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Dissolved oxygen does not directly affect pH levels because there is no physical-chemical connection between the two. However, in some cases there are indirect relationships from external factors, such as additional nutrients increasing algal growth.
Dissolved oxygen and pH are important parameters that determine distributions of aquatic organisms and water quality.
Dissolved oxygen (DO) describes the amount of oxygen (O2) molecules that are dissolved in water or other liquids. As O2 is vital for aquatic organisms and biological processes, understanding if DO affects other water parameters, like pH, is important.
Potential for hydrogen, or pH, is a measurement to quantify the chemical conditions (the number of hydrogen ions) of an aqueous solution. pH measures how acidic or alkaline/basic a solution is, with a numeric scale ranging from 0-14 with 7.0 being neutral, <7.0 acidic, and >7.0 being alkaline or indicating a base.
Dissolved oxygen does not directly affect the pH of a solution because there is no physical-chemical connection between the two.
There is often some confusion that there is a relationship between DO and pH, as pH is related to the presence of free carbon dioxide (CO2) atoms which contain two oxygen atoms. However, DO only refers to the number of oxygen atoms dissolved in water or a solution that does not form other molecules like CO2.
Even though DO is not directly related to pH, there are some ways that they can be indirectly related.
In some cases, DO and pH have a positive correlation, while in other cases they do not. It is thought this is from external factors such as additional nutrients like phosphorus and nitrogen accidentally being added via processes such as runoff may be the cause of this hypothesis.
For example, sewage treatments have an effect on DO levels in water bodies and are suggested that the more pollution released into these water bodies, the less oxygen is available leading to decreases in algae, therefore, increasing the pH of water.
In water, we can look at how minor changes in pH can increase the solvability of nutrients such as phosphorus, encouraging plant growth; if the water contains low nutrients and a high DO concentration, it can have a chain reaction on pH levels. When more nutrients are available, aquatic plants like algae can thrive, increasing the demand for DO.
When pH levels become too low and create acidic environments, the ability for aquatic organisms to effectively absorb DO is decreased. This is because, at a lower pH, the hydrogen ions react with DO and lower it.
Even though DO does not directly affect the pH of water, DO can affect other parameters. When DO levels increase, water is unable to hold other dissolved properties. When DO levels are extremely low, minerals from bottom sediments (i.e. riverbed) will more easily dissolve into the water affecting the water quality.
As mentioned, DO is affected by salinity, temperature, and pressure changes in water and other soluble solutions. The colder a water body is, the more oxygen it can hold, as the temperature increases, the less oxygen can be dissolved into the water.
Salinity is also important to consider when determining the amount of oxygen water bodies can hold. Saltwater cannot absorb as much oxygen as freshwater, therefore the higher the salinity, the lower the DO concentration.
As both atmospheric and hydrostatic pressure increases, DO levels will also increase. This relationship explains “supersaturation” which is when the total dissolved oxygen in water exceeds the concentration of the total dissolved gasses that are normally dissolved.
There are four main factors that affect pH:
When surface temperatures increase, CO2 solubility decreases, therefore, the pH of the water increases, making it more alkaline. When temperatures decrease, the water becomes colder which naturally drops the pH level making the water more acidic.
This is down to the hydrogen ions (H+); an increase in H+ ions decreases the pH of the water, therefore when CO2 levels increase, the pH of the water becomes more acidic. As plants photosynthesis in water, the pH is likely to increase as the plants remove more carbon dioxide from the water, therefore CO2 has many effects on pH.
If the water contains enough carbonate and bicarbonate ions, the pH level will change. When the water has a neutral pH, the presence of such ions will make the water alkaline. If the pH of the water is acidic the carbonate and bicarbonate ions will change the pH of the water to neutral.
As living organisms and organic material contain carbon, when they decompose, they release carbon into the water. As organic compounds are usually unstable and easily oxidized, when they are released, it is actually CO2 that enters the water. Therefore, the same effect happens when carbon dioxide is released as already mentioned above; an increase in H+ decreases the pH and vice versa.
As dissolved oxygen and pH are vital for both aquatic plants and animals, and are strong indications of water quality, it is important to frequently test them in a wide range of applications.
The most reliable way to test DO and pH in applications is using a pH probe/sensor and a dissolved oxygen probe/sensor.
Dissolved oxygen is not affected by pH because there is no physical-chemical connection between the two. However, pH is sometimes indirectly affected by DO, but it is temperature, carbon dioxide, carbonate/bicarbonate ions, and the decomposition of organic material that have the greatest effect on altering the pH in water.
If you have any questions regarding pH or dissolved oxygen, or are unsure which pH/DO probe will best suit your needs, please do not hesitate to contact the world-class team at Atlas Scientific.
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