Final answer:
Boric acid is isotonic as an ophthalmic preparation, but cannot be used in the blood due to osmolarity differences.
Step-by-step explanation:
Boric acid is isotonic as an ophthalmic preparation because it has the same osmolarity as the fluids in the eye, which prevents any fluid shifts that could harm the eye. However, boric acid cannot be used in the blood because it is not compatible with the osmolarity of blood. Blood has a specific osmolarity range that must be maintained for proper cell function, and using boric acid in the blood could disrupt this balance and harm the red blood cells.