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The concentration of oxygen determines the value of the redox potential and largely direction and rate of processes of chemical and biochemical oxidation of organic and inorganic compounds.

Oxygen regime has a deep impact on the life of the reservoir. The minimum content of dissolved oxygen that ensures the normal development of fishes is about 5 mg О2/l. If this measure decreases until 2 mg/l, it will cause massive lethality of fishes. Supersaturation of the water with oxygen also adversely affects its condition. Threshold limited value of dissolved oxygen for the water objects of fishery purposes in the winter period is 4 mg/dm³, in the summer – 6 mg/dm³.

Determination of oxygen in surface waters is included in the program of observations in order to assess the living conditions of aquatic organisms, including fish, indirect water quality characteristics, intensity of processes of producing and destruction of organic matter, self-purification of reservoirs, and so on.

Concentration of oxygen is expressed either in milligrams per liter or as a percentage of saturation, the calculation of oxygen is made according to the formula

(4.1)

where, С_х – concentration of oxygen found experimentally, mg/l;°C _- normal concentration at a given temperature, normal and atmospheric pressure 760 mm (found from Table. 4.1); p – atmospheric pressure at the time of analysis.

The principle of measuring the concentration of oxygen.

Amperometric method of analysis forms the basis for measuring the concentration of dissolved oxygen. The oxygen concentration is determined by current intensity, flowing in the circuit of the electrode system of the sensor О₂.

Electrodes, cathode and anode, of О_{2 sensor} are located in the electrolyte solution and separated from the analyzed liquid by gas permeable membrane. The oxygen freely diffuses through the membrane and electrolyte to the electrodes, which are under the constant voltage coming from the source of polarizing voltage of the device. The current, which is caused by a reduction reaction of molecular oxygen, generates in the electrode circuit and flows by the following scheme:

О₂ + 2Н₂О + 4е־ =4ОН^¯

The current is converted into voltage, which is measured and, in turn, is converted into a value of concentration of oxygen.

Preparation and conducting of measurements.

Measurements of the concentration of dissolved oxygen are produced by special sensors. Sensors «generate» the current that is proportional to the oxygen concentration in the environment. The conversion coefficient depends on several factors and therefore requires periodic calibration of the sensor.

Calibration – put of parameters of the measuring channel sensor → converter, current-voltage of the device obtained in solutions with known concentrations of oxygen into the memory of the device.

Such relatively easy reproducible and available solutions are solutions with zero (0 %) and one hundred percent (100 %) of the dissolved oxygen concentration.

Calibration of electrochemical analyzers of oxygen.

Zero point calibration.

«Zero-solution» can be used as a standard sample with a zero content of oxygen. Beforehand, a solution of sodium sulfide (Na₂S) with temperature 20±0,5 °C and concentration of 80 g/l is prepared. The solution must stand after cooking for at least 8 hours. In another option for making «zero-solution 10±0,5 g of sodium sulfite (Na₂SO₃) is dissolved in 200 ml of tap water and add 20-50 mg of soluble salts of silver or cobalt (e.g. cobalt chloride – CoCl₂). Addition of salts of heavy metals is used as the catalyst for the oxidation of sodium sulfite. After 10 minutes, «Zero-solution» may be used for calibration.

Place the sensor in an environment with zero oxygen content, and to calibrate the zero point of the device.

Calibrating by the air.

At this calibration atmospheric air, which is saturated with water vapor, can be used as a standard sample with known oxygen content.

Place the sensor into a bottle with a small amount of water so that the sensor does not touch the water. Calibrate the top point of the device.

The quality of the calibration and accuracy of the results of subsequent measurements is significantly affected by the temperature measurement accuracy. During measurement should not forget about another important factor – the pressure. Atmospheric pressure has a directly proportional effect on the solubility of oxygen in liquids, freely contacting with atmospheric air, so it must take into account the values during the calibration and measurement.

Table 4.1

The solubility of oxygen in distilled water, saturated with air at a pressure of 101.3 kPa (760 mm Hg).

When working with the amperometric sensor it should be cautious, protecting the glass sleeve from impacts. It is not allowed to apply mechanical stress to the cable. For prolonged storage, when amperometric sensor is off (for more than 6 months) it is necessary to drain the electrolyte solution, wash the corps of the sensor with distilled water and put it on the amperometric sensor.

When there are shorter work breaks, and between the series of analyzes, amperometric sensor is preferably stored in water vapor. In order to do this, select one flask, pour 10-15 ml of distilled water and set amperometric sensor without touching the sensitive part of the sensor to the surface of the flask. Storing the sensor in this position (in water vapor) between measurements and long periods of time can significantly increase the time of routine service of sensor.

Laboratory work № 7
Determination of the oxygen content dissolved in water

Objective: Measurement of the oxygen content in the water.

Tasks:

1. To perform calibration of device on standard solutions.

2. To measure the amount of oxygen in the water

4. To build a graphic of values

5. To make conclusions about the observed phenomena and to prepare a report.

Equipment and materials: Laboratory amperometric analyzer, bottles of 300 ml, microcompressor, barometer, thermometer, 8 % solution of sodium sulfide, salt CoCl_2, electro-mechanical mixing device.

Procedure:

1. To perform the necessary training and the setting of the device (see. Chapter 4);

2. To prepare the workplace, distilled and tap water. To rinse the sensor and detector with distilled water;

3. To lower the detector and the sensor into the solution, stir it to accelerate the achievement of temperature regime. Turn on the device;

4. To turn on the electromechanical mixing device and after 5-7 minutes remove the instrument readings;

5. To measure the temperature of the test sample;

6. To measure the atmospheric pressure;

7. To turn off the electromechanical mixing device.

Task:

1. To measure the oxygen content in distilled water;

2. To measure oxygen content in the distilled water saturated with oxygen in the air for 30 minutes – barbotage;

3. To measure the oxygen content in tap water.

Report design. To make all calculations by all measurements of the oxygen content of the samples taking into the factor of atmospheric pressure, using the formula 4.1 and Table. 4.1. Compare measurement results and make conclusions.

Laboratory work № 8