A sample must not contain more than 2% contaminants to be pure. Are there instances where 2.12% is ok? I do not have any background in chemistry, I am asking this on behalf of someone else, twice translated, so I really hope it makes sense.

"A maximum of 2%" seems quite a clear specification, but since  practice involves rounding of measurements and significant decimals, that might not be the case.

It may seem silly but which of the following options are valid work protocol?


at most 2% means less than 2.5% after rounding
at most 2.0% means less than 2.05% and option 1 still holds
at most 2% is the same as at most 2.0% and means at most 2.000...%




Off topic: An optimal answer would be in the context of the EuAB (Europäisches Arzneibuch).
It depends mainly on the accuracy of your measuring instrument in practice and the accuracy required by your work.

For example:

If you want to make a solution with 2g solute in it, and to measure amount of solute you have an measuring instrument that have least count of 0.1g, then 2.05g is acceptable as the instrument will show 2.0g only but if the least count of intrument is 0.01g then 2.05g is not acceptable if you want 2g.

In real life the least count varies for different situations.

For example if you are manager of a water bottle manufacturing unit you won't need much accuracy to fortify the water with minerals, but if you are carrying out a titration in lab to find strength of some oxidising agent you need very high accuracy that is why burette are made such accurate.