Here's why:

* BaSO₄ is insoluble: Barium sulfate (BaSO₄) is a white, very insoluble solid. If you added BaSO₄ to a sample, you wouldn't be able to observe any reaction or precipitation, which is the basis of the limit test.

* BaCl₂ forms a precipitate: Barium chloride (BaCl₂) is soluble in water. When added to a solution containing sulfate ions (SO₄²⁻), it reacts to form BaSO₄, which precipitates out as a white solid. This visible precipitation is the key indicator of the presence of sulfate.

Here's how the limit test for sulfate works:

1. A known volume of the sample solution is treated with a specific volume of BaCl₂ solution.

2. If sulfate ions are present in the sample, they react with the BaCl₂ to form BaSO₄, which precipitates out of solution.

3. The amount of precipitate formed is compared to a standard reference solution containing a known concentration of sulfate.

4. This comparison allows you to determine if the sulfate content of the sample is within the specified limit.

In summary, BaCl₂ is the preferred reagent for the limit test for sulfate because it reacts with sulfate ions to form a visible precipitate (BaSO₄), allowing for a reliable determination of sulfate content.