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### Topic: Conductivity of salt solutions.  (Read 10020 times)

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#### petkenator

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##### Conductivity of salt solutions.
« on: February 22, 2010, 11:15:22 AM »
I have recently carried out an experiment for a project and have come across some unexpected restults. I tested the current across a sodium chloride solution and recorded my results, i then decided to test a salt solution with +2 -2 charge (magnesium chloride i think but i can't remember of hand). I expected the conductance of the +2 -2 salt to be twice that of the sodium chloride solution but instead it was half. I repeated the experiment and the results were still the same.

Could someone please explain my results, i think it may have something to do with molecular mass of the two salts but my teachers are not willing to commit to an answer because they are unsure.

#### Helper0863

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##### Re: Conductivity of salt solutions.
« Reply #1 on: February 22, 2010, 12:04:39 PM »
Not too sure, but conductance may also be due to the concentration of the different dissolved species in the salt solution.  All salts have different solubility, with some being more soluble than others.  Hence, for example, the concentration of dissolved chloride ions would differ between a sodium chloride and magnesium chloride solution.  It would not be a simple "double" the concentration in sodium chloride solution for the magnesium chloride solution, if you know what I mean.

On a sidenote, the conductance may be also due to the SUM of the different dissolved species in the solution.  For example, if you were to dissolve 1 mole each of sodium chloride (58.44 g) and magnesium chloride (95.21 g) in 1 litre of water, the total concentration of dissolved species would be 2 moles per litre for sodium chloride, and 3 moles per litre for magnesium chloride.  But this is assuming both salts have the same solubility.

Umm, hope this helped.  Reply if it doesn't and I'll try to simplify further.

#### Borek

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##### Re: Conductivity of salt solutions.
« Reply #2 on: February 22, 2010, 12:34:51 PM »
Conductance of the solution is a SUM of conductances of all IONS dissolved in the solution. Basically it is something like

$request=http://www.forkosh.dreamhost.com/mimetex.cgi?{+\kappa+=+\sum+\lambda_i+z_i+c_i+}&hash=0e655eef88f2a6e470f9d7c9b82a035e$

κ - conductance
λi - specific conductance of ith ion
zi - charge of ith ion
ci - concentration of ith ion

Each ion has its own specific condustance that you can read from tables.

No idea about details of your experiment, but to make sense it should take all three parameters into account.

BTW, magnesium chloride is not +2 -2 salt, but +2 2(-1).
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#### petkenator

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##### Re: Conductivity of salt solutions.
« Reply #3 on: February 22, 2010, 02:33:13 PM »
I made sure that the same concentration was present in each solution 0.5 mol/litre i therefore have the same number of ions of each salt in each separate solution. Surely the fact that the same number of ions are present in each solution means that the sum of ions is irrelevant, when comparing results it is relative.

#### FreeTheBee

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##### Re: Conductivity of salt solutions.
« Reply #4 on: February 22, 2010, 02:44:46 PM »
If all solutions were 0.5M than MgCl2 would give 1.5M of ions in total and NaCl only 1. As you can see in the equation Borek posted, the charge and nature of the ions play a role as well.

How did you perform your experiment? In particular, what kind of potential did you apply, direct or alternating?

#### petkenator

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##### Re: Conductivity of salt solutions.
« Reply #5 on: February 22, 2010, 02:58:19 PM »
Ok i now see what you mean there are more individual ions in the Magnesium Chloride but surely if there are more ions then it should conduct more not less, that it what i fail to understand. Also, how can two ions with the same charge conduct differently what are the other factor involved, weight, size?

To perform the experiment i used 1.2V direct current, i know tht this will electrolyse the solution a bit but i could not find any measuring equipment sensitive enough to get any results for AC potential. I used a low voltage to minimise this effect.

#### petkenator

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##### Re: Conductivity of salt solutions.
« Reply #6 on: February 22, 2010, 03:08:08 PM »
Not to jump the gun, but i have another question that arose today. Should the relationship between conductance of a salt solution and temperature be linear?

#### Borek

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##### Re: Conductivity of salt solutions.
« Reply #7 on: February 22, 2010, 04:56:19 PM »
how can two ions with the same charge conduct differently what are the other factor involved, weight, size?

You are on the right track - ions conduct electricity by moving through the solution. Ion mobility depends on its size and interactions with water. It doesn't necessarily mean small ion moves faster and have higher specific conductivity, as they can be surrounded by water molecules which they drag through the solution.
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#### FreeTheBee

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##### Re: Conductivity of salt solutions.
« Reply #8 on: February 23, 2010, 10:03:28 AM »
What order of magnitude are the currents you see and how constant are they? Assuming you have a two electrode system and applying the 1.2V between those I would expect little hydrolysis to occur. Hence, the current you measure would mainly be due to charging of the double layer, and should go to zero in time. The current measured in the first moments should then represent the solution resistance the closest if I am not mistaken.

Usually the conductivity of electrolytes is measured using an AC input, either from a conductivity meter (kHz) or using impedance spectroscopy.