How is dissolved oxygen measured

How is dissolved oxygen measured? There are a few different methods to measure dissolved oxygen in water and the following section provides an overview. Colorimeters, also known as filter photometers, are instruments that measure color intensity. When using these instruments, chemical reagents are mixed with the sample. If the target parameter is present, the solution will have a color, and its intensity will be proportional to the concentration of the parameter being tested.

Light is passed through a test tube containing the sample solution and then through a colored filter onto a photodetector. Filters are chosen so that light of a specific wavelength is selected. When the solution is colorless, all of the light passes through. With colored samples, light is absorbed, and that which passes through the sample is proportionately reduced.

Indigo carmine reacts with DO to form a blue complex. In contrast, Rhodazine D reacts with DO to form a bright pink complex. Reagents are also used when determining DO concentrations via a Winkler titration. In this method, reagents form an acid compound that's titrated with a neutralizing compound. Also, like the colorimetric method, a color change results, and the DO concentration is determined by observing the point at which this color change occurs.

Many standard operating procedures SOPs still call for a Winkler titration, especially at wastewater treatment labs that are determining biological oxygen demand BOD. Winklers need to be done in triplicate, with the results being averaged. Unlike the measurement of DO by performing a Winkler titration or using a colorimeter, electrochemical sensors, also known as membrane-covered DO sensors, don't require reagents.

These sensors provide fast measurements and have a wide range, but water must continuously move across the membrane as oxygen is consumed during the measurement.

There are two types of electrochemical sensors — polarographic and galvanic. In , Dr. The galvanic electrode was developed later on, but it measures DO the same way as the polarographic sensor. Electrochemical DO sensors consist of an anode and a cathode confined in electrolyte solution by an oxygen-permeable membrane. Oxygen molecules dissolved in the sample diffuse through the membrane before being reduced i. The amount of oxygen diffusing through the membrane is proportional to the partial pressure and concentration of oxygen outside the membrane.

As the oxygen concentration varies, so does the oxygen diffusing through the membrane, and this causes the probe current to change proportionally. Polarographic sensors have a silver anode and a gold cathode. These materials require the probe to warm up, or polarize, before use — this takes about 10 minutes. Polarographic sensors have a longer lifespan than galvanic sensors because it is not always on i. Galvanic sensors have a zinc anode and a silver cathode. These materials allow the sensor to be continuously polarized even when the meter is off, so no warm-up period is required.

There is a drawback to always being on — these sensors have a shorter life than polarographic sensors. Optical and electrochemical sensors have some similarities.

For starters, these sensors measure the pressure of oxygen dissolved in the sample. The higher the barometric pressure, the more oxygen will be pushed into the water.

Like electrochemical sensors, no reagents are required when using optical sensors. Both sensor types are also placed directly in the sample when taking a measurement. There are several key structures of an optical DO sensor. The sensor cap of an optical DO sensor contains a diffusion layer across which DO is constantly moving. Unlike electrochemical sensors, oxygen is not consumed during the measurement, so water does not need to flow continuously across the sensor cap. There are also different LEDs, one of which the blue light in most of our YSI sensors causes another layer of the sensor cap — the dye layer — to luminesce i.

As oxygen moves across the diffusion layer, it affects the luminescence of the dye layer. The amount of oxygen passing through the sensing layer is inversely proportional to the lifetime of the luminescence in the sensing layer.

The lifetime of the luminescence is measured by the sensor and compared against the reference the red light in our example , allowing for DO to be determined.

There are several options for measuring dissolved oxygen in water, and it can be challenging for those new to measuring DO to select the right method for them. Colorimeters are not typically used when the only parameter being measured is dissolved oxygen, as they are not convenient — it takes time to mix the reagent and solution! Additionally, there are some pretty tight limitations on the measurement range. Winkler titrations are time-consuming and challenging to perform.

In that case, we recommend using an automated titrator — check out some titration options from YSI — rather than doing the titrations by hand. For customers who require measuring DO in situ or have a high throughput of samples, we recommend using an electrochemical or optical sensor for DO measurement if you have a choice of method. Electrochemical and optical sensors are by far the most commonly used tools when measuring DO. Unlike other water quality sensors e.

Want to learn more about the measurement of DO, differences between DO sensors, and best practices? Download our DO Handbook! While electrochemical and optical DO sensors are suitable for many applications, the instruments they're used with are often designed with specific applications in mind.

Examples include:. The MultiLab W is the ideal instrument to use in a lab e. This is a lab instrument — it's not meant to be used outside! While the sensor technology is the same as is used on field instruments, the sensor bodies are designed for use in a controlled environment e.

The ProDSS is a portable system with a handheld, single cable, and a bulkhead where the sensors are installed. This is a true field instrument — rugged, waterproof case IP rated ; metal, military-spec MS cable connectors; and titanium sensors. This instrument is meant to be used for spot sampling, meaning it is not meant for unattended monitoring.

It has onboard batteries, data logging, and an onboard wiper, all of which allow for months-long deployments in harsh environments. This is the most advanced outdoor water quality monitoring platform we offer. The range goes from 0 to 14, with 7 being neutral. The pH of water is a very important measurement concerning water quality. Yes, water below your feet is moving all the time, but, no, if you have heard there are rivers flowing below ground, that is not true.

Water moves underground downward and sideways, in great quantities, due to gravity and pressure. Eventually it emerges back to the land surface, into rivers, and into the oceans to keep the water cycle going. Water temperature plays an important role in almost all USGS water science. Water temperature exerts a major influence on biological activity and growth, has an effect on water chemistry, can influence water quantity measurements, and governs the kinds of organisms that live in water bodies.

Water and electricity don't mix, right? Well actually, pure water is an excellent insulator and does not conduct electricity. The thing is, you won't find any pure water in nature, so don't mix electricity and water. Our Water Science School page will give you all the details. Lucky for us all, our drinking water is almost always clear very low turbidity. Other water, such as the creek behind your house after a rainstorm, is likely to be highly turbid—brown with floating sediment. Turbidity is the clarity of water and it is an important factor in water quality.

The USGS collaborates with local, state, federal, tribal, university, and industry partners to conduct the science necessary to understand the causes and effects of toxic HABs and inform water management and public health decisions.

USGS is characterizing the life cycle of HABs, their asociated toxins, and the genes responsible for cyanotoxin production. This work is enhancing the ability of Cyanobacterial harmful algal blooms HABs are increasingly a global concern because HABs pose a threat to human and aquatic ecosystem health and cause economic damages. Toxins produced by some species of cyanobacteria called cyanotoxins can cause acute and chronic illnesses in humans and pets. Eutrophication, or excess nutrients in streams, is typically one of the top reasons that a stream is listed as impaired on the d list as part of the Clean Water Act.

How nutrients, primarily nitrogen and phosphorus, are transported to streams and groundwater greatly affects the best management plan to keep them on fields and out of streams and groundwater. Likewise, environmental managers Accurate data for the concentration of dissolved oxygen in surface and ground waters are essential for documenting changes in environmental water resources that result from natural phenomena and human activities.

Dissolved oxygen is necessary in aquatic systems for the survival and growth of many aquatic organisms and is used as an indicator of The Lees Ferry site pictured here is one of six sites on the Colorado River being continuously monitored for dissolved oxygen concentrations. Josh Johnson tests water from the well for dissolved oxygen. The test is one of many performed on site to help the field crew know when to collect samples that will be sent to the laboratories for further testing.

Algal blooms are true to their name—they bloom for relatively short times. But just because they are less than permanent fixtures in the hydrologic landscape doesn't mean that they can't have a big, and nasty, impact on a poor lake subjected to them. Skip to main content.

Search Search. Water Science School. Man-made causes of aeration vary from an aquarium air pump to a hand-turned waterwheel to a large dam. Dissolved oxygen is also produced as a waste product of photosynthesis from phytoplankton, algae, seaweed and other aquatic plants 8. While most photosynthesis takes place at the surface by shallow water plants and algae , a large portion of the process takes place underwater by seaweed, sub-surface algae and phytoplankton.

Light can penetrate water, though the depth that it can reach varies due to dissolved solids and other light-scattering elements present in the water. Depth also affects the wavelengths available to plants, with red being absorbed quickly and blue light being visible past m. In clear water, there is no longer enough light for photosynthesis to occur beyond m, and aquatic plants no longer grow.

In turbid water, this photic light-penetrating zone is often much shallower. The basic reaction of aquatic photosynthesis remains:. At equilibrium, the percentage of each gas in the water would be equivalent to the percentage of that gas in the atmosphere — i.

The water will slowly absorb oxygen and other gasses from the atmosphere until it reaches equilibrium at complete saturation This is true of both atmospheric and hydrostatic pressures. Water at lower altitudes can hold more dissolved oxygen than water at higher altitudes.

As oxygen in the atmosphere is about However, there are several factors that can affect this. Aquatic respiration and decomposition lower DO concentrations, while rapid aeration and photosynthesis can contribute to supersaturation. During the process of photosynthesis, oxygen is produced as a waste product.

In addition, the equalization of water is a slow process except in highly agitated or aerated situations. Unlike small rapids and waves, the water flowing over a dam or waterfall traps and carries air with it, which is then plunged into the water.

As water temperature rises, oxygen solubility decreases. But if there is no wind to move the equilibration along, the lake will still contain that initial 9.

Dissolved oxygen concentrations are constantly affected by diffusion and aeration, photosynthesis, respiration and decomposition. In freshwater systems such as lakes, rivers and streams, dissolved oxygen concentrations will vary by season, location and water depth. Saltwater holds less oxygen than freshwater, so oceanic DO concentrations tend to be lower than those of freshwater. Coldwater fish like trout and salmon are most affected by low dissolved oxygen levels The mean DO level for adult salmonids is 6.

The mean DO levels should remain near 5. The freshwater fish most tolerant to DO levels include fathead minnows and northern pike. Northern pike can survive at dissolved oxygen concentrations as low as 0. If all the oxygen at their water level gets used up, bacteria will start using nitrate to decompose organic matter, a process known as denitrification.

If organic matter accumulates faster than it decomposes, sediment at the bottom of a lake simply becomes enriched by the organic material. This does not mean that saltwater fish can live without dissolved oxygen completely.

The red hake is also extremely sensitive to dissolved oxygen levels, abandoning its preferred habitat near the seafloor if concentrations fall below 4. The dissolved oxygen requirements of open-ocean and deep-ocean fish are a bit harder to track, but there have been some studies in the area. Otherwise, use the chart below to find saturation at a given temperature.

Show Credits Hide Oxygen solubility at different water temperatures. Your Account. Bruckner, Montana State University. Show credits. The Winkler method is done by noting a color change when titrating a fresh water sample.