Winkler Method for Dissolved Oxygen Analysis

...

K2Cr2O7 + H2O + H2SO4 → K2SO4 + Cr2(SO4)3 + S + H2O

For the addition of sodium carbonate in the solution, formation of bubbles and a fizzing sound were observed. This indicates that there is an escape of carbon dioxide gas in the solution; which means that the carbonate ions removed the hydrogen ions in the solution producing the neutral complex. On the other hand, the addition of potassium iodide in the solution is used as an oxidizing agent in the reaction giving up an electron to reduce potassium dichromate (K2Cr2O7). The reaction between the two is shown below:

6 I⁻ + 14 H⁺ + Cr₂O₇⁻² → 3 I₂ + 2 Cr⁺³ + 7H₂O

As shown in the above equation, potassium (K) is not included because it didn’t take part in the reaction, it is a spectator ion, and only I- participates in the reaction; and in this reaction, potassium iodide liberates iodine molecules in an amount that is proportional amount to the original amount of the reduced compound which is K2Cr2O7. The amount of this free iodine molecules is determined by the volume of Na2S2O3 used in titrating the solution with the use of a starch indicator. The table below shows the recorded data on the volume of Na2S2O3 used in the standardization, obtaining the results of 46.50mL, 43.00mL and 43.50mL respectively. Before the addition of starch indicator in the solution, a pale yellow-colored solution was achieved; then after the addition of the starch indicator, an intense blue-colored solution was observed which is because the energy level spacing of the complex are far allowing the absorption of visible light. The use of starch indicator is very useful in this reaction because it involves iodine from which the color change is very sharp since titration was continued from dark blue colored solution to a pale blue colored solution.

Table2. Volume of Na2S2O3 used in the standardization

Trial 1

Trial 2

Trial 3

Initial Burette Reading

0.00+/- 0.05mL

0.00+/- 0.05mL

0.00+/- 0.05mL

Final Burette Reading

46.50+/- 0.05mL

43.00+/- 0.05mL

43.50+/- 0.05mL

Volume of Na2S2O3 used

46.50+/- 0.05mL

43.00+/- 0.05mL

43.50+/- 0.05mL

Table3. Statistical Data of Na2S2O3

Trial 1

Trial 2

Trial 3

Normality of Na2S2O3

0.04702+/- 0.001424N

0.04776+/- 0.001529N

0.04773+/- 0.001512N

Mean Normality of Na2S2O3

0.04750+/- 0.00042N

Standard Deviation

0.00042

Relative Standard Deviation (%)

0.88%

The table above shows the statistical data derived from sodium thiosulfate. Based on the table, the normality of Na2S2O3 was calculated using the formula:

[pic 1]

The calculated results were 0.04702N, 0.04776N and 0.04773N respectively. Its uncertainty on the other hand was computed by getting the square root of the sum of squares of the relative uncertainty of the measurements, from which the values 0.001424N, 0.001529N, and 0.001512N respectively were obtained. The mean normality, however, was calculated by adding the three values from each trial and was divided by the number of trials, which was three. The result was 0.04750N; its uncertainty was similar to the computation of its standard deviation using the formula:

[pic 2]

The calculated value of the standard deviation was 0.00042. The relative standard deviation, seen in table3, was computed using the formula:

[pic 3]

The calculated result for the RSD was 0.88%, which means that the result was accurate and acceptable since it is less than the limit which was 4%.

- Collection and Analysis of Water Samples

Table4. Volumes of Na2S2O3 used in determining DO in water sample

Trial 1

Trial 2

Trial 3

Trial 4

Initial Reading

4.0+/- 0.05mL

5.1+/-0.05mL

6.2+/- 0.05mL

7.4+/- 0.05mL

Final Reading

5.1+/- 0.05mL

6.2+/- 0.05mL

7.4+/- 0.05mL

8.5+/- 0.05mL

Volume of Na2S2O3 used

1.1+/- 0.05mL

1.1+/- 0.05mL

1.2+/- 0.05mL

1.1+/- 0.05mL

For the analysis of water sample, the addition of MnSO4 and alkaline iodide (KI in KOH) leads to the oxidation of Mn2+ to form the solid MnO2, which essentially locks up the dissolved oxygen in a form that allows short term storage before the sample is analyze, in a form of precipitate; and the inversion of the sample bottle was done to allow all the oxygen to react with the MnSO4 solution. It is shown in the chemical reaction below:

2Mn2+ (aq) + 4 OH- (aq) + O2 (aq) → 2MnO2(s) + 2 H2O (l)

The addition of H2SO4 in the water

...