Copper standardization with thiosulfate

Ofner Method. This method is for the determination of invert sugar in products with up to 10% invert in the presence of sucrose and is a copper-reduction method that uses Ofner s solution instead of Fehling s. The reduced cuprous oxide is treated with excess standardized iodine, which is black-titrated with thiosulfate using starch indicator. 

If thiocyanate is not added, the thiosulfate should be standardized with copper metal under the same conditions as for the determination of copper. ... 

As an illustration of the use of electrode potentials, consider the classical method of analysis of copper in brass, which involves dissolving the weighed sample in nitric acid to obtain Cu2+(aq), adjusting the pH to a weakly acidic level, allowing the Cu2+ to react completely with excess potassium iodide to form iodine and the poorly soluble Cul, and then titrating the iodine with sodium thiosulfate solution that has been standardized against pure copper by the same procedure ... 

Repeat the entire procedure with the dextrose standard (5 mL of Dextrose Standard Solution and 5 mL of Copper Reagent Solution), noting the volume of 0.005 N sodium thiosulfate consumed as Vj> Run a corresponding blank titration, Vb> using 5 mL of water and 5 mL of Copper Reagent Solution. 

Other primary standards for sodium thiosulfate are potassium dichromate, potassium bromate, potassium hydrogen iodate, potassium hexacyanoferrate(III), and metallic copper. All these compounds liberate stoichiometric amounts of iodine when treated with excess potassium iodide. 

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