Plural/singular verb agreement with units

Solution 1:

These constructions are called "mass nouns" (or something comparable). Although they refer to more than one thing (e.g., more than one milliliter of water), they treat these things as a single unit.

Mass nouns take singular verbs - not plural (reference and reference).

Take your first example:

I took 2 ml of water, which was/were then added...

The water, despite being more than one milliliter, was added at the same time, as a single unit. It would not be helpful (semantically or grammatically) to think of each milliliter of water as being added separately. Instead, treat the mass noun as the group it is, and use the singular.

Solution 2:

Use singular verbs with units. This usage is so because the measurement results in a quantity of a substance, e.g., 2 litres (better written as 2 L) is a quantity of water with the water still being a non-count noun. To avoid this problem, use the measured quantity as the object of verb as illustrated below:

a. Add 2 L of water and stir. b. Tie 10 m of the rope around the neck of the animal and tether it.

Never write the word name of a unit of measure after a number to represent the result of measurement, e.g., 5 metres is incorrect. Wrte 5 m. However, when used as an adjective, both the symbol and word name are correct, e.g., 5 km journey and 5-kilometre journey. Do not insert a hyphen when the unit symbol is used.