The Folin–Wu and the Somogyi–Nelson methods are both based on the same principles. In the first step, glucose (or a reducing sugar) is oxidised using a solution of Cu(II) ion, which is reduced to Cu(I) by the process. In the second step, the Cu(I) ions are then oxidised back to Cu(II) using a colourless hetero-polymolybdate complex, which is, in the process, reduced to give the characteristic blue colour. Finally the absorption of the hetero-poly molybdenum blue is measured using a colorimeter and compared to standards prepared from reacting sugar solutions of known concentration, to determine the amount of reducing-sugar present. <br />
The Folin–Wu method  uses a reagent that contains sodium tungstate. The exact nature of the blue complex in this procedure is not known.<br />
The Somogyi-Nelson method uses an arsenomolybdate complex formed by the reaction of ammonium molybdate, (NH) MoO, with sodium arsenate, NaHAsO.