a container with metric volume to use, remember than 1 quart is about the same volume as 1 liter. You want to make sure you don’t add too much vinegar, as a high concentration will lower the pH and inhibit the growth of your bacteria.

Please let me know what brand of vinegar you are using.

Donna Hardy

[irregular]

Hi Donna and Craig!

Okay, thanks for your encouragement and suggestion!How could I easily and understandably describe the pictures (http://www.microbialfuelcell.org/www/in ... nisms.html) and (http://www.studentsguide.in/microbiolog ... thways.jpg)?

I followed your advice and decided to test the PVC salt bridge in the current MFC. The salt bridge is about 1.5ft-2ft long, which is very long. The advantage of this is that the salt bridge is completely in both solutions so protons may easily be transferred. I could place the anode/cathode chambers closer together and shorten the length of the salt bridge. So, I don't have to lay the MFC down and worry for leakage.

I was very hopeful today - I got some more bacteria, bought a set of anode/cathode chambers for a new MFC, ready to create my acetic acid MFC...

And then - problem. Right before I switched salt bridges, I took a voltage+resistance+current test. I got 0 mV, infinite resistance, no current. After playing around a bit,turning th multimeter on/off, reinserting leads, etc etc, I got a 50 mV reading. I proceeded to change salt bridges.

Then, I got no reading whatsoever. 0mV. I played around for about half an hour, my dad and I experimented a bit, and we got nothing. It was like the reading would come for one second, and then it would disappear. E.G. I went on resistance, and for 1 second it showed 1200, and then went to infinite. I started to get really, really discouraged. Last thing I want is my multimeter not working.

I tested batteries. No measurement. Finally one measurement of 10V came on a heavy duty battery. After turning the multimeter off and then on, the reading went back to 0.

So tomorrow I will be looking for another multimeter or microammeter at school. I will possibly return the present multimeter I have.

As for the vineger, I am using Heinz brand. It is 5% acetic acid, and the chamber will hold about 1.5L. So, I will need 15ml. Thanks for explaining that part and including the links, I have read them. However, I couldn't open the first link. You mentioned that it's really good - do you mind opening it for me? Thanks!

Thanks so much guys!

[donnahardy2]

Hi Irregular,

Good job troubleshooting! I hope you can obtain a functioning multimeter soon, so you can measure your results. No wonder your results were not making sense.

Excellent. You are planning to add the correct amount of vinegar to your acetate sample.

Can you tell me what you don’t understand about bacterial anaerobic respiration? The detailed enzymatic steps are complex, but the end result of metabolizing carbon sources, as shown in the diagram, is that electrons are transferred from the energy source to the electron acceptor and energy in the form of ATP is formed. ATP is the universal molecule used for intracellular energy.

http://en.wikipedia.org/wiki/Adenosine_triphosphate

I don’t expect everything to make perfect sense to you, since you have not had a chemistry class yet, but if you can let me know what is confusing you I will try to provide additional explanation.

I am attaching the acetate reference.

Donna Hardy

acetate ref part I.pdf

(198.51 KiB) Downloaded 300 times

[donnahardy2]

acetate ref part II.pdf

(138.19 KiB) Downloaded 275 times

[irregular]

Hi Donna!

Okay, so the problem WASN'T the multimeter. We exchanged the multimeter and .. same readings of 0.5mV! We added acetate and more methylene blue to see if anything would change. After waiting for a while, the readings stayed the same.

I realized that the problem is the PVC pipe. Wanting to see if the glass rod would do anything better, my dad and I put that back in. Right away, the multimeter showed 50mV. Since the measurement started decreasing, we applied the caulking compound and will let it solidify until tomorrow. Hopefully it shows better results. Other than the salt bridge, I'm set to go!Thanks for the link!

I read the respiration papers again and am seeming to understand it more. We learned about cells in science last term so I am familiar with ATP. If something becomes confusing, I will definitely ask about it to you, thanks so much!

Thanks Donna!

[deleted-71588]

I't has been a long time since I worked with agar so I've forgotten a lot about its mechanical properties so I'm guessing here.

If you have a 1/2 inch inside diameter pipe, I would guess that you would want your agar plug to be 1/2 to 1-1/4 inches in length in order to make a good seal while minimizing the length of the salt bridge. If you have a 1 inch inside diameter pipe, I would guess that 3/4 to 1-1/4 inches in length would be what you should be trying. For 1-1/2 inside diameter pipe, 1 to 1-1/2 inches whould be what I'd try for.

These ranges assume that the agar is in a horizontal section of the pipe and the liquid levels on both sides is similar and the agar sets up and sticks to the inside of the pipe so that it will deform (melt/flow/plasticize around your finger or whatever you try and push it with) instead of being pushed down the pipe. If it pushes down the pipe, it needs to be longer.

The goal is to get something that just barely seals the liquids in the two chambers from coming in contact with each other inside the pipe without introducing any more electrical resistance than possible in the agar-salt bridge.

larger diameter pipe is better, shorter agar length is better, below the liquid levels required for good solution to agar-salt bridge electrical connection.

[donnahardy2]

Hi Irregular,

How are you doing? And how are the millivolts and resistance on your MFC? Did the acetate improve the respiration rate of the mud bacteria?

Donna Hardy

[deleted-71588]

FYI: Others who are working on MFC's have reported MFC resistances under 10 ohms so your setup has some basic disadvantages to being able to produce any significant amount of power.

[irregular]

Hi guys!

I know that it's been a while since I replied, I'm sorry! I've just been a little busy with exterior activity and running my MFC.

Okay, so we put the glass rod back in as the salt bridge. We spent a LOT of time fixing the connection between the chambers+salt bridge. Amazingly, we ended up with using simple duct tape and it hasn't leaked for 24 hours. Two hose clamps have been purchased to tighten the connection as well.

We are reducing the amount of time each MFC will run from 1 week to a few days. I started the acetate MFC last night. My MFC produced about 175mV, 90milliA, and resistance ranging from 1200-1800 ohms.

The next MFC will be a glucose MFC, then an organic compounds MFC, and then MAYBE one more if time allows.

I'm still concerned about my resistance. According to this group (http://www.engr.psu.edu/ce/enve/logan/b ... al-mfc.pdf), as mentioned on the last page of the report, high resistance was obtained (58,000ohms). My MFC has started obtaining very high resistance right now as well, it's showing infinite on the 2000ohms section, and we are having to measure it at the 200k ohms level. Voltage and Current are also decreasing after only about 24 hours of running.

Thanks so much!

EDIT: Okay, this is weird. So last night I set up my glucose MFC, and everything went perfect. The leakage was solved very quickly, as my dad and I adjusted the jubilee clamps and everything. So, no leakage whatsoever.

But the glucose MFC is producing higher results than the acetate MFC. I'm trying to find some scientific journals/resources to see if some people have tried glucose or acetate and see which one produced more electricity from their results. And according to the Wikipedia article, Glucose is a six carbon molecule! (http://en.wikipedia.org/wiki/Glucose)

Here's some rough calculated average data

Acetate MFC:
microA: 75
Ohms: 1500 (@2000ohms setting)
milliV: 160

Glucose MFC:
microA: 115
Ohms: 5.5 (@20K setting)
milliV: 290