Police said Joanne Rand was sitting on a bench in Frogmoor, High Wycombe, on the afternoon of 3 June when there was a fight between a group of men.
A bottle of liquid, thought to be an alkaline substance, was kicked and went over her.
Ms Rand, 47, of Marlow, was discharged from hospital but was readmitted last Friday and died on Wednesday. A murder investigation is under way.
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A post-mortem is yet to take place.
Police said the bottle of liquid had yet to be recovered.
Det Supt Paul Hayles said the bottle might have been discarded nearby in or with a "dark-coloured shoulder bag".
The bottle is described as white with an orange or red hazard label on the side.
Det Supt Hayles warned members of the public not to touch the bottle.
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