Science Buddies: "Ask an Expert"

Did you ever measure the V immediately after adding urine or water?

Unofirtunately no. This project has gone on so long we have taken the MFC to New York, Washington and several other states as we have traveled.

We should have but it has been hard enough to get my 7th grader to take the measurements once per day.

Kerry

Well, all you really need is ONE reading--immediately after the addition of urine. Do that and you will be able to answer the question of why there is a decline in power when you would predict that the nutrients in urine should stimulate the bacteria to produce more power. If you don't do that one experiment then you won't be able to explain your results.

So would you suggest doing another urine addition then taking hourly
Readings? How much urine would you add? Would you add distilled water
at the same time?

I would do that only if the power comes back up to at least 100 uW. If it stays low like it is now then the effect may not be as clear. If you could take the first reading at 30 min, the second at 60 mins and the rest at hourly intervals that would be great. The more measurements you have, the more information it gives you to use in understanding and explaining the results.

I would use 1 ml of urine since that is the amount you used before when you saw the large drop in V. Take readings for 3 hours then add 2 ml DW. Take a reading at 30 min and 60 min, and then for a couple more hours.

I would suggest if possible to do all the measurements with the same resistor. That would save a lot of time. I don't see why you need to use so many different resistors each time. Does the power really change that much with different R values?

Sybee:

What role does distilled water play in the experiment? I still dont understand the use of the resistors and yes depending on the resistor, the peak power number can vary significantly.

Example:

Day 51

Resistor Power uW
4670 53.53
2190 75.27
1000 108.29
470 115.50
220 72.16
100 16.00
47 4.17

Adding water dilutes the salts from the urine and washes them down from the electrode which, if my theory is correct, eliminates the interference of salts with movement of the electrical charges between anode and cathode. Since this effect would be relatively rapid, that is why I kept asking you to do measurements right after you added the urine or water.

I know what resistors do but I don't know the purpose of adding resistance into the MFC circuit and why the power varies with the R value. My feeling would be that changing the R from reading to reading is introducing another variable in the system. If I were doing the project, I would use a 1000 ohm R for all the measurements so that variable would be constant. I would like to hear what someone who is knowledgeable in electronics would say to explain this.

Has the power output from your MFC returned to 100 uW or better? I do hope you will be able to do the immediate measurements. Without that information I don't see a way to explain your results.

Good luck and let me know what you find out.

Sybee

Peak power is 89 uW today. That is close to 100.

So we should add 1ml of urine. Take measurements then add 2 ml water maybe 3 hours later.

Kerry

89 uW is high enough.

So, just as you said, add 1 ml urine, take reading at 30 mins and 1 hr and 3 hrs; add 2 ml DW and read again at 30 mins, 1 and 3 hrs. Make additional measurements if you have time.

Good luck!

Sybee

Does adding distillled water have any impact on power production other than to perhaps dilute the urine salts?

We are doing the measurements frequently as you instructed. If we experience a decrease in power after
adding 1 ml of urine over a few hours then add 2 ml distilled water and see an increase in power what is our conclusion?
That the salts inhibited electron flow?

It's been years since I took chemistry, but why would the salt impede
Ekectron flow?

Our original hypothesis was that adding urine would lead to increased power produced
by the MFC. Have we proven this hypothesis or not really since we needed
To add distilled water to get the increased power.

Thanks again.

Kerry

Sybee:

Added 1 ml of urine

Measurements

Before adding urine

4670 509
2190 415
1000 297


1/2 hour after adding urine

4670 510
2190 359
1000 304

1 hr after adding urine

4670 430
2190 354
1000 297

1 1/2 hours after

4670 414
2190 367
1000 278

2 hours after

4670 381
2190 348
1000 255

3 hrs

4670 585
2190 446
1000 333

so at the 3 hour mark power went back up without any distilled water added

Well, it looks like my hypothesis may have gotten disproved. I think you should take another reading after 24 hours, the way you have been doing, and see what happens. There's no point adding DW when the power is increasing.

The only variable left that i can think of is the urine itself. The amount of salt in it is going to vary depending on what was eaten before the urine was taken. Why don't you try one more addition--some salt water. Make some normal saline by dissolving 1/2 teaspoon of table salt in 8 ounces of DW. Then add 1 ml of this to the MFC and read the power at 30 mins. If it stays the same then my theory is dead.

Thanks for trying these experiments. I'm glad you did! I was really curious to find out what would happen.

Good luck!

Sybee

Sybee:

would you add the saline solution now or wait until tomorrow?

You can wait till tomorrow. As long as the power is 80-100 uW it should be fine.