Dissolved oxygen test

The measurement of dissolved oxygen (DO) is a very important test for assessing water quality because oxygen is fundamental to life in water. If the oxygen concentration is reduced by polluting substances such as organic matter or reducing agents (for example, sulphide or ferrous ions), then fish and insect life can die or move to cleaner water. The amount of oxygen present in the water is limited by its solubility and this is affected by the water temperature. At normal winter temperatures of 5°C, the equilibrium concentration of oxygen in the water is 12.7 mg/l, but at a summer temperature of 18°C the equilibrium concentration is 9.45 mg/l.

The amount of dissolved oxygen in water is measured either by using a dissolved oxygen meter or else by a titration procedure which is named after Winkler, the Hungarian chemist who discovered it.

The basis of the method is that when concentrated solutions of manganous sulphate and alkali potassium iodide are added to a water sample, a precipitate is formed. This precipitate is initially of manganous hydroxide which is white, but if dissolved oxygen is present then the precipitate immediately turns brown because of the formation of manganic hydroxide. When the sample and its precipitate are treated with strong acid, the precipitate dissolves and the manganic ion oxidizes the iodide present to iodine. This liberated iodine is then titrated with thio-sulphate ion (as described above for the PV test). If the strength of the thiosulphate is exactly 0.0125 M then each ml of titrant used is equivalent to 1 mg/l of dissolved oxygen.

Apparatus required:

Sample bottles. Ideally these should be of 250 ml capacity, with a sloping neck so that air bubbles don't get trapped, and with a well-fitting plastic or glass stopper. 3 x 2 ml dropper pipettes 50 ml burette 100 ml measuring cylinder 250 ml conical flask 10 ml measuring cylinder 5 ml pipette 10 ml pipette

Reagents:

Manganous sulphate solution (50per cent). Dissolve 500 g MnSO4.4H2O in distilled water and make up to 1 litre. Alkali-iodide reagent. Dangerous. Dissolve 500 g NaOH and 150 g KI in distilled water and make up to 1 litre. 50 per cent sulphuric acid. Dangerous. Put 500 ml of distilled water in a beaker of 2 litre capacity, and place the beaker in a sink filled with cold water. Carefully add 500 ml of concentrated sulphuric acid and stir to mix. Heat is given off by this mixing which is why the beaker is cooled. Always add the concentrated acid to the water.

Thiosulphate solution (0.25M). Dissolve 62.05 g Na2S2O3.5H2O in distilled water and make up to 1 litre.

Thiosulphate solution (0.0125 M). This is the working solution and is prepared by diluting 50 ml of the 0.25M solution above to 1 litre with distilled water.

Potassium iodate solution (0.0042 M). This is for standardizing the thiosulphate solution. Dissolve 0.892 g potassium iodate, which has previously been dried at 105°C, in distilled water and make up to 1 litre.

Potassium iodide solution. Dissolve 10 g of KI in 100 ml of distilled water and store in a dark bottle.

Starch solution. Mix 1 g of soluble starch in a beaker with a little water. In a separate beaker, boil 200 ml of distilled water and pour the starch/water mix into the boiling water. Stir the solution as it cools and then transfer it to a glass bottle.

Procedure for standardizing the 0.0125M thiosulphate solution Into a 250 ml conical flask, add 5 ml KI solution, 2 ml of 50 per cent sulphuric acid, 10 ml of KIO3 solution (measured with a pipette) and about 100 ml of distilled water. A brown colour appears as the iodate oxidizes the iodide to iodine in the acid conditions. Titrate the liberated iodine with the thiosulphate until it becomes pale yellow, then add 1 ml of starch solution. This immediately turns the solution blue. Continue the titration until the blue colour just disappears. Ignore the reappearance of the blue colour on standing.

Strength of the thiosulphate solution = 0.0125 x 20

ml of thiosulphate used

Sample collection

When collecting a sample for the measurement of DO, it is important not to trap air bubbles in the bottle as this will give rise to a false high reading. The following procedure should be followed.

At the sample site, make sure the sample is taken in the main flow of the river or effluent. Gently fill the bottle by inclining it at the water surface (Figure 41), then make sure it is brimful by pushing the bottle below the surface. Tap the sides of the bottle with the stopper to bring any bubbles adhering to the inside to the surface.

