# Is there any way to quantitatively find when a reaction has completed?

So I'm designing an experiment for measuring how the reaction time of copper and ferric chloride is dependent on temperature; here's the reaction:

$$\ce{FeCl3 + Cu -> FeCl2 + CuCl}$$

The problem I have right now is how to quantitatively measure when the reaction has ended. I know you can visually see when a small copper rod has completely dissolved, but I wanted there to be a quantitative aspect to it rather than qualitative.

I've thought of using a pH probe to see if the PH changes after the reaction takes places and stays static, which I could leverage as an endpoint of the reaction, but the pH probes my school are damaged, I think, since the reading keeps jumping around by about 3 or 4.

If you have measured reaction time in general before, I'd love to hear how you did it.

• You'd probably want to determine the order of your reaction; further calculations down the line, then, are a cakewalk. May 19, 2017 at 16:39
• UV-vis would make this very simple, you just need to monitor when the absorbance at a certain wavelength (probably that of Fe(III)?) stops changing. May 19, 2017 at 17:18
• I will definitely look into the order of the reaction. May 19, 2017 at 21:01

Option 1: Spectrometry

Easiest method, in most cases accurate enough. If you know that there are no side-reactions, this is a safe bet. Simply measure some characteristic wavelength for one reactant (preferably $\ce{Fe^{3+}}$) and one product (I would go for the $\ce{Cu^{+}}$ there) and then make measurements of your reaction mixture on the fly.

Option 2: Titration

Not that recommendable if you need any reliable results, but if you are low on budget and/or want a didactic effect. Take a little sample from a big flask and do a $\ce{Fe^{2+/3+}}$-redox-titration.

Option 3: Electrolysis

Don't know how to implement that, but basically you would take the sample and measure the amount of electricity needed to convert all $\ce{Fe^{2+}}$ in that sample back to $\ce{Fe^{3+}}$. Probably more accurate than spectrometry as the measurements of currents can be done quite precisely

You could also do some weird stuff like quantitative chlorine-NMR, but I can not think of any benefit that you would have with that.

• Possibly relevant reading: (faculty.sites.uci.edu/chem1l/files/2013/11/…) May 19, 2017 at 20:18
• I think I will go with the Spectrometer method. My school has one of those, so that will be possible. Looks like I have a potential solution; thanks so much for your input! May 19, 2017 at 21:01