A question about capacitor

[aman]

When I think of capacitor I can see there are two conducting parallel
plates seperated by an insulator dielectric. So as there are opposite
charges on the inside of the plates they attract each other thus
containing the charge.

Here is my question. If there is a perfect insulator used as
dielectric, does it mean that the charge is fully contained and if the
dielectric material gets a little conducting the charge on the plates
gets reduced(cannot be retained fully) and some charge flows in the
dielectric. Am I correct to some extent ?

I am asking this because I am constructing a kind of capacitor detector
which is seperated by dielectric which is water(different kind of
samples with different conductivity and am trying to nuetralise the
harmful free ions in water).

 

[aman]

So doesnt capacitance depend on conductivity of the dielectric ? I mean
wont a more conductive sample of water give a different capacitance
than less conductive water ?

 

[aman]

In C= KA/d if K does not depend on conductivity, what makes a
dielectric have different dielectric constant K. What are the
parameters on which K depends on?

 

[aman]

So doesnt this imply that if there are free ions in water there will
be a different K from a nuetral sample of water with no free ions ? I
am actually adding chemicals(ionic) to water which nuetralises harmful
ions in water to form a nuetral particle floc. So I need to detect zero
crossing. As i am adding ions into water i need to detect a zero
crossing in going from postive charge to negetive.

 

[aman]

I am adding salt to water. I read something in one of the forums below
which confuses me. It also says "adding virtually any contaminant to
water will alter its conductivity. That also ruins the diaelectric
constant . Only pure water will have a measurable diaelectric
constant."

http://forum.allaboutcircuits.com/lofiversion/index.php/t1636.html

It says that pure water is insulator and adding salt changes
conductivity. I am adding salts to contaminated water to which makes it
an insulator. But if more salts are added water crosses the point at
which it is an insulator(pure) and again starts conducting. So I need
to detect that zero crossing.

But according to the forum in the link above, conductors cant be
dielectric. But what if I use conducting water as dielectric. What
happens. I know dielectric is supposed to support electric field but
oppose current. But if there is leakage current the capacitor can't
hold charge or what ?

Also what is meant by "Only pure water will have a measurable
diaelectric constant."