How To Measure Dissolved Oxygen In Water

The oxygen in the air dissolved in water is called dissolved oxygen. Dissolved oxygen content in water is closely related to the partial pressure of oxygen in the air and water temperature. Under normal conditions, the oxygen content in the air changes little, so dissolved oxygen is mainly affected by the water temperature. The lower the water temperature, the higher the dissolved oxygen content in the water. The amount of dissolved oxygen in water is an index to measure the self-purification capacity of water.

Measuring principle of dissolved oxygen analyzer

The dissolved oxygen in water depends on temperature, pressure, and salt dissolved in water. The sensor of the dissolved oxygen analyzer is composed of cathode, anode, and potassium chloride or potassium hydroxide electrolyte.

Oxygen diffuses into the electrolyte through the membrane and produces a measurement circuit with cathode and anode. When a polarization voltage of 0.6 ~ 0.8V is applied to the electrode of the dissolved oxygen analyzer, oxygen diffuses through the membrane, the cathode releases electrons, and the anode receives electrons to generate current.

Reaction process:

Anode Ag + Cl → AgCl + 2E – cathode O2 + 2H2O + 4E → 4OH –

According to Faraday’s law, the current flowing through the electrode of the dissolved oxygen analyzer is directly proportional to the oxygen partial pressure, and there is a linear relationship between the current and oxygen concentration under the condition of constant temperature.

Expression of dissolved oxygen content

There are three different ways for the expression of dissolved oxygen content:

oxygen partial pressure (mmHg);

Percent saturation (%);

oxygen concentration (mg / L or 10-6) ;

1)Partial pressure:

Oxygen partial pressure is the primary representation. According to Henry’s law, P = (PO2 + P H2O) × 0.209, where p is the total pressure; PO2 is oxygen partial pressure (mmHg); P H2O is the partial pressure of water vapor; 0.209 is the oxygen content in the air.

2)Percentage saturation:

For example, if the dissolved oxygen is set as 100% at calibration and 0% at zero oxygen, the dissolved oxygen content in the reaction process is the calibration percentage.

3)oxygen concentration:

According to Henry’s law, the oxygen concentration is directly proportional to its partial pressure, i.e., C = PO2 × a.

Where C is the oxygen concentration (mg / L); PO2 is oxygen partial pressure (mmHg); A is the solubility coefficient (mg / mmHg • L). The solubility coefficient A is related to the temperature and composition of the solution.

For an aqueous solution with constant temperature, A is constant, and we can measure oxygen concentration. Oxygen concentration is not commonly used in the fermentation industry, suitable for sewage treatment, drinking water, and other processes.

Factors affecting dissolved oxygen measurement

Dissolved oxygen depends on temperature, pressure, and salt dissolved in water. In addition, oxygen diffuses faster through solution than through membrane. If the flow rate is too slow, interference will occur.

Influence of atmospheric pressure

According to Henry’s law, the solubility of gas is directly proportional to its partial pressure. The oxygen partial pressure is related to the altitude of the area. The difference between Plateau and plain areas is about 20%. We must calibrate it according to the local atmospheric pressure before use.

Influence of temperature

The temperature change will cause the membrane’s diffusion coefficient and oxygen’s solubility to change, which will directly affect the current output of the dissolved oxygen electrode. In this case, we can use a thermistor to eliminate the influence of temperature. When the temperature increases, the diffusion coefficient increases, and the solubility decreases.

We can estimate the effect of temperature on solubility coefficient according to Henry’s law and the effect of temperature on membrane diffusion coefficient β according to Arrhenius law.

1) Solubility coefficient of oxygen: Since the solubility coefficient A is affected by the temperature and composition of the solution. Under the same oxygen partial pressure, the actual oxygen concentration of different components may also be different. According to Henry’s law, the oxygen concentration is directly proportional to its partial pressure. The change of solubility coefficient A for the dilute solution is about 2% / ℃ due to the temperature change.

2) Diffusion coefficient of membrane: According to Arrhenius law, solubility coefficient β varies with temperature T: C = kpo2 • exp (- β/ T), where K and PO2 are assumed to be constants, it can calculate β 2.3% / ℃ at 25 ℃. After the solubility coefficient A is calculated, we can calculate the diffusion coefficient of the membrane by comparing the instrument indication with the laboratory analysis value. The diffusion coefficient of the membrane is 1.5% / ℃ at 25 ℃.

The salt content in the solution

The dissolved oxygen in salt water is significantly lower than that in tap water. We must consider the influence of salt content on dissolved oxygen to measure more accurately. When the temperature is the same, the dissolved oxygen decreases by about 1% by increasing 100mg / L in salt content.

If the salt content of the solution used in the instrument’s calibration is low and the salt content of the measured solution is high, it will also cause an error. So, we must analyze the salt content of the solution to measure it more accurately.

How to measure dissolved oxygen in water?

Electrode polarographic determination

Compared with iodometry, electrode polarography is more straightforward. The principle is to add a constant voltage between the two poles to increase the flow of electrons from the cathode to the anode to produce a certain amount of diffusion current.

