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I am concentrating sulphuric acid. What should I use as boiling chips? I have tried broken mug pieces, the solution became cloudy, and I have tried broken glass (retrieved from a drinking class, not crystal) and the solution became yellow.

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    $\begingroup$ Clean, non-dyed glass won't react $\endgroup$ – The_Vinz Jul 14 at 18:22
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    $\begingroup$ I have no idea, probably residual dirt or dye? You could try with another piece of glass. Also, since I suppose that you did it in a glass container, maybe the container itself was dirty? $\endgroup$ – The_Vinz Jul 14 at 19:19
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    $\begingroup$ Anyway, unless you are doing this in a proper lab setting, note that this is a nasty procedure: the acid has to be heated a lot, and it can make nasty fumes. Some time ago I had to vacuum distill sulfuric acid in the lab, and I can't avoid suggesting great care $\endgroup$ – The_Vinz Jul 14 at 19:23
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    $\begingroup$ The_Vinz Yes I have been treating the acid with the utmost respect. The beaker was clean and so were the glass pieces. Keep in mind when I boiled the acid using porcelain chips and the same beaker, the solution did not turn yellow but instead cloudy. I guess I should try again with another source of glass? Or is the glass most definitely not the problem here? $\endgroup$ – Jonathan Feltrin Jul 14 at 19:34
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    $\begingroup$ A better question is why? It's probably safer, more accurate, and more cost effective to buy concentrated acid and dilute it. $\endgroup$ – Zhe Jul 15 at 0:18
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Clean porcelain or glass shouldn't react with concentrated sulfuric acid. For boiling chips you want to use homogeneous material with a substantial/developed surface:

  • Porous unglazed ceramic. Raschig rings, which are also widely used industrially for the similar distillation purposes, are probably the ideal choice. An alternative for a consumer market would be porous ceramic rings for aquarium filters, which is basically the same thing (stick with the white ones).

  • The majority of glass types (borosilicate glass, quartz, …). Broken pieces of chemical glassware or light bulbs would do. For higher efficiency, use glass capillaries or sintered glass pieces.

Potential cause of cloudy solution when you used mug pieces might be the glaze on top of porcelain or the porcelain itself if it hasn't been properly thermally treated (remaining binder), and, of course, porcelain dust particles leftovers may float around forming a suspension.

Yellow color of sulfuric acid when you used glass pieces is likely an indicator that acid came in contact with organics (e.g. oil, grease). Pure colorless glass won't cause this coloration. On a side note, there is no need to use grease at all as sulfuric acid itself is quite viscous and can serve as a sealant for the joints (if you are using any).

Notes, advises

  • If the liquid has been cooled down, make sure to add at least one new chip before heating again to the boiling point (prevents superheating).
  • Avoid using boiling chips for vacuum distillation.
  • Never add boiling chips to a hot solution as it will result in rapid vapirization and the liquid is going to boil over or bump.
  • Keeping a substantial amount of sodium carbonate nearby might come in handy in neutralizing spilled acid, if there is any.
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