Testing dissolved oxygen (DO) in water is either measured via chemical analysis such as a titrimetric method, electroanalytical (using galvanic & polarographic probes), optical dissolved oxygen, and colorimetric methods. However, modern techniques mainly use electrochemical or optical sensor methods.
Oxygen is not only important in the air we breathe, but it is also an essential element in liquids like water which is why it is important to test dissolved oxygen (DO).
DO is the measurement of the number of free oxygen molecules in water. Measuring DO levels is an important indicator in industries such as water quality systems and aquatic ecosystems as oxygen is an essential chemical element for most forms of life.
In this article we will look at why testing DO is so important and the best ways to measure DO levels in water.
Testing Dissolved Oxygen (DO) In Water
DO is measured using a Dissolved Oxygen meter. The best time to measure DO in water is the same time every day, as concentrations can fluctuate throughout the day. DO is usually measured in milligrams per liter (mg/L) or percentage saturation (% sat), but sometimes it can be measured in parts per million (ppm), which allows measurement comparisons between sites that have different salinity and temperature values.
We can test DO in water through the following methods: titrimetric, electroanalytical (galvanic & polarographic probes), optical dissolved oxygen, and colorimetric.
Titrations use one liquid where the concentration is already determined (titrant), to identify the concentration of another (your sample).
Iodemtry titrations use iodine as an indicator; the iodine indicator will either appear or disappear at the end of the titration. Titrations for testing DO in water are known as the “Winkler Method”. The Winkler Method gives you a “one-time measurement” of the sample being tested.
When using the Winkler Method, water samples are collected, fixed, and titrated in the field or a laboratory setting. As atmospheric contact and agitation can shift DO levels, you must fix the sample with the reagents immediately. Titration methods use a specialized bottle (BOD bottle) that seals without trapping air inside. Usually, all reagents come pre-measured to make it easier and increase accuracy in testing DO in water. To get an accurate DO reading, ensure the titrant solution is proportional to the sample you are testing.
The Winkler Method is still commonly used to test DO in water, however, there are some concerns with inaccuracies, possible sample contaminants/interferences, and human error. That is why new technologies have created easier and more accurate ways to test DO in water.
Electroanyalytical (Galvanic & Polarographic Probes) Method
If you are testing in the lab or the field, this is probably the easiest way to test DO in water. Electroanalytical or electrochemical dissolved oxygen sensors are also known as amperometric or Clark-type Sensors.
There are two types of electroanalytical sensors: galvanic and polarographic probes. The probes work off redox (oxidation-reduction) reactions, providing continuous and live measurements. As both probes have an applied voltage, they require a “warm-up time” before use to polarize the electrodes before measuring the DO in water.
These are membrane probes that have two parts that produce a voltage, acting as a battery (the metals have different electrode potentials). A thin semi-permeable membrane inside the cap of the electrode allows gasses to pass through and block anything else. When oxygen diffuses across the membrane, it dissolves into the probe cap that contains a buffered electrolyte. This allows the oxygen to react with the cathode (usually silver) in the electrode, gaining an electron. The electron given to the oxygen molecule comes from the anode (usually zinc or lead) in the electrode, creating a voltage between the anode and cathode in the probe. It is when this current is formed the meter can convert the reading taken from the probe into a DO concentration value.
Because of self-polarization, these sensors do not require a warm-up time.
These work slightly differently from galvanic probes, but, polarographic probes also contain a thin semi-permeable membrane allowing oxygen into an unbuffered electrolyte. However, instead of acting like a battery, a voltage is applied between the silver anode and gold cathode in the probe. The voltage acts as a catalyst driving an oxygen reaction. When oxygen hits the cathode, an electron is added creating a current, determining the DO concentration.
Polarographic probes can be separated further into steady-state and rapid-pulsing sensors.
Steady-state sensors allow you to measure DO in water without having to stir the sample. When using a Rapid-pulsing sensor, there is also no need to stir the sample, but it contains a third silver electrode as these sensors turn on and off every few seconds to allow the DO to replenish on the cathode surface when it reaches the membrane. Both still utilize a cathode and anode and measure DO by creating a constant voltage to polarize the electrons.
Optical Dissolved Oxygen Method
This method also uses a probe with a semi-permeable membrane to test DO in water, but the probe and meter monitor luminescence instead of monitoring a reaction. Sometimes these DO meters are called fluorescent sensors. However, this is technically incorrect as the probes emit blue light not UV (ultraviolet) light.
The probe emits the blue light that excites (electrons gain energy) light-sensitive material inside the probe cap. When it becomes relaxed (reaches its normal energy state) it emits a red light that is measured as it hits the light sensor inside the probe; the red light is reflected by the dye. If DO is present in water, it suppresses the red light as wavelengths are limited/altered. The frequency, intensity, and decay of the red light are dependent on the amount of DO in the water.
While optical dissolved oxygen probes provide a continuous measurement of DO, they can be affected by humidity.
This method measures color and comes in two variations: Indigo Carmine and the Rhodazine D method. Chemical reagents are added that react with DO in the sample to display a particular color. The chemical reagents used are similar to the modern Winkler Method. How intense the color is, is proportional to how much DO is in the sample.