We can get the dissolved oxygen content in water by measuring the diffusion current because the diffusion current in the water sample is directly proportional to the concentration of dissolved oxygen in water at a certain temperature. Through quantitative analysis, the instrument can get the specific dissolved oxygen value in the water sample.

Fluorescence determination

The fluorescence determination uses the quenching of fluorescent substances to reduce the fluorescence intensity and shorten the life of fluorescence to know the content of dissolved oxygen in the water.

The more dissolved oxygen in water, the shorter the life of fluorescence. When the blue light is irradiated on the fluorescent material, it can emit corresponding red light. Dissolved oxygen in the water will absorb fluorescence energy. Therefore, the duration and intensity of red light are inversely proportional to dissolved oxygen concentration. We can get the dissolved oxygen content in water by measuring the phase difference between red and reference light.

Iodometric determination

Iodometry is one of the primary methods for determining dissolved oxygen in the water. Add manganese sulfate and basic potassium iodide into the water sample to produce manganese hydroxide. While the chemical properties of manganese hydroxide are very unstable, it can react quickly with dissolved oxygen in the water to produce manganese sulfate.

After standing for 15 ~ 20 minutes, add concentrated sulfuric acid to increase the whole reaction between brown precipitation and potassium iodide added in the solution to precipitate iodine gradually.

The more dissolved oxygen in water, the more iodine precipitated and the darker the color of the solution. Take out a certain amount of water sample after reaction through a high-precision pipette, and then use starch as an indicator to titrate through the standard solution to get the specific content of dissolved oxygen in the water.

Reagent:

In manganese sulfate solution, it should not produce a blue color in the case of starch.

Alkaline potassium iodide solution: weigh 480g of manganese sulfate (MnSO4 • 4H2O), dissolve it in water, and dilute it to 1000ml. Add this solution to the acidified potassium iodide solution: weigh 500g sodium hydroxide and dissolve it in 300-400ml water, and weigh 150g potassium iodide and dissolve it in 200ml water.

After the sodium hydroxide solution is cooled, mix the two solutions evenly, and dilute them to 1000ml with water. If there is precipitation, after placing some time, pour out the supernatant, store it in a brown bottle, plug it tightly with a rubber stopper, and keep it away from light. After acidification, it should not be blue in the case of starch.

(1 + 5) sulfuric acid solution: 1 piece of concentrated sulfuric acid + 5 pieces of water, mix and shake well.

1% starch solution:

• Weigh 1g of soluble starch.
• Mix it into a paste with a small amount of water.
• Dilute it to 100ml with just-boiled water.

After cooling, add 0.1g salicylic acid or 0.4g zinc chloride for corrosion prevention.

0.02500mol/l potassium dichromate standard solution:

• Weigh 1.2258g of potassium dichromate dried at 105 ~ 110 ℃ for 2h and cooled.
• Dissolve it in water.
• Transfer it into a 1000ml volumetric flask.
• Dilute it with water.
• Shake it well.

Sodium thiosulfate solution: weigh 3.2g sodium thiosulfate (Na2S2O3 • 5H2O) and dissolve it in boiled and cooled water, add 0.2g sodium carbonate, dilute it with water to 1000ml, store it in a brown bottle, and calibrate it with 0.02500mol/l potassium dichromate standard solution before use.

Sulfuric acid, pH = 1.84.

Steps:

Fixation of dissolved oxygen: insert a pipette into the liquid level of the dissolved oxygen bottle, add 1ml manganese sulfate solution and 2ml alkaline potassium iodide solution, cover the bottle stopper, mix upside down for several times, and stand still. It is commonly fixed at the sampling site.

Open the bottle stopper, insert it immediately into the liquid level with a straw, and add 2.0ml sulfuric acid. Cover the bottle stopper, mix upside down, shake well until all the precipitates are dissolved, and put it in the dark for 5min.

Suck 100.00ml of the above solution into a 250ml conical flask, titrate with sodium thiosulfate standard solution until the solution is light yellow, add 1ml starch solution, continue titrating until the blue just fades, and record the dosage of sodium thiosulfate solution.

Calculation:

Dissolved oxygen (O2, mg / L) = m * V * 8000 / 100

m — concentration of sodium thiosulfate standard solution (mol / L);

V — volume of sodium thiosulfate standard solution consumed in titration (ML).

Comparison of three methods

The above analysis shows their own characteristics for the determination of dissolved oxygen in the water. Among them, iodometry needs more processes, which is suitable for professional laboratories. Suppose there are many algae and other plants in the water. In that case, they will release a certain amount of oxygen under photosynthesis and increase the dissolved oxygen in the water to reach saturation. This state will cause a large measurement error of dissolved oxygen when measured by iodometry.

For electrode polarography, there are few steps and low the price of instruments. It is a relatively standard method for determining dissolved oxygen in the water.

Fluorescence is the simplest and most convenient method for determining dissolved oxygen in the water.

Compared with iodometry and electrode polarography, the fluorescence method does not need to calibrate the dissolved oxygen in water especially. Still, the testing is fast, stable, high accurate, and there is no strict requirement for flow. It is less influenced by external factors, reducing the cleaning frequency and lowering the maintenance cost.