You'll probably damage the glass electrode (via dissolution) putting it in solutions of pH 13 or 14....

If you know the molarity of the NaOH and assuming full dissociation, then you can estimate the pH, yes.  But why do you need to know the pH if you know the starting and finishing molarities?

Thanks Dr Rawle, I know the Molarity of the initial NaOH exactly, I know the stiochiometry also, but I have used different volumes of the alkali in different experiments so I need to know the final pH and the solution volume has changed due to addition of iron salt solution also. So final volume is not the same as the NaOH volume used in the reaction. I wanted to check my calculation but am unable to check the pH of final solution exactly. So can I assume my calculated value is correct?

I still don't know why you need the pH - is it just the desire to 'have a number'?  What additional information does this provide?  I'd measure the OH- concentration with a simple acid-base titration.

I need the pH as the value of pH is going to affect the properties of Iron oxide nanoparticles. They are reported to be synthesized from 8-14 pH. I want to know exactly what is the final pH of my solution at which the particles are synthesized.

In what way does the final pH affect the properties?  What properties in particular?

The co-precipitation of Iron (III) and (II) salts depends upon the pH of the reaction medium.The size of the particles , also crystallinity and thus magnetic properties.

Agreed - but precipitation in alkaline pH's leads to Fe(OH)2 or Fe(OH)3 and this will need roasting to produce iron oxide (Fe2O3 here?).  That leads to sintering of particles. 

Yes but I make Fe3O4, but the question is still that I need to know final pH, as I have used different pH values. The OH-conc.. is high in pH 13/14 so would it be ok to use theoretically  calculated values of pH? Or I should use the titration?