The percentage of oxygen in water can vary depending on several factors such as temperature, pressure, and the presence of dissolved gases. However, under standard conditions (25 degrees Celsius and atmospheric pressure), the percentage of oxygen in water is approximately 0.001%, or 10 parts per million (ppm). This means that for every one million molecules of water, about 10 of them are oxygen molecules.
The measurement of dissolved oxygen in water is typically done using various analytical techniques. The most commonly used method is the Winkler method, which involves a series of chemical reactions to convert dissolved oxygen into a measurable form. Here is a simplified overview of the Winkler method:
Sample collection: A water sample is collected in a clean, airtight container to prevent the loss of dissolved oxygen.
Addition of reagents: Manganous sulfate (MnSO4) and an alkaline iodide solution (KI) are added to the water sample. These reagents react with dissolved oxygen to form manganese dioxide (MnO2) and iodine (I2).
2 MnSO4 + 4 KI + O2 → 2 MnO2 + 2 K2SO4 + 2 I2
- Acidification: The sample is then acidified with sulfuric acid (H2SO4) to release the iodine.
H2SO4 + 2 I2 → 2 HI + H2O + SO2
Titration: The liberated iodine is titrated using a standardized sodium thiosulfate (Na2S2O3) solution. The titration determines the amount of iodine produced, which is proportional to the dissolved oxygen concentration in the water sample.
Calculation: Based on the volume of thiosulfate solution required for the titration, the concentration of dissolved oxygen in the water sample can be determined.
It's important to note that the Winkler method provides a measure of dissolved oxygen rather than the percentage of oxygen in water. The measurement is typically reported in units of milligrams per liter (mg/L) or parts per million (ppm), representing the concentration of dissolved oxygen in the water sample.