Testing Solution Conductivity. I Need Help!

Sashie VII

Sashie VII

Balance of Power
Joined
Apr 11, 2004
Messages
2,370
Location
Where people build cat statues
Chemists/Physicists/anyone :(

I need to test the conductivity of dissolved Zinc Nitrate. The concentration is 50, 100, 150...40ppm, each at 60mL. The problem is that I could not find a conductivity meter in my whole faculty and end up with a GDM-356 Modern Digital Multimeter and a YX360TRF Multitester, which the technician informed me was designed to test solids (eg. batteries etc) only. I thought of using electrodes, and have a pair of stainless steel rods.

However, the readings I got was incomprehensible. First, the values are inconsistent, in that they change continuously for 20-30 mV (I held the electrodes by hand). Second, the values does not show a consistent trend (increasing or decreasing), some are negative.

Spoiler :

50ppm 18.7-19.1

100ppm -8.0 - -4.2

150ppm -8.1 - -14.3

200ppm 14.7-29.7?!

Values are mV, measuring DC


My project supervisor could not help me. Can anyone help me, please?
 
Electrician here.

Those are very inconsistent results. The amperage should be increasing as the concentration goes up. no? Can you try again but this time set it up so the rods are suspended in the liquid an equal distance apart, this time being careful not touch the conducting rods at all with your fingers? Also you could use a solid conductor such as copper, or aluminum stainless isn't exactly a good conductor.

As I see it right now your results are borked and you need to start again to get any accurate readings.

Edit
I don't think you should be measuring in Volts (especially if your're touching the wire with your fingers) there will be a potential differnce between one hand and the other especially when measured in milivolts. Shouldn't amps give you a better idea of conductivity?
 
Mulholland said:
Those are very inconsistent results. The amperage should be increasing as the concentration goes up. no? Can you try again but this time set it up so the rods are suspended in the liquid an equal distance apart, this time being careful not touch the conducting rods at all with your fingers? Also you could use a solid conductor such as copper, or aluminum stainless isn't exactly a good conductor.

As I see it right now your results are borked and you need to start again to get any accurate readings.

Edit
I don't think you should be measuring in Volts (especially if your're touching the wire with your fingers) there will be a potential differnce between one hand and the other especially when measured in milivolts. Shouldn't amps give you a better idea of conductivity?
Click to expand...

My hands didn't touch the electrodes directly, I held the rubber coating of the multitester's probes. But fixing the distance of the rods makes sense, thanks. Does it make a difference if the rods touch the bottom of the beaker?

Another potential problem that I have is that I will also need to conduct the same test with Copper, Lead and Nickel. I do however have Copper, Brass and mild steel rods.

So I should measure mA instead of mV?
 
If you connect a nine volt battery to the conductor you should be able to get a nice amperage reading. It shouldn't make a difference if the rods touch the bottom of the beaker. Electricity will always be pulled across the path of least resistance and since glass is a very poor conductor it shouldn't make a difference.

Edit If you can construct a non conductive device to immerse the conductors in your solution I was thinking of a setup similar to this.
 
Most modern pH meters will read the voltage of a solution. Don't you even have one?
 
GoodGame said:
Most modern pH meters will read the voltage of a solution. Don't you even have one?
Click to expand...

We don't have one. The pH meters I've found at Chemistry etc only read pH, salinity and TDS, unfortunately. My supervisor and I find it hard to believe that the entire faculty doesn't have any :( or perhaps I haven't found it.

Mulholland, I will give your setup a try. Thanks.
 
If you have a biological sciences research section, or anyone that makes biological buffers regularly, they should have one. Short of that, I think a very simple portable pH meter would be rather cheap.
 
DISCLAIMER: PERFECTION HAS NEVER (SERIOUSLY) TRIED TO MEASURE ELECTRICAL CONDUCTIVITY OF SOLUTIONS


That said:

With Mulholland's setup you could measure conductance, but not conductivity. [wiki]Conductivity[/wiki] is a material property whereas [wiki]conductance[/wiki] is current over voltage.

They are related in that the conductance of your setup would be proportional to to conductivity of the solution but the probe sizes, placement, and shape all impact the result. If relative measurments is all you need, then great go ahead (you could circumvent the need for a battery by simply using the multimeter to directly measure [wiki]resistance[/wiki], which is the reciprocal of conductance).

If you really need to know the conductivity and you seriously can't find a meter, then you have two ways to continue:
1. Use a reference solution with a known conductivity to test your conductance measuring rig. Make sure the conductance isn't too small (I wouldn't use something like distilled water), you can use the ratio of conductivity and conductance for the rest of your measurements (make sure you fill up to the same level in your container and that the electrodes are immobile).
2. Analytically figure out the relationship. I'd be willing to lend a hand, but I should warn you it's annoying math and probably won't be very precise.
 
I'm doing this test preceding a Batch Equilibrium Test for my soil samples. The conductivity is for getting a calibration graph as reference later.

Getting this to work is complicated. I think it might be best if I expand my search to other faculties or even to nearby universities. Perhaps discuss the possibility of ordering one with my supervisor. Thanks all.
 
Yeah, if this is serious science, where you really need this to be right, and not some speculative project where you can get away with using creative techniques then getting the proper equipment is definitley the best choice. Just because you can do without something, doesn't make it a good idea.
 
Sashie VII said:
I'm doing this test preceding a Batch Equilibrium Test for my soil samples. The conductivity is for getting a calibration graph as reference later.
Click to expand...
Soil? I thought you were testing solutions?

Anyway: I would suggest that you look at international standards for guidance. ISO 2456 has a section on measuring conductivity of aqueous solutions (it is actually aimed at specifying water used as a solvent for test methods, but Annex A2 deals with measuring electrical conductivity).

Note that conductivity is measured in S/cm - ie the distance between the electrodes is an important variable.
 
ainwood said:
I thought you were testing solutions?
Click to expand...

I am. The test was supposed to provide me with a calibration graph as a reference after BET. After BET, I'll test the conductivity of the residue solution, and use the calibration graph to determine the concentration after BET. Then I'll know how the absorption capacity of the soil types I'm analyzing towards each of the heavy metals.

Thanks for the tip.
 
Perfection said:
DISCLAIMER: PERFECTION HAS NEVER (SERIOUSLY) TRIED TO MEASURE ELECTRICAL CONDUCTIVITY OF SOLUTIONS


That said:

With Mulholland's setup you could measure conductance, but not conductivity. [wiki]Conductivity[/wiki] is a material property whereas [wiki]conductance[/wiki] is current over voltage.

They are related in that the conductance of your setup would be proportional to to conductivity of the solution but the probe sizes, placement, and shape all impact the result. If relative measurments is all you need, then great go ahead (you could circumvent the need for a battery by simply using the multimeter to directly measure [wiki]resistance[/wiki], which is the reciprocal of conductance).

If you really need to know the conductivity and you seriously can't find a meter, then you have two ways to continue:
1. Use a reference solution with a known conductivity to test your conductance measuring rig. Make sure the conductance isn't too small (I wouldn't use something like distilled water), you can use the ratio of conductivity and conductance for the rest of your measurements (make sure you fill up to the same level in your container and that the electrodes are immobile).
2. Analytically figure out the relationship. I'd be willing to lend a hand, but I should warn you it's annoying math and probably won't be very precise.
Click to expand...

Listen to this man he knows what he's talking about.
 
You must log in or register to reply here.
Top Bottom