i heard recently that hydrochloric acid yields only the trimeric form whereas sulphuric acid yields both trimetic- and dimeric form.
Both reactions are to be done in an ice-bath

you can use either acid. i am fairley sure the trimeric form will form if you cool the mixture in an ice bath but if you don't cool the mixture you will get the dimeric. the dimeric form is more unstable and therefore less safe than the trimeric form. as i am not 100% sure can someone please correct me if i am wrong.
also your question isn't that clear.

I belive that was an error in the makeshift aresenal. both acids will yeild trimer version which is more stable and powerful. the dimer version forms when temperature is too high

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It's because H2SO4 tends to be available much more concentrated than HCl (98% Vs 30%) so the H2SO4 has a greater heating affect on the reaction because of it's higher concentration. More heating means higher reaction temp, which means the dimeric form tends to be produced.

And thats why most sources instruct to dilute the H2SO4.

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Thank you for you help and i'm sorry if my question wasn't stated very clear.
This would be becouse I don't live in an englisch speaking country

HCl is also a better acid than H2SO4 since it is more dissociated in water (lower pKa/higher Ka).
THus a better catalyst too!
Much cheaper also!

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