Reference no: EM132381592

AIM

Determination of dissolved Oxygen in Natural Waters

INTRODUCTION

The concentration of dissolved oxygen (DO) in natural waters depends on chemical, physical and biochemical processes in the water and therefore it is one measurement that is widely used to assess water quality. Low oxygen content is indicative of the presence of contaminants or oxygen-consuming substances in the water. DO analysis is an important test in the assessment of water pollution and waste treatment processes.

This experiment involves the use of two of the common methods for the determination of DO; an electrochemical method using the Clark electrode and the classical Winkler method. The electrometric apparatus is portable so that measurements can be made at the point of collection in the field, whereas the Winkler method must be conducted on samples back in the laboratory.

EXPERIMENTAL INSTRUCTIONS

Sample solution:

Two BOD Bottles supplied contains milli-Q water. Students must empty these bottles and collect a water sample (tap water) in them. As special precautions must be taken during collection, consult your demonstrator before collecting your sample. Fill two bottles from the same source (tap water) making sure that there are no air bubbles present. At the same time collect a small sample in a 50 ml beaker to be measured using the Clark electrode

A. Electrometric method: Thermo Scientific Orion star A323 Portable DO meter

A portable, battery operated Oxygen Meter from Thermo Scientific measures oxygen concentrations using an oxygen probe which is placed directly into the test sample. The probe consists of two silver electrodes in an electrolyte solution (1.5 M KCl), separated from the test solution by an oxygen permeable membrane. A voltage is applied across the two electrodes in the probe and oxygen is reduced at the cathode. The resulting diffusion current is proportional to the concentration of oxygen. The probe also includes a thermistor to measure temperature so that any variation in temperature can be compensated for in the measurements.

Air Dissolved Oxygen Calibration method:

An air calibration is performed in water-saturated air using the calibration sleeve included with the DO probe. This is the simplest and most accurate calibration. The highest possible accuracy is reached when the calibration temperature is the same as the measuring temperature.

1. Moisten the sponge in the calibration sleeve with distilled water and insert the probe into the sleeve without touching the water saturated material.

2. In the measurement mode, press f1(cal).

3. Pressto highlight Air and press f3(select).

4. Rinse the optical DO probe with distilled water, blot dry with kimwipes and place into calibration sleeve.

5. When the probe and water-saturated air are ready, press f3(start)

6. Wait for the dissolved oxygen reading on the meter to stabilize and stop flashing. Once the reading is stable, the meter will display ‘Reading is stable'. Accept ‘Auto Calibration Value' of 100%.

7. Press f3(Cal done). The meter will proceed to the measurement mode.

DO measurement:

1. Rinse the DO probe with distilled water, blot dry with kimwipes and place into the supplied sample solution in 50 ml beaker and stir gently. Allow about 3 mins for temperature stabilisation.

2. Press to start the measurement. When the AR icon stops flashing, record the concentration in mg/L and temperature of the sample. Press again to start new measurement.

B. Winkler (iodometric) method

A solution of Mn²⁺ is added to the water sample, followed by sodium hydroxide. The manganous hydroxide precipitate is oxidised by DO to hydroxides of higher valency states. Addition of acid leads to dissolution of the hydroxides and addition of iodide leads to liberation of iodine. Iodine is then titrated with a standard solution of sodium thiosulfate using starch as the end-point indicator:

2 Mn³⁺ + 2 I^- 7 2 Mn²⁺ + I₂I₂ + 2 S₂O₃^2- 2I^- + S₆O₄^2-

Solutions/materials supplied:
a) manganous sulfate (MnSO₄) solution (~ 2 M)
b) conc. phosphoric acid (85 %) (to be used in fumehood)
c) alkaline iodide solution
d) sodium azide solution
e) starch indicator solution
f) sodium thiosulfate (Na₂S₂O₃) solution (~0.025 M), stored in a brown bottle
g) standard potassium iodate (KIO₃) solution - note concentration from container
h) water sample in BOD bottle - use the same sample measured above with the oxygen probe

Step1: Standardisation of sodium thiosulfate solution (students must do three titrations)

1. Pipette 8.00 mL of the standard KIO₃ solution in to a 500 mL conical flask, add approx. 1 g of solid KI and 5 mL of dilute (2M) sulfuric acid. Dilute to approx. 200 mL with ultrapure water and titrate the liberated iodine with the sodium thiosulfate (in burette) solution. Near the end-point of the titration, the solution will turn pale-yellow. Add 2 mL of the starch solution and continue the titration until the blue colour disappears. The expected end-point is less than 8 mL of titrant.

2. Repeat step 1 twice to achieve concordant results (±0.10) for the titration. Calculate the concentration of the sodium thiosulfate solution from your titration results and the following equations for the titration reactions:

IO₃^- + 5 I^- + 6 H⁺ 7 3₂I + 3H₂O + I₂ + 2S₂O₃^2- 7 2 I^- + S₄O₆^2-

Step 2: Procedure for both sample bottles (Student must do two titrations)

Only one student from the group prepares this solution: Add 1mL sodium azide solution to 25 mL alkaline iodide solution and mix well.
(CAUTION: Sodium azide is highly toxic. Refer to risk assessment in your labbook and discuss with your demonstrator about safe handling).

1. Use an automated pipette for reagent addition in this step. The pipette should be calibrated to deliver 1 mL. Remove the stopper from the sample BOD bottles, add 1 mL MnSO4 solution followed by 1 mL of the alkaline-iodide-azide mixture from (1).

For both additions, hold the pipette tip several centimetres below the liquid surface to avoid splashing. Replace the stopper carefully to exclude air bubbles. Hold the stopper in place and invert the bottle several times to mix the contents. A brown precipitate will form in the bottle before and during the mixing.

2. Allow the brown precipitate to settle to approximately half the bottle volume. Remove the stopper and add 3 mL phosphoric acid (to be done carefully in fumehood). Replace the stopper carefully, hold the stopper in place and invert the bottles to mix the contents. Mix the contents until the precipitate dissolves.

The original DO in the sample is now "fixed" in the form of a proportional amount of iodine.

3. To a 500-mL conical flask, transfer accurately a volume corresponding to 150 mL of the "fixed" sample from (3) one BOD bottle, correcting for displacement by the volume of reagents added in the precipitation steps, required for titration.

4. Titrate with the standardised sodium thiosulfate (~0.025 M) solution until the brown colour fades to a pale-yellow colour. Add a few drops of starch indicator solution and the solution turns blue. Continue titrating dropwise until the first disappearance of the blue colour. Record the volume of titrant added.

5. Repeat using the second sample (same as in step 4) from second BOD bottle.

Calculation:

If the concentration of the titrant is not 0.025 M and the sample volume is 150 mL, the DO in the sample may be calculated using the following relationship:

DO in mg/L = (volume of titrant in ml × concentration of titrant in mol/L × 8000)/sample volume titrated in ml

REPORT

ADDITIONAL QUESTIONS

1. Give equations for the half-reactions for the electrochemical conversion of molecular oxygen to (a) water and (b) hydroxide, respectively. Are these half-reaction reductions or oxidations? What are their standard potentials?

2. Determine the formal oxidation states of all elements in the two equations given under ‘standardisation of sodium thiosulfate solution' above.

Quality of writing and presentation:
• clarity, structure (following academic conventions), grammar, spelling, expression, formatting of references, presentation, etc.

Quality of the practical work:
• quality and consistency of the data
• appearance, melting point, spectra and yield of the product (for synthetic experiments)

Attachment:- General Explanation for rebort.rar