Which of the following is a FALSE statement?

Let us evaluate each statement:

Option 1: Carius tube is used in the estimation of sulphur in an organic compound. This is true. In the Carius method, the organic compound is heated with fuming nitric acid in a sealed Carius tube, and sulphur is estimated by precipitating it as $$BaSO_4$$.

Option 2: Phosphoric acid produced on oxidation of phosphorus present in an organic compound is precipitated as $$Mg_2P_2O_7$$ by adding magnesia mixture. This is true. Phosphorus is oxidized to phosphoric acid, which reacts with magnesia mixture ($$MgCl_2 + NH_4Cl + NH_4OH$$) to form magnesium ammonium phosphate, which on ignition gives $$Mg_2P_2O_7$$.

Option 3: Carius method is used for the estimation of nitrogen in an organic compound. This is false. The Carius method is used for the estimation of halogens and sulphur, not nitrogen. Nitrogen is estimated by Kjeldahl's method or Dumas' method.

Option 4: Kjeldahl's method is used for the estimation of nitrogen in an organic compound. This is true.

Therefore, the false statement is: **Carius method is used for the estimation of nitrogen in an organic compound**.