Add about 2 ml of the manganous sulphate solution followed by about

Environmental Pollution Test
Figure 41. Sampling of river water for measurement of dissolved oxygen

2 ml of the alkali-iodide solution. Place the stopper on the bottle firmly and mop up any overflow water with a tissue. This could be alkaline so take care.

A precipitate will have formed in the bottom of the bottle and this now has to be mixed throughout the sample so as to react with all the DO. Holding the bottle with a finger on the lid, shake the bottle vigorously. The precipitate now contains all the dissolved oxygen in the sample and will slowly settle to the bottom of the bottle. This precipitate is stable for a number of days so the laboratory analysis does not need to start immediately.

Analysis procedure:

In the laboratory, remove the stoppers from the samples and carefully add about 4 ml of the 50 per cent sulphuric acid into each bottle using a dropper pipette with the tip below the water surface. The stoppers are then replaced and the excess liquid expelled is mopped up with a tissue. The contents of the bottles are thoroughly mixed by shaking them. The precipitate has now dissolved and iodine has been released to give a clear reddish brown liquid. (At this stage you can assess the DO content because the amount of iodine released, and therefore the intensity of the colour, is directly proportional to the amount of DO. You might consider photographing a row of bottles collected on a survey to illustrate the variation in oxygen content at different sample points.)

For each sample, measure 100 ml from the sample bottle into a conical flask and titrate with the 0.0125 M thiosulphate solution. When the colour has reached a pale yellow, add about 2 ml of starch solution and a deep blue colour is formed. Continue the titration until this just disappears and note down the burette reading. Ignore any blue colour that develops in the flask on standing.

Concentration of dissolved oxygen (mg/l) = ml of 0.0125 M thiosulphate used.

Chemistry of the DO test

MnSO4 + 2NaOH = Mn(OH)2 (white precipitate) + Na2SO4 2Mn(OH)2 + O2 = 2MnO(OH)2 (brown precipitate) MnO(OH)2 + 2H2SO4 = Mn(SO4)2 + 3H2O Mn(SO4)2 + 2KI = MnSO4 + K2SO4 + I2

The iodine released is directly proportional to the amount of dissolved oxygen and is titrated with thiosulphate solution according to the chemistry described for the DO test.

Percentage saturation of dissolved oxygen

As mentioned on page 120, the amount of oxygen that can dissolve in water is dependent mainly on the temperature but also to a minor extent on the atmospheric pressure and the salt content. In fresh water, the variations in the atmospheric pressure have a negligible effect whilst the concentration of dissolved salts hardly alters the DO content. However, the temperature effect must be considered. This is especially important if you are comparing results on different days when the water temperature may vary by a couple of degrees Celsius, or where a heated discharge may be warming the river up from one sampling point to the next.

For this reason, DO results are calculated as the percentage saturation (% sat.). At a particular temperature, there is a concentration of DO in water which is in equilibrium with the air above it. The DO results obtained in the sample are compared against this equilibrium value and expressed as a percentage of it. The equilibrium concentrations for different temperature at normal atmospheric pressure are shown in the Appendix.

As an example, suppose a sample is collected from a river which has a temperature of 14°C, and the titration shows that the DO is 8.6 mg/l. However, at this temperature, from the Appendix, the equilibrium concentration of DO is 10.30 mg/l.

It is possible that you may collect a sample and the DO content is greater than the equilibrium value. This is called super-saturation and is usually caused by the photosynthesis of aquatic weeds or algae in summer months. These green plants use up the carbon dioxide and release oxygen. The oxygen sometimes is released into the water by the algae or plants at a faster rate than it can diffuse into the air so the water becomes supersaturated.

In Chapter 3, the eutrophication of Strathclyde Park Loch was described when excessive amounts of algae are formed in the loch in the summer months. This results in super-saturation of the loch water, as shown in Table 24 and compared with winter values.

Table 24. Percentage saturation of dissolved oxygen

in Strathclyde Park

Loch

Date of sample

% sat.

May 1993

200

November 1993

71

May 1994

170

December 1994

75

Was this article helpful?

0 0

Responses

  • rosario
    How to prepare 0.0125M of Na2S2O3.5H2O?
    3 years ago

Post a comment