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Author: Subject: Electrolyte Conductivity
ChemNub
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[*] posted on 21-10-2010 at 16:37
Electrolyte Conductivity


Hi guys!

Recently I've been playing with some electrolyte solutions, trying to determine adequate conditions for making some metal salts, plating, reducing and the like..

Unfortunately however, I can't get good conductivity in my solutions- even with fairly concentrated strong acids!

5% H2SO4 was reading at about 4.5k ohms (! yes 4000!), and 20% H2SO4 at 5k (makes no sense with my limited knowledge of conductivity..).

Magnetic stirring seemed to drop the resistance temporarily but it just crawled back up over the course of a few minutes. All the connections were good, and the multimeter read the resistance of itself (~0) and a 50Ohm resistor (50.5) with pretty good accuracy..

I am seriously wondering if it's a problem with the multimeter however, as it does not report amperage correctly under any circumstances (perhaps a fuse was blown at some point?) - reading 0.0mA with 12v going through 50Ohms.. (voltage readings tested with a computer PSU seem quite accurate)

Something is definitely amiss... Have I finally lost my mind? It's not necessary to become a little Tesla, sucking kilovolts out of the grid, just to harness a few amps to dissolve a nickel.... is it?
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ChemNub
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[*] posted on 21-10-2010 at 21:25


well, I tried a 15% HCl solution and was able to measure about 20 ohms =D

but still.. why would sulfuric acid be soo much less conductive? is this typical?
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12AX7
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[*] posted on 21-10-2010 at 22:46


Electrode potentials perhaps? Electrochemistry is non-ohmic. In principle, no current flows until the redox potential is reached. Beyond the threshold, current increases exponentially, the same as it does for current flow across a semiconductor junction. Ionic mobility is orders of magnitude less, so there's only a narrow range where it looks exponential, since the resistance swamps it even at low currents.

In short, can you measure with a higher voltage, like the current drawn from 3 or 5V?

Tim




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chemrox
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[*] posted on 22-10-2010 at 00:22


@12AX7 "Electrochemistry is non-ohmic." Is this true? I don't see it this way. Conductivity units are reciprical resistance units (mhos). Sulfuric solutions are slightly buffered and have an order of magnetude less ionization than HCl solutions. I won't work out how this would affect a multimeter reading. Try a conductivity probe and see what that reveals.



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bbartlog
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[*] posted on 22-10-2010 at 02:20


Quote:
Is this true? I don't see it this way.


The point is that V=IR doesn't hold true, so the apparent resistance will be different depending on what voltage you are applying (and it therefore doesn't make much sense to think of the electrolyte as having some fixed value of resistance anyway). Ohm's law doesn't apply, thus non-ohmic...
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watson.fawkes
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[*] posted on 22-10-2010 at 06:10


Quote: Originally posted by chemrox  
@12AX7 "Electrochemistry is non-ohmic." Is this true? I don't see it this way.
Yes, it's true. Go read up on the topic of electrode polarization, for example. There's some non-trivial behavior at the electrode-electrolyte boundary. There's charge buffering, so in part it acts similar to a capacitor. There's also threshold conduction, so in part it acts analogously to a forward-biased diode. In particular, the equivalent circuit of an electrode pair in electrolyte is not just a resistor.
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bquirky
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[*] posted on 22-10-2010 at 10:22


dont forget that electrolyte temprature will efect conductivity as those crazy little ions (you just never know what there going to do next :) can move around more when its hot..


And differences of electrode size, metal type, cleanlyness/oxidisation state and evan the depth of the electrode if there is a concentration or temprature gradiant present can cause stray potentials that can mess up resistance readings of a typical DMM

ive got a CuSO4 plating bath running at the moment conducting 10amps and droping about 4 volts which whould make the whole setup cables,connectors, electrodes,electrolyte etc about half a ohmm

but that specific number is not relly relevent becuse it depends mainly on the area of and distance between the electrodes.





