why sulphuric acid is added in residue on ignition test what ia the exact resion

1)The gravimetric technique is used to perform the residue
ignition test or sulphated ash test. The residue obtained
after sulfuric acid treatemnt is chemically and thermally
stable and it fulfiles principles of gravimetric analysis
techique.
2)The residue obtained without use of sulphuric acid are
sensitive to carbon dioxide and moisture present in lab.
atomosphere. Hence the residues composition changes and
ultimately mass of residues changes during final weighing
of residue and the accuracy and precision of test will be
questionable.