If we are creating a precipitate of (say) Barium Sulfate from a solution of Barium Chloride mixed with Sodium Sulfate, what determines the particle size?

I assume one of the factors will be concentration of reactants in the solutions, but what else?

  • 2
    $\begingroup$ @Poutnik What else? temperature, rate of heating/cooling, local gravitational variations, illumination, purity, electric and magnetic fields, sonics, ionizing radiation which way the wind is blowing... $\endgroup$ – Dirk Bruere Oct 14 '20 at 8:28
  • $\begingroup$ @Poutnik What I want to know is if there are any general rules, and relative magnitudes of the various environmental influences. Of course, these may not exist in any systematic form and be totally reaction dependent. In which case all you have to do is say so $\endgroup$ – Dirk Bruere Oct 14 '20 at 8:43
  • $\begingroup$ @Poutnik I come to SE because I want some quick answer, usually to resolve a go/nogo problem. I am a working scientist/engineer and currently have 4 R&D projects on my plate. Now, I could have waited for Dr Daryl Williams, a professor of chem eng at Imperial College to get in, since his specialty is particle science. But, as you say, I'm lazy. And impatient. But thanks for the answer $\endgroup$ – Dirk Bruere Oct 14 '20 at 9:47

Kinetic aspects:

Low concentrations of interacting ions decrease rate of precipitation, leading to bigger crystals.

A high ionic strength of spectator salts leads to lower activity coeeficients , and decreased rate of precipitation, leading to bigger crystals.

Higher viscosity leads to lower collision rates, decreased rate of precipitation, and to bigger crystals.

Low temperaturee leads to bigger crystals, as rate of collisions leading to crystallisation is lower due higher viscosity, lower molecular speed and lower rates of interactions leading to precipitation.

Note that kinetic aspects may be hard to involve in the proces control, aside of concentrations.

Thermodynamic aspect:

Crystals of low sizes have high surface Gibbs energy and therefore bigger crystals are thermodynamically preferred.

This is initially overrun by the kinetics of the precipitation, but in long term there is ongoing recrystallization in favour of bigger ones. This can be extremely slow for very little soluble salts.

This process is speeded up by high temperature, what increases rates of the opposite processes of dissolution and crystallization. AFAIK, the gravimetric procedure to determinate sulphates as barium sulphate involved heating $\ce{BaSO4}$ suspension near below boiling point for I guess 1-2 hours to speed up recrystallization. As bigger crystals are easier to manage, espacially in this particularly case, as $\ce{BaSO4}$ is known to form too fine crystals.


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