Determination Of Dissolved Oxygen

1. SCOPE

This method is an adaptation from the APHA 20th edition, 4500-O A, C and G, pp. 4 - 129-136. This method covers the determination of dissolved oxygen in natural waters and wastewaters.

A limit of reporting of 0.1 mg/L for waters is achieved by this method.

2. PRINCIPLE

An oxygen meter with probe is used to routinely determine dissolved oxygen. The DO meter is calibrated on use against the Winkler titration method.

The membrane electrode procedure is based on the rate of diffusion of molecular oxygen across a membrane. Oxygen-sensitive membrane electrodes are composed of two solid metal electrodes in contact with supporting electrolyte separated from the test solution by a selective membrane. The ‘diffusion current’ is linearly proportional to the concentration of molecular oxygen. The current is converted to concentration units by a number of calibration procedures.

The DO meter is calibrated against the Winkler titration method. DO rapidly oxidises an equivalent amount of the dispersed divalent manganous hydroxide precipitate to hydroxides of higher valency states. In the presence of iodide ions in an acidic solution, the oxidised manganese reverts to the divalent state, with the liberation of iodine equivalent to the original DO content. The iodine is then titrated with a standard thiosulfate solution using Vitex as an indicator.

MnSO4 + 2NaOH  -->  Mn(OH)2 + Na2SO4

2Mn2+ + O2 +4OH-  -->  2MnO(OH)2
                 acid
Mn4+ + 2I-  ---->  Mn2+ + I2

I2 + 2S2O32-  -->  2I- + S4O62-


3. INTERFERENCES

Plastic films used with membrane electrode systems are permeable to a variety of gases besides oxygen, although none are depolarised easily at the indicator electrode. Prolonged use of membrane electrodes in waters containing such gases as hydrogen sulfide (H2S) tends to lower cell sensitivity. Eliminate this interference by frequently changing the membrane and calibrating the membrane electrode.

DO measurements should ideally be conducted immediately on site upon collection. Any agitation after sample collection will lead to an erroneous estimation of field conditions. In the event that field measurement is not possible, samples should be fixed on site using steps 8.1.1.1 to 8.1.1.4. Steps 8.1.1.5 to 8.1.1.8 may be conducted at the laboratory for field preserved samples.

4. REFERENCES

APHA 20 th ed. Standard Methods - For the Examination of Water and Wastewater
Operating Manual for YSI-5100 Oxygen Meter
Operating Manual for TPS 90-D Oxygen Meter
Operating Manual for YSI-5010 BOD Probe

5. APPARATUS

5.1 DO meter, TPS oxygen / salinity meter, Model 2052A or equivalent

5.2 Probe, YSI oxygen probe, Model 5739 or equivalent

5.3 Balance (0.0001 g accuracy)

5.4 Volumetric flasks, A-grade; 1000 mL

5.5 Erlenmeyer flasks, 250 mL

5.6 Burette; 25 mL, 0.1 mL increments

5.7 300 mL glass bottle having a ground-glass stopper.

5.8 Measuring cylinder, 200 mL

5.9 Volumetric pipettes, 1 mL

6. SAFETY

SAFETY GLASSES, GLOVES AND LABORATORY COAT MUST BE WORN WHEN PREPARING THE FOLLOWING REAGENTS. SULPHURIC ACID AND SODIUM HYDROXIDE ARE HIGHLY CORROSIVE. SODIUM AZIDE IS TOXIC WHEN ABSORBED THROUGH THE SKIN. THE MATERIAL SAFETY DATA SHEETS FOR SULPHURIC ACID, SODIUM HYDROXIDE AND SODIUM AZIDE NEED TO BE READ BEFORE COMMENCING THE PROCEDURE.

7. REAGENTS

Use reagent grade water for all solutions.

7.1 Manganous sulfate, MnSO4.H2O, AR

7.2 Manganous sulfate solution: Dissolve 364 g MnSO4.H2O (7.1) in reagent grade water, filter, and dilute to 1L.

The MnSO4 solution should not give a colour with starch when added to an acidified potassium iodide solution.

