[campycalire]

Hi, I have the plan for OCR, to determine the concentration of NaOH using titration and another method. I know there is a thread already on this topic, however, I am just really stuck. Been going over it all day and getting really stressed. For the second method I have not got a clue, well I have. I am wondering if I can react Fe 2+ ions with NaOH and then get the precipitate and then measure the concnetration using a colorimter? as my teacher said that I can use a reaction that NaOH reacts in and measure the abosrabnce to find the concentration. I am just so confused, I don't no what the second method is. Also I am stuck on the titration, I know I sound thick lol. But just need stuff clayfying, firstly, do I need  to make the NaOH into a solution by using a volumetric flask? Or can I just pippete 25cm3 of the NaOH directly into the conical flask? Also how do I work out the quantities of the reactants that I am going to use?
Please please reply caus im beating myself up like a idot
Thanks
Claire
xxxxxxxxxxxxx

[Arkcon]

But just need stuff clayfying, firstly, do I need  to make the NaOH into a solution by using a volumetric flask? Or can I just pippete 25cm3 of the NaOH directly into the conical flask?

Briefly, you'll get the best accuracy and precision using a volumetric flask or if your 25 mL pipette is likewise a volumetric pipette, then it will work fine in a plain old Erlenmeyer (conical) flask.  If you're not using volumetric glassware, you're not really doing it right, and you're not really learning what they want you to learn for your O-level and A-levels, I'm betting.

As for he rest of the problems you're having, the questions on this forum show lots of confusion all around.  The best suggestion is to just go to the local university library and read a bog-standard college textbook on analytical chemistry.  Then you'll have a good starting point for the requirements and background knowledge, and you have a better reference at write up time, instead of "my teacher says" or "some guy on the internet said"

[campycalire]

Yeah I can see your point. I prefer to use a volemtiric flask and then pippete the sodium hyrdoxide soloution from that, but how do I do this. Like how much sodium hyrdoxide should I place in the Volumetic flask? And then how much water to make up the soloution? Or would it be easier just to pippete the sodium hyrdoxide from a beaker into the conical flask?

Lol thanks for that so confused with it all, have you not got any idea?
Thanks
Claire

[campycalire]

Also what about this for the other method?

FeCl2 + 2NaOH > Fe(OH)2 + 2NaCl Then as Fe(OH)2 is dirty green measure the absorbance of this using a colorimeter, and then measure absorbance of several solutions which I no the concentration to? Then use graph to determined the concentration?
When find the concentration of the Fe(OH)2 then can work out number of moles by, concentration X Volume/ 1000 the volume would be the amount put inside the the curvette, put 10cm3 refer to the experiment  that have done. Then work out that mole use the relationship which is 1 to 2, hence number of moles is twice as much. Then use the experiment, and times the number of moles by 2, then work out concentration by moles/ volume. This will give the concentration.

[Arkcon]

Lol thanks for that so confused with it all, have you not got any idea?
Thanks
Claire

Yes.  Yes I do.  I'd done it, often enough, in US high school classes, university classes, and in industry.  But I can't give you all the answers, because that would be cheating, and the UK is pissed off enough at the US with Dubya's war, and would likely turn their nukes loose on us if I gave it all away.  Furthermore, you might or might not be able to tell, but asking me for "everything", is like asking 50 questions at once, and I don't have time for that.  Try a textbook, you'll see many of the steps, and some of the reasons why we do things a certain way.  That way you can write a good convincing explanation for your decisions.  And from there, we can help you a little bit, with some of the smaller (not simpler) problems you're having.

I worry a little bit about your spectrophotometric measurement of Fe(OH)2, however.  What you're doing is a turbidity analysis -- measuring how light is scattered by an insoluble component in solution, and that for one thing doesn't necessarily follow Beer's law.  Furthermore, you'll have to take into account how the solid forms, to get uniform particles forming from sample to sample, and then there's the issue of settling, and how much time the sample sits out.  Look up turbidity, it is not a simple procedure.

[campycalire]

Ok I understand your point thanks for all your help though.

Yeah well I not gone that far to be honest, I will try thinking about it bit more as got till next tue so gonna work hard

Claire