Indigo Carmine is used for measuring DO concentrations between 0.2 and 15 ppm, whereas Rhodazine D is used to measure much lower DO concentrations (ppb).
Indigo Carmine produces a blue color where the intensity is proportional to the DO concentration. If using this method, keep reagents away from bright lighting, as this can deteriorate the Indigo Carmine. This method is not affected by salinity, temperature, or dissolved gases, but ferric iron, and nitrate and sodium sulfate can. Results are obtained between 30 seconds (low-range tests) and 2-minutes (high-range tests).
Rhodazine D reagents react with DO, producing a rose-colored or pink solution. Oxidizing agents (chlorine, cupric copper, and ferric iron) can interfere with results creating higher DO readings, however, this method is not affected by salinity or sulfide that are usually present in water samples. As this method is time-dependent, ensure you analyze the water sample within 30 seconds of adding the reagent.
Either a spectrophotometer, colorimeter, or simple comparator can be used to measure DO in water using the colorimetric method.
Why Is It Important To Test Dissolved Oxygen (DO) In Water?
Testing DO in water depends on the industry. For example, you may need to test DO when brewing beer or measure the dissolved oxygen in wastewater.
Dissolved oxygen is an essential parameter in monitoring water quality and a key indicator of healthy aquatic ecosystems. Low DO levels in water are problematic for most aquatic life, often creating dead zones where aquatic life dies off.
In wastewater treatments, testing DO levels in water helps us understand the biodegradable organic matter and the biological oxygen demand (BOD). Both these tests indicate general water quality.
Alternatively, too much oxygen in water can also be harmful, this is known as supersaturated oxygen. DO in water originates from the atmosphere and photosynthesis, which can be affected by temperature, salinity, and atmospheric pressure.
What Can Affect Dissolved Oxygen (DO) In Water?
DO concentrations are affected by temperature, salinity, pressure, and humidity, so you will need to take this into account when testing DO.
Temperature is one of the biggest, if not the most, common factors that directly affects DO in water. Colder water contains more oxygen than warmer water, as particle motion decreases. As particles get more excited and bounce around more, they collide and break the bonds that hold them together.
Therefore, the lower the DO, the higher the temperature, and inversely the DO concentration increases as temperature decreases.
Salinity can also affect how much DO is in water. Freshwater contains more oxygen than saltwater because of the charge a salt molecule carries. Salt molecules are attracted to water molecules and easily dissolved in water. If salt is present, oxygen cannot attract to water molecules, therefore as salinity levels increase in a solution, DO decreases.
When we talk about pressure and DO, we are referring to atmospheric pressure. As atmospheric pressure decreases, the partial pressure of oxygen also decreases, therefore, the concentration of DO increases. So, as altitude or atmospheric pressure increases, the number of DO molecules absorbed in water decreases as there is less pressure forcing the oxygen to be diffused in the water, increasing the partial pressure of oxygen.
Water vapor or humidity is another factor that is often not thought about, yet it has major implications for DO concentrations, and can also affect the calibration of some DO meters.
When humidity levels increase, the partial pressure of oxygen increases, which also increases the level of DO.
Which Instrument Is the Best For Measuring Dissolved Oxygen (DO) In Water?
Now that you have an understanding of testing DO in water and why it is important, you may be thinking which equipment is best for testing. First, you need to think about accessibility, will you need a portable meter or will a bench top meter work better for you?
Depending on where you wish to test DO in water, there are two types used: portable meters or benchtop meters.
Portable meters provide flexibility to test wherever you want while still receiving high-level accurate readings.
They either use the colorimetric method, optical DO probe, or the electroanalytical method using galvanic or polarographic probes. Choosing which portable meter you need depends on the sample being measured, the level of accuracy you require, and your personal preference. Some portable meters can test more water parameters than DO so always do your research beforehand to get the right meter for you.
When it comes to electrochemical sensing and measuring DO in water, it can get confusing, which is why we offer a variety of meters to meet your testing needs. Whether you are measuring a simple sample or are working with a PLC, we have you covered when it comes to dissolved oxygen probes. All our portable meters allow you to take highly accurate and interference-free readings of DO in water.
Benchtop meters also come in a variety of types meeting your testing requirements. One thing you need to consider with benchtop meters is space. Some benchtop meters can take up more space than electroanalytical probes. However, there are some benchtop meters that have a “zero-footprint” allowing you to mount them to a wall.
Summing Up How To Test Dissolved Oxygen (DO) In Water & DO Testing Equipment
Dissolved oxygen is an important characteristic of water quality in many industries (hydroponics, food and beverage industries, aquariums, environmental sampling, wastewater, etc.).
Testing dissolved oxygen in water is either measured via chemical analysis such as a titrimetric method, electroanalytical (using galvanic & polarographic probes), optical dissolved oxygen, and colorimetric methods. However, modern techniques mainly use electrochemical probes.
If you would like to learn more about other water quality measurements, characteristics, or applications for DO, do not hesitate to contact our world-class team at Atlas Scientific.