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I use Sodium persulfate to etch a copper board to produce a PCB. I mix 1 dl sodium persulfate with 1 liter hot water. When the water get cold, I heat it up by microwave. After the microwave, I see such crystallized particles at the bottom of the jar. Please note that there were not any solid particles in the acid before the microwave. I wonder what are these particles? is it sodium persulfate? if it is so, why it is crystallized when it is heated up?

Solubility of water increases as the temperature of the water increases, right? Then why it causes solid particles in the car?

enter image description here

Btw, if I put these crystallized particles into the same solvent, they don't dissolve, however, if I put them into a new hot water cup, they dissolve.

If it is sodium persulfate and if they are crystallized because of the microwave, can't I reuse them again?

If it is copper particles, isn't it supposed to be in such color?

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  • $\begingroup$ There are some confusing statements in your post. For example, you describe dissolving sodium persulfate in water and then you mention acid later. Did you dissolve it in water or acid (and which acid?)? That matters. $\endgroup$ – Ben Norris Jul 7 '13 at 19:17
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It looks to me like copper sulphate ($\ce{CuSO4·nH2O}$). If I'm right then you can't use it for etching. For this you need oxidant (persulphate). Try to solve this solid in water. You should get light blue solution. Stick into it a nail (i.e some metal iron source). If it cover itself with a brownish powder like in wiki-link you gave, then it's a copper(+2) salt.

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  • $\begingroup$ thanks for the answer. do you have any guess why I get crystal particles after I heat up the liquid by microwave? There were no such solid particles when the solution was cold $\endgroup$ – sven Jul 7 '13 at 13:40
  • $\begingroup$ Usually warming improves solubility of simple inorganic salts, however higher temperature generally increases reactions rate. That also includes building of crystals. If you leave your solution long enough (over night) there is a good chance that you will see the same solid. Probably the only difference would be that crystals would be bigger and nicer shaped. $\endgroup$ – Kris_R Jul 8 '13 at 18:45

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