[Gareth]

Electrolyte Conductivity.

Hello,

I'm hoping that someone may be able to offer me some advice to help solve a small problem I have.

I have recently been conducting some work concerning voltaic cells and the effect of different electrolyte solutions on peak current values.

I have trialled three different electrolyte solutions to date based on NH4Cl, KOH and ZnCl2. The solutions comprise 75% diH2O (w/w) and 25% chemical salt (w/w), i.e.:

Solution 1: 75% diH2O – 25% NH4Cl
Solution 2: 75% diH2O – 25% KOH
Solution 3: 75% diH2O – 25% ZnCl2

The other voltaic cell components are identical in each case. All consist of a carbon cathode surrounded by an MnO2/C/electrolyte paste, Membrane separator flooded with electrolyte, and a zinc anode. Test conditions (temperature/humidity) are the same during each cell analysis.

Ten cells based on each electrolyte formulation were tested and the results compiled to generate mean peak current values. I discover what is expected, such that the use of each electrolyte creates a slightly different peak current.

NH4Cl – 125mA, KOH – 145mA, ZnCl2 – 265mA.

I would really like some theoretic data to back up these results. The first thing that springs to mind is that the peak current values are different due to the conductivity of the electrolyte solutions (higher conductivity resulting in higher peak currents due to reduced cell internal resistance). I believe differences in conductivity may be due to ion concentration in each solution and hence the number of charge carrying ions, or ion mobility. However, I am not too sure how I would prove this hypothesis.

To date I have calculated the molar fraction for each solution using the method:

M(NH4Cl) = (14 + (4*1) + 35.5) * 1g/mol = 53.5 g/mol
M (KOH) =(39 + 16 + 1) * 1g/mol = 56 g/mol
M(ZnCl2) = (65.4 + (35.5*2)) * 1g/mol = 136.4 g/mol
M (H2O) = ((2*1) + 16) * 1g/mol = 18 g/mol

From these figures it's easy to calculate the amount of each salt in a
75:25 solution in moles..

(NH4Cl) = 25g / 53.5 g/mol = 0.467 mol
(KOH) = 25g / 56 g/mol = 0.446 mol
(ZnCl2) = 25g / 136.4 g/mol = 0.183 mol
(H2O) = 75g / 18 g/mol = 4.167 mol

from this it is possible to work out the mole fraction between chemical salt and
H20 in each electrolyte solution:

(NH4Cl) = 0.467/(0.467 + 4.167) = 0.101 --- hence 0.101 : 0.899
(KOH) =  0.446/(0.446 + 4.167) = 0.097 --- hence 0.097 : 0.903
(ZnCl2) =  0.183/(0.183 + 4.167) = 0.042 --- hence 0.042 : 0.958

From this point I become uncertain.

Can anyone offer some advice/guidance which will help me prove analytically what I have recorded during experimentation.

Many thanks, Gareth.

[Borek]

You are probably on the right track (that is it is conductivity that counts), but I doub't you will be able to calculate it. In so concentrated solutions activity coefficients can't be calculated theoretically.