7.3 Sodium Hydroxide, NaOH, AR

7.4 Sodium Iodide, NaI, AR

7.5 Sodium azide, NaN3, AR

7.6 Alkali-iodide-azide reagent: Dissolve 500 g NaOH (7.3) and 135 g NaI in water and dilute to 1 L. Add 10 g NaN3 (7.5) dissolved in 40 mL of water.

7.7 This reagent should not give a colour with vitex solution when diluted and acidified.

7.8 Sulphuric acid (H2SO4), 98 %, AR


7.9 Vitex, AR

7.10 Sodium thiosulfate, Na2S2O3.5H2O, AR

7.11 Standard sodium thiosulfate titrant, 0.025 M: Dissolve 6.205 g Na2S2O3.5H2O (7.10) in water and add 0.4 g NaOH (7.3) and dilute to 1 L. Standardise as per QWI-EN/48.

8. PROCEDURE

8.1 Calibration DO Meter

8.1.1 Determination DO of Water (reagent grade or other, as long as same water is carried through both Winkler titration and probe reading) by Winkler Titration

8.1.1.1 Fill a 300 mL glass bottle with reagent grade water. Add 1 mL MnSO4 solution (7.2), followed by 1 mL alkali-iodide-azide reagent (7.6). Hold pipette tips just above liquid surface when adding reagents.

8.1.1.2 Stopper carefully to exclude air bubbles and mix by inverting the bottle a few times.

8.1.1.3 When the precipitate has settled sufficiently to leave clear supernatant above the manganese hydroxide floc, add 1.0 mL Conc. H2SO4 (7.7).

8.1.1.4 Restopper and mix by inverting several times until dissolution is complete.

8.1.1.5 Transfer 200 mL sample from 8.1.1.4 to a 250 mL Erlenmeyer flask.

8.1.1.6 Titrate with 0.025 M Na2S2O3 solution (7.9) to a pale straw colour. Add 0.3 g vitex and continue titration until the first disappearance of blue colour.

8.1.1.7 Repeat 8.1.1.1 – 8.1.1.6 two more times. Record on work book.

8.1.1.8 Calculation: When a 200 mL sample aliquot is used:
1 mL of 0.025 M Na2S2O3 = 1 mg DO/L

8.1.2 Calibration of DO Meter

8.1.2.1 Calibration of TPS 90-D DO Meter

8.1.2.1.1 To calibrate the DO meter, use the Span by Air Calibration procedure. Refer to Section 3.1 (b1) of the Instruction Manual for the TPS 90-D Oxygen Meter.

8.1.2.1.2 The DO reading of a water (reagent grade or other, as long as same water is carried through both tests) sample is checked against that of a Winkler titration (procedure 8.1.1).

8.1.2.1.3 If the variation between the DO meter reading and the Winkler titration is < 1.0, the DO meter is then calibrated using the Alternate Oxygen Span Calibration procedure (Section 3.1 (b2) of the Instruction Manual for the TPS 90-D Oxygen Meter). The value of the Winkler titration is keyed into the DO meter. 8.1.2.1.4 If the variation between DO reading and Winkler titration is > 1.0, initiate the following steps:

• Change the filling solution, and replace membrane
• Repeat steps 8.1.2.1.1 – 8.1.2.1.3.

8.1.2.2 Calibration of YSI-5100 Oxygen Meter

8.1.2.2.1 To calibrate the DO meter, use the manual Calibration procedure. Refer to pages 20 –21 of Calibration Instructions for YSI-5100 Oxygen Meter. Calibrate in mg/L.

8.2 Sample measurement

8.2.1 Determine sample DO. If the sample bottle is full with no headspace, carefully insert the DO probe and hold in place with a clamp and stand. Leave probe in place until a stable DO reading is obtained. If a full bottle was not received, accurate DO measurements are not possible.

8.2.2 Record both DO and temperature in DO work sheet. Rinse DO electrode between determinations to prevent cross-contamination of samples.