Winkler Method for Dissolved Oxygen Analysis

Experiment #6

“Winkler Method for Dissolved Oxygen Analysis”

Marriane Mae Beltran-Remigio

Members:

Arnel M. Asuncion

Jerico R. Liwag

October 24, 2014

1. Introduction

Dissolved oxygen is essential for marine life. High amount of oxygen should be present in the water to maintain a healthy environment for aquatic organisms, and when the maximum amount of oxygen was dissolved in water, it is said to be saturated. Dissolved oxygen indicates the health of a water body, wherein higher dissolved oxygen means higher productivity and little pollution (Bruckner, 2013). The reduction on the amount of oxygen either caused by the increase in temperature or by pollution means that it is not suitable for organisms to live there.

In this experiment, the level of dissolved oxygen was determined with the use of Winkler method. The Winkler method was considered the “gold standard” in measuring the concentration of dissolved oxygen in water (Grasshoff, K. 1983). In this method, levels of dissolved oxygen would be tested by the addition of different reagents; then it will be titrated with the standardized solution from which the volume used in the titration is directly related to the amount of oxygen present in the water sample.

The experiment was about the ability of students in analyzing redox reactions involved not only in the Winkler method but also in the standardization of the standard solution. Also the capability of the students in preparing the reagents and different solutions given the techniques and proper handling during the experiment would be observed. Lastly, the experiment was about the use of different statistical tools in determining the precision of the results

The objectives of this experiment are to understand and apply the Winkler method of measuring dissolved oxygen and to undertake an iodometric or indirect method of analysis. The experiment was conducted to know how different factors affect the levels of dissolved oxygen in a water sample, and how the level of dissolved oxygen determines how polluted the water sample could be which then affects not only our lives but also the lives of the organisms living in the aquatic environment.

1. Methodology

1. Preparation and Standardization of 0.05N Sodium Thiosulfate

Six (6) grams of Sodium Thiosulfate (Na2S2O4) was dissolved in previously boiled and cooled distilled water. After that, 0.06g of sodium carbonate (Na2CO3) was added into the solution. Afterwards, it was diluted up to mark in a 500mL volumetric flask. The solution was mixed by inverting the volumetric flask upside down; it was then labeled and was set aside.

For the standardization, three measurements of 0.1g potassium dichromate (K2Cr2O7) was weighed using weighing by difference, and it was dissolved in an Erlenmeyer flask having 75mL of distilled water. Then, 2mL of concentrated sulfuric acid (H2SO4) was added into the dissolved K2Cr2O7. For each of the Erlenmeyer flask, 0.25g Na2CO3 was added into the solution and was gently swirled. After that, 1g of potassium iodide was poured into the solution, was covered with a parafilm and was stored in the cabinet for 5-10 minutes. After that, the Erlenmeyer flask was removed from the cabinet one at a time and sides of the Erlenmeyer flask was washed down with distilled water. Then it was titrated with the prepared solution of Na2S2O4 until a pale yellow color was achieved. After which, a prepared 3mL starch solution was added to the solution and titration was resumed until the dark blue color of the solution became lighter in color. Then, the volume of Na2S2O4 used was recorded.

1. Collection and Analysis of Water Samples

Water sample was collected from Burnham Lake and was placed in a calibrated Winkler bottle.  To the sample, 2mL of 2.4M MnSO4 followed by 2mL alkaline iodide was added below the surface of the water sample using pipette. The solution was mixed by the inversion of bottle for several times. Since the cover of the Winkler bottle could not be removed, the settling of the precipitates was only estimated based on the time the precipitates completely settled of the other groups. After that, 2mL of concentrated H2SO4 was added to the water sample and the solution was mixed by the inversion of the Winkler bottle until all the precipitates were dissolved. Then, 50mL aliquot from the sample was measured and was titrated with the prepared Na2S2O4 solution until a pale yellow color was achieved. After which, 2mL of the freshly prepared starch solution was added into the analyte and the titration was resumed until the blue color disappeared. The final volume of Na2S2O4 used was recorded, and five determinations were made.

1. Results and Discussion

1. Standardization of 0.05N Sodium Thiosulfate (Na2S2O3)

Table1. Weight of K2Cr2O7 used in the standardization of Na2S2O3

The table above shows the data for the mass of K2Cr2O7 used in the standardization of Na2S2O3. The mass of K2Cr2O7 were obtained using weighing by difference, from which the needed mass of K2Cr2O7 was 0.1grams. The obtained weights of 0.1072, 0.1007, and 0.1018 respectively were dissolved in 75mL of distilled water. When K2Cr2O7 is dissolved in water, it breaks into ions which produce an orange-colored solution; these ions are 2K++ Cr2O2/7-. While the addition of H2SO4 acidified the solution and will have the chemical equation: