[williec]

the aq NaOH has an approximate concentration of 2 moldm-3. i think it is to use a pH meter but it said it should involve chemical reactions. What other methords are there to find out the concentration?

[Arkcon]

Your application isn't perfectly clear, but titration seems necessicary for any accuracy.  What else is in solution, how accurately do you need the value, what other instruments do you have, that sort of info is needed for a better answer.

[williec]

there is only NaOH in the solution and you have to find the concentration as accuratly as possible, but it cant be a titration and the this method i am looking for have to involve chemical reaction. you are allowed to use any instruments.

[AWK]

Could you measure a density of solution?

[Arkcon]

So the game is, unique ways to determine concentration?  OK, I'll play.  Can you evaporate a known volume, and mass?  Or evaporate, roast to NaO, and then mass, and calculate backwards? 

Using a pH meter, on this solution, won't give you a useful answer, do you know why?  *Hint* Can you figure out the pH of a 2 M, 2.5 M, 2.25 M, and 1.13 M, or 1.0, or 0.98 M NaOH solution.  How can you fix it so you can get useful info from a pH meter? 

Notice, you still haven't answered my question, how accurately do you need the answer.  What is the likely value -- 2, 2.25, 2.5, or is it far away like 3 or 1.1.  And do you need the answer as -- 2, 2.2, 2.20, or 2.213 M.  This is info you need to know first.

[williec]

the question says find the concentartion as accuratly as possible but it is probably 2.d.p

[Arkcon]

As I understand your original question, you want to use just a pH meter, to determine the concentrations of 2.?? M NaOH, without titrating.  Can you see anything wrong with your plan?  Did you try it, what pH did you get?  Have you looked up what pH you're likely to get?

[williec]

no i havent try it yet. But if I use a pH meter to find the concentration can it involve some kind of chemical reactions?

[Arkcon]

As I study your posts, I still find it hard to understand what the question really is.  The title says you don't want a titration, you've said you want to use a pH meter, and a reaction.  You can use a pH meter, to determine the concentration of NaOH, by a chemical reaction with an acid, watching for the endpoint with a pH meter.  But that is called -- a titration.

[williec]

are ther any other ways without using a pH meter?

[Borek]

Gravimetric analysis.

[Arkcon]

Gravimetric analysis.

Yep.  Like AWK: said, weigh the liquid, or like I said, dry it and weigh it, or dry it, roast it, and weigh it.

[williec]

Dose NaOH reacts with Al and gives Hydrogen gas? if yes what are the conditions?

[Arkcon]

Start with this experiment here: http://www2.uni-siegen.de/~pci/versuche/english/v44-10.html
And work with it to see what's appropriate for your application.

[williec]

i dont really get this. so Al reacts with water to produce hydrogen??? and how do i work out the concentration of the NaOH from the whose equations if i have got the volume of hydrogen of gas produced? Or can i react it with zinc

thank you v much