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Problem

I am trying to do a kinetics analysis on the following reaction:

$$\ce{Na2S2O3 {(aq)} + 2 HCl {(aq)} -> S {(s)} + 2 NaCl {(aq)} + SO2 {(g)} + H2O {(l)}}$$

Suggested Method

However, most practicals online suggest I use an 'X' Mark and let the sulphur precipitate cloud the mark (Here's an Example) . To measure the rate of the reaction, I would begin timing as soon as I mixed the two solutions and end as soon as the 'X' was completely obscured.

Alternatives

However this form of time measurement is somewhat qualitative.

I considered looking at:

  • pH or temperature:       Those readings would not provide substantial data.
  • Change in volume or mass:   For the gas released, but $\ce{SO2_{(g)}}$ is dissolvable in water.
  • Conductance:         Although this is viable, I was told there is an alternative.
  • Colour (Spectrometry):    This probably won't work because the spectra observed and                  measured will suddenly reach its maximum as solids are                becoming present that have complete absorption.

I was told that the last option (colour or spectrometry) was "on the right direction".

Question

So, with all this in mind...

Are there any quantitative means of measuring the rate of the above reaction?

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  • $\begingroup$ You could try arresting the reaction and then measure the important quantities. $\endgroup$ Apr 12 '18 at 12:11
  • $\begingroup$ Hmm, this is all rather too simple methods to get much insight in the reaction. It's much more complicated then it may seem and mechanism is highly non-trivial. Potentiometry may be better, but more sophisticated methods would be needed to study it in depth. $\endgroup$
    – Mithoron
    Apr 12 '18 at 21:26
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Measuring conductivity over time might do the trick (7 ions (8 charge equivalents) in, 4 ions/charge equivalents out).

Regarding spectrometry: you could deliberately make use of the light scattering caused by the precipitating sulphur. Should be somewhere in the 600+ nm range. It's a standard method in biological sciences to follow the growth of bacteria.

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The consumption of [S2O3]2- over time could be monitored using in situ FT-IR spectroscopy (eg. using a react IR instrument). This way the measurement is done in reflectance mode rather than transmission mode which solves the issue of interference from the solids formed such as sulfur. This will provide the reactions profile at different concentrations of [S2O3]2-. Then there are various options for processing the data with the best ones being either initial rates or even better reaction progress kinetic analysis. For mechanistically simple reactions you could use integrated rate equations as well.

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