Whats the role of sodium tungstate in Iron ore Determination ?

we had an experiment ,we are to determine the iron content of an ore.After digesting the sample with HCL we added in SnCl2 then added in sodium tungstate then added titanium chloride then potium dichromate.afterwards put mixed acid of sulfuric and phosphoric then diphelamine sulfonate as indicator then titrated it with potium dichromate. can somebody give me all the reactions and give me the role of each of the chemicals used in this procedure. Thanks.