Science Buddies: "Ask an Expert"

My project is the effect of the salt water concentration of different types of potatoes on conductivity. I know each potato that I'm using (white, red, and purple) have different salt water concentrations, or salinity, but my teacher said I need to prove it by measuring it. I found that I could use an electrical conductivity meter and convert the microSiemens/centimeter to TDS. However, it is usually used for water. Will it accurately measure the salinity of potatoes also?
I want to find which potato conducts electricity the best based on their salinity. I said that this is conductivity, but I was planning on using a multimeter/voltmeter to measure the volts of each potato with a circuit set up. Is this correct? Or should I use what the electrical conductivity meter says?

I'm a little confused about what you are asking. A multimeter will measure the resistance of the different types of potatoes. Using conductivity to measure salinity, however, seems to be begging the question. You would need to measure salinity some *other* way besides electrical conductivity in order to determine the effect of salinity on electrical conductivity. Otherwise it is circular reasoning.

Unfortunately, I don't know of any easy way of measuring salinity other than conductivity. In principle, refractometers can approximately measure salinity in water, but that won't work with a potato. Even if you dissolved the potato in water, the starches would also affect the refractance of the water. You might be able to figure out how to remove the starch, but this is a rather hard project to start at this late date.

I don't like to give advice like this, but unless someone else here can come up with a way of measuring salt content without using conductivity, I think you would be better off answering a question like this by creating water of known salinity (start with distilled and add known quantities of salt) and make your measurements that way.

It may be possible to measure the salinity chemically by measuring the "chlorinity" using titration with silver, and making assumptions about the relationship between chlorine and salt. However, that may not apply to a potato, and again you might have trouble with other constituents of the potato affecting the measurement. It's the best I've found, so far, though. There's some discussion about this here: http://oceanworld.tamu.edu/resources/oc ... r06_01.htm.

I guess I'm a little confused on the difference between voltage and conductivity. I thought a multimeter measures voltage. Maybe I should use another word besides conductivity? I don't really know what word it is, but the definition is how well something conducts electricity.
In the end, I'm trying to find out which type of potato can conduct electricity the best, and I think the potato with the most salinity will be the best because saltwater conducts electricity well. I hadn't planned on measuring the salinity, but now I have to and I found that measuring the electrical conductivity was the easiest way. I'm really just trying to measure how well each type of potato conducts electricity. I can't change much at this point, though, because I'm writing my introduction at the moment and have already done my research. Of course, I could always do more research.

Also, I forgot to mention that I am setting up a circuit in each potato by putting a piece of copper wire and a zinc nail into the potato and connecting wires from them to the multimeter.

Hi cmrpl,

Ray Trent has given you some good advice. Thank you for the additional details!

A multimeter can measure several things, including voltage. That's the "multi-" part of multimeter. Most multimeters can measure current, voltage, and resistance. You can think of voltage as electric pressure, or how hard electrons get "pushed" trough a circuit. Conductivity (which is inverse resistivity) is a measure of how well a material conducts electrical current. The followng Science Buddies article provides really good background on how to use a multimeter and the various associated terms:

https://www.sciencebuddies.org/science- ... rial.shtml

It sounds like you are building a potato battery, like this one: https://www.sciencebuddies.org/science- ... p010.shtml. Is this correct? It would be helpful if you could post a link to the procedure you are using.

Based on what you've said, it sounds like your independent variable is the type of potato. Your hypothesis is that the potato battery made from the potato with the highest salinity will produce the most voltage. Is that correct? If not, please post back with your correct question and hypothesis. If it is correct, let us know that as well. Then we can move forward!

Yes, all you have said is correct. I am currently writing my materials and procedures, so I do not have the procedures finished yet.

My materials and procedure are due tomorrow. I am probably going to use an EC meter with probes and put the probes into the potato, then record the displayed TDS (Total Dissolved Salts). Hopefully this works. If not, I can always change my procedure.

Ok. Let us know how it turns out.