In preparing calibration standards, a stock solution is typically first prepared, then diluted to different concentrations to produce a series of standards. For the analysis of SO2, standards at 5, 10, 25, 50, and 100 mg/L SO2 are required. Describe a procedure to first prepare 100 mL of stock solution (1000 mg/L SO2) using solid potassium metabisulfite salt K2S2O5. Then describe how dilutions would be done to produce 100 mL each of the remaining solutions (5, 10, 25, 50 and 100 mg/L) using the 4000 mg/L stock.

• K2S2O5 has a molecular mass of 222.31 g/mol and yields 2 mole SO2 per mole of K2S2O5
• K2S2O5 + H20 -> 2 SO2 + 2 KOH

 

Preparation of Stock Solution (1000 mg/L SO₂)

Stock Solution: 100 mL or 0.1 L of 1000 mg / L SO₂ concentration

Amount of SO₂ in 100 mL stock solution = 0.1 L x (1000 mg SO₂ / 1 L solution) = 100 mg SO₂

 

Therefore, in your 100 mL stock solution, 100 mg of SO₂ is present.

Now, you need to find the amount of K₂S₂O₅ that you need to make 100 mg of SO₂.

 

Using the equation and the given values (I searched the mol. mass of SO₂ myself since it's not given):

K₂S₂O₅ + H₂O --> 2SO₂ + 2KOH

 

Mol. Mass SO₂ = 64.07 g/mol

Mol. Mass K₂S₂O₅ = 222.31 g/mol

100 mg SO₂ = 0.1 g SO₂

 

Amount of K₂S₂O₅ needed = 0.1 g SO₂ x (1 mol SO₂ / 64.07 g SO₂) x (1 mol K₂S₂O₅ / 2 mol SO₂) x (222.31 g K₂S₂O₅ / 1 mol K₂S₂O₅)

Amount of K₂S₂O₅ needed = 0.1735 g or 17.35 mg K₂S₂O₅

 

The stock solution can then be prepared as follows:

Accurately weigh 17.35 mg of K₂S₂O₅ and dilute it to 100 mL using distilled water

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Preparation of other Standards:

To determine the volume needed to make the lower concentration standards, we need to use the dilution formula:

 

C₁V₁ = C₂V₂, where:

C₁ = Stock concentration = 1000 mg/L

V₁ = Volume of stock needed

C₂ = Final or Target Concentration = 5, 10, 25, 50 and 100 mg/L

V₂ = Volume of target solution = 100 mL = 0.1 L

 

Final formula:

V₁ = C₂V₂ / C₁

 

Let's solve them one by one:

a) 5 mg/L

V₁ = C₂V₂ / C₁

V₁ = (5 mg/L x 0.1 L) / 1000 mg/L

V₁ = 0.0005 L or 0.5 mL

 

Preparation:

Measure 0.5 mL of stock solution (1000 mg/L) and transfer it into a 100 mL volumetric flask. Fill to mark with distilled water to prepare 5 mg/L

Use the same approach for the next concentrations:

 

b) 10 mg/L

V₁ = C₂V₂ / C₁

V₁ = (10 mg/L x 0.1 L) / 1000 mg/L

V₁ = 0.001 L or 1 mL

 

c) 25 mg/L

V₁ = C₂V₂ / C₁

V₁ = (25 mg/L x 0.1 L) / 1000 mg/L

V₁ = 0.0025 L or 2.5 mL

 

d) 50 mg/L

V₁ = C₂V₂ / C₁

V₁ = (50 mg/L x 0.1 L) / 1000 mg/L

V₁ = 0.005 L or 5 mL

 

e) 100 mg/L

V₁ = C₂V₂ / C₁

V₁ = (100 mg/L x 0.1 L) / 1000 mg/L

V₁ = 0.01 L or 10 mL

 

I made a summary of the preparation for the standard solutions below :)

Please see the attached file for the complete solution