[Chem 26.1] Experiment 7

Standardization of Na₂S₂O₃ (Step 1)

Weigh 1.0 g KI in 250-mL Erlenmeyer flask

Standardization of Na₂S₂O₃ (Step 2)

Weigh and record 0.15 g of primary standard KIO₃ in 50-mL beaker

Standardization of Na₂S₂O₃ (Step 3)

Dissolve in 50 mL distilled water

Standardization of Na₂S₂O₃ (Step 4)

Transfer solution to 250-mL volumetric flask, then dilute to mark

Standardization of Na₂S₂O₃ (Step 5)

Take 10.00 mL aliquot of KIO₃ standard solution and transfer to Erlenmeyer flask in step 1

Standardization of Na₂S₂O₃ (Step 6)

Add 20 mL distilled water into flask

Standardization of Na₂S₂O₃ (Step 7)

Add 10 mL of 0.5 M H₂SO₄

Standardization of Na₂S₂O₃ (Step 8)

Titrate solution with Na₂S₂O₃ until pale-yellow color is obtained

Standardization of Na₂S₂O₃ (Step 9)

Add 1.0 mL starch solution

Standardization of Na₂S₂O₃ (Step 10)

Continue titration until blue color disappears

Analysis of Water Sample (Step 1)

Add 0.5 mL of MnSO₄ and 0.5 mL of NaOH (with KI and NaN₃) to water sample using syringe

Analysis of Water Sample (Step 2)

Close bottle and shake thoroughly (should be filled with precipitates)

Analysis of Water Sample (Step 3)

Add 2.0 mL of concentrated H₃PO₄ with pipette just below the surface of the water (should dissolve precipitates and will turn solution to dark/pale yellow color)

Analysis of Water Sample (Step 4)

Shake solution and allow to stand for 10 mins

Analysis of Water Sample (Step 5)

Take 50.0 mL aliquot of solution and transfer to 250-mL Erlenmeyer flask

Analysis of Water Sample (Step 6)

If sample is dark-yellow, titrate with Na₂S₂O₃ until pale yellow color. If sample is pale yellow, add 1.0 mL starch solution.

Analysis of Water Sample (Step 7)

Express DO content in parts per million (ppm)

Clean, good water

8-9 ppm O₂

Slightly polluted

6.7-7.9 ppm O₂

Moderately polluted, can sustain life of warm water fishes

4.5-6.6 ppm O₂

Highly polluted

Below 4.5 ppm O₂

Cannot sustain life

0-2 ppm O₂