Measuring reaction progress is often done by measuring gas volume or mass loss, but it's worth considering other approaches.

One commonly undertaken practical carried out by students when investigating rate is the reaction between iodine and propanone.

As the reaction results in a colour change the progress of the reaction can be monitored using a colorimeter. The chosen filter should let through only the wavelength to be absorbed by the coloured iodine solution. Because iodine solution is brown-red, a blue-green filter is used.

The equation for the reaction is:

I₂(aq) + CH₃COCH₃(aq) _↓ CH₃COCH₂I(aq) + H⁺(aq) + I^–(aq)

As the reaction proceeds so the concentration of iodine decreases and the brown colour is lost and the contents of the cuvette become colourless.

Look at this video showing propanone reacting with iodine.

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This generates curves using colorimetry. 

How do we know what the concentration of iodine is at any point during the experiment?

At the start of the experiment the colorimeter was calibrated using a series of solutions of known concentration of iodine. So the concentration of iodine can be determined at any point during the experiment using the calibration curve.