[Winga]

e.g. When sodium chloride (NaCl) is added to water, two forms of ions are formed, Na+ & Cl-. I want only one form of ions (e.g. Na+) present in water, what can I do?

[jdurg]

You will NEVER have a solution with only positive, or only negative, ions in solution.  Doing so would break so many chemical/physical laws.  

[Winga]

precipitate the Cl- with Ag+. It's not really meaningful to have only Na+ in solution?

This process is not worked, because another anion from the compound of Ag+ ion with that anion is released.

[Winga]

If I am now not isolated one form of ions, I just make a solution with one ion, is there any method?

Can I ionize the Na atom in its gas phase and then pump into the water?

[ssssss]

Tell the exact purpose for which you want Na+ ions.

[jdurg]

You CANNOT have an aqueous solution where only positive, or only negative, ions exist.  The laws of physics will NEVER allow that to happen.  Also, the idea about the "gas phase" will not work since you would need to boil the metal in order to get its ion in a gas phase, and at that temperature liquid water does not exist.  You will just have to realize that you cannot isolate a positive ion without having a negative ion present to balance the charge.  

[budullewraagh]

you could just use a battery to attract anions to one side and cations to another, then use an impermeable material to divide the sections.  the more powerful the battery the better separation you will have.

[jdurg]

you could just use a battery to attract anions to one side and cations to another, then use an impermeable material to divide the sections.  the more powerful the battery the better separation you will have.

You'd need to at least have a salt-bridge working in order for current to flow.  Otherwise it would be like putting an electrical current through a saturated solution of glucose.  Not too much would happen.  

[Winga]

How about if I use nano-technology to make a selective permeable membrane to separate the ions.

[Mitch]

Depending on the reaction, it'll probably just act as a spectator ion.

[Donaldson Tan]

you could consider plasma gas..