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Determination of dissolved oxygen

 The oxygen which is dissolved in water is determined by a portable disolved oxygen meter (DO meter) from Hanna instruments (HI 8543). The principle of the determination is based on polarography. The DO meter has a disolved oxygen probe consisting of a membrane covering the polarographic sensor and a built-in thermistor for temparature measurements and compensation. Dissolved oxygen can be estimated in the range of 0.0 to 19.9 mg/l with a resolution of 0.1mg/l and an accuracy of 0.2 mg/l.

Instructions

 1/ Connect the DOmeter with the DO probe and switch on. Wait about 10 minutes to ensure

    the probe has been polarized.

 2/ Prepare the electrolyte solution by adding 15 to 20 drops of HI 7041 solution into 20 ml of   distilled water.

  3/ Wet the sensor by soaking the bottom of the probe in the dilute electrolyte solution for 5   minutes.

This is done by unscrewing the protective red and black plastic cap. Ensure the o-ring is properly placed inside the membrane. Rinse the membrane cap with electrolyte while shaking it gently. Refill with fresh electrolyte. Gently tap the sides of the membrane with the finger tip to ensure that no air bubbles remain. To avoid damage, do no tap the membrane directly on the bottom. With the sensor facing down, firmly screw the membrane cap clock -wise to the end of the threads. Some electrolyte will overflow.

4/ For slope calibration, rinse the probe throughly (particularly after the zero calibration) with   some water and blot dry in a manner as not to damage the membrane (use cloth or soft   paper). Place the probe in vertical position in air. Wait a few minutes until the  measurement stabilizes.

   Press and hold the red button between the two calibration screws. The decimal point should     disappear from the display. Using a small screwdriver, turn the slope trimmer until the  

   display reads 100. Release the red button and the display will show the saturation value at that ambient temperature. Saturation value is the maximum dissolvable oxygen value in a   liquid at that temperature.

  5/ Dissolved oxygen is determined in the samples after slope calibration. All the   measurements  are referred to sea level and zero salinity..

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