For an experiment, I'm adding several substances (as powders) to hydrochloric acid, and measuring the temperature changes to calculate enthalpy. However when adding the powders, the mixture requires some stirring to get all of the substance reacting.

As I am measuring temperature via a temperature probe hooked up to a data logger (to obtain a temp. against time graph), I need a way to briefly stir the mixture in a controlled way that won't affect the change in temperature (reactions are both endo and exothermic). I have tried stirring with the probe itself, but this is not ideal as the probe is pretty sensitive and friction/bumping with the side of the vessel may affect the recording.

I initially looked at a magnetic stirring to obtain uniform and controlled stirring, but then I realized the magnet would absorb heat and potentially increase the temperature of the mixture through kinetic energy. So I'm after a stirring method that will not affect temperature, so that the temperature change I record is solely due to the reaction.

Any suggestions are welcome, because I can't think of any tool or method that would suit.

  • $\begingroup$ Since you are measuring variation, stirring will not affect the result, I mean, just take the initial temperature under stirring, then then the difference would be the same with or without stirring. $\endgroup$
    – Babounet
    Mar 26, 2015 at 9:08
  • $\begingroup$ I'm not 100% sure what you mean. The graph of temp. against time looks like this, and stirring tends to make the curve a bit bumpy and difficult to extrapolate. $\endgroup$
    – deusy
    Mar 26, 2015 at 9:15
  • $\begingroup$ I mean, to measure temperature change, you have to make a difference. So if the two parts of the difference are impacted by the same perturbation, they'll cancel. $\endgroup$
    – Babounet
    Mar 26, 2015 at 18:02
  • $\begingroup$ Do you mean hydrogen chloride or hydrochloric acid (aqueous) ? $\endgroup$
    – Lighthart
    Mar 26, 2015 at 22:15
  • $\begingroup$ @Lighthart Sorry, I meant hydrochloric acid. Fixed it up. $\endgroup$
    – deusy
    Mar 27, 2015 at 7:15

1 Answer 1


To minimize thermal coupling between the stirring apparatus and the solution, the best choice is a small, slowly stirred, PTFE or equivalent coated magnetic stir bar. Heat generated by friction should be negligible.

A potential source of heat transfer into the system from this configuration is heat from the stirrer motor. To mitigate this, the surface of the stir plate should be covered with a strip of Styrofoam as thermal insulation. Furthermore, the reaction vessel could be physically separated from the stir plate by using a ring stand and clamp or equivalent. It may be necessary to adjust the thickness of the Styrofoam and distance of physical separation in order to keep the stir bar magnetically coupled to the stir plate magnet.

The setup should first be tested by doing several full runs without adding the acid or the powders. This is essentially an analytical blank. If the temperature changes can be considered negligibly small throughout the duration of an experiment, then you are ready to go. If necessary, any temperature change measured from the blank runs can be subtracted from the actual experimental measurements.


Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.