Estimation of Reducing Sugar by Somogyi’s Method

The reducing sugar selectivity reduces copper salt in alkaline medium. The oxidation of CuO by atmospheric O2 is prevented by the addition of Na2SO4 on acidification of the reaction mixture with H2SO4. The KI and KIO3 present in the reagent liberates iodine. In acidic medium, cuprous oxide goes into the solution, producing cuprous ions, and these ions are oxidized by the liberated iodine. The excess of iodine is liberated against N/200 Na2S2O3, using starch as an indicator.

Reagents Required

  1. Copper reagent: Dissolve the following substance in water and make up 1 liter with distilled water and filter.
    (i) Anhydrous sodium phosphate or disodium hydrogen phosphate 28 gms (or) 71 gms, respectively.
    (ii) Sodium potassium tartrate-40 gm.
    (iii) 1N KOH–5.6 gms in 100 mL.
    (iv) 8% CuSO4–8 gms of CuSO4 in 100 mL.
    (v) Anhydrous sodium sulfate 180 mg.
    (vi) KI-8 gms.
    (vii) 1N KIO3-0.892 gm in 50 mL.
  2. In H2SO4—27.8 mL of H2SO4 in 1 liter.
  3. Standard glucose solution, dissolve 100 mg in 100 mL of water.
  4. 0.01 N K2Cr2O7 solution—dissolve 490 mg of K2Cr2O7 in 100 mL of water.
  5. 1% starch indicator: Make paste from 1 gm soluble starch and 5 mL distilled water, pour the suspension into 100 mL boiling water boil for 1 minute. Cool and pressure by adding a drop of chloroform.
Standardization of sodium thiosulfate solution: take 10 mL of 0.01 N K2Cr2O7 in a conical flask to this add 1 gm of KI and half a test tube of dilute H2SO4. One to two drops of starch indicator is then titrated against 0.01 N sodium thiosulfate solution. The end point is blue to colorless. From this 0.005 N sodium thiosulfate is prepared.

Pipette out 0.2 to 1 mL of standard glucose solution into a different test tube until the volume is 2 mL with distilled water. Add 2 mL of Cu-reagent to each and add 2 mL of 1 N H2SO4 to each test tube. Keep it aside for 2–3 minutes and then titrate the liberated iodine against sodium thiosulfate sodium using 1% starch as an indicator. Carry out a blank under the same condition using 2 mL distilled water instead of sugar sodium.

Result: The given unknown sample contains ................ mg / % of sugar.