How to determine the amount of Dissolved-Oxygen

Oxygen-Dissolved

Water in contact with a gas will usually have dissolved in it a small amount of that gas. The atmosphere for example, consists essentially of nitrogen and oxygen, and both are somewhat soluble in water. Nitrogen is an inert gas and of minor importance, but the control of dissolved oxygen in industrial waters is essential for efficient operation.

The term “dissolved oxygen” refers only to that amount of oxygen gas which is actually dissolved and in no way refers to combined oxygen present in the water molecule.

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In a given water sample, the solubility of the gas is determined not only by the pressure of the gas and temperature of the water, but also by the composition of the solution. Mineral constituents in the water, for example, affect the solubility and distilled water absorb more oxygen than waters containing dissolved solids.

In addition to the natural occurrence of oxygen in water supplies, aeration is frequently used to remove other gases such as carbon dioxide and hydrogen sulfide. Efficient aeration results in saturation of the water with dissolved oxygen.

Dissolved oxygen is undesirable in industrial water because of its corrosive effect on iron and steel with which the water comes in contact. Increased temperatures and low pH values intensify corrosion due to oxygen.

Removal of oxygen from boiler system is especially important because of the accelerated effect oxygen has at the elevated pressures and temperatures of boilers. In boiler systems, corrosion may occur in feed lines, heaters, economizers, boilers, steam and return lines. The tests described below focus on measurement or detection of the very low oxygen levels typically associated with boiler operation.

Dissolved-Oxygen, Winkler Method (0-0.8 ppm O2 ) Theory of Test

The winkler determination of dissolved oxygen in water is based on the absorption of oxygen by a flocculant precipitate of manganous sulfate and alkaline potassium iodide.

The reacts with manganous hydroxide to form manganese hydroxide. Following acidification by sulfuric acid, free iodine is released in direct proportion to the amount of oxygen absorbed. The free iodine is then titrated with standard sodium thiosulfate in the presence of starch indicator. The disappearance of the blue color is taken as the endpoint.

Preparation of 0.01N Sodium Thiosulfate Solution

Sodium thiosulfate, 0.01N, is not stable due to absorption of carbon dioxide from the air, and should be freshly prepared (or re-standardized) at least every two weeks. To 90 ml of distilled water (boiled and cooled to room temperature) add 10 ml 0.1N sodium thiosulfate solution. Mix thoroughly and place in tightly stoppered bottle. Avoid unnecessary exposure of the solution to air.

Preparation of Sampling Equipment

If the sample is above 700F a copper cooling coil should be used. The inlet side of the cooling coil should be connected to the sampling point by means of a brass nipple, brass valve and suitable length of copper tubing. All these connections should be of a size equivalent to the size of tubing comprising the coil. Iron should not be used. The discharge side of the cooling coil should be connected to a glass tube with a convenient length of sulfur-free rubber tubing.

The entire system described above must be airtight. The sample must be secured continuously  from a point of the system at which the pressure is greater than atmospheric.

Place the cooling coil in a bucket or similar container into which a regulated flow of cooling water can be discharged and the overflow run to waste.

Secure a clean glass (or metal) container of such proportion that the capacity is approximately three or four times the volume of the 10-ounce glass stoppered bottle used in the test. The height of the container should be at least one inch higher than the height of the bottle with the stopper inserted.

Securing the Sample

Place the 10-ounce bottle in the center of the glass container. Introduce the glass tube into the 10-ounce bottle. Place the thermometer in the bottle.

Turn on the sampling line and by means of a valve, adjust rate of flow into the bottle so that the bottle overflows approximately once per minute. Make certain no air bubbles are being discharged into the sample. Control the cooling water to the coil in the bucket to reduce and maintain the temperature of the sample to 70 F or less. The cooled sample will overflow the sampling bottle and in turn will continue to fill and overflow the outer container. Remove the glass tube from the bottle and place it in the outside space between the bottle and the outer container. Remove the thermometer in the same fashion. The sample should continue to run and overflow the outside  glass container.

Procedure for Test

Fill a 1ml pipet with manganous sulfate solution. To prevent the introduction of air into the  sample, be certain a drop of manganous sulfate hangs from the tip of the pipet. Insert the tip of the pipet through the water layer and well into the sample bottle. Permit exactly 1 ml of manganous sulfate to flow into the sample. Slowly withdraw the pipet and immediately repeat this procedure  to deliver exactly 1.0 ml of alkaline potassium iodide solution by using a second clean 1-ml pipet.

Wet the glass stopper and gently drop it straight into the neck of the sample bottle without removing the sample bottle from the outer container. Allow the stopper to “seat” by its own  weight, then press down firmly. Remove the sample bottle from the outer container and mix the contents by gently rotating. Do not shake. Examine for air bubbles; if any are present, the sample  is worthless and the entire procedure must be repeated. If no air bubbles are present, replace the tightly stoppered bottle in the glass container and allow to stand three minutes to permit the floc to settle.

Carefully remove the stopper (under water) and add exactly 2 ml of 50 per cent sulfuric acid into the bottle following the procedure for pipetting outlined above. Again replace the  stopper.  Remove the stoppered bottle and rotate gently to dissolve floc. The sample is now “fixed” and ready to titrate. The titration must be completed within five minutes to minimize errors from any interference.

Measure 200-ml of the fixed sample and pour into a flask. If the sample is colored yellow, 0.01N sodium thiosulfate until from the microburet with constant swirling, until the yellow color is almost discharged. Add 1 ml (approximately 20 drops) of starch indicator. The sample should turn blue. Continue to add 0.01N sodium thiosulfate until one final drop turns the solution from blue to colorless. This is taken as the endpoint. Record to the second decimal the total number of ml of 0.01N sodium thiosulfate required. If the 200 ml fixed sample is not colored yellow and does not turn blue upon addition of the starch indicator, the dissolved oxygen is recorded as “zero by the winkler method”.

Calculation Of Results

FORMULA: ppm dissolved oxygen = Ml 0.01N sodium thiosulfate x 1000 X .08/ml Sample

With a 200 ml sample, the dissolved oxygen in parts per million is equal to the ml of 0.01N sodium thiosulfate multiplied by 0.4.

Limitations of Test

Nitrites, sulfites, ferric iron and certain types of organic matter interfere with this test. The proper securing of the sample, free from contamination by air, and the proper technique in making the determination are of utmost importance.

This method for determining dissolved oxygen is suitable for rapid determinations where the greatest precision and accuracy are not required. An accuracy of approximately 0.05-ppm may be expected. The method is not recommended for dissolved oxygen concentrations below 0.1 ppm  and is not suitable for checking performance guarantees of deaerating feed water heaters.

A calorimetric comparator procedure for the determination of dissolved oxygen in low concentration, 0 to 100 ppb, is commercially available. The method uses Rhodazine D dye and is based on a color change in the dye on oxidation. In its reduced form the dye is pale yellow, but contact with oxygen changes the color to a deep rose. This produce is recommended for concentrations of dissolved oxygen below 100 ppb.

In addition, several instruments for the determination of dissolved oxygen are commercially available. These instruments work on electrochemical principles, and in the past, were sensitive only to ppm levels of dissolved oxygen. Recent design improvements have made these instruments quite capable of measuring dissolved oxygen levels below 100 parts per billion (ppb).