Measurement of Transport Numbers1. Experimental Objectives In this experiment you will use the...

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Measurement of Transport Numbers1. Experimental Objectives In this experiment you will use the moving boundary method to measure the transport number of the hydrogen ion in dilute hydrochloric acid. Note: VERY HIGH VOLTAGES ARE USED IN THIS EXPERIMENT. READ SAFETY NOTES BELOW.2. Introduction 2.1 Theory of Transport Numbers Electric current is carried through an ionic solution by movement of the ions. The positive ions (cations) move towards the cathode and the negative ions (anions) move towards the anode. The ability of the solution as a whole to carry electric current is measured by its conductance. The fraction of the current carried by a particular species i is the transport number for that species. If we imagine a plane in the solution placed between the two electrodes, the transport number of species i can be measured as the quantity of electricity (current × time) carried by species i across that plane, divided by the total quantity of electricity crossing the plane. The species may be present in different forms: e.g. in a solution of ZnCl2, Zn may be transported as Zn2+ and ZnCl+ (towards the cathode) - and by ZnCl3 and ZnCl4 2- towards the anode. The transport number of Zn refers to the net number of zinc ions migrating towards the cathode when a given quantity of electricity is passed.
	
Document Preview: Second Year Physical Chemistry Laboratory 
DEPARTMENT OF CHEMISTRY 
G3 Measurement of Transport Numbers 
1. Experimental Objectives 
In this experiment you will use the moving boundary method to measure 
the transport number of the hydrogen ion in dilute hydrochloric acid.  Note: VERY 
HIGH VOLTAGES ARE USED IN THIS EXPERIMENT. READ SAFETY NOTES BELOW. 
2. Introduction 
2.1 Theory of Transport Numbers 
Electric current is carried through an ionic solution by movement of the 
ions. The positive ions (cations) move towards the cathode and the 
negative ions (anions) move towards the anode. The ability of the solution 
as a whole to carry electric current is measured by its conductance. The 
fraction of the current carried by a particular species i is the transport 
number for that species. If we imagine a plane in the solution placed 
between the two electrodes, the transport number of species i can be 
measured as the quantity of electricity (current × time) carried by species i 
across that plane, divided by the total quantity of electricity crossing the 
plane. The species may be present in different forms: e.g. in a solution of 
2+ +
ZnCl , Zn may be transported as Zn and ZnCl (towards the cathode) 
2
- 2-
and by ZnCl and ZnCl towards the anode. The transport number of Zn 
3 4
refers to the net number of zinc ions migrating towards the cathode when 
a given quantity of electricity is passed.  
The mobility u , of an ionic species i, is defined by the equation: 
i
s = u E 
i i 
where s is the speed of the ion and E is the strength of the applied electric 
i
-1
field (Vm in SI units). 
The amount of current carried by an ion i is proportional to the magnitude 
of its charge (|z|), its concentration (c), and its mobility. It follows that the 
i i
transport numbers are  
| z | c u | z |c u
+ + + - - -
t = ; t =
+ -
| z |c u + | z |c u | z |c u + |z |c u
- - - + + + - - - + + +
24 April 2009 Revised by Mariam Ayub, 24/04/2009 1since the sum of all transport numbers...

Robert · Accepted Answer

Abstract: 
This report is based on an experiment of moving boundary method to measure transport 
number, in turn to calculate molar ionic conductance of hydrogen ion in dilute hydrochloric acid. 
There are various methods available to measure quantities mentioned above but moving 
boundary method is considered to be the best available as per literature, so this method is 
implemented for the purpose.
Introduction: 
Transport Number:  
Ionic solution contains various ions which act as charge carrier in the solution, in turn are 
carrying electric current. The charges moves towards different electrode according to the 
charge, positive ion (cation) move towards cathode while negative ion (anion) move towards 
anode. The ability of the solution as a whole to carry electric current is measured by its 
conductance. The transport number (sometimes called transference number) of an ion is the 
fraction of charge carried in an electrolyte by that ion. Thus in a simple electrolyte containing 
one cation and one anion, the transport numbers are represented t+ and t- with t+ + t- = 1.  
Now, there are several methods to measure transport number that do not make use of an 
electrochemical cell like Hittorf’s method, moving boundary method. The experiment being 
performed is based on moving boundary method.  
So the transport number can be understood in this way, consider a imaginary vertical plane 
between two of the electrodes, the amount of electricity carried by particular ion i, for a given 
amount of electricity, is the value of transport number of that specie. Thus the transport 
number of H+ ion refers to the net number of hydrogen ion migrating towards the cathode 
when a given quantity of electricity is passed. 
It is also interesting to note that, the mobility of hydrogen ion in aqueous solution is abnormally 
high. This is because the H3O
+ ion are able to transfer a proton to a neighboring water 
molecule. This is explained by the fact that the H+ need not migrate through a protic solvent, 
but move through exchange of a proton between neighboring solvent molecules.  
The amount of current carried by an ion i is proportional to the magnitude of its charge (|Z i|), 
its concentration (Ci) and its mobility.

Measurement of Transport Numbers 1. Experimental Objectives In this experiment you will use the moving boundary method to measure the transport number of the hydrogen ion in dilute hydrochloric acid....

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