[djiang87]

Why is alternating current used to measure the conductivity of solutions?

The literature value of Ka for ethanoic acid is 1.75^10-5 molkg^-1 is approximately equal to  1.75 x 10^-8 at 25 degrees celcius. Do conductivity measurements provide an accurate method of measuring acidity constants and solubilities?

Critically compare the available methods for measuring Ka

Any help would be much appreciated. Thx

[MrTeo]

I'll answer to your first question, even tough the whole request looks like a homework assignment...

Using direct current would cause the solution of which the conductivity has to be measured to start electrolysing thus not following Ohm's Law (IR=∆V) which allows us to find out the conductivity as the inverse of resistivity. On the other hand, if we use alternated current, the adsorbate on each electrode will electrolyse in the short time during which it's subject to a positive charge (for a 1000 Hz AC it's 10^-3 s) and then return to its original state during the next moment, when the direction of the current is inverted. In this way there is no real electrolysis in the solution and it follows Ohm's Law (there are also methods using DC instead of AC to measure conductivity but they're much less precise than the other mentioned).

[Borek]

value of Ka for ethanoic acid is 1.75^10-5 molkg^-1 is approximately equal to  1.75 x 10^-8 at 25 degrees celcius.

No, 1.75e-5 is not - even approximately - equal to 1.75e-8.

[djiang87]

Yeah i know those values arent close but thats wot was written.

And to the first guy to reply, yeah the questions i posted up are the ones i couldnt answer from after performing a prac i did not too long ago. The problem with the questions is that we never learnt anything about conductance. Infact i dont even see a relation between the prac i did and our subject which is 'pharmokinetics'.

[MrTeo]

Well, consider that sometimes with conductivity measurements (especially when the reaction involves free ions) we can find out reaction speed... (the changings in conductivity tell us approximately how many ions have been produced or used)

And sorry if I seemed a bit suspicious about your question... but you can't even imagine how many people asks us to solve their homework 

[djiang87]

MrTeo yeah i can understand that, i wouldnt be surprised if people did.

Also i realised in the second question i asked about the values are approximate as if you look at the difference in SI units it does make them similar in value to each other.