[KBriggs]

I want to make a pH 13.5 KCl solution with a conductivity between 1000 and 5000 uS/cm (roughly the conductivity of a 10-50mM KCl solution). The problem is that in order to get the pH so high, I need to add so much KOH that the conductivity is closer to 50,000uS/cm on the strength of the potassium alone.

Is there any way to raise the pH without adding to the conductivity?

[Arkcon]

Not really.  By definition, pH requires an ionizable salt, so the pH adjuster will simply have to contribute some conductivity.  Does it have to be KCl?  Would a KOH solution not work as well as KCl for your application?

[Borek]

You are asking for impossible. pH 13.5 means OH^- concentration of 0.3M - 300 mM. Thats already about three times more that the sum of concentrations of K⁺ and Cl^- in 50 mM KCl solution. Add to that fact that limiting ion conductivity of OH^- is at least twice higher than limiting ion conductivities of both K⁺ and Cl^- and it is obvious that pH alone rules out conductivity you are aiming at.