39. To determine the formaldehyde content of HSON, a weight of a technical preparation weighing 0.2879 g was dissolved in water, NaOH and 50.00 ml of 0.1004 N were added. solution I2 (feqv(I2)=1/2):
HCOH + 3NaOH + I2 = HCOONa + 2NaI + 2H2O
After the acidification of the solution, 15.20 ml of a thiosulfate solution with Т(Na2S2O3) = 0.01600 g/ml was used for titration of excess iodine. Calculate the mass fraction (%) of formaldehyde in the preparation. Answer: 18.16%.

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