The concentration of SO2 in air is determined by bubbling a sample of air through a trap containing H2O2. Oxidation of SO2 by H2O2 results in the formation of H2SO4, which is then determined by titrating with NaOH.
H2O2 + SO2 →
H2SO4
H2SO4 + 2 NaOH → 2 H2O +
Na2SO4
In a typical analysis, a sample of air was passed through the peroxide trap at a rate of 12.4 L/min for 60 min and required 10.30 mL of 0.0252 M NaOH to reach the phenolphthalein end point. Calculate the mL/L SO2 in the sample of air. The density of SO2 at the temperature of the air sample is 2.86 mg/mL.
Let us consider a reaction,
H2SO4+2NaOH--------->2H2O+ Na2SO4
2 mole of NaOH consumes 1 mole of H2SO4
moles of NaOH = molarity* volume in L
= 0.0252 * 10.30 / 1000
= 0.0002595
moles of H2SO4 = (1/2) * moles of NaOH
= 0.0002595 / 2
= 0.000129
from reaction SO2+ H2O2------>H2SO4
1 mole of H2SO4 requires 1 mole of SO2
hence moles of SO2 = 0.000129 mol
mass of SO2 = moles* molar mass of SO2
= 0.000129 * 64 gm
= 0.0082 gm
= 8.2 mg
density of SO2 = 2.86mg/ml,
volume of SO2 = mass / density
= 8.2 / 2.86 ml
= 2.86 ml
flow rate of air = 12.4 L/min
time air is sent = 60 min,
volume of air sent = 12.4 * 60 L
= 744 L
hence ml of SO2/ L = 2.86 / 744ml/L
= 0.00384 ml/L
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