The problem of measuring calcium in Feed

For the determination of the sample, accurately remove the sample solution 10mL~20mL (calcium content 20mg) into a 200mI beaker, add distilled water 100mL, methyl red indicator 4.82drops, add ammonia solution 4.5until the solution is orange, if excessive, add hydrochloric acid solution to orange, add 2 more drops to make it pink (pH = 2.53.0), boil carefully Slowly heat the oxalate solution for 10 mL and stir continuously. For example, if the solution turns orange, add hydrochloric acid solution to make it red, boil for a few minutes, and let the precipitation age overnight (or heat it in a water bath for 2 h). What is the purpose of adding ammonia and hydrochloric acid in the determination step? Why not add ammonium oxalate directly after methyl red to form calcium oxalate precipitation? Then titrate it with potassium permanganate in an acidic environment?