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not_important
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[*] posted on 22-10-2010 at 11:44


bquirky summarised it nicely - electrode spacing, material + electrolyte composition, electrolyte temperature, applied voltage; all affect measured conductivity. And if you're not using AC to measure with, electrolyte reactions can also be important, often resulting in changing measurements.

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[*] posted on 22-10-2010 at 12:50


An electrolysis cell is highly nonlinear and Ohm's law certainly does not apply to it. A much better electrical equivalence model for an electrolysis cell is a non-linear resistor with the Butler-Volmer characteristic.

I did an experiment with an electrolysis cell and measured the current as function of voltage. Read the following web page I made about this experiment:

http://woelen.homescience.net/science/chem/exps/electrolysis...


At low voltages the cell may take a low current and then its resistance appears to be very high.




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Nicodem
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[*] posted on 23-10-2010 at 00:36


The resistance measuring in multimeters is designed for resistors that obey the Ohm law. The testing potential is often no more than 1V or thereabout. Some multimeters even run on a single 1.5 V battery, so you can be quite sure their testing potential is not higher than that. On the other side, the sum of overpotentials of two electrodes immersed in aqueous H2SO4 is commonly higher than 1V (unless one of the electrodes is dissolving and you get a galvanic cell instead of an electrolytic one).
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12AX7
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[*] posted on 23-10-2010 at 11:50


Yes. The other point of my statement was, you'll get a fairly reasonable measurement (if not physically significant, at least useful), at higher voltage (3-5V), which is sufficiently in excess of whatever electrode polarization and reaction potentials (< 1V) to reveal the bulk resistivity of the cell.

Same thing happens to a silicon junction diode, where you get essentially perfect exponential behavior below 0.7V or so (from pA to mA -- nine orders of magnitude!), turning to resistive at higher levels (due to the resistivity of the bulk silicon layers in the device). A 1N4001 might drop 1V at 1A, 1.5V at 14A and 2V at 30A (peak), which is almost linear, whereas if these points were exponentially related, the voltages would be much closer.

Tim




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merrlin
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[*] posted on 23-10-2010 at 12:28


Electrolyte conductivity measurements can be be made using an AC signal in place of a DC signal. This helps avoid the problem of redox potentials and charging of the double-layer capacitance. I suggest using a variable transformer and/or a resistor divider with your household voltage to obtain a few tenths of a volt at 50 or 60 Hz. You can then use your multimeter(s) to obtain the I-V characteristic of the cell. Large electrodes also help avoid the capacitance charging problem. Highly conductive electrolytes will allow the capacitance to charge quickly, so small voltages and large area electrodes will be required. For accurate measurements you want the capacitance large enough so that most of the potential is dropped across the electrolyte and not across the capacitance. If you have a higher frequency AC source available, it can also provide greater accuracy, but may complicate the I-V measurements.
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[*] posted on 23-10-2010 at 16:03


Quote: Originally posted by merrlin  
Electrolyte conductivity measurements can be made using an AC signal in place of a DC signal (...)

That is right.
Not only can but should be made in this way. In general, all conducivity values from tables are given for f=1000 Hz (standard) or are extrapolated for f going to infinity.
Double-layer capacity is of order 1 microF/cm2, so measurements for 50 Hz can be not very accurate, especially for concentrated electrolytes.
For measurements of small AC voltages, simple but very useful electronic rectifier based on OP can be made. Example:
http://en.wikipedia.org/wiki/Precision_rectifier
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chemrox
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[*] posted on 24-10-2010 at 00:27


Thanks for schooling me up on this!



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[*] posted on 25-10-2010 at 08:05


Quote: Originally posted by ChemNub  

5% H2SO4 was reading at about 4.5k ohms (! yes 4000!), and 20% H2SO4 at 5k (makes no sense with my limited knowledge of conductivity..).

Maybe it makes no sense but can be true :P
See a picture from book "Fundamentals of electrochemistry"(2006) by Bagotsky.

cond.JPG - 18kB
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